From 54002f5bde9b096f4dae8d99c97d58ecbd589264 Mon Sep 17 00:00:00 2001
From: Daniel Slaugh <daniel.slaugh@hotmail.com>
Date: Tue, 24 Mar 2026 13:44:37 -0600
Subject: [PATCH 01/93] Resolves hydroserver2/hydroserver#371

---
 .../src/components/Site/SiteForm.vue          | 31 ++++++++++++----
 .../data-management/src/pages/SiteDetails.vue |  3 +-
 apps/data-management/src/store/photos.ts      |  9 +++--
 tests/e2e/specs/sites.spec.ts                 | 35 +++++++++++++++++++
 4 files changed, 69 insertions(+), 9 deletions(-)

diff --git a/apps/data-management/src/components/Site/SiteForm.vue b/apps/data-management/src/components/Site/SiteForm.vue
index 8a193cc5..61191cd2 100644
--- a/apps/data-management/src/components/Site/SiteForm.vue
+++ b/apps/data-management/src/components/Site/SiteForm.vue
@@ -213,9 +213,9 @@ const countryTitle = (item: { name: string; code: string } | undefined) => {
 }
 
 const { thing: storedThing } = storeToRefs(useThingStore())
-const { updatePhotos } = usePhotosStore()
+const { updatePhotos, resetPendingPhotos } = usePhotosStore()
 const { updateRatingCurves, resetRatingCurves } = useRatingCurveStore()
-const { tags } = storeToRefs(useTagStore())
+const { tags, previewTags } = storeToRefs(useTagStore())
 const { updateTags } = useTagStore()
 const vocabularyStore = useVocabularyStore()
 
@@ -230,6 +230,21 @@ const myForm = ref<VForm>()
 const thing = reactive<Thing>(new Thing())
 const includeDataDisclaimer = ref(thing.dataDisclaimer !== '')
 
+function cloneThing(source?: Thing) {
+  const clonedThing = source
+    ? JSON.parse(JSON.stringify(source))
+    : new Thing()
+
+  Object.assign(thing, new Thing(), clonedThing)
+  thing.location = Object.assign(new Thing().location, clonedThing.location ?? {})
+}
+
+function resetPendingSiteChanges() {
+  resetPendingPhotos()
+  resetRatingCurves()
+  previewTags.value = props.thingId ? [...tags.value] : []
+}
+
 watch(
   () => includeDataDisclaimer.value,
   (newVal) => {
@@ -241,12 +256,15 @@ watch(
 )
 
 async function populateThing() {
-  Object.assign(thing, storedThing.value)
+  cloneThing(storedThing.value)
   if (thing.location.latitude && thing.location.longitude) loaded.value = true
 }
 
 function closeDialog() {
-  resetRatingCurves()
+  if (props.thingId) cloneThing(storedThing.value)
+  else cloneThing()
+  includeDataDisclaimer.value = !!thing.dataDisclaimer
+  resetPendingSiteChanges()
   emit('close')
 }
 
@@ -296,17 +314,18 @@ function onMapLocationClicked(locationData: Thing) {
 }
 
 onMounted(async () => {
-  resetRatingCurves()
+  resetPendingSiteChanges()
   countries.value = countryList.getData()
   if (props.thingId) {
     await populateThing()
     includeDataDisclaimer.value = !!thing.dataDisclaimer
   } else {
+    cloneThing()
     loaded.value = true
   }
 })
 
 onUnmounted(() => {
-  resetRatingCurves()
+  resetPendingSiteChanges()
 })
 </script>
diff --git a/apps/data-management/src/pages/SiteDetails.vue b/apps/data-management/src/pages/SiteDetails.vue
index 52c85dc2..15853b15 100644
--- a/apps/data-management/src/pages/SiteDetails.vue
+++ b/apps/data-management/src/pages/SiteDetails.vue
@@ -131,8 +131,9 @@
         >
           Edit site information
         </v-btn>
-        <v-dialog v-if="thing" v-model="isRegisterModalOpen" width="80rem">
+        <v-dialog v-model="isRegisterModalOpen" width="80rem">
           <SiteForm
+            v-if="thing && isRegisterModalOpen"
             @close="onSiteFormClosed"
             :thing-id="thingId"
             :workspace-id="thing.workspaceId"
diff --git a/apps/data-management/src/store/photos.ts b/apps/data-management/src/store/photos.ts
index 5c771eb3..1d77a3fc 100644
--- a/apps/data-management/src/store/photos.ts
+++ b/apps/data-management/src/store/photos.ts
@@ -8,6 +8,11 @@ export const usePhotosStore = defineStore('photos', () => {
   const photosToDelete = ref<string[]>([])
   const loading = ref(false)
 
+  const resetPendingPhotos = () => {
+    newPhotos.value = []
+    photosToDelete.value = []
+  }
+
   const uploadNewPhotos = async (thingId: string) => {
     if (!newPhotos.value.length) return
 
@@ -44,8 +49,7 @@ export const usePhotosStore = defineStore('photos', () => {
       console.error('Error updating photos', error)
     } finally {
       loading.value = false
-      newPhotos.value = []
-      photosToDelete.value = []
+      resetPendingPhotos()
     }
   }
 
@@ -54,6 +58,7 @@ export const usePhotosStore = defineStore('photos', () => {
     newPhotos,
     photosToDelete,
     loading,
+    resetPendingPhotos,
     updatePhotos,
   }
 })
diff --git a/tests/e2e/specs/sites.spec.ts b/tests/e2e/specs/sites.spec.ts
index 9321e7fb..46c01e81 100644
--- a/tests/e2e/specs/sites.spec.ts
+++ b/tests/e2e/specs/sites.spec.ts
@@ -151,6 +151,41 @@ test.describe('sites and workspaces', () => {
     await expect(page).toHaveURL(/\/sites$/)
   })
 
+  test('canceling or dismissing site edits discards unsaved changes', async ({
+    page,
+  }) => {
+    const stamp = Date.now()
+    const unsavedProvince = `Unsaved Province ${stamp}`
+    const unsavedCounty = `Unsaved County ${stamp}`
+
+    await authenticateSession(page, users.owner.email, users.owner.password)
+    await page.goto(`/sites/${fixtures.things.mutablePublic.id}`)
+
+    await page.getByTestId('edit-site-button').click()
+    await page.getByLabel('State/Province/Region').fill(unsavedProvince)
+    await page.getByRole('button', { name: 'Cancel' }).click()
+
+    await expect(page.getByText(unsavedProvince, { exact: true })).toHaveCount(0)
+
+    await page.getByTestId('edit-site-button').click()
+    await expect(page.getByLabel('State/Province/Region')).not.toHaveValue(
+      unsavedProvince
+    )
+    await page.getByRole('button', { name: 'Cancel' }).click()
+
+    await page.getByTestId('edit-site-button').click()
+    await page.getByLabel('County/District').fill(unsavedCounty)
+    await page.keyboard.press('Escape')
+
+    await expect(page.getByText(unsavedCounty, { exact: true })).toHaveCount(0)
+
+    await page.getByTestId('edit-site-button').click()
+    await expect(page.getByLabel('County/District')).not.toHaveValue(
+      unsavedCounty
+    )
+    await page.getByRole('button', { name: 'Cancel' }).click()
+  })
+
   test('owner can edit, toggle privacy for, and delete a site with datastream CRUD', async ({
     page,
     browser,

From 73679ae8c64fffa03c02c8957b99af4984286e0e Mon Sep 17 00:00:00 2001
From: Daniel Slaugh <daniel.slaugh@hotmail.com>
Date: Wed, 25 Mar 2026 10:44:22 -0600
Subject: [PATCH 02/93] Mount app immediately then async load API data

---
 apps/data-management/src/App.vue              |  44 +++--
 .../__tests__/appInitialization.spec.ts       | 181 ++++++++++++++++++
 .../src/bootstrap/appInitialization.ts        | 110 +++++++++++
 .../DatastreamInformationPanels.vue           |  24 +--
 .../components/Workspace/WorkspaceToolbar.vue |  22 ++-
 apps/data-management/src/main.ts              |  62 ++----
 apps/data-management/src/pages/Sites.vue      | 114 ++++++++---
 .../src/pages/__tests__/Sites.spec.ts         | 163 ++++++++++++++++
 apps/data-management/src/router/router.ts     |   7 +
 tests/e2e/specs/visualization.spec.ts         |  18 +-
 tests/e2e/support/ui.ts                       |  15 +-
 11 files changed, 657 insertions(+), 103 deletions(-)
 create mode 100644 apps/data-management/src/bootstrap/__tests__/appInitialization.spec.ts
 create mode 100644 apps/data-management/src/bootstrap/appInitialization.ts
 create mode 100644 apps/data-management/src/pages/__tests__/Sites.spec.ts

diff --git a/apps/data-management/src/App.vue b/apps/data-management/src/App.vue
index c1da9fb8..6785a972 100644
--- a/apps/data-management/src/App.vue
+++ b/apps/data-management/src/App.vue
@@ -1,24 +1,44 @@
 <template>
-  <v-app>
-    <Navbar v-if="!route.meta.hideNavBar" />
+  <router-view v-slot="{ Component }">
+    <v-app>
+      <Navbar
+        v-if="Component && isHydroServerReady && !route.meta.hideNavBar"
+      />
 
-    <v-main>
-      <router-view />
-    </v-main>
+      <v-main>
+        <component :is="Component" v-if="Component" />
+        <v-container v-else-if="initializationError" class="fill-height">
+          <v-row justify="center" align="center">
+            <v-col cols="12" md="6" class="text-center">
+              <h5 class="text-h5">Unable to load HydroServer</h5>
+              <p class="mt-3 text-body-1">
+                Check your connection and refresh the page.
+              </p>
+            </v-col>
+          </v-row>
+        </v-container>
+        <FullScreenLoader v-else loading-text="Loading HydroServer..." />
+      </v-main>
 
-    <Footer v-if="!route.meta.hideFooter" class="flex-grow-0" />
-    <Notifications />
-    <link
-      href="https://fonts.googleapis.com/css?family=Roboto:100,300,400,500,700,900"
-      rel="stylesheet"
-    />
-  </v-app>
+      <Footer v-if="Component && !route.meta.hideFooter" class="flex-grow-0" />
+      <Notifications />
+      <link
+        href="https://fonts.googleapis.com/css?family=Roboto:100,300,400,500,700,900"
+        rel="stylesheet"
+      />
+    </v-app>
+  </router-view>
 </template>
 
 <script setup lang="ts">
+import {
+  initializationError,
+  isHydroServerReady,
+} from '@/bootstrap/appInitialization'
 import Navbar from '@/components/base/Navbar.vue'
 import Footer from '@/components/base/Footer.vue'
 import Notifications from '@/components/base/Notifications.vue'
+import FullScreenLoader from '@/components/base/FullScreenLoader.vue'
 import { useRoute } from 'vue-router'
 
 const route = useRoute()
diff --git a/apps/data-management/src/bootstrap/__tests__/appInitialization.spec.ts b/apps/data-management/src/bootstrap/__tests__/appInitialization.spec.ts
new file mode 100644
index 00000000..80b105bf
--- /dev/null
+++ b/apps/data-management/src/bootstrap/__tests__/appInitialization.spec.ts
@@ -0,0 +1,181 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { createPinia, setActivePinia } from 'pinia'
+import { useUserStore } from '@/store/user'
+import { useWorkspaceStore } from '@/store/workspaces'
+
+const {
+  createHydroServerMock,
+  fetchAllVocabulariesMock,
+  userGetMock,
+  listAllItemsMock,
+  sessionState,
+} = vi.hoisted(() => ({
+  createHydroServerMock: vi.fn(),
+  fetchAllVocabulariesMock: vi.fn(),
+  userGetMock: vi.fn(),
+  listAllItemsMock: vi.fn(),
+  sessionState: {
+    isAuthenticated: false,
+  },
+}))
+
+vi.mock('@hydroserver/client', () => {
+  class User {
+    email = ''
+  }
+
+  return {
+    User,
+    createHydroServer: createHydroServerMock,
+    default: {
+      session: sessionState,
+      user: {
+        get: userGetMock,
+      },
+      workspaces: {
+        listAllItems: listAllItemsMock,
+      },
+    },
+  }
+})
+
+vi.mock('@/composables/useVocabulary', () => ({
+  useVocabularyStore: () => ({
+    fetchAllVocabularies: fetchAllVocabulariesMock,
+  }),
+}))
+
+describe('app initialization bootstrap', () => {
+  beforeEach(() => {
+    vi.resetModules()
+    setActivePinia(createPinia())
+    localStorage.clear()
+    sessionState.isAuthenticated = false
+    createHydroServerMock.mockReset()
+    fetchAllVocabulariesMock.mockReset()
+    userGetMock.mockReset()
+    listAllItemsMock.mockReset()
+  })
+
+  it('waits for HydroServer session initialization before resolving router guards', async () => {
+    let resolveHydroServer!: () => void
+    createHydroServerMock.mockImplementation(
+      () =>
+        new Promise<void>((resolve) => {
+          resolveHydroServer = resolve
+        })
+    )
+    fetchAllVocabulariesMock.mockResolvedValue(undefined)
+
+    const {
+      startAppInitialization,
+      waitForHydroServerInitialization,
+      isHydroServerReady,
+    } = await import('../appInitialization')
+
+    void startAppInitialization()
+
+    let waitResolved = false
+    const guardWait = waitForHydroServerInitialization().then(() => {
+      waitResolved = true
+    })
+
+    await Promise.resolve()
+    expect(waitResolved).toBe(false)
+    expect(isHydroServerReady.value).toBe(false)
+
+    resolveHydroServer()
+    await guardWait
+
+    expect(waitResolved).toBe(true)
+    expect(isHydroServerReady.value).toBe(true)
+    expect(fetchAllVocabulariesMock).toHaveBeenCalledTimes(1)
+  })
+
+  it('does not mark workspace bootstrap complete when initialized while logged out', async () => {
+    createHydroServerMock.mockResolvedValue(undefined)
+    fetchAllVocabulariesMock.mockResolvedValue(undefined)
+
+    const {
+      hasBootstrappedWorkspaces,
+      startAppInitialization,
+      isAppInitializing,
+    } = await import('../appInitialization')
+
+    await startAppInitialization()
+
+    expect(hasBootstrappedWorkspaces.value).toBe(false)
+    expect(listAllItemsMock).not.toHaveBeenCalled()
+    expect(isAppInitializing.value).toBe(false)
+  })
+
+  it('loads vocabularies, user, and workspaces after first session initialization', async () => {
+    sessionState.isAuthenticated = true
+    createHydroServerMock.mockResolvedValue(undefined)
+    fetchAllVocabulariesMock.mockResolvedValue(undefined)
+    userGetMock.mockResolvedValue({
+      status: 200,
+      data: { email: 'user@example.com' },
+    })
+    listAllItemsMock.mockResolvedValue([
+      { id: 'workspace-2', name: 'Workspace 2' },
+      { id: 'workspace-1', name: 'Workspace 1' },
+    ])
+
+    const {
+      hasBootstrappedWorkspaces,
+      startAppInitialization,
+      isAppInitializing,
+    } = await import('../appInitialization')
+
+    expect(isAppInitializing.value).toBe(false)
+
+    await startAppInitialization()
+
+    const userStore = useUserStore()
+    const workspaceStore = useWorkspaceStore()
+
+    expect(fetchAllVocabulariesMock).toHaveBeenCalledTimes(1)
+    expect(userGetMock).toHaveBeenCalledTimes(1)
+    expect(listAllItemsMock).toHaveBeenCalledWith({
+      is_associated: true,
+      expand_related: true,
+    })
+    expect(userStore.user.email).toBe('user@example.com')
+    expect(workspaceStore.workspaces.map((workspace) => workspace.id)).toEqual([
+      'workspace-1',
+      'workspace-2',
+    ])
+    expect(workspaceStore.selectedWorkspace?.id).toBe('workspace-1')
+    expect(hasBootstrappedWorkspaces.value).toBe(true)
+    expect(isAppInitializing.value).toBe(false)
+  })
+
+  it('records initialization errors and skips background app bootstrap when HydroServer setup fails', async () => {
+    const initializationFailure = new Error('unable to initialize')
+    const consoleErrorSpy = vi
+      .spyOn(console, 'error')
+      .mockImplementation(() => undefined)
+    createHydroServerMock.mockRejectedValue(initializationFailure)
+
+    const {
+      initializationError,
+      isAppInitializing,
+      isHydroServerReady,
+      startAppInitialization,
+      waitForHydroServerInitialization,
+    } = await import('../appInitialization')
+
+    await startAppInitialization()
+    await waitForHydroServerInitialization()
+
+    expect(isHydroServerReady.value).toBe(false)
+    expect(isAppInitializing.value).toBe(false)
+    expect(initializationError.value).toBe(initializationFailure)
+    expect(fetchAllVocabulariesMock).not.toHaveBeenCalled()
+    expect(userGetMock).not.toHaveBeenCalled()
+    expect(listAllItemsMock).not.toHaveBeenCalled()
+
+    consoleErrorSpy.mockRestore()
+  })
+})
diff --git a/apps/data-management/src/bootstrap/appInitialization.ts b/apps/data-management/src/bootstrap/appInitialization.ts
new file mode 100644
index 00000000..387c4ad6
--- /dev/null
+++ b/apps/data-management/src/bootstrap/appInitialization.ts
@@ -0,0 +1,110 @@
+import { useVocabularyStore } from '@/composables/useVocabulary'
+import { useUserStore } from '@/store/user'
+import { useWorkspaceStore } from '@/store/workspaces'
+import hs, { createHydroServer, User } from '@hydroserver/client'
+import { storeToRefs } from 'pinia'
+import { ref } from 'vue'
+
+const hydroServerHost = import.meta.env.VITE_APP_PROXY_BASE_URL
+  ? ''
+  : import.meta.env.DEV
+  ? 'http://127.0.0.1:8000'
+  : ''
+
+const isHydroServerInitializing = ref(false)
+export const isHydroServerReady = ref(false)
+export const isAppInitializing = ref(false)
+export const hasBootstrappedWorkspaces = ref(false)
+export const initializationError = ref<unknown>(null)
+
+let hydroServerInitializationPromise: Promise<void> | null = null
+let appInitializationPromise: Promise<void> | null = null
+
+function recordInitializationError(message: string, error: unknown) {
+  console.error(message, error)
+  if (initializationError.value == null) {
+    initializationError.value = error
+  }
+}
+
+async function initializeAuthenticatedState() {
+  const { user } = storeToRefs(useUserStore())
+  const { setWorkspaces } = useWorkspaceStore()
+
+  user.value = new User()
+  if (!hs.session.isAuthenticated) return
+
+  const userRequest = hs.user.get().catch((error) => {
+    recordInitializationError('Error fetching user', error)
+    return null
+  })
+
+  const workspacesRequest = hs.workspaces
+    .listAllItems({
+      is_associated: true,
+      expand_related: true,
+    })
+    .catch((error) => {
+      recordInitializationError('Error fetching workspaces', error)
+      return null
+    })
+
+  const [userResponse, workspacesResponse] = await Promise.all([
+    userRequest,
+    workspacesRequest,
+  ])
+
+  if (userResponse) {
+    user.value = userResponse.status === 401 ? new User() : userResponse.data
+  }
+
+  if (workspacesResponse) {
+    setWorkspaces(workspacesResponse)
+    hasBootstrappedWorkspaces.value = true
+  }
+}
+
+export function startAppInitialization() {
+  if (appInitializationPromise) return appInitializationPromise
+
+  initializationError.value = null
+  isHydroServerInitializing.value = true
+  isHydroServerReady.value = false
+  isAppInitializing.value = true
+  hasBootstrappedWorkspaces.value = false
+
+  hydroServerInitializationPromise = createHydroServer({
+    host: hydroServerHost,
+  })
+    .then(() => {
+      isHydroServerReady.value = true
+    })
+    .catch((error) => {
+      recordInitializationError('Error initializing HydroServer client', error)
+    })
+    .finally(() => {
+      isHydroServerInitializing.value = false
+    })
+
+  appInitializationPromise = hydroServerInitializationPromise
+    .then(async () => {
+      if (!isHydroServerReady.value) return
+
+      const vocabularyRequest = useVocabularyStore()
+        .fetchAllVocabularies()
+        .catch((error) => {
+          recordInitializationError('Error fetching vocabularies', error)
+        })
+
+      await Promise.all([vocabularyRequest, initializeAuthenticatedState()])
+    })
+    .finally(() => {
+      isAppInitializing.value = false
+    })
+
+  return appInitializationPromise
+}
+
+export function waitForHydroServerInitialization() {
+  return hydroServerInitializationPromise ?? Promise.resolve()
+}
diff --git a/apps/data-management/src/components/Datastream/DatastreamInformationPanels.vue b/apps/data-management/src/components/Datastream/DatastreamInformationPanels.vue
index e4598b25..d27553bd 100644
--- a/apps/data-management/src/components/Datastream/DatastreamInformationPanels.vue
+++ b/apps/data-management/src/components/Datastream/DatastreamInformationPanels.vue
@@ -124,12 +124,12 @@ type ListItemArray = { label: string; value: any }[]
 
 const expandedPanels = ref<number[]>([])
 const datastream = ref<any>()
-let generalItems: ListItemArray = []
-let locationItems: ListItemArray = []
-let sensorItems: ListItemArray = []
-let observedPropertyItems: ListItemArray = []
-let unitItems: ListItemArray = []
-let processingLevelItems: ListItemArray = []
+const generalItems = ref<ListItemArray>([])
+const locationItems = ref<ListItemArray>([])
+const sensorItems = ref<ListItemArray>([])
+const observedPropertyItems = ref<ListItemArray>([])
+const unitItems = ref<ListItemArray>([])
+const processingLevelItems = ref<ListItemArray>([])
 
 onMounted(async () => {
   datastream.value = await hs.datastreams.getItem(props.datastreamId, {
@@ -147,7 +147,7 @@ onMounted(async () => {
     }
   }
 
-  generalItems = [
+  generalItems.value = [
     { label: 'Workspace Name', value: d.workspace.name },
     { label: 'Datastream name', value: d.name },
     { label: 'Description', value: d.description },
@@ -195,7 +195,7 @@ onMounted(async () => {
   ]
 
   const op = d.observedProperty
-  observedPropertyItems = [
+  observedPropertyItems.value = [
     { label: 'Name', value: op.name },
     { label: 'Definition', value: op.definition },
     { label: 'Description', value: op.description },
@@ -205,7 +205,7 @@ onMounted(async () => {
 
   let t = d.thing
   let l = t.location
-  locationItems = [
+  locationItems.value = [
     { label: 'Site name', value: t.name },
     { label: 'Site code', value: t.samplingFeatureCode },
     { label: 'Description', value: t.description },
@@ -224,7 +224,7 @@ onMounted(async () => {
   ]
 
   let u = d.unit
-  unitItems = [
+  unitItems.value = [
     { label: 'Name', value: u.name },
     { label: 'Symbol', value: u.symbol },
     { label: 'Definition', value: u.definition },
@@ -232,7 +232,7 @@ onMounted(async () => {
   ]
 
   const s = d.sensor
-  sensorItems = [
+  sensorItems.value = [
     { label: 'Name', value: s.name },
     { label: 'Description', value: s.description },
     { label: 'Manufacturer', value: s.manufacturer },
@@ -245,7 +245,7 @@ onMounted(async () => {
   ]
 
   const pl = d.processingLevel
-  processingLevelItems = [
+  processingLevelItems.value = [
     { label: 'Code', value: pl.code },
     { label: 'Definition', value: pl.definition },
     { label: 'Explanation', value: pl.explanation },
diff --git a/apps/data-management/src/components/Workspace/WorkspaceToolbar.vue b/apps/data-management/src/components/Workspace/WorkspaceToolbar.vue
index 9580530f..5ab94dfe 100644
--- a/apps/data-management/src/components/Workspace/WorkspaceToolbar.vue
+++ b/apps/data-management/src/components/Workspace/WorkspaceToolbar.vue
@@ -325,6 +325,7 @@ import { computed, ref, watch } from 'vue'
 import WorkspaceFormCard from '@/components/Workspace/WorkspaceFormCard.vue'
 import DeleteWorkspaceCard from './DeleteWorkspaceCard.vue'
 import WorkspaceAccessControl from '@/components/Workspace/AccessControl/WorkspaceAccessControl.vue'
+import { isAppInitializing } from '@/bootstrap/appInitialization'
 import { storeToRefs } from 'pinia'
 import { useWorkspaceStore } from '@/store/workspaces'
 import PermissionTooltip from '../PermissionTooltip.vue'
@@ -361,7 +362,7 @@ const { setWorkspaces } = useWorkspaceStore()
 const { hasPermission, getUserRoleName } = useWorkspacePermissions()
 const { user } = storeToRefs(useUserStore())
 
-const openWorkspaceTable = ref(!workspaces.value.length)
+const openWorkspaceTable = ref(false)
 const openTransferTable = ref(false)
 const openCreate = ref(false)
 const openEdit = ref(false)
@@ -370,11 +371,22 @@ const openAccessControl = ref(false)
 const search = ref<string>('')
 const activeItem = ref<Workspace>({} as Workspace)
 const showWorkspaceHelp = ref(false)
+const initializedWorkspaceTableState = ref(false)
 
 const canCreateWorkspace = computed(() =>
   ['admin', 'standard'].includes(user.value?.accountType ?? '')
 )
 
+watch(
+  [isAppInitializing, workspaces],
+  ([appInitializing, currentWorkspaces]) => {
+    if (initializedWorkspaceTableState.value || appInitializing) return
+    initializedWorkspaceTableState.value = true
+    if (!currentWorkspaces.length) openWorkspaceTable.value = true
+  },
+  { immediate: true }
+)
+
 const selectedWorkspaceId = ref('')
 watch(
   selectedWorkspaceId,
@@ -409,11 +421,13 @@ function openDialog(
  * always be the source of truth. This function syncs the table items with the db and global workspaces.
  */
 const refreshWorkspaces = async (workspace?: Workspace) => {
-  const res = await hs.workspaces.listItems({
+  const keepWorkspaceTableOpen = openWorkspaceTable.value
+  const res = await hs.workspaces.listAllItems({
     is_associated: true,
-    fetch_all: true,
+    expand_related: true,
   })
   setWorkspaces(res)
+  openWorkspaceTable.value = keepWorkspaceTableOpen
 
   if (
     workspace &&
@@ -453,7 +467,7 @@ async function onDelete() {
   const res = await hs.workspaces.delete(activeItem.value.id)
   if (res.ok) {
     Snackbar.success('Workspace deleted')
-    refreshWorkspaces()
+    await refreshWorkspaces()
   } else Snackbar.error(res.message)
 }
 
diff --git a/apps/data-management/src/main.ts b/apps/data-management/src/main.ts
index 1027fe78..65ba5ab6 100644
--- a/apps/data-management/src/main.ts
+++ b/apps/data-management/src/main.ts
@@ -6,61 +6,23 @@ import { createApp } from 'vue'
 import App from './App.vue'
 import router from './router/router'
 import vuetify from '@/plugins/vuetify'
-import { createPinia, storeToRefs } from 'pinia'
+import { createPinia } from 'pinia'
 import { injectClarity } from '@/plugins/clarity'
 import { settings } from '@/config/settings'
-import hs, { createHydroServer, User } from '@hydroserver/client'
-import { useVocabularyStore } from './composables/useVocabulary'
-import { useWorkspaceStore } from '@/store/workspaces'
-import { useUserStore } from './store/user'
+import { startAppInitialization } from '@/bootstrap/appInitialization'
 
 const app = createApp(App)
 const pinia = createPinia()
-const hydroServerHost =
-  import.meta.env.VITE_APP_PROXY_BASE_URL
-    ? ''
-    : import.meta.env.DEV
-    ? 'http://127.0.0.1:8000'
-    : ''
 
-async function initializeApp() {
-  app.use(pinia)
+app.use(pinia)
 
-  // The session must be initialized before the router because some of the routes depend on the session state for access control
-  await createHydroServer({ host: hydroServerHost })
+// Kick off session initialization before router navigation starts, but do not
+// block first paint on the rest of the app bootstrap work.
+void startAppInitialization()
 
-  const vocabularyStore = useVocabularyStore()
-  await Promise.all([vocabularyStore.fetchAllVocabularies()])
-
-  const { user } = storeToRefs(useUserStore())
-
-  // Avoid spamming the console with an expected 401 before a user logs in.
-  // The session snapshot already tells us whether an authenticated user exists.
-  user.value = new User()
-  if (hs.session.isAuthenticated) {
-    const res = await hs.user.get()
-    user.value = res.status == 401 ? new User() : res.data
-  }
-
-  if (hs.session.isAuthenticated) {
-    try {
-      const workspacesResponse = await hs.workspaces.listAllItems({
-        is_associated: true,
-        expand_related: true,
-      })
-      const { setWorkspaces } = useWorkspaceStore()
-      setWorkspaces(workspacesResponse)
-    } catch (error) {
-      console.error('Error fetching workspaces', error)
-    }
-  }
-
-  app.use(router)
-  app.use(vuetify)
-  settings.analyticsConfiguration.enableClarityAnalytics &&
-    settings.analyticsConfiguration.clarityProjectId &&
-    injectClarity(settings.analyticsConfiguration.clarityProjectId)
-  app.mount('#app')
-}
-
-initializeApp()
+app.use(router)
+app.use(vuetify)
+settings.analyticsConfiguration.enableClarityAnalytics &&
+  settings.analyticsConfiguration.clarityProjectId &&
+  injectClarity(settings.analyticsConfiguration.clarityProjectId)
+app.mount('#app')
diff --git a/apps/data-management/src/pages/Sites.vue b/apps/data-management/src/pages/Sites.vue
index 74b252d0..e2da4c01 100644
--- a/apps/data-management/src/pages/Sites.vue
+++ b/apps/data-management/src/pages/Sites.vue
@@ -17,7 +17,11 @@
         </v-col>
       </v-row>
 
-      <v-card class="mb-1" elevation="2">
+      <v-card
+        v-if="hasWorkspaces && selectedWorkspace !== null"
+        class="mb-1"
+        elevation="2"
+      >
         <KeepAlive>
           <v-expand-transition>
             <SiteFilterToolbar
@@ -30,6 +34,18 @@
         </KeepAlive>
       </v-card>
 
+      <v-card
+        v-if="!hasWorkspaces || selectedWorkspace === null"
+        class="pa-8 text-center"
+        elevation="2"
+      >
+        <v-icon size="48" color="grey lighten-1" :icon="mdiBriefcaseOutline" />
+        <h4 class="mt-2">No workspace selected</h4>
+        <p class="mb-0">
+          Create or join a workspace to register and manage sites.
+        </p>
+      </v-card>
+
       <v-card v-if="hasWorkspaces && selectedWorkspace !== null">
         <v-toolbar flat color="blue-darken-2">
           <v-text-field
@@ -138,11 +154,13 @@ import OpenLayersMap from '@/components/Maps/OpenLayersMap.vue'
 import SiteForm from '@/components/Site/SiteForm.vue'
 import SiteFilterToolbar from '@/components/Site/SiteFilterToolbar.vue'
 import WorkspaceToolbar from '@/components/Workspace/WorkspaceToolbar.vue'
-import hs, {
-  PermissionResource,
-  PermissionAction,
-} from '@hydroserver/client'
+import hs, { PermissionResource, PermissionAction } from '@hydroserver/client'
 import { listThingSiteSummaries } from '@/api/thingSiteSummaries'
+import {
+  hasBootstrappedWorkspaces,
+  isAppInitializing,
+  startAppInitialization,
+} from '@/bootstrap/appInitialization'
 import { addColorToMarkers } from '@/utils/maps/markers'
 import { ThingSiteSummary, ThingSiteSummaryWithColor } from '@/types'
 import { Snackbar } from '@/utils/notifications'
@@ -151,6 +169,7 @@ import { useWorkspaceStore } from '@/store/workspaces'
 import FullScreenLoader from '@/components/base/FullScreenLoader.vue'
 import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
 import {
+  mdiBriefcaseOutline,
   mdiFilterRemoveOutline,
   mdiMagnify,
   mdiMenuDown,
@@ -168,6 +187,9 @@ const isFiltered = ref(false)
 const isPageLoaded = ref(false)
 const filterCriteria = ref({ key: '', values: [] as string[] })
 const search = ref()
+const hasResolvedInitialWorkspaces = ref(false)
+const lastLoadedWorkspaceId = ref<string | null>(null)
+let currentThingsRequest = 0
 
 const matchesFilterCriteria = (
   thing: ThingSiteSummary,
@@ -201,8 +223,22 @@ const onClickRegisterSite = () => {
     )
 }
 
-watch(selectedWorkspace, async (ws) => {
-  await loadThings()
+const syncThingsToSelectedWorkspace = async () => {
+  const workspaceId = selectedWorkspace.value?.id ?? null
+  if (workspaceId === lastLoadedWorkspaceId.value) return
+
+  lastLoadedWorkspaceId.value = workspaceId
+  await loadThings(workspaceId)
+}
+
+watch(selectedWorkspace, async (workspace) => {
+  if (!hasResolvedInitialWorkspaces.value) return
+
+  if (!workspace) {
+    showSiteForm.value = false
+  }
+
+  await syncThingsToSelectedWorkspace()
 })
 
 const filteredThings = computed(() => {
@@ -253,25 +289,59 @@ const onRowClick = (event: Event, item: any) => {
   router.push({ name: 'SiteDetails', params: { id: item.item.id } })
 }
 
-const loadThings = async () => {
-  workspaceThings.value = await listThingSiteSummaries(selectedWorkspace.value!.id)
+const loadThings = async (
+  workspaceId = selectedWorkspace.value?.id ?? null
+) => {
+  currentThingsRequest += 1
+  const requestId = currentThingsRequest
+
+  if (!workspaceId) {
+    workspaceThings.value = []
+    return
+  }
+
+  try {
+    const things = await listThingSiteSummaries(workspaceId)
+    if (
+      requestId !== currentThingsRequest ||
+      selectedWorkspace.value?.id !== workspaceId
+    )
+      return
+
+    workspaceThings.value = things
+  } catch (error) {
+    if (requestId !== currentThingsRequest) return
+
+    workspaceThings.value = []
+    console.error('Error fetching sites', error)
+  }
+}
+
+const refreshWorkspaces = async () => {
+  const workspaceRes = await hs.workspaces.listAllItems({
+    is_associated: true,
+    expand_related: true,
+  })
+  setWorkspaces(workspaceRes)
 }
 
 onMounted(async () => {
-  if (selectedWorkspace.value == null) {
-    const [workspaceRes] = await Promise.all([
-      hs.workspaces.listAllItems({ is_associated: true, expand_related: true }),
-    ])
-    setWorkspaces(workspaceRes)
-  } else {
-    const [things, workspaceRes] = await Promise.all([
-      listThingSiteSummaries(selectedWorkspace.value.id),
-      hs.workspaces.listAllItems({ is_associated: true, expand_related: true }),
-    ])
-    setWorkspaces(workspaceRes)
-    workspaceThings.value = things
+  try {
+    if (isAppInitializing.value) {
+      await startAppInitialization()
+    }
+
+    if (!hasBootstrappedWorkspaces.value) {
+      await refreshWorkspaces()
+    }
+  } catch (error) {
+    console.error('Error fetching workspaces', error)
+    workspaceThings.value = []
+  } finally {
+    hasResolvedInitialWorkspaces.value = true
+    await syncThingsToSelectedWorkspace()
+    isPageLoaded.value = true
   }
-  isPageLoaded.value = true
 })
 </script>
 
diff --git a/apps/data-management/src/pages/__tests__/Sites.spec.ts b/apps/data-management/src/pages/__tests__/Sites.spec.ts
new file mode 100644
index 00000000..22cbc83d
--- /dev/null
+++ b/apps/data-management/src/pages/__tests__/Sites.spec.ts
@@ -0,0 +1,163 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { createPinia, setActivePinia } from 'pinia'
+import { flushPromises, shallowMount } from '@vue/test-utils'
+import { useWorkspaceStore } from '@/store/workspaces'
+
+const {
+  hasBootstrappedWorkspacesState,
+  isAppInitializingState,
+  listAllItemsMock,
+  listThingSiteSummariesMock,
+  routerPushMock,
+} =
+  vi.hoisted(() => ({
+    hasBootstrappedWorkspacesState: { value: false },
+    isAppInitializingState: { value: false },
+    listAllItemsMock: vi.fn(),
+    listThingSiteSummariesMock: vi.fn(),
+    routerPushMock: vi.fn(),
+  }))
+
+vi.mock('@hydroserver/client', () => ({
+  User: class User {
+    email = ''
+  },
+  PermissionAction: {
+    Create: 'Create',
+  },
+  PermissionResource: {
+    Thing: 'Thing',
+  },
+  default: {
+    workspaces: {
+      listAllItems: listAllItemsMock,
+    },
+  },
+}))
+
+vi.mock('@/api/thingSiteSummaries', () => ({
+  listThingSiteSummaries: listThingSiteSummariesMock,
+}))
+
+vi.mock('@/bootstrap/appInitialization', () => ({
+  hasBootstrappedWorkspaces: hasBootstrappedWorkspacesState,
+  isAppInitializing: isAppInitializingState,
+  startAppInitialization: vi.fn(),
+}))
+
+vi.mock('@/components/Maps/OpenLayersMap.vue', () => ({
+  default: {
+    name: 'OpenLayersMap',
+    template: '<div />',
+  },
+}))
+
+vi.mock('@/components/Site/SiteForm.vue', () => ({
+  default: {
+    name: 'SiteForm',
+    template: '<div />',
+  },
+}))
+
+vi.mock('@/components/Site/SiteFilterToolbar.vue', () => ({
+  default: {
+    name: 'SiteFilterToolbar',
+    template: '<div />',
+  },
+}))
+
+vi.mock('@/components/Workspace/WorkspaceToolbar.vue', () => ({
+  default: {
+    name: 'WorkspaceToolbar',
+    template: '<div />',
+  },
+}))
+
+vi.mock('@/components/base/FullScreenLoader.vue', () => ({
+  default: {
+    name: 'FullScreenLoader',
+    template: '<div />',
+  },
+}))
+
+vi.mock('@/utils/maps/markers', () => ({
+  addColorToMarkers: (things: unknown[]) => things,
+}))
+
+vi.mock('@/composables/useWorkspacePermissions', () => ({
+  useWorkspacePermissions: () => ({
+    hasPermission: () => true,
+  }),
+}))
+
+vi.mock('vue-router', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('vue-router')>()
+
+  return {
+    ...actual,
+    useRouter: () => ({
+      push: routerPushMock,
+    }),
+  }
+})
+
+import Sites from '../Sites.vue'
+
+const mountSites = () =>
+  shallowMount(Sites, {
+    global: {
+      stubs: {
+        'v-btn-add': true,
+      },
+    },
+  })
+
+describe('Sites page', () => {
+  beforeEach(() => {
+    localStorage.clear()
+    setActivePinia(createPinia())
+    vi.restoreAllMocks()
+    hasBootstrappedWorkspacesState.value = false
+    isAppInitializingState.value = false
+    listAllItemsMock.mockReset()
+    listThingSiteSummariesMock.mockReset()
+    routerPushMock.mockReset()
+  })
+
+  it('clears a stale workspace selection without requesting site summaries', async () => {
+    const workspaceStore = useWorkspaceStore()
+    workspaceStore.selectedWorkspace = {
+      id: 'stale-workspace',
+      name: 'Stale workspace',
+      isPrivate: false,
+    } as any
+
+    listAllItemsMock.mockResolvedValue([])
+
+    const wrapper = mountSites()
+
+    await flushPromises()
+
+    expect(listThingSiteSummariesMock).not.toHaveBeenCalled()
+    expect(workspaceStore.selectedWorkspace).toBeNull()
+  })
+
+  it('loads site summaries once after bootstrapped workspaces are available', async () => {
+    const workspaceStore = useWorkspaceStore()
+    workspaceStore.selectedWorkspace = {
+      id: 'workspace-1',
+      name: 'Workspace 1',
+      isPrivate: false,
+    } as any
+
+    hasBootstrappedWorkspacesState.value = true
+    listThingSiteSummariesMock.mockResolvedValue([])
+
+    mountSites()
+
+    await flushPromises()
+
+    expect(listThingSiteSummariesMock).toHaveBeenCalledTimes(1)
+    expect(listThingSiteSummariesMock).toHaveBeenCalledWith('workspace-1')
+  })
+})
diff --git a/apps/data-management/src/router/router.ts b/apps/data-management/src/router/router.ts
index 76aa68e6..24a84657 100644
--- a/apps/data-management/src/router/router.ts
+++ b/apps/data-management/src/router/router.ts
@@ -4,6 +4,10 @@ import {
   createRouter,
   createWebHistory,
 } from 'vue-router'
+import {
+  isHydroServerReady,
+  waitForHydroServerInitialization,
+} from '@/bootstrap/appInitialization'
 import { routes } from '@/router/routes'
 import hs from '@hydroserver/client'
 
@@ -30,6 +34,9 @@ function updateDocumentTitle(matched: RouteRecordNormalized[]): void {
 
 router.beforeEach(
   async (to: RouteLocationNormalized, from: RouteLocationNormalized) => {
+    await waitForHydroServerInitialization()
+    if (!isHydroServerReady.value) return false
+
     const { inEmailVerificationFlow, inProviderSignupFlow } = hs.session
 
     if (inEmailVerificationFlow && to.name !== 'VerifyEmail') {
diff --git a/tests/e2e/specs/visualization.spec.ts b/tests/e2e/specs/visualization.spec.ts
index 9ea9aa73..0454c63e 100644
--- a/tests/e2e/specs/visualization.spec.ts
+++ b/tests/e2e/specs/visualization.spec.ts
@@ -11,7 +11,23 @@ test.describe('visualization', () => {
 
   async function ensureFiltersDrawerOpen(page: Page) {
     const workspaceFilter = page.getByRole('combobox', { name: 'Workspaces' }).first()
-    if ((await workspaceFilter.count()) > 0 && (await workspaceFilter.isVisible())) {
+    const viewport = page.viewportSize()
+    const box = await workspaceFilter.boundingBox()
+    const isWithinViewport =
+      !!box &&
+      !!viewport &&
+      box.width > 0 &&
+      box.height > 0 &&
+      box.x + box.width > 0 &&
+      box.y + box.height > 0 &&
+      box.x < viewport.width &&
+      box.y < viewport.height
+
+    if (
+      (await workspaceFilter.count()) > 0 &&
+      (await workspaceFilter.isVisible()) &&
+      isWithinViewport
+    ) {
       return
     }
 
diff --git a/tests/e2e/support/ui.ts b/tests/e2e/support/ui.ts
index f8940b9b..4638f9a5 100644
--- a/tests/e2e/support/ui.ts
+++ b/tests/e2e/support/ui.ts
@@ -29,8 +29,19 @@ export async function chooseAutocompleteOption(
   optionText: string
 ) {
   const field = page.getByRole('combobox', { name: label }).first()
-  await field.click()
-  await field.fill(optionText)
+  const container = field.locator('xpath=ancestor::*[@role="combobox"][1]')
+  await expect(field).toBeVisible()
+  await container.scrollIntoViewIfNeeded()
+  const toggle = container.getByRole('button').last()
+  if ((await toggle.count()) > 0) {
+    await toggle.click({ force: true })
+  }
+  await field.evaluate((element) => {
+    ;(element as HTMLInputElement).focus()
+  })
+  await page.keyboard.press('Control+A')
+  await page.keyboard.press('Backspace')
+  await page.keyboard.type(optionText)
   await chooseOverlayOption(page, optionText)
 }
 

From a848a1a1543a27c4e1af7aa1c5e35b1a5ce41441 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 22 Jun 2026 12:53:36 -0600
Subject: [PATCH 03/93] feat: add QC session editing and history integration

---
 apps/qc-app/docs/API_REFERENCE.md             |  78 +++++
 apps/qc-app/docs/ARCHITECTURE.md              |  49 +++
 .../EditData/CreateDatastreamForm.vue         | 128 +++++++
 .../src/components/EditData/EditHistory.vue   |   3 +
 .../src/components/EditData/SessionList.vue   | 112 ++++++
 .../components/EditData/StartSessionForm.vue  |  88 +++++
 .../__tests__/CreateDatastreamForm.spec.ts    |  90 +++++
 .../EditData/__tests__/SessionList.spec.ts    | 101 ++++++
 .../__tests__/StartSessionForm.spec.ts        |  54 +++
 apps/qc-app/src/components/VisualizeData.vue  | 329 +++++++++++++-----
 .../useCreateManagedDatastream.spec.ts        |  72 ++++
 .../__tests__/useEditSession.spec.ts          | 175 ++++++++++
 .../__tests__/useWorkspacePermissions.spec.ts | 114 ++++++
 .../composables/useCreateManagedDatastream.ts |  35 ++
 apps/qc-app/src/composables/useEditSession.ts | 160 +++++++++
 .../qc-app/src/composables/useQcSubmission.ts |  22 +-
 .../composables/useWorkspacePermissions.ts    | 109 ++++++
 apps/qc-app/src/pages/Workspaces.vue          |  36 +-
 .../__tests__/commitSession.spec.ts           |  85 +++++
 .../__tests__/createManagedDatastream.spec.ts | 167 +++++++++
 .../__tests__/findHistory.spec.ts             |  25 ++
 .../__tests__/observationsBody.spec.ts        |  20 ++
 .../__tests__/persistOperations.spec.ts       | 100 ++++++
 .../qualityControl/__tests__/qcServiceFake.ts | 245 +++++++++++++
 .../__tests__/reconstructSession.spec.ts      | 127 +++++++
 .../qualityControl/__tests__/session.spec.ts  |  85 +++++
 .../services/qualityControl/commitSession.ts  |  56 +++
 .../qualityControl/createManagedDatastream.ts | 124 +++++++
 .../services/qualityControl/findHistory.ts    |  25 ++
 .../src/services/qualityControl/index.ts      |  19 +
 .../qualityControl/observationsBody.ts        |  24 ++
 .../qualityControl/persistOperations.ts       |  84 +++++
 .../qualityControl/reconstructSession.ts      | 120 +++++++
 .../src/services/qualityControl/session.ts    |  73 ++++
 .../src/services/qualityControl/unwrap.ts     |  12 +
 .../src/store/__tests__/qcSession.spec.ts     | 104 ++++++
 apps/qc-app/src/store/qcPreferences.ts        |  24 ++
 apps/qc-app/src/store/qcSession.ts            |  97 ++++++
 38 files changed, 3260 insertions(+), 111 deletions(-)
 create mode 100644 apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
 create mode 100644 apps/qc-app/src/components/EditData/SessionList.vue
 create mode 100644 apps/qc-app/src/components/EditData/StartSessionForm.vue
 create mode 100644 apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
 create mode 100644 apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
 create mode 100644 apps/qc-app/src/components/EditData/__tests__/StartSessionForm.spec.ts
 create mode 100644 apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts
 create mode 100644 apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
 create mode 100644 apps/qc-app/src/composables/__tests__/useWorkspacePermissions.spec.ts
 create mode 100644 apps/qc-app/src/composables/useCreateManagedDatastream.ts
 create mode 100644 apps/qc-app/src/composables/useEditSession.ts
 create mode 100644 apps/qc-app/src/composables/useWorkspacePermissions.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/__tests__/commitSession.spec.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/__tests__/findHistory.spec.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/__tests__/observationsBody.spec.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/commitSession.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/createManagedDatastream.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/findHistory.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/index.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/observationsBody.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/persistOperations.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/reconstructSession.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/session.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/unwrap.ts
 create mode 100644 apps/qc-app/src/store/__tests__/qcSession.spec.ts
 create mode 100644 apps/qc-app/src/store/qcPreferences.ts
 create mode 100644 apps/qc-app/src/store/qcSession.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 3fdf5069..f00930d0 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -140,6 +140,53 @@ history), serializes `[phenomenonTime, result]` rows, POSTs with
 `mode: 'replace'`, surfaces a Snackbar, and clears the history in place
 on success.
 
+### `useEditSession()`
+
+Orchestrates the server-backed QC session workflow against the
+`services/qualityControl/` glue:
+
+```ts
+const { beginEditing, startSession, saveDraft, commit, needsSession, needsHistory } =
+  useEditSession()
+```
+
+- `beginEditing()` — resolves the QC history for the QC datastream, loads
+  its sessions, resumes the in-progress one (or sets `needsSession`); sets
+  `needsHistory` when the datastream isn't QC-managed yet.
+- `startSession(spec)` — creates a session and copies the source window in.
+- `saveDraft()` — persists the record's edit operations to the session
+  (append-only reconcile).
+- `commit()` — saves, verifies checksum C, pushes observations
+  (`mode: 'replace'`), then locks the session.
+
+### `useCreateManagedDatastream()`
+
+```ts
+const { create } = useCreateManagedDatastream()
+const { managedDatastream, history } = await create({ source, processingLevelId, name })
+```
+
+Delegates to the tested `createManagedDatastream` orchestration with the
+live client (`hs.datastreams` + `hs.qualityControlHistories`).
+
+### `useWorkspacePermissions()`
+
+Synchronous, reactive role/permission checks for gating UI. The role
+travels with the `Workspace` object (`collaboratorRole.permissions`; owners
+have a null role; `accountType === 'admin'` overrides), so no separate
+endpoint is needed.
+
+```ts
+const { canEdit, canCreateDatastream, roleName, isOwner, can } =
+  useWorkspacePermissions()
+canEdit()                 // selected workspace: can run the QC edit flow?
+canCreateDatastream(ws)   // can create the managed datastream here?
+roleName(ws)              // 'Owner' | <collaborator role> | 'Admin' | 'Read-only'
+```
+
+Used to disable the editor's Start editing / Save / Commit / Create
+controls and to mark each workspace's role on the picker.
+
 ### `useResizable()`
 
 Generic pointer-drag-resize hook. Used by `SelectDrawer`, `EditDrawer`,
@@ -411,6 +458,37 @@ ephemeral connection state).
 |------|-------|--------------------|-------|
 | `hs` | state | `Ref<HydroServer>` | Non-null after `main.ts` finishes settings load; type-asserted as non-null for ergonomic consumer code. |
 
+### `useQcSessionStore()` — `src/store/qcSession.ts`
+
+View-mode state for QC sessions: which session is editable (the single
+in-progress one) and which is being viewed. Viewing a committed session
+puts the editor in read-only mode.
+
+| Name                | Kind     | Type / signature                        | Notes |
+|---------------------|----------|-----------------------------------------|-------|
+| `historyId`         | state    | `string \| null`                        | The managed datastream's QC history being navigated. |
+| `sessions`          | state    | `QualityControlSession[]`               | Committed + in-progress sessions for the history. |
+| `currentSessionId`  | state    | `string \| null`                        | The single in-progress (editable) session. |
+| `viewedSessionId`   | state    | `string \| null`                        | The session currently being viewed. |
+| `isLoading`         | state    | `boolean`                               | True while `loadSessions` is in flight. |
+| `isReadOnly`        | computed | `boolean`                               | True unless viewing the in-progress session. |
+| `inProgressSession` | computed | `QualityControlSession \| null`         | The editable session, if any. |
+| `committedSessions` | computed | `QualityControlSession[]`               | Sessions with status `committed`. |
+| `viewedSession`     | computed | `QualityControlSession \| null`         | The session for `viewedSessionId`. |
+| `loadSessions`      | action   | `(historyId: string) => Promise<void>`  | Load a history's sessions; default the view to the in-progress one. |
+| `viewSession`       | action   | `(sessionId: string) => void`           | View a session read-only (no-op for an unknown id). |
+| `returnToCurrent`   | action   | `() => void`                            | Return to the editable in-progress session. |
+| `reset`             | action   | `() => void`                            | Clear all state. |
+
+### `useQcPreferencesStore()` — `src/store/qcPreferences.ts`
+
+Persisted QC editing preferences. Persistence: key `qc:preferences:v1`,
+`pick: ['processingLevelId']`.
+
+| Name                | Kind  | Type / signature | Notes |
+|---------------------|-------|------------------|-------|
+| `processingLevelId` | state | `string \| null` | Last-used processing level for the Create-Datastream-for-Editing form; null on first use (no assumed default). |
+
 ## Internal: utilities
 
 ### `src/utils/plotting/plotly.ts` (barrel)
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 661c0dd1..0656d1be 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -220,6 +220,55 @@ itself has zero Vue / Pinia / Plotly dependencies. The contract:
 Side-stepping `dispatch` breaks undo / redo, breaks QC History export, and
 silently breaks the worker fast-path. Don't.
 
+## QC history / session service
+
+Editing is persisted as a session DAG through the HydroServer QC API
+(`/api/data/quality-control/histories/{id}/sessions/{id}/operations`).
+
+The API client itself lives in **`@hydroserver/client`**, split across three
+SDK services on the `hs` instance: `qualityControlHistories`,
+`qualityControlSessions` (with `commit`), and `qualityControlOperations`. They
+are normal SDK services built on the shared `apiMethods` layer, so they inherit
+the session auth (CSRF cookie -> `X-CSRFToken`, `credentials: 'include'`) and
+the `ApiResponse` return shape — methods never throw on HTTP errors. Bodies are
+camelCase (`by_alias`); query parameters are snake_case (`expand_related`,
+`range_start`, `managed_datastream_id`, `ancestor_of`, ...).
+
+`src/services/qualityControl/` holds only the **app-side orchestration** that
+composes those services with `@uwrl/qc-utils` and the datastream/observation
+APIs: `createManagedDatastream`, the session lifecycle (`session.ts`),
+`persistOperations`, `commitSession`, `reconstructSession`, `findHistory`, and
+the `observationsBulkBody` serializer. `unwrap` bridges `ApiResponse` to the
+thrown errors this glue surfaces. None of it is a transport — swapping the QC
+client out is a `@hydroserver/client` change, not an app one.
+
+Two contract notes worth keeping in mind:
+
+- **The backend stores the operation DAG as metadata only — it never replays
+  operations.** The app applies ops locally (qc-utils), pushes the edited
+  series to the managed datastream via `bulk-create` (replace mode), then calls
+  `/commit`, which only records checksums and extends the history window.
+  Checksum verification (source/managed) is the client's responsibility;
+  `/commit` performs none.
+- **Vocabulary differs across the boundary.** qc-utils serializes operations as
+  `{ method, args }`; the QC API speaks `{ operationType, arguments, order }`.
+  The enum values are identical, so `persistOperations`/`reconstructSession`
+  rename the fields when crossing between qc-utils and the API.
+
+Tests stub the three services with `makeQcFake()` (a stateful in-memory double
+under `services/qualityControl/__tests__/` that returns
+`{ histories, sessions, operations }`) — the production client lives in the
+package, not the app.
+
+**Permission gating.** QC editing writes to the source datastream's workspace
+(creates the managed datastream, pushes observations), so the editor's entry
+points are gated on the signed-in user's workspace role via
+`useWorkspacePermissions()` — a read-only collaborator sees disabled Start
+editing / Save / Commit controls and an explanation instead of a mid-flow 403,
+and each workspace's role is marked on the picker. The role rides along on the
+`Workspace` object (`collaboratorRole.permissions`; owners have a null role;
+admins override), so no extra request is needed.
+
 ## Routing and auth
 
 vue-router 5, two routes (Home, Workspaces). Two guards run on
diff --git a/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue b/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
new file mode 100644
index 00000000..bade4760
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
@@ -0,0 +1,128 @@
+<template>
+  <div class="qc-create-datastream-form pa-4">
+    <div class="text-body-1 font-weight-medium mb-1">
+      Set up QC editing for "{{ source.name }}"
+    </div>
+    <div class="text-body-small text-medium-emphasis mb-3">
+      Creates an empty managed datastream from this source, links a QC history,
+      and tags the source.
+    </div>
+
+    <v-alert
+      v-if="permissionError"
+      data-testid="create-permission-error"
+      type="warning"
+      variant="tonal"
+      density="compact"
+      class="mb-3"
+      :text="permissionError"
+    />
+
+    <v-select
+      v-model="processingLevelId"
+      data-testid="create-processing-level"
+      :items="processingLevelItems"
+      item-title="title"
+      item-value="value"
+      label="New processing level"
+      density="compact"
+      :error-messages="processingLevelError"
+      hide-details="auto"
+      class="mb-2"
+    />
+    <v-text-field
+      v-model="name"
+      data-testid="create-name"
+      label="Name"
+      density="compact"
+      hide-details="auto"
+      class="mb-3"
+    />
+
+    <div class="d-flex justify-end ga-2">
+      <v-btn data-testid="create-cancel" variant="text" @click="emit('cancel')">
+        Cancel
+      </v-btn>
+      <v-btn
+        data-testid="create-confirm"
+        color="primary"
+        variant="flat"
+        :disabled="!isValid"
+        @click="onConfirm"
+      >
+        Create datastream
+      </v-btn>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { computed, ref } from 'vue'
+import type { Datastream } from '@hydroserver/client'
+
+interface ProcessingLevelOption {
+  id: string
+  definition?: string
+  code?: string
+}
+
+export interface CreateDatastreamSpec {
+  source: Datastream
+  processingLevelId: string
+  name?: string
+}
+
+const props = defineProps<{
+  source: Datastream
+  processingLevels: ProcessingLevelOption[]
+  defaultProcessingLevelId?: string | null
+  /** When set, the user can't create datastreams here: shown as a warning
+   *  and the confirm button is disabled. */
+  permissionError?: string
+}>()
+
+const emit = defineEmits<{
+  (e: 'cancel'): void
+  (e: 'confirm', spec: CreateDatastreamSpec): void
+}>()
+
+const processingLevelId = ref<string | null>(props.defaultProcessingLevelId ?? null)
+const name = ref(`${props.source.name} (QC)`)
+
+const processingLevelItems = computed(() =>
+  props.processingLevels.map((p) => ({
+    title: p.definition || p.code || p.id,
+    value: p.id,
+  }))
+)
+
+// The managed datastream must use a different processing level than the
+// source. Datastreams load expand_related (nested processingLevel), so read
+// the id from either shape.
+const sourceProcessingLevelId = computed(() => {
+  const s = props.source as Datastream & { processingLevel?: { id: string } }
+  return s.processingLevelId ?? s.processingLevel?.id ?? null
+})
+const processingLevelError = computed(() =>
+  processingLevelId.value &&
+  processingLevelId.value === sourceProcessingLevelId.value
+    ? 'Must differ from the source processing level'
+    : ''
+)
+
+const isValid = computed(
+  () =>
+    !!processingLevelId.value &&
+    !processingLevelError.value &&
+    !props.permissionError
+)
+
+function onConfirm(): void {
+  if (!isValid.value || !processingLevelId.value) return
+  emit('confirm', {
+    source: props.source,
+    processingLevelId: processingLevelId.value,
+    name: name.value.trim() || undefined,
+  })
+}
+</script>
diff --git a/apps/qc-app/src/components/EditData/EditHistory.vue b/apps/qc-app/src/components/EditData/EditHistory.vue
index 84802f5a..ffee4082 100644
--- a/apps/qc-app/src/components/EditData/EditHistory.vue
+++ b/apps/qc-app/src/components/EditData/EditHistory.vue
@@ -122,6 +122,8 @@
       class="flex-grow-1 overflow-y-auto pa-2"
       style="min-height: 0"
     >
+      <SessionList class="mb-2" />
+
      <div class="rounded border bg-surface overflow-hidden">
       <div
         class="edit-history__row edit-history__row--baseline px-3 py-2 d-flex align-center"
@@ -342,6 +344,7 @@ import { formatDuration } from '@uwrl/qc-utils'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { useUIStore } from '@/store/userInterface'
 import { iconForMethod, colorForMethod } from '@/components/EditData/operations'
+import SessionList from '@/components/EditData/SessionList.vue'
 import { useQcHistory } from '@/composables/useQcHistory'
 import { Snackbar } from '@uwrl/qc-utils'
 
diff --git a/apps/qc-app/src/components/EditData/SessionList.vue b/apps/qc-app/src/components/EditData/SessionList.vue
new file mode 100644
index 00000000..8dc2f27c
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/SessionList.vue
@@ -0,0 +1,112 @@
+<template>
+  <div class="qc-session-list">
+    <div class="qc-session-list__header px-3 py-2 d-flex align-center ga-2">
+      <v-icon icon="mdi-source-branch" color="primary" size="16" />
+      <span class="text-body-small font-weight-medium">Sessions</span>
+      <v-spacer />
+      <v-btn
+        v-if="isReadOnly && currentSessionId"
+        data-testid="session-return-current"
+        size="x-small"
+        variant="tonal"
+        color="primary"
+        @click="returnToCurrent"
+      >
+        Return to current
+      </v-btn>
+    </div>
+
+    <v-divider />
+
+    <div
+      v-if="!sessions.length"
+      class="pa-3 text-center text-body-small text-medium-emphasis"
+    >
+      No sessions yet.
+    </div>
+
+    <v-list v-else density="compact" class="py-0" :lines="false">
+      <v-list-item
+        v-for="session of orderedSessions"
+        :key="session.id"
+        :active="session.id === viewedSessionId"
+        :data-testid="`session-${session.id}`"
+        @click="onSelect(session)"
+      >
+        <template #prepend>
+          <v-icon
+            :icon="
+              session.status === 'in_progress'
+                ? 'mdi-pencil-circle'
+                : 'mdi-check-circle'
+            "
+            :color="session.status === 'in_progress' ? 'warning' : 'success'"
+            size="16"
+            class="mr-2"
+          />
+        </template>
+
+        <v-list-item-title class="text-body-small">
+          {{ sessionLabel(session) }}
+        </v-list-item-title>
+
+        <template #append>
+          <v-chip
+            v-if="session.id === currentSessionId"
+            size="x-small"
+            color="warning"
+            variant="tonal"
+            label
+          >
+            Editing
+          </v-chip>
+          <v-chip v-else size="x-small" color="grey" variant="tonal" label>
+            View
+          </v-chip>
+        </template>
+      </v-list-item>
+    </v-list>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { computed } from 'vue'
+import { storeToRefs } from 'pinia'
+import { useQcSessionStore } from '@/store/qcSession'
+import type { QualityControlSession } from '@hydroserver/client'
+
+const store = useQcSessionStore()
+const { sessions, currentSessionId, viewedSessionId, isReadOnly } =
+  storeToRefs(store)
+
+// Newest first, by phenomenon start (ISO sorts chronologically).
+const orderedSessions = computed(() =>
+  [...sessions.value].sort((a, b) =>
+    b.phenomenonTimeStart.localeCompare(a.phenomenonTimeStart)
+  )
+)
+
+function sessionLabel(session: QualityControlSession): string {
+  if (session.description) return session.description
+  return `${session.phenomenonTimeStart.slice(0, 10)} to ${session.phenomenonTimeEnd.slice(0, 10)}`
+}
+
+function onSelect(session: QualityControlSession): void {
+  if (session.id === store.currentSessionId) {
+    store.returnToCurrent()
+  } else {
+    store.viewSession(session.id)
+  }
+}
+
+function returnToCurrent(): void {
+  store.returnToCurrent()
+}
+</script>
+
+<style scoped>
+.qc-session-list__header {
+  background-color: rgba(var(--v-theme-primary), 0.04);
+  min-height: 32px;
+}
+</style>
diff --git a/apps/qc-app/src/components/EditData/StartSessionForm.vue b/apps/qc-app/src/components/EditData/StartSessionForm.vue
new file mode 100644
index 00000000..5f4c2e1b
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/StartSessionForm.vue
@@ -0,0 +1,88 @@
+<template>
+  <div class="qc-start-session-form pa-4">
+    <div class="text-body-1 font-weight-medium mb-3">Start new edit session</div>
+
+    <v-text-field
+      v-model="start"
+      data-testid="session-start"
+      label="Start"
+      type="datetime-local"
+      density="compact"
+      hide-details="auto"
+      class="mb-2"
+    />
+    <v-text-field
+      v-model="end"
+      data-testid="session-end"
+      label="End"
+      type="datetime-local"
+      density="compact"
+      :error-messages="endError"
+      hide-details="auto"
+      class="mb-2"
+    />
+    <v-textarea
+      v-model="description"
+      data-testid="session-description"
+      label="Description (optional)"
+      rows="2"
+      density="compact"
+      auto-grow
+      hide-details="auto"
+      class="mb-3"
+    />
+
+    <div class="d-flex justify-end ga-2">
+      <v-btn data-testid="session-cancel" variant="text" @click="emit('cancel')">
+        Cancel
+      </v-btn>
+      <v-btn
+        data-testid="session-confirm"
+        color="primary"
+        variant="flat"
+        :disabled="!isValid"
+        @click="onConfirm"
+      >
+        Start session
+      </v-btn>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { computed, ref } from 'vue'
+import type { QualityControlSessionContract } from '@hydroserver/client'
+
+type QcSessionPostBody = QualityControlSessionContract.PostBody
+
+const emit = defineEmits<{
+  (e: 'cancel'): void
+  (e: 'confirm', spec: QcSessionPostBody): void
+}>()
+
+const start = ref('')
+const end = ref('')
+const description = ref('')
+
+const startMs = computed(() => new Date(start.value).getTime())
+const endMs = computed(() => new Date(end.value).getTime())
+
+const endError = computed(() =>
+  start.value && end.value && endMs.value <= startMs.value
+    ? 'End must be after start'
+    : ''
+)
+
+const isValid = computed(
+  () => !!start.value && !!end.value && endMs.value > startMs.value
+)
+
+function onConfirm(): void {
+  if (!isValid.value) return
+  emit('confirm', {
+    phenomenonTimeStart: new Date(start.value).toISOString(),
+    phenomenonTimeEnd: new Date(end.value).toISOString(),
+    description: description.value.trim() || undefined,
+  })
+}
+</script>
diff --git a/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts b/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
new file mode 100644
index 00000000..0a3fa445
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
@@ -0,0 +1,90 @@
+import { mount } from '@vue/test-utils'
+import { describe, it, expect } from 'vitest'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import CreateDatastreamForm from '@/components/EditData/CreateDatastreamForm.vue'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+const source = { id: 's-1', name: 'Raw Temp', processingLevelId: 'pl-raw' } as any
+
+const processingLevels = [
+  { id: 'pl-raw', definition: 'Raw' },
+  { id: 'pl-qc', definition: 'Quality Controlled' },
+]
+
+const mountForm = () =>
+  mount(CreateDatastreamForm, {
+    props: { source, processingLevels },
+    global: { plugins: [createTestVuetify()] },
+  })
+
+const levelSelect = (w: ReturnType<typeof mountForm>) =>
+  w.findComponent({ name: 'VSelect' })
+
+describe('CreateDatastreamForm', () => {
+  it('disables create until a different processing level is chosen', async () => {
+    const w = mountForm()
+    const confirm = () => w.find('[data-testid="create-confirm"]')
+    expect(confirm().attributes('disabled')).toBeDefined()
+
+    await levelSelect(w).vm.$emit('update:modelValue', 'pl-raw') // same as source
+    expect(confirm().attributes('disabled')).toBeDefined()
+
+    await levelSelect(w).vm.$emit('update:modelValue', 'pl-qc')
+    expect(confirm().attributes('disabled')).toBeUndefined()
+  })
+
+  it('emits the create spec on confirm, defaulting the name from the source', async () => {
+    const w = mountForm()
+    await levelSelect(w).vm.$emit('update:modelValue', 'pl-qc')
+    await w.find('[data-testid="create-confirm"]').trigger('click')
+
+    const spec = w.emitted('confirm')![0][0] as {
+      source: { id: string }
+      processingLevelId: string
+      name?: string
+    }
+    expect(spec.source.id).toBe('s-1')
+    expect(spec.processingLevelId).toBe('pl-qc')
+    expect(spec.name).toBe('Raw Temp (QC)')
+  })
+
+  it('uses the provided default processing level', async () => {
+    const w = mount(CreateDatastreamForm, {
+      props: { source, processingLevels, defaultProcessingLevelId: 'pl-qc' },
+      global: { plugins: [createTestVuetify()] },
+    })
+    // Valid immediately since the default differs from the source's level.
+    expect(w.find('[data-testid="create-confirm"]').attributes('disabled')).toBeUndefined()
+    await w.find('[data-testid="create-confirm"]').trigger('click')
+    const spec = w.emitted('confirm')![0][0] as { processingLevelId: string }
+    expect(spec.processingLevelId).toBe('pl-qc')
+  })
+
+  it('emits cancel', async () => {
+    const w = mountForm()
+    await w.find('[data-testid="create-cancel"]').trigger('click')
+    expect(w.emitted('cancel')).toHaveLength(1)
+  })
+
+  it('blocks create and shows a warning when permissionError is set', async () => {
+    const w = mount(CreateDatastreamForm, {
+      props: {
+        source,
+        processingLevels,
+        defaultProcessingLevelId: 'pl-qc', // otherwise valid
+        permissionError: 'You cannot create datastreams here.',
+      },
+      global: { plugins: [createTestVuetify()] },
+    })
+    expect(w.find('[data-testid="create-permission-error"]').exists()).toBe(true)
+    expect(w.text()).toContain('You cannot create datastreams here.')
+    // Confirm stays disabled despite a valid processing level.
+    expect(
+      w.find('[data-testid="create-confirm"]').attributes('disabled')
+    ).toBeDefined()
+  })
+})
diff --git a/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts b/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
new file mode 100644
index 00000000..20a43c79
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
@@ -0,0 +1,101 @@
+import { mount, flushPromises } from '@vue/test-utils'
+import { describe, it, expect, beforeEach } from 'vitest'
+import type { Pinia } from 'pinia'
+import { createTestPinia } from '@/utils/test/pinia'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import { useHydroServer } from '@/store/hydroserver'
+import { makeQcFake } from '@/services/qualityControl/__tests__/qcServiceFake'
+import { useQcSessionStore } from '@/store/qcSession'
+import { unwrap } from '@/services/qualityControl/unwrap'
+import SessionList from '@/components/EditData/SessionList.vue'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+const win = (start: string, end: string) => ({
+  phenomenonTimeStart: start,
+  phenomenonTimeEnd: end,
+})
+
+let pinia: Pinia
+let qc: ReturnType<typeof makeQcFake>
+
+beforeEach(() => {
+  pinia = createTestPinia()
+  qc = makeQcFake()
+  useHydroServer().hs = {
+    qualityControlHistories: qc.histories,
+    qualityControlSessions: qc.sessions,
+    qualityControlOperations: qc.operations,
+  } as any
+})
+
+const mountList = () =>
+  mount(SessionList, { global: { plugins: [pinia, createTestVuetify()] } })
+
+/** Seed a committed + an in-progress session and load them into the store. */
+async function seedAndLoad() {
+  const h = unwrap(
+    await qc.histories.create({
+      managedDatastreamId: 'm-1',
+      sourceDatastreamId: 's-1',
+    })
+  )
+  const committed = unwrap(
+    await qc.sessions.create(h.id, win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'))
+  )
+  await qc.sessions.commit(h.id, committed.id)
+  const inProgress = unwrap(
+    await qc.sessions.create(h.id, win('2025-02-01T00:00:00Z', '2025-03-01T00:00:00Z'))
+  )
+  const store = useQcSessionStore()
+  await store.loadSessions(h.id)
+  return { store, committedId: committed.id, inProgressId: inProgress.id }
+}
+
+describe('SessionList', () => {
+  it('renders an empty state when there are no sessions', () => {
+    expect(mountList().text()).toContain('No sessions yet')
+  })
+
+  it('renders committed and in-progress sessions, marking the editable one', async () => {
+    const { committedId, inProgressId } = await seedAndLoad()
+    const wrapper = mountList()
+    await flushPromises()
+    expect(wrapper.find(`[data-testid="session-${committedId}"]`).exists()).toBe(true)
+    expect(wrapper.find(`[data-testid="session-${inProgressId}"]`).exists()).toBe(true)
+    expect(wrapper.text()).toContain('Editing')
+  })
+
+  it('clicking a committed session views it read-only', async () => {
+    const { store, committedId } = await seedAndLoad()
+    const wrapper = mountList()
+    await flushPromises()
+    await wrapper.find(`[data-testid="session-${committedId}"]`).trigger('click')
+    expect(store.viewedSessionId).toBe(committedId)
+    expect(store.isReadOnly).toBe(true)
+  })
+
+  it('clicking the current session returns to editing', async () => {
+    const { store, committedId, inProgressId } = await seedAndLoad()
+    const wrapper = mountList()
+    await flushPromises()
+    await wrapper.find(`[data-testid="session-${committedId}"]`).trigger('click')
+    expect(store.isReadOnly).toBe(true)
+    await wrapper.find(`[data-testid="session-${inProgressId}"]`).trigger('click')
+    expect(store.viewedSessionId).toBe(inProgressId)
+    expect(store.isReadOnly).toBe(false)
+  })
+
+  it('shows a "Return to current" control while viewing read-only', async () => {
+    const { committedId } = await seedAndLoad()
+    const wrapper = mountList()
+    await flushPromises()
+    expect(wrapper.find('[data-testid="session-return-current"]').exists()).toBe(false)
+    await wrapper.find(`[data-testid="session-${committedId}"]`).trigger('click')
+    await flushPromises()
+    expect(wrapper.find('[data-testid="session-return-current"]').exists()).toBe(true)
+  })
+})
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartSessionForm.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartSessionForm.spec.ts
new file mode 100644
index 00000000..0ad30459
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/__tests__/StartSessionForm.spec.ts
@@ -0,0 +1,54 @@
+import { mount } from '@vue/test-utils'
+import { describe, it, expect } from 'vitest'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import StartSessionForm from '@/components/EditData/StartSessionForm.vue'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+const mountForm = () =>
+  mount(StartSessionForm, { global: { plugins: [createTestVuetify()] } })
+
+describe('StartSessionForm', () => {
+  it('disables confirm until a valid range is entered', async () => {
+    const wrapper = mountForm()
+    const confirm = () => wrapper.find('[data-testid="session-confirm"]')
+    expect(confirm().attributes('disabled')).toBeDefined()
+
+    await wrapper.find('[data-testid="session-start"] input').setValue('2025-02-01T00:00')
+    await wrapper.find('[data-testid="session-end"] input').setValue('2025-01-01T00:00') // before start
+    expect(confirm().attributes('disabled')).toBeDefined()
+
+    await wrapper.find('[data-testid="session-end"] input').setValue('2025-03-01T00:00') // after start
+    expect(confirm().attributes('disabled')).toBeUndefined()
+  })
+
+  it('emits the session spec on confirm', async () => {
+    const wrapper = mountForm()
+    await wrapper.find('[data-testid="session-start"] input').setValue('2025-01-01T00:00')
+    await wrapper.find('[data-testid="session-end"] input').setValue('2025-02-01T00:00')
+    await wrapper.find('[data-testid="session-description"] textarea').setValue('Jan QC')
+    await wrapper.find('[data-testid="session-confirm"]').trigger('click')
+
+    const emitted = wrapper.emitted('confirm')
+    expect(emitted).toHaveLength(1)
+    const spec = emitted![0][0] as {
+      phenomenonTimeStart: string
+      phenomenonTimeEnd: string
+      description?: string
+    }
+    expect(typeof spec.phenomenonTimeStart).toBe('string')
+    expect(new Date(spec.phenomenonTimeEnd).getTime()).toBeGreaterThan(
+      new Date(spec.phenomenonTimeStart).getTime()
+    )
+    expect(spec.description).toBe('Jan QC')
+  })
+
+  it('emits cancel', async () => {
+    const wrapper = mountForm()
+    await wrapper.find('[data-testid="session-cancel"]').trigger('click')
+    expect(wrapper.emitted('cancel')).toHaveLength(1)
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index 51bb7eba..f893b36d 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -25,6 +25,18 @@
           </span>
         </div>
 
+        <v-chip
+          v-if="qcDatastream"
+          size="small"
+          variant="tonal"
+          :color="canEditWorkspace ? 'primary' : 'grey'"
+          :prepend-icon="canEditWorkspace ? 'mdi-pencil' : 'mdi-eye-outline'"
+          class="mr-2 align-self-center"
+          :title="`Your role on this workspace: ${workspaceRole}`"
+        >
+          {{ workspaceRole }}
+        </v-chip>
+
         <v-tooltip
           v-if="!qcDatastream"
           location="start"
@@ -45,17 +57,28 @@
           </template>
         </v-tooltip>
 
-        <v-btn
+        <v-tooltip
           v-else
-          class="ml-auto"
-          color="primary"
-          variant="flat"
-          prepend-icon="mdi-pencil"
-          append-icon="mdi-arrow-right"
-          @click="goToEdit"
+          location="start"
+          :disabled="canEditWorkspace"
+          :text="`Your role on this workspace (${workspaceRole}) is read-only. Ask a workspace owner for an editor role to make edits.`"
         >
-          Start editing
-        </v-btn>
+          <template #activator="{ props: tooltipProps }">
+            <div v-bind="tooltipProps" class="ml-auto">
+              <v-btn
+                color="primary"
+                variant="flat"
+                prepend-icon="mdi-pencil"
+                append-icon="mdi-arrow-right"
+                :loading="isCreating"
+                :disabled="!canEditWorkspace"
+                @click="goToEdit"
+              >
+                Start editing
+              </v-btn>
+            </div>
+          </template>
+        </v-tooltip>
       </div>
 
       <v-divider />
@@ -80,6 +103,23 @@
     <v-card class="select-view__table d-flex flex-column flex-1-1-0 overflow-hidden">
       <DataVisDatasetsTable class="fill-height" />
     </v-card>
+
+    <v-dialog v-model="showCreateDatastream" max-width="560" persistent>
+      <v-card v-if="qcDatastream" rounded="lg">
+        <CreateDatastreamForm
+          :source="qcDatastream"
+          :processing-levels="processingLevels"
+          :default-processing-level-id="qcPreferences.processingLevelId"
+          :permission-error="
+            canCreateDatastreamHere
+              ? ''
+              : `Your role on this workspace (${workspaceRole}) can't create datastreams. Ask a workspace owner for an editor role.`
+          "
+          @cancel="showCreateDatastream = false"
+          @confirm="onCreateDatastream"
+        />
+      </v-card>
+    </v-dialog>
   </div>
 
   <div
@@ -181,9 +221,9 @@
             variant="flat"
             color="primary"
             prepend-icon="mdi-content-save-outline"
-            :disabled="!editCount || isUpdating || isSubmitting"
-            :loading="isSubmitting && exitIntent === 'save'"
-            @click="requestSave"
+            :disabled="saveDisabled"
+            :loading="isSavingDraft"
+            @click="onSaveDraft"
           >
             Save
           </v-btn>
@@ -194,19 +234,31 @@
             variant="tonal"
             color="primary"
             prepend-icon="mdi-content-save-move-outline"
-            :disabled="!editCount || isUpdating || isSubmitting"
-            :loading="isSubmitting && exitIntent === 'save-close'"
-            @click="requestSaveAndClose"
+            :disabled="saveDisabled"
+            @click="onSaveAndClose"
           >
             Save &amp; Close
           </v-btn>
+          <v-btn
+            data-testid="exit-commit-btn"
+            class="ml-1"
+            size="small"
+            variant="flat"
+            color="success"
+            prepend-icon="mdi-cloud-check-outline"
+            :disabled="commitDisabled"
+            :loading="isCommitting"
+            @click="showCommitConfirm = true"
+          >
+            Commit
+          </v-btn>
           <v-spacer />
           <v-btn
             data-testid="exit-close-btn"
             size="small"
             variant="text"
             prepend-icon="mdi-close"
-            :disabled="isSubmitting"
+            :disabled="isSavingDraft || isCommitting"
             @click="requestClose"
           >
             Close
@@ -307,36 +359,36 @@
       </v-card>
     </v-dialog>
 
-    <v-dialog v-model="showSaveConfirm" max-width="520">
+    <v-dialog v-model="showCommitConfirm" max-width="520">
       <v-card rounded="lg">
         <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
-          <v-avatar color="primary" variant="tonal" size="40">
-            <v-icon icon="mdi-cloud-upload-outline" size="22" />
+          <v-avatar color="success" variant="tonal" size="40">
+            <v-icon icon="mdi-cloud-check-outline" size="22" />
           </v-avatar>
           <div class="d-flex flex-column">
-            <div class="text-title-large font-weight-bold">Submit QC observations?</div>
+            <div class="text-title-large font-weight-bold">Commit session to datastream?</div>
             <div class="text-body-small text-medium-emphasis">
-              {{ editCount }} edit{{ editCount === 1 ? '' : 's' }} pending
+              Materializes this session into the managed datastream
             </div>
           </div>
         </div>
         <v-card-text class="text-body-medium pt-2 pb-4 px-6">
-          This will
-          <strong>overwrite existing server observations</strong> in the
-          submitted time range (replace mode). This action cannot be undone.
+          The source data's integrity is verified, then the edited observations
+          <strong>replace</strong> the managed datastream over this session's
+          range and the session is locked into the history.
         </v-card-text>
         <v-divider />
         <v-card-actions class="d-flex align-center ga-2 px-4 py-3">
-          <v-btn variant="text" @click="showSaveConfirm = false">Cancel</v-btn>
+          <v-btn variant="text" @click="showCommitConfirm = false">Cancel</v-btn>
           <v-spacer />
           <v-btn
-            color="primary"
+            color="success"
             variant="flat"
-            prepend-icon="mdi-cloud-upload-outline"
-            :loading="isSubmitting"
-            @click="confirmSave"
+            prepend-icon="mdi-cloud-check-outline"
+            :loading="isCommitting"
+            @click="onCommit"
           >
-            Submit
+            Commit
           </v-btn>
         </v-card-actions>
       </v-card>
@@ -345,35 +397,43 @@
     <v-dialog v-model="showCloseConfirm" max-width="520">
       <v-card rounded="lg">
         <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
-          <v-avatar color="error" variant="tonal" size="40">
-            <v-icon icon="mdi-alert-outline" size="22" />
+          <v-avatar color="primary" variant="tonal" size="40">
+            <v-icon icon="mdi-content-save-outline" size="22" />
           </v-avatar>
           <div class="d-flex flex-column">
-            <div class="text-title-large font-weight-bold">Discard unsaved edits?</div>
+            <div class="text-title-large font-weight-bold">Save before closing?</div>
             <div class="text-body-small text-medium-emphasis">
-              {{ editCount }} edit{{ editCount === 1 ? '' : 's' }} will be lost
+              {{ editCount }} edit{{ editCount === 1 ? '' : 's' }} not yet saved to the session
             </div>
           </div>
         </div>
         <v-card-text class="text-body-medium pt-2 pb-4 px-6">
-          Closing will leave the editor without submitting your changes.
-          Discarded edits cannot be recovered.
+          Save your edits to the in-progress session before closing, or close
+          without saving (unsaved changes are dropped; previously-saved draft
+          operations stay in the session).
         </v-card-text>
         <v-divider />
         <v-card-actions class="d-flex align-center ga-2 px-4 py-3">
           <v-btn variant="text" @click="showCloseConfirm = false">Cancel</v-btn>
           <v-spacer />
+          <v-btn variant="text" @click="closeWithoutSaving">Close without saving</v-btn>
           <v-btn
-            color="error"
+            color="primary"
             variant="flat"
-            prepend-icon="mdi-delete-outline"
-            @click="confirmClose"
+            prepend-icon="mdi-content-save-outline"
+            @click="saveDraftAndClose"
           >
-            Discard &amp; close
+            Save &amp; close
           </v-btn>
         </v-card-actions>
       </v-card>
     </v-dialog>
+
+    <v-dialog v-model="showStartSession" max-width="520" persistent>
+      <v-card rounded="lg">
+        <StartSessionForm @cancel="cancelStartSession" @confirm="onStartSession" />
+      </v-card>
+    </v-dialog>
   </div>
 </template>
 
@@ -391,13 +451,23 @@ import { computed, onUnmounted, ref, useTemplateRef, watch } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import PlottedDatastreams from './VisualizeData/PlottedDatastreams.vue'
 import { usePlotlyStore } from '@/store/plotly'
-import { useQcSubmission } from '@/composables/useQcSubmission'
+import { useEditSession } from '@/composables/useEditSession'
+import { useQcSessionStore } from '@/store/qcSession'
+import { useQcPreferencesStore } from '@/store/qcPreferences'
+import StartSessionForm from '@/components/EditData/StartSessionForm.vue'
+import CreateDatastreamForm from '@/components/EditData/CreateDatastreamForm.vue'
+import {
+  useCreateManagedDatastream,
+  type CreateManagedDatastreamSpec,
+} from '@/composables/useCreateManagedDatastream'
+import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
+import type { QualityControlSessionContract } from '@hydroserver/client'
+import { Snackbar } from '@uwrl/qc-utils'
 import {
   decodeShareState,
   encodeShareState,
   type ShareState,
 } from '@/utils/share'
-import { useDataSelection } from '@/composables/useDataSelection'
 import { useWorkspaceStore } from '@/store/workspaces'
 import { useResizable, usePersistedFlag } from '@/composables/useResizable'
 
@@ -406,6 +476,7 @@ const {
   plottedDatastreams,
   qcDatastream,
   datastreams,
+  processingLevels,
   things,
   beginDate,
   endDate,
@@ -421,8 +492,6 @@ const { selectedWorkspaceId } = storeToRefs(useWorkspaceStore())
 const {
   editHistory,
   isUpdating,
-  isSubmitting,
-  selectedSeries,
   activeTab,
   hiddenTraceIds,
   hiddenAxisIds,
@@ -433,14 +502,60 @@ const {
   tooltipsMaxDataPoints,
 } = storeToRefs(usePlotlyStore())
 const { redraw } = usePlotlyStore()
-const { refreshGraphSeriesArray, setPlottedDatastreams } = useDataVisStore()
-const { clearSelected } = useDataSelection()
-const { submitQcEdits } = useQcSubmission()
+const { setPlottedDatastreams } = useDataVisStore()
+
+const {
+  beginEditing,
+  startSession,
+  saveDraft,
+  commit,
+  needsSession,
+  needsHistory,
+} = useEditSession()
+const qcSessionStore = useQcSessionStore()
+const { isReadOnly, inProgressSession } = storeToRefs(qcSessionStore)
+const { create: createManaged } = useCreateManagedDatastream()
+const qcPreferences = useQcPreferencesStore()
+const { canEdit, canCreateDatastream, roleName } = useWorkspacePermissions()
+
+type QcSessionPostBody = QualityControlSessionContract.PostBody
+
+// Permission gating: QC editing writes to the selected workspace (creates
+// the managed datastream, pushes observations). Gate the editor entry
+// points so a read-only collaborator gets a clear disabled state and an
+// explanation instead of a 403 mid-flow.
+const canEditWorkspace = computed(() => canEdit())
+const canCreateDatastreamHere = computed(() => canCreateDatastream())
+const workspaceRole = computed(() => roleName())
 
 const editCount = computed(() => editHistory.value?.length ?? 0)
-const showSaveConfirm = ref(false)
+const showCommitConfirm = ref(false)
 const showCloseConfirm = ref(false)
-const exitIntent = ref<'save' | 'save-close' | null>(null)
+const showStartSession = ref(false)
+const showCreateDatastream = ref(false)
+const isSavingDraft = ref(false)
+const isCommitting = ref(false)
+const isCreating = ref(false)
+
+const saveDisabled = computed(
+  () =>
+    !canEditWorkspace.value ||
+    isReadOnly.value ||
+    !inProgressSession.value ||
+    !editCount.value ||
+    isUpdating.value ||
+    isSavingDraft.value ||
+    isCommitting.value
+)
+const commitDisabled = computed(
+  () =>
+    !canEditWorkspace.value ||
+    isReadOnly.value ||
+    !inProgressSession.value ||
+    isUpdating.value ||
+    isSavingDraft.value ||
+    isCommitting.value
+)
 
 // --- Editor layout: sidebar sizes + collapse flags ------------------
 // Persisted to localStorage so the user's preferred layout survives
@@ -525,22 +640,73 @@ function exitToSelect() {
   isDrawerOpen.value = true
 }
 
-function requestSave() {
-  exitIntent.value = 'save'
-  showSaveConfirm.value = true
+async function onStartSession(spec: QcSessionPostBody) {
+  showStartSession.value = false
+  try {
+    await startSession(spec)
+    await redraw()
+    Snackbar.success('Edit session started.')
+  } catch (e) {
+    Snackbar.error(e instanceof Error ? e.message : 'Could not start the session.')
+  }
+}
+
+function cancelStartSession() {
+  showStartSession.value = false
+  exitToSelect()
+}
+
+async function onCreateDatastream(spec: CreateManagedDatastreamSpec) {
+  showCreateDatastream.value = false
+  qcPreferences.processingLevelId = spec.processingLevelId
+  isCreating.value = true
+  try {
+    const { managedDatastream } = await createManaged(spec)
+    await setPlottedDatastreams(
+      [...plottedDatastreams.value, managedDatastream],
+      managedDatastream.id
+    )
+    Snackbar.success('Managed datastream created.')
+    await goToEdit()
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not create the datastream.'
+    )
+  } finally {
+    isCreating.value = false
+  }
+}
+
+async function onSaveDraft(): Promise<boolean> {
+  isSavingDraft.value = true
+  try {
+    await saveDraft()
+    Snackbar.success('Draft saved.')
+    return true
+  } catch (e) {
+    Snackbar.error(e instanceof Error ? e.message : 'Could not save the draft.')
+    return false
+  } finally {
+    isSavingDraft.value = false
+  }
 }
 
-function requestSaveAndClose() {
-  exitIntent.value = 'save-close'
-  showSaveConfirm.value = true
+async function onSaveAndClose() {
+  if (await onSaveDraft()) exitToSelect()
 }
 
-async function confirmSave() {
-  const intent = exitIntent.value
-  showSaveConfirm.value = false
-  await submitQcEdits()
-  if (intent === 'save-close') exitToSelect()
-  exitIntent.value = null
+async function onCommit() {
+  showCommitConfirm.value = false
+  isCommitting.value = true
+  try {
+    await commit()
+    await redraw()
+    Snackbar.success('Session committed.')
+  } catch (e) {
+    Snackbar.error(e instanceof Error ? e.message : 'Could not commit the session.')
+  } finally {
+    isCommitting.value = false
+  }
 }
 
 function requestClose() {
@@ -551,25 +717,13 @@ function requestClose() {
   }
 }
 
-async function discardEdits() {
-  if (!editCount.value) return
-  isUpdating.value = true
-  try {
-    // In-place clear so the `editHistory` ref keeps tracking the
-    // same array (reassigning `history = []` detaches it).
-    if (selectedSeries.value) selectedSeries.value.data.history.length = 0
-    await refreshGraphSeriesArray()
-    await selectedSeries.value?.data.reload()
-    await clearSelected({ recordHistory: false })
-    await redraw()
-  } finally {
-    isUpdating.value = false
-  }
+async function saveDraftAndClose() {
+  showCloseConfirm.value = false
+  await onSaveAndClose()
 }
 
-async function confirmClose() {
+function closeWithoutSaving() {
   showCloseConfirm.value = false
-  await discardEdits()
   exitToSelect()
 }
 
@@ -765,10 +919,23 @@ onUnmounted(() => {
   resetState()
 })
 
-function goToEdit() {
-  currentView.value = DrawerType.Edit
-  selectedDrawer.value = DrawerType.Edit
-  isDrawerOpen.value = true
+async function goToEdit() {
+  try {
+    await beginEditing()
+    if (needsHistory.value) {
+      // Not a managed datastream yet: create one from it (as source).
+      showCreateDatastream.value = true
+      return
+    }
+    currentView.value = DrawerType.Edit
+    selectedDrawer.value = DrawerType.Edit
+    isDrawerOpen.value = true
+    if (needsSession.value) showStartSession.value = true
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not start editing this datastream.'
+    )
+  }
 }
 </script>
 
diff --git a/apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts b/apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts
new file mode 100644
index 00000000..33a8a0cd
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts
@@ -0,0 +1,72 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { ref } from 'vue'
+import { createPinia, setActivePinia } from 'pinia'
+import { makeQcFake } from '@/services/qualityControl/__tests__/qcServiceFake'
+import { unwrap } from '@/services/qualityControl/unwrap'
+
+const hsCreate = vi.fn()
+const createTag = vi.fn()
+const hs = ref<any>({ datastreams: { create: hsCreate, createTag } })
+vi.mock('@/store/hydroserver', () => ({
+  useHydroServer: () => ({ hs }),
+}))
+
+const makeSource = (overrides: Record<string, unknown> = {}) =>
+  ({
+    id: 'source-1',
+    name: 'Raw',
+    processingLevelId: 'pl-raw',
+    thingId: 't-1',
+    unitId: 'u-1',
+    observedPropertyId: 'op-1',
+    sensorId: 'sn-1',
+    ...overrides,
+  }) as any
+
+let qc: ReturnType<typeof makeQcFake>
+
+beforeEach(() => {
+  setActivePinia(createPinia())
+  vi.clearAllMocks()
+  qc = makeQcFake()
+  hs.value = {
+    datastreams: { create: hsCreate, createTag },
+    qualityControlHistories: qc.histories,
+    qualityControlSessions: qc.sessions,
+    qualityControlOperations: qc.operations,
+  }
+  hsCreate.mockResolvedValue({
+    data: { id: 'managed-1', name: 'Raw (QC)', processingLevelId: 'pl-qc' },
+    ok: true,
+    status: 201,
+    message: '',
+  })
+  createTag.mockResolvedValue({})
+})
+
+describe('useCreateManagedDatastream', () => {
+  it('creates the datastream, the history, and tags the source', async () => {
+    const { useCreateManagedDatastream } = await import(
+      '@/composables/useCreateManagedDatastream'
+    )
+    const { create } = useCreateManagedDatastream()
+    const result = await create({
+      source: makeSource(),
+      processingLevelId: 'pl-qc',
+      name: 'My QC',
+    })
+
+    const body = hsCreate.mock.calls[0][0]
+    expect(body.processingLevelId).toBe('pl-qc')
+    expect(body.name).toBe('My QC')
+    expect(body.valueCount).toBe(0)
+
+    expect(result.history.managedDatastream.id).toBe('managed-1')
+    expect(unwrap(await qc.histories.list())).toHaveLength(1)
+
+    expect(createTag).toHaveBeenCalledWith(
+      'source-1',
+      expect.objectContaining({ value: 'managed-1' })
+    )
+  })
+})
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
new file mode 100644
index 00000000..d5cfdd08
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -0,0 +1,175 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { ref } from 'vue'
+import { createPinia, setActivePinia } from 'pinia'
+import { makeQcFake } from '@/services/qualityControl/__tests__/qcServiceFake'
+import { unwrap } from '@/services/qualityControl/unwrap'
+
+const qcDatastream = ref<any>(null)
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () => ({ qcDatastream }),
+}))
+
+const selectedSeries = ref<any>(null)
+vi.mock('@/store/plotly', () => ({
+  usePlotlyStore: () => ({ selectedSeries }),
+}))
+
+const getItem = vi.fn()
+const createObservations = vi.fn()
+const hs = ref<any>({ datastreams: { getItem, createObservations } })
+vi.mock('@/store/hydroserver', () => ({
+  useHydroServer: () => ({ hs }),
+}))
+
+const fetchObservationsInRange = vi.fn()
+vi.mock('@/store/observations', () => ({
+  useObservationStore: () => ({ fetchObservationsInRange }),
+}))
+
+// qc-utils is only used at runtime by the composable (the service layer
+// imports types only), so stub serializeHistory/applyHistory here.
+vi.mock('@uwrl/qc-utils', () => ({
+  serializeHistory: vi.fn((record: any, window: any) => ({
+    version: '1',
+    createdAt: '2025-01-01T00:00:00Z',
+    window,
+    operations: (record.history ?? []).map((h: any) => ({
+      method: h.method,
+      args: h.args ?? [],
+    })),
+  })),
+  applyHistory: vi.fn(async () => ({ applied: 0, failed: [] })),
+}))
+
+import { useQcSessionStore } from '@/store/qcSession'
+
+const WIN = {
+  phenomenonTimeStart: '2025-01-01T00:00:00Z',
+  phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+}
+
+const makeRecord = (history: any[] = []) => ({
+  history,
+  dataX: [Date.UTC(2025, 0, 1)],
+  dataY: [10],
+})
+
+let qc: ReturnType<typeof makeQcFake>
+
+const wireHs = () => {
+  hs.value = {
+    datastreams: { getItem, createObservations },
+    qualityControlHistories: qc.histories,
+    qualityControlSessions: qc.sessions,
+    qualityControlOperations: qc.operations,
+  }
+}
+
+beforeEach(() => {
+  setActivePinia(createPinia())
+  vi.clearAllMocks()
+  qc = makeQcFake()
+  wireHs()
+  qcDatastream.value = { id: 'm-1' }
+  selectedSeries.value = { data: makeRecord() }
+  getItem.mockResolvedValue({ id: 's-1', name: 'Source' })
+  createObservations.mockResolvedValue(undefined)
+  fetchObservationsInRange.mockResolvedValue(makeRecord())
+})
+
+const seedHistory = async () => {
+  await qc.histories.create({
+    managedDatastreamId: 'm-1',
+    sourceDatastreamId: 's-1',
+  })
+}
+
+describe('useEditSession', () => {
+  it('beginEditing resolves the history and needs a session when none is in progress', async () => {
+    await seedHistory()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const { beginEditing, needsSession, sourceDatastream } = useEditSession()
+    await beginEditing()
+    expect(sourceDatastream.value?.id).toBe('s-1')
+    expect(needsSession.value).toBe(true)
+    expect(useQcSessionStore().historyId).toBeTruthy()
+  })
+
+  it('beginEditing flags needsHistory when the datastream is not a managed one', async () => {
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const { beginEditing, needsHistory, needsSession } = useEditSession()
+    await beginEditing()
+    expect(needsHistory.value).toBe(true)
+    expect(needsSession.value).toBe(false)
+  })
+
+  it('beginEditing resumes an in-progress session (replays via applyHistory)', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    await qc.sessions.create(h.id, WIN)
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const qcUtils = await import('@uwrl/qc-utils')
+    const { beginEditing, needsSession } = useEditSession()
+    await beginEditing()
+    expect(needsSession.value).toBe(false)
+    expect(qcUtils.applyHistory).toHaveBeenCalled()
+  })
+
+  it('startSession creates a session and copies the source window', async () => {
+    await seedHistory()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession({ ...WIN, description: 'Jan' })
+    expect(session.needsSession.value).toBe(false)
+    expect(useQcSessionStore().inProgressSession?.description).toBe('Jan')
+    expect(fetchObservationsInRange.mock.calls.at(-1)?.[0].id).toBe('s-1')
+  })
+
+  it('saveDraft persists the record operations to the session', async () => {
+    await seedHistory()
+    selectedSeries.value = {
+      data: makeRecord([
+        { method: 'VALUE_THRESHOLD', args: [] },
+        { method: 'DELETE_POINTS', args: [] },
+      ]),
+    }
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    await session.saveDraft()
+
+    const store = useQcSessionStore()
+    const ops = unwrap(
+      await qc.operations.list(store.historyId!, store.inProgressSession!.id)
+    )
+    expect(ops.map((o) => o.operationType)).toEqual([
+      'VALUE_THRESHOLD',
+      'DELETE_POINTS',
+    ])
+  })
+
+  it('commit pushes observations in replace mode and locks the session', async () => {
+    await seedHistory()
+    selectedSeries.value = { data: makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }]) }
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    await session.commit()
+
+    expect(createObservations).toHaveBeenCalledWith(
+      'm-1',
+      expect.objectContaining({ fields: ['phenomenonTime', 'result'] }),
+      { mode: 'replace' }
+    )
+    const store = useQcSessionStore()
+    expect(store.committedSessions.length).toBe(1)
+    expect(store.inProgressSession).toBeNull()
+  })
+})
diff --git a/apps/qc-app/src/composables/__tests__/useWorkspacePermissions.spec.ts b/apps/qc-app/src/composables/__tests__/useWorkspacePermissions.spec.ts
new file mode 100644
index 00000000..310eed3d
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useWorkspacePermissions.spec.ts
@@ -0,0 +1,114 @@
+import { describe, it, expect, beforeEach } from 'vitest'
+import { setActivePinia, createPinia } from 'pinia'
+import {
+  PermissionAction,
+  PermissionResource,
+  type Workspace,
+  type User,
+} from '@hydroserver/client'
+import { useWorkspacePermissions } from '../useWorkspacePermissions'
+import { useWorkspaceStore } from '@/store/workspaces'
+import { useUserStore } from '@/store/user'
+
+function ws(partial: Partial<Workspace>): Workspace {
+  return {
+    id: 'w',
+    name: 'W',
+    isPrivate: false,
+    owner: null,
+    collaboratorRole: null,
+    ...partial,
+  } as Workspace
+}
+
+const role = (name: string, permissions: { action: string; resource: string }[]) =>
+  ({ name, permissions }) as any
+
+const ownerWs = ws({ owner: { email: 'me@x.org', name: 'Me' } as any })
+const editorWs = ws({
+  id: 'ed',
+  owner: { email: 'other@x.org' } as any,
+  collaboratorRole: role('Editor', [
+    { action: PermissionAction.Create, resource: PermissionResource.Datastream },
+    { action: PermissionAction.Edit, resource: PermissionResource.Observation },
+  ]),
+})
+const viewerWs = ws({
+  id: 'vw',
+  owner: { email: 'other@x.org' } as any,
+  collaboratorRole: role('Viewer', [
+    { action: PermissionAction.View, resource: PermissionResource.Datastream },
+  ]),
+})
+const globalWs = ws({
+  id: 'gl',
+  owner: { email: 'other@x.org' } as any,
+  collaboratorRole: role('Super', [
+    { action: PermissionAction.Global, resource: PermissionResource.Global },
+  ]),
+})
+
+function setUser(email: string, accountType = 'standard') {
+  useUserStore().user = { email, accountType } as unknown as User
+}
+
+describe('useWorkspacePermissions', () => {
+  beforeEach(() => setActivePinia(createPinia()))
+
+  it('owner: can edit and create, role is Owner', () => {
+    setUser('me@x.org')
+    const { isOwner, canEdit, canCreateDatastream, roleName } =
+      useWorkspacePermissions()
+    expect(isOwner(ownerWs)).toBe(true)
+    expect(canEdit(ownerWs)).toBe(true)
+    expect(canCreateDatastream(ownerWs)).toBe(true)
+    expect(roleName(ownerWs)).toBe('Owner')
+  })
+
+  it('editor role: can edit and create datastreams', () => {
+    setUser('me@x.org')
+    const { canEdit, canCreateDatastream, roleName } = useWorkspacePermissions()
+    expect(canEdit(editorWs)).toBe(true)
+    expect(canCreateDatastream(editorWs)).toBe(true)
+    expect(roleName(editorWs)).toBe('Editor')
+  })
+
+  it('viewer role: cannot edit or create', () => {
+    setUser('me@x.org')
+    const { canEdit, canCreateDatastream, roleName } = useWorkspacePermissions()
+    expect(canEdit(viewerWs)).toBe(false)
+    expect(canCreateDatastream(viewerWs)).toBe(false)
+    expect(roleName(viewerWs)).toBe('Viewer')
+  })
+
+  it('global permission grants edit and create', () => {
+    setUser('me@x.org')
+    const { canEdit, canCreateDatastream } = useWorkspacePermissions()
+    expect(canEdit(globalWs)).toBe(true)
+    expect(canCreateDatastream(globalWs)).toBe(true)
+  })
+
+  it('admin overrides a read-only role', () => {
+    setUser('me@x.org', 'admin')
+    const { canEdit, canCreateDatastream } = useWorkspacePermissions()
+    expect(canEdit(viewerWs)).toBe(true)
+    expect(canCreateDatastream(viewerWs)).toBe(true)
+  })
+
+  it('defaults to the selected workspace when no arg is passed', () => {
+    setUser('me@x.org')
+    useWorkspaceStore().selectedWorkspace = editorWs
+    const { canEdit, roleName } = useWorkspacePermissions()
+    expect(canEdit()).toBe(true)
+    expect(roleName()).toBe('Editor')
+  })
+
+  it('workspaceById resolves from availableWorkspaces', () => {
+    setUser('me@x.org')
+    useWorkspaceStore().availableWorkspaces = [editorWs, viewerWs]
+    const { workspaceById } = useWorkspacePermissions()
+    expect(workspaceById('ed')?.id).toBe('ed')
+    expect(workspaceById('missing')).toBeNull()
+    expect(workspaceById(null)).toBeNull()
+  })
+})
diff --git a/apps/qc-app/src/composables/useCreateManagedDatastream.ts b/apps/qc-app/src/composables/useCreateManagedDatastream.ts
new file mode 100644
index 00000000..2d170429
--- /dev/null
+++ b/apps/qc-app/src/composables/useCreateManagedDatastream.ts
@@ -0,0 +1,35 @@
+/**
+ * Wires the "Create Datastream for Editing" flow to the live stores:
+ * delegates to the tested `createManagedDatastream` orchestration with the
+ * HydroServer client and the QC history service.
+ */
+
+import { storeToRefs } from 'pinia'
+import type { Datastream } from '@hydroserver/client'
+import { useHydroServer } from '@/store/hydroserver'
+import {
+  createManagedDatastream,
+  type CreateManagedDatastreamResult,
+} from '@/services/qualityControl'
+
+export interface CreateManagedDatastreamSpec {
+  source: Datastream
+  processingLevelId: string
+  name?: string
+}
+
+export function useCreateManagedDatastream() {
+  const { hs } = storeToRefs(useHydroServer())
+
+  async function create(
+    spec: CreateManagedDatastreamSpec
+  ): Promise<CreateManagedDatastreamResult> {
+    return createManagedDatastream(hs.value, hs.value.qualityControlHistories, {
+      source: spec.source,
+      processingLevelId: spec.processingLevelId,
+      overrides: spec.name ? { name: spec.name } : undefined,
+    })
+  }
+
+  return { create }
+}
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
new file mode 100644
index 00000000..90bcedb5
--- /dev/null
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -0,0 +1,160 @@
+/**
+ * Orchestrates the session-based editing workflow (spec section 5-9),
+ * wiring the QC service layer to the app's stores:
+ *   - beginEditing: resolve the managed datastream's history, load its
+ *     sessions, and resume the in-progress one (or signal that a session
+ *     must be started),
+ *   - startSession: create a session and copy the source window in,
+ *   - saveDraft: persist the record's edit operations to the session
+ *     (append-only, so each user's operations keep their creator),
+ *   - commit: push the final observations (replace) and lock the session.
+ *
+ * The QC API client comes from `hs.qualityControl*`; the server stamps every
+ * operation/session with the authenticated user.
+ */
+
+import { ref } from 'vue'
+import { storeToRefs } from 'pinia'
+import { serializeHistory, applyHistory } from '@uwrl/qc-utils'
+import type { Datastream, QualityControlSessionContract } from '@hydroserver/client'
+import type { ObservationRecord } from '@uwrl/qc-utils'
+import { useDataVisStore } from '@/store/dataVisualization'
+import { usePlotlyStore } from '@/store/plotly'
+import { useHydroServer } from '@/store/hydroserver'
+import { useObservationStore } from '@/store/observations'
+import { useQcSessionStore } from '@/store/qcSession'
+import {
+  findHistoryForDatastream,
+  startOrResumeSession,
+  loadSourceWindow,
+  persistSessionOperations,
+  commitQcSession,
+  reconstructSession,
+  observationsBulkBody,
+} from '@/services/qualityControl'
+
+type QcSessionPostBody = QualityControlSessionContract.PostBody
+
+export function useEditSession() {
+  const { qcDatastream } = storeToRefs(useDataVisStore())
+  const { selectedSeries } = storeToRefs(usePlotlyStore())
+  const { hs } = storeToRefs(useHydroServer())
+  const { fetchObservationsInRange } = useObservationStore()
+  const sessionStore = useQcSessionStore()
+
+  const sourceDatastream = ref<Datastream | null>(null)
+  /** True when the managed datastream has no in-progress session to resume. */
+  const needsSession = ref(false)
+  /** True when the selected datastream has no QC history (not a managed datastream). */
+  const needsHistory = ref(false)
+
+  async function beginEditing(): Promise<void> {
+    const managed = qcDatastream.value
+    if (!managed) return
+    needsHistory.value = false
+
+    const history = await findHistoryForDatastream(
+      hs.value.qualityControlHistories,
+      managed.id
+    )
+    if (!history) {
+      // Not a managed datastream: the caller should offer to create one
+      // from it (with this datastream as the source).
+      needsHistory.value = true
+      return
+    }
+    sourceDatastream.value =
+      (await hs.value.datastreams.getItem(history.sourceDatastream.id)) ?? null
+    await sessionStore.loadSessions(history.id)
+
+    const inProgress = sessionStore.inProgressSession
+    const record = selectedSeries.value?.data
+    if (inProgress && sourceDatastream.value && record) {
+      // Resume: reconstruct the in-progress session's working state.
+      await reconstructSession(
+        {
+          qcSessions: hs.value.qualityControlSessions,
+          fetchInRange: fetchObservationsInRange,
+          applyHistory,
+        },
+        sourceDatastream.value,
+        history.id,
+        inProgress.id
+      )
+      needsSession.value = false
+    } else {
+      needsSession.value = true
+    }
+  }
+
+  async function startSession(spec: QcSessionPostBody): Promise<void> {
+    const historyId = sessionStore.historyId
+    if (!historyId || !sourceDatastream.value) {
+      throw new Error('Load a managed datastream for editing first.')
+    }
+    const session = await startOrResumeSession(
+      hs.value.qualityControlSessions,
+      historyId,
+      spec
+    )
+    await sessionStore.loadSessions(historyId)
+    await loadSourceWindow(fetchObservationsInRange, sourceDatastream.value, session)
+    needsSession.value = false
+  }
+
+  async function saveDraft(): Promise<void> {
+    const historyId = sessionStore.historyId
+    const session = sessionStore.inProgressSession
+    const record = selectedSeries.value?.data
+    if (!historyId || !session || !record) {
+      throw new Error('No active edit session to save.')
+    }
+    const operations = serializeHistory(record as ObservationRecord, {
+      startDate: session.phenomenonTimeStart,
+      endDate: session.phenomenonTimeEnd,
+    }).operations
+    await persistSessionOperations(
+      hs.value.qualityControlOperations,
+      historyId,
+      session.id,
+      operations
+    )
+  }
+
+  async function commit(): Promise<void> {
+    const managed = qcDatastream.value
+    const historyId = sessionStore.historyId
+    const session = sessionStore.inProgressSession
+    const record = selectedSeries.value?.data
+    if (!managed || !historyId || !session || !record) {
+      throw new Error('No active edit session to commit.')
+    }
+    await saveDraft()
+
+    const body = observationsBulkBody(record as ObservationRecord)
+    await commitQcSession({
+      qcSessions: hs.value.qualityControlSessions,
+      historyId,
+      sessionId: session.id,
+      // The real client fetches the source window's X-Checksum header for a
+      // genuine integrity check; here we reuse the checksum captured at create.
+      currentSourceChecksum: session.sourceChecksum,
+      pushObservations: async () => {
+        await hs.value.datastreams.createObservations(managed.id, body, {
+          mode: 'replace',
+        })
+      },
+    })
+    await sessionStore.loadSessions(historyId)
+  }
+
+  return {
+    sourceDatastream,
+    needsSession,
+    needsHistory,
+    beginEditing,
+    startSession,
+    saveDraft,
+    commit,
+  }
+}
diff --git a/apps/qc-app/src/composables/useQcSubmission.ts b/apps/qc-app/src/composables/useQcSubmission.ts
index bc53ebfe..ddec4021 100644
--- a/apps/qc-app/src/composables/useQcSubmission.ts
+++ b/apps/qc-app/src/composables/useQcSubmission.ts
@@ -1,20 +1,10 @@
 import { useDataVisStore } from '@/store/dataVisualization'
 import { usePlotlyStore } from '@/store/plotly'
 import { useHydroServer } from '@/store/hydroserver'
+import { observationsBulkBody } from '@/services/qualityControl/observationsBody'
 import { Snackbar } from '@uwrl/qc-utils'
 import { storeToRefs } from 'pinia'
 
-/**
- * ObservationBulkPostBody is a local type inside @hydroserver/client's
- * datastream.service and is not re-exported from the package entry. Define
- * its structural shape here to keep the composable type-safe without
- * reaching into internal module paths.
- */
-type ObservationBulkPostBody = {
-  fields: ('phenomenonTime' | 'result')[]
-  data: unknown[][]
-}
-
 /**
  * Encapsulates the QC submission flow: guard checks, serialization of the
  * edited ObservationRecord into ObservationBulkPostBody, calling
@@ -37,18 +27,10 @@ export function useQcSubmission() {
       return
     }
 
-    const { dataX, dataY } = selectedSeries.value.data
-
     // TODO: resultQualifierCodes serialization is deferred — qualifier
     // tracking in useDataVisStore is still stubbed. Submit only
     // phenomenonTime + result for now.
-    const body: ObservationBulkPostBody = {
-      fields: ['phenomenonTime', 'result'],
-      data: Array.from(dataX as ArrayLike<number>).map((ts, i) => [
-        new Date(ts).toISOString(),
-        (dataY as ArrayLike<number>)[i],
-      ]),
-    }
+    const body = observationsBulkBody(selectedSeries.value.data)
 
     isSubmitting.value = true
     try {
diff --git a/apps/qc-app/src/composables/useWorkspacePermissions.ts b/apps/qc-app/src/composables/useWorkspacePermissions.ts
new file mode 100644
index 00000000..da743454
--- /dev/null
+++ b/apps/qc-app/src/composables/useWorkspacePermissions.ts
@@ -0,0 +1,109 @@
+/**
+ * Reactive workspace permission checks for the QC app.
+ *
+ * The signed-in user's role on a workspace travels with the `Workspace`
+ * object: owners have a null `collaboratorRole`, collaborators carry their
+ * role's `permissions[]`, and admins (`accountType === 'admin'`) override
+ * everything. No separate "am I an editor" endpoint is needed — the role is
+ * embedded in `hs.workspaces.list()`. (`hs.user.can()` is the async
+ * single-shot equivalent; this composable is the synchronous, reactive one
+ * for gating UI.)
+ */
+
+import { computed } from 'vue'
+import { storeToRefs } from 'pinia'
+import {
+  PermissionAction,
+  PermissionResource,
+  type Permission,
+  type Workspace,
+} from '@hydroserver/client'
+import { useWorkspaceStore } from '@/store/workspaces'
+import { useUserStore } from '@/store/user'
+
+export function useWorkspacePermissions() {
+  const { availableWorkspaces, selectedWorkspace } = storeToRefs(
+    useWorkspaceStore()
+  )
+  const { user } = storeToRefs(useUserStore())
+
+  const isAdmin = computed(
+    () => (user.value?.accountType as string) === 'admin'
+  )
+
+  function isOwner(ws?: Workspace | null): boolean {
+    if (!ws) return false
+    if (ws.owner?.email && user.value?.email) {
+      return ws.owner.email === user.value.email
+    }
+    // Owned workspaces carry a null collaboratorRole.
+    return ws.collaboratorRole == null
+  }
+
+  function hasGlobal(perms: Permission[]): boolean {
+    return perms.some(
+      (p) =>
+        p.resource === PermissionResource.Global &&
+        p.action === PermissionAction.Global
+    )
+  }
+
+  function can(
+    action: PermissionAction,
+    resource: PermissionResource,
+    ws?: Workspace | null
+  ): boolean {
+    const w = ws ?? selectedWorkspace.value
+    if (!w) return false
+    if (isOwner(w) || isAdmin.value) return true
+    const perms = w.collaboratorRole?.permissions ?? []
+    return (
+      hasGlobal(perms) ||
+      perms.some((p) => p.action === action && p.resource === resource)
+    )
+  }
+
+  /** Can create a managed datastream in this workspace (the QC setup step). */
+  function canCreateDatastream(ws?: Workspace | null): boolean {
+    return can(PermissionAction.Create, PermissionResource.Datastream, ws)
+  }
+
+  /**
+   * Can run the QC edit workflow in this workspace — needs to create the
+   * managed datastream and/or write observations. Used to gate the editor's
+   * Start editing / Save / Commit controls.
+   */
+  function canEdit(ws?: Workspace | null): boolean {
+    return (
+      can(PermissionAction.Create, PermissionResource.Datastream, ws) ||
+      can(PermissionAction.Edit, PermissionResource.Datastream, ws) ||
+      can(PermissionAction.Create, PermissionResource.Observation, ws) ||
+      can(PermissionAction.Edit, PermissionResource.Observation, ws)
+    )
+  }
+
+  /** Human-readable role label for display. */
+  function roleName(ws?: Workspace | null): string {
+    const w = ws ?? selectedWorkspace.value
+    if (!w) return ''
+    if (isOwner(w)) return 'Owner'
+    if (w.collaboratorRole?.name) return w.collaboratorRole.name
+    if (isAdmin.value) return 'Admin'
+    return 'Read-only'
+  }
+
+  function workspaceById(id?: string | null): Workspace | null {
+    if (!id) return null
+    return availableWorkspaces.value.find((w) => w.id === id) ?? null
+  }
+
+  return {
+    isAdmin,
+    isOwner,
+    can,
+    canEdit,
+    canCreateDatastream,
+    roleName,
+    workspaceById,
+  }
+}
diff --git a/apps/qc-app/src/pages/Workspaces.vue b/apps/qc-app/src/pages/Workspaces.vue
index c8683ce4..9bade159 100644
--- a/apps/qc-app/src/pages/Workspaces.vue
+++ b/apps/qc-app/src/pages/Workspaces.vue
@@ -47,14 +47,34 @@
           </template>
 
           <template #subtitle>
-            <span class="text-body-small">
-              {{ roleLabel(ws) }}
-              <span v-if="ws.owner?.name"> · {{ ws.owner.name }}</span>
+            <span v-if="ws.owner?.name" class="text-body-small">
+              Owned by {{ ws.owner.name }}
             </span>
           </template>
 
           <template #append>
             <div class="d-flex align-center ga-3">
+              <v-tooltip
+                location="top"
+                :text="
+                  canEdit(ws)
+                    ? `You can edit data in this workspace (role: ${roleName(ws)})`
+                    : `Read-only access (role: ${roleName(ws)})`
+                "
+              >
+                <template #activator="{ props: tp }">
+                  <v-chip
+                    v-bind="tp"
+                    :data-testid="`workspace-role-${ws.id}`"
+                    size="x-small"
+                    variant="tonal"
+                    :color="canEdit(ws) ? 'primary' : 'grey-darken-1'"
+                    :prepend-icon="canEdit(ws) ? 'mdi-pencil' : 'mdi-eye-outline'"
+                  >
+                    {{ roleName(ws) }}
+                  </v-chip>
+                </template>
+              </v-tooltip>
               <v-tooltip
                 location="top"
                 :text="datastreamCountTooltip(ws.id)"
@@ -123,15 +143,17 @@
 import { onMounted, ref } from 'vue'
 import { storeToRefs } from 'pinia'
 import { useRouter, useRoute } from 'vue-router'
-import { Workspace, Datastream, ResultQualifier } from '@hydroserver/client'
+import { Datastream, ResultQualifier } from '@hydroserver/client'
 import { useWorkspaceStore } from '@/store/workspaces'
 import { useHydroServer } from '@/store/hydroserver'
+import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
 
 const router = useRouter()
 const route = useRoute()
 const store = useWorkspaceStore()
 const { availableWorkspaces, selectedWorkspace, isLoading } = storeToRefs(store)
 const { hs } = storeToRefs(useHydroServer())
+const { roleName, canEdit } = useWorkspacePermissions()
 
 // One unscoped listing bucketed by workspaceId is cheaper than N
 // scoped listings: server RBAC already filters to visible datastreams.
@@ -220,12 +242,6 @@ function onPick(id: string) {
   const next = typeof route.query.next === 'string' ? route.query.next : 'Home'
   router.push({ name: next })
 }
-
-function roleLabel(ws: Workspace): string {
-  // Owners have `collaboratorRole === null`.
-  if (!ws.collaboratorRole) return 'Owner'
-  return ws.collaboratorRole.name || 'Collaborator'
-}
 </script>
 
 <style scoped>
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/commitSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/commitSession.spec.ts
new file mode 100644
index 00000000..e96db915
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/__tests__/commitSession.spec.ts
@@ -0,0 +1,85 @@
+import { describe, it, expect, vi } from 'vitest'
+import { makeQcFake } from './qcServiceFake'
+import { commitQcSession } from '../commitSession'
+import { unwrap } from '../unwrap'
+
+const WIN = {
+  phenomenonTimeStart: '2025-01-01T00:00:00Z',
+  phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+}
+
+const setup = async () => {
+  const qc = makeQcFake()
+  const h = unwrap(
+    await qc.histories.create({
+      managedDatastreamId: 'm-1',
+      sourceDatastreamId: 's-1',
+    })
+  )
+  const session = unwrap(await qc.sessions.create(h.id, WIN))
+  return { qc, historyId: h.id, session }
+}
+
+describe('commitQcSession', () => {
+  it('checks checksum C, pushes observations, then commits', async () => {
+    const { qc, historyId, session } = await setup()
+    const order: string[] = []
+    const pushObservations = vi.fn(async () => {
+      order.push('push')
+    })
+
+    const result = await commitQcSession({
+      qcSessions: qc.sessions,
+      historyId,
+      sessionId: session.id,
+      currentSourceChecksum: session.sourceChecksum,
+      pushObservations,
+    })
+    order.push('commit')
+
+    expect(pushObservations).toHaveBeenCalledTimes(1)
+    expect(order).toEqual(['push', 'commit']) // push before commit
+    expect(result.status).toBe('committed')
+    expect(result.committedAt).toBeTruthy()
+    expect(unwrap(await qc.sessions.get(historyId, session.id)).status).toBe(
+      'committed'
+    )
+  })
+
+  it('blocks the commit on a checksum-C mismatch (no push, stays in progress)', async () => {
+    const { qc, historyId, session } = await setup()
+    const pushObservations = vi.fn()
+
+    await expect(
+      commitQcSession({
+        qcSessions: qc.sessions,
+        historyId,
+        sessionId: session.id,
+        currentSourceChecksum: 'changed-checksum',
+        pushObservations,
+      })
+    ).rejects.toThrow(/checksum C/)
+
+    expect(pushObservations).not.toHaveBeenCalled()
+    expect(unwrap(await qc.sessions.get(historyId, session.id)).status).toBe(
+      'in_progress'
+    )
+  })
+
+  it('rejects committing an already-committed session', async () => {
+    const { qc, historyId, session } = await setup()
+    await qc.sessions.commit(historyId, session.id)
+    const pushObservations = vi.fn()
+
+    await expect(
+      commitQcSession({
+        qcSessions: qc.sessions,
+        historyId,
+        sessionId: session.id,
+        currentSourceChecksum: session.sourceChecksum,
+        pushObservations,
+      })
+    ).rejects.toThrow(/in-progress/)
+    expect(pushObservations).not.toHaveBeenCalled()
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
new file mode 100644
index 00000000..38f0b7ca
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
@@ -0,0 +1,167 @@
+import { describe, it, expect, vi } from 'vitest'
+import type { Datastream, HydroServer } from '@hydroserver/client'
+import { makeQcFake } from './qcServiceFake'
+import {
+  SOURCE_TAG_KEY,
+  buildManagedDatastreamBody,
+  createManagedDatastream,
+} from '../createManagedDatastream'
+import { unwrap } from '../unwrap'
+
+const makeSource = (overrides: Partial<Datastream> = {}): Datastream =>
+  ({
+    id: 'source-1',
+    workspaceId: 'ws-1',
+    name: 'Raw Temperature',
+    description: 'raw sensor data',
+    thingId: 'thing-1',
+    observationType: 'OM_Measurement',
+    sampledMedium: 'Air',
+    noDataValue: -9999,
+    aggregationStatistic: 'Continuous',
+    unitId: 'unit-1',
+    observedPropertyId: 'op-1',
+    sensorId: 'sensor-1',
+    processingLevelId: 'pl-raw',
+    isPrivate: false,
+    isVisible: true,
+    timeAggregationInterval: null,
+    valueCount: 1000,
+    phenomenonBeginTime: '2024-01-01T00:00:00Z',
+    phenomenonEndTime: '2024-02-01T00:00:00Z',
+    ...overrides,
+  }) as unknown as Datastream
+
+const makeHs = (
+  created: Datastream | null,
+  { ok = true, status = 201, message = '' } = {}
+) => {
+  const create = vi.fn().mockResolvedValue({ data: created, ok, status, message })
+  const createTag = vi.fn().mockResolvedValue({})
+  const hs = { datastreams: { create, createTag } } as unknown as HydroServer
+  return { hs, create, createTag }
+}
+
+describe('buildManagedDatastreamBody', () => {
+  it('copies source metadata, starts empty, applies the new processing level', () => {
+    const body = buildManagedDatastreamBody(makeSource(), 'pl-qc')
+    expect(body.id).toBe('')
+    expect(body.processingLevelId).toBe('pl-qc')
+    expect(body.valueCount).toBe(0)
+    expect(body.phenomenonBeginTime).toBeNull()
+    expect(body.phenomenonEndTime).toBeNull()
+    // metadata carried over from source
+    expect(body.thingId).toBe('thing-1')
+    expect(body.unitId).toBe('unit-1')
+    expect(body.observedPropertyId).toBe('op-1')
+    expect(body.sensorId).toBe('sensor-1')
+  })
+
+  it('applies overrides last', () => {
+    const body = buildManagedDatastreamBody(makeSource(), 'pl-qc', {
+      name: 'QC Temperature',
+    })
+    expect(body.name).toBe('QC Temperature')
+  })
+
+  it('extracts flat ids from an expand_related (nested) source', () => {
+    const nested = {
+      id: 's-1',
+      name: 'Raw',
+      thing: { id: 'thing-x' },
+      unit: { id: 'unit-x' },
+      observedProperty: { id: 'op-x' },
+      sensor: { id: 'sensor-x' },
+      processingLevel: { id: 'pl-raw' },
+      workspace: { id: 'ws-x' },
+    } as any
+    const body = buildManagedDatastreamBody(nested, 'pl-qc')
+    expect(body.thingId).toBe('thing-x')
+    expect(body.unitId).toBe('unit-x')
+    expect(body.observedPropertyId).toBe('op-x')
+    expect(body.sensorId).toBe('sensor-x')
+    expect(body.processingLevelId).toBe('pl-qc')
+  })
+})
+
+describe('createManagedDatastream', () => {
+  it('creates the datastream, the history, and tags the source', async () => {
+    const source = makeSource()
+    const created = makeSource({ id: 'managed-1', processingLevelId: 'pl-qc', valueCount: 0 })
+    const { hs, create, createTag } = makeHs(created)
+    const qc = makeQcFake()
+
+    const result = await createManagedDatastream(hs, qc.histories, {
+      source,
+      processingLevelId: 'pl-qc',
+    })
+
+    // datastream created from source metadata, empty, new processing level
+    const body = create.mock.calls[0][0] as Datastream
+    expect(body.processingLevelId).toBe('pl-qc')
+    expect(body.valueCount).toBe(0)
+    expect(body.thingId).toBe('thing-1')
+
+    // history links source and managed, and is persisted in the client
+    expect(result.history.managedDatastream.id).toBe('managed-1')
+    expect(result.history.sourceDatastream.id).toBe('source-1')
+    expect(unwrap(await qc.histories.list())).toHaveLength(1)
+
+    // source datastream tagged with the managed id
+    expect(createTag).toHaveBeenCalledWith('source-1', {
+      key: SOURCE_TAG_KEY,
+      value: 'managed-1',
+    })
+  })
+
+  it('rejects a processing level equal to the source', async () => {
+    const { hs } = makeHs(makeSource({ id: 'managed-1' }))
+    const qc = makeQcFake()
+    await expect(
+      createManagedDatastream(hs, qc.histories, {
+        source: makeSource(),
+        processingLevelId: 'pl-raw',
+      })
+    ).rejects.toThrow(/different processing level/)
+  })
+
+  it('rejects a processing level equal to a nested source level', async () => {
+    const { hs } = makeHs(makeSource({ id: 'managed-1' }))
+    const qc = makeQcFake()
+    const nestedSource = { id: 's-1', name: 'Raw', processingLevel: { id: 'pl-raw' } } as any
+    await expect(
+      createManagedDatastream(hs, qc.histories, {
+        source: nestedSource,
+        processingLevelId: 'pl-raw',
+      })
+    ).rejects.toThrow(/different processing level/)
+  })
+
+  it('rejects a missing processing level', async () => {
+    const { hs } = makeHs(makeSource({ id: 'managed-1' }))
+    const qc = makeQcFake()
+    await expect(
+      createManagedDatastream(hs, qc.histories, {
+        source: makeSource(),
+        processingLevelId: '',
+      })
+    ).rejects.toThrow(/processing level is required/)
+  })
+
+  it('surfaces the backend error when creation fails or returns no id', async () => {
+    const qc = makeQcFake()
+    await expect(
+      createManagedDatastream(
+        makeHs(null, { ok: false, status: 403, message: 'Forbidden' }).hs,
+        qc.histories,
+        { source: makeSource(), processingLevelId: 'pl-qc' }
+      )
+    ).rejects.toThrow(/HTTP 403.*Forbidden/)
+    await expect(
+      createManagedDatastream(makeHs({ name: 'no-id' } as any).hs, qc.histories, {
+        source: makeSource(),
+        processingLevelId: 'pl-qc',
+      })
+    ).rejects.toThrow(/Datastream creation failed/)
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/findHistory.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/findHistory.spec.ts
new file mode 100644
index 00000000..26acedc3
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/__tests__/findHistory.spec.ts
@@ -0,0 +1,25 @@
+import { describe, it, expect } from 'vitest'
+import { makeQcFake } from './qcServiceFake'
+import { findHistoryForDatastream } from '../findHistory'
+import { unwrap } from '../unwrap'
+
+describe('findHistoryForDatastream', () => {
+  it('returns the expanded history for a managed datastream', async () => {
+    const qc = makeQcFake()
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const found = await findHistoryForDatastream(qc.histories, 'm-1')
+    expect(found?.id).toBe(h.id)
+    expect(found?.managedDatastream.id).toBe('m-1')
+    expect(found?.sourceDatastream.id).toBe('s-1')
+  })
+
+  it('returns null when the datastream has no history', async () => {
+    const qc = makeQcFake()
+    expect(await findHistoryForDatastream(qc.histories, 'nope')).toBeNull()
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/observationsBody.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/observationsBody.spec.ts
new file mode 100644
index 00000000..8a3bceed
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/__tests__/observationsBody.spec.ts
@@ -0,0 +1,20 @@
+import { describe, it, expect } from 'vitest'
+import { observationsBulkBody } from '../observationsBody'
+
+describe('observationsBulkBody', () => {
+  it('serializes dataX/dataY into a replace-mode bulk body', () => {
+    const body = observationsBulkBody({
+      dataX: [Date.UTC(2025, 0, 1), Date.UTC(2025, 0, 1, 1)],
+      dataY: [10, 20],
+    })
+    expect(body.fields).toEqual(['phenomenonTime', 'result'])
+    expect(body.data).toEqual([
+      ['2025-01-01T00:00:00.000Z', 10],
+      ['2025-01-01T01:00:00.000Z', 20],
+    ])
+  })
+
+  it('handles an empty record', () => {
+    expect(observationsBulkBody({ dataX: [], dataY: [] }).data).toEqual([])
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
new file mode 100644
index 00000000..3b83a072
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
@@ -0,0 +1,100 @@
+import { describe, it, expect } from 'vitest'
+import type { QcHistoryOperation } from '@uwrl/qc-utils'
+import { makeQcFake } from './qcServiceFake'
+import {
+  sessionOperationsFromSerialized,
+  persistSessionOperations,
+} from '../persistOperations'
+import { unwrap } from '../unwrap'
+
+const op = (method: string, args: unknown[] = []): QcHistoryOperation =>
+  ({ method, args }) as unknown as QcHistoryOperation
+
+const WIN = {
+  phenomenonTimeStart: '2025-01-01T00:00:00Z',
+  phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+}
+
+const sessionWith = async (qc: ReturnType<typeof makeQcFake>) => {
+  const h = unwrap(
+    await qc.histories.create({
+      managedDatastreamId: 'm-1',
+      sourceDatastreamId: 's-1',
+    })
+  )
+  const s = unwrap(await qc.sessions.create(h.id, WIN))
+  return { historyId: h.id, sessionId: s.id }
+}
+
+const listOps = async (
+  qc: ReturnType<typeof makeQcFake>,
+  historyId: string,
+  sessionId: string
+) => unwrap(await qc.operations.list(historyId, sessionId))
+
+describe('sessionOperationsFromSerialized', () => {
+  it('maps method/args to operationType/arguments, assigns order, drops execution', () => {
+    const serialized = [
+      { method: 'VALUE_THRESHOLD', args: [{ min: 0 }], execution: { status: 'success' } },
+      { method: 'DELETE_POINTS', args: [] },
+    ] as unknown as QcHistoryOperation[]
+    expect(sessionOperationsFromSerialized(serialized)).toEqual([
+      { operationType: 'VALUE_THRESHOLD', arguments: [{ min: 0 }], order: 0 },
+      { operationType: 'DELETE_POINTS', arguments: [], order: 1 },
+    ])
+  })
+})
+
+describe('persistSessionOperations', () => {
+  it('appends newly-added operations, leaving existing ones in place', async () => {
+    const qc = makeQcFake()
+    const { historyId, sessionId } = await sessionWith(qc)
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('SELECTION'),
+      op('DELETE_POINTS'),
+    ])
+    const firstIds = (await listOps(qc, historyId, sessionId)).map((o) => o.id)
+
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('SELECTION'),
+      op('DELETE_POINTS'),
+      op('CHANGE'),
+    ])
+    const ops = await listOps(qc, historyId, sessionId)
+
+    expect(ops.map((o) => o.operationType)).toEqual([
+      'SELECTION',
+      'DELETE_POINTS',
+      'CHANGE',
+    ])
+    expect(ops.map((o) => o.order)).toEqual([0, 1, 2])
+    // The first two are the same records, not recreated.
+    expect(ops.slice(0, 2).map((o) => o.id)).toEqual(firstIds)
+  })
+
+  it('removes operations undone since the last save (trailing delete)', async () => {
+    const qc = makeQcFake()
+    const { historyId, sessionId } = await sessionWith(qc)
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('SELECTION'),
+      op('DELETE_POINTS'),
+      op('CHANGE'),
+    ])
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('SELECTION'),
+      op('DELETE_POINTS'),
+    ])
+    const ops = await listOps(qc, historyId, sessionId)
+    expect(ops.map((o) => o.operationType)).toEqual(['SELECTION', 'DELETE_POINTS'])
+  })
+
+  it('clears operations when given an empty set', async () => {
+    const qc = makeQcFake()
+    const { historyId, sessionId } = await sessionWith(qc)
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('SELECTION'),
+    ])
+    await persistSessionOperations(qc.operations, historyId, sessionId, [])
+    expect(await listOps(qc, historyId, sessionId)).toHaveLength(0)
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts b/apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts
new file mode 100644
index 00000000..0a68ca7c
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts
@@ -0,0 +1,245 @@
+/**
+ * Test-only in-memory doubles for the SDK's QC services
+ * (`QualityControlHistoryService` / `QualityControlSessionService` /
+ * `QualityControlOperationService`). They return `ApiResponse` like the real
+ * services and enforce the invariants the orchestration relies on (one
+ * in-progress session per history, append-only operations, overlap-based
+ * session dependencies). Not shipped: the real QC client lives in
+ * `@hydroserver/client`.
+ */
+
+import type {
+  ApiResponse,
+  QualityControlHistoryService,
+  QualityControlSessionService,
+  QualityControlOperationService,
+  QualityControlHistoryContract,
+  QualityControlSessionContract,
+  QualityControlOperationContract,
+} from '@hydroserver/client'
+
+type QcHistoryDetail = QualityControlHistoryContract.DetailResponse
+type QcSessionDetail = QualityControlSessionContract.DetailResponse
+type QcSessionSummary = QualityControlSessionContract.SummaryResponse
+type QcOperation = QualityControlOperationContract.SummaryResponse
+type QcOperationPostBody = QualityControlOperationContract.PostBody[number]
+type QcContact = { name: string; email: string }
+
+const ok = <T>(data: T): ApiResponse<T> =>
+  ({ data, status: 200, message: '', ok: true }) as ApiResponse<T>
+const fail = (message: string, status = 400): ApiResponse<any> =>
+  ({ data: null, status, message, ok: false }) as unknown as ApiResponse<any>
+
+const TEST_USER: QcContact = { name: 'Test User', email: 'test@example.org' }
+
+let seq = 0
+const nextId = (prefix: string) => `${prefix}-${(seq += 1)}`
+
+const summary = (s: QcSessionDetail): QcSessionSummary => {
+  const { operations: _o, dependencyIds: _d, ...rest } = s
+  return rest as QcSessionSummary
+}
+
+export interface QcFake {
+  histories: QualityControlHistoryService
+  sessions: QualityControlSessionService
+  operations: QualityControlOperationService
+}
+
+export function makeQcFake(createdBy: QcContact = TEST_USER): QcFake {
+  const histories: QcHistoryDetail[] = []
+  const sessions: QcSessionDetail[] = []
+
+  const forHistory = (historyId: string) =>
+    sessions.filter((s) => s.historyId === historyId)
+  const find = (sessionId: string) => sessions.find((s) => s.id === sessionId)
+
+  /** Transitive committed ancestors of a session (overlap-based dependencies). */
+  const ancestorsOf = (sessionId: string): Set<string> => {
+    const result = new Set<string>()
+    const visit = (id: string) => {
+      const s = find(id)
+      if (!s) return
+      for (const dep of s.dependencyIds) {
+        if (!result.has(dep)) {
+          result.add(dep)
+          visit(dep)
+        }
+      }
+    }
+    visit(sessionId)
+    return result
+  }
+
+  const historiesService = {
+    async create(body: QualityControlHistoryContract.PostBody) {
+      const history = {
+        id: nextId('h'),
+        createdAt: '2025-01-01T00:00:00Z',
+        managedDatastream: {
+          id: body.managedDatastreamId,
+          name: `Datastream ${body.managedDatastreamId}`,
+        },
+        sourceDatastream: {
+          id: body.sourceDatastreamId,
+          name: `Datastream ${body.sourceDatastreamId}`,
+        },
+        phenomenonTimeStart: null,
+        phenomenonTimeEnd: null,
+        sourceChecksum: null,
+        managedChecksum: null,
+      } as unknown as QcHistoryDetail
+      histories.push(history)
+      return ok(history)
+    },
+
+    async list(
+      query?: Partial<QualityControlHistoryContract.QueryParameters>
+    ) {
+      let list = histories
+      const managedIds = query?.managed_datastream_id
+      if (managedIds && managedIds.length) {
+        list = list.filter((h) => managedIds.includes(h.managedDatastream.id))
+      }
+      return ok([...list] as QcHistoryDetail[])
+    },
+
+    async get(historyId: string) {
+      const h = histories.find((x) => x.id === historyId)
+      return h ? ok(h) : fail('history not found', 404)
+    },
+
+    async delete(historyId: string) {
+      const i = histories.findIndex((x) => x.id === historyId)
+      if (i >= 0) histories.splice(i, 1)
+      return ok(null)
+    },
+  }
+
+  const sessionsService = {
+    async create(historyId: string, body: QualityControlSessionContract.PostBody) {
+      if (forHistory(historyId).some((s) => s.status === 'in_progress')) {
+        return fail('history already has an in-progress session')
+      }
+      const id = nextId('s')
+      const start = body.phenomenonTimeStart
+      const end = body.phenomenonTimeEnd
+      const dependencyIds = forHistory(historyId)
+        .filter(
+          (s) =>
+            s.status === 'committed' &&
+            s.phenomenonTimeStart < end &&
+            s.phenomenonTimeEnd > start
+        )
+        .map((s) => s.id)
+      const session = {
+        id,
+        historyId,
+        createdBy,
+        createdAt: '2025-01-01T00:00:00Z',
+        phenomenonTimeStart: start,
+        phenomenonTimeEnd: end,
+        status: 'in_progress',
+        committedAt: null,
+        description: body.description ?? null,
+        sourceChecksum: `chk-${id}`,
+        managedChecksum: null,
+        dependencyIds,
+        operations: [],
+      } as unknown as QcSessionDetail
+      sessions.push(session)
+      return ok({ ...session, operations: [] } as QcSessionDetail)
+    },
+
+    async get(historyId: string, sessionId: string) {
+      const s = find(sessionId)
+      if (!s || s.historyId !== historyId) return fail('session not found', 404)
+      return ok({ ...s, operations: [...s.operations] } as QcSessionDetail)
+    },
+
+    async list(
+      historyId: string,
+      query?: Partial<QualityControlSessionContract.QueryParameters>
+    ) {
+      let list = forHistory(historyId)
+      if (query?.ancestor_of) {
+        const ids = ancestorsOf(query.ancestor_of)
+        list = list.filter((s) => ids.has(s.id))
+      }
+      if (query?.status) list = list.filter((s) => s.status === query.status)
+      return ok(list.map(summary))
+    },
+
+    async update(
+      _historyId: string,
+      sessionId: string,
+      body: { description?: string | null }
+    ) {
+      const s = find(sessionId)
+      if (!s) return fail('session not found', 404)
+      if (body.description !== undefined) s.description = body.description
+      return ok({ ...s, operations: [...s.operations] } as QcSessionDetail)
+    },
+
+    async delete(_historyId: string, sessionId: string) {
+      const i = sessions.findIndex((x) => x.id === sessionId)
+      if (i >= 0) sessions.splice(i, 1)
+      return ok(null)
+    },
+
+    async commit(_historyId: string, sessionId: string) {
+      const s = find(sessionId)
+      if (!s) return fail('session not found', 404)
+      s.status = 'committed'
+      s.committedAt = '2025-06-01T00:00:00Z'
+      s.managedChecksum = `mchk-${sessionId}`
+      return ok({ ...s, operations: [...s.operations] } as QcSessionDetail)
+    },
+  }
+
+  const operationsService = {
+    async list(_historyId: string, sessionId: string) {
+      const s = find(sessionId)
+      return ok(s ? [...s.operations] : [])
+    },
+
+    async create(
+      _historyId: string,
+      sessionId: string,
+      body: QcOperationPostBody[]
+    ) {
+      const s = find(sessionId)
+      if (!s) return fail('session not found', 404)
+      const created = body.map(
+        (b, i): QcOperation =>
+          ({
+            id: nextId('op'),
+            order: b.order ?? s.operations.length + i,
+            operationType: b.operationType,
+            createdAt: '2025-01-01T00:00:00Z',
+            createdBy,
+            comment: b.comment ?? null,
+            arguments: b.arguments ?? null,
+          }) as unknown as QcOperation
+      )
+      s.operations.push(...created)
+      return ok(created)
+    },
+
+    async update() {
+      return fail('not implemented')
+    },
+
+    async delete(_historyId: string, sessionId: string, operationId: string) {
+      const s = find(sessionId)
+      if (s) s.operations = s.operations.filter((o) => o.id !== operationId)
+      return ok(null)
+    },
+  }
+
+  return {
+    histories: historiesService as unknown as QualityControlHistoryService,
+    sessions: sessionsService as unknown as QualityControlSessionService,
+    operations: operationsService as unknown as QualityControlOperationService,
+  }
+}
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
new file mode 100644
index 00000000..cc33aabf
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
@@ -0,0 +1,127 @@
+import { describe, it, expect, vi } from 'vitest'
+import type { Datastream } from '@hydroserver/client'
+import type { ObservationRecord, QcHistory } from '@uwrl/qc-utils'
+import { makeQcFake } from './qcServiceFake'
+import {
+  collectSessionOperations,
+  reconstructSession,
+} from '../reconstructSession'
+import { unwrap } from '../unwrap'
+
+const win = (start: string, end: string) => ({
+  phenomenonTimeStart: start,
+  phenomenonTimeEnd: end,
+})
+
+/** Create a committed session with the given operations. */
+const committedSession = async (
+  qc: ReturnType<typeof makeQcFake>,
+  historyId: string,
+  range: { phenomenonTimeStart: string; phenomenonTimeEnd: string },
+  ops: string[]
+) => {
+  const s = unwrap(await qc.sessions.create(historyId, range))
+  if (ops.length) {
+    await qc.operations.create(
+      historyId,
+      s.id,
+      ops.map((operationType, i) => ({
+        operationType: operationType as any,
+        order: i,
+      }))
+    )
+  }
+  await qc.sessions.commit(historyId, s.id)
+  return s
+}
+
+const newHistory = async (qc: ReturnType<typeof makeQcFake>) =>
+  unwrap(
+    await qc.histories.create({
+      managedDatastreamId: 'm-1',
+      sourceDatastreamId: 's-1',
+    })
+  ).id
+
+describe('collectSessionOperations', () => {
+  it('returns just the session ops when there are no ancestors', async () => {
+    const qc = makeQcFake()
+    const historyId = await newHistory(qc)
+    const s = await committedSession(
+      qc,
+      historyId,
+      win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'),
+      ['SELECTION', 'DELETE_POINTS']
+    )
+    const ops = await collectSessionOperations(qc.sessions, historyId, s.id)
+    expect(ops.map((o) => o.method)).toEqual(['SELECTION', 'DELETE_POINTS'])
+  })
+
+  it('prepends ancestor ops (oldest first) before the target session ops', async () => {
+    const qc = makeQcFake()
+    const historyId = await newHistory(qc)
+    // A (Jan-Mar) then B (Feb-Apr) overlaps A -> B depends on A
+    await committedSession(
+      qc,
+      historyId,
+      win('2025-01-01T00:00:00Z', '2025-03-01T00:00:00Z'),
+      ['VALUE_THRESHOLD']
+    )
+    const b = await committedSession(
+      qc,
+      historyId,
+      win('2025-02-01T00:00:00Z', '2025-04-01T00:00:00Z'),
+      ['CHANGE_VALUES']
+    )
+
+    const ops = await collectSessionOperations(qc.sessions, historyId, b.id)
+    expect(ops.map((o) => o.method)).toEqual(['VALUE_THRESHOLD', 'CHANGE_VALUES'])
+  })
+})
+
+describe('reconstructSession', () => {
+  it('fetches the source window and replays the ordered operations via applyHistory', async () => {
+    const qc = makeQcFake()
+    const historyId = await newHistory(qc)
+    const range = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
+    const s = await committedSession(qc, historyId, range, [
+      'SELECTION',
+      'INTERPOLATE',
+    ])
+
+    const record = {} as ObservationRecord
+    const fetchInRange = vi.fn().mockResolvedValue(record)
+    let captured: QcHistory | undefined
+    const applyHistory = vi.fn(async (_rec: ObservationRecord, history: QcHistory) => {
+      captured = history
+      return { applied: history.operations.length, failed: [] }
+    })
+    const source = { id: 's-1' } as unknown as Datastream
+
+    const result = await reconstructSession(
+      { qcSessions: qc.sessions, fetchInRange, applyHistory },
+      source,
+      historyId,
+      s.id
+    )
+
+    // fetched the source over the session window
+    const [ds, begin, end] = fetchInRange.mock.calls[0]
+    expect(ds).toBe(source)
+    expect((begin as Date).toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect((end as Date).toISOString()).toBe('2025-02-01T00:00:00.000Z')
+
+    // replayed the ordered ops over that window
+    expect(captured?.version).toBe('1')
+    expect(captured?.window).toEqual({
+      startDate: range.phenomenonTimeStart,
+      endDate: range.phenomenonTimeEnd,
+    })
+    expect(captured?.operations.map((o) => o.method)).toEqual([
+      'SELECTION',
+      'INTERPOLATE',
+    ])
+    expect(result.record).toBe(record)
+    expect(result.report.applied).toBe(2)
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
new file mode 100644
index 00000000..bef99a0a
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
@@ -0,0 +1,85 @@
+import { describe, it, expect, vi } from 'vitest'
+import type { Datastream } from '@hydroserver/client'
+import type { ObservationRecord } from '@uwrl/qc-utils'
+import { makeQcFake } from './qcServiceFake'
+import {
+  getInProgressSession,
+  startOrResumeSession,
+  loadSourceWindow,
+} from '../session'
+import { unwrap } from '../unwrap'
+
+const WIN = {
+  phenomenonTimeStart: '2025-01-01T00:00:00Z',
+  phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+}
+
+const historyWith = async (qc: ReturnType<typeof makeQcFake>) => {
+  const h = unwrap(
+    await qc.histories.create({
+      managedDatastreamId: 'm-1',
+      sourceDatastreamId: 's-1',
+    })
+  )
+  return h.id
+}
+
+describe('getInProgressSession', () => {
+  it('returns null when no session is in progress', async () => {
+    const qc = makeQcFake()
+    expect(await getInProgressSession(qc.sessions, await historyWith(qc))).toBeNull()
+  })
+
+  it('returns the in-progress session when one exists', async () => {
+    const qc = makeQcFake()
+    const historyId = await historyWith(qc)
+    const s = unwrap(await qc.sessions.create(historyId, WIN))
+    const found = await getInProgressSession(qc.sessions, historyId)
+    expect(found?.id).toBe(s.id)
+    expect(found?.status).toBe('in_progress')
+  })
+})
+
+describe('startOrResumeSession', () => {
+  it('starts a new session when none is in progress', async () => {
+    const qc = makeQcFake()
+    const historyId = await historyWith(qc)
+    const session = await startOrResumeSession(qc.sessions, historyId, {
+      ...WIN,
+      description: 'Jan QC',
+    })
+    expect(session.status).toBe('in_progress')
+    expect(session.description).toBe('Jan QC')
+    expect(unwrap(await qc.sessions.list(historyId))).toHaveLength(1)
+  })
+
+  it('resumes the existing session instead of creating another (spec ignored)', async () => {
+    const qc = makeQcFake()
+    const historyId = await historyWith(qc)
+    const first = unwrap(await qc.sessions.create(historyId, WIN))
+    const resumed = await startOrResumeSession(qc.sessions, historyId, {
+      phenomenonTimeStart: '2030-01-01T00:00:00Z',
+      phenomenonTimeEnd: '2030-02-01T00:00:00Z',
+    })
+    expect(resumed.id).toBe(first.id)
+    expect(resumed.phenomenonTimeStart).toBe(WIN.phenomenonTimeStart)
+    expect(unwrap(await qc.sessions.list(historyId))).toHaveLength(1)
+  })
+})
+
+describe('loadSourceWindow', () => {
+  it('fetches the session window from the source datastream', async () => {
+    const record = {} as ObservationRecord
+    const fetchInRange = vi.fn().mockResolvedValue(record)
+    const source = { id: 's-1' } as unknown as Datastream
+
+    const result = await loadSourceWindow(fetchInRange, source, WIN)
+
+    expect(result).toBe(record)
+    expect(fetchInRange).toHaveBeenCalledTimes(1)
+    const [ds, begin, end] = fetchInRange.mock.calls[0]
+    expect(ds).toBe(source)
+    expect((begin as Date).toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect((end as Date).toISOString()).toBe('2025-02-01T00:00:00.000Z')
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/commitSession.ts b/apps/qc-app/src/services/qualityControl/commitSession.ts
new file mode 100644
index 00000000..8af5b6fd
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/commitSession.ts
@@ -0,0 +1,56 @@
+/**
+ * Commit handshake for a QC session (spec section 9).
+ *
+ * Order of operations:
+ *   1. ensure the session is in progress,
+ *   2. checksum C: the source-window checksum fetched now (from the
+ *      observations `X-Checksum` header) must equal the checksum captured
+ *      when the session was created, otherwise the source data changed
+ *      out-of-band and the commit is blocked,
+ *   3. push the final observations to the managed datastream (replace mode),
+ *   4. lock the session into the history.
+ *
+ * `pushObservations` is injected as a thunk so this unit-tests without the
+ * HydroServer client. A/B integrity checks and chunked upload are deferred.
+ */
+
+import type {
+  QualityControlSessionService,
+  QualityControlSessionContract,
+} from '@hydroserver/client'
+import { unwrap } from './unwrap'
+
+type QcSessionDetail = QualityControlSessionContract.DetailResponse
+
+export interface CommitQcSessionInput {
+  qcSessions: QualityControlSessionService
+  historyId: string
+  sessionId: string
+  /** Source-window checksum fetched now; compared to the session's checksum C. */
+  currentSourceChecksum: string
+  /** Pushes the final observations to the managed datastream (replace mode). */
+  pushObservations: () => Promise<void>
+}
+
+export async function commitQcSession(
+  input: CommitQcSessionInput
+): Promise<QcSessionDetail> {
+  const { qcSessions, historyId, sessionId, currentSourceChecksum, pushObservations } =
+    input
+
+  const session = unwrap(
+    await qcSessions.get(historyId, sessionId)
+  ) as QcSessionDetail
+  if (session.status !== 'in_progress') {
+    throw new Error('Only an in-progress session can be committed.')
+  }
+  if (currentSourceChecksum !== session.sourceChecksum) {
+    throw new Error(
+      'Source data changed since this session started (checksum C mismatch). ' +
+        'Re-fetch the source and verify your edits before committing again.'
+    )
+  }
+
+  await pushObservations()
+  return unwrap(await qcSessions.commit(historyId, sessionId))
+}
diff --git a/apps/qc-app/src/services/qualityControl/createManagedDatastream.ts b/apps/qc-app/src/services/qualityControl/createManagedDatastream.ts
new file mode 100644
index 00000000..d7f0f319
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/createManagedDatastream.ts
@@ -0,0 +1,124 @@
+/**
+ * Orchestrates creating a managed datastream for QC editing (spec section 5):
+ *   1. create an empty managed datastream from the source's metadata, with
+ *      a new processing level,
+ *   2. create the QC history linking source and managed,
+ *   3. tag the source datastream so the app can mark it read-only.
+ *
+ * Pure (deps injected) so it unit-tests without Pinia/Vuetify; a thin
+ * composable wires it to the live HydroServer client and stores.
+ */
+
+import type {
+  Datastream,
+  DatastreamExtended,
+  HydroServer,
+  QualityControlHistoryService,
+  QualityControlHistoryContract,
+} from '@hydroserver/client'
+import { unwrap } from './unwrap'
+
+type QcHistoryDetail = QualityControlHistoryContract.DetailResponse
+
+/** Tag key placed on a source datastream, pointing at its managed datastream. */
+export const SOURCE_TAG_KEY = 'qc-managed-datastream'
+
+export interface CreateManagedDatastreamInput {
+  /** Source datastream to derive the managed datastream from. */
+  source: Datastream
+  /** Processing level for the new managed datastream (must differ from source). */
+  processingLevelId: string
+  /** Optional metadata overrides (e.g. name, description). */
+  overrides?: Partial<Datastream>
+}
+
+export interface CreateManagedDatastreamResult {
+  managedDatastream: Datastream
+  history: QcHistoryDetail
+}
+
+/**
+ * Extract flat FK ids from a datastream that may be the `expand_related`
+ * (nested objects) shape rather than the bare flat model. Datastreams in the
+ * data-vis store are loaded expanded, so the flat `*Id` fields are absent.
+ */
+function flatDatastreamIds(source: Datastream) {
+  const s = source as Datastream & Partial<DatastreamExtended>
+  return {
+    workspaceId: s.workspaceId ?? s.workspace?.id ?? '',
+    thingId: s.thingId ?? s.thing?.id ?? '',
+    unitId: s.unitId ?? s.unit?.id ?? '',
+    observedPropertyId: s.observedPropertyId ?? s.observedProperty?.id ?? '',
+    sensorId: s.sensorId ?? s.sensor?.id ?? '',
+    processingLevelId: s.processingLevelId ?? s.processingLevel?.id ?? '',
+  }
+}
+
+/**
+ * Build the datastream create body: copy the source's metadata, start
+ * empty (no observations), and apply the new processing level. The id is
+ * cleared so the server assigns one.
+ */
+export function buildManagedDatastreamBody(
+  source: Datastream,
+  processingLevelId: string,
+  overrides: Partial<Datastream> = {}
+): Datastream {
+  return {
+    ...source,
+    id: '',
+    ...flatDatastreamIds(source),
+    processingLevelId,
+    valueCount: 0,
+    phenomenonBeginTime: null,
+    phenomenonEndTime: null,
+    ...overrides,
+  } as Datastream
+}
+
+export async function createManagedDatastream(
+  hs: HydroServer,
+  qcHistories: QualityControlHistoryService,
+  input: CreateManagedDatastreamInput
+): Promise<CreateManagedDatastreamResult> {
+  const { source, processingLevelId, overrides } = input
+
+  if (!processingLevelId) {
+    throw new Error('A processing level is required for the managed datastream.')
+  }
+  if (processingLevelId === flatDatastreamIds(source).processingLevelId) {
+    throw new Error(
+      'The managed datastream must use a different processing level than the source.'
+    )
+  }
+
+  const body = buildManagedDatastreamBody(source, processingLevelId, overrides)
+  const response = await hs.datastreams.create(body)
+  if (!response.ok) {
+    throw new Error(
+      `Datastream creation failed (HTTP ${response.status}): ${
+        response.message || 'the backend returned no datastream id'
+      }`
+    )
+  }
+  const managedDatastream = response.data
+  if (!managedDatastream?.id) {
+    throw new Error(
+      'Datastream creation failed: the backend returned no datastream id.'
+    )
+  }
+
+  const history = unwrap(
+    await qcHistories.create({
+      managedDatastreamId: managedDatastream.id,
+      sourceDatastreamId: source.id,
+    })
+  )
+
+  await hs.datastreams.createTag(source.id, {
+    key: SOURCE_TAG_KEY,
+    value: managedDatastream.id,
+  })
+
+  return { managedDatastream, history }
+}
diff --git a/apps/qc-app/src/services/qualityControl/findHistory.ts b/apps/qc-app/src/services/qualityControl/findHistory.ts
new file mode 100644
index 00000000..325e47be
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/findHistory.ts
@@ -0,0 +1,25 @@
+import type {
+  QualityControlHistoryService,
+  QualityControlHistoryContract,
+} from '@hydroserver/client'
+import { unwrap } from './unwrap'
+
+type QcHistoryDetail = QualityControlHistoryContract.DetailResponse
+
+/**
+ * Resolve the QC history for a managed datastream, or null if it has none
+ * (i.e. the datastream is not set up for QC editing). Returns the expanded
+ * detail so callers get the linked source datastream.
+ */
+export async function findHistoryForDatastream(
+  qcHistories: QualityControlHistoryService,
+  managedDatastreamId: string
+): Promise<QcHistoryDetail | null> {
+  const histories = unwrap(
+    await qcHistories.list({
+      managed_datastream_id: [managedDatastreamId],
+      expand_related: true,
+    })
+  )
+  return (histories[0] as QcHistoryDetail | undefined) ?? null
+}
diff --git a/apps/qc-app/src/services/qualityControl/index.ts b/apps/qc-app/src/services/qualityControl/index.ts
new file mode 100644
index 00000000..44146cdd
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/index.ts
@@ -0,0 +1,19 @@
+/**
+ * Quality Control orchestration layer.
+ *
+ * The QC API client lives in `@hydroserver/client`: the
+ * `qualityControlHistories`, `qualityControlSessions` and
+ * `qualityControlOperations` services on the `hs` instance. This module holds
+ * the app-side glue that composes those services with `@uwrl/qc-utils` and the
+ * HydroServer datastream/observation APIs: creating a managed datastream, the
+ * session lifecycle, persisting/replaying operations, and the commit handshake.
+ */
+
+export * from './createManagedDatastream'
+export * from './session'
+export * from './persistOperations'
+export * from './commitSession'
+export * from './reconstructSession'
+export * from './findHistory'
+export * from './observationsBody'
+export * from './unwrap'
diff --git a/apps/qc-app/src/services/qualityControl/observationsBody.ts b/apps/qc-app/src/services/qualityControl/observationsBody.ts
new file mode 100644
index 00000000..3d13f02d
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/observationsBody.ts
@@ -0,0 +1,24 @@
+/** Bulk observations body for the replace-mode createObservations endpoint. */
+export interface ObservationBulkPostBody {
+  fields: ('phenomenonTime' | 'result')[]
+  data: unknown[][]
+}
+
+/**
+ * Serialize a record's (dataX, dataY) into a replace-mode bulk observations
+ * body: epoch-ms timestamps become ISO strings paired with their results.
+ * (Qualifier serialization is still pending in the data layer.)
+ */
+export function observationsBulkBody(record: {
+  dataX: ArrayLike<number>
+  dataY: ArrayLike<number>
+}): ObservationBulkPostBody {
+  const ys = record.dataY
+  return {
+    fields: ['phenomenonTime', 'result'],
+    data: Array.from(record.dataX).map((ts, i) => [
+      new Date(ts).toISOString(),
+      ys[i],
+    ]),
+  }
+}
diff --git a/apps/qc-app/src/services/qualityControl/persistOperations.ts b/apps/qc-app/src/services/qualityControl/persistOperations.ts
new file mode 100644
index 00000000..cd6f1f44
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/persistOperations.ts
@@ -0,0 +1,84 @@
+/**
+ * Persist an edit session's operations to the QC API (spec section 5-6).
+ *
+ * qc-utils' `serializeHistory` emits operations as `{ method, args }`; the
+ * QC API speaks `{ operationType, arguments, order }`. The enum values are
+ * identical, so the only app-side glue is a field rename plus assigning
+ * `order` by position.
+ */
+
+import type { QcHistoryOperation } from '@uwrl/qc-utils'
+import type {
+  QualityControlOperationService,
+  QualityControlOperation,
+  QualityControlOperationContract,
+} from '@hydroserver/client'
+import { unwrap } from './unwrap'
+
+type QcOperationPostBody = QualityControlOperationContract.PostBody[number]
+
+function operationBody(op: QcHistoryOperation, order: number): QcOperationPostBody {
+  return {
+    operationType: op.method as unknown as QcOperationPostBody['operationType'],
+    arguments: op.args,
+    order,
+  }
+}
+
+/**
+ * Map qc-utils serialized operations to QC API operation bodies: rename
+ * method/args to operationType/arguments and assign order by index.
+ */
+export function sessionOperationsFromSerialized(
+  operations: QcHistoryOperation[]
+): QcOperationPostBody[] {
+  return operations.map((op, index) => operationBody(op, index))
+}
+
+/**
+ * Reconcile a session's persisted operations with the current ordered set
+ * (used when saving an in-progress session).
+ *
+ * Append-only: existing operations are left untouched so they keep their
+ * original creator (drafts are shared, and the server stamps each create
+ * request with the requesting user). Only operations added since the last
+ * save are created; operations removed by undo are deleted from the tail.
+ *
+ * Limitation: reconciles by position, so a mid-history rewrite that keeps
+ * the operation count but changes earlier operations is not re-persisted;
+ * the common append / trailing-undo flow is.
+ */
+export async function persistSessionOperations(
+  qcOperations: QualityControlOperationService,
+  historyId: string,
+  sessionId: string,
+  operations: QcHistoryOperation[]
+): Promise<QualityControlOperation[]> {
+  // fetch_all: the reconcile is position-based, so it needs every persisted
+  // operation, not just the first page.
+  const existing = unwrap(
+    await qcOperations.list(historyId, sessionId, { fetch_all: true })
+  )
+
+  // Undo: drop persisted operations past the current length (newest first).
+  for (let i = existing.length - 1; i >= operations.length; i--) {
+    const op = existing[i]
+    if (op) unwrap(await qcOperations.delete(historyId, sessionId, op.id))
+  }
+
+  // Append the operations added since the last save (server stamps creator).
+  const appended = operations.slice(existing.length)
+  if (appended.length) {
+    unwrap(
+      await qcOperations.create(
+        historyId,
+        sessionId,
+        appended.map((op, index) => operationBody(op, existing.length + index))
+      )
+    )
+  }
+
+  return unwrap(
+    await qcOperations.list(historyId, sessionId, { fetch_all: true })
+  )
+}
diff --git a/apps/qc-app/src/services/qualityControl/reconstructSession.ts b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
new file mode 100644
index 00000000..b8d0e739
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
@@ -0,0 +1,120 @@
+/**
+ * Reconstruct the datastream state "as of" a committed session for
+ * read-only viewing (spec section 4 / 7.3).
+ *
+ * Fetch the source window, then replay the session's operations preceded
+ * by its ancestors' operations (chronological order) via qc-utils
+ * `applyHistory`. `fetchInRange` and `applyHistory` are injected so this
+ * unit-tests without a real ObservationRecord.
+ *
+ * Caveat (deferred): operations are index-coupled to the exact dataset
+ * they were authored against, so reconstructing across sessions authored
+ * over different windows can mis-target. This loads the target session's
+ * window only; full multi-window reconstruction is a later task.
+ */
+
+import type {
+  Datastream,
+  QualityControlSessionService,
+  QualityControlOperation,
+  QualityControlSessionContract,
+} from '@hydroserver/client'
+import type {
+  ApplyHistoryReport,
+  ObservationRecord,
+  QcHistory,
+  QcHistoryOperation,
+} from '@uwrl/qc-utils'
+import { unwrap } from './unwrap'
+import type { FetchObservationsInRange } from './session'
+
+type QcSessionDetail = QualityControlSessionContract.DetailResponse
+
+/** API operation -> qc-utils replayable operation (rename operationType/arguments). */
+const toSerialized = (op: QualityControlOperation): QcHistoryOperation =>
+  ({
+    method: op.operationType as unknown as QcHistoryOperation['method'],
+    args: Array.isArray(op.arguments) ? op.arguments : [],
+  }) as QcHistoryOperation
+
+/**
+ * The ordered operations needed to reconstruct a session: every ancestor's
+ * operations (oldest first) followed by the target session's operations.
+ */
+export async function collectSessionOperations(
+  qcSessions: QualityControlSessionService,
+  historyId: string,
+  sessionId: string
+): Promise<QcHistoryOperation[]> {
+  const session = unwrap(
+    await qcSessions.get(historyId, sessionId)
+  ) as QcSessionDetail
+  const ancestors = unwrap(
+    await qcSessions.list(historyId, {
+      ancestor_of: sessionId,
+      fetch_all: true,
+    })
+  )
+  const ordered = [...ancestors].sort(
+    (a, b) =>
+      new Date(a.phenomenonTimeStart).getTime() -
+      new Date(b.phenomenonTimeStart).getTime()
+  )
+
+  const operations: QcHistoryOperation[] = []
+  for (const ancestor of ordered) {
+    const detail = unwrap(
+      await qcSessions.get(historyId, ancestor.id)
+    ) as QcSessionDetail
+    operations.push(...detail.operations.map(toSerialized))
+  }
+  operations.push(...session.operations.map(toSerialized))
+  return operations
+}
+
+export interface ReconstructSessionDeps {
+  qcSessions: QualityControlSessionService
+  fetchInRange: FetchObservationsInRange
+  applyHistory: (
+    record: ObservationRecord,
+    history: QcHistory
+  ) => Promise<ApplyHistoryReport>
+}
+
+export interface ReconstructSessionResult {
+  record: ObservationRecord
+  report: ApplyHistoryReport
+}
+
+export async function reconstructSession(
+  deps: ReconstructSessionDeps,
+  source: Datastream,
+  historyId: string,
+  sessionId: string
+): Promise<ReconstructSessionResult> {
+  const { qcSessions, fetchInRange, applyHistory } = deps
+
+  const session = unwrap(
+    await qcSessions.get(historyId, sessionId)
+  ) as QcSessionDetail
+  const operations = await collectSessionOperations(qcSessions, historyId, sessionId)
+
+  const record = await fetchInRange(
+    source,
+    new Date(session.phenomenonTimeStart),
+    new Date(session.phenomenonTimeEnd)
+  )
+
+  const history: QcHistory = {
+    version: '1',
+    createdAt: session.createdAt,
+    window: {
+      startDate: session.phenomenonTimeStart,
+      endDate: session.phenomenonTimeEnd,
+    },
+    operations,
+  }
+
+  const report = await applyHistory(record, history)
+  return { record, report }
+}
diff --git a/apps/qc-app/src/services/qualityControl/session.ts b/apps/qc-app/src/services/qualityControl/session.ts
new file mode 100644
index 00000000..30b1b8ef
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/session.ts
@@ -0,0 +1,73 @@
+/**
+ * Session lifecycle helpers (spec section 5): resume the in-progress
+ * session for a history or start a new one, and load the source-data
+ * window for editing ("Copy data from source").
+ *
+ * Pure (deps injected) for unit testing; a composable wires these to the
+ * QC client and the observation store.
+ */
+
+import type {
+  Datastream,
+  QualityControlSessionService,
+  QualityControlSessionContract,
+} from '@hydroserver/client'
+import type { ObservationRecord } from '@uwrl/qc-utils'
+import { unwrap } from './unwrap'
+
+type QcSessionDetail = QualityControlSessionContract.DetailResponse
+type QcSessionSummary = QualityControlSessionContract.SummaryResponse
+type QcSessionPostBody = QualityControlSessionContract.PostBody
+
+/** The history's single in-progress session, or null. */
+export async function getInProgressSession(
+  qcSessions: QualityControlSessionService,
+  historyId: string
+): Promise<QcSessionSummary | null> {
+  const list = unwrap(await qcSessions.list(historyId, { status: 'in_progress' }))
+  return (list[0] as QcSessionSummary | undefined) ?? null
+}
+
+/**
+ * Resume the history's in-progress session if one exists, otherwise start
+ * a new one with the given window/description. Returns the session detail
+ * (with its operations) either way. On resume the spec is ignored — the
+ * existing session keeps its own window.
+ */
+export async function startOrResumeSession(
+  qcSessions: QualityControlSessionService,
+  historyId: string,
+  spec: QcSessionPostBody
+): Promise<QcSessionDetail> {
+  const existing = await getInProgressSession(qcSessions, historyId)
+  if (existing) {
+    // A single-resource GET returns the detail shape (with operations).
+    return unwrap(await qcSessions.get(historyId, existing.id)) as QcSessionDetail
+  }
+  return unwrap(await qcSessions.create(historyId, spec))
+}
+
+/** Fetch signature matching `useObservationStore.fetchObservationsInRange`. */
+export type FetchObservationsInRange = (
+  datastream: Datastream,
+  beginTime: Date,
+  endTime: Date
+) => Promise<ObservationRecord>
+
+/**
+ * "Copy data from source": fetch the session's phenomenon-time window from
+ * the source datastream into an ObservationRecord for editing. The mixed
+ * managed+source fetch (windows overlapping committed sessions) is
+ * deferred; this loads source-only.
+ */
+export async function loadSourceWindow(
+  fetchInRange: FetchObservationsInRange,
+  source: Datastream,
+  session: Pick<QcSessionDetail, 'phenomenonTimeStart' | 'phenomenonTimeEnd'>
+): Promise<ObservationRecord> {
+  return fetchInRange(
+    source,
+    new Date(session.phenomenonTimeStart),
+    new Date(session.phenomenonTimeEnd)
+  )
+}
diff --git a/apps/qc-app/src/services/qualityControl/unwrap.ts b/apps/qc-app/src/services/qualityControl/unwrap.ts
new file mode 100644
index 00000000..dee74065
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/unwrap.ts
@@ -0,0 +1,12 @@
+import type { ApiResponse } from '@hydroserver/client'
+
+/**
+ * Unwrap a HydroServer `ApiResponse`, returning its data or throwing the
+ * server message. The QC service methods (like the rest of the SDK) never
+ * throw on HTTP errors; the session orchestration wants a thrown error it can
+ * surface, so this bridges the two.
+ */
+export function unwrap<T>(res: ApiResponse<T>): T {
+  if (!res.ok) throw new Error(res.message || 'QC API request failed.')
+  return res.data
+}
diff --git a/apps/qc-app/src/store/__tests__/qcSession.spec.ts b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
new file mode 100644
index 00000000..21a2b9fe
--- /dev/null
+++ b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
@@ -0,0 +1,104 @@
+import { describe, it, expect, beforeEach } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+import { useHydroServer } from '@/store/hydroserver'
+import { makeQcFake } from '@/services/qualityControl/__tests__/qcServiceFake'
+import { useQcSessionStore } from '@/store/qcSession'
+import { unwrap } from '@/services/qualityControl/unwrap'
+
+const win = (start: string, end: string) => ({
+  phenomenonTimeStart: start,
+  phenomenonTimeEnd: end,
+})
+
+let qc: ReturnType<typeof makeQcFake>
+
+beforeEach(() => {
+  createTestPinia()
+  qc = makeQcFake()
+  useHydroServer().hs = {
+    qualityControlHistories: qc.histories,
+    qualityControlSessions: qc.sessions,
+    qualityControlOperations: qc.operations,
+  } as any
+})
+
+/** Seed a history with one committed session and one in-progress session. */
+async function seed() {
+  const h = unwrap(
+    await qc.histories.create({
+      managedDatastreamId: 'm-1',
+      sourceDatastreamId: 's-1',
+    })
+  )
+  const committed = unwrap(
+    await qc.sessions.create(h.id, win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'))
+  )
+  await qc.sessions.commit(h.id, committed.id)
+  const inProgress = unwrap(
+    await qc.sessions.create(h.id, win('2025-02-01T00:00:00Z', '2025-03-01T00:00:00Z'))
+  )
+  return { historyId: h.id, committedId: committed.id, inProgressId: inProgress.id }
+}
+
+describe('useQcSessionStore', () => {
+  it('loads sessions and defaults the view to the in-progress session', async () => {
+    const { historyId, inProgressId } = await seed()
+    const store = useQcSessionStore()
+    await store.loadSessions(historyId)
+
+    expect(store.historyId).toBe(historyId)
+    expect(store.sessions).toHaveLength(2)
+    expect(store.currentSessionId).toBe(inProgressId)
+    expect(store.viewedSessionId).toBe(inProgressId)
+    expect(store.isReadOnly).toBe(false)
+    expect(store.committedSessions).toHaveLength(1)
+    expect(store.inProgressSession?.id).toBe(inProgressId)
+  })
+
+  it('viewing a committed session is read-only; returnToCurrent restores editing', async () => {
+    const { historyId, committedId, inProgressId } = await seed()
+    const store = useQcSessionStore()
+    await store.loadSessions(historyId)
+
+    store.viewSession(committedId)
+    expect(store.viewedSessionId).toBe(committedId)
+    expect(store.viewedSession?.status).toBe('committed')
+    expect(store.isReadOnly).toBe(true)
+
+    store.returnToCurrent()
+    expect(store.viewedSessionId).toBe(inProgressId)
+    expect(store.isReadOnly).toBe(false)
+  })
+
+  it('ignores viewSession for an unknown session id', async () => {
+    const { historyId, inProgressId } = await seed()
+    const store = useQcSessionStore()
+    await store.loadSessions(historyId)
+    store.viewSession('does-not-exist')
+    expect(store.viewedSessionId).toBe(inProgressId)
+  })
+
+  it('with no in-progress session, defaults to the latest committed and is read-only', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const a = unwrap(
+      await qc.sessions.create(h.id, win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'))
+    )
+    await qc.sessions.commit(h.id, a.id)
+    const b = unwrap(
+      await qc.sessions.create(h.id, win('2025-02-01T00:00:00Z', '2025-03-01T00:00:00Z'))
+    )
+    await qc.sessions.commit(h.id, b.id)
+
+    const store = useQcSessionStore()
+    await store.loadSessions(h.id)
+
+    expect(store.currentSessionId).toBeNull()
+    expect(store.viewedSessionId).toBe(b.id)
+    expect(store.isReadOnly).toBe(true)
+  })
+})
diff --git a/apps/qc-app/src/store/qcPreferences.ts b/apps/qc-app/src/store/qcPreferences.ts
new file mode 100644
index 00000000..e3e229ab
--- /dev/null
+++ b/apps/qc-app/src/store/qcPreferences.ts
@@ -0,0 +1,24 @@
+/**
+ * Persisted QC editing preferences.
+ *
+ * Currently just the last-used processing level for the "Create Datastream
+ * for Editing" form, so the user's choice is remembered across reloads.
+ * Defaults to null (no assumed processing level on first use).
+ */
+
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+
+export const useQcPreferencesStore = defineStore(
+  'qcPreferences',
+  () => {
+    const processingLevelId = ref<string | null>(null)
+    return { processingLevelId }
+  },
+  {
+    persist: {
+      key: 'qc:preferences:v1',
+      pick: ['processingLevelId'],
+    },
+  }
+)
diff --git a/apps/qc-app/src/store/qcSession.ts b/apps/qc-app/src/store/qcSession.ts
new file mode 100644
index 00000000..418211f2
--- /dev/null
+++ b/apps/qc-app/src/store/qcSession.ts
@@ -0,0 +1,97 @@
+/**
+ * View-mode state for QC sessions (spec section 4 history navigation).
+ *
+ * Tracks the history's sessions, which one is editable (the single
+ * in-progress session) and which one is being viewed. Viewing a committed
+ * session puts the editor in read-only mode; `returnToCurrent` restores
+ * editing of the in-progress session.
+ */
+
+import { defineStore, storeToRefs } from 'pinia'
+import { computed, ref } from 'vue'
+import { useHydroServer } from '@/store/hydroserver'
+import { unwrap } from '@/services/qualityControl'
+import type { QualityControlSession } from '@hydroserver/client'
+
+export const useQcSessionStore = defineStore('qcSession', () => {
+  const historyId = ref<string | null>(null)
+  const sessions = ref<QualityControlSession[]>([])
+  /** The single in-progress (editable) session, if any. */
+  const currentSessionId = ref<string | null>(null)
+  /** The session currently being viewed. */
+  const viewedSessionId = ref<string | null>(null)
+  const isLoading = ref(false)
+
+  /** Editing is allowed only while viewing the in-progress session. */
+  const isReadOnly = computed(
+    () => !currentSessionId.value || viewedSessionId.value !== currentSessionId.value
+  )
+
+  const inProgressSession = computed(
+    () => sessions.value.find((s) => s.status === 'in_progress') ?? null
+  )
+  const committedSessions = computed(() =>
+    sessions.value.filter((s) => s.status === 'committed')
+  )
+  const viewedSession = computed(
+    () => sessions.value.find((s) => s.id === viewedSessionId.value) ?? null
+  )
+
+  /** Load a history's sessions; default the view to the in-progress session. */
+  async function loadSessions(id: string): Promise<void> {
+    isLoading.value = true
+    try {
+      const { hs } = storeToRefs(useHydroServer())
+      const list = unwrap(
+        await hs.value.qualityControlSessions.list(id, { fetch_all: true })
+      )
+      historyId.value = id
+      sessions.value = list
+      const inProgress = list.find((s) => s.status === 'in_progress') ?? null
+      currentSessionId.value = inProgress?.id ?? null
+      // Default view: the editable session, else the latest committed.
+      const latestCommitted = [...list]
+        .filter((s) => s.status === 'committed')
+        .sort((a, b) => b.phenomenonTimeStart.localeCompare(a.phenomenonTimeStart))[0]
+      viewedSessionId.value = inProgress?.id ?? latestCommitted?.id ?? null
+    } finally {
+      isLoading.value = false
+    }
+  }
+
+  /** View a session read-only (no-op for an unknown id). */
+  function viewSession(sessionId: string): void {
+    if (sessions.value.some((s) => s.id === sessionId)) {
+      viewedSessionId.value = sessionId
+    }
+  }
+
+  /** Return to the editable in-progress session. */
+  function returnToCurrent(): void {
+    viewedSessionId.value = currentSessionId.value
+  }
+
+  function reset(): void {
+    historyId.value = null
+    sessions.value = []
+    currentSessionId.value = null
+    viewedSessionId.value = null
+    isLoading.value = false
+  }
+
+  return {
+    historyId,
+    sessions,
+    currentSessionId,
+    viewedSessionId,
+    isLoading,
+    isReadOnly,
+    inProgressSession,
+    committedSessions,
+    viewedSession,
+    loadSessions,
+    viewSession,
+    returnToCurrent,
+    reset,
+  }
+})

From cb16a2338aae653403a4bcbb49db341bc7c1c669 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 22 Jun 2026 12:53:48 -0600
Subject: [PATCH 04/93] fix: return values from router guards instead of next()

---
 apps/qc-app/src/router/guards.ts |  8 +++-----
 apps/qc-app/src/router/router.ts | 22 +++++++++-------------
 2 files changed, 12 insertions(+), 18 deletions(-)

diff --git a/apps/qc-app/src/router/guards.ts b/apps/qc-app/src/router/guards.ts
index e19bbcac..737e07b9 100644
--- a/apps/qc-app/src/router/guards.ts
+++ b/apps/qc-app/src/router/guards.ts
@@ -1,7 +1,6 @@
 import { useWorkspaceStore } from '@/store/workspaces'
 import hs from '@hydroserver/client'
 import {
-  NavigationGuardNext,
   RouteLocationNormalized,
   RouteLocationRaw,
 } from 'vue-router'
@@ -10,8 +9,7 @@ type RouteGuardResult = RouteLocationRaw | false | null | undefined | void
 
 export type RouteGuard = (
   to: RouteLocationNormalized,
-  from: RouteLocationNormalized,
-  next: NavigationGuardNext
+  from: RouteLocationNormalized
 ) => RouteGuardResult | Promise<RouteGuardResult>
 
 const getQcReturnPath = (to: RouteLocationNormalized) => {
@@ -66,7 +64,7 @@ export const guards: RouteGuard[] = [
   // HydroServer workspace context. If none is selected, bounce to the
   // picker and carry a `next` hint so we can come back here once the
   // user commits to a workspace.
-  (to, _from, _next) => {
+  (to) => {
     if (!to.meta?.hasWorkspaceGuard) return null
     const { hasSelection } = useWorkspaceStore()
     if (hasSelection) return null
@@ -78,7 +76,7 @@ export const guards: RouteGuard[] = [
 
   // https://www.digitalocean.com/community/tutorials/vuejs-vue-router-modify-head
   // Append head tags and update page title
-  (to, from, _next) => {
+  (to, from) => {
     // This goes through the matched routes from last to first, finding the closest route with a title.
     // e.g., if we have `/some/deep/nested/route` and `/some`, `/deep`, and `/nested` have titles,
     // `/nested`'s will be chosen.
diff --git a/apps/qc-app/src/router/router.ts b/apps/qc-app/src/router/router.ts
index ee65e185..ced30f75 100644
--- a/apps/qc-app/src/router/router.ts
+++ b/apps/qc-app/src/router/router.ts
@@ -8,19 +8,15 @@ const router = createRouter({
 })
 
 export function setupRouteGuards() {
-  guards.map((fn) => {
-    router.beforeEach(async (to, from, next) => {
-      const activatedRouteGuard = await fn(to, from, next)
-      if (activatedRouteGuard === false) {
-        next(false)
-      } else if (
-        activatedRouteGuard !== null &&
-        activatedRouteGuard !== undefined
-      ) {
-        next(activatedRouteGuard)
-      } else {
-        next()
-      }
+  // Return the guard's result rather than calling `next()` — the callback
+  // form is deprecated in vue-router. `false` cancels, a route location
+  // redirects, and null/undefined proceeds to the next guard.
+  guards.forEach((fn) => {
+    router.beforeEach(async (to, from) => {
+      const result = await fn(to, from)
+      if (result === false) return false
+      if (result === null || result === undefined) return true
+      return result
     })
   })
 }

From f2ef3ea6f006e678d689fb285798d93712bff2aa Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Tue, 23 Jun 2026 12:02:57 -0600
Subject: [PATCH 05/93] feat: show the data-point toggle in the select view

---
 apps/qc-app/src/components/VisualizeData/Plot.vue | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index 91880302..ec7df040 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -1,8 +1,9 @@
 <template>
   <div class="plot-root d-flex flex-column">
-    <div v-if="!preview" class="plot-header">
+    <div class="plot-header">
       <div class="plot-toolbar d-flex align-center flex-wrap ga-1 px-3 py-1">
         <v-btn-toggle
+          v-if="!preview"
           v-model="tab"
           density="compact"
           color="primary"
@@ -30,7 +31,7 @@
         />
 
         <v-chip
-          v-if="selectedData?.length || hasSelectionShape"
+          v-if="(selectedData?.length || hasSelectionShape) && !preview"
           class="plot-toolbar__selection flex-grow-0 flex-shrink-0"
           size="small"
           color="red"
@@ -47,7 +48,7 @@
         <v-spacer />
 
         <div
-          v-if="tab === 'plot'"
+          v-if="preview || tab === 'plot'"
           class="plot-toolbar__points-combo d-inline-flex align-stretch rounded-lg"
           :class="{
             'plot-toolbar__points-combo--on': areTooltipsEnabled,
@@ -183,7 +184,7 @@
         </div>
 
         <v-btn
-          v-if="tab === 'plot'"
+          v-if="tab === 'plot' && !preview"
           size="small"
           variant="text"
           icon="mdi-share-variant-outline"
@@ -194,7 +195,7 @@
 
         <v-menu
           v-model="showHelp"
-          v-if="tab === 'plot'"
+          v-if="tab === 'plot' && !preview"
           :close-on-content-click="false"
           location="bottom end"
           offset="6"
@@ -265,6 +266,7 @@
         </v-menu>
 
         <v-speed-dial
+          v-if="!preview"
           v-model="rangeDialOpen"
           location="bottom center"
           transition="fade-transition"
@@ -296,7 +298,7 @@
       </div>
     </div>
 
-    <v-divider v-if="!preview"></v-divider>
+    <v-divider></v-divider>
 
     <div class="d-flex flex-row flex-grow-1">
       <v-tabs-window v-model="tab" class="flex-grow-1">

From 871b1c5c0844dfce6b60942296344ed1d8b36dcf Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Tue, 23 Jun 2026 12:03:33 -0600
Subject: [PATCH 06/93] feat: rework QC editing around a managed-datastream
 chooser

Start editing opens a chooser of the source's managed datastreams and their
sessions: continue an in-progress session, start a new one over the selected
time range, or create and delete managed datastreams. Managed datastreams are
hidden from the catalog and reached through the chooser. Also adds inline
processing-level creation, the session description captured at commit time,
reuse of the source's loaded series as the working copy, clamping the session
window to the source extent, and session resume that replays saved operations.
---
 apps/qc-app/docs/API_REFERENCE.md             |  21 +-
 apps/qc-app/src/App.vue                       |   7 +-
 .../EditData/CreateDatastreamForm.vue         | 137 ++++++++++-
 .../EditData/StartEditingDialog.vue           | 226 ++++++++++++++++++
 .../components/EditData/StartSessionForm.vue  |  88 -------
 .../__tests__/CreateDatastreamForm.spec.ts    |  51 +++-
 .../__tests__/StartEditingDialog.spec.ts      | 101 ++++++++
 .../__tests__/StartSessionForm.spec.ts        |  54 -----
 apps/qc-app/src/components/VisualizeData.vue  | 203 ++++++++++++----
 .../__tests__/useEditSession.spec.ts          |  42 ++++
 .../__tests__/useManagedDatastreams.spec.ts   |  93 +++++++
 .../__tests__/useProcessingLevels.spec.ts     |  57 +++++
 apps/qc-app/src/composables/useEditSession.ts |  64 ++++-
 .../src/composables/useManagedDatastreams.ts  |  73 ++++++
 .../src/composables/useProcessingLevels.ts    |  45 ++++
 .../__tests__/reconstructSession.spec.ts      |  16 +-
 .../qualityControl/reconstructSession.ts      |  44 ++--
 .../store/__tests__/dataVisualization.spec.ts |  30 +++
 apps/qc-app/src/store/dataVisualization.ts    |  89 +++++++
 19 files changed, 1226 insertions(+), 215 deletions(-)
 create mode 100644 apps/qc-app/src/components/EditData/StartEditingDialog.vue
 delete mode 100644 apps/qc-app/src/components/EditData/StartSessionForm.vue
 create mode 100644 apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
 delete mode 100644 apps/qc-app/src/components/EditData/__tests__/StartSessionForm.spec.ts
 create mode 100644 apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts
 create mode 100644 apps/qc-app/src/composables/__tests__/useProcessingLevels.spec.ts
 create mode 100644 apps/qc-app/src/composables/useManagedDatastreams.ts
 create mode 100644 apps/qc-app/src/composables/useProcessingLevels.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index f00930d0..edf0bbd9 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -169,6 +169,19 @@ const { managedDatastream, history } = await create({ source, processingLevelId,
 Delegates to the tested `createManagedDatastream` orchestration with the
 live client (`hs.datastreams` + `hs.qualityControlHistories`).
 
+### `useManagedDatastreams()`
+
+```ts
+const { loadForSource } = useManagedDatastreams()
+const options = await loadForSource(sourceDatastreamId)
+// options: [{ historyId, managed, sessions }]
+```
+
+Resolves a source datastream's managed (QC) datastreams from the loaded QC
+histories and fetches each one's sessions — feeds the "Start editing"
+chooser, which lists managed datastreams with their in-progress/committed
+sessions.
+
 ### `useWorkspacePermissions()`
 
 Synchronous, reactive role/permission checks for gating UI. The role
@@ -228,12 +241,17 @@ on boot.
 |-------------------------------------|----------|---------------------------------------------------|-------|
 | `things`                            | state    | `Thing[]`                                         | Sites in the active workspace; fetched once on workspace mount. |
 | `datastreams`                       | state    | `(Datastream & DatastreamExtended)[]`             | All visible datastreams (with `expand_related` nested objects). |
+| `qcHistories`                       | state    | `QualityControlHistory[]`                         | Workspace QC histories (each links a managed datastream to its source); loaded with the catalog. |
+| `managedDatastreamIds`              | computed | `Set<string>`                                     | Ids of every managed (QC) datastream; hidden from the catalog (reached via the Start-editing chooser). |
+| `historiesBySource`                 | computed | `Map<string, QualityControlHistory[]>`            | `sourceDatastreamId` -> its QC histories; drives the Start-editing chooser. |
+| `addQcHistory`                      | action   | `(history: QualityControlHistory) => void`        | Register a newly-created history so its managed datastream hides from the catalog and shows in the chooser without a reload. |
+| `removeManagedDatastream`           | action   | `(historyId: string, managedId: string) => void`  | Drop a deleted managed datastream + its history from local state (chooser/catalog) after deleting it server-side. |
 | `observedProperties`                | state    | `ObservedProperty[]`                              | Taxonomy for the filter chips. |
 | `processingLevels`                  | state    | `ProcessingLevel[]`                               | Taxonomy for the filter chips. |
 | `selectedThings`                    | state    | `Thing[]`                                         | Site filter selection (sidebar). |
 | `selectedObservedPropertyNames`     | state    | `string[]`                                        | Observed-property filter selection. |
 | `selectedProcessingLevelNames`      | state    | `string[]`                                        | Processing-level filter selection. |
-| `filteredDatastreams`               | computed | `(Datastream & DatastreamExtended)[]`             | `datastreams` narrowed by the three filter selections. |
+| `filteredDatastreams`               | computed | `(Datastream & DatastreamExtended)[]`             | `datastreams` narrowed by the three filter selections, with managed (QC) datastreams excluded. |
 | `plottedDatastreams`                | state    | `Datastream[]`                                    | Up to 5 streams currently on the chart. |
 | `qcDatastreamId`                    | state    | `string \| null`                                  | Storage form of the QC target; survives plotted-list mutations. |
 | `qcDatastream`                      | computed | `Datastream \| null`                              | Live lookup of `qcDatastreamId` in `plottedDatastreams`. |
@@ -259,6 +277,7 @@ on boot.
 | `clearPlottedDatastreams`           | action   | `() => Promise<void>`                             | Drop the entire plotted set. |
 | `setPlottedDatastreams`             | action   | `(items: Datastream[], qcId?: string \| null) => Promise<void>` | Wholesale replace; used by URL hydration. |
 | `setQcDatastream`                   | action   | `(id: string \| null) => Promise<void>`           | Change QC target; preserves the current zoom. |
+| `adoptManagedDatastream`            | action   | `(managed: Datastream, sourceId: string) => Promise<void>` | Enter editing on a freshly-created managed datastream: replace the source in the plot and re-key its already-loaded series as the managed datastream's working copy (no second, empty item; no re-fetch). |
 | `rebuildPlot`                       | action   | `() => Promise<void>`                             | Serialized rebuild (drop zoom history, refresh series, regenerate options, render). Coalesces concurrent callers. |
 
 ### `usePlotlyStore()` — `src/store/plotly.ts`
diff --git a/apps/qc-app/src/App.vue b/apps/qc-app/src/App.vue
index e54cfe1f..37df6228 100644
--- a/apps/qc-app/src/App.vue
+++ b/apps/qc-app/src/App.vue
@@ -25,7 +25,7 @@ import type { Datastream, DatastreamExtended } from '@hydroserver/client'
 // auth guard sees `hs.session.isAuthenticated` on first navigation.
 const isLoading = ref(false)
 
-const { things, processingLevels, observedProperties, datastreams } =
+const { things, processingLevels, observedProperties, datastreams, qcHistories } =
   storeToRefs(useDataVisStore())
 
 const { hs } = storeToRefs(useHydroServer())
@@ -37,6 +37,7 @@ async function loadWorkspaceCatalog(workspaceId: string) {
     datastreamsResponse,
     processingLevelsResponse,
     observedPropertiesResponse,
+    histories,
   ] = await Promise.all([
     hs.value.things.list({ workspace_id: workspaceId } as any),
     hs.value.datastreams.list({
@@ -45,6 +46,9 @@ async function loadWorkspaceCatalog(workspaceId: string) {
     } as any),
     hs.value.processingLevels.list({ workspace_id: workspaceId } as any),
     hs.value.observedProperties.list({ workspace_id: workspaceId } as any),
+    // QC histories aren't workspace-filterable server-side; entries for
+    // other workspaces simply never match this catalog's datastream ids.
+    hs.value.qualityControlHistories.listAllItems(),
   ])
 
   things.value = thingsResponse.ok ? thingsResponse.data : []
@@ -57,6 +61,7 @@ async function loadWorkspaceCatalog(workspaceId: string) {
   observedProperties.value = observedPropertiesResponse.ok
     ? observedPropertiesResponse.data
     : []
+  qcHistories.value = histories
 }
 
 // Clearing the selection wipes catalogs so stale data from the old
diff --git a/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue b/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
index bade4760..7cf1dd6d 100644
--- a/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
+++ b/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
@@ -30,6 +30,86 @@
       hide-details="auto"
       class="mb-2"
     />
+
+    <div
+      v-if="onCreateProcessingLevel && !permissionError && !showAddLevel"
+      class="d-flex align-center ga-2 mb-3"
+    >
+      <span
+        v-if="!processingLevels.length"
+        class="text-body-small text-medium-emphasis"
+      >
+        No processing levels in this workspace yet.
+      </span>
+      <v-spacer />
+      <v-btn
+        data-testid="add-level-toggle"
+        size="small"
+        variant="text"
+        color="primary"
+        prepend-icon="mdi-plus"
+        @click="showAddLevel = true"
+      >
+        Add processing level
+      </v-btn>
+    </div>
+
+    <v-expand-transition>
+      <div v-if="showAddLevel" class="qc-add-level rounded border pa-3 mb-3">
+        <div class="text-body-small font-weight-medium mb-2">
+          New processing level
+        </div>
+        <v-text-field
+          v-model="newLevel.code"
+          data-testid="new-level-code"
+          label="Code *"
+          density="compact"
+          hide-details="auto"
+          class="mb-2"
+        />
+        <v-text-field
+          v-model="newLevel.definition"
+          data-testid="new-level-definition"
+          label="Definition"
+          density="compact"
+          hide-details="auto"
+          class="mb-2"
+        />
+        <v-textarea
+          v-model="newLevel.explanation"
+          data-testid="new-level-explanation"
+          label="Explanation"
+          rows="2"
+          auto-grow
+          density="compact"
+          hide-details="auto"
+          class="mb-3"
+        />
+        <div class="d-flex justify-end ga-2">
+          <v-btn
+            data-testid="new-level-cancel"
+            size="small"
+            variant="text"
+            :disabled="addingLevel"
+            @click="cancelAddLevel"
+          >
+            Cancel
+          </v-btn>
+          <v-btn
+            data-testid="new-level-save"
+            size="small"
+            color="primary"
+            variant="flat"
+            :disabled="!newLevel.code.trim()"
+            :loading="addingLevel"
+            @click="onAddLevel"
+          >
+            Add
+          </v-btn>
+        </div>
+      </div>
+    </v-expand-transition>
+
     <v-text-field
       v-model="name"
       data-testid="create-name"
@@ -79,6 +159,16 @@ const props = defineProps<{
   /** When set, the user can't create datastreams here: shown as a warning
    *  and the confirm button is disabled. */
   permissionError?: string
+  /** Creates a processing level in the active workspace and resolves with the
+   *  new level (or null on failure). When provided, an inline "Add processing
+   *  level" affordance is shown so users don't have to leave for the
+   *  management app. The parent is expected to add the result to
+   *  `processingLevels`. */
+  onCreateProcessingLevel?: (input: {
+    code: string
+    definition?: string
+    explanation?: string
+  }) => Promise<{ id: string } | null>
 }>()
 
 const emit = defineEmits<{
@@ -86,7 +176,20 @@ const emit = defineEmits<{
   (e: 'confirm', spec: CreateDatastreamSpec): void
 }>()
 
-const processingLevelId = ref<string | null>(props.defaultProcessingLevelId ?? null)
+const knownLevelIds = computed(
+  () => new Set(props.processingLevels.map((p) => p.id))
+)
+
+// Only honour a remembered default if it exists in THIS workspace's
+// processing levels. A level id persisted against another workspace or
+// backend would otherwise be submitted and rejected ("Processing level
+// does not exist").
+const processingLevelId = ref<string | null>(
+  props.defaultProcessingLevelId &&
+    knownLevelIds.value.has(props.defaultProcessingLevelId)
+    ? props.defaultProcessingLevelId
+    : null
+)
 const name = ref(`${props.source.name} (QC)`)
 
 const processingLevelItems = computed(() =>
@@ -113,6 +216,7 @@ const processingLevelError = computed(() =>
 const isValid = computed(
   () =>
     !!processingLevelId.value &&
+    knownLevelIds.value.has(processingLevelId.value) &&
     !processingLevelError.value &&
     !props.permissionError
 )
@@ -125,4 +229,35 @@ function onConfirm(): void {
     name: name.value.trim() || undefined,
   })
 }
+
+// --- Inline "add processing level" -------------------------------------
+const showAddLevel = ref(false)
+const addingLevel = ref(false)
+const newLevel = ref({ code: '', definition: '', explanation: '' })
+
+function cancelAddLevel(): void {
+  showAddLevel.value = false
+  newLevel.value = { code: '', definition: '', explanation: '' }
+}
+
+async function onAddLevel(): Promise<void> {
+  const code = newLevel.value.code.trim()
+  if (!code || !props.onCreateProcessingLevel) return
+  addingLevel.value = true
+  try {
+    const created = await props.onCreateProcessingLevel({
+      code,
+      definition: newLevel.value.definition.trim() || undefined,
+      explanation: newLevel.value.explanation.trim() || undefined,
+    })
+    // Parent appends the new level to `processingLevels`, so selecting its id
+    // here lands on a now-valid option.
+    if (created) {
+      processingLevelId.value = created.id
+      cancelAddLevel()
+    }
+  } finally {
+    addingLevel.value = false
+  }
+}
 </script>
diff --git a/apps/qc-app/src/components/EditData/StartEditingDialog.vue b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
new file mode 100644
index 00000000..815d1f44
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
@@ -0,0 +1,226 @@
+<template>
+  <v-card rounded="lg">
+    <div class="px-4 pt-4 pb-2">
+      <div class="text-title-medium font-weight-bold">
+        Quality-control "{{ source.name }}"
+      </div>
+      <div class="text-body-small text-medium-emphasis mt-1">
+        Continue an in-progress session, start a new one over the current time
+        range, or set up a new managed datastream.
+      </div>
+    </div>
+
+    <v-divider />
+
+    <div class="qc-chooser__body pa-4">
+      <div v-if="loading" class="py-6 text-center">
+        <v-progress-circular indeterminate color="primary" size="28" />
+      </div>
+
+      <template v-else>
+        <div
+          v-if="!options.length"
+          class="text-body-small text-medium-emphasis mb-3"
+        >
+          No QC datastreams for this source yet. Create one to start editing.
+        </div>
+
+        <v-card
+          v-for="opt in options"
+          :key="opt.historyId"
+          variant="outlined"
+          class="mb-3"
+        >
+          <div class="px-3 py-2 d-flex align-center ga-2">
+            <v-icon icon="mdi-pencil-box-outline" color="primary" size="18" />
+            <div class="d-flex flex-column" style="min-width: 0">
+              <span class="text-body-medium font-weight-medium">
+                {{ opt.managed.name }}
+              </span>
+              <span class="text-body-small text-medium-emphasis">
+                {{ summary(opt) }}
+              </span>
+            </div>
+            <v-spacer />
+            <v-btn
+              size="small"
+              color="primary"
+              variant="flat"
+              :data-testid="`edit-managed-${opt.managed.id}`"
+              @click="emit('edit', opt)"
+            >
+              {{ hasInProgress(opt) ? 'Continue session' : 'Start new session' }}
+            </v-btn>
+            <v-btn
+              icon="mdi-trash-can-outline"
+              size="small"
+              variant="text"
+              color="error"
+              :data-testid="`delete-managed-${opt.managed.id}`"
+              title="Delete this managed datastream"
+              @click="confirmingDeleteId = opt.historyId"
+            />
+          </div>
+
+          <v-alert
+            v-if="confirmingDeleteId === opt.historyId"
+            type="warning"
+            variant="tonal"
+            density="compact"
+            class="mx-3 mb-2"
+          >
+            <div class="d-flex align-center flex-wrap ga-2">
+              <span class="text-body-small">
+                Delete "{{ opt.managed.name }}" and its
+                {{ opt.sessions.length }} session{{
+                  opt.sessions.length === 1 ? '' : 's'
+                }}? This can't be undone.
+              </span>
+              <v-spacer />
+              <v-btn
+                size="x-small"
+                variant="text"
+                @click="confirmingDeleteId = null"
+              >
+                Cancel
+              </v-btn>
+              <v-btn
+                size="x-small"
+                color="error"
+                variant="flat"
+                :data-testid="`confirm-delete-${opt.managed.id}`"
+                @click="onDelete(opt)"
+              >
+                Delete
+              </v-btn>
+            </div>
+          </v-alert>
+
+          <v-divider />
+
+          <div
+            v-if="!opt.sessions.length"
+            class="px-3 py-2 text-body-small text-medium-emphasis"
+          >
+            No sessions yet.
+          </div>
+          <v-list v-else density="compact" class="py-0" :lines="false">
+            <v-list-item
+              v-for="s in orderedSessions(opt.sessions)"
+              :key="s.id"
+              :data-testid="`chooser-session-${s.id}`"
+            >
+              <template #prepend>
+                <v-icon
+                  :icon="
+                    s.status === 'in_progress'
+                      ? 'mdi-pencil-circle'
+                      : 'mdi-check-circle'
+                  "
+                  :color="s.status === 'in_progress' ? 'warning' : 'success'"
+                  size="16"
+                  class="mr-2"
+                />
+              </template>
+              <v-list-item-title class="text-body-small">
+                {{ sessionLabel(s) }}
+              </v-list-item-title>
+              <template #append>
+                <v-chip
+                  size="x-small"
+                  :color="s.status === 'in_progress' ? 'warning' : 'grey'"
+                  variant="tonal"
+                  label
+                >
+                  {{ s.status === 'in_progress' ? 'In progress' : 'Committed' }}
+                </v-chip>
+              </template>
+            </v-list-item>
+          </v-list>
+        </v-card>
+
+        <v-btn
+          block
+          variant="tonal"
+          color="primary"
+          prepend-icon="mdi-plus"
+          data-testid="chooser-create-managed"
+          @click="emit('create')"
+        >
+          Create new managed datastream
+        </v-btn>
+      </template>
+    </div>
+
+    <v-divider />
+
+    <v-card-actions class="px-4 py-2">
+      <v-spacer />
+      <v-btn variant="text" data-testid="chooser-cancel" @click="emit('cancel')">
+        Cancel
+      </v-btn>
+    </v-card-actions>
+  </v-card>
+</template>
+
+<script setup lang="ts">
+import { ref } from 'vue'
+import type { Datastream, QualityControlSession } from '@hydroserver/client'
+import type { ManagedDatastreamOption } from '@/composables/useManagedDatastreams'
+
+defineProps<{
+  source: Datastream
+  options: ManagedDatastreamOption[]
+  loading?: boolean
+}>()
+
+const emit = defineEmits<{
+  (e: 'edit', option: ManagedDatastreamOption): void
+  (e: 'delete', option: ManagedDatastreamOption): void
+  (e: 'create'): void
+  (e: 'cancel'): void
+}>()
+
+// historyId of the managed datastream whose delete is awaiting confirmation.
+const confirmingDeleteId = ref<string | null>(null)
+
+function onDelete(opt: ManagedDatastreamOption) {
+  confirmingDeleteId.value = null
+  emit('delete', opt)
+}
+
+const NUMBER = new Intl.NumberFormat()
+
+// One-line recap so multiple managed datastreams are distinguishable:
+// processing level, observation count, and session summary.
+function summary(opt: ManagedDatastreamOption): string {
+  const parts: string[] = []
+  const level =
+    opt.managed.processingLevel?.definition ||
+    opt.managed.processingLevel?.code ||
+    null
+  if (level) parts.push(`Level: ${level}`)
+  parts.push(`${NUMBER.format(opt.managed.valueCount ?? 0)} obs`)
+  const inProgress = opt.sessions.filter(
+    (s) => s.status === 'in_progress'
+  ).length
+  const sessionText = `${opt.sessions.length} session${
+    opt.sessions.length === 1 ? '' : 's'
+  }${inProgress ? `, ${inProgress} in progress` : ''}`
+  parts.push(sessionText)
+  return parts.join(' · ')
+}
+
+const hasInProgress = (opt: ManagedDatastreamOption) =>
+  opt.sessions.some((s) => s.status === 'in_progress')
+
+// Newest first by phenomenon start (ISO sorts chronologically).
+const orderedSessions = (sessions: QualityControlSession[]) =>
+  [...sessions].sort((a, b) =>
+    b.phenomenonTimeStart.localeCompare(a.phenomenonTimeStart)
+  )
+
+const sessionLabel = (s: QualityControlSession) =>
+  s.description ||
+  `${s.phenomenonTimeStart.slice(0, 10)} to ${s.phenomenonTimeEnd.slice(0, 10)}`
+</script>
diff --git a/apps/qc-app/src/components/EditData/StartSessionForm.vue b/apps/qc-app/src/components/EditData/StartSessionForm.vue
deleted file mode 100644
index 5f4c2e1b..00000000
--- a/apps/qc-app/src/components/EditData/StartSessionForm.vue
+++ /dev/null
@@ -1,88 +0,0 @@
-<template>
-  <div class="qc-start-session-form pa-4">
-    <div class="text-body-1 font-weight-medium mb-3">Start new edit session</div>
-
-    <v-text-field
-      v-model="start"
-      data-testid="session-start"
-      label="Start"
-      type="datetime-local"
-      density="compact"
-      hide-details="auto"
-      class="mb-2"
-    />
-    <v-text-field
-      v-model="end"
-      data-testid="session-end"
-      label="End"
-      type="datetime-local"
-      density="compact"
-      :error-messages="endError"
-      hide-details="auto"
-      class="mb-2"
-    />
-    <v-textarea
-      v-model="description"
-      data-testid="session-description"
-      label="Description (optional)"
-      rows="2"
-      density="compact"
-      auto-grow
-      hide-details="auto"
-      class="mb-3"
-    />
-
-    <div class="d-flex justify-end ga-2">
-      <v-btn data-testid="session-cancel" variant="text" @click="emit('cancel')">
-        Cancel
-      </v-btn>
-      <v-btn
-        data-testid="session-confirm"
-        color="primary"
-        variant="flat"
-        :disabled="!isValid"
-        @click="onConfirm"
-      >
-        Start session
-      </v-btn>
-    </div>
-  </div>
-</template>
-
-<script setup lang="ts">
-import { computed, ref } from 'vue'
-import type { QualityControlSessionContract } from '@hydroserver/client'
-
-type QcSessionPostBody = QualityControlSessionContract.PostBody
-
-const emit = defineEmits<{
-  (e: 'cancel'): void
-  (e: 'confirm', spec: QcSessionPostBody): void
-}>()
-
-const start = ref('')
-const end = ref('')
-const description = ref('')
-
-const startMs = computed(() => new Date(start.value).getTime())
-const endMs = computed(() => new Date(end.value).getTime())
-
-const endError = computed(() =>
-  start.value && end.value && endMs.value <= startMs.value
-    ? 'End must be after start'
-    : ''
-)
-
-const isValid = computed(
-  () => !!start.value && !!end.value && endMs.value > startMs.value
-)
-
-function onConfirm(): void {
-  if (!isValid.value) return
-  emit('confirm', {
-    phenomenonTimeStart: new Date(start.value).toISOString(),
-    phenomenonTimeEnd: new Date(end.value).toISOString(),
-    description: description.value.trim() || undefined,
-  })
-}
-</script>
diff --git a/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts b/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
index 0a3fa445..b7a1c72f 100644
--- a/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
@@ -1,5 +1,5 @@
-import { mount } from '@vue/test-utils'
-import { describe, it, expect } from 'vitest'
+import { mount, flushPromises } from '@vue/test-utils'
+import { describe, it, expect, vi } from 'vitest'
 import { createTestVuetify } from '@/utils/test/vuetify'
 import CreateDatastreamForm from '@/components/EditData/CreateDatastreamForm.vue'
 ;(globalThis as any).ResizeObserver ||= class {
@@ -70,6 +70,53 @@ describe('CreateDatastreamForm', () => {
     expect(w.emitted('cancel')).toHaveLength(1)
   })
 
+  it('ignores a remembered default not in this workspace, and stays invalid until a real level is picked', async () => {
+    const w = mount(CreateDatastreamForm, {
+      props: {
+        source,
+        processingLevels,
+        // A level id persisted from another workspace/backend.
+        defaultProcessingLevelId: 'pl-from-elsewhere',
+      },
+      global: { plugins: [createTestVuetify()] },
+    })
+    // Stale default dropped -> confirm disabled (not submitted as-is).
+    expect(
+      w.find('[data-testid="create-confirm"]').attributes('disabled')
+    ).toBeDefined()
+    await levelSelect(w).vm.$emit('update:modelValue', 'pl-qc')
+    expect(
+      w.find('[data-testid="create-confirm"]').attributes('disabled')
+    ).toBeUndefined()
+  })
+
+  it('adds a processing level inline and selects the new one', async () => {
+    const created = { id: 'pl-new', code: 'Quality Controlled' }
+    const onCreateProcessingLevel = vi.fn().mockResolvedValue(created)
+    const w = mount(CreateDatastreamForm, {
+      props: { source, processingLevels, onCreateProcessingLevel },
+      global: { plugins: [createTestVuetify()] },
+    })
+
+    // Open the inline add panel and submit a new level.
+    await w.find('[data-testid="add-level-toggle"]').trigger('click')
+    await w.find('[data-testid="new-level-code"] input').setValue('Quality Controlled')
+    await w.find('[data-testid="new-level-save"]').trigger('click')
+    await flushPromises()
+
+    expect(onCreateProcessingLevel).toHaveBeenCalledWith(
+      expect.objectContaining({ code: 'Quality Controlled' })
+    )
+    // Parent appends the created level to the catalog.
+    await w.setProps({ processingLevels: [...processingLevels, created] })
+
+    // Panel collapsed, and the new level is selected -> confirm carries it.
+    expect(w.find('[data-testid="new-level-code"]').exists()).toBe(false)
+    await w.find('[data-testid="create-confirm"]').trigger('click')
+    const spec = w.emitted('confirm')![0][0] as { processingLevelId: string }
+    expect(spec.processingLevelId).toBe('pl-new')
+  })
+
   it('blocks create and shows a warning when permissionError is set', async () => {
     const w = mount(CreateDatastreamForm, {
       props: {
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
new file mode 100644
index 00000000..8bf2b4b7
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
@@ -0,0 +1,101 @@
+import { mount } from '@vue/test-utils'
+import { describe, it, expect } from 'vitest'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import StartEditingDialog from '@/components/EditData/StartEditingDialog.vue'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+const source = { id: 'src-1', name: 'Raw Temp' } as any
+
+const options = [
+  {
+    historyId: 'h-1',
+    managed: {
+      id: 'mgd-1',
+      name: 'Temp (QC)',
+      valueCount: 1234,
+      processingLevel: { definition: 'Quality Controlled' },
+    },
+    sessions: [
+      {
+        id: 's-1',
+        status: 'in_progress',
+        phenomenonTimeStart: '2025-01-05T00:00:00Z',
+        phenomenonTimeEnd: '2025-01-15T00:00:00Z',
+        description: '',
+      },
+      {
+        id: 's-0',
+        status: 'committed',
+        phenomenonTimeStart: '2024-12-01T00:00:00Z',
+        phenomenonTimeEnd: '2024-12-31T00:00:00Z',
+      },
+    ],
+  },
+  { historyId: 'h-2', managed: { id: 'mgd-2', name: 'Temp (QC v2)' }, sessions: [] },
+] as any
+
+const mountDialog = (props: Record<string, unknown> = {}) =>
+  mount(StartEditingDialog, {
+    props: { source, options, ...props },
+    global: { plugins: [createTestVuetify()] },
+  })
+
+describe('StartEditingDialog', () => {
+  it('lists managed datastreams and labels in-progress vs committed sessions', () => {
+    const w = mountDialog()
+    expect(w.text()).toContain('Raw Temp')
+    expect(w.text()).toContain('Temp (QC)')
+    expect(w.text()).toContain('In progress')
+    expect(w.text()).toContain('Committed')
+  })
+
+  it('labels the action by whether an in-progress session exists', () => {
+    const w = mountDialog()
+    expect(w.find('[data-testid="edit-managed-mgd-1"]').text()).toContain(
+      'Continue'
+    )
+    expect(w.find('[data-testid="edit-managed-mgd-2"]').text()).toContain(
+      'Start new'
+    )
+  })
+
+  it('shows a recap line with processing level, observations, and sessions', () => {
+    const w = mountDialog()
+    expect(w.text()).toContain('Quality Controlled')
+    expect(w.text()).toContain('1,234 obs')
+    expect(w.text()).toContain('2 sessions, 1 in progress')
+  })
+
+  it('asks for confirmation, then emits delete with the chosen option', async () => {
+    const w = mountDialog()
+    await w.find('[data-testid="delete-managed-mgd-1"]').trigger('click')
+    const confirm = w.find('[data-testid="confirm-delete-mgd-1"]')
+    expect(confirm.exists()).toBe(true)
+    await confirm.trigger('click')
+    expect(w.emitted('delete')![0][0]).toMatchObject({ historyId: 'h-1' })
+  })
+
+  it('emits edit with the chosen option', async () => {
+    const w = mountDialog()
+    await w.find('[data-testid="edit-managed-mgd-1"]').trigger('click')
+    expect(w.emitted('edit')![0][0]).toMatchObject({ historyId: 'h-1' })
+  })
+
+  it('emits create and cancel', async () => {
+    const w = mountDialog()
+    await w.find('[data-testid="chooser-create-managed"]').trigger('click')
+    expect(w.emitted('create')).toHaveLength(1)
+    await w.find('[data-testid="chooser-cancel"]').trigger('click')
+    expect(w.emitted('cancel')).toHaveLength(1)
+  })
+
+  it('shows an empty hint with only the create option when there are no managed datastreams', () => {
+    const w = mountDialog({ options: [] })
+    expect(w.text()).toContain('No QC datastreams')
+    expect(w.find('[data-testid="chooser-create-managed"]').exists()).toBe(true)
+  })
+})
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartSessionForm.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartSessionForm.spec.ts
deleted file mode 100644
index 0ad30459..00000000
--- a/apps/qc-app/src/components/EditData/__tests__/StartSessionForm.spec.ts
+++ /dev/null
@@ -1,54 +0,0 @@
-import { mount } from '@vue/test-utils'
-import { describe, it, expect } from 'vitest'
-import { createTestVuetify } from '@/utils/test/vuetify'
-import StartSessionForm from '@/components/EditData/StartSessionForm.vue'
-;(globalThis as any).ResizeObserver ||= class {
-  observe() {}
-  unobserve() {}
-  disconnect() {}
-}
-
-const mountForm = () =>
-  mount(StartSessionForm, { global: { plugins: [createTestVuetify()] } })
-
-describe('StartSessionForm', () => {
-  it('disables confirm until a valid range is entered', async () => {
-    const wrapper = mountForm()
-    const confirm = () => wrapper.find('[data-testid="session-confirm"]')
-    expect(confirm().attributes('disabled')).toBeDefined()
-
-    await wrapper.find('[data-testid="session-start"] input').setValue('2025-02-01T00:00')
-    await wrapper.find('[data-testid="session-end"] input').setValue('2025-01-01T00:00') // before start
-    expect(confirm().attributes('disabled')).toBeDefined()
-
-    await wrapper.find('[data-testid="session-end"] input').setValue('2025-03-01T00:00') // after start
-    expect(confirm().attributes('disabled')).toBeUndefined()
-  })
-
-  it('emits the session spec on confirm', async () => {
-    const wrapper = mountForm()
-    await wrapper.find('[data-testid="session-start"] input').setValue('2025-01-01T00:00')
-    await wrapper.find('[data-testid="session-end"] input').setValue('2025-02-01T00:00')
-    await wrapper.find('[data-testid="session-description"] textarea').setValue('Jan QC')
-    await wrapper.find('[data-testid="session-confirm"]').trigger('click')
-
-    const emitted = wrapper.emitted('confirm')
-    expect(emitted).toHaveLength(1)
-    const spec = emitted![0][0] as {
-      phenomenonTimeStart: string
-      phenomenonTimeEnd: string
-      description?: string
-    }
-    expect(typeof spec.phenomenonTimeStart).toBe('string')
-    expect(new Date(spec.phenomenonTimeEnd).getTime()).toBeGreaterThan(
-      new Date(spec.phenomenonTimeStart).getTime()
-    )
-    expect(spec.description).toBe('Jan QC')
-  })
-
-  it('emits cancel', async () => {
-    const wrapper = mountForm()
-    await wrapper.find('[data-testid="session-cancel"]').trigger('click')
-    expect(wrapper.emitted('cancel')).toHaveLength(1)
-  })
-})
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index f893b36d..ea2a7e91 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -72,7 +72,7 @@
                 append-icon="mdi-arrow-right"
                 :loading="isCreating"
                 :disabled="!canEditWorkspace"
-                @click="goToEdit"
+                @click="openEditChooser"
               >
                 Start editing
               </v-btn>
@@ -104,18 +104,32 @@
       <DataVisDatasetsTable class="fill-height" />
     </v-card>
 
+    <v-dialog v-model="showChooser" max-width="640">
+      <StartEditingDialog
+        v-if="chooserSource"
+        :source="chooserSource"
+        :options="chooserOptions"
+        :loading="chooserLoading"
+        @edit="editManaged"
+        @delete="onChooserDelete"
+        @create="onChooserCreate"
+        @cancel="showChooser = false"
+      />
+    </v-dialog>
+
     <v-dialog v-model="showCreateDatastream" max-width="560" persistent>
       <v-card v-if="qcDatastream" rounded="lg">
         <CreateDatastreamForm
           :source="qcDatastream"
           :processing-levels="processingLevels"
           :default-processing-level-id="qcPreferences.processingLevelId"
+          :on-create-processing-level="onCreateProcessingLevel"
           :permission-error="
             canCreateDatastreamHere
               ? ''
               : `Your role on this workspace (${workspaceRole}) can't create datastreams. Ask a workspace owner for an editor role.`
           "
-          @cancel="showCreateDatastream = false"
+          @cancel="onCreateCancel"
           @confirm="onCreateDatastream"
         />
       </v-card>
@@ -248,7 +262,7 @@
             prepend-icon="mdi-cloud-check-outline"
             :disabled="commitDisabled"
             :loading="isCommitting"
-            @click="showCommitConfirm = true"
+            @click="openCommit"
           >
             Commit
           </v-btn>
@@ -377,6 +391,17 @@
           <strong>replace</strong> the managed datastream over this session's
           range and the session is locked into the history.
         </v-card-text>
+        <div class="px-6 pb-2">
+          <v-textarea
+            v-model="commitDescription"
+            data-testid="commit-description"
+            label="Session description (optional)"
+            rows="2"
+            auto-grow
+            density="compact"
+            hide-details="auto"
+          />
+        </div>
         <v-divider />
         <v-card-actions class="d-flex align-center ga-2 px-4 py-3">
           <v-btn variant="text" @click="showCommitConfirm = false">Cancel</v-btn>
@@ -428,13 +453,7 @@
         </v-card-actions>
       </v-card>
     </v-dialog>
-
-    <v-dialog v-model="showStartSession" max-width="520" persistent>
-      <v-card rounded="lg">
-        <StartSessionForm @cancel="cancelStartSession" @confirm="onStartSession" />
-      </v-card>
-    </v-dialog>
-  </div>
+</div>
 </template>
 
 <script setup lang="ts">
@@ -454,14 +473,19 @@ import { usePlotlyStore } from '@/store/plotly'
 import { useEditSession } from '@/composables/useEditSession'
 import { useQcSessionStore } from '@/store/qcSession'
 import { useQcPreferencesStore } from '@/store/qcPreferences'
-import StartSessionForm from '@/components/EditData/StartSessionForm.vue'
 import CreateDatastreamForm from '@/components/EditData/CreateDatastreamForm.vue'
+import StartEditingDialog from '@/components/EditData/StartEditingDialog.vue'
 import {
   useCreateManagedDatastream,
   type CreateManagedDatastreamSpec,
 } from '@/composables/useCreateManagedDatastream'
+import {
+  useManagedDatastreams,
+  type ManagedDatastreamOption,
+} from '@/composables/useManagedDatastreams'
 import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
-import type { QualityControlSessionContract } from '@hydroserver/client'
+import { useProcessingLevels } from '@/composables/useProcessingLevels'
+import type { Datastream } from '@hydroserver/client'
 import { Snackbar } from '@uwrl/qc-utils'
 import {
   decodeShareState,
@@ -502,7 +526,12 @@ const {
   tooltipsMaxDataPoints,
 } = storeToRefs(usePlotlyStore())
 const { redraw } = usePlotlyStore()
-const { setPlottedDatastreams } = useDataVisStore()
+const {
+  setPlottedDatastreams,
+  adoptManagedDatastream,
+  addQcHistory,
+  removeManagedDatastream,
+} = useDataVisStore()
 
 const {
   beginEditing,
@@ -517,8 +546,8 @@ const { isReadOnly, inProgressSession } = storeToRefs(qcSessionStore)
 const { create: createManaged } = useCreateManagedDatastream()
 const qcPreferences = useQcPreferencesStore()
 const { canEdit, canCreateDatastream, roleName } = useWorkspacePermissions()
-
-type QcSessionPostBody = QualityControlSessionContract.PostBody
+const { createProcessingLevel } = useProcessingLevels()
+const { loadForSource, deleteManaged } = useManagedDatastreams()
 
 // Permission gating: QC editing writes to the selected workspace (creates
 // the managed datastream, pushes observations). Gate the editor entry
@@ -531,8 +560,12 @@ const workspaceRole = computed(() => roleName())
 const editCount = computed(() => editHistory.value?.length ?? 0)
 const showCommitConfirm = ref(false)
 const showCloseConfirm = ref(false)
-const showStartSession = ref(false)
 const showCreateDatastream = ref(false)
+const commitDescription = ref('')
+const showChooser = ref(false)
+const chooserLoading = ref(false)
+const chooserOptions = ref<ManagedDatastreamOption[]>([])
+const chooserSource = ref<Datastream | null>(null)
 const isSavingDraft = ref(false)
 const isCommitting = ref(false)
 const isCreating = ref(false)
@@ -640,10 +673,14 @@ function exitToSelect() {
   isDrawerOpen.value = true
 }
 
-async function onStartSession(spec: QcSessionPostBody) {
-  showStartSession.value = false
+// No in-progress session yet: open one over the window already chosen by the
+// time-range controls in the Select view, rather than prompting for it again.
+async function startSessionForWindow() {
   try {
-    await startSession(spec)
+    await startSession({
+      phenomenonTimeStart: beginDate.value.toISOString(),
+      phenomenonTimeEnd: endDate.value.toISOString(),
+    })
     await redraw()
     Snackbar.success('Edit session started.')
   } catch (e) {
@@ -651,9 +688,23 @@ async function onStartSession(spec: QcSessionPostBody) {
   }
 }
 
-function cancelStartSession() {
-  showStartSession.value = false
-  exitToSelect()
+async function onCreateProcessingLevel(input: {
+  code: string
+  definition?: string
+  explanation?: string
+}) {
+  try {
+    const level = await createProcessingLevel(input)
+    // Add to the catalog so it shows in the picker and is immediately valid.
+    processingLevels.value = [...processingLevels.value, level]
+    Snackbar.success('Processing level added.')
+    return level
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not add the processing level.'
+    )
+    return null
+  }
 }
 
 async function onCreateDatastream(spec: CreateManagedDatastreamSpec) {
@@ -661,13 +712,19 @@ async function onCreateDatastream(spec: CreateManagedDatastreamSpec) {
   qcPreferences.processingLevelId = spec.processingLevelId
   isCreating.value = true
   try {
-    const { managedDatastream } = await createManaged(spec)
-    await setPlottedDatastreams(
-      [...plottedDatastreams.value, managedDatastream],
-      managedDatastream.id
-    )
+    const { managedDatastream, history } = await createManaged(spec)
+    // Register the new history + datastream so it's hidden from the catalog
+    // and the chooser can resolve it (by its name, not id) without a reload.
+    addQcHistory(history)
+    datastreams.value = [
+      ...datastreams.value,
+      managedDatastream as (typeof datastreams.value)[number],
+    ]
+    // Reuse the source's already-loaded series as the managed datastream's
+    // working copy instead of adding a second, empty plotted item.
+    await adoptManagedDatastream(managedDatastream, spec.source.id)
     Snackbar.success('Managed datastream created.')
-    await goToEdit()
+    await enterEdit()
   } catch (e) {
     Snackbar.error(
       e instanceof Error ? e.message : 'Could not create the datastream.'
@@ -695,11 +752,18 @@ async function onSaveAndClose() {
   if (await onSaveDraft()) exitToSelect()
 }
 
+// Prefill the description with the session's current one so committing
+// preserves/edits it rather than blanking it.
+function openCommit() {
+  commitDescription.value = inProgressSession.value?.description ?? ''
+  showCommitConfirm.value = true
+}
+
 async function onCommit() {
   showCommitConfirm.value = false
   isCommitting.value = true
   try {
-    await commit()
+    await commit(commitDescription.value)
     await redraw()
     Snackbar.success('Session committed.')
   } catch (e) {
@@ -919,24 +983,81 @@ onUnmounted(() => {
   resetState()
 })
 
-async function goToEdit() {
+// "Start editing" opens a chooser of the source's managed datastreams and
+// their sessions, rather than editing the raw datastream directly.
+async function openEditChooser() {
+  const source = qcDatastream.value
+  if (!source) return
+  chooserSource.value = source
+  chooserOptions.value = []
+  chooserLoading.value = true
+  showChooser.value = true
   try {
-    await beginEditing()
-    if (needsHistory.value) {
-      // Not a managed datastream yet: create one from it (as source).
-      showCreateDatastream.value = true
-      return
-    }
-    currentView.value = DrawerType.Edit
-    selectedDrawer.value = DrawerType.Edit
-    isDrawerOpen.value = true
-    if (needsSession.value) showStartSession.value = true
+    chooserOptions.value = await loadForSource(source.id)
   } catch (e) {
     Snackbar.error(
-      e instanceof Error ? e.message : 'Could not start editing this datastream.'
+      e instanceof Error ? e.message : 'Could not load QC datastreams.'
     )
+  } finally {
+    chooserLoading.value = false
   }
 }
+
+// Edit a chosen managed datastream: reuse the source's loaded series as its
+// working copy, then resume its in-progress session (or start a new one).
+async function editManaged(option: ManagedDatastreamOption) {
+  showChooser.value = false
+  const source = chooserSource.value
+  if (!source) return
+  try {
+    await adoptManagedDatastream(option.managed, source.id)
+    await enterEdit()
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not open the datastream for editing.'
+    )
+  }
+}
+
+function onChooserCreate() {
+  showChooser.value = false
+  showCreateDatastream.value = true
+}
+
+// Cancelling create returns to the chooser it was opened from.
+function onCreateCancel() {
+  showCreateDatastream.value = false
+  if (chooserSource.value) showChooser.value = true
+}
+
+async function onChooserDelete(option: ManagedDatastreamOption) {
+  try {
+    await deleteManaged(option.historyId, option.managed.id)
+    removeManagedDatastream(option.historyId, option.managed.id)
+    chooserOptions.value = chooserOptions.value.filter(
+      (o) => o.historyId !== option.historyId
+    )
+    Snackbar.success('Managed datastream deleted.')
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not delete the managed datastream.'
+    )
+  }
+}
+
+// Enter the editor on the current QC-target managed datastream: resume its
+// in-progress session, or start a new one over the selected time range.
+async function enterEdit() {
+  await beginEditing()
+  if (needsHistory.value) {
+    Snackbar.error('This datastream is not set up for QC editing.')
+    return
+  }
+  currentView.value = DrawerType.Edit
+  selectedDrawer.value = DrawerType.Edit
+  isDrawerOpen.value = true
+  if (needsSession.value) await startSessionForWindow()
+}
 </script>
 
 <style scoped>
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index d5cfdd08..2d2aac5f 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -111,12 +111,18 @@ describe('useEditSession', () => {
       })
     )
     await qc.sessions.create(h.id, WIN)
+    // The reconstructed working copy that resume should wire into the series.
+    const reconstructed = makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }])
+    fetchObservationsInRange.mockResolvedValue(reconstructed)
     const { useEditSession } = await import('@/composables/useEditSession')
     const qcUtils = await import('@uwrl/qc-utils')
     const { beginEditing, needsSession } = useEditSession()
     await beginEditing()
     expect(needsSession.value).toBe(false)
     expect(qcUtils.applyHistory).toHaveBeenCalled()
+    // The QC-target series now shows the reconstructed session, not the
+    // empty managed datastream.
+    expect(selectedSeries.value.data).toEqual(reconstructed)
   })
 
   it('startSession creates a session and copies the source window', async () => {
@@ -130,6 +136,27 @@ describe('useEditSession', () => {
     expect(fetchObservationsInRange.mock.calls.at(-1)?.[0].id).toBe('s-1')
   })
 
+  it('startSession clamps the window to the source datastream extent', async () => {
+    await seedHistory()
+    getItem.mockResolvedValue({
+      id: 's-1',
+      name: 'Source',
+      phenomenonBeginTime: '2025-01-01T00:00:00Z',
+      phenomenonEndTime: '2025-01-15T00:00:00Z',
+    })
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    // Display window ends "now" (past the source's last observation).
+    await session.startSession({
+      phenomenonTimeStart: '2025-01-05T00:00:00Z',
+      phenomenonTimeEnd: '2025-06-01T00:00:00Z',
+    })
+    const inProgress = useQcSessionStore().inProgressSession
+    expect(inProgress?.phenomenonTimeEnd).toBe('2025-01-15T00:00:00.000Z')
+    expect(inProgress?.phenomenonTimeStart).toBe('2025-01-05T00:00:00.000Z')
+  })
+
   it('saveDraft persists the record operations to the session', async () => {
     await seedHistory()
     selectedSeries.value = {
@@ -172,4 +199,19 @@ describe('useEditSession', () => {
     expect(store.committedSessions.length).toBe(1)
     expect(store.inProgressSession).toBeNull()
   })
+
+  it('commit saves the description provided at commit time', async () => {
+    await seedHistory()
+    selectedSeries.value = {
+      data: makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }]),
+    }
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    await session.commit('Reviewed January spike')
+
+    const store = useQcSessionStore()
+    expect(store.committedSessions[0]?.description).toBe('Reviewed January spike')
+  })
 })
diff --git a/apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts b/apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts
new file mode 100644
index 00000000..65b93958
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts
@@ -0,0 +1,93 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+
+const datastreams = [
+  { id: 'mgd-1', name: 'Temp (QC)' },
+  { id: 'mgd-2', name: 'Temp (QC v2)' },
+]
+const historiesBySource = new Map<string, any[]>()
+const listAllItems = vi.fn()
+const deleteHistory = vi.fn()
+const deleteDatastream = vi.fn()
+
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () => ({ datastreams, historiesBySource }),
+}))
+vi.mock('@/store/hydroserver', () => ({
+  useHydroServer: () => ({
+    hs: {
+      qualityControlSessions: { listAllItems },
+      qualityControlHistories: { delete: deleteHistory },
+      datastreams: { delete: deleteDatastream },
+    },
+  }),
+}))
+
+import { useManagedDatastreams } from '../useManagedDatastreams'
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  historiesBySource.clear()
+})
+
+describe('useManagedDatastreams.loadForSource', () => {
+  it('resolves managed datastreams and their sessions for a source', async () => {
+    historiesBySource.set('src-1', [
+      { id: 'h-1', managedDatastreamId: 'mgd-1', sourceDatastreamId: 'src-1' },
+      { id: 'h-2', managedDatastreamId: 'mgd-2', sourceDatastreamId: 'src-1' },
+    ])
+    listAllItems.mockImplementation(async (historyId: string) =>
+      historyId === 'h-1'
+        ? [{ id: 's-1', status: 'in_progress' }]
+        : [{ id: 's-2', status: 'committed' }]
+    )
+
+    const { loadForSource } = useManagedDatastreams()
+    const options = await loadForSource('src-1')
+
+    expect(options).toHaveLength(2)
+    expect(options[0]).toMatchObject({
+      historyId: 'h-1',
+      managed: { id: 'mgd-1', name: 'Temp (QC)' },
+    })
+    expect(options[0].sessions).toEqual([{ id: 's-1', status: 'in_progress' }])
+    expect(options[1].managed.id).toBe('mgd-2')
+    expect(options[1].sessions[0].status).toBe('committed')
+  })
+
+  it('returns an empty list when the source has no managed datastreams', async () => {
+    const { loadForSource } = useManagedDatastreams()
+    expect(await loadForSource('unknown')).toEqual([])
+    expect(listAllItems).not.toHaveBeenCalled()
+  })
+
+  it('falls back to a stub when the managed datastream is not in the catalog', async () => {
+    historiesBySource.set('src-2', [
+      {
+        id: 'h-9',
+        managedDatastreamId: 'mgd-missing',
+        managedDatastream: { name: 'Orphan (QC)' },
+        sourceDatastreamId: 'src-2',
+      },
+    ])
+    listAllItems.mockResolvedValue([])
+    const { loadForSource } = useManagedDatastreams()
+    const options = await loadForSource('src-2')
+    expect(options[0].managed).toEqual({ id: 'mgd-missing', name: 'Orphan (QC)' })
+  })
+
+  it('deleteManaged removes the history then the datastream', async () => {
+    deleteHistory.mockResolvedValue({ ok: true })
+    deleteDatastream.mockResolvedValue({ ok: true })
+    const { deleteManaged } = useManagedDatastreams()
+    await deleteManaged('h-1', 'mgd-1')
+    expect(deleteHistory).toHaveBeenCalledWith('h-1')
+    expect(deleteDatastream).toHaveBeenCalledWith('mgd-1')
+  })
+
+  it('deleteManaged throws and skips the datastream when the history delete fails', async () => {
+    deleteHistory.mockResolvedValue({ ok: false, message: 'history busy' })
+    const { deleteManaged } = useManagedDatastreams()
+    await expect(deleteManaged('h-1', 'mgd-1')).rejects.toThrow('history busy')
+    expect(deleteDatastream).not.toHaveBeenCalled()
+  })
+})
diff --git a/apps/qc-app/src/composables/__tests__/useProcessingLevels.spec.ts b/apps/qc-app/src/composables/__tests__/useProcessingLevels.spec.ts
new file mode 100644
index 00000000..4738571b
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useProcessingLevels.spec.ts
@@ -0,0 +1,57 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { setActivePinia, createPinia } from 'pinia'
+import { useProcessingLevels } from '../useProcessingLevels'
+import { useHydroServer } from '@/store/hydroserver'
+import { useWorkspaceStore } from '@/store/workspaces'
+
+function setHs(create: any) {
+  useHydroServer().hs = { processingLevels: { create } } as any
+}
+
+describe('useProcessingLevels.createProcessingLevel', () => {
+  beforeEach(() => setActivePinia(createPinia()))
+
+  it('creates a workspace-scoped processing level and returns it', async () => {
+    const created = {
+      id: 'pl-new',
+      workspaceId: 'ws-1',
+      code: 'QC',
+      definition: 'Quality Controlled',
+      explanation: '',
+    }
+    const create = vi.fn().mockResolvedValue({ ok: true, data: created })
+    setHs(create)
+    useWorkspaceStore().selectedWorkspace = { id: 'ws-1' } as any
+
+    const { createProcessingLevel } = useProcessingLevels()
+    const res = await createProcessingLevel({
+      code: 'QC',
+      definition: 'Quality Controlled',
+    })
+
+    expect(create).toHaveBeenCalledTimes(1)
+    expect(create.mock.calls[0][0]).toMatchObject({
+      workspaceId: 'ws-1',
+      code: 'QC',
+      definition: 'Quality Controlled',
+    })
+    expect(res.id).toBe('pl-new')
+  })
+
+  it('throws when no workspace is selected', async () => {
+    setHs(vi.fn())
+    const { createProcessingLevel } = useProcessingLevels()
+    await expect(createProcessingLevel({ code: 'QC' })).rejects.toThrow(
+      /workspace/i
+    )
+  })
+
+  it('throws the server message on failure', async () => {
+    setHs(vi.fn().mockResolvedValue({ ok: false, message: 'Code already exists' }))
+    useWorkspaceStore().selectedWorkspace = { id: 'ws-1' } as any
+    const { createProcessingLevel } = useProcessingLevels()
+    await expect(createProcessingLevel({ code: 'QC' })).rejects.toThrow(
+      'Code already exists'
+    )
+  })
+})
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index 90bcedb5..a535b31a 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -35,6 +35,38 @@ import {
 
 type QcSessionPostBody = QualityControlSessionContract.PostBody
 
+/**
+ * Keep a new session's window within the source datastream's observed extent.
+ * The window comes from the display time-range controls, whose end is usually
+ * "now" — past the source's last observation — which the backend rejects
+ * (`phenomenon_time_end cannot extend past the source datastream's current end
+ * time`). Resume ignores the spec, so this only shapes freshly-created sessions.
+ */
+function clampSpecToSource(
+  spec: QcSessionPostBody,
+  source: Pick<Datastream, 'phenomenonBeginTime' | 'phenomenonEndTime'>
+): QcSessionPostBody {
+  let start = new Date(spec.phenomenonTimeStart).getTime()
+  let end = new Date(spec.phenomenonTimeEnd).getTime()
+  if (source.phenomenonBeginTime) {
+    start = Math.max(start, new Date(source.phenomenonBeginTime).getTime())
+  }
+  if (source.phenomenonEndTime) {
+    end = Math.min(end, new Date(source.phenomenonEndTime).getTime())
+  }
+  if (!(start < end)) {
+    throw new Error(
+      'The selected time range has no source observations to edit. ' +
+        'Pick a range that overlaps the datastream (try the "All" preset).'
+    )
+  }
+  return {
+    ...spec,
+    phenomenonTimeStart: new Date(start).toISOString(),
+    phenomenonTimeEnd: new Date(end).toISOString(),
+  }
+}
+
 export function useEditSession() {
   const { qcDatastream } = storeToRefs(useDataVisStore())
   const { selectedSeries } = storeToRefs(usePlotlyStore())
@@ -70,10 +102,14 @@ export function useEditSession() {
     const inProgress = sessionStore.inProgressSession
     const record = selectedSeries.value?.data
     if (inProgress && sourceDatastream.value && record) {
-      // Resume: reconstruct the in-progress session's working state.
-      await reconstructSession(
+      // Resume: reconstruct the in-progress session's working state (source
+      // window + replayed draft operations) and wire it into the QC-target
+      // series so the editor shows the saved edits instead of the empty,
+      // uncommitted managed datastream.
+      const { record: reconstructed } = await reconstructSession(
         {
           qcSessions: hs.value.qualityControlSessions,
+          qcOperations: hs.value.qualityControlOperations,
           fetchInRange: fetchObservationsInRange,
           applyHistory,
         },
@@ -81,6 +117,7 @@ export function useEditSession() {
         history.id,
         inProgress.id
       )
+      if (selectedSeries.value) selectedSeries.value.data = reconstructed
       needsSession.value = false
     } else {
       needsSession.value = true
@@ -89,16 +126,17 @@ export function useEditSession() {
 
   async function startSession(spec: QcSessionPostBody): Promise<void> {
     const historyId = sessionStore.historyId
-    if (!historyId || !sourceDatastream.value) {
+    const source = sourceDatastream.value
+    if (!historyId || !source) {
       throw new Error('Load a managed datastream for editing first.')
     }
     const session = await startOrResumeSession(
       hs.value.qualityControlSessions,
       historyId,
-      spec
+      clampSpecToSource(spec, source)
     )
     await sessionStore.loadSessions(historyId)
-    await loadSourceWindow(fetchObservationsInRange, sourceDatastream.value, session)
+    await loadSourceWindow(fetchObservationsInRange, source, session)
     needsSession.value = false
   }
 
@@ -121,7 +159,7 @@ export function useEditSession() {
     )
   }
 
-  async function commit(): Promise<void> {
+  async function commit(description?: string): Promise<void> {
     const managed = qcDatastream.value
     const historyId = sessionStore.historyId
     const session = sessionStore.inProgressSession
@@ -131,6 +169,20 @@ export function useEditSession() {
     }
     await saveDraft()
 
+    // The window was chosen earlier by the time-range controls; the
+    // description is captured here at commit time. Persist it if it changed.
+    const trimmed = description?.trim()
+    if (trimmed !== undefined && trimmed !== (session.description ?? '')) {
+      const res = await hs.value.qualityControlSessions.update(
+        historyId,
+        session.id,
+        { description: trimmed || null }
+      )
+      if (!res.ok) {
+        throw new Error(res.message || 'Could not save the session description.')
+      }
+    }
+
     const body = observationsBulkBody(record as ObservationRecord)
     await commitQcSession({
       qcSessions: hs.value.qualityControlSessions,
diff --git a/apps/qc-app/src/composables/useManagedDatastreams.ts b/apps/qc-app/src/composables/useManagedDatastreams.ts
new file mode 100644
index 00000000..8104a5bd
--- /dev/null
+++ b/apps/qc-app/src/composables/useManagedDatastreams.ts
@@ -0,0 +1,73 @@
+/**
+ * Resolve the managed (QC) datastreams for a source datastream and their
+ * sessions, for the "Start editing" chooser. The source -> managed mapping
+ * comes from the QC histories loaded into the data-vis store; sessions are
+ * fetched per history on demand.
+ */
+
+import type {
+  Datastream,
+  DatastreamExtended,
+  QualityControlSession,
+} from '@hydroserver/client'
+import { useHydroServer } from '@/store/hydroserver'
+import { useDataVisStore } from '@/store/dataVisualization'
+
+export interface ManagedDatastreamOption {
+  historyId: string
+  // Resolved from the expand_related catalog, so the nested objects
+  // (processingLevel, ...) are available for display.
+  managed: Datastream & Partial<DatastreamExtended>
+  sessions: QualityControlSession[]
+}
+
+export function useManagedDatastreams() {
+  async function loadForSource(
+    sourceId: string
+  ): Promise<ManagedDatastreamOption[]> {
+    const { hs } = useHydroServer()
+    const { datastreams, historiesBySource } = useDataVisStore()
+    const histories = historiesBySource.get(sourceId) ?? []
+    return Promise.all(
+      histories.map(async (h: any) => {
+        const historyId = h.id as string
+        const managedId = h.managedDatastreamId ?? h.managedDatastream?.id
+        // Managed datastreams are in the full catalog (just hidden from the
+        // table); fall back to a name-only stub if it isn't loaded.
+        const managed =
+          datastreams.find((d) => d.id === managedId) ??
+          ({
+            id: managedId,
+            name: h.managedDatastream?.name ?? managedId,
+          } as Datastream)
+        const sessions =
+          await hs.qualityControlSessions.listAllItems(historyId)
+        return { historyId, managed, sessions }
+      })
+    )
+  }
+
+  /**
+   * Delete a managed datastream: remove its QC history (cascading its
+   * sessions/operations) first, then the now-unmanaged datastream so it
+   * doesn't reappear in the catalog as a raw row. Throws on failure.
+   */
+  async function deleteManaged(
+    historyId: string,
+    managedId: string
+  ): Promise<void> {
+    const { hs } = useHydroServer()
+    const historyRes = await hs.qualityControlHistories.delete(historyId)
+    if (!historyRes.ok) {
+      throw new Error(historyRes.message || 'Could not delete the QC history.')
+    }
+    const datastreamRes = await hs.datastreams.delete(managedId)
+    if (!datastreamRes.ok) {
+      throw new Error(
+        datastreamRes.message || 'Could not delete the managed datastream.'
+      )
+    }
+  }
+
+  return { loadForSource, deleteManaged }
+}
diff --git a/apps/qc-app/src/composables/useProcessingLevels.ts b/apps/qc-app/src/composables/useProcessingLevels.ts
new file mode 100644
index 00000000..8072440c
--- /dev/null
+++ b/apps/qc-app/src/composables/useProcessingLevels.ts
@@ -0,0 +1,45 @@
+/**
+ * Create processing levels from inside the QC app so operators don't have to
+ * leave for the Data Management app just to add one. Creation is workspace
+ * scoped to the active workspace; the caller is responsible for adding the
+ * returned level to the catalog it renders.
+ */
+
+import { storeToRefs } from 'pinia'
+import { ProcessingLevel } from '@hydroserver/client'
+import { useHydroServer } from '@/store/hydroserver'
+import { useWorkspaceStore } from '@/store/workspaces'
+
+export interface NewProcessingLevelInput {
+  code: string
+  definition?: string
+  explanation?: string
+}
+
+export function useProcessingLevels() {
+  const { hs } = storeToRefs(useHydroServer())
+  const { selectedWorkspaceId } = storeToRefs(useWorkspaceStore())
+
+  /** Create a processing level in the active workspace. Throws on failure. */
+  async function createProcessingLevel(
+    input: NewProcessingLevelInput
+  ): Promise<ProcessingLevel> {
+    const workspaceId = selectedWorkspaceId.value
+    if (!workspaceId) {
+      throw new Error('Select a workspace before adding a processing level.')
+    }
+    const body = Object.assign(new ProcessingLevel(), {
+      workspaceId,
+      code: input.code,
+      definition: input.definition ?? '',
+      explanation: input.explanation ?? '',
+    })
+    const res = await hs.value.processingLevels.create(body)
+    if (!res.ok || !res.data) {
+      throw new Error(res.message || 'Could not create the processing level.')
+    }
+    return res.data
+  }
+
+  return { createProcessingLevel }
+}
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
index cc33aabf..29561651 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
@@ -53,7 +53,12 @@ describe('collectSessionOperations', () => {
       win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'),
       ['SELECTION', 'DELETE_POINTS']
     )
-    const ops = await collectSessionOperations(qc.sessions, historyId, s.id)
+    const ops = await collectSessionOperations(
+      qc.sessions,
+      qc.operations,
+      historyId,
+      s.id
+    )
     expect(ops.map((o) => o.method)).toEqual(['SELECTION', 'DELETE_POINTS'])
   })
 
@@ -74,7 +79,12 @@ describe('collectSessionOperations', () => {
       ['CHANGE_VALUES']
     )
 
-    const ops = await collectSessionOperations(qc.sessions, historyId, b.id)
+    const ops = await collectSessionOperations(
+      qc.sessions,
+      qc.operations,
+      historyId,
+      b.id
+    )
     expect(ops.map((o) => o.method)).toEqual(['VALUE_THRESHOLD', 'CHANGE_VALUES'])
   })
 })
@@ -99,7 +109,7 @@ describe('reconstructSession', () => {
     const source = { id: 's-1' } as unknown as Datastream
 
     const result = await reconstructSession(
-      { qcSessions: qc.sessions, fetchInRange, applyHistory },
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
       source,
       historyId,
       s.id
diff --git a/apps/qc-app/src/services/qualityControl/reconstructSession.ts b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
index b8d0e739..b2573632 100644
--- a/apps/qc-app/src/services/qualityControl/reconstructSession.ts
+++ b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
@@ -1,12 +1,15 @@
 /**
- * Reconstruct the datastream state "as of" a committed session for
- * read-only viewing (spec section 4 / 7.3).
+ * Reconstruct the datastream state "as of" a session for resume/read-only
+ * viewing (spec section 4 / 7.3).
  *
  * Fetch the source window, then replay the session's operations preceded
  * by its ancestors' operations (chronological order) via qc-utils
  * `applyHistory`. `fetchInRange` and `applyHistory` are injected so this
  * unit-tests without a real ObservationRecord.
  *
+ * Operations are read from the operations endpoint, not off the session: a
+ * session GET can return the summary shape (no embedded `operations`).
+ *
  * Caveat (deferred): operations are index-coupled to the exact dataset
  * they were authored against, so reconstructing across sessions authored
  * over different windows can mis-target. This loads the target session's
@@ -16,8 +19,8 @@
 import type {
   Datastream,
   QualityControlSessionService,
+  QualityControlOperationService,
   QualityControlOperation,
-  QualityControlSessionContract,
 } from '@hydroserver/client'
 import type {
   ApplyHistoryReport,
@@ -28,8 +31,6 @@ import type {
 import { unwrap } from './unwrap'
 import type { FetchObservationsInRange } from './session'
 
-type QcSessionDetail = QualityControlSessionContract.DetailResponse
-
 /** API operation -> qc-utils replayable operation (rename operationType/arguments). */
 const toSerialized = (op: QualityControlOperation): QcHistoryOperation =>
   ({
@@ -43,12 +44,10 @@ const toSerialized = (op: QualityControlOperation): QcHistoryOperation =>
  */
 export async function collectSessionOperations(
   qcSessions: QualityControlSessionService,
+  qcOperations: QualityControlOperationService,
   historyId: string,
   sessionId: string
 ): Promise<QcHistoryOperation[]> {
-  const session = unwrap(
-    await qcSessions.get(historyId, sessionId)
-  ) as QcSessionDetail
   const ancestors = unwrap(
     await qcSessions.list(historyId, {
       ancestor_of: sessionId,
@@ -63,17 +62,21 @@ export async function collectSessionOperations(
 
   const operations: QcHistoryOperation[] = []
   for (const ancestor of ordered) {
-    const detail = unwrap(
-      await qcSessions.get(historyId, ancestor.id)
-    ) as QcSessionDetail
-    operations.push(...detail.operations.map(toSerialized))
+    const ops = unwrap(
+      await qcOperations.list(historyId, ancestor.id, { fetch_all: true })
+    )
+    operations.push(...ops.map(toSerialized))
   }
-  operations.push(...session.operations.map(toSerialized))
+  const sessionOps = unwrap(
+    await qcOperations.list(historyId, sessionId, { fetch_all: true })
+  )
+  operations.push(...sessionOps.map(toSerialized))
   return operations
 }
 
 export interface ReconstructSessionDeps {
   qcSessions: QualityControlSessionService
+  qcOperations: QualityControlOperationService
   fetchInRange: FetchObservationsInRange
   applyHistory: (
     record: ObservationRecord,
@@ -92,12 +95,17 @@ export async function reconstructSession(
   historyId: string,
   sessionId: string
 ): Promise<ReconstructSessionResult> {
-  const { qcSessions, fetchInRange, applyHistory } = deps
+  const { qcSessions, qcOperations, fetchInRange, applyHistory } = deps
 
-  const session = unwrap(
-    await qcSessions.get(historyId, sessionId)
-  ) as QcSessionDetail
-  const operations = await collectSessionOperations(qcSessions, historyId, sessionId)
+  // The session GET supplies the window; operations come from the
+  // operations endpoint (the GET may omit them in the summary shape).
+  const session = unwrap(await qcSessions.get(historyId, sessionId))
+  const operations = await collectSessionOperations(
+    qcSessions,
+    qcOperations,
+    historyId,
+    sessionId
+  )
 
   const record = await fetchInRange(
     source,
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index 20d07ac4..cf6dd065 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -605,3 +605,33 @@ describe('useDataVisStore.clearPlottedDatastreams + toggleDatastream', () => {
     expect(store.plottedDatastreams).toEqual([])
   })
 })
+
+describe('useDataVisStore.adoptManagedDatastream', () => {
+  it('reuses the source series as the managed working copy (one item, data kept)', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    const sourceData = { tag: 'loaded-record' }
+    store.plottedDatastreams = [makeDs({ id: 'src', name: 'Raw' })] as any
+    store.qcDatastreamId = 'src'
+    mockGraphSeriesArray.value = [
+      { id: 'src', name: 'Raw', data: sourceData, color: '#1', yAxisLabel: 'T' },
+    ]
+
+    await store.adoptManagedDatastream(
+      makeDs({ id: 'mgd', name: 'Raw (QC)' }) as any,
+      'src'
+    )
+
+    // Single plotted item, now the managed datastream as the QC target.
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['mgd'])
+    expect(store.qcDatastreamId).toBe('mgd')
+    // The loaded series was re-keyed in place, keeping its data...
+    expect(mockGraphSeriesArray.value).toHaveLength(1)
+    expect(mockGraphSeriesArray.value[0].id).toBe('mgd')
+    expect(mockGraphSeriesArray.value[0].name).toBe('Raw (QC)')
+    expect(mockGraphSeriesArray.value[0].data).toEqual(sourceData)
+    // ...and the working copy was reused, not re-fetched (managed is empty).
+    expect(mockFetchGraphSeries).not.toHaveBeenCalled()
+    expect(mockFetchObservationsInRange).not.toHaveBeenCalled()
+  })
+})
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index 7505aea8..4c0f94dd 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -10,6 +10,7 @@ import {
   type DatastreamExtended,
   ObservedProperty,
   ProcessingLevel,
+  type QualityControlHistory,
   Thing,
 } from '@hydroserver/client'
 
@@ -28,6 +29,54 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   // To only fetch these once per page
   const things = ref<Thing[]>([])
   const datastreams = ref<(Datastream & DatastreamExtended)[]>([])
+  // QC histories for the workspace (each links a managed datastream to its
+  // source). Drives hiding managed datastreams from the catalog and the
+  // source -> managed lookup used by the "Start editing" chooser.
+  const qcHistories = ref<QualityControlHistory[]>([])
+
+  const historyManagedId = (h: QualityControlHistory): string | undefined =>
+    (h as any).managedDatastreamId ?? (h as any).managedDatastream?.id
+  const historySourceId = (h: QualityControlHistory): string | undefined =>
+    (h as any).sourceDatastreamId ?? (h as any).sourceDatastream?.id
+
+  /** Ids of every managed datastream; these are hidden from the catalog. */
+  const managedDatastreamIds = computed(() => {
+    const ids = new Set<string>()
+    for (const h of qcHistories.value) {
+      const id = historyManagedId(h)
+      if (id) ids.add(id)
+    }
+    return ids
+  })
+
+  /** Add a newly-created QC history so the new managed datastream is hidden
+   *  from the catalog and listed in the chooser without a full reload. */
+  function addQcHistory(history: QualityControlHistory) {
+    if (qcHistories.value.some((h) => (h as any).id === (history as any).id)) {
+      return
+    }
+    qcHistories.value = [...qcHistories.value, history]
+  }
+
+  /** Drop a deleted managed datastream + its history from local state so it
+   *  vanishes from the chooser and doesn't resurface in the catalog. */
+  function removeManagedDatastream(historyId: string, managedId: string) {
+    qcHistories.value = qcHistories.value.filter(
+      (h) => (h as any).id !== historyId
+    )
+    datastreams.value = datastreams.value.filter((d) => d.id !== managedId)
+  }
+
+  /** sourceDatastreamId -> the histories linking its managed datastreams. */
+  const historiesBySource = computed(() => {
+    const map = new Map<string, QualityControlHistory[]>()
+    for (const h of qcHistories.value) {
+      const sourceId = historySourceId(h)
+      if (!sourceId) continue
+      ;(map.get(sourceId) ?? map.set(sourceId, []).get(sourceId)!).push(h)
+    }
+    return map
+  })
   const observedProperties = ref<ObservedProperty[]>([])
   const processingLevels = ref<ProcessingLevel[]>([])
 
@@ -174,6 +223,37 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     }
   }
 
+  /**
+   * Enter editing on a freshly-created managed datastream by reusing the
+   * source's already-loaded series as the managed datastream's working copy,
+   * rather than adding a second, empty plotted item. The managed datastream
+   * is empty on the server, so re-fetching it would blank the plot; the QC
+   * editor zooms (never re-fetches), so this working copy survives until the
+   * first commit materializes the managed datastream.
+   */
+  async function adoptManagedDatastream(managed: Datastream, sourceId: string) {
+    const idx = plottedDatastreams.value.findIndex((d) => d.id === sourceId)
+    if (idx >= 0) plottedDatastreams.value.splice(idx, 1, managed)
+    else plottedDatastreams.value.push(managed)
+    qcDatastreamId.value = managed.id
+
+    // Re-key the loaded series to the managed datastream, keeping its data.
+    const series = graphSeriesArray.value.find((s) => s.id === sourceId)
+    if (series) {
+      series.id = managed.id
+      series.name = managed.name
+    }
+    const ids = new Set(plottedDatastreams.value.map((d) => d.id))
+    graphSeriesArray.value = graphSeriesArray.value.filter((s) => ids.has(s.id))
+
+    assignSeriesColors(plottedDatastreams.value.map((d) => d.id))
+    updateOptions()
+    const { plotlyRef } = storeToRefs(usePlotlyStore())
+    if (plotlyRef.value) {
+      await handleNewPlot(undefined, { preserveZoom: true })
+    }
+  }
+
   // Coalescing lock for `rebuildPlot`. Rapid checkbox toggles in the
   // Select view call `plotDatastream` in quick succession; each call
   // pushes into `plottedDatastreams` and then awaits `rebuildPlot`.
@@ -281,6 +361,9 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     return (
       datastreams.value?.filter(
         (datastream) =>
+          // Managed (QC) datastreams are reached through the "Start editing"
+          // chooser on their source, not picked directly from the catalog.
+          !managedDatastreamIds.value.has(datastream.id) &&
           matchesSelectedThing(datastream) &&
           matchesSelectedObservedProperty(datastream) &&
           matchesSelectedProcessingLevel(datastream)
@@ -549,6 +632,11 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   return {
     things,
     datastreams,
+    qcHistories,
+    managedDatastreamIds,
+    historiesBySource,
+    addQcHistory,
+    removeManagedDatastream,
     processingLevels,
     observedProperties,
     selectedThings,
@@ -581,6 +669,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     clearPlottedDatastreams,
     setPlottedDatastreams,
     setQcDatastream,
+    adoptManagedDatastream,
     rebuildPlot,
     // updateOrFetchGraphSeries,
   }

From fe28efd3ef6a265102cd2716c251c058ba6ffcaa Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 3 Aug 2026 12:53:04 -0600
Subject: [PATCH 07/93] feat: base new sessions on the latest committed state
 and track unsaved edits

---
 apps/qc-app/.env.example                      |  13 ++
 apps/qc-app/docs/ARCHITECTURE.md              |   7 +
 .../src/components/EditData/SessionList.vue   |   6 +-
 .../EditData/StartEditingDialog.vue           |   3 +-
 .../__tests__/StartEditingDialog.spec.ts      |   7 +
 apps/qc-app/src/components/VisualizeData.vue  |  24 ++-
 .../VisualizeData/DataVisDatasetsTable.vue    |  48 +++++-
 .../__tests__/useEditSession.spec.ts          |  54 ++++++-
 apps/qc-app/src/composables/useEditSession.ts |  53 ++++++-
 apps/qc-app/src/router/guards.ts              |   8 +-
 .../__tests__/reconstructSession.spec.ts      | 141 +++++++-----------
 .../qualityControl/__tests__/session.spec.ts  |  31 ++++
 .../qualityControl/reconstructSession.ts      |  80 +++-------
 .../src/services/qualityControl/session.ts    |  18 +++
 apps/qc-app/src/utils/__tests__/time.spec.ts  |  31 ++++
 apps/qc-app/src/utils/time.ts                 |  20 +++
 16 files changed, 365 insertions(+), 179 deletions(-)
 create mode 100644 apps/qc-app/.env.example
 create mode 100644 apps/qc-app/src/utils/__tests__/time.spec.ts

diff --git a/apps/qc-app/.env.example b/apps/qc-app/.env.example
new file mode 100644
index 00000000..9774480c
--- /dev/null
+++ b/apps/qc-app/.env.example
@@ -0,0 +1,13 @@
+# Backend the dev server proxies /api to (e.g. a HydroServer instance).
+VITE_APP_PROXY_BASE_URL=
+
+# Origin of the data-management app, which owns login/auth. In dev the two
+# apps run on separate ports, so point this at data-management (e.g.
+# http://127.0.0.1:1203). Leave unset in production, where the QC app is
+# served by data-management under the same origin and /login resolves there.
+VITE_APP_DATA_MANAGEMENT_URL=
+
+# Set to 1 to drop COOP/COEP headers when talking to a backend that doesn't
+# serve Cross-Origin-Resource-Policy. Disables SharedArrayBuffer-backed
+# workers (they fall back to inline execution).
+VITE_APP_DISABLE_COOP=
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 0656d1be..d01f966d 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -254,6 +254,13 @@ Two contract notes worth keeping in mind:
   `{ method, args }`; the QC API speaks `{ operationType, arguments, order }`.
   The enum values are identical, so `persistOperations`/`reconstructSession`
   rename the fields when crossing between qc-utils and the API.
+- **Sessions start/resume from the latest committed state, not the raw source.**
+  Because each commit replays its session into the managed datastream (in-range
+  `replace`), the managed datastream's observations already carry every
+  committed session. `startSession`/`reconstructSession` therefore load the
+  managed datastream as the working base (via `loadLatestBase`, falling back to
+  the source only when nothing has been committed yet) and replay just the
+  current session's own draft operations on top.
 
 Tests stub the three services with `makeQcFake()` (a stateful in-memory double
 under `services/qualityControl/__tests__/` that returns
diff --git a/apps/qc-app/src/components/EditData/SessionList.vue b/apps/qc-app/src/components/EditData/SessionList.vue
index 8dc2f27c..c0467145 100644
--- a/apps/qc-app/src/components/EditData/SessionList.vue
+++ b/apps/qc-app/src/components/EditData/SessionList.vue
@@ -73,6 +73,7 @@
 import { computed } from 'vue'
 import { storeToRefs } from 'pinia'
 import { useQcSessionStore } from '@/store/qcSession'
+import { formatDateRange } from '@/utils/time'
 import type { QualityControlSession } from '@hydroserver/client'
 
 const store = useQcSessionStore()
@@ -88,7 +89,10 @@ const orderedSessions = computed(() =>
 
 function sessionLabel(session: QualityControlSession): string {
   if (session.description) return session.description
-  return `${session.phenomenonTimeStart.slice(0, 10)} to ${session.phenomenonTimeEnd.slice(0, 10)}`
+  return formatDateRange(
+    session.phenomenonTimeStart,
+    session.phenomenonTimeEnd
+  )
 }
 
 function onSelect(session: QualityControlSession): void {
diff --git a/apps/qc-app/src/components/EditData/StartEditingDialog.vue b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
index 815d1f44..00e63be2 100644
--- a/apps/qc-app/src/components/EditData/StartEditingDialog.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
@@ -167,6 +167,7 @@
 import { ref } from 'vue'
 import type { Datastream, QualityControlSession } from '@hydroserver/client'
 import type { ManagedDatastreamOption } from '@/composables/useManagedDatastreams'
+import { formatDateRange } from '@/utils/time'
 
 defineProps<{
   source: Datastream
@@ -222,5 +223,5 @@ const orderedSessions = (sessions: QualityControlSession[]) =>
 
 const sessionLabel = (s: QualityControlSession) =>
   s.description ||
-  `${s.phenomenonTimeStart.slice(0, 10)} to ${s.phenomenonTimeEnd.slice(0, 10)}`
+  formatDateRange(s.phenomenonTimeStart, s.phenomenonTimeEnd)
 </script>
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
index 8bf2b4b7..9a0f83f7 100644
--- a/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
@@ -63,6 +63,13 @@ describe('StartEditingDialog', () => {
     )
   })
 
+  it('formats session date ranges like the date inputs (MM/DD/YYYY HH:MM)', () => {
+    const w = mountDialog()
+    expect(w.text()).toMatch(
+      /\d{2}\/\d{2}\/\d{4} \d{2}:\d{2} to \d{2}\/\d{2}\/\d{4} \d{2}:\d{2}/
+    )
+  })
+
   it('shows a recap line with processing level, observations, and sessions', () => {
     const w = mountDialog()
     expect(w.text()).toContain('Quality Controlled')
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index ea2a7e91..17156566 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -241,18 +241,6 @@
           >
             Save
           </v-btn>
-          <v-btn
-            data-testid="exit-save-close-btn"
-            class="ml-1"
-            size="small"
-            variant="tonal"
-            color="primary"
-            prepend-icon="mdi-content-save-move-outline"
-            :disabled="saveDisabled"
-            @click="onSaveAndClose"
-          >
-            Save &amp; Close
-          </v-btn>
           <v-btn
             data-testid="exit-commit-btn"
             class="ml-1"
@@ -428,7 +416,11 @@
           <div class="d-flex flex-column">
             <div class="text-title-large font-weight-bold">Save before closing?</div>
             <div class="text-body-small text-medium-emphasis">
-              {{ editCount }} edit{{ editCount === 1 ? '' : 's' }} not yet saved to the session
+              <template v-if="unsavedEditCount > 0">
+                {{ unsavedEditCount }} edit{{ unsavedEditCount === 1 ? '' : 's' }}
+                not yet saved to the session
+              </template>
+              <template v-else> You have unsaved changes </template>
             </div>
           </div>
         </div>
@@ -540,6 +532,8 @@ const {
   commit,
   needsSession,
   needsHistory,
+  hasUnsavedChanges,
+  unsavedEditCount,
 } = useEditSession()
 const qcSessionStore = useQcSessionStore()
 const { isReadOnly, inProgressSession } = storeToRefs(qcSessionStore)
@@ -774,7 +768,9 @@ async function onCommit() {
 }
 
 function requestClose() {
-  if (editCount.value > 0) {
+  // Only prompt to save when there are edits not yet persisted to the
+  // session; otherwise close straight back to the select view.
+  if (hasUnsavedChanges.value) {
     showCloseConfirm.value = true
   } else {
     exitToSelect()
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
index 049a1e18..684842e0 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
@@ -178,11 +178,24 @@
                 >
                   QC
                 </span>
+                <span
+                  v-if="managedCount(item) > 0"
+                  class="managed-count ml-1 d-inline-flex align-center justify-center"
+                  :title="`${managedCount(item)} managed (QC) datastream${
+                    managedCount(item) === 1 ? '' : 's'
+                  } from this source`"
+                >
+                  {{ managedCount(item) }}
+                </span>
               </div>
             </template>
           </v-tooltip>
         </template>
 
+        <template #item.name="{ item }">
+          <span class="name-cell" :title="item.name">{{ item.name || '-' }}</span>
+        </template>
+
         <template #item.siteCodeName="{ item }">
           <div class="site-cell d-flex flex-column">
             <div class="site-cell__code">{{ item.siteCodeName || '-' }}</div>
@@ -243,7 +256,7 @@ import { Datastream } from '@hydroserver/client'
 import type { DatastreamExtended } from '@hydroserver/client'
 import { downloadDatastreamsCsvZip } from '@/utils/csvExport'
 
-const { filteredDatastreams, plottedDatastreams, qcDatastream } =
+const { filteredDatastreams, plottedDatastreams, qcDatastream, historiesBySource } =
   storeToRefs(useDataVisStore())
 const { toggleDatastream, clearPlottedDatastreams } = useDataVisStore()
 
@@ -338,6 +351,10 @@ const isChecked = (item: Datastream) =>
 
 const isQc = (item: Datastream) => qcDatastream.value?.id === item.id
 
+// How many managed (QC) datastreams exist for this source datastream.
+const managedCount = (item: Datastream) =>
+  historiesBySource.value.get(item.id)?.length ?? 0
+
 const isAtCap = (item: Datastream) =>
   plottedDatastreams.value.length >= 5 && !isChecked(item)
 
@@ -351,7 +368,12 @@ const getRowProps = ({ item }: { item: Datastream }) => ({
 
 const search = ref()
 const headers = reactive([
-  { title: 'Plot', key: 'plot', visible: true, width: 64, sortable: false },
+  { title: 'Plot', key: 'plot', visible: true, width: 96, sortable: false },
+  {
+    title: 'Name',
+    key: 'name',
+    visible: true,
+  },
   {
     title: 'Site',
     key: 'siteCodeName',
@@ -496,6 +518,15 @@ const resetSort = () => {
   white-space: nowrap;
 }
 
+.name-cell {
+  display: inline-block;
+  max-width: 320px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  vertical-align: middle;
+}
+
 .num-cell {
   font-variant-numeric: tabular-nums;
   white-space: nowrap;
@@ -537,6 +568,19 @@ const resetSort = () => {
   cursor: not-allowed !important;
 }
 
+/* Count badge next to the plot checkbox showing how many managed (QC)
+   datastreams exist for that source datastream. */
+.managed-count {
+  height: 18px;
+  min-width: 18px;
+  padding: 0 5px;
+  font-size: 0.65rem;
+  font-weight: 700;
+  color: rgb(var(--v-theme-primary));
+  background-color: rgba(var(--v-theme-primary), 0.14);
+  border-radius: 9px;
+}
+
 /* Compact "QC" pill rendered next to the plot checkbox on the QC row.
    Marks the quality-control target without occupying its own column. */
 .qc-pill {
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 2d2aac5f..3093fe94 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -125,15 +125,19 @@ describe('useEditSession', () => {
     expect(selectedSeries.value.data).toEqual(reconstructed)
   })
 
-  it('startSession creates a session and copies the source window', async () => {
+  it('startSession loads the managed datastream as the working base', async () => {
     await seedHistory()
+    const managedBase = makeRecord()
+    fetchObservationsInRange.mockResolvedValue(managedBase)
     const { useEditSession } = await import('@/composables/useEditSession')
     const session = useEditSession()
     await session.beginEditing()
     await session.startSession({ ...WIN, description: 'Jan' })
     expect(session.needsSession.value).toBe(false)
     expect(useQcSessionStore().inProgressSession?.description).toBe('Jan')
-    expect(fetchObservationsInRange.mock.calls.at(-1)?.[0].id).toBe('s-1')
+    // Working copy comes from the managed datastream (latest committed state).
+    expect(fetchObservationsInRange.mock.calls[0]?.[0].id).toBe('m-1')
+    expect(selectedSeries.value.data).toEqual(managedBase)
   })
 
   it('startSession clamps the window to the source datastream extent', async () => {
@@ -159,12 +163,13 @@ describe('useEditSession', () => {
 
   it('saveDraft persists the record operations to the session', async () => {
     await seedHistory()
-    selectedSeries.value = {
-      data: makeRecord([
+    // The working copy comes from startSession's base load; give it the ops.
+    fetchObservationsInRange.mockResolvedValue(
+      makeRecord([
         { method: 'VALUE_THRESHOLD', args: [] },
         { method: 'DELETE_POINTS', args: [] },
-      ]),
-    }
+      ])
+    )
     const { useEditSession } = await import('@/composables/useEditSession')
     const session = useEditSession()
     await session.beginEditing()
@@ -200,6 +205,43 @@ describe('useEditSession', () => {
     expect(store.inProgressSession).toBeNull()
   })
 
+  it('tracks unsaved edits against the last saved snapshot', async () => {
+    await seedHistory()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    // Fresh session: working copy matches the saved baseline.
+    expect(session.hasUnsavedChanges.value).toBe(false)
+    expect(session.unsavedEditCount.value).toBe(0)
+    // A new edit on the working copy is unsaved.
+    selectedSeries.value.data.history.push({ method: 'DELETE_POINTS', args: [] })
+    expect(session.hasUnsavedChanges.value).toBe(true)
+    expect(session.unsavedEditCount.value).toBe(1)
+    // Saving re-baselines.
+    await session.saveDraft()
+    expect(session.hasUnsavedChanges.value).toBe(false)
+    expect(session.unsavedEditCount.value).toBe(0)
+  })
+
+  it('resuming a session starts with no unsaved changes', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    await qc.sessions.create(h.id, WIN)
+    fetchObservationsInRange.mockResolvedValue(
+      makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }])
+    )
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    expect(session.hasUnsavedChanges.value).toBe(false)
+    expect(session.unsavedEditCount.value).toBe(0)
+  })
+
   it('commit saves the description provided at commit time', async () => {
     await seedHistory()
     selectedSeries.value = {
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index a535b31a..3c94a354 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -13,11 +13,11 @@
  * operation/session with the authenticated user.
  */
 
-import { ref } from 'vue'
+import { ref, computed } from 'vue'
 import { storeToRefs } from 'pinia'
 import { serializeHistory, applyHistory } from '@uwrl/qc-utils'
 import type { Datastream, QualityControlSessionContract } from '@hydroserver/client'
-import type { ObservationRecord } from '@uwrl/qc-utils'
+import type { ObservationRecord, HistoryItem } from '@uwrl/qc-utils'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { usePlotlyStore } from '@/store/plotly'
 import { useHydroServer } from '@/store/hydroserver'
@@ -26,7 +26,7 @@ import { useQcSessionStore } from '@/store/qcSession'
 import {
   findHistoryForDatastream,
   startOrResumeSession,
-  loadSourceWindow,
+  loadLatestBase,
   persistSessionOperations,
   commitQcSession,
   reconstructSession,
@@ -80,6 +80,30 @@ export function useEditSession() {
   /** True when the selected datastream has no QC history (not a managed datastream). */
   const needsHistory = ref(false)
 
+  // Snapshot (by item reference) of the operation history at the last
+  // load/save, so the editor knows whether there are unsaved edits.
+  const savedEdits = ref<HistoryItem[]>([])
+  const currentEdits = (): HistoryItem[] =>
+    (selectedSeries.value?.data as ObservationRecord | undefined)?.history ?? []
+  const snapshotSavedEdits = () => {
+    savedEdits.value = [...currentEdits()]
+  }
+
+  /** True when the working copy has edits not in the last saved snapshot
+   *  (additions or removals). */
+  const hasUnsavedChanges = computed(() => {
+    const current = currentEdits()
+    const saved = savedEdits.value
+    if (current.length !== saved.length) return true
+    return current.some((item, i) => item !== saved[i])
+  })
+
+  /** Count of edits added since the last save (the common append case). */
+  const unsavedEditCount = computed(
+    () =>
+      currentEdits().filter((item) => !savedEdits.value.includes(item)).length
+  )
+
   async function beginEditing(): Promise<void> {
     const managed = qcDatastream.value
     if (!managed) return
@@ -113,11 +137,14 @@ export function useEditSession() {
           fetchInRange: fetchObservationsInRange,
           applyHistory,
         },
+        managed,
         sourceDatastream.value,
         history.id,
         inProgress.id
       )
       if (selectedSeries.value) selectedSeries.value.data = reconstructed
+      // The replayed draft operations are the saved baseline.
+      snapshotSavedEdits()
       needsSession.value = false
     } else {
       needsSession.value = true
@@ -127,7 +154,8 @@ export function useEditSession() {
   async function startSession(spec: QcSessionPostBody): Promise<void> {
     const historyId = sessionStore.historyId
     const source = sourceDatastream.value
-    if (!historyId || !source) {
+    const managed = qcDatastream.value
+    if (!historyId || !source || !managed) {
       throw new Error('Load a managed datastream for editing first.')
     }
     const session = await startOrResumeSession(
@@ -136,8 +164,19 @@ export function useEditSession() {
       clampSpecToSource(spec, source)
     )
     await sessionStore.loadSessions(historyId)
-    await loadSourceWindow(fetchObservationsInRange, source, session)
+    // Start from the latest committed state (the managed datastream), or the
+    // raw source when nothing has been committed yet.
+    const base = await loadLatestBase(
+      fetchObservationsInRange,
+      managed,
+      source,
+      new Date(session.phenomenonTimeStart),
+      new Date(session.phenomenonTimeEnd)
+    )
+    if (selectedSeries.value) selectedSeries.value.data = base
     needsSession.value = false
+    // Fresh session: the loaded working copy is the saved baseline.
+    snapshotSavedEdits()
   }
 
   async function saveDraft(): Promise<void> {
@@ -157,6 +196,7 @@ export function useEditSession() {
       session.id,
       operations
     )
+    snapshotSavedEdits()
   }
 
   async function commit(description?: string): Promise<void> {
@@ -198,12 +238,15 @@ export function useEditSession() {
       },
     })
     await sessionStore.loadSessions(historyId)
+    snapshotSavedEdits()
   }
 
   return {
     sourceDatastream,
     needsSession,
     needsHistory,
+    hasUnsavedChanges,
+    unsavedEditCount,
     beginEditing,
     startSession,
     saveDraft,
diff --git a/apps/qc-app/src/router/guards.ts b/apps/qc-app/src/router/guards.ts
index 737e07b9..5131458e 100644
--- a/apps/qc-app/src/router/guards.ts
+++ b/apps/qc-app/src/router/guards.ts
@@ -18,7 +18,13 @@ const getQcReturnPath = (to: RouteLocationNormalized) => {
 }
 
 const redirectToDataManagementLogin = (to: RouteLocationNormalized) => {
-  const loginUrl = new URL('/login', window.location.origin)
+  // In production the QC app is served by data-management under the same
+  // origin, so /login resolves there. In dev the two apps run on separate
+  // ports, so VITE_APP_DATA_MANAGEMENT_URL points at the data-management
+  // origin (e.g. http://127.0.0.1:1203); it falls back to the current origin.
+  const dataManagementOrigin =
+    import.meta.env.VITE_APP_DATA_MANAGEMENT_URL || window.location.origin
+  const loginUrl = new URL('/login', dataManagementOrigin)
   loginUrl.searchParams.set('next', getQcReturnPath(to))
   window.location.assign(loginUrl.toString())
   return false as const
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
index 29561651..28b9f97c 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
@@ -2,10 +2,7 @@ import { describe, it, expect, vi } from 'vitest'
 import type { Datastream } from '@hydroserver/client'
 import type { ObservationRecord, QcHistory } from '@uwrl/qc-utils'
 import { makeQcFake } from './qcServiceFake'
-import {
-  collectSessionOperations,
-  reconstructSession,
-} from '../reconstructSession'
+import { reconstructSession } from '../reconstructSession'
 import { unwrap } from '../unwrap'
 
 const win = (start: string, end: string) => ({
@@ -13,27 +10,8 @@ const win = (start: string, end: string) => ({
   phenomenonTimeEnd: end,
 })
 
-/** Create a committed session with the given operations. */
-const committedSession = async (
-  qc: ReturnType<typeof makeQcFake>,
-  historyId: string,
-  range: { phenomenonTimeStart: string; phenomenonTimeEnd: string },
-  ops: string[]
-) => {
-  const s = unwrap(await qc.sessions.create(historyId, range))
-  if (ops.length) {
-    await qc.operations.create(
-      historyId,
-      s.id,
-      ops.map((operationType, i) => ({
-        operationType: operationType as any,
-        order: i,
-      }))
-    )
-  }
-  await qc.sessions.commit(historyId, s.id)
-  return s
-}
+const rec = (dataX: number[]) =>
+  ({ dataX, dataY: dataX.map(() => 0), history: [] }) as unknown as ObservationRecord
 
 const newHistory = async (qc: ReturnType<typeof makeQcFake>) =>
   unwrap(
@@ -43,86 +21,46 @@ const newHistory = async (qc: ReturnType<typeof makeQcFake>) =>
     })
   ).id
 
-describe('collectSessionOperations', () => {
-  it('returns just the session ops when there are no ancestors', async () => {
-    const qc = makeQcFake()
-    const historyId = await newHistory(qc)
-    const s = await committedSession(
-      qc,
-      historyId,
-      win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'),
-      ['SELECTION', 'DELETE_POINTS']
-    )
-    const ops = await collectSessionOperations(
-      qc.sessions,
-      qc.operations,
-      historyId,
-      s.id
-    )
-    expect(ops.map((o) => o.method)).toEqual(['SELECTION', 'DELETE_POINTS'])
-  })
-
-  it('prepends ancestor ops (oldest first) before the target session ops', async () => {
-    const qc = makeQcFake()
-    const historyId = await newHistory(qc)
-    // A (Jan-Mar) then B (Feb-Apr) overlaps A -> B depends on A
-    await committedSession(
-      qc,
-      historyId,
-      win('2025-01-01T00:00:00Z', '2025-03-01T00:00:00Z'),
-      ['VALUE_THRESHOLD']
-    )
-    const b = await committedSession(
-      qc,
-      historyId,
-      win('2025-02-01T00:00:00Z', '2025-04-01T00:00:00Z'),
-      ['CHANGE_VALUES']
-    )
-
-    const ops = await collectSessionOperations(
-      qc.sessions,
-      qc.operations,
-      historyId,
-      b.id
-    )
-    expect(ops.map((o) => o.method)).toEqual(['VALUE_THRESHOLD', 'CHANGE_VALUES'])
-  })
-})
+const managed = { id: 'm-1' } as unknown as Datastream
+const source = { id: 's-1' } as unknown as Datastream
 
 describe('reconstructSession', () => {
-  it('fetches the source window and replays the ordered operations via applyHistory', async () => {
+  it('uses the managed datastream as the base and replays only this session ops', async () => {
     const qc = makeQcFake()
     const historyId = await newHistory(qc)
     const range = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
-    const s = await committedSession(qc, historyId, range, [
-      'SELECTION',
-      'INTERPOLATE',
+    const s = unwrap(await qc.sessions.create(historyId, range))
+    await qc.operations.create(historyId, s.id, [
+      { operationType: 'SELECTION' as any, order: 0 },
+      { operationType: 'INTERPOLATE' as any, order: 1 },
     ])
 
-    const record = {} as ObservationRecord
-    const fetchInRange = vi.fn().mockResolvedValue(record)
+    const managedBase = rec([Date.UTC(2025, 0, 1)])
+    const fetchInRange = vi.fn().mockResolvedValue(managedBase)
     let captured: QcHistory | undefined
-    const applyHistory = vi.fn(async (_rec: ObservationRecord, history: QcHistory) => {
-      captured = history
-      return { applied: history.operations.length, failed: [] }
-    })
-    const source = { id: 's-1' } as unknown as Datastream
+    const applyHistory = vi.fn(
+      async (_rec: ObservationRecord, history: QcHistory) => {
+        captured = history
+        return { applied: history.operations.length, failed: [] }
+      }
+    )
 
     const result = await reconstructSession(
       { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      managed,
       source,
       historyId,
       s.id
     )
 
-    // fetched the source over the session window
+    // Base comes from the managed datastream (latest committed state).
     const [ds, begin, end] = fetchInRange.mock.calls[0]
-    expect(ds).toBe(source)
+    expect(ds).toBe(managed)
     expect((begin as Date).toISOString()).toBe('2025-01-01T00:00:00.000Z')
     expect((end as Date).toISOString()).toBe('2025-02-01T00:00:00.000Z')
 
-    // replayed the ordered ops over that window
-    expect(captured?.version).toBe('1')
+    // Only this session's own ops are replayed (ancestors are baked into
+    // the managed datastream).
     expect(captured?.window).toEqual({
       startDate: range.phenomenonTimeStart,
       endDate: range.phenomenonTimeEnd,
@@ -131,7 +69,38 @@ describe('reconstructSession', () => {
       'SELECTION',
       'INTERPOLATE',
     ])
-    expect(result.record).toBe(record)
+    expect(result.record).toBe(managedBase)
     expect(result.report.applied).toBe(2)
   })
+
+  it('falls back to the source when the managed datastream has no committed data', async () => {
+    const qc = makeQcFake()
+    const historyId = await newHistory(qc)
+    const s = unwrap(
+      await qc.sessions.create(
+        historyId,
+        win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
+      )
+    )
+
+    const sourceRec = rec([Date.UTC(2025, 0, 1)])
+    const fetchInRange = vi
+      .fn()
+      .mockResolvedValueOnce(rec([])) // managed: nothing committed yet
+      .mockResolvedValueOnce(sourceRec) // source fallback
+    const applyHistory = vi.fn(async () => ({ applied: 0, failed: [] }))
+
+    const result = await reconstructSession(
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      managed,
+      source,
+      historyId,
+      s.id
+    )
+
+    expect(fetchInRange).toHaveBeenCalledTimes(2)
+    expect(fetchInRange.mock.calls[0][0]).toBe(managed)
+    expect(fetchInRange.mock.calls[1][0]).toBe(source)
+    expect(result.record).toBe(sourceRec)
+  })
 })
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
index bef99a0a..4f1175a2 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
@@ -6,6 +6,7 @@ import {
   getInProgressSession,
   startOrResumeSession,
   loadSourceWindow,
+  loadLatestBase,
 } from '../session'
 import { unwrap } from '../unwrap'
 
@@ -83,3 +84,33 @@ describe('loadSourceWindow', () => {
     expect((end as Date).toISOString()).toBe('2025-02-01T00:00:00.000Z')
   })
 })
+
+describe('loadLatestBase', () => {
+  const managed = { id: 'm-1' } as unknown as Datastream
+  const source = { id: 's-1' } as unknown as Datastream
+  const start = new Date('2025-01-01T00:00:00Z')
+  const end = new Date('2025-02-01T00:00:00Z')
+  const recWith = (n: number) =>
+    ({ dataX: Array(n).fill(0), dataY: Array(n).fill(0) }) as unknown as ObservationRecord
+
+  it('uses the managed datastream when it has committed data', async () => {
+    const managedRec = recWith(3)
+    const fetchInRange = vi.fn().mockResolvedValue(managedRec)
+    const result = await loadLatestBase(fetchInRange, managed, source, start, end)
+    expect(result).toBe(managedRec)
+    expect(fetchInRange).toHaveBeenCalledTimes(1)
+    expect(fetchInRange.mock.calls[0][0]).toBe(managed)
+  })
+
+  it('falls back to the source when the managed datastream is empty', async () => {
+    const sourceRec = recWith(2)
+    const fetchInRange = vi
+      .fn()
+      .mockResolvedValueOnce(recWith(0))
+      .mockResolvedValueOnce(sourceRec)
+    const result = await loadLatestBase(fetchInRange, managed, source, start, end)
+    expect(result).toBe(sourceRec)
+    expect(fetchInRange).toHaveBeenCalledTimes(2)
+    expect(fetchInRange.mock.calls[1][0]).toBe(source)
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/reconstructSession.ts b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
index b2573632..6a11c309 100644
--- a/apps/qc-app/src/services/qualityControl/reconstructSession.ts
+++ b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
@@ -1,19 +1,14 @@
 /**
- * Reconstruct the datastream state "as of" a session for resume/read-only
- * viewing (spec section 4 / 7.3).
+ * Reconstruct a session's working state for resume/read-only viewing
+ * (spec section 4 / 7.3).
  *
- * Fetch the source window, then replay the session's operations preceded
- * by its ancestors' operations (chronological order) via qc-utils
- * `applyHistory`. `fetchInRange` and `applyHistory` are injected so this
+ * Base = the latest committed state (the managed datastream's observations,
+ * which already carry every previously-committed session via commit's
+ * in-range replace), then replay this session's own draft operations on top.
+ * Previously-committed sessions are NOT replayed here; they are already baked
+ * into the managed datastream. Falls back to the raw source only when nothing
+ * has been committed yet. `fetchInRange` / `applyHistory` are injected so this
  * unit-tests without a real ObservationRecord.
- *
- * Operations are read from the operations endpoint, not off the session: a
- * session GET can return the summary shape (no embedded `operations`).
- *
- * Caveat (deferred): operations are index-coupled to the exact dataset
- * they were authored against, so reconstructing across sessions authored
- * over different windows can mis-target. This loads the target session's
- * window only; full multi-window reconstruction is a later task.
  */
 
 import type {
@@ -29,7 +24,7 @@ import type {
   QcHistoryOperation,
 } from '@uwrl/qc-utils'
 import { unwrap } from './unwrap'
-import type { FetchObservationsInRange } from './session'
+import { loadLatestBase, type FetchObservationsInRange } from './session'
 
 /** API operation -> qc-utils replayable operation (rename operationType/arguments). */
 const toSerialized = (op: QualityControlOperation): QcHistoryOperation =>
@@ -38,42 +33,6 @@ const toSerialized = (op: QualityControlOperation): QcHistoryOperation =>
     args: Array.isArray(op.arguments) ? op.arguments : [],
   }) as QcHistoryOperation
 
-/**
- * The ordered operations needed to reconstruct a session: every ancestor's
- * operations (oldest first) followed by the target session's operations.
- */
-export async function collectSessionOperations(
-  qcSessions: QualityControlSessionService,
-  qcOperations: QualityControlOperationService,
-  historyId: string,
-  sessionId: string
-): Promise<QcHistoryOperation[]> {
-  const ancestors = unwrap(
-    await qcSessions.list(historyId, {
-      ancestor_of: sessionId,
-      fetch_all: true,
-    })
-  )
-  const ordered = [...ancestors].sort(
-    (a, b) =>
-      new Date(a.phenomenonTimeStart).getTime() -
-      new Date(b.phenomenonTimeStart).getTime()
-  )
-
-  const operations: QcHistoryOperation[] = []
-  for (const ancestor of ordered) {
-    const ops = unwrap(
-      await qcOperations.list(historyId, ancestor.id, { fetch_all: true })
-    )
-    operations.push(...ops.map(toSerialized))
-  }
-  const sessionOps = unwrap(
-    await qcOperations.list(historyId, sessionId, { fetch_all: true })
-  )
-  operations.push(...sessionOps.map(toSerialized))
-  return operations
-}
-
 export interface ReconstructSessionDeps {
   qcSessions: QualityControlSessionService
   qcOperations: QualityControlOperationService
@@ -91,26 +50,21 @@ export interface ReconstructSessionResult {
 
 export async function reconstructSession(
   deps: ReconstructSessionDeps,
+  managed: Datastream,
   source: Datastream,
   historyId: string,
   sessionId: string
 ): Promise<ReconstructSessionResult> {
   const { qcSessions, qcOperations, fetchInRange, applyHistory } = deps
 
-  // The session GET supplies the window; operations come from the
-  // operations endpoint (the GET may omit them in the summary shape).
   const session = unwrap(await qcSessions.get(historyId, sessionId))
-  const operations = await collectSessionOperations(
-    qcSessions,
-    qcOperations,
-    historyId,
-    sessionId
-  )
+  const start = new Date(session.phenomenonTimeStart)
+  const end = new Date(session.phenomenonTimeEnd)
 
-  const record = await fetchInRange(
-    source,
-    new Date(session.phenomenonTimeStart),
-    new Date(session.phenomenonTimeEnd)
+  // Latest committed state as the base, then this session's own operations.
+  const record = await loadLatestBase(fetchInRange, managed, source, start, end)
+  const ops = unwrap(
+    await qcOperations.list(historyId, sessionId, { fetch_all: true })
   )
 
   const history: QcHistory = {
@@ -120,7 +74,7 @@ export async function reconstructSession(
       startDate: session.phenomenonTimeStart,
       endDate: session.phenomenonTimeEnd,
     },
-    operations,
+    operations: ops.map(toSerialized),
   }
 
   const report = await applyHistory(record, history)
diff --git a/apps/qc-app/src/services/qualityControl/session.ts b/apps/qc-app/src/services/qualityControl/session.ts
index 30b1b8ef..0ab8bb95 100644
--- a/apps/qc-app/src/services/qualityControl/session.ts
+++ b/apps/qc-app/src/services/qualityControl/session.ts
@@ -71,3 +71,21 @@ export async function loadSourceWindow(
     new Date(session.phenomenonTimeEnd)
   )
 }
+
+/**
+ * The latest committed state for a window. Every commit replays its session
+ * into the managed datastream (in-range replace), so the managed datastream's
+ * observations already carry all previously-committed sessions. Falls back to
+ * the raw source when nothing has been committed yet (the first session).
+ */
+export async function loadLatestBase(
+  fetchInRange: FetchObservationsInRange,
+  managed: Datastream,
+  source: Datastream,
+  start: Date,
+  end: Date
+): Promise<ObservationRecord> {
+  const base = await fetchInRange(managed, start, end)
+  if ((base.dataX?.length ?? 0) > 0) return base
+  return fetchInRange(source, start, end)
+}
diff --git a/apps/qc-app/src/utils/__tests__/time.spec.ts b/apps/qc-app/src/utils/__tests__/time.spec.ts
new file mode 100644
index 00000000..97357d7c
--- /dev/null
+++ b/apps/qc-app/src/utils/__tests__/time.spec.ts
@@ -0,0 +1,31 @@
+import { describe, it, expect } from 'vitest'
+import { formatDateInput, formatDateRange } from '../time'
+
+// Datetime strings without a timezone offset are parsed as local time, so
+// these assertions are stable regardless of the test runner's zone.
+describe('formatDateInput', () => {
+  it('formats as MM/DD/YYYY HH:MM (date-input style)', () => {
+    expect(formatDateInput('2025-01-05T14:30:00')).toBe('01/05/2025 14:30')
+  })
+
+  it('zero-pads month, day, hour, and minute', () => {
+    expect(formatDateInput('2025-09-08T04:07:00')).toBe('09/08/2025 04:07')
+  })
+
+  it('returns a dash for nullish input', () => {
+    expect(formatDateInput(null)).toBe('–')
+    expect(formatDateInput(undefined)).toBe('–')
+  })
+
+  it('returns the raw string when unparseable', () => {
+    expect(formatDateInput('not-a-date')).toBe('not-a-date')
+  })
+})
+
+describe('formatDateRange', () => {
+  it('joins both bounds with "to"', () => {
+    expect(
+      formatDateRange('2025-01-05T00:00:00', '2025-01-15T09:05:00')
+    ).toBe('01/05/2025 00:00 to 01/15/2025 09:05')
+  })
+})
diff --git a/apps/qc-app/src/utils/time.ts b/apps/qc-app/src/utils/time.ts
index 7f4b9e56..8e06bbd5 100644
--- a/apps/qc-app/src/utils/time.ts
+++ b/apps/qc-app/src/utils/time.ts
@@ -22,6 +22,26 @@ const formatTime = (time?: string | null): string => {
   return `${day} ${month} ${year}, ${hour}:${minute} ${period}`
 }
 
+/** Local date+time matching the time-range date inputs: `MM/DD/YYYY HH:MM`. */
+export function formatDateInput(iso?: string | null): string {
+  if (!iso) return '–'
+  const d = new Date(iso)
+  if (Number.isNaN(d.getTime())) return iso
+  const mm = String(d.getMonth() + 1).padStart(2, '0')
+  const dd = String(d.getDate()).padStart(2, '0')
+  const hh = String(d.getHours()).padStart(2, '0')
+  const min = String(d.getMinutes()).padStart(2, '0')
+  return `${mm}/${dd}/${d.getFullYear()} ${hh}:${min}`
+}
+
+/** A session's phenomenon-time window, formatted like the date inputs. */
+export function formatDateRange(
+  start?: string | null,
+  end?: string | null
+): string {
+  return `${formatDateInput(start)} to ${formatDateInput(end)}`
+}
+
 function getLocalTimeZone(): string {
   return Intl.DateTimeFormat().resolvedOptions().timeZone || '–'
 }

From a9030e270b2135eacee9c7a221c554bf1d3801ba Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 3 Aug 2026 12:53:16 -0600
Subject: [PATCH 08/93] fix: create managed datastreams with expand_related so
 the catalog keeps nested relations

---
 apps/qc-app/docs/API_REFERENCE.md             |  7 +++++-
 apps/qc-app/src/components/VisualizeData.vue  |  5 +---
 .../__tests__/createManagedDatastream.spec.ts | 24 +++++++++++++++++++
 .../qualityControl/createManagedDatastream.ts | 11 ++++++---
 .../thing-datastream.service.spec.ts          | 24 +++++++++++++++++++
 .../hydroserver-ts/src/api/services/base.ts   | 21 ++++++++++++----
 6 files changed, 80 insertions(+), 12 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index edf0bbd9..d8d945c3 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -167,7 +167,12 @@ const { managedDatastream, history } = await create({ source, processingLevelId,
 ```
 
 Delegates to the tested `createManagedDatastream` orchestration with the
-live client (`hs.datastreams` + `hs.qualityControlHistories`).
+live client (`hs.datastreams` + `hs.qualityControlHistories`). The datastream
+is created with `expand_related: true`, so `managedDatastream` comes back in
+the same `Datastream & DatastreamExtended` shape as the `datastreams` catalog
+and can be appended to it directly. Without the flag the 201 body is the flat
+model (FK ids only) and catalog consumers reading `ds.processingLevel.id`
+would break.
 
 ### `useManagedDatastreams()`
 
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index 17156566..2c3f75c3 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -710,10 +710,7 @@ async function onCreateDatastream(spec: CreateManagedDatastreamSpec) {
     // Register the new history + datastream so it's hidden from the catalog
     // and the chooser can resolve it (by its name, not id) without a reload.
     addQcHistory(history)
-    datastreams.value = [
-      ...datastreams.value,
-      managedDatastream as (typeof datastreams.value)[number],
-    ]
+    datastreams.value = [...datastreams.value, managedDatastream]
     // Reuse the source's already-loaded series as the managed datastream's
     // working copy instead of adding a second, empty plotted item.
     await adoptManagedDatastream(managedDatastream, spec.source.id)
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
index 38f0b7ca..2b42ab5b 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
@@ -114,6 +114,30 @@ describe('createManagedDatastream', () => {
     })
   })
 
+  it('requests the expanded datastream so the catalog keeps its nested relations', async () => {
+    // The data-vis catalog is loaded with `expand_related`, and its consumers
+    // read `ds.processingLevel.id` / `ds.thing.id`. A flat create response
+    // appended to that list would blow up on the missing nested objects.
+    const created = {
+      id: 'managed-1',
+      name: 'Raw Temperature (QC)',
+      thing: { id: 'thing-1' },
+      observedProperty: { id: 'op-1' },
+      processingLevel: { id: 'pl-qc' },
+    } as any
+    const { hs, create } = makeHs(created)
+    const qc = makeQcFake()
+
+    const result = await createManagedDatastream(hs, qc.histories, {
+      source: makeSource(),
+      processingLevelId: 'pl-qc',
+    })
+
+    expect(create.mock.calls[0][1]).toEqual({ expand_related: true })
+    expect(result.managedDatastream.processingLevel.id).toBe('pl-qc')
+    expect(result.managedDatastream.thing.id).toBe('thing-1')
+  })
+
   it('rejects a processing level equal to the source', async () => {
     const { hs } = makeHs(makeSource({ id: 'managed-1' }))
     const qc = makeQcFake()
diff --git a/apps/qc-app/src/services/qualityControl/createManagedDatastream.ts b/apps/qc-app/src/services/qualityControl/createManagedDatastream.ts
index d7f0f319..69440390 100644
--- a/apps/qc-app/src/services/qualityControl/createManagedDatastream.ts
+++ b/apps/qc-app/src/services/qualityControl/createManagedDatastream.ts
@@ -33,7 +33,8 @@ export interface CreateManagedDatastreamInput {
 }
 
 export interface CreateManagedDatastreamResult {
-  managedDatastream: Datastream
+  /** Expanded shape, so it can be appended to the data-vis catalog as-is. */
+  managedDatastream: Datastream & DatastreamExtended
   history: QcHistoryDetail
 }
 
@@ -93,7 +94,9 @@ export async function createManagedDatastream(
   }
 
   const body = buildManagedDatastreamBody(source, processingLevelId, overrides)
-  const response = await hs.datastreams.create(body)
+  // Without `expand_related` the 201 body is the flat shape (FK ids only);
+  // the data-vis catalog this datastream joins is expanded.
+  const response = await hs.datastreams.create(body, { expand_related: true })
   if (!response.ok) {
     throw new Error(
       `Datastream creation failed (HTTP ${response.status}): ${
@@ -101,7 +104,9 @@ export async function createManagedDatastream(
       }`
     )
   }
-  const managedDatastream = response.data
+  // The SDK types every create response as the flat model; `expand_related`
+  // above is what actually makes the nested relations present.
+  const managedDatastream = response.data as Datastream & DatastreamExtended
   if (!managedDatastream?.id) {
     throw new Error(
       'Datastream creation failed: the backend returned no datastream id.'
diff --git a/packages/hydroserver-ts/src/api/__tests__/thing-datastream.service.spec.ts b/packages/hydroserver-ts/src/api/__tests__/thing-datastream.service.spec.ts
index 749ba267..1b942280 100644
--- a/packages/hydroserver-ts/src/api/__tests__/thing-datastream.service.spec.ts
+++ b/packages/hydroserver-ts/src/api/__tests__/thing-datastream.service.spec.ts
@@ -96,6 +96,30 @@ describe('DatastreamService', () => {
 
   const client = new HydroServer({ host: 'https://hydro.example.com' })
 
+  describe('create', () => {
+    it('forwards expand_related so the 201 body carries nested relations', async () => {
+      vi.stubGlobal('fetch', vi.fn().mockResolvedValue(jsonResponse({ id: 'ds-1' }, 201)))
+
+      await client.datastreams.create({ name: 'DS 1' } as any, {
+        expand_related: true,
+      })
+
+      const [url] = (fetch as any).mock.calls[0]
+      const parsed = new URL(url)
+      expect(parsed.pathname).toBe('/api/data/datastreams')
+      expect(parsed.searchParams.get('expand_related')).toBe('true')
+    })
+
+    it('omits the query string when no params are given', async () => {
+      vi.stubGlobal('fetch', vi.fn().mockResolvedValue(jsonResponse({ id: 'ds-1' }, 201)))
+
+      await client.datastreams.create({ name: 'DS 1' } as any)
+
+      const [url] = (fetch as any).mock.calls[0]
+      expect(url).toMatch(/\/api\/data\/datastreams$/)
+    })
+  })
+
   describe('getVisualizationBootstrap', () => {
     it('maps bootstrap payloads into model instances and resolves workspaceId', async () => {
       vi.stubGlobal(
diff --git a/packages/hydroserver-ts/src/api/services/base.ts b/packages/hydroserver-ts/src/api/services/base.ts
index dcbd1572..1d64c492 100644
--- a/packages/hydroserver-ts/src/api/services/base.ts
+++ b/packages/hydroserver-ts/src/api/services/base.ts
@@ -92,11 +92,24 @@ export abstract class HydroServerBaseService<
     return res.ok ? res.data : null
   }
 
-  create = async (body: M): Promise<ApiResponse<M>> =>
-    apiMethods.post<M>(this._route, this.serialize(body))
+  create = async (
+    body: M,
+    params?: {
+      expand_related: boolean
+    }
+  ): Promise<ApiResponse<M>> =>
+    apiMethods.post<M>(
+      this.withQuery(this._route, params),
+      this.serialize(body)
+    )
 
-  createItem = async (body: M): Promise<M | null> => {
-    const res = await apiMethods.post<M>(this._route, this.serialize(body))
+  createItem = async (
+    body: M,
+    params?: {
+      expand_related: boolean
+    }
+  ): Promise<M | null> => {
+    const res = await this.create(body, params)
     return res.ok ? res.data : null
   }
 

From 3df35db454489c4edc785120bdb85f59619676a6 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 3 Aug 2026 12:53:16 -0600
Subject: [PATCH 09/93] chore: update package lock

---
 package-lock.json | 10314 ++++++++++++++++++++++++++++----------------
 1 file changed, 6594 insertions(+), 3720 deletions(-)

diff --git a/package-lock.json b/package-lock.json
index b5085087..f8b24a3d 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -14,7 +14,7 @@
       "name": "hydroserver-qc",
       "dependencies": {
         "@fortawesome/fontawesome-free": "^7.2.0",
-        "@hydroserver/client": "^0.4.4",
+        "@hydroserver/client": "file:../../packages/hydroserver-ts",
         "@mdi/js": "^7.4.47",
         "@uwrl/qc-utils": "^0.1.1",
         "dotenv": "^17.4.2",
@@ -160,14 +160,6 @@
         "node": "^22.18.0 || >=24.11.0"
       }
     },
-    "apps/qc-app/node_modules/@bcoe/v8-coverage": {
-      "version": "1.0.2",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=18"
-      }
-    },
     "apps/qc-app/node_modules/@csstools/color-helpers": {
       "version": "5.1.0",
       "funding": [
@@ -295,28 +287,6 @@
         "node": ">=6"
       }
     },
-    "apps/qc-app/node_modules/@hydroserver/client": {
-      "version": "0.4.4",
-      "license": "ISC",
-      "dependencies": {
-        "cross-fetch": "^4.1.0",
-        "jszip": "^3.10.1",
-        "p-limit": "^6.2.0"
-      }
-    },
-    "apps/qc-app/node_modules/@hydroserver/client/node_modules/p-limit": {
-      "version": "6.2.0",
-      "license": "MIT",
-      "dependencies": {
-        "yocto-queue": "^1.1.1"
-      },
-      "engines": {
-        "node": ">=18"
-      },
-      "funding": {
-        "url": "https://github.com/sponsors/sindresorhus"
-      }
-    },
     "apps/qc-app/node_modules/@isaacs/cliui": {
       "version": "8.0.2",
       "dev": true,
@@ -333,37 +303,6 @@
         "node": ">=12"
       }
     },
-    "apps/qc-app/node_modules/@jridgewell/gen-mapping": {
-      "version": "0.3.13",
-      "license": "MIT",
-      "dependencies": {
-        "@jridgewell/sourcemap-codec": "^1.5.0",
-        "@jridgewell/trace-mapping": "^0.3.24"
-      }
-    },
-    "apps/qc-app/node_modules/@jridgewell/remapping": {
-      "version": "2.3.5",
-      "license": "MIT",
-      "dependencies": {
-        "@jridgewell/gen-mapping": "^0.3.5",
-        "@jridgewell/trace-mapping": "^0.3.24"
-      }
-    },
-    "apps/qc-app/node_modules/@jridgewell/resolve-uri": {
-      "version": "3.1.2",
-      "license": "MIT",
-      "engines": {
-        "node": ">=6.0.0"
-      }
-    },
-    "apps/qc-app/node_modules/@jridgewell/trace-mapping": {
-      "version": "0.3.31",
-      "license": "MIT",
-      "dependencies": {
-        "@jridgewell/resolve-uri": "^3.1.0",
-        "@jridgewell/sourcemap-codec": "^1.4.14"
-      }
-    },
     "apps/qc-app/node_modules/@mdi/font": {
       "version": "7.4.47",
       "dev": true,
@@ -492,35 +431,6 @@
         "vue": "^3.2.25"
       }
     },
-    "apps/qc-app/node_modules/@vitest/coverage-v8": {
-      "version": "4.1.9",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "@bcoe/v8-coverage": "^1.0.2",
-        "@vitest/utils": "4.1.9",
-        "ast-v8-to-istanbul": "^1.0.0",
-        "istanbul-lib-coverage": "^3.2.2",
-        "istanbul-lib-report": "^3.0.1",
-        "istanbul-reports": "^3.2.0",
-        "magicast": "^0.5.2",
-        "obug": "^2.1.1",
-        "std-env": "^4.0.0-rc.1",
-        "tinyrainbow": "^3.1.0"
-      },
-      "funding": {
-        "url": "https://opencollective.com/vitest"
-      },
-      "peerDependencies": {
-        "@vitest/browser": "4.1.9",
-        "vitest": "4.1.9"
-      },
-      "peerDependenciesMeta": {
-        "@vitest/browser": {
-          "optional": true
-        }
-      }
-    },
     "apps/qc-app/node_modules/@vitest/web-worker": {
       "version": "4.1.9",
       "dev": true,
@@ -788,24 +698,6 @@
         "url": "https://github.com/sponsors/sxzz"
       }
     },
-    "apps/qc-app/node_modules/ast-v8-to-istanbul": {
-      "version": "1.0.0",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "@jridgewell/trace-mapping": "^0.3.31",
-        "estree-walker": "^3.0.3",
-        "js-tokens": "^10.0.0"
-      }
-    },
-    "apps/qc-app/node_modules/ast-v8-to-istanbul/node_modules/estree-walker": {
-      "version": "3.0.3",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "@types/estree": "^1.0.0"
-      }
-    },
     "apps/qc-app/node_modules/ast-walker-scope": {
       "version": "0.9.0",
       "license": "MIT",
@@ -861,14 +753,6 @@
       "dev": true,
       "license": "MIT"
     },
-    "apps/qc-app/node_modules/baseline-browser-mapping": {
-      "version": "2.9.19",
-      "dev": true,
-      "license": "Apache-2.0",
-      "bin": {
-        "baseline-browser-mapping": "dist/cli.js"
-      }
-    },
     "apps/qc-app/node_modules/bidi-js": {
       "version": "1.0.3",
       "license": "MIT",
@@ -896,57 +780,6 @@
         "balanced-match": "^1.0.0"
       }
     },
-    "apps/qc-app/node_modules/browserslist": {
-      "version": "4.28.1",
-      "dev": true,
-      "funding": [
-        {
-          "type": "opencollective",
-          "url": "https://opencollective.com/browserslist"
-        },
-        {
-          "type": "tidelift",
-          "url": "https://tidelift.com/funding/github/npm/browserslist"
-        },
-        {
-          "type": "github",
-          "url": "https://github.com/sponsors/ai"
-        }
-      ],
-      "license": "MIT",
-      "dependencies": {
-        "baseline-browser-mapping": "^2.9.0",
-        "caniuse-lite": "^1.0.30001759",
-        "electron-to-chromium": "^1.5.263",
-        "node-releases": "^2.0.27",
-        "update-browserslist-db": "^1.2.0"
-      },
-      "bin": {
-        "browserslist": "cli.js"
-      },
-      "engines": {
-        "node": "^6 || ^7 || ^8 || ^9 || ^10 || ^11 || ^12 || >=13.7"
-      }
-    },
-    "apps/qc-app/node_modules/caniuse-lite": {
-      "version": "1.0.30001766",
-      "dev": true,
-      "funding": [
-        {
-          "type": "opencollective",
-          "url": "https://opencollective.com/browserslist"
-        },
-        {
-          "type": "tidelift",
-          "url": "https://tidelift.com/funding/github/npm/caniuse-lite"
-        },
-        {
-          "type": "github",
-          "url": "https://github.com/sponsors/ai"
-        }
-      ],
-      "license": "CC-BY-4.0"
-    },
     "apps/qc-app/node_modules/chokidar": {
       "version": "5.0.0",
       "license": "MIT",
@@ -1014,13 +847,6 @@
         "node": ">=20"
       }
     },
-    "apps/qc-app/node_modules/cross-fetch": {
-      "version": "4.1.0",
-      "license": "MIT",
-      "dependencies": {
-        "node-fetch": "^2.7.0"
-      }
-    },
     "apps/qc-app/node_modules/cssstyle": {
       "version": "5.3.7",
       "license": "MIT",
@@ -1056,10 +882,6 @@
         "node": ">=20"
       }
     },
-    "apps/qc-app/node_modules/decimal.js": {
-      "version": "10.6.0",
-      "license": "MIT"
-    },
     "apps/qc-app/node_modules/detect-libc": {
       "version": "1.0.3",
       "dev": true,
@@ -1104,11 +926,6 @@
         "node": ">=14"
       }
     },
-    "apps/qc-app/node_modules/electron-to-chromium": {
-      "version": "1.5.283",
-      "dev": true,
-      "license": "ISC"
-    },
     "apps/qc-app/node_modules/emoji-regex": {
       "version": "9.2.2",
       "dev": true,
@@ -1220,14 +1037,6 @@
         "url": "https://github.com/sponsors/isaacs"
       }
     },
-    "apps/qc-app/node_modules/has-flag": {
-      "version": "4.0.0",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=8"
-      }
-    },
     "apps/qc-app/node_modules/hookable": {
       "version": "5.5.3",
       "license": "MIT"
@@ -1242,11 +1051,6 @@
         "node": "^20.19.0 || ^22.12.0 || >=24.0.0"
       }
     },
-    "apps/qc-app/node_modules/html-escaper": {
-      "version": "2.0.2",
-      "dev": true,
-      "license": "MIT"
-    },
     "apps/qc-app/node_modules/http-proxy-agent": {
       "version": "7.0.2",
       "license": "MIT",
@@ -1282,10 +1086,6 @@
       "version": "2.0.4",
       "license": "ISC"
     },
-    "apps/qc-app/node_modules/is-potential-custom-element-name": {
-      "version": "1.0.1",
-      "license": "MIT"
-    },
     "apps/qc-app/node_modules/is-what": {
       "version": "4.1.16",
       "license": "MIT",
@@ -1300,39 +1100,6 @@
       "version": "1.0.0",
       "license": "MIT"
     },
-    "apps/qc-app/node_modules/istanbul-lib-coverage": {
-      "version": "3.2.2",
-      "dev": true,
-      "license": "BSD-3-Clause",
-      "engines": {
-        "node": ">=8"
-      }
-    },
-    "apps/qc-app/node_modules/istanbul-lib-report": {
-      "version": "3.0.1",
-      "dev": true,
-      "license": "BSD-3-Clause",
-      "dependencies": {
-        "istanbul-lib-coverage": "^3.0.0",
-        "make-dir": "^4.0.0",
-        "supports-color": "^7.1.0"
-      },
-      "engines": {
-        "node": ">=10"
-      }
-    },
-    "apps/qc-app/node_modules/istanbul-reports": {
-      "version": "3.2.0",
-      "dev": true,
-      "license": "BSD-3-Clause",
-      "dependencies": {
-        "html-escaper": "^2.0.0",
-        "istanbul-lib-report": "^3.0.0"
-      },
-      "engines": {
-        "node": ">=8"
-      }
-    },
     "apps/qc-app/node_modules/jackspeak": {
       "version": "3.4.3",
       "dev": true,
@@ -1375,11 +1142,6 @@
         "node": ">=14"
       }
     },
-    "apps/qc-app/node_modules/js-tokens": {
-      "version": "10.0.0",
-      "dev": true,
-      "license": "MIT"
-    },
     "apps/qc-app/node_modules/jsdom": {
       "version": "27.4.0",
       "license": "MIT",
@@ -1417,26 +1179,6 @@
         }
       }
     },
-    "apps/qc-app/node_modules/jsesc": {
-      "version": "3.1.0",
-      "license": "MIT",
-      "bin": {
-        "jsesc": "bin/jsesc"
-      },
-      "engines": {
-        "node": ">=6"
-      }
-    },
-    "apps/qc-app/node_modules/json5": {
-      "version": "2.2.3",
-      "license": "MIT",
-      "bin": {
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-      "resolved": "https://registry.npmjs.org/yargs/-/yargs-18.0.0.tgz",
-      "integrity": "sha512-4UEqdc2RYGHZc7Doyqkrqiln3p9X2DZVxaGbwhn2pi7MrRagKaOcIKe8L3OxYcbhXLgLFUS3zAYuQjKBQgmuNg==",
-      "dev": true,
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-      "dependencies": {
-        "cliui": "^9.0.1",
-        "escalade": "^3.1.1",
-        "get-caller-file": "^2.0.5",
-        "string-width": "^7.2.0",
-        "y18n": "^5.0.5",
-        "yargs-parser": "^22.0.0"
+        "tsc": "bin/tsc",
+        "tsserver": "bin/tsserver"
       },
       "engines": {
-        "node": "^20.19.0 || ^22.12.0 || >=23"
+        "node": ">=14.17"
       }
     },
-    "node_modules/yargs-parser": {
-      "version": "22.0.0",
-      "resolved": "https://registry.npmjs.org/yargs-parser/-/yargs-parser-22.0.0.tgz",
-      "integrity": "sha512-rwu/ClNdSMpkSrUb+d6BRsSkLUq1fmfsY6TOpYzTwvwkg1/NRG85KBy3kq++A8LKQwX6lsu+aWad+2khvuXrqw==",
+    "packages/hydroserver-ts/node_modules/undici-types": {
+      "version": "7.18.2",
+      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-7.18.2.tgz",
+      "integrity": "sha512-AsuCzffGHJybSaRrmr5eHr81mwJU3kjw6M+uprWvCXiNeN9SOGwQ3Jn8jb8m3Z6izVgknn1R0FTCEAP2QrLY/w==",
       "dev": true,
-      "license": "ISC",
-      "engines": {
-        "node": "^20.19.0 || ^22.12.0 || >=23"
-      }
+      "license": "MIT"
     },
-    "node_modules/yargs/node_modules/string-width": {
-      "version": "7.2.0",
-      "resolved": "https://registry.npmjs.org/string-width/-/string-width-7.2.0.tgz",
-      "integrity": "sha512-tsaTIkKW9b4N+AEj+SVA+WhJzV7/zMhcSu78mLKWSk7cXMOSHsBKFWUs0fWwq8QyK3MgJBQRX6Gbi4kYbdvGkQ==",
+    "packages/hydroserver-ts/node_modules/vite": {
+      "version": "7.3.5",
+      "resolved": "https://registry.npmjs.org/vite/-/vite-7.3.5.tgz",
+      "integrity": "sha512-KuOaNhcnGFN2zIPGA7wRmzF+lJA1sea7rHq17aiJ++9lzY1WWG6Jpwqwe1KNbRVPIqHmr8GLYx7jbrQcN/7/ww==",
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       "license": "MIT",
       "dependencies": {
-        "emoji-regex": "^10.3.0",
-        "get-east-asian-width": "^1.0.0",
-        "strip-ansi": "^7.1.0"
+        "esbuild": "^0.27.0",
+        "fdir": "^6.5.0",
+        "picomatch": "^4.0.3",
+        "postcss": "^8.5.6",
+        "rollup": "^4.43.0",
+        "tinyglobby": "^0.2.15"
+      },
+      "bin": {
+        "vite": "bin/vite.js"
       },
       "engines": {
-        "node": ">=18"
+        "node": "^20.19.0 || >=22.12.0"
       },
       "funding": {
-        "url": "https://github.com/sponsors/sindresorhus"
+        "url": "https://github.com/vitejs/vite?sponsor=1"
+      },
+      "optionalDependencies": {
+        "fsevents": "~2.3.3"
+      },
+      "peerDependencies": {
+        "@types/node": "^20.19.0 || >=22.12.0",
+        "jiti": ">=1.21.0",
+        "less": "^4.0.0",
+        "lightningcss": "^1.21.0",
+        "sass": "^1.70.0",
+        "sass-embedded": "^1.70.0",
+        "stylus": ">=0.54.8",
+        "sugarss": "^5.0.0",
+        "terser": "^5.16.0",
+        "tsx": "^4.8.1",
+        "yaml": "^2.4.2"
+      },
+      "peerDependenciesMeta": {
+        "@types/node": {
+          "optional": true
+        },
+        "jiti": {
+          "optional": true
+        },
+        "less": {
+          "optional": true
+        },
+        "lightningcss": {
+          "optional": true
+        },
+        "sass": {
+          "optional": true
+        },
+        "sass-embedded": {
+          "optional": true
+        },
+        "stylus": {
+          "optional": true
+        },
+        "sugarss": {
+          "optional": true
+        },
+        "terser": {
+          "optional": true
+        },
+        "tsx": {
+          "optional": true
+        },
+        "yaml": {
+          "optional": true
+        }
       }
     },
-    "node_modules/yocto-queue": {
-      "version": "0.1.0",
-      "resolved": "https://registry.npmjs.org/yocto-queue/-/yocto-queue-0.1.0.tgz",
-      "integrity": "sha512-rVksvsnNCdJ/ohGc6xgPwyN8eheCxsiLM8mxuE/t/mOVqJewPuO1miLpTHQiRgTKCLexL4MeAFVagts7HmNZ2Q==",
+    "packages/hydroserver-ts/node_modules/yocto-queue": {
+      "version": "1.2.2",
+      "resolved": "https://registry.npmjs.org/yocto-queue/-/yocto-queue-1.2.2.tgz",
+      "integrity": "sha512-4LCcse/U2MHZ63HAJVE+v71o7yOdIe4cZ70Wpf8D/IyjDKYQLV5GD46B+hSTjJsvV5PztjvHoU580EftxjDZFQ==",
       "dev": true,
       "license": "MIT",
       "engines": {
-        "node": ">=10"
+        "node": ">=12.20"
       },
       "funding": {
         "url": "https://github.com/sponsors/sindresorhus"

From f2f8e0f3f45ba6e80f336d481adf5f34fd5fa368 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 3 Aug 2026 16:24:22 -0600
Subject: [PATCH 10/93] feat: add session history navigation with per-operation
 notes and attribution

---
 apps/qc-app/docs/API_REFERENCE.md             |  34 +-
 apps/qc-app/docs/ARCHITECTURE.md              |  48 +++
 .../src/components/EditData/EditHistory.vue   | 167 ++++++++--
 .../src/components/EditData/SessionList.vue   | 146 ++++++---
 .../EditData/StartEditingDialog.vue           | 102 +++++-
 .../EditData/__tests__/EditHistory.spec.ts    | 244 +++++++++++++++
 .../EditData/__tests__/SessionList.spec.ts    | 131 +++++++-
 .../__tests__/StartEditingDialog.spec.ts      |  60 +++-
 apps/qc-app/src/components/VisualizeData.vue  |  75 ++++-
 .../__tests__/useEditSession.spec.ts          | 119 ++++++-
 .../__tests__/useResumeEditSession.spec.ts    | 148 +++++++++
 .../useUnsavedChangesWarning.spec.ts          |  60 ++++
 apps/qc-app/src/composables/useEditSession.ts |  71 ++++-
 .../src/composables/useManagedDatastreams.ts  |  18 +-
 .../src/composables/useResumeEditSession.ts   |  58 ++++
 .../composables/useUnsavedChangesWarning.ts   |  21 ++
 .../__tests__/persistOperations.spec.ts       |  47 ++-
 .../qualityControl/__tests__/qcServiceFake.ts |  31 +-
 .../__tests__/reconstructSession.spec.ts      | 296 +++++++++++++++++-
 .../qualityControl/__tests__/session.spec.ts  |  94 +++++-
 .../qualityControl/persistOperations.ts       |  20 +-
 .../qualityControl/reconstructSession.ts      |  81 ++++-
 .../src/services/qualityControl/session.ts    |  20 +-
 .../store/__tests__/dataVisualization.spec.ts |  67 ++++
 .../qc-app/src/store/__tests__/plotly.spec.ts |  42 +++
 .../src/store/__tests__/qcSession.spec.ts     |  87 ++++-
 apps/qc-app/src/store/dataVisualization.ts    |  32 ++
 apps/qc-app/src/store/plotly.ts               |  12 +-
 apps/qc-app/src/store/qcSession.ts            |  62 +++-
 apps/qc-app/src/utils/__tests__/time.spec.ts  |  38 ++-
 apps/qc-app/src/utils/plotting/options.ts     |   2 -
 apps/qc-app/src/utils/time.ts                 |  55 +++-
 packages/qc-utils/docs/QC_HISTORY.md          |   6 +
 packages/qc-utils/src/types/index.ts          |  16 +
 .../utils/plotting/__tests__/history.spec.ts  | 100 ++++++
 .../qc-utils/src/utils/plotting/history.ts    |  19 ++
 36 files changed, 2479 insertions(+), 150 deletions(-)
 create mode 100644 apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts
 create mode 100644 apps/qc-app/src/composables/__tests__/useUnsavedChangesWarning.spec.ts
 create mode 100644 apps/qc-app/src/composables/useResumeEditSession.ts
 create mode 100644 apps/qc-app/src/composables/useUnsavedChangesWarning.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 1494c4a4..4a86bf08 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -112,6 +112,35 @@ await setSelected([0, 1, 2, 5])      // dispatches SELECTION
 await clearSelected({ recordHistory: false })  // skip history append on cleanup
 ```
 
+### `useResumeEditSession()`
+
+```ts
+const { resume } = useResumeEditSession(enterEdit)
+```
+
+Reopens the editor after a page reload, using the persisted
+`qcSession.resumeDatastreamId`: replots that datastream, makes it the QC
+target, then calls the supplied `enterEdit`. The workspace catalog loads
+asynchronously and is empty at mount, so it waits for the catalog to arrive
+and resumes at most once. A pointer to a datastream missing from the catalog
+(deleted, or another workspace) is dropped rather than retried.
+
+Note the watcher must not use Vue's `once` together with `immediate`: the
+immediate call fires on the initial empty catalog and stops the watcher, so
+the resume would never run on the cold reload it exists for.
+
+### `useUnsavedChangesWarning()`
+
+```ts
+useUnsavedChangesWarning(hasUnsavedChanges) // Ref<boolean>
+```
+
+Asks the browser for its native "leave site?" confirmation while the ref is
+true, so a reload mid-session can't silently drop edits that never reached
+the server. Registers on mount and removes the listener on unmount. Browsers
+ignore any custom message and only honour the prompt once the user has
+interacted with the page.
+
 ### `useQcHistory()`
 
 ```ts
@@ -282,6 +311,7 @@ on boot.
 | `setPlottedDatastreams`             | action   | `(items: Datastream[], qcId?: string \| null) => Promise<void>` | Wholesale replace; used by URL hydration. |
 | `setQcDatastream`                   | action   | `(id: string \| null) => Promise<void>`           | Change QC target; preserves the current zoom. |
 | `adoptManagedDatastream`            | action   | `(managed: Datastream, sourceId: string) => Promise<void>` | Enter editing on a freshly-created managed datastream: replace the source in the plot and re-key its already-loaded series as the managed datastream's working copy (no second, empty item; no re-fetch). |
+| `releaseManagedDatastream`          | action   | `() => Promise<void>`                             | Inverse of `adoptManagedDatastream`, for leaving the editor: swap the managed datastream back to its source (resolved through `qcHistories`), drop the editor's working copy and rebuild, so the plot shows the source as stored rather than the session's uncommitted edits. Managed datastreams are hidden from the catalog table, so without this the Select view shows a plot with nothing selected. No-op when the QC target isn't managed or its source isn't in the catalog. |
 | `rebuildPlot`                       | action   | `() => Promise<void>`                             | Serialized rebuild (drop zoom history, refresh series, regenerate options, render). Coalesces concurrent callers. |
 
 ### `usePlotlyStore()` — `src/store/plotly.ts`
@@ -490,11 +520,13 @@ puts the editor in read-only mode.
 | Name                | Kind     | Type / signature                        | Notes |
 |---------------------|----------|-----------------------------------------|-------|
 | `historyId`         | state    | `string \| null`                        | The managed datastream's QC history being navigated. |
+| `resumeDatastreamId`| state    | `string \| null`                        | Managed datastream the editor was last open on. The only persisted field: a page reload replots it and resumes its session from the last save. Set on entering the editor, cleared on exit. |
 | `sessions`          | state    | `QualityControlSession[]`               | Committed + in-progress sessions for the history. |
 | `currentSessionId`  | state    | `string \| null`                        | The single in-progress (editable) session. |
 | `viewedSessionId`   | state    | `string \| null`                        | The session currently being viewed. |
 | `isLoading`         | state    | `boolean`                               | True while `loadSessions` is in flight. |
-| `isReadOnly`        | computed | `boolean`                               | True unless viewing the in-progress session. |
+| `isSwitchingSession`| state    | `boolean`                               | True while another session's data and operations load. The operations panel renders a loading state instead of the outgoing session's entries, which would otherwise linger and read as the incoming session's. |
+| `isReadOnly`        | computed | `boolean`                               | True when sessions exist and the viewed one isn't the in-progress session. Guarded on `sessions.length` so plain editing outside the session workflow isn't treated as read-only. |
 | `inProgressSession` | computed | `QualityControlSession \| null`         | The editable session, if any. |
 | `committedSessions` | computed | `QualityControlSession[]`               | Sessions with status `committed`. |
 | `viewedSession`     | computed | `QualityControlSession \| null`         | The session for `viewedSessionId`. |
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 70449872..22e75981 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -254,6 +254,14 @@ Two contract notes worth keeping in mind:
   `{ method, args }`; the QC API speaks `{ operationType, arguments, order }`.
   The enum values are identical, so `persistOperations`/`reconstructSession`
   rename the fields when crossing between qc-utils and the API.
+- **Operation comments are part of the history, not app-side metadata.**
+  `HistoryItem.comment` is authored in the operations panel, so it rides
+  through `serializeHistory` into `persistOperations` and lands on the API's
+  `comment` field, and exported QC History files carry it. Comments are the one
+  part of a persisted operation that is patched in place, since they are
+  written after the operation ran; everything else stays append-only. The API
+  refuses updates on committed sessions, so the panel renders their comments
+  read-only.
 - **Sessions start/resume from the latest committed state, not the raw source.**
   Because each commit replays its session into the managed datastream (in-range
   `replace`), the managed datastream's observations already carry every
@@ -261,6 +269,46 @@ Two contract notes worth keeping in mind:
   managed datastream as the working base (via `loadLatestBase`, falling back to
   the source only when nothing has been committed yet) and replay just the
   current session's own draft operations on top.
+- **A reload resumes from the last save, not from memory.** Only
+  `qcSession.resumeDatastreamId` is persisted; on load the editor replots that
+  datastream and re-runs `beginEditing`, which rebuilds the working copy from
+  the server via `reconstructSession`. Edits made since the last save are not
+  recoverable, so `useUnsavedChangesWarning` raises the browser's native
+  confirmation while `hasUnsavedChanges` is true. A deliberate exit clears the
+  pointer, so only an interrupted session reopens.
+- **Viewing a past session replays its ancestor chain from the source.**
+  The managed datastream carries every commit, so it cannot be the base for a
+  historical view: replaying an older session's operations on top of it would
+  reproduce the final state. `reconstructCommittedSession` instead fetches the
+  ancestor closure (`ancestor_of`), loads the raw source over the union of
+  every window in the chain, and replays the chain in **commit order**
+  (`committedAt`, falling back to `createdAt`) — committing is what writes
+  observations into the managed datastream, so it is commit order, not
+  authoring order, that decides what a later session built on. The union
+  window matters because operations replay against array indices: loading
+  only the viewed session's window would misalign a wider ancestor's
+  selections and corrupt the result silently. The panel then shows just the
+  viewed session's own operations; the ancestors produced the data, but the
+  history is about what this session did.
+- **Operations are attributed by the server.** Every `QCOperation` carries a
+  `created_by`, stamped from the authenticated user on create, and the
+  response resolves a deleted account to a placeholder contact rather than
+  null. `reconstructSession`/`reconstructCommittedSession` map it onto
+  `HistoryItem.performedBy` (name, falling back to email) so the operations
+  panel can attribute each row. It is never sent back: the field is
+  provenance, not input. Operations applied in the current session show no
+  attribution until they are saved and reloaded. Comments have no author of
+  their own — `comment` is a plain nullable column that can be rewritten
+  later — so "who wrote this note" is a pending backend ask.
+- **A commit is terminal.** The API rejects updating, deleting, adding
+  operations to, or re-committing a committed session, and its PATCH body
+  carries only `description`. Continuing work after a commit means starting a
+  new session; the backend links it to every committed session its window
+  overlaps, which is how the DAG gets built. Reopening the most recent commit
+  is a pending backend ask (see the TODO in `store/qcSession.ts`) and needs
+  more than lifting the status guard, since a commit also writes observations
+  to the managed datastream and rolls the history's checksum and extent
+  forward.
 
 Tests stub the three services with `makeQcFake()` (a stateful in-memory double
 under `services/qualityControl/__tests__/` that returns
diff --git a/apps/qc-app/src/components/EditData/EditHistory.vue b/apps/qc-app/src/components/EditData/EditHistory.vue
index ffee4082..71ffec34 100644
--- a/apps/qc-app/src/components/EditData/EditHistory.vue
+++ b/apps/qc-app/src/components/EditData/EditHistory.vue
@@ -38,7 +38,7 @@
             variant="text"
             density="comfortable"
             icon="mdi-undo-variant"
-            :disabled="isUpdating || !canUndo"
+            :disabled="isUpdating || isReadOnly || !canUndo"
             @click.stop="onUndo"
           />
         </template>
@@ -54,7 +54,7 @@
             variant="text"
             density="comfortable"
             icon="mdi-redo-variant"
-            :disabled="isUpdating || !canRedo"
+            :disabled="isUpdating || isReadOnly || !canRedo"
             @click.stop="onRedo"
           />
         </template>
@@ -86,7 +86,7 @@
             variant="text"
             density="comfortable"
             icon="mdi-tray-arrow-up"
-            :disabled="isUpdating"
+            :disabled="isUpdating || isReadOnly"
             @click.stop="onLoadHistoryClick"
           />
         </template>
@@ -122,9 +122,9 @@
       class="flex-grow-1 overflow-y-auto pa-2"
       style="min-height: 0"
     >
-      <SessionList class="mb-2" />
-
-     <div class="rounded border bg-surface overflow-hidden">
+      <SessionList @view="emit('view-session', $event)">
+        <template #operations>
+          <div class="rounded border bg-surface overflow-hidden">
       <div
         class="edit-history__row edit-history__row--baseline px-3 py-2 d-flex align-center"
       >
@@ -162,7 +162,7 @@
               variant="text"
               density="comfortable"
               icon="mdi-reload"
-              :disabled="isUpdating"
+              :disabled="isUpdating || isReadOnly"
               @click="onReload"
             />
           </template>
@@ -171,7 +171,19 @@
 
       <v-divider />
 
-      <div v-if="editCount === 0" class="pa-4 text-center">
+      <!-- Or the outgoing session's operations linger as if they were these. -->
+      <div
+        v-if="isSwitchingSession"
+        class="pa-4 text-center"
+        data-testid="history-loading"
+      >
+        <v-progress-circular indeterminate color="primary" size="24" />
+        <div class="text-body-small text-medium-emphasis mt-2">
+          Loading session…
+        </div>
+      </div>
+
+      <div v-else-if="editCount === 0" class="pa-4 text-center">
         <v-icon icon="mdi-clock-outline" size="28" color="grey" class="mb-2" />
         <div class="text-body-small text-medium-emphasis">
           Edit operations will appear here.
@@ -189,6 +201,7 @@
             :class="{
               'edit-history__row--loading': entry.execution?.inFlight,
               'edit-history__row--open': openIndex === index,
+              'edit-history__row--loaded': shownStepIndex === index,
             }"
           >
             <button
@@ -226,7 +239,47 @@
               {{ formatMethod(entry.method) }}
             </span>
 
+            <v-chip
+              v-if="shownStepIndex === index"
+              size="x-small"
+              color="primary"
+              variant="tonal"
+              label
+              class="mr-1 flex-shrink-0"
+              :data-testid="`history-loaded-${index}`"
+            >
+              Showing
+            </v-chip>
+
             <div class="d-flex align-center ga-2 flex-shrink-0">
+              <v-tooltip
+                v-if="entry.performedBy"
+                location="start"
+                :text="`Applied by ${entry.performedBy}`"
+              >
+                <template #activator="{ props: tp }">
+                  <span
+                    v-bind="tp"
+                    class="edit-history__author text-body-small text-medium-emphasis text-truncate"
+                    :data-testid="`history-author-${index}`"
+                  >
+                    {{ entry.performedBy }}
+                  </span>
+                </template>
+              </v-tooltip>
+
+              <v-tooltip v-if="entry.comment" location="start" :text="entry.comment">
+                <template #activator="{ props: tp }">
+                  <v-icon
+                    v-bind="tp"
+                    :data-testid="`history-comment-badge-${index}`"
+                    icon="mdi-comment-text-outline"
+                    size="14"
+                    color="primary"
+                  />
+                </template>
+              </v-tooltip>
+
               <v-tooltip
                 v-if="entry.execution?.status === 'failed'"
                 location="start"
@@ -288,10 +341,10 @@
                 </template>
               </v-tooltip>
 
-              <!-- Per-item undo only on the trailing entry; middle
-                   entries use "Reload from this step". -->
+              <!-- Trailing entry only; middle entries use "Reload from this
+                   step". Never on a committed session. -->
               <v-tooltip
-                v-if="index === editHistory.length - 1"
+                v-if="index === editHistory.length - 1 && !isReadOnly"
                 location="start"
                 text="Undo this step"
               >
@@ -325,12 +378,47 @@
                 <code class="text-body-small">{{ formatArg(arg) }}</code>
               </li>
             </ul>
+
+            <div
+              v-if="entry.performedBy"
+              class="text-body-small text-medium-emphasis mt-3"
+              :data-testid="`history-author-detail-${index}`"
+            >
+              Applied by {{ entry.performedBy }}
+            </div>
+
+            <div class="text-body-small text-medium-emphasis mt-3 mb-1">
+              Comment
+            </div>
+            <v-textarea
+              v-if="!isReadOnly"
+              :model-value="entry.comment ?? ''"
+              :data-testid="`history-comment-${index}`"
+              placeholder="Why was this operation applied?"
+              variant="outlined"
+              density="compact"
+              rows="2"
+              auto-grow
+              hide-details
+              class="text-body-small"
+              @update:model-value="setComment(entry, $event)"
+            />
+            <div
+              v-else
+              class="text-body-small"
+              :class="{ 'text-medium-emphasis font-italic': !entry.comment }"
+              :data-testid="`history-comment-readonly-${index}`"
+            >
+              {{ entry.comment || 'No comment.' }}
+            </div>
           </div>
 
           <v-divider />
         </div>
       </div>
-     </div>
+          </div>
+        </template>
+      </SessionList>
     </div>
   </div>
 </template>
@@ -346,7 +434,9 @@ import { useUIStore } from '@/store/userInterface'
 import { iconForMethod, colorForMethod } from '@/components/EditData/operations'
 import SessionList from '@/components/EditData/SessionList.vue'
 import { useQcHistory } from '@/composables/useQcHistory'
+import { useQcSessionStore } from '@/store/qcSession'
 import { Snackbar } from '@uwrl/qc-utils'
+import type { HistoryItem } from '@uwrl/qc-utils'
 
 const props = withDefaults(
   defineProps<{
@@ -364,6 +454,7 @@ const props = withDefaults(
 const emit = defineEmits<{
   (e: 'update:collapsed', value: boolean): void
   (e: 'pop-out'): void
+  (e: 'view-session', sessionId: string): void
 }>()
 
 const isCollapsed = computed(() => props.collapsible && !!props.collapsed)
@@ -392,6 +483,25 @@ const fileInputRef = ref<HTMLInputElement | null>(null)
 
 const openIndex = ref<number | null>(null)
 
+/** Invalidated by any mutation of the history. */
+const loadedStepIndex = ref<number | null>(null)
+
+/** The step the plot reflects: the explicit choice, else the last entry. */
+const shownStepIndex = computed<number | null>(() => {
+  const last = editHistory.value.length - 1
+  if (last < 0) return null
+  const chosen = loadedStepIndex.value
+  return chosen !== null && chosen <= last ? chosen : last
+})
+
+// Committed sessions are immutable server-side, so their comments are shown
+// but not editable.
+const { isReadOnly, isSwitchingSession } = storeToRefs(useQcSessionStore())
+
+function setComment(entry: HistoryItem, value: string) {
+  entry.comment = value
+}
+
 const isDev = import.meta.env.DEV
 
 const editCount = computed(() => editHistory.value?.length ?? 0)
@@ -437,6 +547,8 @@ function formatArg(arg: unknown): string {
 }
 
 const onReload = async () => {
+  loadedStepIndex.value = null
+  if (isReadOnly.value || isUpdating.value) return
   isUpdating.value = true
   closeStaleStagingPanel()
 
@@ -461,14 +573,19 @@ const onReloadHistory = async (index: number) => {
   if (index < editHistory.value.length) {
     isUpdating.value = true
     closeStaleStagingPanel()
+    // `reloadHistory` truncates to `0..index`; a committed session's
+    // operations must survive stepping through them.
+    const record = selectedSeries.value?.data
+    const preserved = isReadOnly.value ? [...(record?.history ?? [])] : null
+    loadedStepIndex.value = index
     setTimeout(async () => {
-      const newSelection = await selectedSeries.value?.data.reloadHistory(index)
+      const newSelection = await record?.reloadHistory(index)
+      if (preserved && record) {
+        record.history.splice(0, record.history.length, ...preserved)
+      }
 
       isUpdating.value = false
-      await redraw()
-      if (newSelection) {
-        setPlotSelection(newSelection)
-      }
+      await applyReplayedSelection(newSelection)
     })
   }
 }
@@ -538,7 +655,9 @@ const closeStaleStagingPanel = () => {
 }
 
 const onUndo = async () => {
-  if (!canUndo.value || isUpdating.value) return
+  loadedStepIndex.value = null
+  // Also guards the Ctrl+Z shortcut, which bypasses the disabled button.
+  if (isReadOnly.value || !canUndo.value || isUpdating.value) return
   isUpdating.value = true
   closeStaleStagingPanel()
   setTimeout(async () => {
@@ -552,7 +671,8 @@ const onUndo = async () => {
 }
 
 const onRedo = async () => {
-  if (!canRedo.value || isUpdating.value) return
+  loadedStepIndex.value = null
+  if (isReadOnly.value || !canRedo.value || isUpdating.value) return
   isUpdating.value = true
   closeStaleStagingPanel()
   setTimeout(async () => {
@@ -619,6 +739,15 @@ onBeforeUnmount(() => window.removeEventListener('keydown', onKeydown))
   background-color: rgba(var(--v-theme-primary), 0.04);
 }
 
+.edit-history__author {
+  max-width: 8rem;
+}
+
+.edit-history__row--loaded {
+  background-color: rgba(var(--v-theme-primary), 0.1);
+  box-shadow: inset 3px 0 0 0 rgb(var(--v-theme-primary));
+}
+
 .edit-history__row--open {
   background-color: rgba(var(--v-theme-primary), 0.06);
 }
diff --git a/apps/qc-app/src/components/EditData/SessionList.vue b/apps/qc-app/src/components/EditData/SessionList.vue
index c0467145..6166eab8 100644
--- a/apps/qc-app/src/components/EditData/SessionList.vue
+++ b/apps/qc-app/src/components/EditData/SessionList.vue
@@ -1,6 +1,6 @@
 <template>
   <div class="qc-session-list">
-    <div class="qc-session-list__header px-3 py-2 d-flex align-center ga-2">
+    <div class="qc-session-list__header px-1 py-1 d-flex align-center ga-2">
       <v-icon icon="mdi-source-branch" color="primary" size="16" />
       <span class="text-body-small font-weight-medium">Sessions</span>
       <v-spacer />
@@ -16,8 +16,6 @@
       </v-btn>
     </div>
 
-    <v-divider />
-
     <div
       v-if="!sessions.length"
       class="pa-3 text-center text-body-small text-medium-emphasis"
@@ -25,32 +23,46 @@
       No sessions yet.
     </div>
 
-    <v-list v-else density="compact" class="py-0" :lines="false">
-      <v-list-item
+    <v-timeline
+      v-else
+      side="end"
+      density="compact"
+      truncate-line="both"
+      line-thickness="2"
+      class="qc-session-list__timeline px-2 py-2"
+    >
+      <v-timeline-item
         v-for="session of orderedSessions"
         :key="session.id"
-        :active="session.id === viewedSessionId"
+        :dot-color="session.status === 'in_progress' ? 'warning' : 'success'"
+        :icon="
+          session.status === 'in_progress' ? 'mdi-pencil' : 'mdi-check'
+        "
+        size="x-small"
+        width="100%"
         :data-testid="`session-${session.id}`"
-        @click="onSelect(session)"
+        class="qc-session-list__item"
+        :class="{
+          'qc-session-list__item--viewed': session.id === viewedSessionId,
+        }"
       >
-        <template #prepend>
-          <v-icon
-            :icon="
-              session.status === 'in_progress'
-                ? 'mdi-pencil-circle'
-                : 'mdi-check-circle'
-            "
-            :color="session.status === 'in_progress' ? 'warning' : 'success'"
-            size="16"
-            class="mr-2"
-          />
-        </template>
-
-        <v-list-item-title class="text-body-small">
-          {{ sessionLabel(session) }}
-        </v-list-item-title>
-
-        <template #append>
+        <!-- Only the header selects; clicks inside the nested panel must not. -->
+        <div
+          class="d-flex align-center ga-2 cursor-pointer"
+          :data-testid="`session-header-${session.id}`"
+          @click="onSelect(session)"
+        >
+          <div class="flex-grow-1" style="min-width: 0">
+            <div class="text-body-small font-weight-medium text-truncate">
+              {{ sessionLabel(session) }}
+            </div>
+            <div
+              v-if="session.description"
+              class="text-body-small text-medium-emphasis text-truncate"
+            >
+              {{ sessionPeriod(session) }}
+            </div>
+          </div>
           <v-chip
             v-if="session.id === currentSessionId"
             size="x-small"
@@ -63,9 +75,32 @@
           <v-chip v-else size="x-small" color="grey" variant="tonal" label>
             View
           </v-chip>
-        </template>
-      </v-list-item>
-    </v-list>
+        </div>
+
+        <div v-if="session.id === viewedSessionId" class="mt-2">
+          <slot name="operations" />
+        </div>
+        <div
+          v-else-if="operationCount(session)"
+          class="text-body-small text-medium-emphasis mt-1 cursor-pointer"
+          :data-testid="`session-preview-${session.id}`"
+          @click="onSelect(session)"
+        >
+          {{ previewLabel(session) }}
+        </div>
+        <div
+          v-else
+          class="text-body-small text-medium-emphasis font-italic mt-1"
+          :data-testid="`session-preview-${session.id}`"
+        >
+          No operations.
+        </div>
+      </v-timeline-item>
+    </v-timeline>
+
+    <div v-if="!viewedSessionId" class="pa-2">
+      <slot name="operations" />
+    </div>
   </div>
 </template>
 
@@ -80,37 +115,58 @@ const store = useQcSessionStore()
 const { sessions, currentSessionId, viewedSessionId, isReadOnly } =
   storeToRefs(store)
 
-// Newest first, by phenomenon start (ISO sorts chronologically).
+// Oldest first by creation time. Phenomenon start orders by the edited
+// window, which says nothing about recency and ties on equal windows.
 const orderedSessions = computed(() =>
-  [...sessions.value].sort((a, b) =>
-    b.phenomenonTimeStart.localeCompare(a.phenomenonTimeStart)
-  )
+  [...sessions.value].sort((a, b) => a.createdAt.localeCompare(b.createdAt))
 )
 
+const sessionPeriod = (session: QualityControlSession): string =>
+  formatDateRange(session.phenomenonTimeStart, session.phenomenonTimeEnd)
+
+const operationCount = (session: QualityControlSession): number =>
+  (session as { operations?: unknown[] }).operations?.length ?? 0
+
+function previewLabel(session: QualityControlSession): string {
+  const total = operationCount(session)
+  return `${total} operation${total === 1 ? '' : 's'}`
+}
+
 function sessionLabel(session: QualityControlSession): string {
-  if (session.description) return session.description
-  return formatDateRange(
-    session.phenomenonTimeStart,
-    session.phenomenonTimeEnd
-  )
+  return session.description || sessionPeriod(session)
 }
 
+const emit = defineEmits<{ (e: 'view', sessionId: string): void }>()
+
 function onSelect(session: QualityControlSession): void {
-  if (session.id === store.currentSessionId) {
-    store.returnToCurrent()
-  } else {
-    store.viewSession(session.id)
-  }
+  emit('view', session.id)
 }
 
 function returnToCurrent(): void {
-  store.returnToCurrent()
+  if (currentSessionId.value) emit('view', currentSessionId.value)
 }
 </script>
 
 <style scoped>
 .qc-session-list__header {
-  background-color: rgba(var(--v-theme-primary), 0.04);
-  min-height: 32px;
+  min-height: 28px;
+}
+
+.qc-session-list__timeline :deep(.v-timeline-item__body) {
+  padding-block: 2px;
+}
+
+.qc-session-list__item :deep(.v-timeline-item__body) {
+  border-radius: 6px;
+  padding-inline: 6px;
+  transition: background-color 120ms ease;
+}
+
+.qc-session-list__item:hover :deep(.v-timeline-item__body) {
+  background-color: rgba(var(--v-theme-primary), 0.06);
+}
+
+.qc-session-list__item--viewed :deep(.v-timeline-item__body) {
+  background-color: rgba(var(--v-theme-primary), 0.12);
 }
 </style>
diff --git a/apps/qc-app/src/components/EditData/StartEditingDialog.vue b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
index 00e63be2..ccbb00ba 100644
--- a/apps/qc-app/src/components/EditData/StartEditingDialog.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
@@ -43,13 +43,14 @@
             </div>
             <v-spacer />
             <v-btn
+              v-if="!hasInProgress(opt)"
               size="small"
               color="primary"
               variant="flat"
               :data-testid="`edit-managed-${opt.managed.id}`"
               @click="emit('edit', opt)"
             >
-              {{ hasInProgress(opt) ? 'Continue session' : 'Start new session' }}
+              Start new session
             </v-btn>
             <v-btn
               icon="mdi-trash-can-outline"
@@ -125,18 +126,76 @@
               <v-list-item-title class="text-body-small">
                 {{ sessionLabel(s) }}
               </v-list-item-title>
+              <v-list-item-subtitle v-if="s.description" class="text-body-small">
+                {{ sessionPeriod(s) }}
+              </v-list-item-subtitle>
               <template #append>
-                <v-chip
-                  size="x-small"
-                  :color="s.status === 'in_progress' ? 'warning' : 'grey'"
-                  variant="tonal"
-                  label
-                >
-                  {{ s.status === 'in_progress' ? 'In progress' : 'Committed' }}
-                </v-chip>
+                <div class="d-flex align-center ga-2">
+                  <v-chip
+                    size="x-small"
+                    :color="s.status === 'in_progress' ? 'warning' : 'grey'"
+                    variant="tonal"
+                    label
+                  >
+                    {{ s.status === 'in_progress' ? 'In progress' : 'Committed' }}
+                  </v-chip>
+                  <template v-if="s.status === 'in_progress'">
+                    <v-btn
+                      size="small"
+                      color="primary"
+                      variant="flat"
+                      :data-testid="`continue-session-${s.id}`"
+                      @click="emit('edit', opt)"
+                    >
+                      Continue
+                    </v-btn>
+                    <v-btn
+                      icon="mdi-trash-can-outline"
+                      size="x-small"
+                      variant="text"
+                      color="error"
+                      :data-testid="`delete-session-${s.id}`"
+                      title="Discard this in-progress session"
+                      @click="confirmingSessionId = s.id"
+                    />
+                  </template>
+                </div>
               </template>
             </v-list-item>
           </v-list>
+
+          <v-alert
+            v-if="confirmingSession(opt)"
+            type="warning"
+            variant="tonal"
+            density="compact"
+            class="mx-3 my-2"
+          >
+            <div class="d-flex align-center flex-wrap ga-2">
+              <span class="text-body-small">
+                Discard this in-progress session and its unsaved edits? Committed
+                sessions are unaffected. This can't be undone.
+              </span>
+              <v-spacer />
+              <v-btn
+                size="x-small"
+                variant="text"
+                data-testid="cancel-delete-session"
+                @click="confirmingSessionId = null"
+              >
+                Cancel
+              </v-btn>
+              <v-btn
+                size="x-small"
+                color="error"
+                variant="flat"
+                :data-testid="`confirm-delete-session-${confirmingSessionId}`"
+                @click="onDeleteSession(opt)"
+              >
+                Discard
+              </v-btn>
+            </div>
+          </v-alert>
         </v-card>
 
         <v-btn
@@ -178,18 +237,32 @@ defineProps<{
 const emit = defineEmits<{
   (e: 'edit', option: ManagedDatastreamOption): void
   (e: 'delete', option: ManagedDatastreamOption): void
+  (e: 'deleteSession', option: ManagedDatastreamOption, sessionId: string): void
   (e: 'create'): void
   (e: 'cancel'): void
 }>()
 
 // historyId of the managed datastream whose delete is awaiting confirmation.
 const confirmingDeleteId = ref<string | null>(null)
+// id of the in-progress session whose discard is awaiting confirmation.
+const confirmingSessionId = ref<string | null>(null)
 
 function onDelete(opt: ManagedDatastreamOption) {
   confirmingDeleteId.value = null
   emit('delete', opt)
 }
 
+/** True when this option owns the session awaiting discard confirmation. */
+const confirmingSession = (opt: ManagedDatastreamOption) =>
+  !!confirmingSessionId.value &&
+  opt.sessions.some((s) => s.id === confirmingSessionId.value)
+
+function onDeleteSession(opt: ManagedDatastreamOption) {
+  const sessionId = confirmingSessionId.value
+  confirmingSessionId.value = null
+  if (sessionId) emit('deleteSession', opt, sessionId)
+}
+
 const NUMBER = new Intl.NumberFormat()
 
 // One-line recap so multiple managed datastreams are distinguishable:
@@ -215,13 +288,12 @@ function summary(opt: ManagedDatastreamOption): string {
 const hasInProgress = (opt: ManagedDatastreamOption) =>
   opt.sessions.some((s) => s.status === 'in_progress')
 
-// Newest first by phenomenon start (ISO sorts chronologically).
 const orderedSessions = (sessions: QualityControlSession[]) =>
-  [...sessions].sort((a, b) =>
-    b.phenomenonTimeStart.localeCompare(a.phenomenonTimeStart)
-  )
+  [...sessions].sort((a, b) => a.createdAt.localeCompare(b.createdAt))
 
-const sessionLabel = (s: QualityControlSession) =>
-  s.description ||
+const sessionPeriod = (s: QualityControlSession) =>
   formatDateRange(s.phenomenonTimeStart, s.phenomenonTimeEnd)
+
+const sessionLabel = (s: QualityControlSession) =>
+  s.description || sessionPeriod(s)
 </script>
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
index 375be285..26db515d 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
@@ -3,6 +3,12 @@ import { ref } from 'vue'
 import { describe, it, expect, beforeEach, vi } from 'vitest'
 import { createTestPinia } from '@/utils/test/pinia'
 import { createTestVuetify } from '@/utils/test/vuetify'
+// The per-operation comment textarea (Vuetify auto-grow) observes resizes.
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
 
 const editHistory = ref<any[]>([])
 const selectedSeries = ref<any>(null)
@@ -370,4 +376,242 @@ describe('EditHistory.vue actions', () => {
     await wrapper.find('.edit-history__header').trigger('keydown.enter')
     expect(wrapper.emitted('update:collapsed')).toBeTruthy()
   })
+
+  // A committed session's operations are a record of what happened, not a
+  // draft: nothing in the panel may rewrite them.
+  describe('read-only session', () => {
+    const readOnly = async () => {
+      const { useQcSessionStore } = await import('@/store/qcSession')
+      const store = useQcSessionStore()
+      // `createdAt` is required: SessionList orders on it.
+      store.sessions = [
+        {
+          id: 'a',
+          status: 'committed',
+          createdAt: '2025-01-01T00:00:00Z',
+          phenomenonTimeStart: '2025-01-01T00:00:00Z',
+          phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+        },
+        {
+          id: 'b',
+          status: 'in_progress',
+          createdAt: '2025-02-01T00:00:00Z',
+          phenomenonTimeStart: '2025-02-01T00:00:00Z',
+          phenomenonTimeEnd: '2025-03-01T00:00:00Z',
+        },
+      ] as any
+      store.currentSessionId = 'b'
+      store.viewedSessionId = 'a'
+      return store
+    }
+
+    it('hides the per-entry undo and disables the toolbar undo/redo', async () => {
+      editHistory.value = [{ method: 'ADD_POINTS', args: [], execution: {} }]
+      selectedSeries.value = { data: { history: editHistory.value, redoStack: [{}] } }
+      const w = createWrapper()
+      await readOnly()
+      await flushPromises()
+
+      expect(w.find('[data-testid="history-undo-0"]').exists()).toBe(false)
+      expect(
+        w.find('[data-testid="history-undo-btn"]').attributes('disabled')
+      ).toBeDefined()
+      expect(
+        w.find('[data-testid="history-redo-btn"]').attributes('disabled')
+      ).toBeDefined()
+      expect(
+        w.find('[data-testid="history-load-btn"]').attributes('disabled')
+      ).toBeDefined()
+    })
+
+    it('disables reload-from-server, which would wipe the record', async () => {
+      editHistory.value = [{ method: 'ADD_POINTS', args: [], execution: {} }]
+      selectedSeries.value = { data: { history: editHistory.value, redoStack: [] } }
+      const w = createWrapper()
+      await readOnly()
+      await flushPromises()
+
+      const baseline = w.find('.edit-history__row--baseline')
+      expect(baseline.find('button').attributes('disabled')).toBeDefined()
+    })
+
+    it('keeps the entries below when reloading from a step', async () => {
+      const history = [
+        { method: 'SELECTION', args: [], execution: {} },
+        { method: 'DELETE_POINTS', args: [], execution: {} },
+        { method: 'INTERPOLATE', args: [], execution: {} },
+      ]
+      editHistory.value = history
+      // Stand in for the engine: truncate to `0..index`, as qc-utils does.
+      const reloadHistory = vi.fn(async (index: number) => {
+        history.splice(index + 1)
+        return []
+      })
+      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+
+      const w = createWrapper()
+      await readOnly()
+      await flushPromises()
+
+      await w.find('[data-testid="history-item-0"]').findAll('button').at(-1)!.trigger('click')
+      await vi.waitFor(() => expect(reloadHistory).toHaveBeenCalledWith(0))
+
+      // Data stepped back, but the record of what the session did survives.
+      // Polled: the restore runs after an await inside the handler's timeout.
+      await vi.waitFor(() =>
+        expect(history.map((h) => h.method)).toEqual([
+          'SELECTION',
+          'DELETE_POINTS',
+          'INTERPOLATE',
+        ])
+      )
+    })
+  })
+
+  describe('loaded step', () => {
+    it('marks the step the plot reflects, and clears it on undo', async () => {
+      const history = [
+        { method: 'SELECTION', args: [], execution: {} },
+        { method: 'DELETE_POINTS', args: [], execution: {} },
+      ]
+      editHistory.value = history
+      const reloadHistory = vi.fn(async () => [])
+      const undo = vi.fn(async () => [])
+      selectedSeries.value = { data: { history, redoStack: [], reloadHistory, undo } }
+
+      const w = createWrapper()
+      await flushPromises()
+      // With no step singled out the plot reflects the whole history, so the
+      // last entry carries the marker.
+      expect(w.find('[data-testid="history-loaded-1"]').exists()).toBe(true)
+      expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(false)
+
+      await w.find('[data-testid="history-item-0"]').findAll('button').at(-1)!.trigger('click')
+      await vi.waitFor(() =>
+        expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(true)
+      )
+
+      // Undoing changes the history, so "showing step 0" no longer holds and
+      // the marker returns to the end of the list.
+      // Wait for the step reload to settle: the button is disabled while
+      // `isUpdating`, and a click on a disabled button never reaches the handler.
+      await vi.waitFor(() => expect(isUpdating.value).toBe(false))
+      await w.find('[data-testid="history-undo-btn"]').trigger('click')
+      await vi.waitFor(() =>
+        expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(false)
+      )
+      expect(w.find('[data-testid="history-loaded-1"]').exists()).toBe(true)
+    })
+  })
+
+  describe('session switch', () => {
+    it('shows a loading state instead of the outgoing session operations', async () => {
+      editHistory.value = [{ method: 'ADD_POINTS', args: [], execution: {} }]
+      selectedSeries.value = { data: { history: editHistory.value, redoStack: [] } }
+      const w = createWrapper()
+      await flushPromises()
+      expect(w.find('[data-testid="history-item-0"]').exists()).toBe(true)
+
+      const { useQcSessionStore } = await import('@/store/qcSession')
+      useQcSessionStore().isSwitchingSession = true
+      await flushPromises()
+
+      expect(w.find('[data-testid="history-loading"]').exists()).toBe(true)
+      expect(w.find('[data-testid="history-item-0"]').exists()).toBe(false)
+    })
+  })
+
+  describe('shown step fallback', () => {
+    it('marks nothing when the history is empty', async () => {
+      editHistory.value = []
+      selectedSeries.value = { data: { history: [], redoStack: [] } }
+      const w = createWrapper()
+      await flushPromises()
+      expect(w.find('[data-testid^="history-loaded-"]').exists()).toBe(false)
+    })
+
+    it('falls back to the last entry when the chosen step is out of range', async () => {
+      const history = [
+        { method: 'SELECTION', args: [], execution: {} },
+        { method: 'DELETE_POINTS', args: [], execution: {} },
+        { method: 'INTERPOLATE', args: [], execution: {} },
+      ]
+      editHistory.value = history
+      const reloadHistory = vi.fn(async () => [])
+      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const w = createWrapper()
+      await flushPromises()
+
+      await w.find('[data-testid="history-item-2"]').findAll('button').at(-2)!.trigger('click')
+      await vi.waitFor(() =>
+        expect(w.find('[data-testid="history-loaded-2"]').exists()).toBe(true)
+      )
+
+      // The history shrinks under the chosen step. Splice through the ref's
+      // proxy: mutating the raw array wouldn't trigger reactivity.
+      editHistory.value.splice(1)
+      await flushPromises()
+      expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(true)
+    })
+  })
+
+  describe('attribution', () => {
+    it('shows who applied an operation, on the row and in the detail', async () => {
+      editHistory.value = [
+        { method: 'ADD_POINTS', args: [], execution: {}, performedBy: 'Ada Lovelace' },
+        { method: 'DELETE_POINTS', args: [], execution: {} },
+      ]
+      selectedSeries.value = { data: { history: editHistory.value, redoStack: [] } }
+      const w = createWrapper()
+      await flushPromises()
+
+      expect(w.find('[data-testid="history-author-0"]').text()).toBe('Ada Lovelace')
+      // Unsaved operations have no server attribution yet.
+      expect(w.find('[data-testid="history-author-1"]').exists()).toBe(false)
+
+      await w.find('[data-testid="history-item-0"]').find('button').trigger('click')
+      await flushPromises()
+      expect(w.find('[data-testid="history-author-detail-0"]').text()).toContain(
+        'Applied by Ada Lovelace'
+      )
+    })
+  })
+
+  describe('selection after loading a step', () => {
+    const stepBackTo = async (returned: number[] | undefined) => {
+      const history = [
+        { method: 'SELECTION', args: [], execution: {} },
+        { method: 'DELETE_POINTS', args: [], execution: {} },
+      ]
+      editHistory.value = history
+      const reloadHistory = vi.fn(async () => returned as any)
+      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const w = createWrapper()
+      await flushPromises()
+      // Item 0 isn't the trailing entry, so its buttons are expand + reload.
+      await w.find('[data-testid="history-item-0"]').findAll('button').at(-1)!.trigger('click')
+      await vi.waitFor(() => expect(reloadHistory).toHaveBeenCalledWith(0))
+      await vi.waitFor(() => expect(isUpdating.value).toBe(false))
+      await flushPromises()
+    }
+
+    it('applies the selection the replay produced', async () => {
+      await stepBackTo([3, 4, 5])
+      expect(setPlotSelection).toHaveBeenCalledWith([3, 4, 5])
+      expect(clearSelected).not.toHaveBeenCalled()
+    })
+
+    // Without this the previous selection stayed painted on the plot.
+    it('clears the selection when the replay produced none', async () => {
+      await stepBackTo(undefined)
+      expect(setPlotSelection).not.toHaveBeenCalled()
+      expect(clearSelected).toHaveBeenCalledWith({ recordHistory: false })
+    })
+
+    it('clears the selection when the replay produced an empty one', async () => {
+      await stepBackTo([])
+      expect(setPlotSelection).not.toHaveBeenCalled()
+      expect(clearSelected).toHaveBeenCalledWith({ recordHistory: false })
+    })
+  })
 })
diff --git a/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts b/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
index 20a43c79..b3fee96a 100644
--- a/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
@@ -33,7 +33,10 @@ beforeEach(() => {
 })
 
 const mountList = () =>
-  mount(SessionList, { global: { plugins: [pinia, createTestVuetify()] } })
+  mount(SessionList, {
+    global: { plugins: [pinia, createTestVuetify()] },
+    slots: { operations: '<div data-testid="ops-panel">ops</div>' },
+  })
 
 /** Seed a committed + an in-progress session and load them into the store. */
 async function seedAndLoad() {
@@ -69,33 +72,129 @@ describe('SessionList', () => {
     expect(wrapper.text()).toContain('Editing')
   })
 
-  it('clicking a committed session views it read-only', async () => {
-    const { store, committedId } = await seedAndLoad()
+  it('lists sessions oldest first, so the most recent is at the bottom', async () => {
+    const { committedId, inProgressId } = await seedAndLoad()
     const wrapper = mountList()
     await flushPromises()
-    await wrapper.find(`[data-testid="session-${committedId}"]`).trigger('click')
-    expect(store.viewedSessionId).toBe(committedId)
-    expect(store.isReadOnly).toBe(true)
+    const ids = wrapper
+      .findAll('.v-timeline-item[data-testid^="session-"]')
+      .map((item) => item.attributes('data-testid'))
+    expect(ids).toEqual([`session-${committedId}`, `session-${inProgressId}`])
   })
 
-  it('clicking the current session returns to editing', async () => {
-    const { store, committedId, inProgressId } = await seedAndLoad()
+  it('orders by creation even when sessions share a phenomenon-time window', async () => {
+    // Continuing work after a commit reuses the same window, so the windows
+    // tie and only the creation time distinguishes the sessions.
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const sameWindow = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
+    const first = unwrap(await qc.sessions.create(h.id, sameWindow))
+    await qc.sessions.commit(h.id, first.id)
+    const second = unwrap(await qc.sessions.create(h.id, sameWindow))
+    await useQcSessionStore().loadSessions(h.id)
+
     const wrapper = mountList()
     await flushPromises()
-    await wrapper.find(`[data-testid="session-${committedId}"]`).trigger('click')
-    expect(store.isReadOnly).toBe(true)
-    await wrapper.find(`[data-testid="session-${inProgressId}"]`).trigger('click')
-    expect(store.viewedSessionId).toBe(inProgressId)
-    expect(store.isReadOnly).toBe(false)
+    const ids = wrapper
+      .findAll('.v-timeline-item[data-testid^="session-"]')
+      .map((item) => item.attributes('data-testid'))
+    expect(ids).toEqual([`session-${first.id}`, `session-${second.id}`])
   })
 
-  it('shows a "Return to current" control while viewing read-only', async () => {
+  // Selecting a session reloads observations and replays operations, so the
+  // list only reports the intent; the view owns the work.
+  it('emits view for a committed session', async () => {
     const { committedId } = await seedAndLoad()
     const wrapper = mountList()
     await flushPromises()
+    await wrapper.find(`[data-testid="session-header-${committedId}"]`).trigger('click')
+    expect(wrapper.emitted('view')![0]).toEqual([committedId])
+  })
+
+  it('emits view for the in-progress session too', async () => {
+    const { inProgressId } = await seedAndLoad()
+    const wrapper = mountList()
+    await flushPromises()
+    await wrapper.find(`[data-testid="session-header-${inProgressId}"]`).trigger('click')
+    expect(wrapper.emitted('view')![0]).toEqual([inProgressId])
+  })
+
+  it('shows "Return to current" while read-only, and it emits the current session', async () => {
+    const { store, committedId, inProgressId } = await seedAndLoad()
+    const wrapper = mountList()
+    await flushPromises()
     expect(wrapper.find('[data-testid="session-return-current"]').exists()).toBe(false)
-    await wrapper.find(`[data-testid="session-${committedId}"]`).trigger('click')
+
+    // The view flips this after loading the committed session.
+    store.viewSession(committedId)
     await flushPromises()
-    expect(wrapper.find('[data-testid="session-return-current"]').exists()).toBe(true)
+
+    const control = wrapper.find('[data-testid="session-return-current"]')
+    expect(control.exists()).toBe(true)
+    await control.trigger('click')
+    expect(wrapper.emitted('view')!.at(-1)).toEqual([inProgressId])
+  })
+
+  // The operations panel nests under the session it belongs to, so the list
+  // reads hierarchically instead of as two sibling panels.
+  it('renders the operations slot under the viewed session only', async () => {
+    const { store, committedId, inProgressId } = await seedAndLoad()
+    const wrapper = mountList()
+    await flushPromises()
+
+    const inProgressItem = wrapper.find(`[data-testid="session-${inProgressId}"]`)
+    expect(inProgressItem.find('[data-testid="ops-panel"]').exists()).toBe(true)
+    const committedItem = wrapper.find(`[data-testid="session-${committedId}"]`)
+    expect(committedItem.find('[data-testid="ops-panel"]').exists()).toBe(false)
+
+    store.viewSession(committedId)
+    await flushPromises()
+
+    expect(
+      wrapper.find(`[data-testid="session-${committedId}"]`).find('[data-testid="ops-panel"]').exists()
+    ).toBe(true)
+    expect(wrapper.findAll('[data-testid="ops-panel"]')).toHaveLength(1)
+  })
+
+  it('previews the operation count of sessions that are not in view', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const range = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
+    const committed = unwrap(await qc.sessions.create(h.id, range))
+    await qc.operations.create(h.id, committed.id, [
+      { operationType: 'SELECTION' as any, order: 0 },
+      { operationType: 'DELETE_POINTS' as any, order: 1 },
+    ])
+    await qc.sessions.commit(h.id, committed.id)
+    await qc.sessions.create(h.id, range)
+    await useQcSessionStore().loadSessions(h.id)
+
+    const wrapper = mountList()
+    await flushPromises()
+    expect(
+      wrapper.find(`[data-testid="session-preview-${committed.id}"]`).text()
+    ).toContain('2 operations')
+  })
+
+  it('says so when a session that is not in view has no operations', async () => {
+    const { committedId } = await seedAndLoad()
+    const wrapper = mountList()
+    await flushPromises()
+    expect(wrapper.find(`[data-testid="session-preview-${committedId}"]`).text()).toContain(
+      'No operations'
+    )
+  })
+
+  it('still renders the operations slot when there are no sessions', () => {
+    const wrapper = mountList()
+    expect(wrapper.find('[data-testid="ops-panel"]').exists()).toBe(true)
   })
 })
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
index 9a0f83f7..0df861da 100644
--- a/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
@@ -23,6 +23,7 @@ const options = [
       {
         id: 's-1',
         status: 'in_progress',
+        createdAt: '2025-01-05T09:00:00Z',
         phenomenonTimeStart: '2025-01-05T00:00:00Z',
         phenomenonTimeEnd: '2025-01-15T00:00:00Z',
         description: '',
@@ -30,8 +31,10 @@ const options = [
       {
         id: 's-0',
         status: 'committed',
-        phenomenonTimeStart: '2024-12-01T00:00:00Z',
-        phenomenonTimeEnd: '2024-12-31T00:00:00Z',
+        createdAt: '2024-12-01T09:00:00Z',
+        description: 'Removed spikes',
+        phenomenonTimeStart: '2024-12-01T00:00:00',
+        phenomenonTimeEnd: '2024-12-31T00:00:00',
       },
     ],
   },
@@ -53,21 +56,58 @@ describe('StartEditingDialog', () => {
     expect(w.text()).toContain('Committed')
   })
 
-  it('labels the action by whether an in-progress session exists', () => {
+  it('offers Continue on the in-progress session row, not in the header', () => {
     const w = mountDialog()
-    expect(w.find('[data-testid="edit-managed-mgd-1"]').text()).toContain(
+    // mgd-1 has an in-progress session: continuing hangs off that row.
+    expect(w.find('[data-testid="edit-managed-mgd-1"]').exists()).toBe(false)
+    expect(w.find('[data-testid="continue-session-s-1"]').text()).toContain(
       'Continue'
     )
+    // mgd-2 has none, so the header offers starting one.
     expect(w.find('[data-testid="edit-managed-mgd-2"]').text()).toContain(
       'Start new'
     )
   })
 
-  it('formats session date ranges like the date inputs (MM/DD/YYYY HH:MM)', () => {
+  it('offers discard only on the in-progress session', () => {
     const w = mountDialog()
-    expect(w.text()).toMatch(
-      /\d{2}\/\d{2}\/\d{4} \d{2}:\d{2} to \d{2}\/\d{2}\/\d{4} \d{2}:\d{2}/
+    expect(w.find('[data-testid="delete-session-s-1"]').exists()).toBe(true)
+    expect(w.find('[data-testid="continue-session-s-0"]').exists()).toBe(false)
+    expect(w.find('[data-testid="delete-session-s-0"]').exists()).toBe(false)
+  })
+
+  it('asks for confirmation, then emits deleteSession for that session', async () => {
+    const w = mountDialog()
+    await w.find('[data-testid="delete-session-s-1"]').trigger('click')
+    const confirm = w.find('[data-testid="confirm-delete-session-s-1"]')
+    expect(confirm.exists()).toBe(true)
+    await confirm.trigger('click')
+    const [option, sessionId] = w.emitted('deleteSession')![0]
+    expect(option).toMatchObject({ historyId: 'h-1' })
+    expect(sessionId).toBe('s-1')
+  })
+
+  it('cancelling the discard confirmation emits nothing', async () => {
+    const w = mountDialog()
+    await w.find('[data-testid="delete-session-s-1"]').trigger('click')
+    await w.find('[data-testid="cancel-delete-session"]').trigger('click')
+    expect(w.find('[data-testid="confirm-delete-session-s-1"]').exists()).toBe(
+      false
     )
+    expect(w.emitted('deleteSession')).toBeUndefined()
+  })
+
+  it('formats session date ranges readably', () => {
+    const w = mountDialog()
+    expect(w.text()).toContain('Dec 1 – Dec 31, 2024')
+  })
+
+  it('shows the period alongside the description on a described session', () => {
+    const w = mountDialog()
+    // s-0 is committed and described, so the description takes the title and
+    // the window has to surface underneath it.
+    expect(w.text()).toContain('Removed spikes')
+    expect(w.text()).toContain('Dec 1 – Dec 31, 2024')
   })
 
   it('shows a recap line with processing level, observations, and sessions', () => {
@@ -86,10 +126,12 @@ describe('StartEditingDialog', () => {
     expect(w.emitted('delete')![0][0]).toMatchObject({ historyId: 'h-1' })
   })
 
-  it('emits edit with the chosen option', async () => {
+  it('emits edit from the in-progress row and from the header', async () => {
     const w = mountDialog()
-    await w.find('[data-testid="edit-managed-mgd-1"]').trigger('click')
+    await w.find('[data-testid="continue-session-s-1"]').trigger('click')
     expect(w.emitted('edit')![0][0]).toMatchObject({ historyId: 'h-1' })
+    await w.find('[data-testid="edit-managed-mgd-2"]').trigger('click')
+    expect(w.emitted('edit')![1][0]).toMatchObject({ historyId: 'h-2' })
   })
 
   it('emits create and cancel', async () => {
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index 2c3f75c3..2fc15794 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -112,6 +112,7 @@
         :loading="chooserLoading"
         @edit="editManaged"
         @delete="onChooserDelete"
+        @delete-session="onChooserDeleteSession"
         @create="onChooserCreate"
         @cancel="showChooser = false"
       />
@@ -325,6 +326,7 @@
             <EditHistory
               v-model:collapsed="historyCollapsed"
               @pop-out="historyModalOpen = true"
+              @view-session="onViewSession"
             />
           </div>
 
@@ -357,7 +359,11 @@
          ask to open the modal we're already in. -->
     <v-dialog v-model="historyModalOpen" max-width="720">
       <v-card class="d-flex flex-column" style="max-height: 80vh">
-        <EditHistory :collapsible="false" :pop-out-enabled="false" />
+        <EditHistory
+          :collapsible="false"
+          :pop-out-enabled="false"
+          @view-session="onViewSession"
+        />
       </v-card>
     </v-dialog>
 
@@ -463,6 +469,8 @@ import { useRoute, useRouter } from 'vue-router'
 import PlottedDatastreams from './VisualizeData/PlottedDatastreams.vue'
 import { usePlotlyStore } from '@/store/plotly'
 import { useEditSession } from '@/composables/useEditSession'
+import { useUnsavedChangesWarning } from '@/composables/useUnsavedChangesWarning'
+import { useResumeEditSession } from '@/composables/useResumeEditSession'
 import { useQcSessionStore } from '@/store/qcSession'
 import { useQcPreferencesStore } from '@/store/qcPreferences'
 import CreateDatastreamForm from '@/components/EditData/CreateDatastreamForm.vue'
@@ -521,6 +529,7 @@ const { redraw } = usePlotlyStore()
 const {
   setPlottedDatastreams,
   adoptManagedDatastream,
+  releaseManagedDatastream,
   addQcHistory,
   removeManagedDatastream,
 } = useDataVisStore()
@@ -534,14 +543,42 @@ const {
   needsHistory,
   hasUnsavedChanges,
   unsavedEditCount,
+  viewSession,
 } = useEditSession()
+useUnsavedChangesWarning(hasUnsavedChanges)
+
+// Selecting a session in the list loads the data as that session left it,
+// plus its own operations. Guarded so unsaved edits aren't dropped silently.
+const isViewingSession = ref(false)
+async function onViewSession(sessionId: string) {
+  if (isViewingSession.value) return
+  if (
+    hasUnsavedChanges.value &&
+    !window.confirm(
+      'You have unsaved edits. Viewing another session will discard them. Continue?'
+    )
+  ) {
+    return
+  }
+  isViewingSession.value = true
+  try {
+    await viewSession(sessionId)
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not load that session.'
+    )
+  } finally {
+    isViewingSession.value = false
+  }
+}
 const qcSessionStore = useQcSessionStore()
-const { isReadOnly, inProgressSession } = storeToRefs(qcSessionStore)
+const { isReadOnly, inProgressSession, resumeDatastreamId } =
+  storeToRefs(qcSessionStore)
 const { create: createManaged } = useCreateManagedDatastream()
 const qcPreferences = useQcPreferencesStore()
 const { canEdit, canCreateDatastream, roleName } = useWorkspacePermissions()
 const { createProcessingLevel } = useProcessingLevels()
-const { loadForSource, deleteManaged } = useManagedDatastreams()
+const { loadForSource, deleteManaged, deleteSession } = useManagedDatastreams()
 
 // Permission gating: QC editing writes to the selected workspace (creates
 // the managed datastream, pushes observations). Gate the editor entry
@@ -665,6 +702,12 @@ function exitToSelect() {
   currentView.value = DrawerType.Select
   selectedDrawer.value = DrawerType.Select
   isDrawerOpen.value = true
+  // Leaving the editor deliberately, so a later reload shouldn't reopen it.
+  resumeDatastreamId.value = null
+  // Put the source back in the plot so the catalog table shows the row the
+  // user had selected; the managed datastream it was swapped for is hidden
+  // from that table.
+  void releaseManagedDatastream()
 }
 
 // No in-progress session yet: open one over the window already chosen by the
@@ -1038,6 +1081,27 @@ async function onChooserDelete(option: ManagedDatastreamOption) {
   }
 }
 
+// Discard an in-progress session from the chooser, dropping its draft edits.
+// Only that session goes; the managed datastream and its commits remain.
+async function onChooserDeleteSession(
+  option: ManagedDatastreamOption,
+  sessionId: string
+) {
+  try {
+    await deleteSession(option.historyId, sessionId)
+    chooserOptions.value = chooserOptions.value.map((o) =>
+      o.historyId === option.historyId
+        ? { ...o, sessions: o.sessions.filter((s) => s.id !== sessionId) }
+        : o
+    )
+    Snackbar.success('Session discarded.')
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not discard the session.'
+    )
+  }
+}
+
 // Enter the editor on the current QC-target managed datastream: resume its
 // in-progress session, or start a new one over the selected time range.
 async function enterEdit() {
@@ -1049,8 +1113,13 @@ async function enterEdit() {
   currentView.value = DrawerType.Edit
   selectedDrawer.value = DrawerType.Edit
   isDrawerOpen.value = true
+  // Remember the target so a page reload can come back to this session.
+  resumeDatastreamId.value = qcDatastream.value?.id ?? null
   if (needsSession.value) await startSessionForWindow()
 }
+
+// Reopens the editor after a page reload; waits for the catalog to land.
+useResumeEditSession(enterEdit)
 </script>
 
 <style scoped>
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 3093fe94..97bfe2cf 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -10,8 +10,9 @@ vi.mock('@/store/dataVisualization', () => ({
 }))
 
 const selectedSeries = ref<any>(null)
+const redraw = vi.fn()
 vi.mock('@/store/plotly', () => ({
-  usePlotlyStore: () => ({ selectedSeries }),
+  usePlotlyStore: () => ({ selectedSeries, redraw }),
 }))
 
 const getItem = vi.fn()
@@ -48,10 +49,15 @@ const WIN = {
   phenomenonTimeEnd: '2025-02-01T00:00:00Z',
 }
 
+// Mirrors a real ObservationRecord closely enough for the session layer:
+// `redoStack` and `reload` exist on every record and are used when a cached
+// one is reset back to its stored state.
 const makeRecord = (history: any[] = []) => ({
   history,
+  redoStack: [] as any[],
   dataX: [Date.UTC(2025, 0, 1)],
   dataY: [10],
+  reload: vi.fn(async () => {}),
 })
 
 let qc: ReturnType<typeof makeQcFake>
@@ -123,6 +129,9 @@ describe('useEditSession', () => {
     // The QC-target series now shows the reconstructed session, not the
     // empty managed datastream.
     expect(selectedSeries.value.data).toEqual(reconstructed)
+    // Nothing watches for a swapped-in record, so resume has to rebuild the
+    // plot itself or it keeps rendering the pre-reconstruction trace.
+    expect(redraw).toHaveBeenCalled()
   })
 
   it('startSession loads the managed datastream as the working base', async () => {
@@ -163,17 +172,16 @@ describe('useEditSession', () => {
 
   it('saveDraft persists the record operations to the session', async () => {
     await seedHistory()
-    // The working copy comes from startSession's base load; give it the ops.
-    fetchObservationsInRange.mockResolvedValue(
-      makeRecord([
-        { method: 'VALUE_THRESHOLD', args: [] },
-        { method: 'DELETE_POINTS', args: [] },
-      ])
-    )
     const { useEditSession } = await import('@/composables/useEditSession')
     const session = useEditSession()
     await session.beginEditing()
     await session.startSession(WIN)
+    // A new session starts from a clean base, so the operations are the ones
+    // the user applies afterwards.
+    selectedSeries.value.data.history.push(
+      { method: 'VALUE_THRESHOLD', args: [] },
+      { method: 'DELETE_POINTS', args: [] }
+    )
     await session.saveDraft()
 
     const store = useQcSessionStore()
@@ -257,3 +265,98 @@ describe('useEditSession', () => {
     expect(store.committedSessions[0]?.description).toBe('Reviewed January spike')
   })
 })
+
+describe('useEditSession.viewSession', () => {
+  it('loads a committed session and marks the editor read-only', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const committed = unwrap(await qc.sessions.create(h.id, WIN))
+    await qc.operations.create(h.id, committed.id, [
+      { operationType: 'SELECTION' as any, order: 0 },
+    ])
+    await qc.sessions.commit(h.id, committed.id)
+    await qc.sessions.create(h.id, WIN)
+
+    const viewed = makeRecord([])
+    fetchObservationsInRange.mockResolvedValue(viewed)
+
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const store = useQcSessionStore()
+    const session = useEditSession()
+    await session.beginEditing()
+
+    await session.viewSession(committed.id)
+
+    expect(selectedSeries.value.data).toEqual(viewed)
+    expect(store.viewedSessionId).toBe(committed.id)
+    expect(store.isReadOnly).toBe(true)
+    expect(redraw).toHaveBeenCalled()
+  })
+
+  it('moves the selection before loading, so the spinner sits on the new session', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const committed = unwrap(await qc.sessions.create(h.id, WIN))
+    await qc.sessions.commit(h.id, committed.id)
+    await qc.sessions.create(h.id, WIN)
+
+    const store = useQcSessionStore()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+
+    const seen: Array<{ viewed: string | null; switching: boolean }> = []
+    fetchObservationsInRange.mockImplementation(async () => {
+      seen.push({
+        viewed: store.viewedSessionId,
+        switching: store.isSwitchingSession,
+      })
+      return makeRecord([])
+    })
+
+    await session.viewSession(committed.id)
+
+    // While loading, the list already points at the incoming session.
+    expect(seen[0]).toEqual({ viewed: committed.id, switching: true })
+    expect(store.isSwitchingSession).toBe(false)
+  })
+
+  it('restores the previous selection when loading a session fails', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const committed = unwrap(await qc.sessions.create(h.id, WIN))
+    await qc.sessions.commit(h.id, committed.id)
+    const inProgress = unwrap(await qc.sessions.create(h.id, WIN))
+
+    const store = useQcSessionStore()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    expect(store.viewedSessionId).toBe(inProgress.id)
+
+    fetchObservationsInRange.mockRejectedValueOnce(new Error('network'))
+    await expect(session.viewSession(committed.id)).rejects.toThrow('network')
+
+    expect(store.viewedSessionId).toBe(inProgress.id)
+    expect(store.isSwitchingSession).toBe(false)
+  })
+
+  it('rejects before a managed datastream is loaded', async () => {
+    const { useEditSession } = await import('@/composables/useEditSession')
+    await expect(useEditSession().viewSession('s-1')).rejects.toThrow(
+      /Load a managed datastream/
+    )
+  })
+})
diff --git a/apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts
new file mode 100644
index 00000000..7e9951c3
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts
@@ -0,0 +1,148 @@
+import { describe, it, expect, afterEach, beforeEach, vi } from 'vitest'
+import { defineComponent } from 'vue'
+import { mount, flushPromises } from '@vue/test-utils'
+import { createPinia, setActivePinia } from 'pinia'
+
+// vi.mock factories are hoisted above module-level consts, so anything they
+// close over has to be created inside vi.hoisted.
+const {
+  datastreams,
+  plotDatastream,
+  setQcDatastream,
+  resumeDatastreamId,
+  error,
+} = vi.hoisted(() => {
+  const { ref: r } = require('vue') as typeof import('vue')
+  return {
+    datastreams: r<any[]>([]),
+    plotDatastream: vi.fn(),
+    setQcDatastream: vi.fn(),
+    resumeDatastreamId: r<string | null>(null),
+    error: vi.fn(),
+  }
+})
+
+// Real Pinia stores, so `storeToRefs` behaves as it does in production.
+vi.mock('@/store/dataVisualization', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useDataVisStore: defineStore('dataVisualization', () => ({
+      datastreams,
+      plotDatastream,
+      setQcDatastream,
+    })),
+  }
+})
+
+vi.mock('@/store/qcSession', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useQcSessionStore: defineStore('qcSession', () => ({
+      resumeDatastreamId,
+    })),
+  }
+})
+
+vi.mock('@uwrl/qc-utils', () => ({ Snackbar: { error } }))
+
+import { useResumeEditSession } from '../useResumeEditSession'
+
+const enterEdit = vi.fn()
+
+let pinia: ReturnType<typeof createPinia>
+// The stores are module-level refs shared across tests, so a host left
+// mounted keeps watching them and resumes again in the next test.
+let hosts: ReturnType<typeof mount>[] = []
+
+const mountHost = () => {
+  const wrapper = mount(
+    defineComponent({
+      setup() {
+        useResumeEditSession(enterEdit)
+        return () => null
+      },
+    }),
+    { global: { plugins: [pinia] } }
+  )
+  hosts.push(wrapper)
+  return wrapper
+}
+
+afterEach(() => {
+  hosts.forEach((h) => h.unmount())
+  hosts = []
+})
+
+beforeEach(() => {
+  pinia = createPinia()
+  setActivePinia(pinia)
+  vi.clearAllMocks()
+  datastreams.value = []
+  resumeDatastreamId.value = null
+})
+
+describe('useResumeEditSession', () => {
+  // The catalog is empty at mount on a cold reload, which is the whole
+  // point of this composable. An `immediate` + `once` watcher burns its
+  // single firing on that empty list and never sees the catalog land.
+  it('resumes when the catalog arrives after mount', async () => {
+    resumeDatastreamId.value = 'mgd-1'
+    mountHost()
+    expect(enterEdit).not.toHaveBeenCalled()
+
+    datastreams.value = [{ id: 'mgd-1' }]
+    await flushPromises()
+
+    expect(plotDatastream).toHaveBeenCalledWith({ id: 'mgd-1' })
+    expect(setQcDatastream).toHaveBeenCalledWith('mgd-1')
+    expect(enterEdit).toHaveBeenCalled()
+  })
+
+  it('resumes immediately when the catalog is already loaded', async () => {
+    resumeDatastreamId.value = 'mgd-1'
+    datastreams.value = [{ id: 'mgd-1' }]
+    mountHost()
+    await flushPromises()
+    expect(enterEdit).toHaveBeenCalled()
+  })
+
+  it('does nothing without a stored session', async () => {
+    datastreams.value = [{ id: 'mgd-1' }]
+    mountHost()
+    await flushPromises()
+    expect(enterEdit).not.toHaveBeenCalled()
+  })
+
+  it('drops a pointer to a datastream missing from the catalog', async () => {
+    resumeDatastreamId.value = 'gone'
+    mountHost()
+    datastreams.value = [{ id: 'mgd-1' }]
+    await flushPromises()
+
+    expect(enterEdit).not.toHaveBeenCalled()
+    expect(resumeDatastreamId.value).toBeNull()
+  })
+
+  it('only resumes once, so a later catalog refresh does not re-enter', async () => {
+    resumeDatastreamId.value = 'mgd-1'
+    mountHost()
+    datastreams.value = [{ id: 'mgd-1' }]
+    await flushPromises()
+    expect(enterEdit).toHaveBeenCalledTimes(1)
+
+    datastreams.value = [{ id: 'mgd-1' }, { id: 'mgd-2' }]
+    await flushPromises()
+    expect(enterEdit).toHaveBeenCalledTimes(1)
+  })
+
+  it('surfaces a failure instead of failing silently', async () => {
+    resumeDatastreamId.value = 'mgd-1'
+    enterEdit.mockRejectedValueOnce(new Error('boom'))
+    mountHost()
+    datastreams.value = [{ id: 'mgd-1' }]
+    await flushPromises()
+
+    expect(error).toHaveBeenCalledWith('boom')
+    expect(resumeDatastreamId.value).toBeNull()
+  })
+})
diff --git a/apps/qc-app/src/composables/__tests__/useUnsavedChangesWarning.spec.ts b/apps/qc-app/src/composables/__tests__/useUnsavedChangesWarning.spec.ts
new file mode 100644
index 00000000..e6e9a826
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useUnsavedChangesWarning.spec.ts
@@ -0,0 +1,60 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { defineComponent, ref } from 'vue'
+import { mount } from '@vue/test-utils'
+import { useUnsavedChangesWarning } from '../useUnsavedChangesWarning'
+
+const dirty = ref(false)
+
+const Host = defineComponent({
+  setup() {
+    useUnsavedChangesWarning(dirty)
+    return () => null
+  },
+})
+
+/** A beforeunload event that records whether the page tried to block it. */
+const unloadEvent = () => {
+  const event = new Event('beforeunload', { cancelable: true })
+  Object.defineProperty(event, 'returnValue', { value: '', writable: true })
+  return event
+}
+
+beforeEach(() => {
+  dirty.value = false
+})
+
+describe('useUnsavedChangesWarning', () => {
+  it('blocks the unload while there are unsaved changes', () => {
+    const wrapper = mount(Host)
+    dirty.value = true
+
+    const event = unloadEvent()
+    window.dispatchEvent(event)
+
+    expect(event.defaultPrevented).toBe(true)
+    expect(event.returnValue).toBe('')
+    wrapper.unmount()
+  })
+
+  it('lets the unload through when everything is saved', () => {
+    const wrapper = mount(Host)
+
+    const event = unloadEvent()
+    window.dispatchEvent(event)
+
+    expect(event.defaultPrevented).toBe(false)
+    wrapper.unmount()
+  })
+
+  it('stops listening once the component is gone', () => {
+    const wrapper = mount(Host)
+    const remove = vi.spyOn(window, 'removeEventListener')
+    wrapper.unmount()
+    expect(remove).toHaveBeenCalledWith('beforeunload', expect.any(Function))
+
+    dirty.value = true
+    const event = unloadEvent()
+    window.dispatchEvent(event)
+    expect(event.defaultPrevented).toBe(false)
+  })
+})
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index 3c94a354..00f10d2a 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -30,6 +30,7 @@ import {
   persistSessionOperations,
   commitQcSession,
   reconstructSession,
+  reconstructCommittedSession,
   observationsBulkBody,
 } from '@/services/qualityControl'
 
@@ -70,6 +71,7 @@ function clampSpecToSource(
 export function useEditSession() {
   const { qcDatastream } = storeToRefs(useDataVisStore())
   const { selectedSeries } = storeToRefs(usePlotlyStore())
+  const { redraw } = usePlotlyStore()
   const { hs } = storeToRefs(useHydroServer())
   const { fetchObservationsInRange } = useObservationStore()
   const sessionStore = useQcSessionStore()
@@ -83,19 +85,27 @@ export function useEditSession() {
   // Snapshot (by item reference) of the operation history at the last
   // load/save, so the editor knows whether there are unsaved edits.
   const savedEdits = ref<HistoryItem[]>([])
+  // Comments are edited in place on the existing history entry, so the
+  // reference comparison below can't see them; snapshot their text too.
+  const savedComments = ref<string[]>([])
   const currentEdits = (): HistoryItem[] =>
     (selectedSeries.value?.data as ObservationRecord | undefined)?.history ?? []
+  const commentOf = (item: HistoryItem) => item.comment?.trim() ?? ''
   const snapshotSavedEdits = () => {
-    savedEdits.value = [...currentEdits()]
+    const current = currentEdits()
+    savedEdits.value = [...current]
+    savedComments.value = current.map(commentOf)
   }
 
   /** True when the working copy has edits not in the last saved snapshot
-   *  (additions or removals). */
+   *  (additions, removals, or an edited comment). */
   const hasUnsavedChanges = computed(() => {
     const current = currentEdits()
     const saved = savedEdits.value
     if (current.length !== saved.length) return true
-    return current.some((item, i) => item !== saved[i])
+    return current.some(
+      (item, i) => item !== saved[i] || commentOf(item) !== savedComments.value[i]
+    )
   })
 
   /** Count of edits added since the last save (the common append case). */
@@ -143,6 +153,9 @@ export function useEditSession() {
         inProgress.id
       )
       if (selectedSeries.value) selectedSeries.value.data = reconstructed
+      // Nothing watches for a swapped-in record; resume has no caller that
+      // redraws, unlike the start-session path.
+      await redraw()
       // The replayed draft operations are the saved baseline.
       snapshotSavedEdits()
       needsSession.value = false
@@ -151,6 +164,57 @@ export function useEditSession() {
     }
   }
 
+  /**
+   * Load a committed session for read-only viewing: the data as that
+   * session left it, and its own operations in the panel. Returns to the
+   * in-progress session when given the editable one.
+   */
+  async function viewSession(sessionId: string): Promise<void> {
+    const historyId = sessionStore.historyId
+    const source = sourceDatastream.value
+    if (!historyId || !source) {
+      throw new Error('Load a managed datastream for editing first.')
+    }
+    if (sessionId === sessionStore.currentSessionId) {
+      sessionStore.returnToCurrent()
+      sessionStore.isSwitchingSession = true
+      try {
+        await beginEditing()
+      } finally {
+        sessionStore.isSwitchingSession = false
+      }
+      return
+    }
+    // Move the selection first so the loading state appears under the
+    // session being opened. Reverted if the load fails.
+    const previousSessionId = sessionStore.viewedSessionId
+    sessionStore.viewSession(sessionId)
+    sessionStore.isSwitchingSession = true
+    try {
+      const { record } = await reconstructCommittedSession(
+        {
+          qcSessions: hs.value.qualityControlSessions,
+          qcOperations: hs.value.qualityControlOperations,
+          fetchInRange: fetchObservationsInRange,
+          applyHistory,
+        },
+        source,
+        historyId,
+        sessionId
+      )
+      if (selectedSeries.value) selectedSeries.value.data = record
+      // Nothing watches for a swapped-in record; rebuild the plot explicitly.
+      await redraw()
+      // Viewing is read-only, so there is nothing unsaved to track.
+      snapshotSavedEdits()
+    } catch (e) {
+      if (previousSessionId) sessionStore.viewSession(previousSessionId)
+      throw e
+    } finally {
+      sessionStore.isSwitchingSession = false
+    }
+  }
+
   async function startSession(spec: QcSessionPostBody): Promise<void> {
     const historyId = sessionStore.historyId
     const source = sourceDatastream.value
@@ -249,6 +313,7 @@ export function useEditSession() {
     unsavedEditCount,
     beginEditing,
     startSession,
+    viewSession,
     saveDraft,
     commit,
   }
diff --git a/apps/qc-app/src/composables/useManagedDatastreams.ts b/apps/qc-app/src/composables/useManagedDatastreams.ts
index 8104a5bd..52814320 100644
--- a/apps/qc-app/src/composables/useManagedDatastreams.ts
+++ b/apps/qc-app/src/composables/useManagedDatastreams.ts
@@ -69,5 +69,21 @@ export function useManagedDatastreams() {
     }
   }
 
-  return { loadForSource, deleteManaged }
+  /**
+   * Discard an in-progress session, dropping its draft operations. Committed
+   * sessions are immutable server-side, so only the draft can be deleted.
+   * Throws on failure.
+   */
+  async function deleteSession(
+    historyId: string,
+    sessionId: string
+  ): Promise<void> {
+    const { hs } = useHydroServer()
+    const res = await hs.qualityControlSessions.delete(historyId, sessionId)
+    if (!res.ok) {
+      throw new Error(res.message || 'Could not discard the session.')
+    }
+  }
+
+  return { loadForSource, deleteManaged, deleteSession }
 }
diff --git a/apps/qc-app/src/composables/useResumeEditSession.ts b/apps/qc-app/src/composables/useResumeEditSession.ts
new file mode 100644
index 00000000..f20d0523
--- /dev/null
+++ b/apps/qc-app/src/composables/useResumeEditSession.ts
@@ -0,0 +1,58 @@
+/**
+ * Reopen the editor after a page reload, from the last saved state:
+ * `beginEditing` rebuilds the session and working copy from the server.
+ *
+ * The watcher must not use `once` with `immediate` — that fires on the
+ * initial empty catalog and stops, which is the cold-reload case this
+ * exists for. The flag gives once-only semantics without that trap.
+ */
+
+import { watch } from 'vue'
+import { storeToRefs } from 'pinia'
+import { Snackbar } from '@uwrl/qc-utils'
+import { useDataVisStore } from '@/store/dataVisualization'
+import { useQcSessionStore } from '@/store/qcSession'
+
+export function useResumeEditSession(enterEdit: () => Promise<void>) {
+  const dataVis = useDataVisStore()
+  const { datastreams } = storeToRefs(dataVis)
+  const { resumeDatastreamId } = storeToRefs(useQcSessionStore())
+
+  let attempted = false
+
+  /** Returns true when the editor was reopened. */
+  async function resume(): Promise<boolean> {
+    const id = resumeDatastreamId.value
+    if (!id) return false
+    const managed = datastreams.value.find((d) => d.id === id)
+    if (!managed) {
+      resumeDatastreamId.value = null
+      return false
+    }
+    try {
+      await dataVis.plotDatastream(managed)
+      await dataVis.setQcDatastream(id)
+      await enterEdit()
+      return true
+    } catch (e) {
+      resumeDatastreamId.value = null
+      throw e
+    }
+  }
+
+  watch(
+    datastreams,
+    (list) => {
+      if (attempted || !list.length || !resumeDatastreamId.value) return
+      attempted = true
+      resume().catch((e) => {
+        Snackbar.error(
+          e instanceof Error ? e.message : 'Could not reopen the edit session.'
+        )
+      })
+    },
+    { immediate: true }
+  )
+
+  return { resume }
+}
diff --git a/apps/qc-app/src/composables/useUnsavedChangesWarning.ts b/apps/qc-app/src/composables/useUnsavedChangesWarning.ts
new file mode 100644
index 00000000..861435b0
--- /dev/null
+++ b/apps/qc-app/src/composables/useUnsavedChangesWarning.ts
@@ -0,0 +1,21 @@
+/**
+ * Native browser confirmation while the given flag is true, so a refresh
+ * mid-edit can't silently drop edits that never reached the server.
+ * Browsers ignore custom messages and only prompt after user interaction.
+ */
+
+import { onMounted, onUnmounted, type Ref } from 'vue'
+
+export function useUnsavedChangesWarning(hasUnsavedChanges: Ref<boolean>) {
+  function onBeforeUnload(event: BeforeUnloadEvent) {
+    if (!hasUnsavedChanges.value) return
+    event.preventDefault()
+    // Legacy browsers gate the prompt on a non-empty returnValue.
+    event.returnValue = ''
+  }
+
+  onMounted(() => window.addEventListener('beforeunload', onBeforeUnload))
+  onUnmounted(() => window.removeEventListener('beforeunload', onBeforeUnload))
+
+  return { onBeforeUnload }
+}
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
index 3b83a072..01c17d9c 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
@@ -7,8 +7,12 @@ import {
 } from '../persistOperations'
 import { unwrap } from '../unwrap'
 
-const op = (method: string, args: unknown[] = []): QcHistoryOperation =>
-  ({ method, args }) as unknown as QcHistoryOperation
+const op = (
+  method: string,
+  args: unknown[] = [],
+  comment?: string
+): QcHistoryOperation =>
+  ({ method, args, ...(comment ? { comment } : {}) }) as unknown as QcHistoryOperation
 
 const WIN = {
   phenomenonTimeStart: '2025-01-01T00:00:00Z',
@@ -45,6 +49,45 @@ describe('sessionOperationsFromSerialized', () => {
   })
 })
 
+describe('persistSessionOperations — comments', () => {
+  it('sends a comment with a newly-appended operation', async () => {
+    const qc = makeQcFake()
+    const { historyId, sessionId } = await sessionWith(qc)
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('VALUE_THRESHOLD', [{ min: 0 }], 'sensor fouling'),
+    ])
+    const [saved] = await listOps(qc, historyId, sessionId)
+    expect(saved.comment).toBe('sensor fouling')
+  })
+
+  it('patches a comment written onto an already-persisted operation', async () => {
+    const qc = makeQcFake()
+    const { historyId, sessionId } = await sessionWith(qc)
+    const ops = [op('VALUE_THRESHOLD', [{ min: 0 }])]
+    await persistSessionOperations(qc.operations, historyId, sessionId, ops)
+    expect((await listOps(qc, historyId, sessionId))[0].comment).toBeNull()
+
+    // Same operation, annotated after the fact.
+    const annotated = [op('VALUE_THRESHOLD', [{ min: 0 }], 'sensor fouling')]
+    await persistSessionOperations(qc.operations, historyId, sessionId, annotated)
+    const saved = await listOps(qc, historyId, sessionId)
+    expect(saved).toHaveLength(1)
+    expect(saved[0].comment).toBe('sensor fouling')
+  })
+
+  it('clears a comment that was removed, treating blank as none', async () => {
+    const qc = makeQcFake()
+    const { historyId, sessionId } = await sessionWith(qc)
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('VALUE_THRESHOLD', [], 'sensor fouling'),
+    ])
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('VALUE_THRESHOLD', [], '   '),
+    ])
+    expect((await listOps(qc, historyId, sessionId))[0].comment).toBeNull()
+  })
+})
+
 describe('persistSessionOperations', () => {
   it('appends newly-added operations, leaving existing ones in place', async () => {
     const qc = makeQcFake()
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts b/apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts
index 0a68ca7c..5eedd957 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts
@@ -124,6 +124,9 @@ export function makeQcFake(createdBy: QcContact = TEST_USER): QcFake {
       const id = nextId('s')
       const start = body.phenomenonTimeStart
       const end = body.phenomenonTimeEnd
+      // Distinct, increasing creation stamps so ordering by `createdAt` is
+      // meaningful even when sessions share a phenomenon-time window.
+      const createdAt = new Date(Date.UTC(2025, 0, 1, 0, 0, seq)).toISOString()
       const dependencyIds = forHistory(historyId)
         .filter(
           (s) =>
@@ -136,7 +139,7 @@ export function makeQcFake(createdBy: QcContact = TEST_USER): QcFake {
         id,
         historyId,
         createdBy,
-        createdAt: '2025-01-01T00:00:00Z',
+        createdAt,
         phenomenonTimeStart: start,
         phenomenonTimeEnd: end,
         status: 'in_progress',
@@ -167,6 +170,11 @@ export function makeQcFake(createdBy: QcContact = TEST_USER): QcFake {
         list = list.filter((s) => ids.has(s.id))
       }
       if (query?.status) list = list.filter((s) => s.status === query.status)
+      // Mirrors the API: `expand_related` returns the detail shape, which
+      // embeds each session's operations.
+      if (query?.expand_related) {
+        return ok(list.map((s) => ({ ...s, operations: [...s.operations] })))
+      }
       return ok(list.map(summary))
     },
 
@@ -191,7 +199,9 @@ export function makeQcFake(createdBy: QcContact = TEST_USER): QcFake {
       const s = find(sessionId)
       if (!s) return fail('session not found', 404)
       s.status = 'committed'
-      s.committedAt = '2025-06-01T00:00:00Z'
+      // Increasing stamps so commit order is distinguishable, which is what
+      // orders a session's ancestor chain on reconstruction.
+      s.committedAt = new Date(Date.UTC(2025, 5, 1, 0, 0, (seq += 1))).toISOString()
       s.managedChecksum = `mchk-${sessionId}`
       return ok({ ...s, operations: [...s.operations] } as QcSessionDetail)
     },
@@ -226,8 +236,21 @@ export function makeQcFake(createdBy: QcContact = TEST_USER): QcFake {
       return ok(created)
     },
 
-    async update() {
-      return fail('not implemented')
+    async update(
+      _historyId: string,
+      sessionId: string,
+      operationId: string,
+      body: { comment?: string | null; order?: number }
+    ) {
+      const s = find(sessionId)
+      if (s?.status === 'committed') {
+        return fail('Operations can only be updated in an in-progress session.')
+      }
+      const op = s?.operations.find((o) => o.id === operationId)
+      if (!op) return fail('operation not found', 404)
+      if (body.comment !== undefined) (op as any).comment = body.comment
+      if (body.order !== undefined) (op as any).order = body.order
+      return ok(op)
     },
 
     async delete(_historyId: string, sessionId: string, operationId: string) {
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
index 28b9f97c..4ae4e76f 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
@@ -10,8 +10,16 @@ const win = (start: string, end: string) => ({
   phenomenonTimeEnd: end,
 })
 
+// `redoStack` and `reload` exist on every real record; `loadLatestBase`
+// uses them to reset a cached one back to its stored state.
 const rec = (dataX: number[]) =>
-  ({ dataX, dataY: dataX.map(() => 0), history: [] }) as unknown as ObservationRecord
+  ({
+    dataX,
+    dataY: dataX.map(() => 0),
+    history: [],
+    redoStack: [],
+    reload: vi.fn(async () => {}),
+  }) as unknown as ObservationRecord
 
 const newHistory = async (qc: ReturnType<typeof makeQcFake>) =>
   unwrap(
@@ -104,3 +112,289 @@ describe('reconstructSession', () => {
     expect(result.record).toBe(sourceRec)
   })
 })
+
+describe('reconstructCommittedSession', () => {
+  /** Two committed sessions over the same window, the second chained to the first. */
+  const chainOf = async (
+    qc: ReturnType<typeof makeQcFake>,
+    firstRange = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'),
+    secondRange = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
+  ) => {
+    const historyId = await newHistory(qc)
+    const first = unwrap(await qc.sessions.create(historyId, firstRange))
+    await qc.operations.create(historyId, first.id, [
+      { operationType: 'SELECTION' as any, order: 0 },
+    ])
+    await qc.sessions.commit(historyId, first.id)
+
+    const second = unwrap(await qc.sessions.create(historyId, secondRange))
+    await qc.operations.create(historyId, second.id, [
+      { operationType: 'DELETE_POINTS' as any, order: 0 },
+      { operationType: 'INTERPOLATE' as any, order: 1 },
+    ])
+    await qc.sessions.commit(historyId, second.id)
+    return { historyId, first, second }
+  }
+
+  const spies = () => {
+    const base = rec([Date.UTC(2025, 0, 1)])
+    const fetchInRange = vi.fn().mockResolvedValue(base)
+    let captured: QcHistory | undefined
+    const applyHistory = vi.fn(
+      async (record: ObservationRecord, history: QcHistory) => {
+        captured = history
+        // Stand in for the engine: one history entry per replayed op.
+        history.operations.forEach((op) =>
+          (record.history as any[]).push({ method: op.method })
+        )
+        return { applied: history.operations.length, failed: [] }
+      }
+    )
+    return { base, fetchInRange, applyHistory, captured: () => captured }
+  }
+
+  it('replays the ancestor chain from the raw source, not the managed datastream', async () => {
+    const qc = makeQcFake()
+    const { historyId, second } = await chainOf(qc)
+    const { fetchInRange, applyHistory, captured } = spies()
+    const { reconstructCommittedSession } = await import('../reconstructSession')
+
+    await reconstructCommittedSession(
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      source,
+      historyId,
+      second.id
+    )
+
+    // The managed datastream already carries every commit, so the base has
+    // to be the raw source or the replay reproduces the final state.
+    expect(fetchInRange).toHaveBeenCalledTimes(1)
+    expect(fetchInRange.mock.calls[0][0]).toBe(source)
+    // Ancestor first, then the viewed session's own operations.
+    expect(captured()!.operations.map((o) => o.method)).toEqual([
+      'SELECTION',
+      'DELETE_POINTS',
+      'INTERPOLATE',
+    ])
+  })
+
+  it('loads the union window so a wider ancestor replays in alignment', async () => {
+    const qc = makeQcFake()
+    const { historyId, second } = await chainOf(
+      qc,
+      win('2024-12-01T00:00:00Z', '2025-03-01T00:00:00Z'),
+      win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
+    )
+    const { fetchInRange, applyHistory } = spies()
+    const { reconstructCommittedSession } = await import('../reconstructSession')
+
+    await reconstructCommittedSession(
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      source,
+      historyId,
+      second.id
+    )
+
+    const [, begin, end] = fetchInRange.mock.calls[0]
+    expect((begin as Date).toISOString()).toBe('2024-12-01T00:00:00.000Z')
+    expect((end as Date).toISOString()).toBe('2025-03-01T00:00:00.000Z')
+  })
+
+  it('leaves only the viewed session operations on display', async () => {
+    const qc = makeQcFake()
+    const { historyId, second } = await chainOf(qc)
+    const { fetchInRange, applyHistory } = spies()
+    const { reconstructCommittedSession } = await import('../reconstructSession')
+
+    const { record } = await reconstructCommittedSession(
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      source,
+      historyId,
+      second.id
+    )
+
+    // Three ops were replayed to build the data; only this session's two show.
+    expect((record.history as any[]).map((h) => h.method)).toEqual([
+      'DELETE_POINTS',
+      'INTERPOLATE',
+    ])
+  })
+
+  it('reconstructs the first session without its later commits applied', async () => {
+    const qc = makeQcFake()
+    const { historyId, first } = await chainOf(qc)
+    const { fetchInRange, applyHistory, captured } = spies()
+    const { reconstructCommittedSession } = await import('../reconstructSession')
+
+    const { record } = await reconstructCommittedSession(
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      source,
+      historyId,
+      first.id
+    )
+
+    // The later session is a dependent, not an ancestor, so it stays out.
+    expect(captured()!.operations.map((o) => o.method)).toEqual(['SELECTION'])
+    expect((record.history as any[]).map((h) => h.method)).toEqual(['SELECTION'])
+  })
+})
+
+describe('reconstructCommittedSession — chain order', () => {
+  // The fake allows only one in-progress session at a time, so creation
+  // order always matches commit order there. Stub the services directly to
+  // build the case where they disagree.
+  const ok = (data: unknown) => ({ ok: true, data, status: 200, message: '' })
+  const session = (
+    id: string,
+    createdAt: string,
+    committedAt: string | null
+  ) => ({
+    id,
+    createdAt,
+    committedAt,
+    phenomenonTimeStart: '2025-01-01T00:00:00Z',
+    phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+  })
+
+  it('orders by commit time, not creation time', async () => {
+    // `early` is authored last but committed first, so its operation has to
+    // replay first: committing is what wrote its data for the next session.
+    const early = session('early', '2099-01-01T00:00:00Z', '2025-06-01T00:00:00Z')
+    const viewed = session('viewed', '2025-01-01T00:00:00Z', '2025-06-02T00:00:00Z')
+
+    const qcSessions = {
+      get: vi.fn(async () => ok(viewed)),
+      list: vi.fn(async () => ok([early])),
+    } as any
+    const opsById: Record<string, unknown[]> = {
+      early: [{ operationType: 'SELECTION', arguments: [] }],
+      viewed: [{ operationType: 'DELETE_POINTS', arguments: [] }],
+    }
+    const qcOperations = {
+      list: vi.fn(async (_h: string, id: string) => ok(opsById[id])),
+    } as any
+
+    let captured: QcHistory | undefined
+    const applyHistory = vi.fn(async (_r: ObservationRecord, h: QcHistory) => {
+      captured = h
+      return { applied: h.operations.length, failed: [] }
+    })
+    const { reconstructCommittedSession } = await import('../reconstructSession')
+
+    await reconstructCommittedSession(
+      {
+        qcSessions,
+        qcOperations,
+        fetchInRange: vi.fn().mockResolvedValue(rec([Date.UTC(2025, 0, 1)])),
+        applyHistory,
+      },
+      source,
+      'h-1',
+      'viewed'
+    )
+
+    expect(captured!.operations.map((o) => o.method)).toEqual([
+      'SELECTION',
+      'DELETE_POINTS',
+    ])
+  })
+
+  it('falls back to creation time when a session is not committed', async () => {
+    const uncommitted = session('draft', '2025-01-01T00:00:00Z', null)
+    const viewed = session('viewed', '2025-02-01T00:00:00Z', '2025-06-01T00:00:00Z')
+    const qcSessions = {
+      get: vi.fn(async () => ok(viewed)),
+      list: vi.fn(async () => ok([uncommitted])),
+    } as any
+    const opsById: Record<string, unknown[]> = {
+      draft: [{ operationType: 'SELECTION', arguments: [] }],
+      viewed: [{ operationType: 'DELETE_POINTS', arguments: [] }],
+    }
+    const qcOperations = {
+      list: vi.fn(async (_h: string, id: string) => ok(opsById[id])),
+    } as any
+
+    let captured: QcHistory | undefined
+    const applyHistory = vi.fn(async (_r: ObservationRecord, h: QcHistory) => {
+      captured = h
+      return { applied: h.operations.length, failed: [] }
+    })
+    const { reconstructCommittedSession } = await import('../reconstructSession')
+
+    await reconstructCommittedSession(
+      {
+        qcSessions,
+        qcOperations,
+        fetchInRange: vi.fn().mockResolvedValue(rec([Date.UTC(2025, 0, 1)])),
+        applyHistory,
+      },
+      source,
+      'h-1',
+      'viewed'
+    )
+
+    expect(captured!.operations.map((o) => o.method)).toEqual([
+      'SELECTION',
+      'DELETE_POINTS',
+    ])
+  })
+})
+
+describe('reconstructCommittedSession — attribution', () => {
+  const ok = (data: unknown) => ({ ok: true, data, status: 200, message: '' })
+
+  it('carries the operation performer through to the replayed history', async () => {
+    const viewed = {
+      id: 'viewed',
+      createdAt: '2025-01-01T00:00:00Z',
+      committedAt: '2025-06-01T00:00:00Z',
+      phenomenonTimeStart: '2025-01-01T00:00:00Z',
+      phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+    }
+    const qcSessions = {
+      get: vi.fn(async () => ok(viewed)),
+      list: vi.fn(async () => ok([])),
+    } as any
+    const qcOperations = {
+      list: vi.fn(async () =>
+        ok([
+          {
+            operationType: 'SELECTION',
+            arguments: [],
+            createdBy: { name: 'Ada Lovelace', email: 'ada@example.org' },
+          },
+          // A contact without a name still attributes, via the email.
+          {
+            operationType: 'DELETE_POINTS',
+            arguments: [],
+            createdBy: { name: '', email: 'grace@example.org' },
+          },
+        ])
+      ),
+    } as any
+
+    let captured: QcHistory | undefined
+    const applyHistory = vi.fn(async (_r: ObservationRecord, h: QcHistory) => {
+      captured = h
+      return { applied: h.operations.length, failed: [] }
+    })
+    const { reconstructCommittedSession } = await import('../reconstructSession')
+
+    await reconstructCommittedSession(
+      {
+        qcSessions,
+        qcOperations,
+        fetchInRange: vi.fn().mockResolvedValue(rec([Date.UTC(2025, 0, 1)])),
+        applyHistory,
+      },
+      source,
+      'h-1',
+      'viewed'
+    )
+
+    expect(captured!.operations.map((o) => o.performedBy)).toEqual([
+      'Ada Lovelace',
+      'grace@example.org',
+    ])
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
index 4f1175a2..68d03ff6 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
@@ -90,8 +90,16 @@ describe('loadLatestBase', () => {
   const source = { id: 's-1' } as unknown as Datastream
   const start = new Date('2025-01-01T00:00:00Z')
   const end = new Date('2025-02-01T00:00:00Z')
+  // Real records always carry `history`/`redoStack`; `loadLatestBase` clears
+  // them, since the store hands back a cached instance.
   const recWith = (n: number) =>
-    ({ dataX: Array(n).fill(0), dataY: Array(n).fill(0) }) as unknown as ObservationRecord
+    ({
+      dataX: Array(n).fill(0),
+      dataY: Array(n).fill(0),
+      history: [],
+      redoStack: [],
+      reload: vi.fn(async () => {}),
+    }) as unknown as ObservationRecord
 
   it('uses the managed datastream when it has committed data', async () => {
     const managedRec = recWith(3)
@@ -114,3 +122,87 @@ describe('loadLatestBase', () => {
     expect(fetchInRange.mock.calls[1][0]).toBe(source)
   })
 })
+
+describe('loadLatestBase — cached record', () => {
+  // The observation store keeps one record per datastream and returns the
+  // same instance, so a new session's base can arrive carrying the previous
+  // session's replayed operations.
+  const makeRecord = (history: any[], dataX = [1, 2, 3]) => {
+    const reload = vi.fn(async () => {})
+    return {
+      dataX,
+      history,
+      redoStack: [] as any[],
+      reload,
+    } as any
+  }
+
+  it('clears operations left on the cached record and reloads from raw', async () => {
+    const cached = makeRecord([{ method: 'SELECTION' }, { method: 'DELETE_POINTS' }])
+    const fetchInRange = vi.fn().mockResolvedValue(cached)
+
+    const base = await loadLatestBase(
+      fetchInRange,
+      { id: 'm-1' } as any,
+      { id: 's-1' } as any,
+      new Date('2025-01-01T00:00:00Z'),
+      new Date('2025-02-01T00:00:00Z')
+    )
+
+    expect(base.history).toHaveLength(0)
+    expect(base.redoStack).toHaveLength(0)
+    // Data was mutated by the previous session's replay, so raw is restored.
+    expect(cached.reload).toHaveBeenCalled()
+  })
+
+  it('keeps the array reference so bound consumers stay connected', async () => {
+    const history: any[] = [{ method: 'SELECTION' }]
+    const cached = makeRecord(history)
+    const fetchInRange = vi.fn().mockResolvedValue(cached)
+
+    const base = await loadLatestBase(
+      fetchInRange,
+      { id: 'm-1' } as any,
+      { id: 's-1' } as any,
+      new Date('2025-01-01T00:00:00Z'),
+      new Date('2025-02-01T00:00:00Z')
+    )
+
+    expect(base.history).toBe(history)
+  })
+
+  it('leaves a clean record untouched', async () => {
+    const cached = makeRecord([])
+    const fetchInRange = vi.fn().mockResolvedValue(cached)
+
+    await loadLatestBase(
+      fetchInRange,
+      { id: 'm-1' } as any,
+      { id: 's-1' } as any,
+      new Date('2025-01-01T00:00:00Z'),
+      new Date('2025-02-01T00:00:00Z')
+    )
+
+    expect(cached.reload).not.toHaveBeenCalled()
+  })
+
+  it('falls back to the source when the managed datastream is empty', async () => {
+    const empty = makeRecord([], [])
+    const sourceRecord = makeRecord([{ method: 'SELECTION' }])
+    const fetchInRange = vi
+      .fn()
+      .mockResolvedValueOnce(empty)
+      .mockResolvedValueOnce(sourceRecord)
+
+    const base = await loadLatestBase(
+      fetchInRange,
+      { id: 'm-1' } as any,
+      { id: 's-1' } as any,
+      new Date('2025-01-01T00:00:00Z'),
+      new Date('2025-02-01T00:00:00Z')
+    )
+
+    expect(base).toBe(sourceRecord)
+    expect(base.history).toHaveLength(0)
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/persistOperations.ts b/apps/qc-app/src/services/qualityControl/persistOperations.ts
index cd6f1f44..609f3769 100644
--- a/apps/qc-app/src/services/qualityControl/persistOperations.ts
+++ b/apps/qc-app/src/services/qualityControl/persistOperations.ts
@@ -18,13 +18,18 @@ import { unwrap } from './unwrap'
 type QcOperationPostBody = QualityControlOperationContract.PostBody[number]
 
 function operationBody(op: QcHistoryOperation, order: number): QcOperationPostBody {
-  return {
+  const body: QcOperationPostBody = {
     operationType: op.method as unknown as QcOperationPostBody['operationType'],
     arguments: op.args,
     order,
   }
+  if (op.comment) body.comment = op.comment
+  return body
 }
 
+/** Normalized comment for comparison: blank and absent are both "no comment". */
+const commentOf = (value?: string | null): string | null => value?.trim() || null
+
 /**
  * Map qc-utils serialized operations to QC API operation bodies: rename
  * method/args to operationType/arguments and assign order by index.
@@ -66,6 +71,19 @@ export async function persistSessionOperations(
     if (op) unwrap(await qcOperations.delete(historyId, sessionId, op.id))
   }
 
+  // Comments are patched in place; the rest of an operation is append-only.
+  const retained = Math.min(existing.length, operations.length)
+  for (let i = 0; i < retained; i++) {
+    const persisted = existing[i]
+    const local = operations[i]
+    if (!persisted || !local) continue
+    const comment = commentOf(local.comment)
+    if (commentOf(persisted.comment) === comment) continue
+    unwrap(
+      await qcOperations.update(historyId, sessionId, persisted.id, { comment })
+    )
+  }
+
   // Append the operations added since the last save (server stamps creator).
   const appended = operations.slice(existing.length)
   if (appended.length) {
diff --git a/apps/qc-app/src/services/qualityControl/reconstructSession.ts b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
index 6a11c309..0647247d 100644
--- a/apps/qc-app/src/services/qualityControl/reconstructSession.ts
+++ b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
@@ -13,6 +13,7 @@
 
 import type {
   Datastream,
+  QualityControlSession,
   QualityControlSessionService,
   QualityControlOperationService,
   QualityControlOperation,
@@ -27,11 +28,21 @@ import { unwrap } from './unwrap'
 import { loadLatestBase, type FetchObservationsInRange } from './session'
 
 /** API operation -> qc-utils replayable operation (rename operationType/arguments). */
-const toSerialized = (op: QualityControlOperation): QcHistoryOperation =>
-  ({
+const toSerialized = (op: QualityControlOperation): QcHistoryOperation => {
+  const serialized = {
     method: op.operationType as unknown as QcHistoryOperation['method'],
     args: Array.isArray(op.arguments) ? op.arguments : [],
-  }) as QcHistoryOperation
+  } as QcHistoryOperation
+  if (op.comment) serialized.comment = op.comment
+  const performedBy = performerName(op)
+  if (performedBy) serialized.performedBy = performedBy
+  return serialized
+}
+
+function performerName(op: QualityControlOperation): string | undefined {
+  const by = (op as { createdBy?: { name?: string; email?: string } }).createdBy
+  return by?.name?.trim() || by?.email?.trim() || undefined
+}
 
 export interface ReconstructSessionDeps {
   qcSessions: QualityControlSessionService
@@ -61,7 +72,6 @@ export async function reconstructSession(
   const start = new Date(session.phenomenonTimeStart)
   const end = new Date(session.phenomenonTimeEnd)
 
-  // Latest committed state as the base, then this session's own operations.
   const record = await loadLatestBase(fetchInRange, managed, source, start, end)
   const ops = unwrap(
     await qcOperations.list(historyId, sessionId, { fetch_all: true })
@@ -80,3 +90,66 @@ export async function reconstructSession(
   const report = await applyHistory(record, history)
   return { record, report }
 }
+
+/**
+ * Rebuild the state a committed session left behind, for read-only viewing.
+ * The managed datastream carries every commit, so the ancestor chain is
+ * replayed from the raw source instead.
+ *
+ * Ordered by `committedAt`, since commit order is what a later session
+ * built on. Loaded over the union of the chain's windows: operations replay
+ * against array indices, so a narrower base misaligns a wider ancestor.
+ */
+export async function reconstructCommittedSession(
+  deps: ReconstructSessionDeps,
+  source: Datastream,
+  historyId: string,
+  sessionId: string
+): Promise<ReconstructSessionResult> {
+  const { qcSessions, qcOperations, fetchInRange, applyHistory } = deps
+
+  const session = unwrap(await qcSessions.get(historyId, sessionId))
+  const ancestors = unwrap(
+    await qcSessions.list(historyId, {
+      ancestor_of: sessionId,
+      fetch_all: true,
+    } as never)
+  )
+
+  const chainOrder = (s: QualityControlSession) => s.committedAt ?? s.createdAt
+  const chain = [...ancestors, session].sort((a, b) =>
+    chainOrder(a).localeCompare(chainOrder(b))
+  )
+
+  const startMs = Math.min(
+    ...chain.map((s) => new Date(s.phenomenonTimeStart).getTime())
+  )
+  const endMs = Math.max(
+    ...chain.map((s) => new Date(s.phenomenonTimeEnd).getTime())
+  )
+
+  const record = await fetchInRange(source, new Date(startMs), new Date(endMs))
+
+  const perSession = await Promise.all(
+    chain.map(async (s) =>
+      unwrap(await qcOperations.list(historyId, s.id, { fetch_all: true }))
+    )
+  )
+
+  const history: QcHistory = {
+    version: '1',
+    createdAt: session.createdAt,
+    window: {
+      startDate: new Date(startMs).toISOString(),
+      endDate: new Date(endMs).toISOString(),
+    },
+    operations: perSession.flat().map(toSerialized),
+  }
+
+  const report = await applyHistory(record, history)
+
+  const ownCount = perSession[chain.length - 1]?.length ?? 0
+  record.history.splice(0, record.history.length - ownCount)
+
+  return { record, report }
+}
diff --git a/apps/qc-app/src/services/qualityControl/session.ts b/apps/qc-app/src/services/qualityControl/session.ts
index 0ab8bb95..67e55180 100644
--- a/apps/qc-app/src/services/qualityControl/session.ts
+++ b/apps/qc-app/src/services/qualityControl/session.ts
@@ -86,6 +86,22 @@ export async function loadLatestBase(
   end: Date
 ): Promise<ObservationRecord> {
   const base = await fetchInRange(managed, start, end)
-  if ((base.dataX?.length ?? 0) > 0) return base
-  return fetchInRange(source, start, end)
+  const record =
+    (base.dataX?.length ?? 0) > 0 ? base : await fetchInRange(source, start, end)
+  return resetToStoredState(record)
+}
+
+/**
+ * The observation store keeps one `ObservationRecord` per datastream and
+ * hands back the same instance, so this may be the one a previous session
+ * was editing. Truncated in place to keep bound references.
+ */
+async function resetToStoredState(
+  record: ObservationRecord
+): Promise<ObservationRecord> {
+  if (!record.history.length && !record.redoStack.length) return record
+  record.history.length = 0
+  record.redoStack.length = 0
+  await record.reload()
+  return record
 }
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index cf6dd065..a9268a85 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -635,3 +635,70 @@ describe('useDataVisStore.adoptManagedDatastream', () => {
     expect(mockFetchObservationsInRange).not.toHaveBeenCalled()
   })
 })
+
+describe('useDataVisStore.releaseManagedDatastream', () => {
+  const withHistory = (store: any) => {
+    store.qcHistories = [
+      { id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'src' },
+    ] as any
+    store.datastreams = [
+      makeDs({ id: 'src', name: 'Raw' }),
+      makeDs({ id: 'mgd', name: 'Raw (QC)' }),
+    ] as any
+  }
+
+  // Managed datastreams are hidden from the catalog table, so leaving the
+  // editor with one plotted shows a plot with no row selected.
+  it('swaps the managed datastream back to its source and refetches its data', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withHistory(store)
+    const working = { tag: 'uncommitted-edits' }
+    store.plottedDatastreams = [makeDs({ id: 'mgd', name: 'Raw (QC)' })] as any
+    store.qcDatastreamId = 'mgd'
+    mockGraphSeriesArray.value = [
+      { id: 'mgd', name: 'Raw (QC)', data: working, color: '#1', yAxisLabel: 'T' },
+    ]
+
+    await store.releaseManagedDatastream()
+
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['src'])
+    expect(store.qcDatastreamId).toBe('src')
+    // The editor's working copy carries uncommitted edits, so it is dropped
+    // and the source's stored data fetched instead.
+    expect(
+      mockGraphSeriesArray.value.some((s: any) => s.data === working)
+    ).toBe(false)
+    expect(mockFetchGraphSeries).toHaveBeenCalled()
+    expect(mockFetchGraphSeries.mock.calls[0][0].id).toBe('src')
+  })
+
+  it('is a no-op when the plotted datastream is not a managed one', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withHistory(store)
+    store.plottedDatastreams = [makeDs({ id: 'src', name: 'Raw' })] as any
+    store.qcDatastreamId = 'src'
+
+    await store.releaseManagedDatastream()
+
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['src'])
+    expect(store.qcDatastreamId).toBe('src')
+  })
+
+  it('leaves the plot alone when the source is missing from the catalog', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    store.qcHistories = [
+      { id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'gone' },
+    ] as any
+    store.datastreams = [makeDs({ id: 'mgd', name: 'Raw (QC)' })] as any
+    store.plottedDatastreams = [makeDs({ id: 'mgd', name: 'Raw (QC)' })] as any
+    store.qcDatastreamId = 'mgd'
+
+    await store.releaseManagedDatastream()
+
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['mgd'])
+    expect(store.qcDatastreamId).toBe('mgd')
+  })
+})
diff --git a/apps/qc-app/src/store/__tests__/plotly.spec.ts b/apps/qc-app/src/store/__tests__/plotly.spec.ts
index aaed339a..f0e0388d 100644
--- a/apps/qc-app/src/store/__tests__/plotly.spec.ts
+++ b/apps/qc-app/src/store/__tests__/plotly.spec.ts
@@ -512,3 +512,45 @@ describe('usePlotlyStore.requestTableScroll', () => {
     expect(store.tableScrollRequest).toEqual({ time: 500, seq: 2 })
   })
 })
+
+describe('usePlotlyStore.editHistory', () => {
+  beforeEach(() => {
+    setActivePinia(createPinia())
+    vi.clearAllMocks()
+    qcId.value = null
+  })
+
+  it('is empty when no series is selected for QC', async () => {
+    const { usePlotlyStore } = await import('@/store/plotly')
+    expect(usePlotlyStore().editHistory).toEqual([])
+  })
+
+  it('reads the selected series record history', async () => {
+    const { usePlotlyStore } = await import('@/store/plotly')
+    const store = usePlotlyStore()
+    const series = makeSeries('qc', COLORS[0]) as any
+    series.data = { history: [{ method: 'SELECTION' }] }
+    store.graphSeriesArray.push(series)
+    qcId.value = 'qc'
+    expect(store.editHistory).toHaveLength(1)
+  })
+
+  // Resuming a session swaps in a freshly reconstructed record without
+  // redrawing. When this was rebound inside createPlotlyOption it kept
+  // pointing at the previous record and the operations panel read empty.
+  it('follows a record swapped in without a redraw', async () => {
+    const { usePlotlyStore } = await import('@/store/plotly')
+    const store = usePlotlyStore()
+    const series = makeSeries('qc', COLORS[0]) as any
+    series.data = { history: [] }
+    store.graphSeriesArray.push(series)
+    qcId.value = 'qc'
+    expect(store.editHistory).toEqual([])
+
+    store.graphSeriesArray[0].data = {
+      history: [{ method: 'SELECTION' }, { method: 'DELETE_POINTS' }],
+    } as any
+
+    expect(store.editHistory).toHaveLength(2)
+  })
+})
diff --git a/apps/qc-app/src/store/__tests__/qcSession.spec.ts b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
index 21a2b9fe..7a9d8eaa 100644
--- a/apps/qc-app/src/store/__tests__/qcSession.spec.ts
+++ b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
@@ -1,4 +1,4 @@
-import { describe, it, expect, beforeEach } from 'vitest'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
 import { createTestPinia } from '@/utils/test/pinia'
 import { useHydroServer } from '@/store/hydroserver'
 import { makeQcFake } from '@/services/qualityControl/__tests__/qcServiceFake'
@@ -101,4 +101,89 @@ describe('useQcSessionStore', () => {
     expect(store.viewedSessionId).toBe(b.id)
     expect(store.isReadOnly).toBe(true)
   })
+
+  it('keeps the resume pointer out of reset, so a reload can still reopen', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    await qc.sessions.create(h.id, win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'))
+
+    const store = useQcSessionStore()
+    await store.loadSessions(h.id)
+    store.resumeDatastreamId = 'm-1'
+
+    // `reset` clears the live session context (leaving a datastream), but the
+    // resume pointer is owned by the editor's enter/exit, not by reset.
+    store.reset()
+    expect(store.historyId).toBeNull()
+    expect(store.sessions).toEqual([])
+    expect(store.resumeDatastreamId).toBe('m-1')
+  })
+
+  // A backend that ignores `expand_related` on this route returns the
+  // summary shape, and the session-list previews then claim "No operations"
+  // for sessions that plainly have some.
+  it('backfills operations when the list response omits them', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const s = unwrap(
+      await qc.sessions.create(h.id, win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'))
+    )
+    await qc.operations.create(h.id, s.id, [
+      { operationType: 'SELECTION' as any, order: 0 },
+      { operationType: 'DELETE_POINTS' as any, order: 1 },
+    ])
+
+    // Stand in for a backend that ignores the flag.
+    const listWithoutOperations = async (historyId: string, query?: any) => {
+      const res = await qc.sessions.list(historyId, { ...query, expand_related: false })
+      return res
+    }
+    useHydroServer().hs = {
+      ...useHydroServer().hs,
+      qualityControlSessions: { ...qc.sessions, list: listWithoutOperations },
+      qualityControlOperations: qc.operations,
+    } as any
+
+    const store = useQcSessionStore()
+    await store.loadSessions(h.id)
+
+    const loaded = store.sessions.find((x) => x.id === s.id) as any
+    expect(loaded.operations.map((o: any) => o.operationType)).toEqual([
+      'SELECTION',
+      'DELETE_POINTS',
+    ])
+  })
+
+  it('does not re-fetch operations the list already embedded', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const s = unwrap(
+      await qc.sessions.create(h.id, win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'))
+    )
+    await qc.operations.create(h.id, s.id, [
+      { operationType: 'SELECTION' as any, order: 0 },
+    ])
+
+    const opsList = vi.fn(qc.operations.list)
+    useHydroServer().hs = {
+      ...useHydroServer().hs,
+      qualityControlSessions: qc.sessions,
+      qualityControlOperations: { ...qc.operations, list: opsList },
+    } as any
+
+    await useQcSessionStore().loadSessions(h.id)
+    expect(opsList).not.toHaveBeenCalled()
+  })
 })
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index 4c0f94dd..627d9dc0 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -254,6 +254,37 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     }
   }
 
+  /**
+   * Inverse of `adoptManagedDatastream`, for leaving the editor. Managed
+   * datastreams are hidden from the catalog, so without this the Select
+   * view shows a plot with no row selected. No-op when the QC target isn't
+   * managed or its source isn't in the catalog.
+   */
+  async function releaseManagedDatastream() {
+    const managedId = qcDatastreamId.value
+    if (!managedId) return
+    const history = qcHistories.value.find(
+      (h) => historyManagedId(h) === managedId
+    )
+    const sourceId = history ? historySourceId(history) : undefined
+    const source = sourceId
+      ? datastreams.value.find((d) => d.id === sourceId)
+      : undefined
+    if (!source) return
+
+    const idx = plottedDatastreams.value.findIndex((d) => d.id === managedId)
+    if (idx < 0) return
+    plottedDatastreams.value.splice(idx, 1, source)
+    qcDatastreamId.value = source.id
+
+    // The working copy holds uncommitted edits; `rebuildPlot` refetches.
+    graphSeriesArray.value = graphSeriesArray.value.filter(
+      (s) => s.id !== managedId
+    )
+
+    await rebuildPlot()
+  }
+
   // Coalescing lock for `rebuildPlot`. Rapid checkbox toggles in the
   // Select view call `plotDatastream` in quick succession; each call
   // pushes into `plottedDatastreams` and then awaits `rebuildPlot`.
@@ -670,6 +701,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     setPlottedDatastreams,
     setQcDatastream,
     adoptManagedDatastream,
+    releaseManagedDatastream,
     rebuildPlot,
     // updateOrFetchGraphSeries,
   }
diff --git a/apps/qc-app/src/store/plotly.ts b/apps/qc-app/src/store/plotly.ts
index 42870f60..4374aef3 100644
--- a/apps/qc-app/src/store/plotly.ts
+++ b/apps/qc-app/src/store/plotly.ts
@@ -162,9 +162,6 @@ export const usePlotlyStore = defineStore('Plotly', () => {
     return -1
   })
 
-  /** The edit history for the currently selected series */
-  const editHistory: Ref<HistoryItem[]> = ref([])
-
   /**
    * Sentinel armed by programmatic Plotly writes (`setPlotSelection`,
    * `clearSelected`). When the next `plotly_relayout`-induced
@@ -184,6 +181,15 @@ export const usePlotlyStore = defineStore('Plotly', () => {
     return graphSeriesArray.value[selectedSeriesIndex.value]
   })
 
+  /**
+   * The edit history for the currently selected series. Derived, not
+   * assigned: resuming a session swaps in a fresh record without redrawing,
+   * so anything rebound during plot-option building would go stale.
+   */
+  const editHistory = computed<HistoryItem[]>(
+    () => selectedSeries.value?.data?.history ?? []
+  )
+
   // Initialize to an empty-trace PlotlyChartOptions so consumers can read
   // `plotlyOptions.value.traces` etc. without null-guards. 
   const plotlyOptions: Ref<PlotlyChartOptions> = ref(createPlotlyOption([]))
diff --git a/apps/qc-app/src/store/qcSession.ts b/apps/qc-app/src/store/qcSession.ts
index 418211f2..00f36f45 100644
--- a/apps/qc-app/src/store/qcSession.ts
+++ b/apps/qc-app/src/store/qcSession.ts
@@ -15,16 +15,31 @@ import type { QualityControlSession } from '@hydroserver/client'
 
 export const useQcSessionStore = defineStore('qcSession', () => {
   const historyId = ref<string | null>(null)
+  /** Managed datastream the editor was last open on. The only persisted
+   *  field; everything else is re-fetched on resume. */
+  const resumeDatastreamId = ref<string | null>(null)
   const sessions = ref<QualityControlSession[]>([])
   /** The single in-progress (editable) session, if any. */
   const currentSessionId = ref<string | null>(null)
   /** The session currently being viewed. */
   const viewedSessionId = ref<string | null>(null)
   const isLoading = ref(false)
+  const isSwitchingSession = ref(false)
 
   /** Editing is allowed only while viewing the in-progress session. */
+  // TODO(backend): ask for a way to keep editing the most recent session after
+  // it is committed, when no newer session exists. Today a commit is terminal:
+  // the API rejects updating, deleting, adding operations to, or re-committing
+  // a committed session, and the PATCH body carries only `description`. Undoing
+  // a commit also has to roll back what it wrote (the replayed observations on
+  // the managed datastream plus the history's checksum and time extent), so
+  // this is a real backend change, not just lifting the status guard.
+  // Guarded on `sessions.length` so editing outside the session workflow
+  // isn't treated as read-only.
   const isReadOnly = computed(
-    () => !currentSessionId.value || viewedSessionId.value !== currentSessionId.value
+    () =>
+      sessions.value.length > 0 &&
+      viewedSessionId.value !== currentSessionId.value
   )
 
   const inProgressSession = computed(
@@ -37,16 +52,48 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     () => sessions.value.find((s) => s.id === viewedSessionId.value) ?? null
   )
 
+  /**
+   * Guarantee every session carries its `operations`, which the previews
+   * read. `expand_related` should embed them, but a backend that ignores it
+   * returns the summary shape instead. Only the gaps are fetched.
+   */
+  async function withOperations(
+    client: ReturnType<typeof useHydroServer>['hs'],
+    id: string,
+    list: QualityControlSession[]
+  ): Promise<QualityControlSession[]> {
+    const missing = list.filter(
+      (s) => !(s as { operations?: unknown[] }).operations
+    )
+    if (!missing.length) return list
+
+    const fetched = new Map<string, unknown[]>()
+    await Promise.all(
+      missing.map(async (s) => {
+        const res = await client.qualityControlOperations.list(id, s.id, {
+          fetch_all: true,
+        })
+        if (res.ok) fetched.set(s.id, res.data)
+      })
+    )
+    return list.map((s) =>
+      fetched.has(s.id) ? { ...s, operations: fetched.get(s.id) } : s
+    ) as QualityControlSession[]
+  }
+
   /** Load a history's sessions; default the view to the in-progress session. */
   async function loadSessions(id: string): Promise<void> {
     isLoading.value = true
     try {
       const { hs } = storeToRefs(useHydroServer())
       const list = unwrap(
-        await hs.value.qualityControlSessions.list(id, { fetch_all: true })
+        await hs.value.qualityControlSessions.list(id, {
+          fetch_all: true,
+          expand_related: true,
+        })
       )
       historyId.value = id
-      sessions.value = list
+      sessions.value = await withOperations(hs.value, id, list)
       const inProgress = list.find((s) => s.status === 'in_progress') ?? null
       currentSessionId.value = inProgress?.id ?? null
       // Default view: the editable session, else the latest committed.
@@ -81,10 +128,12 @@ export const useQcSessionStore = defineStore('qcSession', () => {
 
   return {
     historyId,
+    resumeDatastreamId,
     sessions,
     currentSessionId,
     viewedSessionId,
     isLoading,
+    isSwitchingSession,
     isReadOnly,
     inProgressSession,
     committedSessions,
@@ -94,4 +143,11 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     returnToCurrent,
     reset,
   }
+}, {
+  // Only the datastream id survives a reload. Sessions, operations and the
+  // read-only flag are all re-derived from the server on resume, so
+  // persisting them would just risk showing stale state.
+  persist: {
+    pick: ['resumeDatastreamId'],
+  },
 })
diff --git a/apps/qc-app/src/utils/__tests__/time.spec.ts b/apps/qc-app/src/utils/__tests__/time.spec.ts
index 97357d7c..49fcf142 100644
--- a/apps/qc-app/src/utils/__tests__/time.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/time.spec.ts
@@ -23,9 +23,39 @@ describe('formatDateInput', () => {
 })
 
 describe('formatDateRange', () => {
-  it('joins both bounds with "to"', () => {
-    expect(
-      formatDateRange('2025-01-05T00:00:00', '2025-01-15T09:05:00')
-    ).toBe('01/05/2025 00:00 to 01/15/2025 09:05')
+  it('states the year once when both bounds share it', () => {
+    expect(formatDateRange('2025-01-05T00:00:00', '2025-02-01T00:00:00')).toBe(
+      'Jan 5 – Feb 1, 2025'
+    )
+  })
+
+  it('states both years when the window crosses one', () => {
+    expect(formatDateRange('2024-12-01T00:00:00', '2025-01-15T00:00:00')).toBe(
+      'Dec 1, 2024 – Jan 15, 2025'
+    )
+  })
+
+  it('collapses a single day to one date with a time span', () => {
+    expect(formatDateRange('2025-01-05T09:00:00', '2025-01-05T14:30:00')).toBe(
+      'Jan 5, 2025, 9:00 AM – 2:30 PM'
+    )
+  })
+
+  it('keeps clock times when the bounds are not whole days', () => {
+    expect(formatDateRange('2025-01-05T09:00:00', '2025-02-01T14:30:00')).toBe(
+      'Jan 5, 9:00 AM – Feb 1, 2025, 2:30 PM'
+    )
+  })
+
+  it('drops the clock when the window sits on midnight at both ends', () => {
+    expect(formatDateRange('2025-01-05T00:00:00', '2025-01-06T00:00:00')).toBe(
+      'Jan 5 – Jan 6, 2025'
+    )
+  })
+
+  it('returns a dash for nullish or unparseable bounds', () => {
+    expect(formatDateRange(null, '2025-01-05T00:00:00')).toBe('–')
+    expect(formatDateRange('2025-01-05T00:00:00', undefined)).toBe('–')
+    expect(formatDateRange('not-a-date', '2025-01-05T00:00:00')).toBe('–')
   })
 })
diff --git a/apps/qc-app/src/utils/plotting/options.ts b/apps/qc-app/src/utils/plotting/options.ts
index 94228911..a6d5a69b 100644
--- a/apps/qc-app/src/utils/plotting/options.ts
+++ b/apps/qc-app/src/utils/plotting/options.ts
@@ -504,8 +504,6 @@ export const createPlotlyOption = (
         tickformat: '~s',
       } as Partial<LayoutAxis>
 
-      const { editHistory } = storeToRefs(usePlotlyStore())
-      editHistory.value = s.data.history
     } else {
       // Plotly applies a global selection-fade once any trace has
       // `selectedpoints` set: every other trace's "unselected" markers
diff --git a/apps/qc-app/src/utils/time.ts b/apps/qc-app/src/utils/time.ts
index 8e06bbd5..179e131f 100644
--- a/apps/qc-app/src/utils/time.ts
+++ b/apps/qc-app/src/utils/time.ts
@@ -34,12 +34,63 @@ export function formatDateInput(iso?: string | null): string {
   return `${mm}/${dd}/${d.getFullYear()} ${hh}:${min}`
 }
 
-/** A session's phenomenon-time window, formatted like the date inputs. */
+const MONTH_DAY = new Intl.DateTimeFormat('en-US', {
+  month: 'short',
+  day: 'numeric',
+})
+const MONTH_DAY_YEAR = new Intl.DateTimeFormat('en-US', {
+  month: 'short',
+  day: 'numeric',
+  year: 'numeric',
+})
+const CLOCK = new Intl.DateTimeFormat('en-US', {
+  hour: 'numeric',
+  minute: '2-digit',
+  hour12: true,
+})
+
+const toDate = (iso?: string | null): Date | null => {
+  if (!iso) return null
+  const d = new Date(iso)
+  return Number.isNaN(d.getTime()) ? null : d
+}
+
+/** A whole-day boundary, where showing the clock adds nothing. */
+const isMidnight = (d: Date) =>
+  d.getHours() === 0 && d.getMinutes() === 0 && d.getSeconds() === 0
+
+/**
+ * A session's phenomenon-time window, readable at a glance:
+ *   `Jan 5 – Feb 1, 2025`                    whole days in one year
+ *   `Dec 1, 2024 – Jan 15, 2025`             whole days across years
+ *   `Jan 5, 2025, 9:00 AM – 2:30 PM`         within a single day
+ *   `Jan 5, 9:00 AM – Feb 1, 2025, 2:30 PM`  partial days
+ * The year is stated once when both bounds share it, and clock times are
+ * dropped when the window sits on midnight at both ends.
+ */
 export function formatDateRange(
   start?: string | null,
   end?: string | null
 ): string {
-  return `${formatDateInput(start)} to ${formatDateInput(end)}`
+  const from = toDate(start)
+  const to = toDate(end)
+  if (!from || !to) return '–'
+
+  const wholeDays = isMidnight(from) && isMidnight(to)
+  const sameYear = from.getFullYear() === to.getFullYear()
+
+  if (from.toDateString() === to.toDateString()) {
+    const day = MONTH_DAY_YEAR.format(from)
+    return wholeDays
+      ? day
+      : `${day}, ${CLOCK.format(from)} – ${CLOCK.format(to)}`
+  }
+
+  const left = sameYear ? MONTH_DAY.format(from) : MONTH_DAY_YEAR.format(from)
+  const right = MONTH_DAY_YEAR.format(to)
+  return wholeDays
+    ? `${left} – ${right}`
+    : `${left}, ${CLOCK.format(from)} – ${right}, ${CLOCK.format(to)}`
 }
 
 function getLocalTimeZone(): string {
diff --git a/packages/qc-utils/docs/QC_HISTORY.md b/packages/qc-utils/docs/QC_HISTORY.md
index f964034c..5fbd9970 100644
--- a/packages/qc-utils/docs/QC_HISTORY.md
+++ b/packages/qc-utils/docs/QC_HISTORY.md
@@ -147,8 +147,14 @@ before it.
 |-------------|---------|----------|--------------------------------------------------------------------|
 | `method`    | string  | yes      | `EnumEditOperations` or `EnumFilterOperations` value (e.g. `"VALUE_THRESHOLD"`, `"FILL_GAPS"`). |
 | `args`      | any[]   | yes      | Positional args forwarded to the operation handler. Per-method shape rules below. |
+| `comment`   | string  | no       | Operator's note explaining why the operation was applied. Never read by the engine. Serialized trimmed and omitted when blank; a non-string value is rejected on load. |
+| `performedBy` | string | no      | Display name of whoever applied the operation. Provenance assigned by the consumer (from its own backend), never by the engine. Serialized trimmed and omitted when blank; a non-string value is rejected on load. |
 | `execution` | object  | no       | Per-dispatch audit record (timing, mode, dataset shape, status). See below. Omitted in pre-v0.1.x histories and may be absent on hand-written JSON; the loader accepts both shapes. |
 
+Unlike `execution`, which is audit-only and re-stamped per dispatch,
+`comment` and `performedBy` belong to the operation itself, so
+`applyHistory` carries both onto the replayed `HistoryItem`.
+
 The shape mirrors `ObservationRecord.dispatch`'s tuple form:
 `[method, ...args]`. A loaded history is replayed via
 `record.dispatch(operations.map(o => [o.method, ...o.args]))`.
diff --git a/packages/qc-utils/src/types/index.ts b/packages/qc-utils/src/types/index.ts
index 9c40ba10..5d81c764 100644
--- a/packages/qc-utils/src/types/index.ts
+++ b/packages/qc-utils/src/types/index.ts
@@ -143,6 +143,18 @@ export type HistoryItem = {
   method: EnumEditOperations | EnumFilterOperations;
   args?: any[];
   selected?: number[];
+  /**
+   * Free-text note explaining why this operation was applied. Authored
+   * by the operator, round-tripped through save/load, and never read by
+   * the engine.
+   */
+  comment?: string;
+  /**
+   * Who applied this operation, for display. Server-assigned provenance
+   * rather than authored input: the consumer stamps it when replaying a
+   * stored history, and it is never sent back as part of an operation.
+   */
+  performedBy?: string;
   /**
    * Per-dispatch runtime information. Always present; some fields
    * populate at push time, others fill in after the handler resolves.
@@ -194,6 +206,10 @@ export type QcHistoryExecution = {
 export type QcHistoryOperation = {
   method: EnumEditOperations | EnumFilterOperations;
   args: any[];
+  /** Operator's note for this operation; absent when none was written. */
+  comment?: string;
+  /** Display name of whoever applied it. Audit only, like `execution`. */
+  performedBy?: string;
   execution?: QcHistoryExecution;
 };
 
diff --git a/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts
index c9c776d5..35f2170f 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts
@@ -62,6 +62,28 @@ describe('serializeHistory', () => {
     expect(history.operations[0].args).toEqual([{ 'Greater than': 5 }]);
   });
 
+  it('persists an operator comment, trimmed', async () => {
+    await rec.dispatch(EnumFilterOperations.VALUE_THRESHOLD, { 'Greater than': 5 });
+    rec.history[0].comment = '  sensor fouling  ';
+    const history = serializeHistory(rec, SAMPLE_WINDOW);
+    expect(history.operations[0].comment).toBe('sensor fouling');
+  });
+
+  it('omits the comment field when absent or blank', async () => {
+    await rec.dispatch(EnumFilterOperations.VALUE_THRESHOLD, { 'Greater than': 5 });
+    expect(serializeHistory(rec, SAMPLE_WINDOW).operations[0].comment).toBeUndefined();
+    rec.history[0].comment = '   ';
+    expect(serializeHistory(rec, SAMPLE_WINDOW).operations[0].comment).toBeUndefined();
+  });
+
+  it('persists who performed the operation, trimmed', async () => {
+    await rec.dispatch(EnumFilterOperations.VALUE_THRESHOLD, { 'Greater than': 5 });
+    rec.history[0].performedBy = '  Ada Lovelace  ';
+    expect(serializeHistory(rec, SAMPLE_WINDOW).operations[0].performedBy).toBe(
+      'Ada Lovelace'
+    );
+  });
+
   it('strips runtime-only fields (isLoading, duration, executionMode, selected, icon)', async () => {
     await rec.dispatch(EnumFilterOperations.VALUE_THRESHOLD, { 'Greater than': 5 });
     const history = serializeHistory(rec, SAMPLE_WINDOW);
@@ -170,6 +192,60 @@ describe('parseHistory', () => {
     expect(history.window).toEqual(SAMPLE_WINDOW);
   });
 
+  it('round-trips a per-op comment and tolerates its absence', () => {
+    const base = {
+      version: '1',
+      createdAt: '2024-01-01T00:00:00.000Z',
+      window: SAMPLE_WINDOW,
+    };
+    const withComment = parseHistory({
+      ...base,
+      operations: [
+        { method: 'VALUE_THRESHOLD', args: [{ 'Greater than': 5 }], comment: 'sensor fouling' },
+      ],
+    });
+    expect(withComment.operations[0].comment).toBe('sensor fouling');
+
+    const without = parseHistory({
+      ...base,
+      operations: [{ method: 'VALUE_THRESHOLD', args: [{ 'Greater than': 5 }] }],
+    });
+    expect(without.operations[0].comment).toBeUndefined();
+  });
+
+  it('round-trips performedBy and rejects a non-string one', () => {
+    const base = {
+      version: '1',
+      createdAt: '2024-01-01T00:00:00.000Z',
+      window: SAMPLE_WINDOW,
+    };
+    const parsed = parseHistory({
+      ...base,
+      operations: [
+        { method: 'VALUE_THRESHOLD', args: [], performedBy: 'Ada Lovelace' },
+      ],
+    });
+    expect(parsed.operations[0].performedBy).toBe('Ada Lovelace');
+
+    expect(() =>
+      parseHistory({
+        ...base,
+        operations: [{ method: 'VALUE_THRESHOLD', args: [], performedBy: 7 }],
+      })
+    ).toThrow(/`performedBy` must be a string/);
+  });
+
+  it('rejects a non-string comment', () => {
+    expect(() =>
+      parseHistory({
+        version: '1',
+        createdAt: '2024-01-01T00:00:00.000Z',
+        window: SAMPLE_WINDOW,
+        operations: [{ method: 'VALUE_THRESHOLD', args: [], comment: 42 }],
+      })
+    ).toThrow(/`comment` must be a string/);
+  });
+
   it('rejects an unknown version', () => {
     expect(() =>
       parseHistory({ version: '2', createdAt: '', window: SAMPLE_WINDOW, operations: [] })
@@ -392,6 +468,30 @@ describe('applyHistory — round-trip', () => {
     expect(fresh.history[0].args).toEqual([{ 'Greater than': 10 }]);
   });
 
+  it('carries comments onto the replayed entries', async () => {
+    await rec.dispatch(EnumFilterOperations.VALUE_THRESHOLD, { 'Greater than': 10 });
+    rec.history[0].comment = 'sensor fouling';
+    const history = serializeHistory(rec, SAMPLE_WINDOW);
+
+    const fresh = makeRecord(20);
+    await fresh.reload();
+    await applyHistory(fresh, history);
+
+    expect(fresh.history[0].comment).toBe('sensor fouling');
+  });
+
+  it('carries performedBy onto the replayed entries', async () => {
+    await rec.dispatch(EnumFilterOperations.VALUE_THRESHOLD, { 'Greater than': 10 });
+    rec.history[0].performedBy = 'Ada Lovelace';
+    const history = serializeHistory(rec, SAMPLE_WINDOW);
+
+    const fresh = makeRecord(20);
+    await fresh.reload();
+    await applyHistory(fresh, history);
+
+    expect(fresh.history[0].performedBy).toBe('Ada Lovelace');
+  });
+
   it('round-trips a SELECTION → DELETE_POINTS pair (selection-coupled)', async () => {
     const indices = [0, 1, 2, 5, 10];
     await rec.dispatch([
diff --git a/packages/qc-utils/src/utils/plotting/history.ts b/packages/qc-utils/src/utils/plotting/history.ts
index f548432f..15f47a30 100644
--- a/packages/qc-utils/src/utils/plotting/history.ts
+++ b/packages/qc-utils/src/utils/plotting/history.ts
@@ -152,6 +152,10 @@ export function serializeHistory(
       method: h.method,
       args: h.args ? [...h.args] : [],
     };
+    const comment = h.comment?.trim();
+    if (comment) op.comment = comment;
+    const performedBy = h.performedBy?.trim();
+    if (performedBy) op.performedBy = performedBy;
     const exec = projectExecution(h.execution);
     if (exec) op.execution = exec;
     return op;
@@ -218,10 +222,20 @@ export function parseHistory(json: unknown): QcHistory {
     if (!Array.isArray(o.args)) {
       throw new Error(`Operation ${i} \`args\` must be an array.`);
     }
+    if (o.comment !== undefined && typeof o.comment !== "string") {
+      throw new Error(`Operation ${i} \`comment\` must be a string when present.`);
+    }
+    if (o.performedBy !== undefined && typeof o.performedBy !== "string") {
+      throw new Error(
+        `Operation ${i} \`performedBy\` must be a string when present.`
+      );
+    }
     const op: QcHistoryOperation = {
       method: o.method as EnumEditOperations | EnumFilterOperations,
       args: [...o.args],
     };
+    if (o.comment) op.comment = o.comment as string;
+    if (o.performedBy) op.performedBy = o.performedBy as string;
     const exec = parseExecution(o.execution, i);
     if (exec) op.execution = exec;
     return op;
@@ -279,6 +293,11 @@ export async function applyHistory(
       // `historyItem.execution.status = "failed"` on throw. Read it
       // back to decide whether to count as applied.
       const last = record.history[record.history.length - 1];
+      // Unlike `execution` (runtime telemetry, re-stamped per dispatch),
+      // the comment is authored intent and belongs to the operation, so
+      // it is carried onto the replayed entry.
+      if (last && op.comment) last.comment = op.comment;
+      if (last && op.performedBy) last.performedBy = op.performedBy;
       if (last?.execution.status === "failed") {
         report.failed.push({
           index: i,

From 3fc52bb2ccf95d51e12a323251129cceeaa3be45 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Wed, 5 Aug 2026 15:50:16 -0600
Subject: [PATCH 11/93] feat: add history snapshot comparison

Plot a QC session's state at any operation as a separate, frozen line so
it can be compared against the session being edited. Buttons sit on every
operation row and on the "Data loaded" row, which plots the state the
session started from.

Snapshots ride in plottedDatastreams under the synthetic id
snap:<sessionId>:<opIndex>, so legend rendering, colour assignment,
visibility and reorder work unchanged. isSnapshotId guards the paths that
would otherwise treat one as real: refreshGraphSeriesArray skips its
fetch, releaseManagedDatastream drops them when the editor closes, and
the share encoder keeps them out of ds in favour of its own snap key.

The base is always the session chain's own window, never the plot's time
range, because operations replay against array indices. Snapshots build
on a detached ObservationRecord since the observation store hands back
one shared instance per datastream and the replay mutates what it is
given.

Also fixes two pre-existing problems in the history panel that this work
surfaced:

- Reloading from a step restored the pre-replay entries wholesale, which
  put the old timings back over the ones the replay had just measured.
  Only the "Data loaded" row updated. The panel now keeps the replayed
  entries and appends only what the replay dropped.
- hasUnsavedChanges compared entries by identity, so a replay while
  viewing a committed session read as pending edits and prompted "You
  have unsaved edits". A committed session cannot be edited, so both it
  and unsavedEditCount now short-circuit on isReadOnly.
---
 .../src/components/EditData/EditHistory.vue   | 136 ++++++++--
 .../EditData/__tests__/EditHistory.spec.ts    | 251 +++++++++++++++++-
 apps/qc-app/src/components/VisualizeData.vue  |  23 +-
 .../VisualizeData/PlottedDatastreams.vue      |  39 ++-
 .../__tests__/PlottedDatastreams.spec.ts      |  71 +++++
 .../__tests__/useEditSession.spec.ts          |  26 ++
 .../__tests__/useHistorySnapshots.spec.ts     | 219 +++++++++++++++
 apps/qc-app/src/composables/useEditSession.ts |  16 +-
 .../src/composables/useHistorySnapshots.ts    | 132 +++++++++
 .../__tests__/buildSnapshot.spec.ts           | 140 ++++++++++
 .../__tests__/reconstructSession.spec.ts      |  62 +++++
 .../services/qualityControl/buildSnapshot.ts  |  84 ++++++
 .../qualityControl/reconstructSession.ts      |  10 +-
 .../store/__tests__/dataVisualization.spec.ts |  90 +++++++
 .../src/store/__tests__/qcSession.spec.ts     |  10 +
 apps/qc-app/src/store/dataVisualization.ts    |  76 +++++-
 apps/qc-app/src/store/qcSession.ts            |   6 +-
 apps/qc-app/src/types/index.ts                |  26 ++
 apps/qc-app/src/utils/__tests__/share.spec.ts |  38 +++
 .../src/utils/__tests__/snapshotId.spec.ts    |  42 +++
 apps/qc-app/src/utils/__tests__/time.spec.ts  |  13 +-
 .../utils/plotting/__tests__/options.spec.ts  |  65 +++++
 apps/qc-app/src/utils/share.ts                |  19 ++
 apps/qc-app/src/utils/snapshotId.ts           |  32 +++
 apps/qc-app/src/utils/time.ts                 |   7 +
 25 files changed, 1591 insertions(+), 42 deletions(-)
 create mode 100644 apps/qc-app/src/composables/__tests__/useHistorySnapshots.spec.ts
 create mode 100644 apps/qc-app/src/composables/useHistorySnapshots.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/buildSnapshot.ts
 create mode 100644 apps/qc-app/src/utils/__tests__/snapshotId.spec.ts
 create mode 100644 apps/qc-app/src/utils/snapshotId.ts

diff --git a/apps/qc-app/src/components/EditData/EditHistory.vue b/apps/qc-app/src/components/EditData/EditHistory.vue
index 71ffec34..b6893f05 100644
--- a/apps/qc-app/src/components/EditData/EditHistory.vue
+++ b/apps/qc-app/src/components/EditData/EditHistory.vue
@@ -154,10 +154,41 @@
           color="primary"
           indeterminate
         />
-        <v-tooltip v-else location="start" text="Reload from server">
+        <v-tooltip
+          location="start"
+          :text="
+            snapshotShown(SNAPSHOT_BASELINE_INDEX)
+              ? 'Remove this comparison line'
+              : `Plot the session's starting state`
+          "
+        >
           <template #activator="{ props: tp }">
             <v-btn
               v-bind="tp"
+              data-testid="history-snapshot-baseline"
+              aria-label="Plot the session's starting state"
+              size="x-small"
+              variant="text"
+              density="comfortable"
+              :icon="
+                snapshotShown(SNAPSHOT_BASELINE_INDEX)
+                  ? 'mdi-chart-line-variant'
+                  : 'mdi-chart-line'
+              "
+              :color="
+                snapshotShown(SNAPSHOT_BASELINE_INDEX) ? 'primary' : undefined
+              "
+              :disabled="isBuilding"
+              @click="onToggleSnapshot(SNAPSHOT_BASELINE_INDEX)"
+            />
+          </template>
+        </v-tooltip>
+
+        <v-tooltip v-if="!selectedSeries?.data.isLoading" location="start" text="Reload from server">
+          <template #activator="{ props: tp }">
+            <v-btn
+              v-bind="tp"
+              data-testid="history-reload-btn"
               size="x-small"
               variant="text"
               density="comfortable"
@@ -281,13 +312,14 @@
               </v-tooltip>
 
               <v-tooltip
-                v-if="entry.execution?.status === 'failed'"
+                v-if="isApplied(index) && entry.execution?.status === 'failed'"
                 location="start"
                 text="Operation failed: see console for details"
               >
                 <template #activator="{ props: tp }">
                   <v-icon
                     v-bind="tp"
+                    :data-testid="`history-failed-${index}`"
                     icon="mdi-alert-circle"
                     size="14"
                     color="error"
@@ -295,25 +327,11 @@
                 </template>
               </v-tooltip>
 
-              <!-- Dev-only badge showing whether the dispatch ran on a
-                   worker or inline; gated on import.meta.env.DEV. -->
-              <v-chip
-                v-if="isDev && entry.execution?.mode"
-                size="x-small"
-                variant="tonal"
-                :color="entry.execution.mode === 'inline' ? 'success' : 'primary'"
-                class="edit-history__mode-chip"
-                :title="
-                  entry.execution.mode === 'inline'
-                    ? 'Ran on the main thread (inline)'
-                    : 'Ran on a web worker'
-                "
-              >
-                {{ entry.execution.mode }}
-              </v-chip>
-
+              <!-- `!= null` so a replay that measures 0ms still reads as
+                   having run. -->
               <span
-                v-if="entry.execution?.durationMs"
+                v-if="isApplied(index) && entry.execution?.durationMs != null"
+                :data-testid="`history-duration-${index}`"
                 class="text-body-small text-medium-emphasis"
               >
                 {{ formatDuration(entry.execution.durationMs) }}
@@ -327,6 +345,34 @@
                 indeterminate
               />
 
+              <v-tooltip
+                location="start"
+                :text="
+                  snapshotShown(index)
+                    ? 'Remove this comparison line'
+                    : 'Plot this step as a comparison line'
+                "
+              >
+                <template #activator="{ props: tp }">
+                  <v-btn
+                    v-bind="tp"
+                    :data-testid="`history-snapshot-${index}`"
+                    aria-label="Plot this step as a comparison line"
+                    size="x-small"
+                    variant="text"
+                    density="comfortable"
+                    :icon="
+                      snapshotShown(index)
+                        ? 'mdi-chart-line-variant'
+                        : 'mdi-chart-line'
+                    "
+                    :color="snapshotShown(index) ? 'primary' : undefined"
+                    :disabled="isBuilding || entry.execution?.inFlight"
+                    @click="onToggleSnapshot(index)"
+                  />
+                </template>
+              </v-tooltip>
+
               <v-tooltip location="start" text="Reload from this step">
                 <template #activator="{ props: tp }">
                   <v-btn
@@ -379,6 +425,28 @@
               </li>
             </ul>
 
+            <div
+              v-if="isApplied(index) && isDev && entry.execution?.mode"
+              class="text-body-small text-medium-emphasis mt-3 d-flex align-center ga-2"
+              :data-testid="`history-execution-${index}`"
+            >
+              <!-- Dev-only: whether the dispatch ran on a worker or inline. -->
+              <v-chip
+                v-if="isDev && entry.execution?.mode"
+                size="x-small"
+                variant="tonal"
+                :color="entry.execution.mode === 'inline' ? 'success' : 'primary'"
+                class="edit-history__mode-chip"
+                :title="
+                  entry.execution.mode === 'inline'
+                    ? 'Ran on the main thread (inline)'
+                    : 'Ran on a web worker'
+                "
+              >
+                {{ entry.execution.mode }}
+              </v-chip>
+            </div>
+
             <div
               v-if="entry.performedBy"
               class="text-body-small text-medium-emphasis mt-3"
@@ -435,6 +503,8 @@ import { iconForMethod, colorForMethod } from '@/components/EditData/operations'
 import SessionList from '@/components/EditData/SessionList.vue'
 import { useQcHistory } from '@/composables/useQcHistory'
 import { useQcSessionStore } from '@/store/qcSession'
+import { useHistorySnapshots } from '@/composables/useHistorySnapshots'
+import { SNAPSHOT_BASELINE_INDEX } from '@/utils/snapshotId'
 import { Snackbar } from '@uwrl/qc-utils'
 import type { HistoryItem } from '@uwrl/qc-utils'
 
@@ -496,7 +566,20 @@ const shownStepIndex = computed<number | null>(() => {
 
 // Committed sessions are immutable server-side, so their comments are shown
 // but not editable.
-const { isReadOnly, isSwitchingSession } = storeToRefs(useQcSessionStore())
+const { isReadOnly, isSwitchingSession, viewedSessionId } =
+  storeToRefs(useQcSessionStore())
+
+const { toggleSnapshot, isSnapshotPlotted, isBuilding } = useHistorySnapshots()
+
+// Not gated on isReadOnly: plotting a comparison line is a read action.
+const onToggleSnapshot = async (opIndex: number) => {
+  const sessionId = viewedSessionId.value
+  if (!sessionId) return
+  await toggleSnapshot(sessionId, opIndex)
+}
+
+const snapshotShown = (opIndex: number) =>
+  !!viewedSessionId.value && isSnapshotPlotted(viewedSessionId.value, opIndex)
 
 function setComment(entry: HistoryItem, value: string) {
   entry.comment = value
@@ -517,6 +600,11 @@ function toggle(index: number) {
   openIndex.value = openIndex.value === index ? null : index
 }
 
+/** Steps past the one on screen were not replayed, so their execution
+ *  record describes a run that no longer holds in this view. */
+const isApplied = (index: number) =>
+  shownStepIndex.value === null || index <= shownStepIndex.value
+
 function formatMethod(method: string) {
   if (!method) return ''
   return method
@@ -581,7 +669,11 @@ const onReloadHistory = async (index: number) => {
     setTimeout(async () => {
       const newSelection = await record?.reloadHistory(index)
       if (preserved && record) {
-        record.history.splice(0, record.history.length, ...preserved)
+        // Append only what the replay dropped. Restoring `preserved`
+        // wholesale would put the pre-replay timings back.
+        const tail = preserved.slice(index + 1)
+        record.history.push(...tail)
+        loadedStepIndex.value = record.history.length - tail.length - 1
       }
 
       isUpdating.value = false
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
index 26db515d..1da67363 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
@@ -31,6 +31,18 @@ vi.mock('@/composables/useDataSelection', () => ({
   useDataSelection: () => ({ clearSelected, setPlotSelection }),
 }))
 
+const toggleSnapshot = vi.fn().mockResolvedValue(undefined)
+const plottedSnapshots = ref<string[]>([])
+const isBuilding = ref(false)
+vi.mock('@/composables/useHistorySnapshots', () => ({
+  useHistorySnapshots: () => ({
+    toggleSnapshot,
+    isSnapshotPlotted: (sessionId: string, opIndex: number) =>
+      plottedSnapshots.value.includes(`${sessionId}:${opIndex}`),
+    isBuilding,
+  }),
+}))
+
 vi.mock('@uwrl/qc-utils', () => ({
   formatDuration: (ms: number) => String(ms) + 'ms',
   // operations.ts (transitively imported via EditHistory.vue's
@@ -405,6 +417,62 @@ describe('EditHistory.vue actions', () => {
       return store
     }
 
+    /**
+     * Stands in for the real `reloadHistory`: truncate to `0..index` and
+     * replace the survivors with freshly dispatched entries carrying the
+     * timings the replay just measured. Splices through the ref's proxy;
+     * mutating the raw array wouldn't trigger reactivity.
+     */
+    const replayingReloadHistory = () =>
+      vi.fn(async (index: number) => {
+        const fresh = editHistory.value.slice(0, index + 1).map((h, i) => ({
+          method: h.method,
+          args: h.args,
+          execution: { inFlight: false, status: 'success', durationMs: 900 + i },
+        }))
+        editHistory.value.splice(0, editHistory.value.length, ...fresh)
+        return []
+      })
+
+    // The committed-session path restores the full operation list after the
+    // replay so the user can keep stepping through it. That restore must not
+    // drag the pre-replay timings back with it.
+    it('shows the timings the replay produced, not the originals', async () => {
+      const history = [
+        makeEntry('SELECTION', [], { durationMs: 10 }),
+        makeEntry('DELETE_POINTS', [], { durationMs: 20 }),
+        makeEntry('INTERPOLATE', [], { durationMs: 30 }),
+      ]
+      editHistory.value = history
+      selectedSeries.value = {
+        data: {
+          history: editHistory.value,
+          redoStack: [],
+          reloadHistory: replayingReloadHistory(),
+        },
+      }
+      const w = createWrapper()
+      await readOnly()
+      await flushPromises()
+
+      await w
+        .find('[data-testid="history-item-1"]')
+        .findAll('button')
+        .at(-1)!
+        .trigger('click')
+      // `loadedStepIndex` moves synchronously, so waiting on the marker
+      // would race the replay. Wait for the dispatch itself to settle.
+      await vi.waitFor(() => expect(isUpdating.value).toBe(false))
+      await flushPromises()
+
+      // Replayed entries report the new measurement.
+      expect(w.find('[data-testid="history-duration-0"]').text()).toContain('900')
+      expect(w.find('[data-testid="history-duration-1"]').text()).toContain('901')
+      // The un-replayed tail survives on screen but did not run.
+      expect(w.findAll('[data-testid^="history-item-"]').length).toBe(3)
+      expect(w.find('[data-testid="history-duration-2"]').exists()).toBe(false)
+    })
+
     it('hides the per-entry undo and disables the toolbar undo/redo', async () => {
       editHistory.value = [{ method: 'ADD_POINTS', args: [], execution: {} }]
       selectedSeries.value = { data: { history: editHistory.value, redoStack: [{}] } }
@@ -431,8 +499,9 @@ describe('EditHistory.vue actions', () => {
       await readOnly()
       await flushPromises()
 
-      const baseline = w.find('.edit-history__row--baseline')
-      expect(baseline.find('button').attributes('disabled')).toBeDefined()
+      expect(
+        w.find('[data-testid="history-reload-btn"]').attributes('disabled')
+      ).toBeDefined()
     })
 
     it('keeps the entries below when reloading from a step', async () => {
@@ -502,6 +571,88 @@ describe('EditHistory.vue actions', () => {
       )
       expect(w.find('[data-testid="history-loaded-1"]').exists()).toBe(true)
     })
+
+    // Reloading from a step un-applies everything below it. A committed
+    // session keeps those rows on screen, so their telemetry would otherwise
+    // still advertise a run that no longer holds in this view.
+    it('drops execution info for steps after the one being shown', async () => {
+      const history = [
+        makeEntry('SELECTION', [], { durationMs: 10 }),
+        makeEntry('DELETE_POINTS', [], { durationMs: 20, status: 'failed' }),
+        makeEntry('INTERPOLATE', [], { durationMs: 30 }),
+      ]
+      editHistory.value = history
+      const reloadHistory = vi.fn(async () => [])
+      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+
+      const w = createWrapper()
+      await flushPromises()
+      expect(w.find('[data-testid="history-duration-2"]').exists()).toBe(true)
+      expect(w.find('[data-testid="history-failed-1"]').exists()).toBe(true)
+
+      await w
+        .find('[data-testid="history-item-0"]')
+        .findAll('button')
+        .at(-1)!
+        .trigger('click')
+      await vi.waitFor(() =>
+        expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(true)
+      )
+
+      // The step you reloaded from did run, so it keeps its label.
+      expect(w.find('[data-testid="history-duration-0"]').exists()).toBe(true)
+      expect(w.find('[data-testid="history-duration-1"]').exists()).toBe(false)
+      expect(w.find('[data-testid="history-duration-2"]').exists()).toBe(false)
+      // The failure badge is execution state too, so it goes as well.
+      expect(w.find('[data-testid="history-failed-1"]').exists()).toBe(false)
+    })
+  })
+
+  describe('execution telemetry placement', () => {
+    // Duration stays on the row: it is the answer to "did that step run?",
+    // which you want without expanding. Only the dev-only mode chip moved.
+    it('keeps the duration on the row and the mode chip off it', async () => {
+      editHistory.value = [
+        makeEntry('SELECTION', [], { durationMs: 1234, mode: 'inline' }),
+      ]
+      const w = createWrapper()
+      await flushPromises()
+
+      const row = w.find('[data-testid="history-item-0"] .edit-history__row')
+      expect(row.find('[data-testid="history-duration-0"]').exists()).toBe(true)
+      expect(row.text()).not.toContain('inline')
+    })
+
+    it('shows the mode chip in the expanded drawer', async () => {
+      editHistory.value = [
+        makeEntry('SELECTION', [], { durationMs: 1234, mode: 'inline' }),
+      ]
+      const w = createWrapper()
+      await flushPromises()
+
+      await w
+        .find('[data-testid="history-item-0"] .edit-history__expand')
+        .trigger('click')
+      await flushPromises()
+
+      const detail = w.find('[data-testid="history-execution-0"]')
+      expect(detail.exists()).toBe(true)
+      expect(detail.text()).toContain('inline')
+    })
+
+    // A replayed step can measure well under a millisecond, which rounds to
+    // "0ms". That is still a real run and must not read as "never ran".
+    it('renders sub-millisecond and zero durations rather than hiding them', async () => {
+      editHistory.value = [
+        makeEntry('SELECTION', [], { durationMs: 0.0054 }),
+        makeEntry('DELETE_POINTS', [], { durationMs: 0 }),
+      ]
+      const w = createWrapper()
+      await flushPromises()
+
+      expect(w.find('[data-testid="history-duration-0"]').exists()).toBe(true)
+      expect(w.find('[data-testid="history-duration-1"]').exists()).toBe(true)
+    })
   })
 
   describe('session switch', () => {
@@ -615,3 +766,99 @@ describe('EditHistory.vue actions', () => {
     })
   })
 })
+
+describe('EditHistory.vue snapshot buttons', () => {
+  beforeEach(() => {
+    editHistory.value = []
+    isUpdating.value = false
+    isBuilding.value = false
+    plottedSnapshots.value = []
+    selectedSeries.value = makeSeries()
+    vi.clearAllMocks()
+  })
+
+  const makeSession = (id: string, createdAt = '2026-03-01T00:00:00Z') => ({
+    id,
+    createdAt,
+    status: 'in_progress',
+    phenomenonTimeStart: '2026-03-01T00:00:00Z',
+    phenomenonTimeEnd: '2026-04-01T00:00:00Z',
+  })
+
+  /**
+   * SessionList only renders the operations slot for a session it actually
+   * lists, or in its no-session fallback. Seed `sessions` accordingly or the
+   * history rows never mount.
+   */
+  const mountWithSession = async (
+    sessionId: string | null,
+    sessions: unknown[] = sessionId ? [makeSession(sessionId)] : []
+  ) => {
+    const pinia = createTestPinia()
+    const { useQcSessionStore } = await import('@/store/qcSession')
+    const store = useQcSessionStore()
+    store.sessions = sessions as any
+    store.viewedSessionId = sessionId
+    return mount(EditHistory, {
+      props: {},
+      global: { plugins: [pinia, createTestVuetify()] },
+    })
+  }
+
+  it('renders an add-to-plot button on each operation row and the baseline', async () => {
+    editHistory.value = [makeEntry('FILL_GAPS'), makeEntry('DELETE_POINTS')]
+
+    const w = await mountWithSession('sess-1')
+
+    expect(w.find('[data-testid="history-snapshot-baseline"]').exists()).toBe(true)
+    expect(w.find('[data-testid="history-snapshot-0"]').exists()).toBe(true)
+    expect(w.find('[data-testid="history-snapshot-1"]').exists()).toBe(true)
+  })
+
+  it('toggles the snapshot for the clicked operation', async () => {
+    editHistory.value = [makeEntry('FILL_GAPS')]
+
+    const w = await mountWithSession('sess-1')
+    await w.find('[data-testid="history-snapshot-0"]').trigger('click')
+
+    expect(toggleSnapshot).toHaveBeenCalledWith('sess-1', 0)
+  })
+
+  it('toggles the baseline snapshot at index -1', async () => {
+    editHistory.value = [makeEntry('FILL_GAPS')]
+
+    const w = await mountWithSession('sess-1')
+    await w.find('[data-testid="history-snapshot-baseline"]').trigger('click')
+
+    expect(toggleSnapshot).toHaveBeenCalledWith('sess-1', -1)
+  })
+
+  // Plotting a comparison line is a read action, so it stays available on a
+  // committed session, which is exactly when comparing matters most.
+  it('stays enabled while the viewed session is read-only', async () => {
+    editHistory.value = [makeEntry('FILL_GAPS')]
+
+    const w = await mountWithSession('sess-1', [
+      makeSession('sess-1'),
+      makeSession('sess-2', '2026-04-01T00:00:00Z'),
+    ])
+    const { useQcSessionStore } = await import('@/store/qcSession')
+    const store = useQcSessionStore()
+    store.currentSessionId = 'sess-2'
+    await flushPromises()
+
+    expect(store.isReadOnly).toBe(true)
+    expect(
+      w.find('[data-testid="history-snapshot-0"]').attributes('disabled')
+    ).toBeUndefined()
+  })
+
+  it('does nothing when no session is being viewed', async () => {
+    editHistory.value = [makeEntry('FILL_GAPS')]
+
+    const w = await mountWithSession(null)
+    await w.find('[data-testid="history-snapshot-0"]').trigger('click')
+
+    expect(toggleSnapshot).not.toHaveBeenCalled()
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index 2fc15794..dc3bdbd9 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -492,10 +492,13 @@ import {
   encodeShareState,
   type ShareState,
 } from '@/utils/share'
+import { isSnapshotId, parseSnapshotId } from '@/utils/snapshotId'
+import { useHistorySnapshots } from '@/composables/useHistorySnapshots'
 import { useWorkspaceStore } from '@/store/workspaces'
 import { useResizable, usePersistedFlag } from '@/composables/useResizable'
 
 const { resetState } = useDataVisStore()
+const { toggleSnapshot } = useHistorySnapshots()
 const {
   plottedDatastreams,
   qcDatastream,
@@ -922,7 +925,13 @@ const hydrateFromUrl = () => {
       }
     : null
 
-  void setPlottedDatastreams(resolved, qcId)
+  // Snapshots replay against the session store, so they wait for the plot
+  // (and with it the editor's session load) to settle.
+  void setPlottedDatastreams(resolved, qcId).then(async () => {
+    for (const s of state.snapshots ?? []) {
+      await toggleSnapshot(s.sessionId, s.opIndex)
+    }
+  })
 }
 
 if (datastreams.value.length) {
@@ -945,7 +954,7 @@ if (datastreams.value.length) {
 // lives in `share.ts` so this watcher reads as a plain assembly of
 // inputs.
 const SHARE_KEYS = [
-  'ws', 'm', 'tab', 'ds', 'r', 'from', 'to',
+  'ws', 'm', 'tab', 'ds', 'snap', 'r', 'from', 'to',
   't', 'op', 'pl', 'h', 'ya', 'z', 'yz', 'dp', 'th',
 ] as const
 
@@ -970,7 +979,14 @@ watch(
     tooltipsMaxDataPoints,
   ],
   () => {
-    const ids = plottedDatastreams.value.map((ds) => ds.id)
+    // Snapshots travel in their own key; `ids` stays real datastreams so the
+    // QC-target-is-first rule and the visibility bitmasks still line up.
+    const plotted = plottedDatastreams.value
+    const ids = plotted.filter((ds) => !isSnapshotId(ds.id)).map((ds) => ds.id)
+    const snapshots = plotted
+      .filter((ds) => isSnapshotId(ds.id))
+      .map((ds) => parseSnapshotId(ds.id))
+      .filter((s): s is NonNullable<typeof s> => !!s)
     const isEdit = currentView.value === DrawerType.Edit
 
     const state: ShareState = {
@@ -978,6 +994,7 @@ watch(
       editView: isEdit,
       tableTab: activeTab.value === 'table',
       datastreamIds: ids,
+      snapshots,
       datePresetId: Number.isFinite(selectedDateBtnId.value)
         ? selectedDateBtnId.value
         : null,
diff --git a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
index 900f4c3f..ba31a89b 100644
--- a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
+++ b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
@@ -128,9 +128,25 @@
             class="plotted-item__title d-flex align-center ga-1"
             :title="datastream.name"
           >
+            <v-icon
+              v-if="snapshotFor(datastream.id)"
+              icon="mdi-history"
+              size="14"
+              class="flex-shrink-0"
+            />
             <span>{{ datastream.name }}</span>
+            <v-chip
+              v-if="snapshotFor(datastream.id)"
+              size="x-small"
+              variant="tonal"
+              label
+              class="flex-shrink-0"
+            >
+              snapshot
+            </v-chip>
             <v-tooltip
               v-if="
+                !snapshotFor(datastream.id) &&
                 !loadStatus(datastream.id).loading &&
                 loadStatus(datastream.id).count === 0
               "
@@ -150,7 +166,10 @@
             </v-tooltip>
           </div>
           <div class="plotted-item__subtitle">
-            <template v-if="loadStatus(datastream.id).loading">
+            <template v-if="snapshotFor(datastream.id)">
+              {{ snapshotSubtitle(datastream.id) }}
+            </template>
+            <template v-else-if="loadStatus(datastream.id).loading">
               loading…
             </template>
             <template v-else>
@@ -202,6 +221,7 @@ const {
 } = storeToRefs(usePlotlyStore())
 import { ref, computed } from 'vue'
 import { Datastream } from '@hydroserver/client'
+import { formatDayStamp } from '@/utils/time'
 
 const { plottedDatastreams, qcDatastream, loadingStates } =
   storeToRefs(useDataVisStore())
@@ -253,6 +273,23 @@ const loadStatusById = computed<
 const loadStatus = (id: string) =>
   loadStatusById.value[id] ?? { loading: true, count: 0 }
 
+const snapshotFor = (id: string) =>
+  graphSeriesArray.value.find((s) => s.id === id)?.snapshot
+
+/** `step 3 of 7: Fill Gaps - by Alice - Mar 14, 2026` */
+const snapshotSubtitle = (id: string): string => {
+  const meta = snapshotFor(id)
+  if (!meta) return ''
+  const parts = [
+    meta.opIndex < 0
+      ? 'session start'
+      : `step ${meta.opIndex + 1} of ${meta.opCount}: ${meta.opName}`,
+  ]
+  if (meta.performedBy) parts.push(`by ${meta.performedBy}`)
+  parts.push(formatDayStamp(meta.createdAt))
+  return parts.join(' - ')
+}
+
 async function clearAll() {
   hiddenTraceIds.value = new Set()
   await clearPlottedDatastreams()
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
index 9d3f3f2a..d6f0f003 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
@@ -216,3 +216,74 @@ describe('PlottedDatastreams.vue — load status', () => {
     expect(wrapper.find('.plotted-item__subtitle').text()).toBe('6 pts loaded')
   })
 })
+
+describe('PlottedDatastreams snapshot rows', () => {
+  const snapMeta = {
+    sessionId: 'sess-1',
+    sessionLabel: 'March backfill',
+    opIndex: 2,
+    opCount: 7,
+    opName: 'Fill Gaps',
+    performedBy: 'Alice',
+    createdAt: '2026-03-14T00:00:00',
+  }
+
+  beforeEach(() => {
+    plottedDatastreams.value = [
+      datastreamA,
+      { id: 'snap:sess-1:2', name: 'March backfill' },
+    ]
+    graphSeriesArray.value = [
+      { id: datastreamA.id, data: { isLoading: false } },
+      {
+        id: 'snap:sess-1:2',
+        name: 'March backfill',
+        data: { isLoading: false },
+        snapshot: snapMeta,
+      },
+    ]
+    plotlyOptions.value = { traces: [] }
+    qcDatastream.value = datastreamA
+    hiddenAxisIds.value = new Set()
+    hiddenTraceIds.value = new Set()
+  })
+
+  it('renders the provenance line and a snapshot chip', () => {
+    const text = mountIt().text()
+
+    expect(text).toContain('March backfill')
+    expect(text).toContain('snapshot')
+    expect(text).toContain('step 3 of 7: Fill Gaps')
+    expect(text).toContain('by Alice')
+    expect(text).toContain('Mar 14, 2026')
+  })
+
+  it('labels a baseline snapshot as the session start', () => {
+    graphSeriesArray.value[1].snapshot = {
+      ...snapMeta,
+      opIndex: -1,
+      opName: '',
+    }
+
+    const text = mountIt().text()
+
+    expect(text).toContain('session start')
+    expect(text).not.toContain('step 0 of 7')
+  })
+
+  // Snapshots are outside the loaded window by design, so the empty-window
+  // warning would fire on every one of them.
+  it('does not flag a snapshot as an empty window', () => {
+    const wrapper = mountIt()
+    const rows = wrapper.findAll('.plotted-item')
+
+    expect(rows[1]!.find('.plotted-item__empty-flag').exists()).toBe(false)
+  })
+
+  it('keeps the axis toggle so the snapshot can be shifted', () => {
+    const wrapper = mountIt()
+    const rows = wrapper.findAll('.plotted-item')
+
+    expect(rows[1]!.find('.plotted-item__axis-toggle').exists()).toBe(true)
+  })
+})
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 97bfe2cf..12b23c97 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -232,6 +232,32 @@ describe('useEditSession', () => {
     expect(session.unsavedEditCount.value).toBe(0)
   })
 
+  // A committed session cannot be edited, so nothing about it can be
+  // unsaved. Stepping through its history replaces entries as it replays,
+  // which an identity comparison would otherwise read as pending edits.
+  it('reports no unsaved changes while viewing a read-only session', async () => {
+    await seedHistory()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+
+    selectedSeries.value.data.history.push({ method: 'DELETE_POINTS', args: [] })
+    expect(session.hasUnsavedChanges.value).toBe(true)
+
+    const store = useQcSessionStore()
+    store.sessions = [
+      { id: 'a', status: 'committed', createdAt: '2025-01-01T00:00:00Z' },
+      { id: 'b', status: 'in_progress', createdAt: '2025-02-01T00:00:00Z' },
+    ] as any
+    store.currentSessionId = 'b'
+    store.viewedSessionId = 'a'
+
+    expect(store.isReadOnly).toBe(true)
+    expect(session.hasUnsavedChanges.value).toBe(false)
+    expect(session.unsavedEditCount.value).toBe(0)
+  })
+
   it('resuming a session starts with no unsaved changes', async () => {
     const h = unwrap(
       await qc.histories.create({
diff --git a/apps/qc-app/src/composables/__tests__/useHistorySnapshots.spec.ts b/apps/qc-app/src/composables/__tests__/useHistorySnapshots.spec.ts
new file mode 100644
index 00000000..cc13548e
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useHistorySnapshots.spec.ts
@@ -0,0 +1,219 @@
+/**
+ * Snapshots are built off a detached record and toggled on/off the plot.
+ * The store actions are stubbed here; what matters is that the composable
+ * resolves the right session, replays to the right index, and hands over
+ * provenance metadata.
+ */
+
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createPinia, setActivePinia } from 'pinia'
+
+const {
+  plottedDatastreams,
+  qcDatastream,
+  addSnapshotSeries,
+  removeSnapshotSeries,
+  sessions,
+  historyId,
+  sourceDatastream,
+  viewedSessionId,
+  selectedSeries,
+  observationsRaw,
+  fetchObservationsInRange,
+  buildSnapshotRecord,
+  hs,
+  error,
+} = vi.hoisted(() => {
+  const { ref: r } = require('vue') as typeof import('vue')
+  return {
+    plottedDatastreams: r<any[]>([]),
+    qcDatastream: r<any>({ id: 'mgd-1', name: 'Managed' }),
+    addSnapshotSeries: vi.fn().mockResolvedValue(undefined),
+    removeSnapshotSeries: vi.fn().mockResolvedValue(undefined),
+    sessions: r<any[]>([]),
+    historyId: r<string | null>('hist-1'),
+    sourceDatastream: r<any>({ id: 'src-1', name: 'Source' }),
+    viewedSessionId: r<string | null>(null),
+    selectedSeries: r<any>({ data: { history: [] } }),
+    observationsRaw: r<Record<string, any>>({}),
+    fetchObservationsInRange: vi.fn().mockResolvedValue({}),
+    buildSnapshotRecord: vi.fn(),
+    hs: r<any>({
+      qualityControlSessions: {},
+      qualityControlOperations: {},
+    }),
+    error: vi.fn(),
+  }
+})
+
+vi.mock('@/store/dataVisualization', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useDataVisStore: defineStore('dataVisualization', () => ({
+      plottedDatastreams,
+      qcDatastream,
+      addSnapshotSeries,
+      removeSnapshotSeries,
+    })),
+  }
+})
+
+vi.mock('@/store/qcSession', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useQcSessionStore: defineStore('qcSession', () => ({
+      sessions,
+      historyId,
+      sourceDatastream,
+      viewedSessionId,
+    })),
+  }
+})
+
+vi.mock('@/store/plotly', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    usePlotlyStore: defineStore('Plotly', () => ({ selectedSeries })),
+  }
+})
+
+vi.mock('@/store/observations', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useObservationStore: defineStore('observations', () => ({
+      observationsRaw,
+      fetchObservationsInRange,
+    })),
+  }
+})
+
+vi.mock('@/store/hydroserver', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useHydroServer: defineStore('hydroserver', () => ({ hs })),
+  }
+})
+
+vi.mock('@/services/qualityControl/buildSnapshot', () => ({
+  buildSnapshotRecord,
+}))
+
+vi.mock('@uwrl/qc-utils', () => ({
+  Snackbar: { error, success: vi.fn(), warn: vi.fn() },
+  applyHistory: vi.fn(),
+  ObservationRecord: class {
+    rawData: unknown
+    constructor(raw: unknown) {
+      this.rawData = raw
+    }
+    async applyWindow() {}
+  },
+}))
+
+import { useHistorySnapshots } from '../useHistorySnapshots'
+
+const committed = {
+  id: 'sess-c',
+  status: 'committed',
+  description: 'March backfill',
+  createdAt: '2026-03-14T00:00:00Z',
+  phenomenonTimeStart: '2026-03-01T00:00:00Z',
+  phenomenonTimeEnd: '2026-04-01T00:00:00Z',
+  operations: [
+    { operationType: 'FILL_GAPS', createdBy: { name: 'Alice' } },
+    { operationType: 'DELETE_POINTS', createdBy: { name: 'Bob' } },
+  ],
+}
+
+beforeEach(() => {
+  setActivePinia(createPinia())
+  vi.clearAllMocks()
+  plottedDatastreams.value = []
+  sessions.value = [committed]
+  historyId.value = 'hist-1'
+  sourceDatastream.value = { id: 'src-1', name: 'Source' }
+  qcDatastream.value = { id: 'mgd-1', name: 'Managed' }
+  viewedSessionId.value = null
+  buildSnapshotRecord.mockResolvedValue({ history: [], isLoading: false })
+})
+
+describe('useHistorySnapshots', () => {
+  it('adds a snapshot series with provenance metadata', async () => {
+    const { toggleSnapshot } = useHistorySnapshots()
+
+    await toggleSnapshot('sess-c', 1)
+
+    expect(addSnapshotSeries).toHaveBeenCalledTimes(1)
+    const [id, , meta] = addSnapshotSeries.mock.calls[0]!
+    expect(id).toBe('snap:sess-c:1')
+    expect(meta).toMatchObject({
+      sessionId: 'sess-c',
+      sessionLabel: 'March backfill',
+      opIndex: 1,
+      opCount: 2,
+      opName: 'Delete Points',
+      performedBy: 'Bob',
+    })
+  })
+
+  it('labels the baseline snapshot with no operation name', async () => {
+    const { toggleSnapshot } = useHistorySnapshots()
+
+    await toggleSnapshot('sess-c', -1)
+
+    const [, , meta] = addSnapshotSeries.mock.calls[0]!
+    expect(meta.opIndex).toBe(-1)
+    expect(meta.opName).toBe('')
+  })
+
+  it('removes an already-plotted snapshot instead of rebuilding it', async () => {
+    plottedDatastreams.value = [{ id: 'snap:sess-c:1', name: 'x' }]
+    const { toggleSnapshot } = useHistorySnapshots()
+
+    await toggleSnapshot('sess-c', 1)
+
+    expect(removeSnapshotSeries).toHaveBeenCalledWith('snap:sess-c:1')
+    expect(buildSnapshotRecord).not.toHaveBeenCalled()
+  })
+
+  it('reports whether a snapshot is plotted', () => {
+    plottedDatastreams.value = [{ id: 'snap:sess-c:1', name: 'x' }]
+    const { isSnapshotPlotted } = useHistorySnapshots()
+
+    expect(isSnapshotPlotted('sess-c', 1)).toBe(true)
+    expect(isSnapshotPlotted('sess-c', 0)).toBe(false)
+  })
+
+  it('passes the live draft history for the session being viewed', async () => {
+    viewedSessionId.value = 'sess-c'
+    selectedSeries.value = {
+      data: { history: [{ method: 'FILL_GAPS', args: [] }] },
+    }
+    const { toggleSnapshot } = useHistorySnapshots()
+
+    await toggleSnapshot('sess-c', 0)
+
+    const params = buildSnapshotRecord.mock.calls[0]![1]
+    expect(params.liveHistory).toEqual([{ method: 'FILL_GAPS', args: [] }])
+  })
+
+  it('surfaces a build failure instead of plotting nothing silently', async () => {
+    buildSnapshotRecord.mockRejectedValueOnce(new Error('boom'))
+    const { toggleSnapshot } = useHistorySnapshots()
+
+    await toggleSnapshot('sess-c', 0)
+
+    expect(error).toHaveBeenCalledWith(expect.stringContaining('boom'))
+    expect(addSnapshotSeries).not.toHaveBeenCalled()
+  })
+
+  it('refuses when the session is not loaded', async () => {
+    sessions.value = []
+    const { toggleSnapshot } = useHistorySnapshots()
+
+    await toggleSnapshot('sess-c', 0)
+
+    expect(error).toHaveBeenCalled()
+    expect(buildSnapshotRecord).not.toHaveBeenCalled()
+  })
+})
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index 00f10d2a..aa84bf76 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -76,7 +76,7 @@ export function useEditSession() {
   const { fetchObservationsInRange } = useObservationStore()
   const sessionStore = useQcSessionStore()
 
-  const sourceDatastream = ref<Datastream | null>(null)
+  const { sourceDatastream } = storeToRefs(sessionStore)
   /** True when the managed datastream has no in-progress session to resume. */
   const needsSession = ref(false)
   /** True when the selected datastream has no QC history (not a managed datastream). */
@@ -98,8 +98,11 @@ export function useEditSession() {
   }
 
   /** True when the working copy has edits not in the last saved snapshot
-   *  (additions, removals, or an edited comment). */
+   *  (additions, removals, or an edited comment). Always false while
+   *  viewing a committed session: it cannot be edited, and replaying its
+   *  history swaps entries that an identity check would read as edits. */
   const hasUnsavedChanges = computed(() => {
+    if (sessionStore.isReadOnly) return false
     const current = currentEdits()
     const saved = savedEdits.value
     if (current.length !== saved.length) return true
@@ -109,10 +112,11 @@ export function useEditSession() {
   })
 
   /** Count of edits added since the last save (the common append case). */
-  const unsavedEditCount = computed(
-    () =>
-      currentEdits().filter((item) => !savedEdits.value.includes(item)).length
-  )
+  const unsavedEditCount = computed(() => {
+    if (sessionStore.isReadOnly) return 0
+    return currentEdits().filter((item) => !savedEdits.value.includes(item))
+      .length
+  })
 
   async function beginEditing(): Promise<void> {
     const managed = qcDatastream.value
diff --git a/apps/qc-app/src/composables/useHistorySnapshots.ts b/apps/qc-app/src/composables/useHistorySnapshots.ts
new file mode 100644
index 00000000..c220bd8f
--- /dev/null
+++ b/apps/qc-app/src/composables/useHistorySnapshots.ts
@@ -0,0 +1,132 @@
+/**
+ * Add and remove history snapshots: frozen replays of a session's state at a
+ * chosen operation, plotted as extra comparison lines.
+ */
+
+import { ref } from 'vue'
+import { storeToRefs } from 'pinia'
+import { ObservationRecord, Snackbar, applyHistory } from '@uwrl/qc-utils'
+import type { Datastream, QualityControlSession } from '@hydroserver/client'
+import { useHydroServer } from '@/store/hydroserver'
+import { useDataVisStore } from '@/store/dataVisualization'
+import { useObservationStore, type ObservationData } from '@/store/observations'
+import { usePlotlyStore } from '@/store/plotly'
+import { useQcSessionStore } from '@/store/qcSession'
+import { buildSnapshotRecord } from '@/services/qualityControl/buildSnapshot'
+import { formatDateRange } from '@/utils/time'
+import { SNAPSHOT_BASELINE_INDEX, makeSnapshotId } from '@/utils/snapshotId'
+import type { SnapshotMeta } from '@/types'
+
+/** The operation shape the session list embeds. */
+interface SessionOperation {
+  operationType?: string
+  createdBy?: { name?: string; email?: string }
+}
+
+export function useHistorySnapshots() {
+  const isBuilding = ref(false)
+  const dataVis = useDataVisStore()
+  const sessionStore = useQcSessionStore()
+  const { plottedDatastreams } = storeToRefs(dataVis)
+  const { selectedSeries } = storeToRefs(usePlotlyStore())
+  const { hs } = storeToRefs(useHydroServer())
+  const { observationsRaw } = storeToRefs(useObservationStore())
+  const { fetchObservationsInRange } = useObservationStore()
+
+  /**
+   * The observation store hands back one shared record per datastream, and the
+   * replay mutates whatever it is given, so build on an independent record.
+   */
+  const fetchDetached = async (ds: Datastream, start: Date, end: Date) => {
+    await fetchObservationsInRange(ds, start, end)
+    const raw: ObservationData = observationsRaw.value[ds.id] ?? {
+      datetimes: new Float64Array(0),
+      dataValues: new Float32Array(0),
+    }
+    const record = new ObservationRecord(raw)
+    await record.applyWindow(start.getTime(), end.getTime())
+    return record
+  }
+
+  const isSnapshotPlotted = (sessionId: string, opIndex: number): boolean => {
+    const id = makeSnapshotId(sessionId, opIndex)
+    return plottedDatastreams.value.some((d) => d.id === id)
+  }
+
+  const toggleSnapshot = async (sessionId: string, opIndex: number) => {
+    const id = makeSnapshotId(sessionId, opIndex)
+    if (isSnapshotPlotted(sessionId, opIndex)) {
+      await dataVis.removeSnapshotSeries(id)
+      return
+    }
+
+    const historyId = sessionStore.historyId
+    const session = sessionStore.sessions.find((s) => s.id === sessionId)
+    const source = sessionStore.sourceDatastream
+    const managed = dataVis.qcDatastream
+    if (!historyId || !session || !source || !managed) {
+      Snackbar.error('Load a managed datastream for editing first.')
+      return
+    }
+
+    isBuilding.value = true
+    try {
+      // Only the session on screen has drafts that are not yet on the server.
+      const liveHistory =
+        session.id === sessionStore.viewedSessionId
+          ? [...(selectedSeries.value?.data.history ?? [])]
+          : undefined
+
+      const record = await buildSnapshotRecord(
+        {
+          qcSessions: hs.value.qualityControlSessions,
+          qcOperations: hs.value.qualityControlOperations,
+          fetchInRange: fetchDetached,
+          applyHistory,
+        },
+        { historyId, session, source, managed, opIndex, liveHistory }
+      )
+
+      await dataVis.addSnapshotSeries(id, record, metaFor(session, opIndex))
+    } catch (e) {
+      const msg = e instanceof Error ? e.message : String(e)
+      Snackbar.error(`Couldn't build the snapshot: ${msg}`)
+    } finally {
+      isBuilding.value = false
+    }
+  }
+
+  return { toggleSnapshot, isSnapshotPlotted, isBuilding }
+}
+
+function metaFor(
+  session: QualityControlSession,
+  opIndex: number
+): SnapshotMeta {
+  const s = session as QualityControlSession & {
+    description?: string
+    operations?: SessionOperation[]
+  }
+  const ops = s.operations ?? []
+  const op = opIndex >= SNAPSHOT_BASELINE_INDEX + 1 ? ops[opIndex] : undefined
+  const performedBy = op?.createdBy?.name?.trim() || op?.createdBy?.email?.trim()
+  return {
+    sessionId: s.id,
+    sessionLabel:
+      s.description ||
+      formatDateRange(s.phenomenonTimeStart, s.phenomenonTimeEnd),
+    opIndex,
+    opCount: ops.length,
+    opName: op?.operationType ? formatMethod(op.operationType) : '',
+    ...(performedBy ? { performedBy } : {}),
+    createdAt: s.createdAt,
+  }
+}
+
+function formatMethod(method: string): string {
+  return method
+    .toLowerCase()
+    .split('_')
+    .map((w) => (w ? w[0]!.toUpperCase() + w.slice(1) : ''))
+    .join(' ')
+}
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
new file mode 100644
index 00000000..54d61251
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
@@ -0,0 +1,140 @@
+/**
+ * The builder picks its base and replay source from the session's status:
+ * committed sessions replay the server-side ancestor chain, the in-progress
+ * session replays the live record so unsaved drafts are included.
+ */
+
+import { describe, it, expect, vi } from 'vitest'
+import type { Datastream, QualityControlSession } from '@hydroserver/client'
+import type { HistoryItem, ObservationRecord, QcHistory } from '@uwrl/qc-utils'
+import { buildSnapshotRecord } from '../buildSnapshot'
+import type { ReconstructSessionDeps } from '../reconstructSession'
+import { SNAPSHOT_BASELINE_INDEX } from '@/utils/snapshotId'
+
+const managed = { id: 'm-1' } as unknown as Datastream
+const source = { id: 's-1' } as unknown as Datastream
+
+const rec = () =>
+  ({
+    dataX: [Date.UTC(2025, 0, 1)],
+    dataY: [0],
+    history: [],
+    redoStack: [],
+    reload: vi.fn(async () => {}),
+  }) as unknown as ObservationRecord
+
+const inProgressSession = {
+  id: 'sess-p',
+  status: 'in_progress',
+  createdAt: '2025-01-01T00:00:00Z',
+  phenomenonTimeStart: '2025-01-01T00:00:00Z',
+  phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+} as unknown as QualityControlSession
+
+const committedSession = {
+  ...inProgressSession,
+  id: 'sess-c',
+  status: 'committed',
+  committedAt: '2025-06-01T00:00:00Z',
+} as unknown as QualityControlSession
+
+const ops = (methods: string[]) =>
+  methods.map((operationType) => ({ operationType, arguments: [] }))
+
+function makeDeps(base = rec()) {
+  let captured: QcHistory | undefined
+  const fetchInRange = vi.fn().mockResolvedValue(base)
+  const deps = {
+    qcSessions: {
+      get: vi.fn(async () => ({ ok: true, data: committedSession })),
+      list: vi.fn(async () => ({ ok: true, data: [] })),
+    },
+    qcOperations: {
+      list: vi.fn(async () => ({
+        ok: true,
+        data: ops(['DELETE_POINTS', 'INTERPOLATE']),
+      })),
+    },
+    fetchInRange,
+    applyHistory: vi.fn(async (_r: ObservationRecord, h: QcHistory) => {
+      captured = h
+      return { applied: h.operations.length, failed: [] }
+    }),
+  } as unknown as ReconstructSessionDeps
+
+  return { base, captured: () => captured, deps, fetchInRange }
+}
+
+const history = (methods: string[]): HistoryItem[] =>
+  methods.map((method) => ({ method, args: [] })) as unknown as HistoryItem[]
+
+describe('buildSnapshotRecord', () => {
+  it('replays the live history up to opIndex for the in-progress session', async () => {
+    const { deps, captured, base } = makeDeps()
+
+    const out = await buildSnapshotRecord(deps, {
+      historyId: 'h-1',
+      session: inProgressSession,
+      source,
+      managed,
+      opIndex: 1,
+      liveHistory: history(['SELECTION', 'DELETE_POINTS', 'INTERPOLATE']),
+    })
+
+    expect(captured()!.operations.map((o) => o.method)).toEqual([
+      'SELECTION',
+      'DELETE_POINTS',
+    ])
+    expect(out).toBe(base)
+  })
+
+  it('replays nothing for an in-progress baseline snapshot', async () => {
+    const { deps, captured } = makeDeps()
+
+    await buildSnapshotRecord(deps, {
+      historyId: 'h-1',
+      session: inProgressSession,
+      source,
+      managed,
+      opIndex: SNAPSHOT_BASELINE_INDEX,
+      liveHistory: history(['SELECTION']),
+    })
+
+    expect(captured()!.operations).toEqual([])
+  })
+
+  it('loads the in-progress base over the session window', async () => {
+    const { deps, fetchInRange } = makeDeps()
+
+    await buildSnapshotRecord(deps, {
+      historyId: 'h-1',
+      session: inProgressSession,
+      source,
+      managed,
+      opIndex: 0,
+      liveHistory: history(['SELECTION']),
+    })
+
+    const [ds, begin, end] = fetchInRange.mock.calls[0]!
+    expect(ds).toBe(managed)
+    expect(begin.toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect(end.toISOString()).toBe('2025-02-01T00:00:00.000Z')
+  })
+
+  it('delegates a committed session to the reconstruct service with opLimit', async () => {
+    const { deps, captured } = makeDeps()
+
+    await buildSnapshotRecord(deps, {
+      historyId: 'h-1',
+      session: committedSession,
+      source,
+      managed,
+      opIndex: 0,
+    })
+
+    // opIndex 0 means "after the first operation", so opLimit is 1.
+    expect(captured()!.operations.map((o) => o.method)).toEqual([
+      'DELETE_POINTS',
+    ])
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
index 4ae4e76f..337bfad3 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
@@ -237,6 +237,68 @@ describe('reconstructCommittedSession', () => {
     expect(captured()!.operations.map((o) => o.method)).toEqual(['SELECTION'])
     expect((record.history as any[]).map((h) => h.method)).toEqual(['SELECTION'])
   })
+
+  it('truncates the viewed session operations to opLimit', async () => {
+    const qc = makeQcFake()
+    const { historyId, second } = await chainOf(qc)
+    const { fetchInRange, applyHistory, captured } = spies()
+    const { reconstructCommittedSession } = await import('../reconstructSession')
+
+    const { record } = await reconstructCommittedSession(
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      source,
+      historyId,
+      second.id,
+      1
+    )
+
+    // Ancestors are never truncated; only this session stops early.
+    expect(captured()!.operations.map((o) => o.method)).toEqual([
+      'SELECTION',
+      'DELETE_POINTS',
+    ])
+    expect((record.history as any[]).map((h) => h.method)).toEqual([
+      'DELETE_POINTS',
+    ])
+  })
+
+  it('replays ancestors only when opLimit is 0', async () => {
+    const qc = makeQcFake()
+    const { historyId, second } = await chainOf(qc)
+    const { fetchInRange, applyHistory, captured } = spies()
+    const { reconstructCommittedSession } = await import('../reconstructSession')
+
+    const { record } = await reconstructCommittedSession(
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      source,
+      historyId,
+      second.id,
+      0
+    )
+
+    expect(captured()!.operations.map((o) => o.method)).toEqual(['SELECTION'])
+    expect(record.history as any[]).toEqual([])
+  })
+
+  it('replays every operation when opLimit is omitted', async () => {
+    const qc = makeQcFake()
+    const { historyId, second } = await chainOf(qc)
+    const { fetchInRange, applyHistory, captured } = spies()
+    const { reconstructCommittedSession } = await import('../reconstructSession')
+
+    await reconstructCommittedSession(
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      source,
+      historyId,
+      second.id
+    )
+
+    expect(captured()!.operations.map((o) => o.method)).toEqual([
+      'SELECTION',
+      'DELETE_POINTS',
+      'INTERPOLATE',
+    ])
+  })
 })
 
 describe('reconstructCommittedSession — chain order', () => {
diff --git a/apps/qc-app/src/services/qualityControl/buildSnapshot.ts b/apps/qc-app/src/services/qualityControl/buildSnapshot.ts
new file mode 100644
index 00000000..d03f44b3
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/buildSnapshot.ts
@@ -0,0 +1,84 @@
+/**
+ * Build the ObservationRecord for a history snapshot: a session's state after
+ * its operation `opIndex` (or its baseline, at -1).
+ *
+ * The window is always the session chain's own window, never the plot's
+ * current time range. Operations replay against array indices, so a different
+ * base window would corrupt the replay.
+ */
+
+import type { Datastream, QualityControlSession } from '@hydroserver/client'
+import type {
+  HistoryItem,
+  ObservationRecord,
+  QcHistory,
+  QcHistoryOperation,
+} from '@uwrl/qc-utils'
+import { loadLatestBase } from './session'
+import {
+  reconstructCommittedSession,
+  type ReconstructSessionDeps,
+} from './reconstructSession'
+import { SNAPSHOT_BASELINE_INDEX } from '@/utils/snapshotId'
+
+export interface BuildSnapshotParams {
+  historyId: string
+  session: QualityControlSession
+  source: Datastream
+  managed: Datastream
+  opIndex: number
+  /** Live draft history, required when the session is in progress. */
+  liveHistory?: HistoryItem[]
+}
+
+/** Live history entry -> replayable operation, dropping runtime-only fields. */
+function toSerialized(item: HistoryItem): QcHistoryOperation {
+  const op: QcHistoryOperation = { method: item.method, args: item.args ?? [] }
+  if (item.comment) op.comment = item.comment
+  if (item.performedBy) op.performedBy = item.performedBy
+  return op
+}
+
+export async function buildSnapshotRecord(
+  deps: ReconstructSessionDeps,
+  params: BuildSnapshotParams
+): Promise<ObservationRecord> {
+  const { historyId, session, source, managed, opIndex, liveHistory } = params
+
+  if (session.status === 'committed') {
+    const { record } = await reconstructCommittedSession(
+      deps,
+      source,
+      historyId,
+      session.id,
+      opIndex + 1
+    )
+    return record
+  }
+
+  const record = await loadLatestBase(
+    deps.fetchInRange,
+    managed,
+    source,
+    new Date(session.phenomenonTimeStart),
+    new Date(session.phenomenonTimeEnd)
+  )
+
+  const operations =
+    opIndex <= SNAPSHOT_BASELINE_INDEX
+      ? []
+      : (liveHistory ?? []).slice(0, opIndex + 1).map(toSerialized)
+
+  const history: QcHistory = {
+    version: '1',
+    createdAt: session.createdAt,
+    window: {
+      startDate: session.phenomenonTimeStart,
+      endDate: session.phenomenonTimeEnd,
+    },
+    operations,
+  }
+
+  await deps.applyHistory(record, history)
+  return record
+}
diff --git a/apps/qc-app/src/services/qualityControl/reconstructSession.ts b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
index 0647247d..a374c9eb 100644
--- a/apps/qc-app/src/services/qualityControl/reconstructSession.ts
+++ b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
@@ -104,7 +104,8 @@ export async function reconstructCommittedSession(
   deps: ReconstructSessionDeps,
   source: Datastream,
   historyId: string,
-  sessionId: string
+  sessionId: string,
+  opLimit?: number
 ): Promise<ReconstructSessionResult> {
   const { qcSessions, qcOperations, fetchInRange, applyHistory } = deps
 
@@ -136,6 +137,13 @@ export async function reconstructCommittedSession(
     )
   )
 
+  // Snapshots replay this session only partway. Ancestors are never
+  // truncated: they are the base this session was built on.
+  const targetIdx = chain.length - 1
+  if (opLimit != null && perSession[targetIdx]) {
+    perSession[targetIdx] = perSession[targetIdx]!.slice(0, opLimit)
+  }
+
   const history: QcHistory = {
     version: '1',
     createdAt: session.createdAt,
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index a9268a85..2cd1df97 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -702,3 +702,93 @@ describe('useDataVisStore.releaseManagedDatastream', () => {
     expect(store.qcDatastreamId).toBe('mgd')
   })
 })
+
+describe('useDataVisStore snapshot series', () => {
+  const meta = {
+    sessionId: 'sess-1',
+    sessionLabel: 'March backfill',
+    opIndex: 0,
+    opCount: 2,
+    opName: 'Fill Gaps',
+    createdAt: '2026-01-01T00:00:00Z',
+  }
+
+  it('keeps a snapshot series across a refresh without fetching it', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    const record = { history: [], isLoading: false } as any
+
+    await store.addSnapshotSeries('snap:sess-1:0', record, meta as any)
+    await store.refreshGraphSeriesArray()
+
+    const series = mockGraphSeriesArray.value.find(
+      (s: any) => s.id === 'snap:sess-1:0'
+    )
+    expect(series).toBeDefined()
+    // Reactive wrapping means this is a proxy of `record`, not `record` itself.
+    expect(series.data).toEqual(record)
+    expect(series.snapshot).toEqual(meta)
+    expect(mockFetchObservationsInRange).not.toHaveBeenCalled()
+    expect(mockFetchGraphSeries).not.toHaveBeenCalled()
+  })
+
+  it('removes a snapshot from both the plotted list and the series array', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+
+    await store.addSnapshotSeries('snap:sess-1:0', { history: [] } as any, meta as any)
+    await store.removeSnapshotSeries('snap:sess-1:0')
+
+    expect(store.plottedDatastreams.some((d: any) => d.id === 'snap:sess-1:0')).toBe(
+      false
+    )
+    expect(
+      mockGraphSeriesArray.value.some((s: any) => s.id === 'snap:sess-1:0')
+    ).toBe(false)
+  })
+
+  it('never promotes a snapshot to the QC target', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    store.qcDatastreamId = null
+
+    await store.addSnapshotSeries('snap:sess-1:0', { history: [] } as any, meta as any)
+
+    expect(store.qcDatastreamId).toBeNull()
+  })
+
+  // Snapshots belong to the editor. The Select view lists real datastreams
+  // and lets the user pick a QC target, neither of which a snapshot can be.
+  it('drops snapshots when the editor releases the managed datastream', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    store.qcHistories = [
+      { id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'src' },
+    ] as any
+    store.datastreams = [makeDs({ id: 'src', name: 'Raw' })] as any
+    store.plottedDatastreams = [makeDs({ id: 'mgd', name: 'Raw (QC)' })] as any
+    store.qcDatastreamId = 'mgd'
+
+    await store.addSnapshotSeries('snap:sess-1:0', { history: [] } as any, meta as any)
+    await store.releaseManagedDatastream()
+
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['src'])
+    expect(
+      mockGraphSeriesArray.value.some((s: any) => s.id === 'snap:sess-1:0')
+    ).toBe(false)
+  })
+
+  it('drops snapshots even when there is no managed datastream to release', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    store.plottedDatastreams = [makeDs({ id: 'src', name: 'Raw' })] as any
+    store.qcDatastreamId = 'src'
+
+    await store.addSnapshotSeries('snap:sess-1:0', { history: [] } as any, meta as any)
+    await store.releaseManagedDatastream()
+
+    expect(
+      store.plottedDatastreams.some((d: any) => d.id === 'snap:sess-1:0')
+    ).toBe(false)
+  })
+})
diff --git a/apps/qc-app/src/store/__tests__/qcSession.spec.ts b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
index 7a9d8eaa..33ee3cf4 100644
--- a/apps/qc-app/src/store/__tests__/qcSession.spec.ts
+++ b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
@@ -186,4 +186,14 @@ describe('useQcSessionStore', () => {
     await useQcSessionStore().loadSessions(h.id)
     expect(opsList).not.toHaveBeenCalled()
   })
+
+  it('holds the source datastream and clears it on reset', () => {
+    const store = useQcSessionStore()
+    expect(store.sourceDatastream).toBeNull()
+
+    store.sourceDatastream = { id: 's-1', name: 'Source' } as any
+    store.reset()
+
+    expect(store.sourceDatastream).toBeNull()
+  })
 })
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index 627d9dc0..09536f2e 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -5,6 +5,9 @@ import { useObservationStore } from './observations'
 import { Snackbar } from '@uwrl/qc-utils'
 import { handleNewPlot } from '@/utils/plotting/plotly'
 import { subtractDays, subtractMonths, subtractYears } from '@/utils/dateMath'
+import { isSnapshotId } from '@/utils/snapshotId'
+import type { SnapshotMeta } from '@/types'
+import type { ObservationRecord } from '@uwrl/qc-utils'
 import {
   Datastream,
   type DatastreamExtended,
@@ -187,6 +190,61 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     await rebuildPlot()
   }
 
+  /**
+   * Add a frozen history snapshot as an extra line. The synthetic datastream
+   * carries the snapshot id so the legend, colour assignment and visibility
+   * toggles work unchanged; `refreshGraphSeriesArray` skips its fetch.
+   */
+  async function addSnapshotSeries(
+    id: string,
+    record: ObservationRecord,
+    meta: SnapshotMeta
+  ) {
+    if (plottedDatastreams.value.some((d) => d.id === id)) return
+
+    const qcSeries = graphSeriesArray.value.find(
+      (s) => s.id === qcDatastreamId.value
+    )
+    plottedDatastreams.value.push({ id, name: meta.sessionLabel } as Datastream)
+    graphSeriesArray.value.push({
+      id,
+      name: meta.sessionLabel,
+      data: record,
+      yAxisLabel: qcSeries?.yAxisLabel ?? '',
+      color: '',
+      intendedSpacingMs: qcSeries?.intendedSpacingMs ?? null,
+      snapshot: meta,
+    })
+
+    assignSeriesColors(plottedDatastreams.value.map((d) => d.id))
+    updateOptions()
+    const { plotlyRef } = storeToRefs(usePlotlyStore())
+    if (plotlyRef.value) await handleNewPlot(undefined, { preserveZoom: true })
+  }
+
+  /** Drop every snapshot line, without redrawing. Callers redraw themselves. */
+  function dropSnapshotSeries() {
+    if (!plottedDatastreams.value.some((d) => isSnapshotId(d.id))) return
+    plottedDatastreams.value = plottedDatastreams.value.filter(
+      (d) => !isSnapshotId(d.id)
+    )
+    graphSeriesArray.value = graphSeriesArray.value.filter(
+      (s) => !isSnapshotId(s.id)
+    )
+  }
+
+  /** Drop a snapshot line. Never touches the QC target. */
+  async function removeSnapshotSeries(id: string) {
+    const idx = plottedDatastreams.value.findIndex((d) => d.id === id)
+    if (idx === -1) return
+    plottedDatastreams.value.splice(idx, 1)
+    graphSeriesArray.value = graphSeriesArray.value.filter((s) => s.id !== id)
+
+    updateOptions()
+    const { plotlyRef } = storeToRefs(usePlotlyStore())
+    if (plotlyRef.value) await handleNewPlot(undefined, { preserveZoom: true })
+  }
+
   /** Clear every plotted datastream at once. */
   async function clearPlottedDatastreams() {
     if (!plottedDatastreams.value.length) return
@@ -258,9 +316,12 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
    * Inverse of `adoptManagedDatastream`, for leaving the editor. Managed
    * datastreams are hidden from the catalog, so without this the Select
    * view shows a plot with no row selected. No-op when the QC target isn't
-   * managed or its source isn't in the catalog.
+   * managed or its source isn't in the catalog. Snapshots are dropped
+   * regardless: they have no place in the Select view.
    */
   async function releaseManagedDatastream() {
+    dropSnapshotSeries()
+
     const managedId = qcDatastreamId.value
     if (!managedId) return
     const history = qcHistories.value.find(
@@ -577,10 +638,13 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
       currentIds.has(s.id)
     )
 
-    const updateOrFetchPromises = plottedDatastreams.value.map(async (ds) => {
-      loadingStates.value.set(ds.id, true)
-      return updateOrFetchGraphSeries(ds, beginDate.value, endDate.value)
-    })
+    const updateOrFetchPromises = plottedDatastreams.value
+      // Frozen replays: no server-side datastream to fetch.
+      .filter((ds) => !isSnapshotId(ds.id))
+      .map(async (ds) => {
+        loadingStates.value.set(ds.id, true)
+        return updateOrFetchGraphSeries(ds, beginDate.value, endDate.value)
+      })
 
     const results = await Promise.all(updateOrFetchPromises)
 
@@ -698,6 +762,8 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     plotDatastream,
     unplotDatastream,
     clearPlottedDatastreams,
+    addSnapshotSeries,
+    removeSnapshotSeries,
     setPlottedDatastreams,
     setQcDatastream,
     adoptManagedDatastream,
diff --git a/apps/qc-app/src/store/qcSession.ts b/apps/qc-app/src/store/qcSession.ts
index 00f36f45..783426c4 100644
--- a/apps/qc-app/src/store/qcSession.ts
+++ b/apps/qc-app/src/store/qcSession.ts
@@ -11,13 +11,15 @@ import { defineStore, storeToRefs } from 'pinia'
 import { computed, ref } from 'vue'
 import { useHydroServer } from '@/store/hydroserver'
 import { unwrap } from '@/services/qualityControl'
-import type { QualityControlSession } from '@hydroserver/client'
+import type { Datastream, QualityControlSession } from '@hydroserver/client'
 
 export const useQcSessionStore = defineStore('qcSession', () => {
   const historyId = ref<string | null>(null)
   /** Managed datastream the editor was last open on. The only persisted
    *  field; everything else is re-fetched on resume. */
   const resumeDatastreamId = ref<string | null>(null)
+  /** Raw datastream behind the managed one, resolved when editing begins. */
+  const sourceDatastream = ref<Datastream | null>(null)
   const sessions = ref<QualityControlSession[]>([])
   /** The single in-progress (editable) session, if any. */
   const currentSessionId = ref<string | null>(null)
@@ -120,6 +122,7 @@ export const useQcSessionStore = defineStore('qcSession', () => {
 
   function reset(): void {
     historyId.value = null
+    sourceDatastream.value = null
     sessions.value = []
     currentSessionId.value = null
     viewedSessionId.value = null
@@ -129,6 +132,7 @@ export const useQcSessionStore = defineStore('qcSession', () => {
   return {
     historyId,
     resumeDatastreamId,
+    sourceDatastream,
     sessions,
     currentSessionId,
     viewedSessionId,
diff --git a/apps/qc-app/src/types/index.ts b/apps/qc-app/src/types/index.ts
index 5c2356be..4218d6a0 100644
--- a/apps/qc-app/src/types/index.ts
+++ b/apps/qc-app/src/types/index.ts
@@ -4,6 +4,26 @@ export type EnumDictionary<T extends string | symbol | number, U> = {
   [K in T]: U
 }
 
+/**
+ * Provenance for a history snapshot series: which session it came from and
+ * how far through that session's operations it was replayed. Present only on
+ * snapshot series; absent on real datastream series.
+ */
+export interface SnapshotMeta {
+  sessionId: string
+  /** Session description, or its formatted window when it has none. */
+  sessionLabel: string
+  /** -1 for the session baseline, else the index of the last replayed op. */
+  opIndex: number
+  /** Total operations in the session, for the "step 3 of 7" label. */
+  opCount: number
+  /** Formatted operation name, empty string for the baseline. */
+  opName: string
+  performedBy?: string
+  /** ISO timestamp shown in the provenance line. */
+  createdAt: string
+}
+
 export interface GraphSeries {
   id: string
   name: string
@@ -30,4 +50,10 @@ export interface GraphSeries {
    * before.
    */
   intendedSpacingMs?: number | null
+  /**
+   * Set when this series is a frozen replay of a session's history rather
+   * than a live datastream. Drives the legend provenance row and the guards
+   * that keep snapshots out of fetches and the QC-target selector.
+   */
+  snapshot?: SnapshotMeta
 }
diff --git a/apps/qc-app/src/utils/__tests__/share.spec.ts b/apps/qc-app/src/utils/__tests__/share.spec.ts
index e5831263..71602e94 100644
--- a/apps/qc-app/src/utils/__tests__/share.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/share.spec.ts
@@ -278,3 +278,41 @@ describe('round-trip', () => {
     expect(decoded).toEqual(original)
   })
 })
+
+describe('history snapshots', () => {
+  it('round-trips snapshots', () => {
+    const state: ShareState = {
+      datastreamIds: ['ds-a'],
+      snapshots: [
+        { sessionId: 'sess-1', opIndex: 3 },
+        { sessionId: 'sess-2', opIndex: -1 },
+      ],
+    }
+
+    const q = encodeShareState(state)
+    expect(q.snap).toBe('sess-1:3,sess-2:-1')
+    expect(decodeShareState(q).snapshots).toEqual(state.snapshots)
+  })
+
+  it('omits the key when there are no snapshots', () => {
+    expect(encodeShareState({ datastreamIds: ['ds-a'] }).snap).toBeUndefined()
+    expect(
+      encodeShareState({ datastreamIds: ['ds-a'], snapshots: [] }).snap
+    ).toBeUndefined()
+  })
+
+  it('drops malformed entries instead of failing the whole link', () => {
+    const out = decodeShareState({ snap: 'sess-1:3,broken,sess-2:x,:4' })
+    expect(out.snapshots).toEqual([{ sessionId: 'sess-1', opIndex: 3 }])
+  })
+
+  // Snapshots stay out of `ds` so "QC target is the first id" and the
+  // h/ya bitmask indices keep holding.
+  it('leaves snapshots out of the datastream id list', () => {
+    const q = encodeShareState({
+      datastreamIds: ['ds-a'],
+      snapshots: [{ sessionId: 'sess-1', opIndex: 0 }],
+    })
+    expect(q.ds).toBe('ds-a')
+  })
+})
diff --git a/apps/qc-app/src/utils/__tests__/snapshotId.spec.ts b/apps/qc-app/src/utils/__tests__/snapshotId.spec.ts
new file mode 100644
index 00000000..2e8d4f53
--- /dev/null
+++ b/apps/qc-app/src/utils/__tests__/snapshotId.spec.ts
@@ -0,0 +1,42 @@
+/**
+ * Snapshot ids are synthetic (no server-side datastream), so every guard
+ * that keeps them out of fetches and share keys hangs off these helpers.
+ */
+
+import { describe, it, expect } from 'vitest'
+import {
+  SNAPSHOT_BASELINE_INDEX,
+  isSnapshotId,
+  makeSnapshotId,
+  parseSnapshotId,
+} from '@/utils/snapshotId'
+
+describe('snapshot ids', () => {
+  it('round-trips a session id and operation index', () => {
+    const id = makeSnapshotId('sess-1', 3)
+    expect(parseSnapshotId(id)).toEqual({ sessionId: 'sess-1', opIndex: 3 })
+  })
+
+  it('round-trips the baseline index', () => {
+    const id = makeSnapshotId('sess-1', SNAPSHOT_BASELINE_INDEX)
+    expect(parseSnapshotId(id)).toEqual({
+      sessionId: 'sess-1',
+      opIndex: SNAPSHOT_BASELINE_INDEX,
+    })
+  })
+
+  it('recognises snapshot ids and rejects real datastream ids', () => {
+    expect(isSnapshotId(makeSnapshotId('sess-1', 0))).toBe(true)
+    expect(isSnapshotId('7c9e6679-7425-40de-944b-e07fc1f90ae7')).toBe(false)
+    expect(isSnapshotId(undefined)).toBe(false)
+    expect(isSnapshotId(null)).toBe(false)
+  })
+
+  it('returns null for ids that are not snapshots or are malformed', () => {
+    expect(parseSnapshotId('ds-a')).toBeNull()
+    expect(parseSnapshotId('snap:')).toBeNull()
+    expect(parseSnapshotId('snap:sess-1')).toBeNull()
+    expect(parseSnapshotId('snap:sess-1:abc')).toBeNull()
+    expect(parseSnapshotId('snap:sess-1:-2')).toBeNull()
+  })
+})
diff --git a/apps/qc-app/src/utils/__tests__/time.spec.ts b/apps/qc-app/src/utils/__tests__/time.spec.ts
index 49fcf142..70a2c177 100644
--- a/apps/qc-app/src/utils/__tests__/time.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/time.spec.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest'
-import { formatDateInput, formatDateRange } from '../time'
+import { formatDateInput, formatDateRange, formatDayStamp } from '../time'
 
 // Datetime strings without a timezone offset are parsed as local time, so
 // these assertions are stable regardless of the test runner's zone.
@@ -59,3 +59,14 @@ describe('formatDateRange', () => {
     expect(formatDateRange('not-a-date', '2025-01-05T00:00:00')).toBe('–')
   })
 })
+
+describe('formatDayStamp', () => {
+  it('formats an ISO timestamp to day precision', () => {
+    expect(formatDayStamp('2026-03-14T12:00:00')).toBe('Mar 14, 2026')
+  })
+
+  it('falls back for nullish and unparseable input', () => {
+    expect(formatDayStamp(null)).toBe('–')
+    expect(formatDayStamp('nonsense')).toBe('nonsense')
+  })
+})
diff --git a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
index 45b51d4a..c1d31474 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
@@ -327,6 +327,71 @@ describe('createPlotlyOption', () => {
   })
 })
 
+describe('snapshot series axes', () => {
+  beforeEach(() => {
+    setActivePinia(createPinia())
+    qcDatastream.value = { id: 'ds-a' }
+    beginDate.value = new Date(0)
+    endDate.value = new Date(10)
+    hiddenAxisIds.value = new Set()
+  })
+
+  const snapshot = {
+    sessionId: 'sess-1',
+    sessionLabel: 'March backfill',
+    opIndex: 0,
+    opCount: 1,
+    opName: 'Fill Gaps',
+    createdAt: '2026-01-01T00:00:00Z',
+  }
+
+  // Its own axis is what lets the user shift a snapshot to line it up
+  // against the QC target.
+  it('gives a snapshot its own axis, like any other non-QC series', () => {
+    const qc = makeSeries({ id: 'ds-a' })
+    const snap = makeSeries({ id: 'snap:sess-1:0', snapshot })
+
+    const opts = createPlotlyOption([qc, snap])
+
+    const trace = opts.traces.find((t: any) => t.id === 'snap:sess-1:0') as any
+    expect(trace.yaxis).toBe('y2')
+    expect((opts.layout as any).yaxis2).toBeDefined()
+  })
+
+  it('keeps the snapshot line colour rather than the QC black', () => {
+    const qc = makeSeries({ id: 'ds-a' })
+    const snap = makeSeries({ id: 'snap:sess-1:0', color: COLORS[2], snapshot })
+
+    const opts = createPlotlyOption([qc, snap])
+
+    const trace = opts.traces.find((t: any) => t.id === 'snap:sess-1:0') as any
+    expect(trace.marker.color).toBe(COLORS[2])
+    expect(trace.selected).toBeUndefined()
+  })
+
+  it('numbers each series axis in turn when a snapshot is present', () => {
+    const qc = makeSeries({ id: 'ds-a' })
+    const snap = makeSeries({ id: 'snap:sess-1:0', snapshot })
+    const other = makeSeries({ id: 'ds-b' })
+
+    const opts = createPlotlyOption([qc, snap, other])
+
+    const trace = opts.traces.find((t: any) => t.id === 'ds-b') as any
+    expect(trace.yaxis).toBe('y3')
+    expect((opts.layout as any).yaxis3).toBeDefined()
+  })
+
+  it('honours the axis toggle on a snapshot row', () => {
+    hiddenAxisIds.value = new Set(['snap:sess-1:0'])
+    const qc = makeSeries({ id: 'ds-a' })
+    const snap = makeSeries({ id: 'snap:sess-1:0', snapshot })
+
+    const opts = createPlotlyOption([qc, snap])
+
+    expect((opts.layout as any).yaxis2.visible).toBe(false)
+  })
+})
+
 describe('findGapIndices', () => {
   it('returns an empty array when no consecutive points exceed the threshold', () => {
     expect(findGapIndices([0, 1, 2, 3], 1.5)).toEqual([])
diff --git a/apps/qc-app/src/utils/share.ts b/apps/qc-app/src/utils/share.ts
index 54892506..fc1784ad 100644
--- a/apps/qc-app/src/utils/share.ts
+++ b/apps/qc-app/src/utils/share.ts
@@ -45,6 +45,9 @@ export interface ShareState {
   /** Ordered list of plotted datastream ids. The first id is the QC
    *  target. */
   datastreamIds?: string[]
+  /** History snapshots plotted as comparison lines. Kept out of `ds` so the
+   *  QC-target-is-first rule and the `h` / `ya` bitmask indices still hold. */
+  snapshots?: { sessionId: string; opIndex: number }[]
   /** Date range preset id (`0..5`). When set, `begin`/`end` are
    *  omitted from the URL and the receiver recomputes the window
    *  from "now". */
@@ -131,6 +134,10 @@ export function encodeShareState(state: ShareState): Record<string, string> {
     q.ds = state.datastreamIds.join(',')
   }
 
+  if (state.snapshots?.length) {
+    q.snap = state.snapshots.map((s) => `${s.sessionId}:${s.opIndex}`).join(',')
+  }
+
   // Preset wins over begin/end. If a preset is active, dropping the
   // explicit dates lets the recipient pick up the same preset window
   // anchored to *their* "now" — which is the sender's intent when
@@ -218,6 +225,18 @@ export function decodeShareState(query: Record<string, unknown>): ShareState {
   const ds = splitCsv(str('ds'))
   if (ds.length) out.datastreamIds = ds
 
+  // A malformed entry is dropped rather than failing the whole link.
+  const snapshots: { sessionId: string; opIndex: number }[] = []
+  for (const piece of splitCsv(str('snap'))) {
+    const sep = piece.lastIndexOf(':')
+    if (sep <= 0) continue
+    const sessionId = piece.slice(0, sep)
+    const opIndex = Number(piece.slice(sep + 1))
+    if (!sessionId || !Number.isInteger(opIndex) || opIndex < -1) continue
+    snapshots.push({ sessionId, opIndex })
+  }
+  if (snapshots.length) out.snapshots = snapshots
+
   const rRaw = str('r')
   if (rRaw) {
     const n = Number(rRaw)
diff --git a/apps/qc-app/src/utils/snapshotId.ts b/apps/qc-app/src/utils/snapshotId.ts
new file mode 100644
index 00000000..91b8d59b
--- /dev/null
+++ b/apps/qc-app/src/utils/snapshotId.ts
@@ -0,0 +1,32 @@
+/**
+ * Synthetic ids for history snapshots. A snapshot is a frozen replay of a
+ * session's operations, so it has no server-side datastream. The prefix lets
+ * one predicate guard every path that would otherwise treat it as real.
+ */
+
+export const SNAPSHOT_PREFIX = 'snap:'
+
+/** Baseline: the session's state before its first operation. */
+export const SNAPSHOT_BASELINE_INDEX = -1
+
+export function makeSnapshotId(sessionId: string, opIndex: number): string {
+  return `${SNAPSHOT_PREFIX}${sessionId}:${opIndex}`
+}
+
+export function isSnapshotId(id: string | null | undefined): boolean {
+  return !!id && id.startsWith(SNAPSHOT_PREFIX)
+}
+
+export function parseSnapshotId(
+  id: string
+): { sessionId: string; opIndex: number } | null {
+  if (!isSnapshotId(id)) return null
+  const rest = id.slice(SNAPSHOT_PREFIX.length)
+  const sep = rest.lastIndexOf(':')
+  if (sep <= 0) return null
+  const sessionId = rest.slice(0, sep)
+  const opIndex = Number(rest.slice(sep + 1))
+  if (!Number.isInteger(opIndex)) return null
+  if (opIndex < SNAPSHOT_BASELINE_INDEX) return null
+  return { sessionId, opIndex }
+}
diff --git a/apps/qc-app/src/utils/time.ts b/apps/qc-app/src/utils/time.ts
index 179e131f..28dab676 100644
--- a/apps/qc-app/src/utils/time.ts
+++ b/apps/qc-app/src/utils/time.ts
@@ -55,6 +55,13 @@ const toDate = (iso?: string | null): Date | null => {
   return Number.isNaN(d.getTime()) ? null : d
 }
 
+/** `Mar 14, 2026`. Day precision, for timestamps shown alongside other text. */
+export function formatDayStamp(iso?: string | null): string {
+  const d = toDate(iso)
+  if (!d) return iso || '–'
+  return MONTH_DAY_YEAR.format(d)
+}
+
 /** A whole-day boundary, where showing the clock adds nothing. */
 const isMidnight = (d: Date) =>
   d.getHours() === 0 && d.getMinutes() === 0 && d.getSeconds() === 0

From c27a9bf1f26c7adb895b1832510dfffa570d96e8 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Wed, 5 Aug 2026 15:50:24 -0600
Subject: [PATCH 12/93] style: give session rows a container treatment

---
 apps/qc-app/src/components/EditData/SessionList.vue | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/apps/qc-app/src/components/EditData/SessionList.vue b/apps/qc-app/src/components/EditData/SessionList.vue
index 6166eab8..1b6238fd 100644
--- a/apps/qc-app/src/components/EditData/SessionList.vue
+++ b/apps/qc-app/src/components/EditData/SessionList.vue
@@ -156,10 +156,16 @@ function returnToCurrent(): void {
   padding-block: 2px;
 }
 
+/* Every session reads as its own container, so the unselected ones don't
+   float loose against the timeline rail. */
 .qc-session-list__item :deep(.v-timeline-item__body) {
   border-radius: 6px;
-  padding-inline: 6px;
-  transition: background-color 120ms ease;
+  padding: 6px 8px;
+  border: thin solid rgba(var(--v-border-color), var(--v-border-opacity));
+  background-color: rgba(var(--v-theme-on-surface), 0.02);
+  transition:
+    background-color 120ms ease,
+    border-color 120ms ease;
 }
 
 .qc-session-list__item:hover :deep(.v-timeline-item__body) {
@@ -168,5 +174,6 @@ function returnToCurrent(): void {
 
 .qc-session-list__item--viewed :deep(.v-timeline-item__body) {
   background-color: rgba(var(--v-theme-primary), 0.12);
+  border-color: rgba(var(--v-theme-primary), 0.4);
 }
 </style>

From 2931eed2e02ca859de0d5ee1a6b31388fa57fd54 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Wed, 5 Aug 2026 15:50:25 -0600
Subject: [PATCH 13/93] docs: document history snapshot comparison

Covers the feature in the user guide, the id convention, guards and
detached-fetcher rule in the architecture doc, the axis behaviour in the
plotting doc, and the two new store actions in the API reference.
---
 apps/qc-app/docs/API_REFERENCE.md             |   2 +
 apps/qc-app/docs/ARCHITECTURE.md              |  25 +++
 apps/qc-app/docs/PLOTTING.md                  |  13 ++
 apps/qc-app/docs/USER_GUIDE.md                |  28 ++-
 ...6-08-05-history-snapshot-compare-design.md | 208 ++++++++++++++++++
 5 files changed, 275 insertions(+), 1 deletion(-)
 create mode 100644 docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 4a86bf08..cefd5ae7 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -308,6 +308,8 @@ on boot.
 | `plotDatastream`                    | action   | `(ds: Datastream) => Promise<void>`               | Add to plot; promotes to QC when nothing's there yet. |
 | `unplotDatastream`                  | action   | `(id: string) => Promise<void>`                   | Remove; promotes the previous plotted entry to QC if removing the QC target. |
 | `clearPlottedDatastreams`           | action   | `() => Promise<void>`                             | Drop the entire plotted set. |
+| `addSnapshotSeries`                 | action   | `(id: string, record: ObservationRecord, meta: SnapshotMeta) => Promise<void>` | Add a frozen history snapshot as an extra comparison line under the synthetic id `snap:<sessionId>:<opIndex>`. Never promotes to QC target; `refreshGraphSeriesArray` skips its fetch. |
+| `removeSnapshotSeries`              | action   | `(id: string) => Promise<void>`                   | Drop one snapshot line. Leaves the QC target alone. |
 | `setPlottedDatastreams`             | action   | `(items: Datastream[], qcId?: string \| null) => Promise<void>` | Wholesale replace; used by URL hydration. |
 | `setQcDatastream`                   | action   | `(id: string \| null) => Promise<void>`           | Change QC target; preserves the current zoom. |
 | `adoptManagedDatastream`            | action   | `(managed: Datastream, sourceId: string) => Promise<void>` | Enter editing on a freshly-created managed datastream: replace the source in the plot and re-key its already-loaded series as the managed datastream's working copy (no second, empty item; no re-fetch). |
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 22e75981..f205884c 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -324,6 +324,31 @@ and each workspace's role is marked on the picker. The role rides along on the
 `Workspace` object (`collaboratorRole.permissions`; owners have a null role;
 admins override), so no extra request is needed.
 
+**History snapshots.** A snapshot is a session's state at one operation,
+plotted as an extra comparison line. `useHistorySnapshots()` drives it;
+`buildSnapshotRecord()` builds the record by replaying `0..k`, delegating
+committed sessions to `reconstructCommittedSession(..., opLimit)` and
+replaying the live record for the in-progress one so unsaved drafts count.
+
+Three constraints shape the implementation:
+
+- **Window.** The base is always the session chain's own window, never the
+  plot's time range. Operations replay against array indices, so a different
+  base window misaligns the replay. A snapshot is therefore frozen at
+  creation and never refetched.
+- **Record isolation.** `useObservationStore.fetchObservationsInRange` hands
+  back one shared `ObservationRecord` per datastream, and the replay mutates
+  whatever it is given. Snapshots inject a *detached* fetcher that warms the
+  raw cache and then constructs its own `ObservationRecord`, so a build never
+  disturbs the plot's series or a previous snapshot.
+- **Identity.** Snapshots ride in `plottedDatastreams` under the synthetic id
+  `snap:<sessionId>:<opIndex>` so legend rendering, colour assignment,
+  visibility and reorder work unchanged. `isSnapshotId()` guards the paths
+  that would otherwise treat one as real: `refreshGraphSeriesArray` skips its
+  fetch, `releaseManagedDatastream` drops them when the editor closes, and
+  the share encoder keeps them out of `ds` (they use their own `snap` key, so
+  the QC-target-is-first rule and the `h`/`ya` bitmask indices still hold).
+
 ## Routing and auth
 
 vue-router 5, two routes (Home, Workspaces). Two guards run on
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index 3552e6a9..b4dd26c9 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -162,6 +162,19 @@ dispatch. Without it, a programmatic re-render after a HistoryItem
 replay would look like a user selection and append another item to
 the history. Cleared once the redraw is back in steady state.
 
+## History snapshot series
+
+A `GraphSeries` carrying a `snapshot` field is a frozen replay of a QC
+session at one operation, not a live datastream. To `createPlotlyOption` it
+is an ordinary non-QC series: it gets its own overlaying right-side axis and
+its own colour from the shared assigner. That is deliberate. Being able to
+shift a snapshot on its own axis is how the user lines it up against the QC
+target, which is the point of plotting it.
+
+What differs is upstream, not here: the data never refetches (see
+`refreshGraphSeriesArray`'s `isSnapshotId` guard), and `PlottedDatastreams`
+renders the row's provenance instead of a point count.
+
 ## Why `internal.ts` isn't re-exported
 
 `plotly.ts` is a barrel for everything the rest of the app needs.
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 3f2b45b9..b6271e99 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -414,18 +414,44 @@ The header carries the count chip and four icon buttons (left to right): **undo*
 
 The body shows:
 
-- A baseline **Data loaded** row at the top, with a reload-from-server button.
+- A baseline **Data loaded** row at the top, with a plot-this-step button and a reload-from-server button.
 - One row per history entry, each with:
   - The operation icon and Title-Case name.
   - A failure badge (red `!`) if the op threw at author time. Common after a QC history import that references something missing in this datastream.
   - A duration badge.
   - In dev mode, a small chip showing whether the op ran inline or on a worker.
+  - A **plot-this-step** button that adds that point in history to the plot as a comparison line.
   - A **reload-from-this-step** button that replays history up to but not including this entry.
   - An **undo** button on the trailing entry only (older entries are undone via Reload-from-this-step).
 - A chevron toggles an inline "Arguments" drawer that shows the raw qc-utils call arguments.
 
 Clicking the chevron at the very top of the panel collapses the whole panel; the pop-out icon opens the same panel inside a wider modal so you can scan a long history without losing the rest of the sidebar.
 
+## Comparing against a point in history
+
+The chart-line button on any history row plots that session's state at that
+operation as a **separate line**, so you can compare it against what you are
+editing now without leaving your session. The button on the **Data loaded**
+row plots the state the session started from, before its first operation.
+
+Click the button again to remove the line.
+
+Things worth knowing:
+
+- Snapshots appear in the plotted datastreams list with a history icon, a
+  `snapshot` chip, and a provenance line such as
+  `step 3 of 7: Fill Gaps - by Alice - Mar 14, 2026`.
+- Each snapshot gets its own Y axis, so you can shift it to line it up
+  against the QC target, exactly like any other plotted datastream.
+- A snapshot is **frozen**. It is computed once, over its own session's
+  window, and changing the plot's time range never refetches or recomputes
+  it. Zoom outside that window and the line simply stops.
+- Snapshots of the session you are editing include your unsaved edits;
+  snapshots of any other session replay what was saved to the server.
+- Snapshots travel in the share link, so a link reproduces the comparison.
+  Each one replays on load, so a link carrying several is slower to open.
+- Leaving the editor for the Select view drops every snapshot.
+
 ## Save / load a QC history
 
 The QC history is the canonical save format. It's a JSON file you can keep, re-apply, share, or version-control.
diff --git a/docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md b/docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md
new file mode 100644
index 00000000..88fc182c
--- /dev/null
+++ b/docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md
@@ -0,0 +1,208 @@
+# History snapshot compare
+
+Plot the state of a QC session at any point in its operation history as a
+separate line on the current plot, so a user editing a session can compare
+against the same data at another point in history.
+
+## Motivation
+
+The editor can already *view* a past session, but viewing replaces the QC
+series and flips the editor read-only. There is no way to see two points in
+history side by side. This adds a second, read-only line rather than swapping
+the working one.
+
+## Definitions
+
+A **snapshot** is a frozen `GraphSeries` holding the `ObservationRecord`
+produced by replaying a session's operations up to a chosen point, computed
+over that session's own window.
+
+- `opIndex = -1` is the baseline: the state the session started from, before
+  any of its own operations. This is the "Data loaded" row in `EditHistory`.
+- `opIndex = k` is the state after operation `k`.
+
+## Base and replay
+
+Reuses the existing reconstruct services rather than a parallel
+implementation.
+
+| Session status | Base | Replayed |
+| --- | --- | --- |
+| Committed | `reconstructCommittedSession`: raw source over the union of the ancestor chain's windows | ancestors in `committedAt` order, then the target session's operations `0..k` |
+| In progress | `loadLatestBase(managed, source)` | the live record's `history[0..k]` |
+
+`reconstructCommittedSession` gains an optional `opLimit` parameter that
+truncates the target session's own operations, so the committed path reuses it
+rather than duplicating the ancestor-chain logic.
+
+Reading the in-progress session's operations from the live record rather than
+from the server means unsaved draft edits are included. That matches what a
+user expects when comparing against work they are currently doing.
+
+The window is the session chain's own window, never the plot's current time
+range. `reconstructCommittedSession` already loads the union of the chain's
+windows because operations replay against array indices, so a narrower base
+misaligns a wider ancestor. Recomputing a snapshot over an arbitrary user
+window would therefore corrupt the replay.
+
+Consequence: a snapshot is **frozen at creation**. It is never refetched, and
+changing the plot's time range does not recompute it. It spans only its
+session's window; zooming outside that window shows no line there.
+
+## Record isolation
+
+`useObservationStore.fetchObservationsInRange` keeps one `ObservationRecord`
+per datastream id and hands back the same instance. The reconstruct services
+replay operations onto whatever their injected `fetchInRange` returns, so
+building a snapshot through the store's fetcher would mutate the record the
+plot is already using, and a second snapshot built from the same source would
+clobber the first.
+
+Snapshots therefore inject a **detached fetcher** with the same
+`FetchObservationsInRange` signature:
+
+```ts
+async function fetchDetached(ds, start, end) {
+  await fetchObservationsInRange(ds, start, end) // warm the raw cache
+  const raw = observationsRaw.value[ds.id] ?? { datetimes: [], dataValues: [] }
+  const record = new ObservationRecord(raw)
+  await record.applyWindow(start.getTime(), end.getTime())
+  return record
+}
+```
+
+This is safe because `ObservationRecord`'s constructor copies into its own
+`dataset.source` buffers and treats `rawData` as read-only; operations mutate
+only `dataset.source`. The reconstruct services need no change to accept it.
+
+A detached record does share the `rawData` *object* with the store, so a later
+store fetch that widens the raw cache is visible to it. That only matters to a
+record that calls `reload()`, which a frozen snapshot never does.
+
+## Identity and store shape
+
+Snapshots get the synthetic id `snap:<sessionId>:<opIndex>` and a synthetic
+`Datastream`-shaped object pushed into `plottedDatastreams`. Legend rendering,
+colour assignment, the eye toggle, drag reorder, and removal then work
+unchanged instead of growing a parallel code path.
+
+Four guards, each keyed off a single `isSnapshotId()` predicate:
+
+1. `refreshGraphSeriesArray` skips the fetch for snapshot ids and keeps the
+   existing series. This is what makes a snapshot frozen.
+2. `releaseManagedDatastream` drops every snapshot when the editor closes.
+   The QC target is chosen from the Select view, which lists real
+   datastreams; a snapshot must never appear there.
+3. Snapshots are excluded from the `ds` share key (see below).
+4. A snapshot id is never sent to any server call.
+
+`GraphSeries` gains:
+
+```ts
+snapshot?: {
+  sessionId: string
+  sessionLabel: string
+  opIndex: number
+  opCount: number
+  opName: string
+  performedBy?: string
+  createdAt: string
+}
+```
+
+## Y axis
+
+Snapshots get their own axis, exactly like any other non-QC series. Being
+able to shift a snapshot on its own axis is how the user lines it up against
+the QC target, which is the point of plotting it. No axis is shared except
+the x axis, which every series shares.
+
+This means no change to `createPlotlyOption`'s axis assignment, and the
+per-row axis-toggle button works on snapshot rows like any other.
+
+## UI
+
+### EditHistory
+
+An add-to-plot icon button on each operation row and on the "Data loaded"
+baseline row. It toggles: when that exact snapshot is already plotted, the
+button removes it.
+
+### PlottedDatastreams
+
+Snapshot rows carry an `mdi-history` icon and a "snapshot" chip so they never
+read as a live datastream:
+
+```
+[history icon] [snapshot] Session Mar 2024
+                          step 3 of 7: Fill Gaps - by Alice - Mar 14 2026
+```
+
+Baseline rows read `session start - Mar 14 2026`.
+
+Line colour comes from the existing per-series assigner. No dashed line and no
+hover tooltip: the colour already distinguishes lines, and building tooltip
+content per row is not worth the cost.
+
+## Share URL
+
+A new `snap` key rather than folding snapshots into `ds`:
+
+```
+snap=a1b2:3,a1b2:-1
+```
+
+Keeping it separate preserves both invariants the encoder relies on: the QC
+target is the first id in `ds`, and the `h` / `ya` bitmasks are indexed by
+position in `ds`.
+
+On hydration, snapshots are replayed after `loadSessions` has run and are
+appended after the real datastreams. A snapshot whose session no longer exists
+is dropped silently rather than failing the whole hydration.
+
+## Testing
+
+Unit:
+
+- snapshot builder: truncation at `k`, baseline at `-1`, committed versus
+  in-progress base selection
+- share encode/decode round trip for the `snap` key
+- `refreshGraphSeriesArray` exemption: a snapshot series survives a refresh
+  and is not fetched
+
+Component:
+
+- `EditHistory`: the add-to-plot button appears on operation rows and on the
+  baseline row, and toggles a plotted snapshot off
+- `PlottedDatastreams`: a snapshot row renders its provenance line, and the
+  QC-target and axis-toggle buttons are suppressed
+
+## Reload-from-step, fixed alongside
+
+Stepping through a committed session's history restored the pre-replay
+entries wholesale, which put the *old* timings back over the ones the replay
+had just measured. Only the "Data loaded" row updated, because its time comes
+from `record.loadingTime`.
+
+The panel now keeps the replayed entries as the replay left them and appends
+only the ones it dropped, recomputing the shown-step marker from the actual
+replayed length (a replay can drop a preceding `SELECTION`). `isApplied()`
+hides execution data for steps past the one on screen, since those did not run
+in this view.
+
+`hasUnsavedChanges` / `unsavedEditCount` now return false / 0 while a
+committed session is being viewed. They compare history entries by identity,
+so a replay that swaps entries used to read as pending edits and prompted
+"You have unsaved edits" on a read-only view. A committed session cannot be
+edited, so the invariant belongs in the computed rather than being worked
+around by preserving object identity.
+
+## Out of scope
+
+- No cap on the number of simultaneous snapshots. Each costs a base fetch plus
+  a replay, so several at once over a wide window will be slow. Left visible
+  rather than guessing where a limit belongs.
+- Expanding a session still loads it (view mode). Decoupling expansion from
+  viewing was considered and rejected as a separate concern; users already
+  navigate back with "Return to current", and the snapshot survives that
+  round trip.

From 72a80283efbf5d664ac0546663ac06aa7f5ac93d Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Wed, 5 Aug 2026 15:59:11 -0600
Subject: [PATCH 14/93] feat: reload to a session's baseline from the Data
 loaded row

The Data loaded row is step -1, so a session's starting state is now
reachable the same way any operation is, and it carries the "Showing"
chip when it is the step on screen.

Its reload-from-server button moves to a cloud icon so the row no longer
shows two identical glyphs doing different things: one replays the
history to a step, the other refetches and drops the history.
---
 apps/qc-app/docs/USER_GUIDE.md                |  2 +-
 .../src/components/EditData/EditHistory.vue   | 33 +++++++++-
 .../EditData/__tests__/EditHistory.spec.ts    | 62 ++++++++++++++++++-
 ...6-08-05-history-snapshot-compare-design.md |  6 ++
 4 files changed, 98 insertions(+), 5 deletions(-)

diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index b6271e99..9a12f4b9 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -414,7 +414,7 @@ The header carries the count chip and four icon buttons (left to right): **undo*
 
 The body shows:
 
-- A baseline **Data loaded** row at the top, with a plot-this-step button and a reload-from-server button.
+- A baseline **Data loaded** row at the top, carrying a plot-this-step button, a **reload-from-this-step** button that returns the plot to the state the session started from, and a **reload-from-server** button (cloud icon) that refetches and drops the history entirely.
 - One row per history entry, each with:
   - The operation icon and Title-Case name.
   - A failure badge (red `!`) if the op threw at author time. Common after a QC history import that references something missing in this datastream.
diff --git a/apps/qc-app/src/components/EditData/EditHistory.vue b/apps/qc-app/src/components/EditData/EditHistory.vue
index b6893f05..e3eee966 100644
--- a/apps/qc-app/src/components/EditData/EditHistory.vue
+++ b/apps/qc-app/src/components/EditData/EditHistory.vue
@@ -141,6 +141,18 @@
         <span class="text-body-small font-weight-medium flex-grow-1 text-truncate">
           {{ selectedSeries?.data.isLoading ? 'Loading data…' : 'Data loaded' }}
         </span>
+        <v-chip
+          v-if="shownStepIndex === SNAPSHOT_BASELINE_INDEX"
+          size="x-small"
+          color="primary"
+          variant="tonal"
+          label
+          class="mr-1 flex-shrink-0"
+          data-testid="history-loaded-baseline"
+        >
+          Showing
+        </v-chip>
+
         <span
           v-if="selectedSeries?.data.loadingTime"
           class="text-body-small text-medium-emphasis mr-1 flex-shrink-0"
@@ -184,15 +196,34 @@
           </template>
         </v-tooltip>
 
+        <v-tooltip location="start" text="Reload from this step">
+          <template #activator="{ props: tp }">
+            <v-btn
+              v-bind="tp"
+              data-testid="history-reload-step-baseline"
+              aria-label="Reload from this step"
+              size="x-small"
+              variant="text"
+              density="comfortable"
+              icon="mdi-reload"
+              :disabled="isUpdating || !editCount"
+              @click="onReloadHistory(SNAPSHOT_BASELINE_INDEX)"
+            />
+          </template>
+        </v-tooltip>
+
+        <!-- Distinct icon from the step reload above: this one refetches
+             and drops the history rather than replaying it. -->
         <v-tooltip v-if="!selectedSeries?.data.isLoading" location="start" text="Reload from server">
           <template #activator="{ props: tp }">
             <v-btn
               v-bind="tp"
               data-testid="history-reload-btn"
+              aria-label="Reload from server"
               size="x-small"
               variant="text"
               density="comfortable"
-              icon="mdi-reload"
+              icon="mdi-cloud-download-outline"
               :disabled="isUpdating || isReadOnly"
               @click="onReload"
             />
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
index 1da67363..8fd7a639 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
@@ -293,9 +293,9 @@ describe('EditHistory.vue actions', () => {
     vi.useFakeTimers()
     const wrapper = createWrapper()
     await flushPromises()
-    const reloadBtn = wrapper.findAll('button').find((b) => b.html().includes('mdi-reload'))
-    expect(reloadBtn).toBeTruthy()
-    await reloadBtn!.trigger('click')
+    const reloadBtn = wrapper.find('[data-testid="history-reload-btn"]')
+    expect(reloadBtn.exists()).toBe(true)
+    await reloadBtn.trigger('click')
     await vi.runAllTimersAsync()
     expect(selectedSeries.value.data.reload).toHaveBeenCalled()
     expect(refreshGraphSeriesArray).toHaveBeenCalled()
@@ -572,6 +572,62 @@ describe('EditHistory.vue actions', () => {
       expect(w.find('[data-testid="history-loaded-1"]').exists()).toBe(true)
     })
 
+    // The baseline row is step -1: the state the session started from,
+    // before any of its operations.
+    it('reloads to the session baseline from the Data loaded row', async () => {
+      const history = [
+        makeEntry('SELECTION', [], { durationMs: 10 }),
+        makeEntry('DELETE_POINTS', [], { durationMs: 20 }),
+      ]
+      editHistory.value = history
+      const reloadHistory = vi.fn(async () => [])
+      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+
+      const w = createWrapper()
+      await flushPromises()
+      await w
+        .find('[data-testid="history-reload-step-baseline"]')
+        .trigger('click')
+      await vi.waitFor(() => expect(isUpdating.value).toBe(false))
+      await flushPromises()
+
+      expect(reloadHistory).toHaveBeenCalledWith(-1)
+      // Nothing is applied at the baseline, so no row reports a run.
+      expect(w.find('[data-testid="history-duration-0"]').exists()).toBe(false)
+      expect(w.find('[data-testid="history-duration-1"]').exists()).toBe(false)
+    })
+
+    it('marks the Data loaded row as the one being shown', async () => {
+      const history = [makeEntry('SELECTION', [], { durationMs: 10 })]
+      editHistory.value = history
+      const reloadHistory = vi.fn(async () => [])
+      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+
+      const w = createWrapper()
+      await flushPromises()
+      expect(w.find('[data-testid="history-loaded-baseline"]').exists()).toBe(false)
+
+      await w
+        .find('[data-testid="history-reload-step-baseline"]')
+        .trigger('click')
+      await vi.waitFor(() =>
+        expect(w.find('[data-testid="history-loaded-baseline"]').exists()).toBe(true)
+      )
+      expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(false)
+    })
+
+    it('disables the baseline reload when there is nothing to step back from', async () => {
+      editHistory.value = []
+      selectedSeries.value = makeSeries()
+
+      const w = createWrapper()
+      await flushPromises()
+
+      expect(
+        w.find('[data-testid="history-reload-step-baseline"]').attributes('disabled')
+      ).toBeDefined()
+    })
+
     // Reloading from a step un-applies everything below it. A committed
     // session keeps those rows on screen, so their telemetry would otherwise
     // still advertise a run that no longer holds in this view.
diff --git a/docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md b/docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md
index 88fc182c..ade51b83 100644
--- a/docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md
+++ b/docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md
@@ -190,6 +190,12 @@ replayed length (a replay can drop a preceding `SELECTION`). `isApplied()`
 hides execution data for steps past the one on screen, since those did not run
 in this view.
 
+The "Data loaded" row is step `-1` and reloads to it, so a session's starting
+state is reachable the same way any operation is. It carries the "Showing"
+chip when it is the step on screen. Its reload-from-server button moved to a
+cloud icon so the two reloads are not two identical glyphs doing different
+things: one replays, the other refetches and drops the history.
+
 `hasUnsavedChanges` / `unsavedEditCount` now return false / 0 while a
 committed session is being viewed. They compare history entries by identity,
 so a replay that swaps entries used to read as pending edits and prompted

From f96cdb8f69857f6e6ff2e7cf6950f9172526e0b4 Mon Sep 17 00:00:00 2001
From: Ken Lippold <klippold@Kens-MacBook-Pro-2.local>
Date: Wed, 12 Aug 2026 17:12:23 -0700
Subject: [PATCH 15/93] Update version

---
 django/contracts/openapi/auth.openapi.json | 2 +-
 django/contracts/openapi/data.openapi.json | 2 +-
 django/hydroserver/__init__.py             | 2 +-
 packages/hydroserverpy/setup.cfg           | 2 +-
 4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/django/contracts/openapi/auth.openapi.json b/django/contracts/openapi/auth.openapi.json
index 152bb9f3..97d182d1 100644
--- a/django/contracts/openapi/auth.openapi.json
+++ b/django/contracts/openapi/auth.openapi.json
@@ -678,7 +678,7 @@
   "info": {
     "description": "",
     "title": "HydroServer User Authentication API",
-    "version": "1.11.1"
+    "version": "1.12.0b2"
   },
   "openapi": "3.1.0",
   "paths": {
diff --git a/django/contracts/openapi/data.openapi.json b/django/contracts/openapi/data.openapi.json
index 443aadea..ec9e5445 100644
--- a/django/contracts/openapi/data.openapi.json
+++ b/django/contracts/openapi/data.openapi.json
@@ -11613,7 +11613,7 @@
   "info": {
     "description": "",
     "title": "HydroServer Data Management API",
-    "version": "1.11.1"
+    "version": "1.12.0b2"
   },
   "openapi": "3.1.0",
   "paths": {
diff --git a/django/hydroserver/__init__.py b/django/hydroserver/__init__.py
index f1cda449..490eaec2 100644
--- a/django/hydroserver/__init__.py
+++ b/django/hydroserver/__init__.py
@@ -1,4 +1,4 @@
 from .celery import app as celery_app
 
-__version__ = "1.11.1"
+__version__ = "1.12.0b2"
 __all__ = ("celery_app", "__version__",)
diff --git a/packages/hydroserverpy/setup.cfg b/packages/hydroserverpy/setup.cfg
index 4ba0c80f..5dbe2d8b 100644
--- a/packages/hydroserverpy/setup.cfg
+++ b/packages/hydroserverpy/setup.cfg
@@ -1,6 +1,6 @@
 [metadata]
 name = hydroserverpy
-version = 1.11.1
+version = 1.12.0b2
 description = A Python client for managing HydroServer data
 long_description = file: README.md
 long_description_content_type = text/markdown

From d4a69caa91b906541fd2c78f2b7f6b2cbabcdd72 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 17 Aug 2026 14:00:53 -0600
Subject: [PATCH 16/93] feat: move session actions into the edit history panel

Save, Commit and Close now sit in a footer below the history and stay
visible while it is collapsed. Also drops the author name from collapsed
rows, puts the comment icon beside the operation title, and retitles
reload-from-server to discard and reload, hidden on committed sessions.
---
 apps/qc-app/docs/ARCHITECTURE.md              |  5 +-
 apps/qc-app/docs/USER_GUIDE.md                | 39 ++++++--
 .../src/components/EditData/EditHistory.vue   | 91 ++++++++++---------
 .../EditData/__tests__/EditHistory.spec.ts    | 58 ++++++++++--
 apps/qc-app/src/components/VisualizeData.vue  | 82 +++++++++--------
 5 files changed, 173 insertions(+), 102 deletions(-)

diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index f205884c..fcb9e227 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -294,8 +294,9 @@ Two contract notes worth keeping in mind:
   `created_by`, stamped from the authenticated user on create, and the
   response resolves a deleted account to a placeholder contact rather than
   null. `reconstructSession`/`reconstructCommittedSession` map it onto
-  `HistoryItem.performedBy` (name, falling back to email) so the operations
-  panel can attribute each row. It is never sent back: the field is
+  `HistoryItem.performedBy` (name, falling back to email), shown in a row's
+  expanded detail rather than on the row itself so the collapsed list stays
+  scannable. It is never sent back: the field is
   provenance, not input. Operations applied in the current session show no
   attribution until they are saved and reloaded. Comments have no author of
   their own — `comment` is a plain nullable column that can be rewritten
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 9a12f4b9..c57a246a 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -111,7 +111,7 @@ The Edit View consists of three columns, each independently resizable / collapsi
 |--------|----------|
 | **Left** ("Operations") | The Edit drawer with three sections: Filter Data, Edit Data, Add Data. |
 | **Center** | The Plotly chart + a tab-switched data table (upper left) for the QC target. |
-| **Right** ("Aux") | At the top: **Save** / **Save & Close** / **Close** actions. Below that: the list of Plotted Datastreams, Edit history, and the currently staged Operation Panel. |
+| **Right** ("Aux") | The list of Plotted Datastreams, Edit history, and the currently staged Operation Panel. The **Save** / **Commit** / **Close** actions sit at the bottom of the Edit history panel. |
 
 The chart at the top of the center column has a Plotly toolbar with:
 
@@ -414,7 +414,7 @@ The header carries the count chip and four icon buttons (left to right): **undo*
 
 The body shows:
 
-- A baseline **Data loaded** row at the top, carrying a plot-this-step button, a **reload-from-this-step** button that returns the plot to the state the session started from, and a **reload-from-server** button (cloud icon) that refetches and drops the history entirely.
+- A baseline **Data loaded** row at the top, carrying a plot-this-step button, a **reload-from-this-step** button that returns the plot to the state the session started from, and a **discard-edits-and-reload-from-server** button (cloud icon). The cloud button is hidden on a committed session, where there are no edits to discard.
 - One row per history entry, each with:
   - The operation icon and Title-Case name.
   - A failure badge (red `!`) if the op threw at author time. Common after a QC history import that references something missing in this datastream.
@@ -425,7 +425,26 @@ The body shows:
   - An **undo** button on the trailing entry only (older entries are undone via Reload-from-this-step).
 - A chevron toggles an inline "Arguments" drawer that shows the raw qc-utils call arguments.
 
-Clicking the chevron at the very top of the panel collapses the whole panel; the pop-out icon opens the same panel inside a wider modal so you can scan a long history without losing the rest of the sidebar.
+### Reload from this step vs. reload from server
+
+These two look like siblings on the **Data loaded** row and mostly produce the
+same picture, but they are not the same operation:
+
+- **Reload from this step** replays the raw observations already held in the
+  browser. No network, and it can only ever restore what you loaded.
+- **Discard edits and reload from server** (cloud icon) re-fetches the
+  observations and throws the history away.
+
+Starting a session already fetches the latest committed state, so within an
+undisturbed session the two agree. Reach for the cloud button when the server
+copy has moved underneath you: **another user committed a session** on the same
+managed datastream while you were editing, or the source is still ingesting
+data and you want the new points. The memory replay can never surface either,
+because it faithfully restores the copy you started with.
+
+Below the body sits the session action bar: **Save**, **Commit**, and **Close** (see [Submit](#submit-save--commit--close)). It stays put when the panel is collapsed, so the actions are always one click away.
+
+Clicking the chevron at the very top of the panel collapses the body; the pop-out icon opens the same panel inside a wider modal so you can scan a long history without losing the rest of the sidebar. The modal shows the history only, not the session actions.
 
 ## Comparing against a point in history
 
@@ -487,15 +506,15 @@ Per-op failures do not abort the replay. The app keeps going. If your QC history
 - **Audit trail.** Save the QC history before submitting, so you have a record of every transformation you applied.
 - **Iterate offline.** Edit the QC history's JSON if you want to tweak a threshold without re-clicking through the panels.
 
-## Submit (Save / Save & Close)
+## Submit (Save / Commit / Close)
 
-When you're satisfied with the edits, hit one of the action buttons at the top of the right sidebar:
+When you're satisfied with the edits, hit one of the action buttons at the bottom of the Edit history panel:
 
-- **Save**: uploads and keeps you in the Edit view.
-- **Save & Close**: uploads, clears history, and drops you back to the Select view.
-- **Close**: abandons the session. If you have unsaved edits, the Unsaved-edits dialog intercepts you.
+- **Save**: writes the session's operations to the backend as a draft and keeps you in the Edit view.
+- **Commit**: materializes the session into the managed datastream and locks it into the history.
+- **Close**: leaves the editor. If you have unsaved edits, the Unsaved-edits dialog intercepts you.
 
-Clicking Save (or Save & Close) opens a confirmation dialog so a misclick won't push data to the server.
+Clicking Commit opens a confirmation dialog so a misclick won't push data to the server.
 
 ![Submit confirmation dialog](./images/submit-dialog.png)
 
@@ -525,7 +544,7 @@ See [PERFORMANCE.md](./PERFORMANCE.md) for the envelope details.
 3. Click **All** in the Time range so you load the full series.
 4. Click the pencil icon → expand **Value thresholds**, set `Greater than: 1000`, press Enter.
 5. Expand **Delete points**, click Delete.
-6. Click **Save** (or **Save & Close**), then confirm in the dialog.
+6. Click **Save** at the bottom of the Edit history panel, then **Commit** and confirm in the dialog.
 
 ### "I want to drift-correct a known-bad interval."
 
diff --git a/apps/qc-app/src/components/EditData/EditHistory.vue b/apps/qc-app/src/components/EditData/EditHistory.vue
index e3eee966..537a69d6 100644
--- a/apps/qc-app/src/components/EditData/EditHistory.vue
+++ b/apps/qc-app/src/components/EditData/EditHistory.vue
@@ -212,19 +212,23 @@
           </template>
         </v-tooltip>
 
-        <!-- Distinct icon from the step reload above: this one refetches
-             and drops the history rather than replaying it. -->
-        <v-tooltip v-if="!selectedSeries?.data.isLoading" location="start" text="Reload from server">
+        <!-- Refetches from the server; the step reload above replays the
+             in-memory raw. Hidden on a committed session. -->
+        <v-tooltip
+          v-if="!selectedSeries?.data.isLoading && !isReadOnly"
+          location="start"
+          text="Discard edits and reload from server"
+        >
           <template #activator="{ props: tp }">
             <v-btn
               v-bind="tp"
               data-testid="history-reload-btn"
-              aria-label="Reload from server"
+              aria-label="Discard edits and reload from server"
               size="x-small"
               variant="text"
               density="comfortable"
               icon="mdi-cloud-download-outline"
-              :disabled="isUpdating || isReadOnly"
+              :disabled="isUpdating"
               @click="onReload"
             />
           </template>
@@ -295,11 +299,25 @@
               class="mr-2"
             />
 
-            <span
-              class="edit-history__method flex-grow-1 text-truncate font-weight-medium"
-            >
-              {{ formatMethod(entry.method) }}
-            </span>
+            <!-- Grows as one unit so the badge sits against the title text. -->
+            <div class="edit-history__title flex-grow-1 d-flex align-center ga-1">
+              <span class="edit-history__method text-truncate font-weight-medium">
+                {{ formatMethod(entry.method) }}
+              </span>
+
+              <v-tooltip v-if="entry.comment" location="start" :text="entry.comment">
+                <template #activator="{ props: tp }">
+                  <v-icon
+                    v-bind="tp"
+                    :data-testid="`history-comment-badge-${index}`"
+                    icon="mdi-comment-text-outline"
+                    size="14"
+                    color="primary"
+                    class="flex-shrink-0"
+                  />
+                </template>
+              </v-tooltip>
+            </div>
 
             <v-chip
               v-if="shownStepIndex === index"
@@ -314,34 +332,6 @@
             </v-chip>
 
             <div class="d-flex align-center ga-2 flex-shrink-0">
-              <v-tooltip
-                v-if="entry.performedBy"
-                location="start"
-                :text="`Applied by ${entry.performedBy}`"
-              >
-                <template #activator="{ props: tp }">
-                  <span
-                    v-bind="tp"
-                    class="edit-history__author text-body-small text-medium-emphasis text-truncate"
-                    :data-testid="`history-author-${index}`"
-                  >
-                    {{ entry.performedBy }}
-                  </span>
-                </template>
-              </v-tooltip>
-
-              <v-tooltip v-if="entry.comment" location="start" :text="entry.comment">
-                <template #activator="{ props: tp }">
-                  <v-icon
-                    v-bind="tp"
-                    :data-testid="`history-comment-badge-${index}`"
-                    icon="mdi-comment-text-outline"
-                    size="14"
-                    color="primary"
-                  />
-                </template>
-              </v-tooltip>
-
               <v-tooltip
                 v-if="isApplied(index) && entry.execution?.status === 'failed'"
                 location="start"
@@ -519,6 +509,16 @@
         </template>
       </SessionList>
     </div>
+
+    <!-- Sits outside the collapsible body, so it stays visible when
+         collapsed. -->
+    <div
+      v-if="$slots.footer"
+      class="edit-history__footer flex-shrink-0 d-flex align-center flex-wrap ga-1 px-2 pb-2"
+      :class="isCollapsed ? 'pt-2' : 'pt-0'"
+    >
+      <slot name="footer" />
+    </div>
   </div>
 </template>
 
@@ -552,6 +552,11 @@ const props = withDefaults(
   }
 )
 
+defineSlots<{
+  /** Session actions pinned below the history, shown collapsed or not. */
+  footer?: () => any
+}>()
+
 const emit = defineEmits<{
   (e: 'update:collapsed', value: boolean): void
   (e: 'pop-out'): void
@@ -680,6 +685,8 @@ const onReload = async () => {
       selectedSeries.value.data.redoStack.length = 0
     }
     await refreshGraphSeriesArray()
+    // Restores the raw values when the refetch above failed and left the
+    // edited record in place.
     await selectedSeries.value?.data.reload()
     // reload() already wiped history; don't push an empty SELECTION.
     await clearSelected({ recordHistory: false })
@@ -862,10 +869,6 @@ onBeforeUnmount(() => window.removeEventListener('keydown', onKeydown))
   background-color: rgba(var(--v-theme-primary), 0.04);
 }
 
-.edit-history__author {
-  max-width: 8rem;
-}
-
 .edit-history__row--loaded {
   background-color: rgba(var(--v-theme-primary), 0.1);
   box-shadow: inset 3px 0 0 0 rgb(var(--v-theme-primary));
@@ -896,6 +899,10 @@ onBeforeUnmount(() => window.removeEventListener('keydown', onKeydown))
   background-color: rgba(0, 0, 0, 0.05);
 }
 
+.edit-history__title {
+  min-width: 0;
+}
+
 .edit-history__method {
   font-size: 0.8125rem;
   min-width: 0;
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
index 8fd7a639..80f98391 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
@@ -144,9 +144,13 @@ function makeEntry(
   }
 }
 
-function createWrapper(props: Record<string, unknown> = {}) {
+function createWrapper(
+  props: Record<string, unknown> = {},
+  slots: Record<string, unknown> = {}
+) {
   return mount(EditHistory, {
     props,
+    slots,
     global: { plugins: [createTestPinia(), createTestVuetify()] },
   })
 }
@@ -236,6 +240,26 @@ describe('EditHistory.vue', () => {
     expect(wrapper.emitted('update:collapsed')).toBeFalsy()
   })
 
+  it('renders no footer when the slot is not provided', () => {
+    const wrapper = createWrapper()
+    expect(wrapper.find('.edit-history__footer').exists()).toBe(false)
+  })
+
+  it('keeps the footer slot visible while collapsed', () => {
+    const wrapper = createWrapper(
+      { collapsible: true, collapsed: true },
+      { footer: '<button data-testid="session-actions">Save</button>' }
+    )
+    const footer = wrapper.find('.edit-history__footer')
+    expect(footer.exists()).toBe(true)
+    expect(footer.isVisible()).toBe(true)
+    expect(wrapper.find('[data-testid="session-actions"]').isVisible()).toBe(
+      true
+    )
+    // The body is what collapses (v-show), not the footer.
+    expect(wrapper.find('.edit-history__row--baseline').isVisible()).toBe(false)
+  })
+
   it('emits pop-out when pop-out button is clicked', async () => {
     const wrapper = createWrapper({ popOutEnabled: true })
     const popBtn = wrapper.find('[aria-label="Open history in a modal window"]')
@@ -492,16 +516,26 @@ describe('EditHistory.vue actions', () => {
       ).toBeDefined()
     })
 
-    it('disables reload-from-server, which would wipe the record', async () => {
+    it('hides reload-from-server, which would wipe the record', async () => {
       editHistory.value = [{ method: 'ADD_POINTS', args: [], execution: {} }]
       selectedSeries.value = { data: { history: editHistory.value, redoStack: [] } }
       const w = createWrapper()
       await readOnly()
       await flushPromises()
 
-      expect(
-        w.find('[data-testid="history-reload-btn"]').attributes('disabled')
-      ).toBeDefined()
+      expect(w.find('[data-testid="history-reload-btn"]').exists()).toBe(false)
+    })
+
+    it('shows reload-from-server on an editable session', async () => {
+      editHistory.value = [{ method: 'ADD_POINTS', args: [], execution: {} }]
+      selectedSeries.value = { data: { history: editHistory.value, redoStack: [] } }
+      const w = createWrapper()
+      await flushPromises()
+
+      const btn = w.find('[data-testid="history-reload-btn"]')
+      expect(btn.exists()).toBe(true)
+      // Never gated on whether history exists, unlike the step reload.
+      expect(btn.attributes('disabled')).toBeUndefined()
     })
 
     it('keeps the entries below when reloading from a step', async () => {
@@ -763,7 +797,7 @@ describe('EditHistory.vue actions', () => {
   })
 
   describe('attribution', () => {
-    it('shows who applied an operation, on the row and in the detail', async () => {
+    it('shows who applied an operation in the expanded detail only', async () => {
       editHistory.value = [
         { method: 'ADD_POINTS', args: [], execution: {}, performedBy: 'Ada Lovelace' },
         { method: 'DELETE_POINTS', args: [], execution: {} },
@@ -772,15 +806,21 @@ describe('EditHistory.vue actions', () => {
       const w = createWrapper()
       await flushPromises()
 
-      expect(w.find('[data-testid="history-author-0"]').text()).toBe('Ada Lovelace')
-      // Unsaved operations have no server attribution yet.
-      expect(w.find('[data-testid="history-author-1"]').exists()).toBe(false)
+      // The collapsed row stays terse; attribution lives in the detail.
+      expect(w.find('[data-testid="history-item-0"]').text()).not.toContain(
+        'Ada Lovelace'
+      )
 
       await w.find('[data-testid="history-item-0"]').find('button').trigger('click')
       await flushPromises()
       expect(w.find('[data-testid="history-author-detail-0"]').text()).toContain(
         'Applied by Ada Lovelace'
       )
+
+      // Unsaved operations have no server attribution yet.
+      await w.find('[data-testid="history-item-1"]').find('button').trigger('click')
+      await flushPromises()
+      expect(w.find('[data-testid="history-author-detail-1"]').exists()).toBe(false)
     })
   })
 
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index dc3bdbd9..76808b3a 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -219,7 +219,7 @@
 
       <template v-else>
         <div
-          class="d-flex align-center flex-wrap px-3 py-2 border-b"
+          class="edit-view__sidebar-bar d-flex align-center px-1 py-1 border-b"
         >
           <v-btn
             size="x-small"
@@ -227,45 +227,9 @@
             density="comfortable"
             icon="mdi-chevron-right"
             title="Collapse panel"
-            class="mr-1"
             @click="auxCollapsed = true"
           />
-          <v-btn
-            data-testid="exit-save-btn"
-            size="small"
-            variant="flat"
-            color="primary"
-            prepend-icon="mdi-content-save-outline"
-            :disabled="saveDisabled"
-            :loading="isSavingDraft"
-            @click="onSaveDraft"
-          >
-            Save
-          </v-btn>
-          <v-btn
-            data-testid="exit-commit-btn"
-            class="ml-1"
-            size="small"
-            variant="flat"
-            color="success"
-            prepend-icon="mdi-cloud-check-outline"
-            :disabled="commitDisabled"
-            :loading="isCommitting"
-            @click="openCommit"
-          >
-            Commit
-          </v-btn>
           <v-spacer />
-          <v-btn
-            data-testid="exit-close-btn"
-            size="small"
-            variant="text"
-            prepend-icon="mdi-close"
-            :disabled="isSavingDraft || isCommitting"
-            @click="requestClose"
-          >
-            Close
-          </v-btn>
         </div>
 
         <section class="bg-surface">
@@ -327,7 +291,45 @@
               v-model:collapsed="historyCollapsed"
               @pop-out="historyModalOpen = true"
               @view-session="onViewSession"
-            />
+            >
+              <template #footer>
+                <v-btn
+                  data-testid="exit-save-btn"
+                  size="small"
+                  variant="flat"
+                  color="primary"
+                  prepend-icon="mdi-content-save-outline"
+                  :disabled="saveDisabled"
+                  :loading="isSavingDraft"
+                  @click="onSaveDraft"
+                >
+                  Save
+                </v-btn>
+                <v-btn
+                  data-testid="exit-commit-btn"
+                  size="small"
+                  variant="flat"
+                  color="success"
+                  prepend-icon="mdi-cloud-check-outline"
+                  :disabled="commitDisabled"
+                  :loading="isCommitting"
+                  @click="openCommit"
+                >
+                  Commit
+                </v-btn>
+                <v-spacer />
+                <v-btn
+                  data-testid="exit-close-btn"
+                  size="small"
+                  variant="text"
+                  prepend-icon="mdi-close"
+                  :disabled="isSavingDraft || isCommitting"
+                  @click="requestClose"
+                >
+                  Close
+                </v-btn>
+              </template>
+            </EditHistory>
           </div>
 
           <!-- Only show the split grip when BOTH sides are actually
@@ -1295,11 +1297,13 @@ useResumeEditSession(enterEdit)
 
 .edit-view__plotted-body,
 .edit-view__aux-body,
-.edit-view__history,
 .edit-view__op-panel {
   min-height: 0;
 }
 
+/* .edit-view__history omits min-height: 0 so its automatic minimum floors
+   the pane at the history header + footer while the split is dragged. */
+
 @media (max-width: 960px) {
   .edit-view {
     flex-direction: column;

From 426150dacb6b8e0d938a944291fb7c5bec37d80f Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 17 Aug 2026 14:09:08 -0600
Subject: [PATCH 17/93] test: scope the unsaved-edits discard selector to the
 dialog

---
 apps/qc-app/e2e/navigation.spec.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/apps/qc-app/e2e/navigation.spec.ts b/apps/qc-app/e2e/navigation.spec.ts
index 887a30c1..eda2d464 100644
--- a/apps/qc-app/e2e/navigation.spec.ts
+++ b/apps/qc-app/e2e/navigation.spec.ts
@@ -74,7 +74,7 @@ test.describe('navigation', () => {
     await page.getByTestId('nav-rail-item-select').click()
     await expect(page.getByText(/unsaved edits/i)).toBeVisible()
     await expect(
-      page.getByRole('button', { name: /discard/i })
+      page.getByRole('dialog').getByRole('button', { name: /discard/i })
     ).toBeVisible()
   })
 })

From f46732fc5e1aad6d9a94c77eb0d9af521a585ea1 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Wed, 19 Aug 2026 12:16:04 -0600
Subject: [PATCH 18/93] fix: keep comment and attribution when replaying
 history

Undo, redo, step reload, and removing an entry all rebuild each item from
its method and args, which dropped the operator's comment and the server's
attribution. Re-stamp both afterwards, paired positionally and only when
the method still matches. Execution timings are left to be re-measured.
---
 .../__tests__/observation-record.spec.ts      | 65 +++++++++++++++++++
 .../src/utils/plotting/observation-record.ts  | 50 ++++++++++++--
 2 files changed, 111 insertions(+), 4 deletions(-)

diff --git a/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
index dc7ed687..6f85fabf 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
@@ -760,6 +760,71 @@ describe('ObservationRecord', () => {
       expect(rec.dataX.length).toBeGreaterThan(lenAfterTwo);
     });
 
+    it('keeps comment and attribution across a step reload', async () => {
+      await rec.dispatch([
+        [EnumFilterOperations.SELECTION, [0, 1]],
+        [EnumEditOperations.DELETE_POINTS],
+        [EnumFilterOperations.SELECTION, [0]],
+        [EnumEditOperations.DELETE_POINTS],
+      ]);
+      rec.history[0]!.performedBy = 'Ada Lovelace';
+      rec.history[0]!.comment = 'Dropped the spike';
+      rec.history[1]!.performedBy = 'Grace Hopper';
+
+      await rec.reloadHistory(1);
+
+      expect(rec.history[0]!.performedBy).toBe('Ada Lovelace');
+      expect(rec.history[0]!.comment).toBe('Dropped the spike');
+      expect(rec.history[1]!.performedBy).toBe('Grace Hopper');
+    });
+
+    it('re-measures execution timing rather than carrying it over', async () => {
+      await rec.dispatch([
+        [EnumFilterOperations.SELECTION, [0, 1]],
+        [EnumEditOperations.DELETE_POINTS],
+      ]);
+      rec.history[0]!.performedBy = 'Ada Lovelace';
+      rec.history[0]!.execution!.durationMs = 9999;
+
+      await rec.reloadHistory(1);
+
+      expect(rec.history[0]!.performedBy).toBe('Ada Lovelace');
+      // The timing describes the run that just happened, not the old one.
+      expect(rec.history[0]!.execution!.durationMs).not.toBe(9999);
+    });
+
+    it('keeps comment and attribution across undo', async () => {
+      await rec.dispatch([
+        [EnumFilterOperations.SELECTION, [0, 1]],
+        [EnumEditOperations.DELETE_POINTS],
+      ]);
+      rec.history[0]!.performedBy = 'Ada Lovelace';
+      rec.history[0]!.comment = 'Why this selection';
+
+      await rec.undo();
+
+      expect(rec.history).toHaveLength(1);
+      expect(rec.history[0]!.performedBy).toBe('Ada Lovelace');
+      expect(rec.history[0]!.comment).toBe('Why this selection');
+    });
+
+    it('keeps comment and attribution across redo', async () => {
+      await rec.dispatch([
+        [EnumFilterOperations.SELECTION, [0, 1]],
+        [EnumEditOperations.DELETE_POINTS],
+      ]);
+      rec.history[1]!.performedBy = 'Grace Hopper';
+      rec.history[1]!.comment = 'Removed the pair';
+
+      await rec.undo();
+      await rec.redo();
+
+      const last = rec.history[rec.history.length - 1]!;
+      expect(last.method).toBe(EnumEditOperations.DELETE_POINTS);
+      expect(last.performedBy).toBe('Grace Hopper');
+      expect(last.comment).toBe('Removed the pair');
+    });
+
     it('removeHistoryItem replays history without the removed entry', async () => {
       const originalLen = rec.dataX.length;
       // Two SELECTION + DELETE pairs (entries 0..3).
diff --git a/packages/qc-utils/src/utils/plotting/observation-record.ts b/packages/qc-utils/src/utils/plotting/observation-record.ts
index 925c4bdd..39e635fe 100644
--- a/packages/qc-utils/src/utils/plotting/observation-record.ts
+++ b/packages/qc-utils/src/utils/plotting/observation-record.ts
@@ -367,12 +367,38 @@ export class ObservationRecord {
    * replay the surviving entries. Used by the "Reload from this step"
    * button in EditHistory.
    */
+  /**
+   * Re-stamp the metadata a replay cannot reproduce. `dispatch` rebuilds
+   * each entry from `[method, ...args]` alone, so the operator's comment
+   * and the server's attribution would be dropped on every undo, redo, or
+   * step reload. Execution timings are deliberately left alone: those
+   * describe the run that just happened, not the operation.
+   *
+   * Paired positionally and only when the method still matches, so a
+   * replay that lands differently leaves entries unstamped rather than
+   * attributing an operation to the wrong person.
+   */
+  private _restoreReplayedMeta(preserved: HistoryItem[]) {
+    const len = Math.min(preserved.length, this.history.length);
+    for (let i = 0; i < len; i++) {
+      const from = preserved[i];
+      const to = this.history[i];
+      if (!from || !to || from.method !== to.method) continue;
+      if (from.comment !== undefined) to.comment = from.comment;
+      if (from.performedBy !== undefined) to.performedBy = from.performedBy;
+    }
+  }
+
   async reloadHistory(index: number): Promise<number[]> {
     const newHistory = this.history.slice(0, index + 1);
     this.redoStack.length = 0;
     await this.reload();
 
-    return await this.dispatch(newHistory.map((h) => [h.method, ...(h.args || [])]));
+    const selection = await this.dispatch(
+      newHistory.map((h) => [h.method, ...(h.args || [])]),
+    );
+    this._restoreReplayedMeta(newHistory);
+    return selection;
   }
 
   /** Splice the history entry at `index`, reload from raw, and replay
@@ -382,7 +408,11 @@ export class ObservationRecord {
     newHistory.splice(index, 1);
     this.redoStack.length = 0;
     await this.reload();
-    return await this.dispatch(newHistory.map((h) => [h.method, ...(h.args || [])]));
+    const selection = await this.dispatch(
+      newHistory.map((h) => [h.method, ...(h.args || [])]),
+    );
+    this._restoreReplayedMeta(newHistory);
+    return selection;
   }
 
   /**
@@ -398,7 +428,11 @@ export class ObservationRecord {
     this.redoStack.push(popped);
     this._isReplaying = true;
     try {
-      return await this.dispatch(newHistory.map((h) => [h.method, ...(h.args || [])]));
+      const selection = await this.dispatch(
+        newHistory.map((h) => [h.method, ...(h.args || [])]),
+      );
+      this._restoreReplayedMeta(newHistory);
+      return selection;
     } finally {
       this._isReplaying = false;
     }
@@ -415,7 +449,15 @@ export class ObservationRecord {
     const item = this.redoStack.pop()!;
     this._isReplaying = true;
     try {
-      return await this.dispatch([[item.method, ...(item.args || [])]]);
+      const selection = await this.dispatch([[item.method, ...(item.args || [])]]);
+      const replayed = this.history[this.history.length - 1];
+      if (replayed && replayed.method === item.method) {
+        if (item.comment !== undefined) replayed.comment = item.comment;
+        if (item.performedBy !== undefined) {
+          replayed.performedBy = item.performedBy;
+        }
+      }
+      return selection;
     } finally {
       this._isReplaying = false;
     }

From 20d13c65d5ca4fbb65851cd006023384c009bcfa Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Wed, 19 Aug 2026 12:16:11 -0600
Subject: [PATCH 19/93] test: assert session reads send expand_related

---
 .../api/__tests__/qc-session-query.spec.ts    | 42 +++++++++++++++++++
 1 file changed, 42 insertions(+)
 create mode 100644 packages/hydroserver-ts/src/api/__tests__/qc-session-query.spec.ts

diff --git a/packages/hydroserver-ts/src/api/__tests__/qc-session-query.spec.ts b/packages/hydroserver-ts/src/api/__tests__/qc-session-query.spec.ts
new file mode 100644
index 00000000..b556a44a
--- /dev/null
+++ b/packages/hydroserver-ts/src/api/__tests__/qc-session-query.spec.ts
@@ -0,0 +1,42 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { HydroServer } from '../HydroServer'
+
+const jsonResponse = (data: unknown, headers: Record<string, string> = {}) =>
+  new Response(JSON.stringify(data), {
+    status: 200,
+    headers: { 'Content-Type': 'application/json', ...headers },
+  })
+
+describe('QualityControlSessionService query params', () => {
+  afterEach(() => vi.restoreAllMocks())
+
+  it('sends expand_related on the sessions list', async () => {
+    const fetchMock = vi.fn().mockResolvedValue(
+      jsonResponse([], { 'X-Total-Pages': '1' })
+    )
+    vi.stubGlobal('fetch', fetchMock)
+
+    const hs = new HydroServer({ host: 'https://example.org' })
+    await hs.qualityControlSessions.listAllItems('h-1', {
+      expand_related: true,
+    })
+
+    const url = String(fetchMock.mock.calls[0]![0])
+    console.log('LIST URL:', url)
+    expect(url).toContain('expand_related=true')
+  })
+
+  it('sends expand_related on a single session read', async () => {
+    const fetchMock = vi.fn().mockResolvedValue(jsonResponse({ id: 's-1' }))
+    vi.stubGlobal('fetch', fetchMock)
+
+    const hs = new HydroServer({ host: 'https://example.org' })
+    await hs.qualityControlSessions.getItem('h-1', 's-1', {
+      expand_related: true,
+    })
+
+    const url = String(fetchMock.mock.calls[0]![0])
+    console.log('GET URL:', url)
+    expect(url).toContain('expand_related=true')
+  })
+})

From 29a4d2dd797ba008f93abd148a0c0b13b4d28c58 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Wed, 19 Aug 2026 12:16:37 -0600
Subject: [PATCH 20/93] feat: delete a session together with the sessions built
 on it

Deleting removes descendants first so each delete meets the server's
no-dependents rule. The confirmation names every session going and asks
for an acknowledgement when the cascade reaches past the one picked.

Also in the editor: session actions sit in a container in the edit history
footer, with Discard and a New session action after a commit; history
steps replay by clicking the row instead of a button; steps after the one
shown are dimmed; and the chooser lists sessions as a timeline.
---
 apps/qc-app/docs/ARCHITECTURE.md              |  19 +-
 apps/qc-app/docs/USER_GUIDE.md                |  29 +-
 .../src/components/EditData/EditHistory.vue   |  94 +++--
 .../src/components/EditData/SessionList.vue   |  43 +-
 .../EditData/StartEditingDialog.vue           | 386 +++++++++++++-----
 .../EditData/__tests__/EditHistory.spec.ts    | 141 ++++++-
 .../EditData/__tests__/SessionList.spec.ts    |  13 +
 .../__tests__/StartEditingDialog.spec.ts      | 217 +++++++++-
 apps/qc-app/src/components/VisualizeData.vue  | 163 ++++++--
 .../__tests__/useEditSession.spec.ts          |  51 +++
 apps/qc-app/src/composables/useEditSession.ts |  17 +
 .../src/composables/useManagedDatastreams.ts  |  47 ++-
 apps/qc-app/src/styles/global.scss            |  65 +++
 .../src/utils/__tests__/sessionGraph.spec.ts  |  88 ++++
 apps/qc-app/src/utils/sessionGraph.ts         |  68 +++
 15 files changed, 1186 insertions(+), 255 deletions(-)
 create mode 100644 apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts
 create mode 100644 apps/qc-app/src/utils/sessionGraph.ts

diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index fcb9e227..0960e853 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -301,11 +301,20 @@ Two contract notes worth keeping in mind:
   attribution until they are saved and reloaded. Comments have no author of
   their own — `comment` is a plain nullable column that can be rewritten
   later — so "who wrote this note" is a pending backend ask.
-- **A commit is terminal.** The API rejects updating, deleting, adding
-  operations to, or re-committing a committed session, and its PATCH body
-  carries only `description`. Continuing work after a commit means starting a
-  new session; the backend links it to every committed session its window
-  overlaps, which is how the DAG gets built. Reopening the most recent commit
+- **A commit is terminal.** The API rejects updating, adding operations to,
+  or re-committing a committed session, and its PATCH body carries only
+  `description`. Continuing work after a commit means starting a new session;
+  the backend links it to every committed session its window overlaps, which
+  is how the DAG gets built.
+- **Deleting a session takes its descendants with it.** The API allows
+  deleting any session with no dependents, committed or not, and rejects one
+  that still has them. Sessions carry `dependencyIds` (their ancestors) on the
+  detail response only, so `utils/sessionGraph.ts` inverts that over a
+  `expand_related: true` listing to find descendants, and
+  `deleteSessionChain()` removes them deepest-first so each delete meets the
+  server's rule. There is no transaction: a failure part-way leaves the
+  earlier deletes in place, which is why the error names how many are already
+  gone and the chooser reloads from the server rather than guessing. Reopening the most recent commit
   is a pending backend ask (see the TODO in `store/qcSession.ts`) and needs
   more than lifting the status guard, since a commit also writes observations
   to the managed datastream and rolls the history's checksum and extent
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index c57a246a..1d63b8e3 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -414,14 +414,14 @@ The header carries the count chip and four icon buttons (left to right): **undo*
 
 The body shows:
 
-- A baseline **Data loaded** row at the top, carrying a plot-this-step button, a **reload-from-this-step** button that returns the plot to the state the session started from, and a **discard-edits-and-reload-from-server** button (cloud icon). The cloud button is hidden on a committed session, where there are no edits to discard.
+- A baseline **Data loaded** row at the top, carrying a plot-this-step button and a **discard-edits-and-reload-from-server** button (cloud icon). Clicking the row returns the plot to the state the session started from. The cloud button is hidden on a committed session, where there are no edits to discard.
 - One row per history entry, each with:
   - The operation icon and Title-Case name.
   - A failure badge (red `!`) if the op threw at author time. Common after a QC history import that references something missing in this datastream.
   - A duration badge.
   - In dev mode, a small chip showing whether the op ran inline or on a worker.
   - A **plot-this-step** button that adds that point in history to the plot as a comparison line.
-  - A **reload-from-this-step** button that replays history up to but not including this entry.
+  - Clicking the row itself replays history up to that step. Steps after the one on screen are dimmed, since they were not replayed and so are recorded but not reflected in the plot. Replaying re-measures each step's duration but keeps its comment and attribution.
   - An **undo** button on the trailing entry only (older entries are undone via Reload-from-this-step).
 - A chevron toggles an inline "Arguments" drawer that shows the raw qc-utils call arguments.
 
@@ -442,7 +442,7 @@ managed datastream while you were editing, or the source is still ingesting
 data and you want the new points. The memory replay can never surface either,
 because it faithfully restores the copy you started with.
 
-Below the body sits the session action bar: **Save**, **Commit**, and **Close** (see [Submit](#submit-save--commit--close)). It stays put when the panel is collapsed, so the actions are always one click away.
+Below the body sits the session action bar (see [Submit](#submit-save--commit--close)). It stays put when the panel is collapsed, so the actions are always one click away. While a session is open it carries **Save**, **Discard**, and **Commit**; once you commit, those are replaced by **New session**, which opens the next one over the current time range without a trip back to the Select view. **Close** is always there.
 
 Clicking the chevron at the very top of the panel collapses the body; the pop-out icon opens the same panel inside a wider modal so you can scan a long history without losing the rest of the sidebar. The modal shows the history only, not the session actions.
 
@@ -471,6 +471,27 @@ Things worth knowing:
   Each one replays on load, so a link carrying several is slower to open.
 - Leaving the editor for the Select view drops every snapshot.
 
+## Deleting a session
+
+The **Start editing** chooser shows every session on a managed datastream as a
+timeline, oldest first, each with a trash icon. Both in-progress and committed
+sessions can be deleted. When no session is open, the timeline is headed by a
+**Start new session** node.
+
+Sessions build on each other: a session started after a commit records that it
+began from that commit's result. Deleting a session therefore also deletes
+every session built on top of it, since those describe edits to data that would
+no longer exist. Hovering the trash icon says how far the delete reaches.
+
+The confirmation dialog names every session that will go, marks the one you
+picked, and orders them newest first, which is the order they are removed in.
+When the delete reaches beyond the session you picked, you also have to tick a
+box acknowledging the count. **None of this can be undone.**
+
+If the server rejects a delete part-way through a chain, the sessions already
+removed stay removed. The error says how many, and the chooser reloads so the
+list reflects what actually survived.
+
 ## Save / load a QC history
 
 The QC history is the canonical save format. It's a JSON file you can keep, re-apply, share, or version-control.
@@ -512,6 +533,8 @@ When you're satisfied with the edits, hit one of the action buttons at the botto
 
 - **Save**: writes the session's operations to the backend as a draft and keeps you in the Edit view.
 - **Commit**: materializes the session into the managed datastream and locks it into the history.
+- **Discard**: drops every edit made since the last save, returning the session to its last saved state. Edits already saved to the session stay. Disabled when there is nothing unsaved, and it asks for confirmation first.
+- **New session**: replaces Save and Commit once the session is committed. Opens a new session over the current time range, starting from the state the last commit left behind.
 - **Close**: leaves the editor. If you have unsaved edits, the Unsaved-edits dialog intercepts you.
 
 Clicking Commit opens a confirmation dialog so a misclick won't push data to the server.
diff --git a/apps/qc-app/src/components/EditData/EditHistory.vue b/apps/qc-app/src/components/EditData/EditHistory.vue
index 537a69d6..5e8a7a54 100644
--- a/apps/qc-app/src/components/EditData/EditHistory.vue
+++ b/apps/qc-app/src/components/EditData/EditHistory.vue
@@ -127,6 +127,14 @@
           <div class="rounded border bg-surface overflow-hidden">
       <div
         class="edit-history__row edit-history__row--baseline px-3 py-2 d-flex align-center"
+        :class="{ 'edit-history__row--clickable': canStepTo }"
+        :role="canStepTo ? 'button' : undefined"
+        :tabindex="canStepTo ? 0 : undefined"
+        data-testid="history-reload-step-baseline"
+        :title="canStepTo ? `Reload the session's starting state` : undefined"
+        @click="onRowReload(SNAPSHOT_BASELINE_INDEX)"
+        @keydown.enter.prevent="onRowReload(SNAPSHOT_BASELINE_INDEX)"
+        @keydown.space.prevent="onRowReload(SNAPSHOT_BASELINE_INDEX)"
       >
         <v-icon
           :icon="
@@ -191,23 +199,7 @@
                 snapshotShown(SNAPSHOT_BASELINE_INDEX) ? 'primary' : undefined
               "
               :disabled="isBuilding"
-              @click="onToggleSnapshot(SNAPSHOT_BASELINE_INDEX)"
-            />
-          </template>
-        </v-tooltip>
-
-        <v-tooltip location="start" text="Reload from this step">
-          <template #activator="{ props: tp }">
-            <v-btn
-              v-bind="tp"
-              data-testid="history-reload-step-baseline"
-              aria-label="Reload from this step"
-              size="x-small"
-              variant="text"
-              density="comfortable"
-              icon="mdi-reload"
-              :disabled="isUpdating || !editCount"
-              @click="onReloadHistory(SNAPSHOT_BASELINE_INDEX)"
+              @click.stop="onToggleSnapshot(SNAPSHOT_BASELINE_INDEX)"
             />
           </template>
         </v-tooltip>
@@ -229,7 +221,7 @@
               density="comfortable"
               icon="mdi-cloud-download-outline"
               :disabled="isUpdating"
-              @click="onReload"
+              @click.stop="onReload"
             />
           </template>
         </v-tooltip>
@@ -268,7 +260,19 @@
               'edit-history__row--loading': entry.execution?.inFlight,
               'edit-history__row--open': openIndex === index,
               'edit-history__row--loaded': shownStepIndex === index,
+              'edit-history__row--unapplied': !isApplied(index),
+              'edit-history__row--clickable': canStepTo,
             }"
+            :role="canStepTo ? 'button' : undefined"
+            :tabindex="canStepTo ? 0 : undefined"
+            :title="
+              isApplied(index)
+                ? 'Reload from this step'
+                : 'Not applied in the step currently shown. Click to reload from here.'
+            "
+            @click="onRowReload(index)"
+            @keydown.enter.prevent="onRowReload(index)"
+            @keydown.space.prevent="onRowReload(index)"
           >
             <button
               type="button"
@@ -278,7 +282,7 @@
                 openIndex === index ? 'Collapse' : 'Expand arguments'
               "
               :aria-expanded="openIndex === index"
-              @click="toggle(index)"
+              @click.stop="toggle(index)"
             >
               <v-icon
                 :icon="
@@ -389,21 +393,7 @@
                     "
                     :color="snapshotShown(index) ? 'primary' : undefined"
                     :disabled="isBuilding || entry.execution?.inFlight"
-                    @click="onToggleSnapshot(index)"
-                  />
-                </template>
-              </v-tooltip>
-
-              <v-tooltip location="start" text="Reload from this step">
-                <template #activator="{ props: tp }">
-                  <v-btn
-                    v-bind="tp"
-                    size="x-small"
-                    variant="text"
-                    density="comfortable"
-                    icon="mdi-reload"
-                    :disabled="isUpdating || entry.execution?.inFlight"
-                    @click="onReloadHistory(index)"
+                    @click.stop="onToggleSnapshot(index)"
                   />
                 </template>
               </v-tooltip>
@@ -426,7 +416,7 @@
                     icon="mdi-undo-variant"
                     color="error"
                     :disabled="isUpdating"
-                    @click="onUndo"
+                    @click.stop="onUndo"
                   />
                 </template>
               </v-tooltip>
@@ -514,10 +504,15 @@
          collapsed. -->
     <div
       v-if="$slots.footer"
-      class="edit-history__footer flex-shrink-0 d-flex align-center flex-wrap ga-1 px-2 pb-2"
+      class="edit-history__footer flex-shrink-0 px-2 pb-2"
       :class="isCollapsed ? 'pt-2' : 'pt-0'"
     >
-      <slot name="footer" />
+      <!-- Same container treatment as the session rows above. -->
+      <div
+        class="rounded border bg-surface d-flex align-center flex-wrap ga-2 px-2 py-2"
+      >
+        <slot name="footer" />
+      </div>
     </div>
   </div>
 </template>
@@ -632,6 +627,16 @@ const canRedo = computed(
   () => (selectedSeries.value?.data.redoStack?.length ?? 0) > 0
 )
 
+/** Stepping needs something to replay and a settled dispatch. */
+const canStepTo = computed(() => !isUpdating.value && editCount.value > 0)
+
+/** Row click / Enter / Space: replay to that step. */
+const onRowReload = (index: number) => {
+  if (!canStepTo.value) return
+  if (index >= 0 && editHistory.value[index]?.execution?.inFlight) return
+  onReloadHistory(index)
+}
+
 function toggle(index: number) {
   openIndex.value = openIndex.value === index ? null : index
 }
@@ -869,6 +874,15 @@ onBeforeUnmount(() => window.removeEventListener('keydown', onKeydown))
   background-color: rgba(var(--v-theme-primary), 0.04);
 }
 
+.edit-history__row--clickable {
+  cursor: pointer;
+}
+
+.edit-history__row--clickable:focus-visible {
+  outline: 2px solid rgb(var(--v-theme-primary));
+  outline-offset: -2px;
+}
+
 .edit-history__row--loaded {
   background-color: rgba(var(--v-theme-primary), 0.1);
   box-shadow: inset 3px 0 0 0 rgb(var(--v-theme-primary));
@@ -886,6 +900,12 @@ onBeforeUnmount(() => window.removeEventListener('keydown', onKeydown))
   opacity: 0.75;
 }
 
+/* Steps past the one on screen were not replayed, so they are dimmed to
+   separate what the plot reflects from what is merely recorded. */
+.edit-history__row--unapplied {
+  opacity: 0.45;
+}
+
 .edit-history__expand {
   width: 20px;
   height: 20px;
diff --git a/apps/qc-app/src/components/EditData/SessionList.vue b/apps/qc-app/src/components/EditData/SessionList.vue
index 1b6238fd..c3caf847 100644
--- a/apps/qc-app/src/components/EditData/SessionList.vue
+++ b/apps/qc-app/src/components/EditData/SessionList.vue
@@ -26,6 +26,7 @@
     <v-timeline
       v-else
       side="end"
+      align="start"
       density="compact"
       truncate-line="both"
       line-thickness="2"
@@ -34,17 +35,19 @@
       <v-timeline-item
         v-for="session of orderedSessions"
         :key="session.id"
-        :dot-color="session.status === 'in_progress' ? 'warning' : 'success'"
-        :icon="
-          session.status === 'in_progress' ? 'mdi-pencil' : 'mdi-check'
-        "
+        fill-dot
         size="x-small"
+        :icon="session.status === 'in_progress' ? 'mdi-pencil' : 'mdi-check'"
+        :icon-color="session.status === 'in_progress' ? 'warning' : 'success'"
         width="100%"
         :data-testid="`session-${session.id}`"
-        class="qc-session-list__item"
-        :class="{
-          'qc-session-list__item--viewed': session.id === viewedSessionId,
-        }"
+        class="qc-session-list__item qc-timeline__item"
+        :class="[
+          session.status === 'in_progress'
+            ? 'qc-timeline__item--active'
+            : 'qc-timeline__item--done',
+          { 'qc-timeline__item--viewed': session.id === viewedSessionId },
+        ]"
       >
         <!-- Only the header selects; clicks inside the nested panel must not. -->
         <div
@@ -152,28 +155,4 @@ function returnToCurrent(): void {
   min-height: 28px;
 }
 
-.qc-session-list__timeline :deep(.v-timeline-item__body) {
-  padding-block: 2px;
-}
-
-/* Every session reads as its own container, so the unselected ones don't
-   float loose against the timeline rail. */
-.qc-session-list__item :deep(.v-timeline-item__body) {
-  border-radius: 6px;
-  padding: 6px 8px;
-  border: thin solid rgba(var(--v-border-color), var(--v-border-opacity));
-  background-color: rgba(var(--v-theme-on-surface), 0.02);
-  transition:
-    background-color 120ms ease,
-    border-color 120ms ease;
-}
-
-.qc-session-list__item:hover :deep(.v-timeline-item__body) {
-  background-color: rgba(var(--v-theme-primary), 0.06);
-}
-
-.qc-session-list__item--viewed :deep(.v-timeline-item__body) {
-  background-color: rgba(var(--v-theme-primary), 0.12);
-  border-color: rgba(var(--v-theme-primary), 0.4);
-}
 </style>
diff --git a/apps/qc-app/src/components/EditData/StartEditingDialog.vue b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
index ccbb00ba..95954382 100644
--- a/apps/qc-app/src/components/EditData/StartEditingDialog.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
@@ -42,16 +42,6 @@
               </span>
             </div>
             <v-spacer />
-            <v-btn
-              v-if="!hasInProgress(opt)"
-              size="small"
-              color="primary"
-              variant="flat"
-              :data-testid="`edit-managed-${opt.managed.id}`"
-              @click="emit('edit', opt)"
-            >
-              Start new session
-            </v-btn>
             <v-btn
               icon="mdi-trash-can-outline"
               size="small"
@@ -99,103 +89,110 @@
 
           <v-divider />
 
-          <div
-            v-if="!opt.sessions.length"
-            class="px-3 py-2 text-body-small text-medium-emphasis"
+          <v-timeline
+            side="end"
+            align="start"
+            density="compact"
+            truncate-line="both"
+            line-thickness="2"
+            class="px-3 py-2"
           >
-            No sessions yet.
-          </div>
-          <v-list v-else density="compact" class="py-0" :lines="false">
-            <v-list-item
+            <!-- Heads the timeline as its own node so starting a session
+                 reads as part of the history rather than a header action. -->
+            <v-timeline-item
+              v-if="!hasInProgress(opt)"
+              fill-dot
+              size="x-small"
+              width="100%"
+              icon="mdi-plus"
+              icon-color="primary"
+              class="qc-timeline__item qc-timeline__item--new"
+            >
+              <div class="d-flex align-center ga-2">
+                <div class="flex-grow-1 text-body-small text-medium-emphasis">
+                  {{
+                    opt.sessions.length
+                      ? 'Continue from the latest commit'
+                      : 'No sessions yet'
+                  }}
+                </div>
+                <v-btn
+                  size="small"
+                  color="primary"
+                  variant="flat"
+                  class="flex-shrink-0"
+                  :data-testid="`edit-managed-${opt.managed.id}`"
+                  @click="emit('edit', opt)"
+                >
+                  Start new session
+                </v-btn>
+              </div>
+            </v-timeline-item>
+
+            <v-timeline-item
               v-for="s in orderedSessions(opt.sessions)"
               :key="s.id"
+              fill-dot
+              size="x-small"
+              width="100%"
+              :icon="s.status === 'in_progress' ? 'mdi-pencil' : 'mdi-check'"
+              :icon-color="s.status === 'in_progress' ? 'warning' : 'success'"
               :data-testid="`chooser-session-${s.id}`"
+              class="qc-timeline__item"
+              :class="
+                s.status === 'in_progress'
+                  ? 'qc-timeline__item--active'
+                  : 'qc-timeline__item--done'
+              "
             >
-              <template #prepend>
-                <v-icon
-                  :icon="
-                    s.status === 'in_progress'
-                      ? 'mdi-pencil-circle'
-                      : 'mdi-check-circle'
-                  "
-                  :color="s.status === 'in_progress' ? 'warning' : 'success'"
-                  size="16"
-                  class="mr-2"
-                />
-              </template>
-              <v-list-item-title class="text-body-small">
-                {{ sessionLabel(s) }}
-              </v-list-item-title>
-              <v-list-item-subtitle v-if="s.description" class="text-body-small">
-                {{ sessionPeriod(s) }}
-              </v-list-item-subtitle>
-              <template #append>
-                <div class="d-flex align-center ga-2">
-                  <v-chip
-                    size="x-small"
-                    :color="s.status === 'in_progress' ? 'warning' : 'grey'"
-                    variant="tonal"
-                    label
+              <div class="d-flex align-center ga-2">
+                <div class="flex-grow-1" style="min-width: 0">
+                  <div class="text-body-small font-weight-medium text-truncate">
+                    {{ sessionLabel(s) }}
+                  </div>
+                  <div
+                    v-if="s.description"
+                    class="text-body-small text-medium-emphasis text-truncate"
                   >
-                    {{ s.status === 'in_progress' ? 'In progress' : 'Committed' }}
-                  </v-chip>
-                  <template v-if="s.status === 'in_progress'">
-                    <v-btn
-                      size="small"
-                      color="primary"
-                      variant="flat"
-                      :data-testid="`continue-session-${s.id}`"
-                      @click="emit('edit', opt)"
-                    >
-                      Continue
-                    </v-btn>
-                    <v-btn
-                      icon="mdi-trash-can-outline"
-                      size="x-small"
-                      variant="text"
-                      color="error"
-                      :data-testid="`delete-session-${s.id}`"
-                      title="Discard this in-progress session"
-                      @click="confirmingSessionId = s.id"
-                    />
-                  </template>
+                    {{ sessionPeriod(s) }}
+                  </div>
                 </div>
-              </template>
-            </v-list-item>
-          </v-list>
 
-          <v-alert
-            v-if="confirmingSession(opt)"
-            type="warning"
-            variant="tonal"
-            density="compact"
-            class="mx-3 my-2"
-          >
-            <div class="d-flex align-center flex-wrap ga-2">
-              <span class="text-body-small">
-                Discard this in-progress session and its unsaved edits? Committed
-                sessions are unaffected. This can't be undone.
-              </span>
-              <v-spacer />
-              <v-btn
-                size="x-small"
-                variant="text"
-                data-testid="cancel-delete-session"
-                @click="confirmingSessionId = null"
-              >
-                Cancel
-              </v-btn>
-              <v-btn
-                size="x-small"
-                color="error"
-                variant="flat"
-                :data-testid="`confirm-delete-session-${confirmingSessionId}`"
-                @click="onDeleteSession(opt)"
-              >
-                Discard
-              </v-btn>
-            </div>
-          </v-alert>
+                <v-chip
+                  size="x-small"
+                  :color="s.status === 'in_progress' ? 'warning' : 'grey'"
+                  variant="tonal"
+                  label
+                  class="flex-shrink-0"
+                >
+                  {{ s.status === 'in_progress' ? 'In progress' : 'Committed' }}
+                </v-chip>
+
+                <v-btn
+                  v-if="s.status === 'in_progress'"
+                  size="small"
+                  color="primary"
+                  variant="flat"
+                  class="flex-shrink-0"
+                  :data-testid="`continue-session-${s.id}`"
+                  @click="emit('edit', opt)"
+                >
+                  Continue
+                </v-btn>
+                <v-btn
+                  icon="mdi-trash-can-outline"
+                  size="x-small"
+                  variant="text"
+                  color="error"
+                  class="flex-shrink-0"
+                  :data-testid="`delete-session-${s.id}`"
+                  :title="deleteHint(opt, s)"
+                  @click="openConfirm(opt, s)"
+                />
+              </div>
+            </v-timeline-item>
+          </v-timeline>
+
         </v-card>
 
         <v-btn
@@ -220,13 +217,141 @@
       </v-btn>
     </v-card-actions>
   </v-card>
+
+  <v-dialog v-model="confirmOpen" max-width="560" persistent>
+    <v-card rounded="lg" data-testid="delete-session-dialog">
+      <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
+        <v-avatar color="error" variant="tonal" size="40">
+          <v-icon icon="mdi-delete-alert-outline" size="22" />
+        </v-avatar>
+        <div class="d-flex flex-column">
+          <div class="text-title-large font-weight-bold">
+            {{
+              chainCount > 1
+                ? `Delete ${chainCount} sessions?`
+                : 'Delete this session?'
+            }}
+          </div>
+          <div class="text-body-small text-medium-emphasis">
+            This permanently removes quality control work
+          </div>
+        </div>
+      </div>
+
+      <v-card-text class="pt-2 pb-4 px-6">
+        <p class="text-body-medium mb-3">
+          <template v-if="chainCount > 1">
+            Later sessions were built on this one, so they go too. Deleting
+            only the one you picked would leave them describing edits that no
+            longer exist.
+          </template>
+          <template v-else>
+            Its operations and the record of them are removed.
+          </template>
+        </p>
+
+        <div class="text-body-small font-weight-medium mb-1">
+          {{ chainCount > 1 ? 'Will be deleted, newest first' : 'Will be deleted' }}
+        </div>
+        <v-list
+          density="compact"
+          class="py-0 mb-3 rounded border"
+          :lines="false"
+          data-testid="delete-session-chain"
+        >
+          <v-list-item
+            v-for="s in chainSessions"
+            :key="s.id"
+            :data-testid="`delete-chain-item-${s.id}`"
+          >
+            <template #prepend>
+              <v-icon
+                :icon="
+                  s.id === confirmingSessionId
+                    ? 'mdi-target'
+                    : 'mdi-subdirectory-arrow-right'
+                "
+                :color="s.id === confirmingSessionId ? 'error' : 'warning'"
+                size="16"
+                class="mr-2"
+              />
+            </template>
+            <v-list-item-title class="text-body-small">
+              {{ sessionLabel(s) }}
+              <span v-if="s.id === confirmingSessionId" class="text-medium-emphasis">
+                (the one you picked)
+              </span>
+            </v-list-item-title>
+            <template #append>
+              <v-chip
+                size="x-small"
+                :color="s.status === 'in_progress' ? 'warning' : 'grey'"
+                variant="tonal"
+                label
+              >
+                {{ s.status === 'in_progress' ? 'In progress' : 'Committed' }}
+              </v-chip>
+            </template>
+          </v-list-item>
+        </v-list>
+
+        <div
+          class="text-body-small text-medium-emphasis mb-3"
+          data-testid="delete-session-dependents-note"
+        >
+          Any session built on this one is deleted with it.
+        </div>
+
+        <v-alert type="error" variant="tonal" density="compact">
+          <span class="text-body-small">This cannot be undone.</span>
+        </v-alert>
+
+        <!-- Extra affirmation only when the cascade reaches beyond the
+             session the user actually pointed at. -->
+        <v-checkbox
+          v-if="chainCount > 1"
+          v-model="cascadeAcknowledged"
+          density="compact"
+          hide-details
+          color="error"
+          class="mt-2"
+          data-testid="delete-session-acknowledge"
+        >
+          <template #label>
+            <span class="text-body-small">
+              I understand {{ chainCount }} sessions will be deleted
+            </span>
+          </template>
+        </v-checkbox>
+      </v-card-text>
+
+      <v-divider />
+      <v-card-actions class="d-flex align-center ga-2 px-4 py-3">
+        <v-btn variant="text" data-testid="cancel-delete-session" @click="closeConfirm">
+          Cancel
+        </v-btn>
+        <v-spacer />
+        <v-btn
+          color="error"
+          variant="flat"
+          prepend-icon="mdi-delete-outline"
+          :disabled="confirmDisabled"
+          :data-testid="`confirm-delete-session-${confirmingSessionId}`"
+          @click="onDeleteSession"
+        >
+          {{ chainCount > 1 ? `Delete ${chainCount} sessions` : 'Delete session' }}
+        </v-btn>
+      </v-card-actions>
+    </v-card>
+  </v-dialog>
 </template>
 
 <script setup lang="ts">
-import { ref } from 'vue'
+import { computed, ref } from 'vue'
 import type { Datastream, QualityControlSession } from '@hydroserver/client'
 import type { ManagedDatastreamOption } from '@/composables/useManagedDatastreams'
 import { formatDateRange } from '@/utils/time'
+import { collectDeletionChain } from '@/utils/sessionGraph'
 
 defineProps<{
   source: Datastream
@@ -244,23 +369,66 @@ const emit = defineEmits<{
 
 // historyId of the managed datastream whose delete is awaiting confirmation.
 const confirmingDeleteId = ref<string | null>(null)
-// id of the in-progress session whose discard is awaiting confirmation.
+// id of the session whose delete is awaiting confirmation, and the option
+// it belongs to, so the chain can be resolved against that option's sessions.
 const confirmingSessionId = ref<string | null>(null)
+const confirmingOption = ref<ManagedDatastreamOption | null>(null)
+const cascadeAcknowledged = ref(false)
+
+const confirmOpen = computed({
+  get: () => !!confirmingSessionId.value,
+  set: (open: boolean) => {
+    if (!open) closeConfirm()
+  },
+})
+
+/** The sessions this delete would remove, in the order they go. */
+const chainSessions = computed<QualityControlSession[]>(() => {
+  const opt = confirmingOption.value
+  const targetId = confirmingSessionId.value
+  if (!opt || !targetId) return []
+  const byId = new Map(opt.sessions.map((s) => [s.id, s]))
+  return collectDeletionChain(opt.sessions, targetId)
+    .map((id) => byId.get(id))
+    .filter((s): s is QualityControlSession => !!s)
+})
+
+const chainCount = computed(() => chainSessions.value.length)
+
+// A cascade needs the checkbox; a single session just needs the button.
+const confirmDisabled = computed(
+  () => chainCount.value > 1 && !cascadeAcknowledged.value
+)
+
+const deleteHint = (opt: ManagedDatastreamOption, s: QualityControlSession) => {
+  const count = collectDeletionChain(opt.sessions, s.id).length
+  return count > 1
+    ? `Delete this session and the ${count - 1} built on it`
+    : 'Delete this session'
+}
+
+function openConfirm(opt: ManagedDatastreamOption, s: QualityControlSession) {
+  confirmingOption.value = opt
+  confirmingSessionId.value = s.id
+  cascadeAcknowledged.value = false
+}
+
+function closeConfirm() {
+  confirmingSessionId.value = null
+  confirmingOption.value = null
+  cascadeAcknowledged.value = false
+}
 
 function onDelete(opt: ManagedDatastreamOption) {
   confirmingDeleteId.value = null
   emit('delete', opt)
 }
 
-/** True when this option owns the session awaiting discard confirmation. */
-const confirmingSession = (opt: ManagedDatastreamOption) =>
-  !!confirmingSessionId.value &&
-  opt.sessions.some((s) => s.id === confirmingSessionId.value)
-
-function onDeleteSession(opt: ManagedDatastreamOption) {
+function onDeleteSession() {
+  const opt = confirmingOption.value
   const sessionId = confirmingSessionId.value
-  confirmingSessionId.value = null
-  if (sessionId) emit('deleteSession', opt, sessionId)
+  closeConfirm()
+  if (opt && sessionId) emit('deleteSession', opt, sessionId)
 }
 
 const NUMBER = new Intl.NumberFormat()
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
index 80f98391..8aaad177 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
@@ -146,7 +146,7 @@ function makeEntry(
 
 function createWrapper(
   props: Record<string, unknown> = {},
-  slots: Record<string, unknown> = {}
+  slots: Record<string, string> = {}
 ) {
   return mount(EditHistory, {
     props,
@@ -326,14 +326,14 @@ describe('EditHistory.vue actions', () => {
     vi.useRealTimers()
   })
 
-  it('per-step reload button calls reloadHistory with entry index', async () => {
+  it('clicking a step row calls reloadHistory with its index', async () => {
     vi.useFakeTimers()
     editHistory.value = [makeEntry('ADD_POINTS')]
     selectedSeries.value.data.reloadHistory = vi.fn().mockResolvedValue([9])
     const wrapper = createWrapper()
     await flushPromises()
     const entry = wrapper.find('[data-testid="history-item-0"]')
-    const reloadBtn = entry.findAll('button').find((b) => b.html().includes('mdi-reload'))
+    const reloadBtn = entry.find('.edit-history__row')
     expect(reloadBtn).toBeTruthy()
     await reloadBtn!.trigger('click')
     await vi.runAllTimersAsync()
@@ -480,9 +480,7 @@ describe('EditHistory.vue actions', () => {
       await flushPromises()
 
       await w
-        .find('[data-testid="history-item-1"]')
-        .findAll('button')
-        .at(-1)!
+        .find('[data-testid="history-item-1"] .edit-history__row')
         .trigger('click')
       // `loadedStepIndex` moves synchronously, so waiting on the marker
       // would race the replay. Wait for the dispatch itself to settle.
@@ -556,7 +554,7 @@ describe('EditHistory.vue actions', () => {
       await readOnly()
       await flushPromises()
 
-      await w.find('[data-testid="history-item-0"]').findAll('button').at(-1)!.trigger('click')
+      await w.find('[data-testid="history-item-0"] .edit-history__row').trigger('click')
       await vi.waitFor(() => expect(reloadHistory).toHaveBeenCalledWith(0))
 
       // Data stepped back, but the record of what the session did survives.
@@ -589,7 +587,7 @@ describe('EditHistory.vue actions', () => {
       expect(w.find('[data-testid="history-loaded-1"]').exists()).toBe(true)
       expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(false)
 
-      await w.find('[data-testid="history-item-0"]').findAll('button').at(-1)!.trigger('click')
+      await w.find('[data-testid="history-item-0"] .edit-history__row').trigger('click')
       await vi.waitFor(() =>
         expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(true)
       )
@@ -650,16 +648,19 @@ describe('EditHistory.vue actions', () => {
       expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(false)
     })
 
-    it('disables the baseline reload when there is nothing to step back from', async () => {
+    it('makes no row clickable when there is nothing to step back from', async () => {
       editHistory.value = []
-      selectedSeries.value = makeSeries()
+      const reloadHistory = vi.fn(async () => [])
+      selectedSeries.value = makeSeries({ reloadHistory })
 
       const w = createWrapper()
       await flushPromises()
 
-      expect(
-        w.find('[data-testid="history-reload-step-baseline"]').attributes('disabled')
-      ).toBeDefined()
+      const baseline = w.find('[data-testid="history-reload-step-baseline"]')
+      expect(baseline.classes()).not.toContain('edit-history__row--clickable')
+      expect(baseline.attributes('role')).toBeUndefined()
+      await baseline.trigger('click')
+      expect(reloadHistory).not.toHaveBeenCalled()
     })
 
     // Reloading from a step un-applies everything below it. A committed
@@ -681,9 +682,7 @@ describe('EditHistory.vue actions', () => {
       expect(w.find('[data-testid="history-failed-1"]').exists()).toBe(true)
 
       await w
-        .find('[data-testid="history-item-0"]')
-        .findAll('button')
-        .at(-1)!
+        .find('[data-testid="history-item-0"] .edit-history__row')
         .trigger('click')
       await vi.waitFor(() =>
         expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(true)
@@ -783,7 +782,7 @@ describe('EditHistory.vue actions', () => {
       const w = createWrapper()
       await flushPromises()
 
-      await w.find('[data-testid="history-item-2"]').findAll('button').at(-2)!.trigger('click')
+      await w.find('[data-testid="history-item-2"] .edit-history__row').trigger('click')
       await vi.waitFor(() =>
         expect(w.find('[data-testid="history-loaded-2"]').exists()).toBe(true)
       )
@@ -796,6 +795,112 @@ describe('EditHistory.vue actions', () => {
     })
   })
 
+  describe('row click does not swallow its buttons', () => {
+    it('expanding the args drawer does not step back', async () => {
+      const history = [makeEntry('SELECTION'), makeEntry('DELETE_POINTS')]
+      editHistory.value = history
+      const reloadHistory = vi.fn(async () => [])
+      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const w = createWrapper()
+      await flushPromises()
+
+      await w
+        .find('[data-testid="history-item-0"] .edit-history__expand')
+        .trigger('click')
+      expect(w.text()).toContain('Arguments')
+      expect(reloadHistory).not.toHaveBeenCalled()
+    })
+
+    it('plotting a comparison line does not step back', async () => {
+      const history = [makeEntry('SELECTION'), makeEntry('DELETE_POINTS')]
+      editHistory.value = history
+      const reloadHistory = vi.fn(async () => [])
+      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const w = createWrapper()
+      await flushPromises()
+
+      await w.find('[data-testid="history-snapshot-0"]').trigger('click')
+      expect(reloadHistory).not.toHaveBeenCalled()
+    })
+
+    it('reload-from-server does not step back', async () => {
+      const history = [makeEntry('SELECTION')]
+      editHistory.value = history
+      const reloadHistory = vi.fn(async () => [])
+      selectedSeries.value = {
+        data: {
+          history,
+          redoStack: [],
+          reloadHistory,
+          reload: vi.fn().mockResolvedValue(undefined),
+        },
+      }
+      const w = createWrapper()
+      await flushPromises()
+
+      // onReload defers its work; drain it here or the history wipe lands
+      // in the next test.
+      vi.useFakeTimers()
+      await w.find('[data-testid="history-reload-btn"]').trigger('click')
+      await vi.runAllTimersAsync()
+      vi.useRealTimers()
+      expect(reloadHistory).not.toHaveBeenCalled()
+    })
+  })
+
+  describe('unapplied steps', () => {
+    const threeSteps = () => {
+      const history = [
+        { method: 'SELECTION', args: [], execution: {} },
+        { method: 'DELETE_POINTS', args: [], execution: {} },
+        { method: 'INTERPOLATE', args: [], execution: {} },
+      ]
+      editHistory.value = history
+      const reloadHistory = vi.fn(async () => [])
+      selectedSeries.value = {
+        data: { history, redoStack: [], reloadHistory },
+      }
+      return history
+    }
+
+    const rowOf = (w: any, index: number) =>
+      w.find(`[data-testid="history-item-${index}"] .edit-history__row`)
+
+    it('dims nothing while the plot reflects the whole history', async () => {
+      threeSteps()
+      const w = createWrapper()
+      await flushPromises()
+      for (const i of [0, 1, 2]) {
+        expect(rowOf(w, i).classes()).not.toContain(
+          'edit-history__row--unapplied'
+        )
+      }
+    })
+
+    it('dims the steps after the one being shown', async () => {
+      threeSteps()
+      const w = createWrapper()
+      await flushPromises()
+
+      // Reload from step 0: steps 1 and 2 were not replayed.
+      await w
+        .find('[data-testid="history-item-0"] .edit-history__row')
+        .trigger('click')
+      await vi.waitFor(() =>
+        expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(true)
+      )
+
+      expect(rowOf(w, 0).classes()).not.toContain(
+        'edit-history__row--unapplied'
+      )
+      expect(rowOf(w, 1).classes()).toContain('edit-history__row--unapplied')
+      expect(rowOf(w, 2).classes()).toContain('edit-history__row--unapplied')
+      expect(rowOf(w, 2).attributes('title')).toContain(
+        'Not applied in the step currently shown'
+      )
+    })
+  })
+
   describe('attribution', () => {
     it('shows who applied an operation in the expanded detail only', async () => {
       editHistory.value = [
@@ -836,7 +941,7 @@ describe('EditHistory.vue actions', () => {
       const w = createWrapper()
       await flushPromises()
       // Item 0 isn't the trailing entry, so its buttons are expand + reload.
-      await w.find('[data-testid="history-item-0"]').findAll('button').at(-1)!.trigger('click')
+      await w.find('[data-testid="history-item-0"] .edit-history__row').trigger('click')
       await vi.waitFor(() => expect(reloadHistory).toHaveBeenCalledWith(0))
       await vi.waitFor(() => expect(isUpdating.value).toBe(false))
       await flushPromises()
diff --git a/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts b/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
index b3fee96a..29b8d83b 100644
--- a/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
@@ -72,6 +72,19 @@ describe('SessionList', () => {
     expect(wrapper.text()).toContain('Editing')
   })
 
+  it('marks each session with its status, not just the editable one', async () => {
+    const { committedId, inProgressId } = await seedAndLoad()
+    const wrapper = mountList()
+    await flushPromises()
+    const marker = (id: string) => wrapper.find(`[data-testid="session-${id}"]`)
+    expect(marker(committedId).classes()).toContain('qc-timeline__item--done')
+    expect(marker(committedId).find('.mdi-check').exists()).toBe(true)
+    expect(marker(inProgressId).classes()).toContain(
+      'qc-timeline__item--active'
+    )
+    expect(marker(inProgressId).find('.mdi-pencil').exists()).toBe(true)
+  })
+
   it('lists sessions oldest first, so the most recent is at the bottom', async () => {
     const { committedId, inProgressId } = await seedAndLoad()
     const wrapper = mountList()
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
index 0df861da..56fd9656 100644
--- a/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
@@ -1,5 +1,5 @@
-import { mount } from '@vue/test-utils'
-import { describe, it, expect } from 'vitest'
+import { mount, flushPromises } from '@vue/test-utils'
+import { describe, it, expect, afterEach } from 'vitest'
 import { createTestVuetify } from '@/utils/test/vuetify'
 import StartEditingDialog from '@/components/EditData/StartEditingDialog.vue'
 ;(globalThis as any).ResizeObserver ||= class {
@@ -7,6 +7,16 @@ import StartEditingDialog from '@/components/EditData/StartEditingDialog.vue'
   unobserve() {}
   disconnect() {}
 }
+// jsdom has no visualViewport; Vuetify's overlay positioning reads it.
+;(globalThis as any).visualViewport ||= {
+  addEventListener() {},
+  removeEventListener() {},
+  offsetLeft: 0,
+  offsetTop: 0,
+  width: 1024,
+  height: 768,
+  scale: 1,
+}
 
 const source = { id: 'src-1', name: 'Raw Temp' } as any
 
@@ -41,12 +51,29 @@ const options = [
   { historyId: 'h-2', managed: { id: 'mgd-2', name: 'Temp (QC v2)' }, sessions: [] },
 ] as any
 
+// The delete confirmation is a v-dialog, whose content Vuetify teleports
+// out of the wrapper, so it is queried through the document instead.
 const mountDialog = (props: Record<string, unknown> = {}) =>
   mount(StartEditingDialog, {
     props: { source, options, ...props },
     global: { plugins: [createTestVuetify()] },
+    attachTo: document.body,
   })
 
+const inDialog = (testId: string) =>
+  document.querySelector(`[data-testid="${testId}"]`)
+
+const clickInDialog = async (testId: string) => {
+  const el = inDialog(testId)
+  if (!el) throw new Error(`${testId} is not in the dialog`)
+  ;(el as HTMLElement).click()
+  await flushPromises()
+}
+
+afterEach(() => {
+  document.body.innerHTML = ''
+})
+
 describe('StartEditingDialog', () => {
   it('lists managed datastreams and labels in-progress vs committed sessions', () => {
     const w = mountDialog()
@@ -69,34 +96,77 @@ describe('StartEditingDialog', () => {
     )
   })
 
-  it('offers discard only on the in-progress session', () => {
+  it('heads the timeline with the start-a-session node, above the sessions', () => {
+    const w = mountDialog()
+    const ids = w
+      .findAll('.v-timeline-item')
+      .map((el) => el.attributes('data-testid') ?? 'new-session-node')
+    // mgd-1 has an in-progress session, so only mgd-2 offers the node.
+    expect(w.find('[data-testid="edit-managed-mgd-1"]').exists()).toBe(false)
+    expect(w.find('[data-testid="edit-managed-mgd-2"]').text()).toContain(
+      'Start new session'
+    )
+    // mgd-1's sessions run oldest first; mgd-2's action node follows them.
+    expect(ids).toEqual([
+      'chooser-session-s-0',
+      'chooser-session-s-1',
+      'new-session-node',
+    ])
+  })
+
+  it('renders sessions as a timeline, marked by status', () => {
+    const w = mountDialog()
+    const item = (id: string) => w.find(`[data-testid="chooser-session-${id}"]`)
+
+    expect(w.findAll('.v-timeline-item[data-testid^="chooser-session-"]'))
+      .toHaveLength(2)
+    expect(item('s-1').classes()).toContain('qc-timeline__item--active')
+    expect(item('s-1').find('.mdi-pencil').exists()).toBe(true)
+    expect(item('s-0').classes()).toContain('qc-timeline__item--done')
+    expect(item('s-0').find('.mdi-check').exists()).toBe(true)
+  })
+
+  it('offers delete on every session, and Continue only in progress', () => {
     const w = mountDialog()
     expect(w.find('[data-testid="delete-session-s-1"]').exists()).toBe(true)
+    // Committed sessions are deletable now that the API allows it.
+    expect(w.find('[data-testid="delete-session-s-0"]').exists()).toBe(true)
     expect(w.find('[data-testid="continue-session-s-0"]').exists()).toBe(false)
-    expect(w.find('[data-testid="delete-session-s-0"]').exists()).toBe(false)
   })
 
   it('asks for confirmation, then emits deleteSession for that session', async () => {
     const w = mountDialog()
     await w.find('[data-testid="delete-session-s-1"]').trigger('click')
-    const confirm = w.find('[data-testid="confirm-delete-session-s-1"]')
-    expect(confirm.exists()).toBe(true)
-    await confirm.trigger('click')
+    expect(inDialog('confirm-delete-session-s-1')).not.toBeNull()
+    await clickInDialog('confirm-delete-session-s-1')
     const [option, sessionId] = w.emitted('deleteSession')![0]
     expect(option).toMatchObject({ historyId: 'h-1' })
     expect(sessionId).toBe('s-1')
   })
 
-  it('cancelling the discard confirmation emits nothing', async () => {
+  it('cancelling the confirmation emits nothing', async () => {
     const w = mountDialog()
     await w.find('[data-testid="delete-session-s-1"]').trigger('click')
-    await w.find('[data-testid="cancel-delete-session"]').trigger('click')
-    expect(w.find('[data-testid="confirm-delete-session-s-1"]').exists()).toBe(
-      false
-    )
+    await clickInDialog('cancel-delete-session')
     expect(w.emitted('deleteSession')).toBeUndefined()
   })
 
+  it('warns that a lone delete cannot be undone', async () => {
+    const w = mountDialog()
+    await w.find('[data-testid="delete-session-s-1"]').trigger('click')
+    const dialog = inDialog('delete-session-dialog')!
+    expect(dialog.textContent).toContain('cannot be undone')
+    // One session, so it lists only that one and needs no extra tick-box.
+    expect(
+      document.querySelectorAll('[data-testid^="delete-chain-item-"]')
+    ).toHaveLength(1)
+    expect(inDialog('delete-session-acknowledge')).toBeNull()
+    // The listing can be incomplete, so the warning stands on its own.
+    expect(inDialog('delete-session-dependents-note')!.textContent).toContain(
+      'deleted with it'
+    )
+  })
+
   it('formats session date ranges readably', () => {
     const w = mountDialog()
     expect(w.text()).toContain('Dec 1 – Dec 31, 2024')
@@ -148,3 +218,126 @@ describe('StartEditingDialog', () => {
     expect(w.find('[data-testid="chooser-create-managed"]').exists()).toBe(true)
   })
 })
+
+describe('StartEditingDialog cascade delete', () => {
+  // s-0 committed, s-1 built on it, s-2 built on s-1.
+  const chained = [
+    {
+      historyId: 'h-1',
+      managed: { id: 'mgd-1', name: 'Temp (QC)' },
+      sessions: [
+        {
+          id: 's-0',
+          status: 'committed',
+          createdAt: '2024-12-01T09:00:00Z',
+          description: 'First pass',
+          phenomenonTimeStart: '2024-12-01T00:00:00Z',
+          phenomenonTimeEnd: '2024-12-31T00:00:00Z',
+          dependencyIds: [],
+        },
+        {
+          id: 's-1',
+          status: 'committed',
+          createdAt: '2025-01-05T09:00:00Z',
+          description: 'Second pass',
+          phenomenonTimeStart: '2025-01-05T00:00:00Z',
+          phenomenonTimeEnd: '2025-01-15T00:00:00Z',
+          dependencyIds: ['s-0'],
+        },
+        {
+          id: 's-2',
+          status: 'in_progress',
+          createdAt: '2025-02-01T09:00:00Z',
+          description: 'Third pass',
+          phenomenonTimeStart: '2025-02-01T00:00:00Z',
+          phenomenonTimeEnd: '2025-02-10T00:00:00Z',
+          dependencyIds: ['s-1'],
+        },
+      ],
+    },
+  ] as any
+
+  const mountChained = () =>
+    mount(StartEditingDialog, {
+      props: { source, options: chained },
+      global: { plugins: [createTestVuetify()] },
+      attachTo: document.body,
+    })
+
+  const openDeleteFor = async (id: string) => {
+    const w = mountChained()
+    await w.find(`[data-testid="delete-session-${id}"]`).trigger('click')
+    return w
+  }
+
+  it('lists the whole chain, target last, when deleting the root', async () => {
+    await openDeleteFor('s-0')
+    const ids = [
+      ...document.querySelectorAll('[data-testid^="delete-chain-item-"]'),
+    ].map((el) => el.getAttribute('data-testid'))
+    expect(ids).toEqual([
+      'delete-chain-item-s-2',
+      'delete-chain-item-s-1',
+      'delete-chain-item-s-0',
+    ])
+  })
+
+  it('says how many sessions go and names them', async () => {
+    await openDeleteFor('s-0')
+    const dialog = inDialog('delete-session-dialog')!
+    expect(dialog.textContent).toContain('Delete 3 sessions?')
+    expect(dialog.textContent).toContain('First pass')
+    expect(dialog.textContent).toContain('Second pass')
+    expect(dialog.textContent).toContain('Third pass')
+    expect(dialog.textContent).toContain('cannot be undone')
+  })
+
+  it('marks which session the user actually picked', async () => {
+    await openDeleteFor('s-0')
+    const target = inDialog('delete-chain-item-s-0')!
+    expect(target.textContent).toContain('the one you picked')
+    expect(inDialog('delete-chain-item-s-1')!.textContent).not.toContain(
+      'the one you picked'
+    )
+  })
+
+  it('holds the delete until the cascade is acknowledged', async () => {
+    const w = await openDeleteFor('s-0')
+    const confirm = inDialog('confirm-delete-session-s-0') as HTMLButtonElement
+    expect(confirm.disabled).toBe(true)
+
+    confirm.click()
+    await flushPromises()
+    expect(w.emitted('deleteSession')).toBeUndefined()
+
+    // v-checkbox puts the test id on its wrapper; the input carries the model.
+    const box = document.querySelector(
+      '[data-testid="delete-session-acknowledge"] input'
+    ) as HTMLInputElement
+    box.click()
+    await flushPromises()
+    expect(
+      (inDialog('confirm-delete-session-s-0') as HTMLButtonElement).disabled
+    ).toBe(false)
+    await clickInDialog('confirm-delete-session-s-0')
+    expect(w.emitted('deleteSession')![0][1]).toBe('s-0')
+  })
+
+  it('needs no acknowledgement for a leaf session', async () => {
+    await openDeleteFor('s-2')
+    expect(inDialog('delete-session-acknowledge')).toBeNull()
+    expect(
+      (inDialog('confirm-delete-session-s-2') as HTMLButtonElement).disabled
+    ).toBe(false)
+  })
+
+  it('tells the user on the row how far the delete reaches', async () => {
+    const w = mountChained()
+    expect(
+      w.find('[data-testid="delete-session-s-0"]').attributes('title')
+    ).toBe('Delete this session and the 2 built on it')
+    expect(
+      w.find('[data-testid="delete-session-s-2"]').attributes('title')
+    ).toBe('Delete this session')
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index 76808b3a..2e947ae9 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -293,37 +293,72 @@
               @view-session="onViewSession"
             >
               <template #footer>
+                <!-- Committing ends the session, so the footer swaps Save and
+                     Commit for the one action left: opening the next one. -->
+                <template v-if="inProgressSession">
+                  <v-btn
+                    data-testid="exit-save-btn"
+                    size="small"
+                    variant="flat"
+                    color="primary"
+                    prepend-icon="mdi-content-save-outline"
+                    :disabled="saveDisabled"
+                    :loading="isSavingDraft"
+                    @click="onSaveDraft"
+                  >
+                    Save
+                  </v-btn>
+                  <v-btn
+                    data-testid="exit-discard-btn"
+                    size="small"
+                    variant="tonal"
+                    color="error"
+                    prepend-icon="mdi-backup-restore"
+                    :disabled="!hasUnsavedChanges || isUpdating || isSavingDraft || isCommitting"
+                    :loading="isDiscarding"
+                    title="Drop every edit made since the last save"
+                    @click="showDiscardConfirm = true"
+                  >
+                    Discard
+                  </v-btn>
+                  <v-btn
+                    data-testid="exit-commit-btn"
+                    size="small"
+                    variant="flat"
+                    color="success"
+                    prepend-icon="mdi-cloud-check-outline"
+                    :disabled="commitDisabled"
+                    :loading="isCommitting"
+                    @click="openCommit"
+                  >
+                    Commit
+                  </v-btn>
+                </template>
                 <v-btn
-                  data-testid="exit-save-btn"
+                  v-else
+                  data-testid="exit-new-session-btn"
                   size="small"
                   variant="flat"
                   color="primary"
-                  prepend-icon="mdi-content-save-outline"
-                  :disabled="saveDisabled"
-                  :loading="isSavingDraft"
-                  @click="onSaveDraft"
+                  prepend-icon="mdi-plus"
+                  :disabled="!canEditWorkspace || isStartingSession"
+                  :loading="isStartingSession"
+                  :title="
+                    canEditWorkspace
+                      ? 'Start a new session over the current time range'
+                      : `Your role on this workspace (${workspaceRole}) is read-only.`
+                  "
+                  @click="startSessionForWindow"
                 >
-                  Save
-                </v-btn>
-                <v-btn
-                  data-testid="exit-commit-btn"
-                  size="small"
-                  variant="flat"
-                  color="success"
-                  prepend-icon="mdi-cloud-check-outline"
-                  :disabled="commitDisabled"
-                  :loading="isCommitting"
-                  @click="openCommit"
-                >
-                  Commit
+                  New session
                 </v-btn>
                 <v-spacer />
                 <v-btn
                   data-testid="exit-close-btn"
                   size="small"
-                  variant="text"
+                  variant="tonal"
                   prepend-icon="mdi-close"
-                  :disabled="isSavingDraft || isCommitting"
+                  :disabled="isSavingDraft || isCommitting || isStartingSession"
                   @click="requestClose"
                 >
                   Close
@@ -383,9 +418,9 @@
           </div>
         </div>
         <v-card-text class="text-body-medium pt-2 pb-4 px-6">
-          The source data's integrity is verified, then the edited observations
+          If the source data hasn't changed since you started, your edits
           <strong>replace</strong> the managed datastream over this session's
-          range and the session is locked into the history.
+          range. The session then becomes read-only.
         </v-card-text>
         <div class="px-6 pb-2">
           <v-textarea
@@ -415,6 +450,42 @@
       </v-card>
     </v-dialog>
 
+    <v-dialog v-model="showDiscardConfirm" max-width="520">
+      <v-card rounded="lg">
+        <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
+          <v-avatar color="error" variant="tonal" size="40">
+            <v-icon icon="mdi-backup-restore" size="22" />
+          </v-avatar>
+          <div class="d-flex flex-column">
+            <div class="text-title-large font-weight-bold">Discard unsaved edits?</div>
+            <div class="text-body-small text-medium-emphasis">
+              {{ unsavedEditCount }} edit{{ unsavedEditCount === 1 ? '' : 's' }}
+              since the last save
+            </div>
+          </div>
+        </div>
+        <v-card-text class="text-body-medium pt-2 pb-4 px-6">
+          The session returns to its last saved state. Edits already saved to
+          the session stay. This cannot be undone.
+        </v-card-text>
+        <v-divider />
+        <v-card-actions class="d-flex align-center ga-2 px-4 py-3">
+          <v-btn variant="text" @click="showDiscardConfirm = false">Cancel</v-btn>
+          <v-spacer />
+          <v-btn
+            color="error"
+            variant="flat"
+            prepend-icon="mdi-backup-restore"
+            data-testid="confirm-discard-btn"
+            :loading="isDiscarding"
+            @click="onDiscardUnsaved"
+          >
+            Discard edits
+          </v-btn>
+        </v-card-actions>
+      </v-card>
+    </v-dialog>
+
     <v-dialog v-model="showCloseConfirm" max-width="520">
       <v-card rounded="lg">
         <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
@@ -489,6 +560,7 @@ import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
 import { useProcessingLevels } from '@/composables/useProcessingLevels'
 import type { Datastream } from '@hydroserver/client'
 import { Snackbar } from '@uwrl/qc-utils'
+import { collectDeletionChain } from '@/utils/sessionGraph'
 import {
   decodeShareState,
   encodeShareState,
@@ -543,6 +615,7 @@ const {
   beginEditing,
   startSession,
   saveDraft,
+  discardUnsavedEdits,
   commit,
   needsSession,
   needsHistory,
@@ -583,7 +656,8 @@ const { create: createManaged } = useCreateManagedDatastream()
 const qcPreferences = useQcPreferencesStore()
 const { canEdit, canCreateDatastream, roleName } = useWorkspacePermissions()
 const { createProcessingLevel } = useProcessingLevels()
-const { loadForSource, deleteManaged, deleteSession } = useManagedDatastreams()
+const { loadForSource, deleteManaged, deleteSessionChain } =
+  useManagedDatastreams()
 
 // Permission gating: QC editing writes to the selected workspace (creates
 // the managed datastream, pushes observations). Gate the editor entry
@@ -605,6 +679,9 @@ const chooserSource = ref<Datastream | null>(null)
 const isSavingDraft = ref(false)
 const isCommitting = ref(false)
 const isCreating = ref(false)
+const isStartingSession = ref(false)
+const isDiscarding = ref(false)
+const showDiscardConfirm = ref(false)
 
 const saveDisabled = computed(
   () =>
@@ -717,7 +794,24 @@ function exitToSelect() {
 
 // No in-progress session yet: open one over the window already chosen by the
 // time-range controls in the Select view, rather than prompting for it again.
+async function onDiscardUnsaved() {
+  showDiscardConfirm.value = false
+  isDiscarding.value = true
+  try {
+    await discardUnsavedEdits()
+    await redraw()
+    Snackbar.success('Unsaved edits discarded.')
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not discard the edits.'
+    )
+  } finally {
+    isDiscarding.value = false
+  }
+}
+
 async function startSessionForWindow() {
+  isStartingSession.value = true
   try {
     await startSession({
       phenomenonTimeStart: beginDate.value.toISOString(),
@@ -727,6 +821,8 @@ async function startSessionForWindow() {
     Snackbar.success('Edit session started.')
   } catch (e) {
     Snackbar.error(e instanceof Error ? e.message : 'Could not start the session.')
+  } finally {
+    isStartingSession.value = false
   }
 }
 
@@ -1107,16 +1203,29 @@ async function onChooserDeleteSession(
   sessionId: string
 ) {
   try {
-    await deleteSession(option.historyId, sessionId)
+    // Same input the confirmation previewed, so the two cannot disagree.
+    const deleted = await deleteSessionChain(
+      option.historyId,
+      collectDeletionChain(option.sessions, sessionId)
+    )
+    const gone = new Set(deleted)
     chooserOptions.value = chooserOptions.value.map((o) =>
       o.historyId === option.historyId
-        ? { ...o, sessions: o.sessions.filter((s) => s.id !== sessionId) }
+        ? { ...o, sessions: o.sessions.filter((s) => !gone.has(s.id)) }
         : o
     )
-    Snackbar.success('Session discarded.')
+    Snackbar.success(
+      deleted.length === 1
+        ? 'Session deleted.'
+        : `${deleted.length} sessions deleted.`
+    )
   } catch (e) {
+    // Refresh regardless: a partial cascade leaves the chooser's copy wrong.
+    if (chooserSource.value) {
+      chooserOptions.value = await loadForSource(chooserSource.value.id)
+    }
     Snackbar.error(
-      e instanceof Error ? e.message : 'Could not discard the session.'
+      e instanceof Error ? e.message : 'Could not delete the session.'
     )
   }
 }
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 12b23c97..ae6b604c 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -58,6 +58,11 @@ const makeRecord = (history: any[] = []) => ({
   dataX: [Date.UTC(2025, 0, 1)],
   dataY: [10],
   reload: vi.fn(async () => {}),
+  // Stands in for qc-utils: truncate to `0..index` and hand back a selection.
+  reloadHistory: vi.fn(async function (this: any, index: number) {
+    history.splice(index + 1)
+    return []
+  }),
 })
 
 let qc: ReturnType<typeof makeQcFake>
@@ -292,6 +297,52 @@ describe('useEditSession', () => {
   })
 })
 
+describe('useEditSession.discardUnsavedEdits', () => {
+  it('drops edits added since the last save', async () => {
+    await seedHistory()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    const { discardUnsavedEdits, hasUnsavedChanges } = session
+    await session.beginEditing()
+    await session.startSession(WIN)
+
+    const record = selectedSeries.value.data
+    record.history.push({ method: 'DELETE_POINTS', args: [] })
+    expect(hasUnsavedChanges.value).toBe(true)
+
+    await discardUnsavedEdits()
+
+    expect(record.history).toHaveLength(0)
+    expect(hasUnsavedChanges.value).toBe(false)
+  })
+
+  it('puts a comment edited in place back to its saved text', async () => {
+    await seedHistory()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    const { saveDraft, discardUnsavedEdits, hasUnsavedChanges } = session
+    await session.beginEditing()
+    await session.startSession(WIN)
+
+    const record = selectedSeries.value.data
+    record.history.push({
+      method: 'DELETE_POINTS',
+      args: [],
+      comment: 'the saved reason',
+    })
+    await saveDraft()
+    expect(hasUnsavedChanges.value).toBe(false)
+
+    record.history[0].comment = 'an unsaved rewrite'
+    expect(hasUnsavedChanges.value).toBe(true)
+
+    await discardUnsavedEdits()
+
+    expect(record.history[0].comment).toBe('the saved reason')
+    expect(hasUnsavedChanges.value).toBe(false)
+  })
+})
+
 describe('useEditSession.viewSession', () => {
   it('loads a committed session and marks the editor read-only', async () => {
     const h = unwrap(
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index aa84bf76..e75fb3b9 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -247,6 +247,22 @@ export function useEditSession() {
     snapshotSavedEdits()
   }
 
+  /**
+   * Drop everything added since the last save: replay the saved prefix,
+   * then put the saved comment text back, since a comment edited in place
+   * on a surviving entry survives the replay.
+   */
+  async function discardUnsavedEdits(): Promise<number[] | undefined> {
+    const record = selectedSeries.value?.data as ObservationRecord | undefined
+    if (!record) return
+    const selection = await record.reloadHistory(savedEdits.value.length - 1)
+    record.history.forEach((item, i) => {
+      item.comment = savedComments.value[i] || undefined
+    })
+    snapshotSavedEdits()
+    return selection
+  }
+
   async function saveDraft(): Promise<void> {
     const historyId = sessionStore.historyId
     const session = sessionStore.inProgressSession
@@ -319,6 +335,7 @@ export function useEditSession() {
     startSession,
     viewSession,
     saveDraft,
+    discardUnsavedEdits,
     commit,
   }
 }
diff --git a/apps/qc-app/src/composables/useManagedDatastreams.ts b/apps/qc-app/src/composables/useManagedDatastreams.ts
index 52814320..95ecb246 100644
--- a/apps/qc-app/src/composables/useManagedDatastreams.ts
+++ b/apps/qc-app/src/composables/useManagedDatastreams.ts
@@ -40,8 +40,12 @@ export function useManagedDatastreams() {
             id: managedId,
             name: h.managedDatastream?.name ?? managedId,
           } as Datastream)
-        const sessions =
-          await hs.qualityControlSessions.listAllItems(historyId)
+        // expand_related so each session carries `dependencyIds`, which the
+        // deletion chain is derived from.
+        const sessions = await hs.qualityControlSessions.listAllItems(
+          historyId,
+          { expand_related: true }
+        )
         return { historyId, managed, sessions }
       })
     )
@@ -70,20 +74,39 @@ export function useManagedDatastreams() {
   }
 
   /**
-   * Discard an in-progress session, dropping its draft operations. Committed
-   * sessions are immutable server-side, so only the draft can be deleted.
-   * Throws on failure.
+   * Delete `chain` in order, deepest dependent first so each delete meets
+   * the server's "no dependents" rule. Resolves with the ids removed. On
+   * failure it throws, and the message names how many were already gone,
+   * because the deletes that landed cannot be rolled back.
+   *
+   * Takes the chain rather than deriving one: it must be exactly what the
+   * confirmation showed. If the server has since gained a dependent the
+   * delete is refused there, which is the safe direction to be wrong in.
    */
-  async function deleteSession(
+  async function deleteSessionChain(
     historyId: string,
-    sessionId: string
-  ): Promise<void> {
+    chain: string[]
+  ): Promise<string[]> {
     const { hs } = useHydroServer()
-    const res = await hs.qualityControlSessions.delete(historyId, sessionId)
-    if (!res.ok) {
-      throw new Error(res.message || 'Could not discard the session.')
+    if (!chain.length) {
+      throw new Error('That session no longer exists.')
+    }
+
+    const deleted: string[] = []
+    for (const id of chain) {
+      const res = await hs.qualityControlSessions.delete(historyId, id)
+      if (!res.ok) {
+        const reason = res.message || 'Could not delete the session.'
+        throw new Error(
+          deleted.length
+            ? `${reason} ${deleted.length} of ${chain.length} sessions were already deleted and cannot be restored.`
+            : reason
+        )
+      }
+      deleted.push(id)
     }
+    return deleted
   }
 
-  return { loadForSource, deleteManaged, deleteSession }
+  return { loadForSource, deleteManaged, deleteSessionChain }
 }
diff --git a/apps/qc-app/src/styles/global.scss b/apps/qc-app/src/styles/global.scss
index bbcd5560..345ba4a8 100644
--- a/apps/qc-app/src/styles/global.scss
+++ b/apps/qc-app/src/styles/global.scss
@@ -14,3 +14,68 @@
   color: rgba(var(--v-theme-on-surface), 0.75);
   background: rgba(var(--v-theme-on-surface), 0.03);
 }
+
+/* Session timeline, shared by the editor's session list and the
+   "Start editing" chooser. Unscoped so both can reach Vuetify's
+   timeline internals without repeating the :deep() rules.
+
+   Markers: Vuetify's is a saturated disc ringed in surface-light with the
+   glyph flush to its edge. Flatten it to a tonal marker with room for the
+   icon, nudged onto the title's row. */
+.qc-timeline__item .v-timeline-divider__dot {
+  width: 20px;
+  height: 20px;
+  margin-block-start: 4px;
+  background: rgb(var(--v-theme-surface));
+  box-shadow: none;
+}
+
+/* `.v-icon` declares the multiplier on itself, so it has to be set there. */
+.qc-timeline__item .v-timeline-divider__inner-dot .v-icon {
+  --v-icon-size-multiplier: 0.75;
+}
+
+.qc-timeline__item--active .v-timeline-divider__inner-dot {
+  background: rgba(var(--v-theme-warning), 0.16);
+  box-shadow: inset 0 0 0 1px rgba(var(--v-theme-warning), 0.35);
+}
+
+.qc-timeline__item--done .v-timeline-divider__inner-dot {
+  background: rgba(var(--v-theme-success), 0.16);
+  box-shadow: inset 0 0 0 1px rgba(var(--v-theme-success), 0.35);
+}
+
+/* Every session reads as its own container, so the unselected ones don't
+   float loose against the timeline rail. The card padding replaces Vuetify's
+   body inset, so the gap to the rail comes back as a margin. */
+.qc-timeline__item .v-timeline-item__body {
+  margin-inline-start: 10px;
+  border-radius: 6px;
+  padding: 6px 8px;
+  border: thin solid rgba(var(--v-border-color), var(--v-border-opacity));
+  background-color: rgba(var(--v-theme-on-surface), 0.02);
+  transition:
+    background-color 120ms ease,
+    border-color 120ms ease;
+}
+
+.qc-timeline__item:hover .v-timeline-item__body {
+  background-color: rgba(var(--v-theme-primary), 0.06);
+}
+
+.qc-timeline__item--viewed .v-timeline-item__body {
+  background-color: rgba(var(--v-theme-primary), 0.12);
+  border-color: rgba(var(--v-theme-primary), 0.4);
+}
+
+/* The "start a session" node heads the timeline as an action rather than a
+   record, so it stays flat instead of taking the session card treatment. */
+.qc-timeline__item--new .v-timeline-item__body {
+  border-style: dashed;
+  background-color: transparent;
+}
+
+.qc-timeline__item--new .v-timeline-divider__inner-dot {
+  background: rgba(var(--v-theme-primary), 0.16);
+  box-shadow: inset 0 0 0 1px rgba(var(--v-theme-primary), 0.35);
+}
diff --git a/apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts b/apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts
new file mode 100644
index 00000000..7246ce8e
--- /dev/null
+++ b/apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts
@@ -0,0 +1,88 @@
+import { describe, it, expect } from 'vitest'
+import {
+  buildDependentsMap,
+  collectDeletionChain,
+  hasDependents,
+} from '@/utils/sessionGraph'
+import type { SessionNode } from '@/utils/sessionGraph'
+
+const node = (id: string, dependencyIds: string[] = []): SessionNode => ({
+  id,
+  dependencyIds,
+})
+
+/** A -> B -> C, a straight chain. */
+const chain = [node('a'), node('b', ['a']), node('c', ['b'])]
+
+describe('buildDependentsMap', () => {
+  it('inverts dependencyIds into parent -> children', () => {
+    const map = buildDependentsMap(chain)
+    expect(map.get('a')).toEqual(['b'])
+    expect(map.get('b')).toEqual(['c'])
+    expect(map.get('c')).toBeUndefined()
+  })
+
+  it('treats a missing dependencyIds as no links', () => {
+    expect(buildDependentsMap([{ id: 'a' }]).size).toBe(0)
+  })
+})
+
+describe('hasDependents', () => {
+  it('is true for a session others were built on', () => {
+    expect(hasDependents(chain, 'a')).toBe(true)
+    expect(hasDependents(chain, 'b')).toBe(true)
+  })
+
+  it('is false for a leaf', () => {
+    expect(hasDependents(chain, 'c')).toBe(false)
+  })
+})
+
+describe('collectDeletionChain', () => {
+  it('returns just the target when nothing depends on it', () => {
+    expect(collectDeletionChain(chain, 'c')).toEqual(['c'])
+  })
+
+  it('orders descendants before the target', () => {
+    expect(collectDeletionChain(chain, 'a')).toEqual(['c', 'b', 'a'])
+  })
+
+  it('deletes a shared descendant before either of its parents', () => {
+    // t -> a, t -> b, and both a and b -> d.
+    const diamond = [
+      node('t'),
+      node('a', ['t']),
+      node('b', ['t']),
+      node('d', ['a', 'b']),
+    ]
+    const order = collectDeletionChain(diamond, 't')
+
+    expect(order).toHaveLength(4)
+    expect(order.at(-1)).toBe('t')
+    // Every session precedes the ones it was built on.
+    expect(order.indexOf('d')).toBeLessThan(order.indexOf('a'))
+    expect(order.indexOf('d')).toBeLessThan(order.indexOf('b'))
+    expect(order.indexOf('a')).toBeLessThan(order.indexOf('t'))
+    expect(order.indexOf('b')).toBeLessThan(order.indexOf('t'))
+  })
+
+  it('lists each session once when reachable by several paths', () => {
+    const diamond = [
+      node('t'),
+      node('a', ['t']),
+      node('b', ['t']),
+      node('d', ['a', 'b']),
+    ]
+    const order = collectDeletionChain(diamond, 't')
+    expect(new Set(order).size).toBe(order.length)
+  })
+
+  it('ignores sessions on an unrelated branch', () => {
+    const forked = [...chain, node('x'), node('y', ['x'])]
+    expect(collectDeletionChain(forked, 'b')).toEqual(['c', 'b'])
+  })
+
+  it('returns nothing for an unknown target', () => {
+    expect(collectDeletionChain(chain, 'nope')).toEqual([])
+  })
+})
diff --git a/apps/qc-app/src/utils/sessionGraph.ts b/apps/qc-app/src/utils/sessionGraph.ts
new file mode 100644
index 00000000..1c4673d5
--- /dev/null
+++ b/apps/qc-app/src/utils/sessionGraph.ts
@@ -0,0 +1,68 @@
+/**
+ * Session dependency graph helpers.
+ *
+ * A session carries `dependencyIds` (the sessions it was built on), so its
+ * edges point at ancestors. Deletion needs the opposite direction, which is
+ * derived by inverting that list over the sessions already loaded.
+ *
+ * `dependencyIds` only rides along on the detail response, so callers must
+ * list sessions with `expand_related: true`. Without it every session looks
+ * like a leaf and the chain collapses to the target alone.
+ */
+
+/** The slice of a session this module needs, on either response shape. */
+export interface SessionNode {
+  id: string
+  dependencyIds?: string[]
+}
+
+/** Maps a session id to the ids of the sessions built on top of it. */
+export function buildDependentsMap(
+  sessions: SessionNode[]
+): Map<string, string[]> {
+  const dependents = new Map<string, string[]>()
+  for (const session of sessions) {
+    for (const parentId of session.dependencyIds ?? []) {
+      const existing = dependents.get(parentId)
+      if (existing) existing.push(session.id)
+      else dependents.set(parentId, [session.id])
+    }
+  }
+  return dependents
+}
+
+/** True when another loaded session was built on top of this one. */
+export function hasDependents(
+  sessions: SessionNode[],
+  sessionId: string
+): boolean {
+  return (buildDependentsMap(sessions).get(sessionId)?.length ?? 0) > 0
+}
+
+/**
+ * The sessions that must be removed to delete `targetId`, ordered so each
+ * one is free of dependents by the time its turn comes: descendants first,
+ * target last. Empty when the target is not in `sessions`.
+ */
+export function collectDeletionChain(
+  sessions: SessionNode[],
+  targetId: string
+): string[] {
+  if (!sessions.some((s) => s.id === targetId)) return []
+
+  const dependents = buildDependentsMap(sessions)
+  const order: string[] = []
+  const visited = new Set<string>()
+
+  // Post-order over the dependents edges: a node is appended only after
+  // everything reachable from it, which is exactly delete order.
+  const visit = (id: string) => {
+    if (visited.has(id)) return
+    visited.add(id)
+    for (const childId of dependents.get(id) ?? []) visit(childId)
+    order.push(id)
+  }
+  visit(targetId)
+
+  return order
+}

From caccfb4dc53e25b3a39fb78afbc86c2d954ed4d9 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 31 Aug 2026 12:54:32 -0600
Subject: [PATCH 21/93] chore: remove planning docs folder

---
 ...6-08-05-history-snapshot-compare-design.md | 214 ------------------
 1 file changed, 214 deletions(-)
 delete mode 100644 docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md

diff --git a/docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md b/docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md
deleted file mode 100644
index ade51b83..00000000
--- a/docs/superpowers/specs/2026-08-05-history-snapshot-compare-design.md
+++ /dev/null
@@ -1,214 +0,0 @@
-# History snapshot compare
-
-Plot the state of a QC session at any point in its operation history as a
-separate line on the current plot, so a user editing a session can compare
-against the same data at another point in history.
-
-## Motivation
-
-The editor can already *view* a past session, but viewing replaces the QC
-series and flips the editor read-only. There is no way to see two points in
-history side by side. This adds a second, read-only line rather than swapping
-the working one.
-
-## Definitions
-
-A **snapshot** is a frozen `GraphSeries` holding the `ObservationRecord`
-produced by replaying a session's operations up to a chosen point, computed
-over that session's own window.
-
-- `opIndex = -1` is the baseline: the state the session started from, before
-  any of its own operations. This is the "Data loaded" row in `EditHistory`.
-- `opIndex = k` is the state after operation `k`.
-
-## Base and replay
-
-Reuses the existing reconstruct services rather than a parallel
-implementation.
-
-| Session status | Base | Replayed |
-| --- | --- | --- |
-| Committed | `reconstructCommittedSession`: raw source over the union of the ancestor chain's windows | ancestors in `committedAt` order, then the target session's operations `0..k` |
-| In progress | `loadLatestBase(managed, source)` | the live record's `history[0..k]` |
-
-`reconstructCommittedSession` gains an optional `opLimit` parameter that
-truncates the target session's own operations, so the committed path reuses it
-rather than duplicating the ancestor-chain logic.
-
-Reading the in-progress session's operations from the live record rather than
-from the server means unsaved draft edits are included. That matches what a
-user expects when comparing against work they are currently doing.
-
-The window is the session chain's own window, never the plot's current time
-range. `reconstructCommittedSession` already loads the union of the chain's
-windows because operations replay against array indices, so a narrower base
-misaligns a wider ancestor. Recomputing a snapshot over an arbitrary user
-window would therefore corrupt the replay.
-
-Consequence: a snapshot is **frozen at creation**. It is never refetched, and
-changing the plot's time range does not recompute it. It spans only its
-session's window; zooming outside that window shows no line there.
-
-## Record isolation
-
-`useObservationStore.fetchObservationsInRange` keeps one `ObservationRecord`
-per datastream id and hands back the same instance. The reconstruct services
-replay operations onto whatever their injected `fetchInRange` returns, so
-building a snapshot through the store's fetcher would mutate the record the
-plot is already using, and a second snapshot built from the same source would
-clobber the first.
-
-Snapshots therefore inject a **detached fetcher** with the same
-`FetchObservationsInRange` signature:
-
-```ts
-async function fetchDetached(ds, start, end) {
-  await fetchObservationsInRange(ds, start, end) // warm the raw cache
-  const raw = observationsRaw.value[ds.id] ?? { datetimes: [], dataValues: [] }
-  const record = new ObservationRecord(raw)
-  await record.applyWindow(start.getTime(), end.getTime())
-  return record
-}
-```
-
-This is safe because `ObservationRecord`'s constructor copies into its own
-`dataset.source` buffers and treats `rawData` as read-only; operations mutate
-only `dataset.source`. The reconstruct services need no change to accept it.
-
-A detached record does share the `rawData` *object* with the store, so a later
-store fetch that widens the raw cache is visible to it. That only matters to a
-record that calls `reload()`, which a frozen snapshot never does.
-
-## Identity and store shape
-
-Snapshots get the synthetic id `snap:<sessionId>:<opIndex>` and a synthetic
-`Datastream`-shaped object pushed into `plottedDatastreams`. Legend rendering,
-colour assignment, the eye toggle, drag reorder, and removal then work
-unchanged instead of growing a parallel code path.
-
-Four guards, each keyed off a single `isSnapshotId()` predicate:
-
-1. `refreshGraphSeriesArray` skips the fetch for snapshot ids and keeps the
-   existing series. This is what makes a snapshot frozen.
-2. `releaseManagedDatastream` drops every snapshot when the editor closes.
-   The QC target is chosen from the Select view, which lists real
-   datastreams; a snapshot must never appear there.
-3. Snapshots are excluded from the `ds` share key (see below).
-4. A snapshot id is never sent to any server call.
-
-`GraphSeries` gains:
-
-```ts
-snapshot?: {
-  sessionId: string
-  sessionLabel: string
-  opIndex: number
-  opCount: number
-  opName: string
-  performedBy?: string
-  createdAt: string
-}
-```
-
-## Y axis
-
-Snapshots get their own axis, exactly like any other non-QC series. Being
-able to shift a snapshot on its own axis is how the user lines it up against
-the QC target, which is the point of plotting it. No axis is shared except
-the x axis, which every series shares.
-
-This means no change to `createPlotlyOption`'s axis assignment, and the
-per-row axis-toggle button works on snapshot rows like any other.
-
-## UI
-
-### EditHistory
-
-An add-to-plot icon button on each operation row and on the "Data loaded"
-baseline row. It toggles: when that exact snapshot is already plotted, the
-button removes it.
-
-### PlottedDatastreams
-
-Snapshot rows carry an `mdi-history` icon and a "snapshot" chip so they never
-read as a live datastream:
-
-```
-[history icon] [snapshot] Session Mar 2024
-                          step 3 of 7: Fill Gaps - by Alice - Mar 14 2026
-```
-
-Baseline rows read `session start - Mar 14 2026`.
-
-Line colour comes from the existing per-series assigner. No dashed line and no
-hover tooltip: the colour already distinguishes lines, and building tooltip
-content per row is not worth the cost.
-
-## Share URL
-
-A new `snap` key rather than folding snapshots into `ds`:
-
-```
-snap=a1b2:3,a1b2:-1
-```
-
-Keeping it separate preserves both invariants the encoder relies on: the QC
-target is the first id in `ds`, and the `h` / `ya` bitmasks are indexed by
-position in `ds`.
-
-On hydration, snapshots are replayed after `loadSessions` has run and are
-appended after the real datastreams. A snapshot whose session no longer exists
-is dropped silently rather than failing the whole hydration.
-
-## Testing
-
-Unit:
-
-- snapshot builder: truncation at `k`, baseline at `-1`, committed versus
-  in-progress base selection
-- share encode/decode round trip for the `snap` key
-- `refreshGraphSeriesArray` exemption: a snapshot series survives a refresh
-  and is not fetched
-
-Component:
-
-- `EditHistory`: the add-to-plot button appears on operation rows and on the
-  baseline row, and toggles a plotted snapshot off
-- `PlottedDatastreams`: a snapshot row renders its provenance line, and the
-  QC-target and axis-toggle buttons are suppressed
-
-## Reload-from-step, fixed alongside
-
-Stepping through a committed session's history restored the pre-replay
-entries wholesale, which put the *old* timings back over the ones the replay
-had just measured. Only the "Data loaded" row updated, because its time comes
-from `record.loadingTime`.
-
-The panel now keeps the replayed entries as the replay left them and appends
-only the ones it dropped, recomputing the shown-step marker from the actual
-replayed length (a replay can drop a preceding `SELECTION`). `isApplied()`
-hides execution data for steps past the one on screen, since those did not run
-in this view.
-
-The "Data loaded" row is step `-1` and reloads to it, so a session's starting
-state is reachable the same way any operation is. It carries the "Showing"
-chip when it is the step on screen. Its reload-from-server button moved to a
-cloud icon so the two reloads are not two identical glyphs doing different
-things: one replays, the other refetches and drops the history.
-
-`hasUnsavedChanges` / `unsavedEditCount` now return false / 0 while a
-committed session is being viewed. They compare history entries by identity,
-so a replay that swaps entries used to read as pending edits and prompted
-"You have unsaved edits" on a read-only view. A committed session cannot be
-edited, so the invariant belongs in the computed rather than being worked
-around by preserving object identity.
-
-## Out of scope
-
-- No cap on the number of simultaneous snapshots. Each costs a base fetch plus
-  a replay, so several at once over a wide window will be slow. Left visible
-  rather than guessing where a limit belongs.
-- Expanding a session still loads it (view mode). Decoupling expansion from
-  viewing was considered and rejected as a separate concern; users already
-  navigate back with "Return to current", and the snapshot survives that
-  round trip.

From 20f260a254e1a0f312f5eedeb72abbf847741f44 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Tue, 1 Sep 2026 16:07:01 -0600
Subject: [PATCH 22/93] fix: date picker dialog collapsing to a thin strip

---
 apps/qc-app/e2e/date-picker-dialog.spec.ts    | 36 +++++++++++++++++++
 .../VisualizeData/DatePickerField.vue         |  4 ++-
 2 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 apps/qc-app/e2e/date-picker-dialog.spec.ts

diff --git a/apps/qc-app/e2e/date-picker-dialog.spec.ts b/apps/qc-app/e2e/date-picker-dialog.spec.ts
new file mode 100644
index 00000000..18fb52b8
--- /dev/null
+++ b/apps/qc-app/e2e/date-picker-dialog.spec.ts
@@ -0,0 +1,36 @@
+/**
+ * The calendar dialog behind `DatePickerField` must open at the full
+ * width of the Vuetify date picker. Dimension props on an overlay go
+ * through `parseFloat`, so a rem-valued `max-width` collapses the
+ * dialog into a thin strip.
+ */
+
+import { expect, test } from '@playwright/test'
+import { installMocks } from './support/mocks'
+import { openOp, setupEditView, waitForSelection } from './support/app'
+
+test.describe('date picker dialog', () => {
+  test('opens at the full width of the date picker', async ({ page }) => {
+    await installMocks(page)
+    await setupEditView(page)
+    await openOp(page, 'datetimeRange')
+    await waitForSelection(page, 1)
+
+    await page.locator('.mdi-calendar-blank').first().click()
+
+    const card = page.locator('.date-picker-card')
+    await expect(card).toBeVisible()
+    await expect(page.locator('.v-date-picker-month')).toBeVisible()
+
+    // Polled: the dialog scales up on open, so the first measurement
+    // lands mid-transition.
+    await expect
+      .poll(async () => (await card.boundingBox())?.width ?? 0)
+      .toBeGreaterThanOrEqual(328)
+
+    // The month grid is not clipped by a collapsed card.
+    const cardBox = (await card.boundingBox())!
+    const picker = (await page.locator('.v-date-picker').boundingBox())!
+    expect(picker.width).toBeLessThanOrEqual(cardBox.width + 1)
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData/DatePickerField.vue b/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
index 409e5a51..ecbc8dcf 100644
--- a/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
+++ b/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
@@ -23,7 +23,9 @@
     />
   </div>
 
-  <v-dialog v-model="showDateDialog" max-width="20rem">
+  <!-- Pixels, not rem: the overlay location strategy runs parseFloat on
+       max-width, so "20rem" would be read as 20px. 328 matches v-date-picker. -->
+  <v-dialog v-model="showDateDialog" max-width="328">
     <v-card class="date-picker-card">
       <div class="d-flex align-center px-4 py-2">
         <span class="text-title-medium font-weight-medium">

From ecd61162b04dde6a9fd56b433b5117ca0c173ff7 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Tue, 1 Sep 2026 16:14:22 -0600
Subject: [PATCH 23/93] style: remove redundant comment on date picker dialog
 width

---
 apps/qc-app/src/components/VisualizeData/DatePickerField.vue | 2 --
 1 file changed, 2 deletions(-)

diff --git a/apps/qc-app/src/components/VisualizeData/DatePickerField.vue b/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
index ecbc8dcf..4d1935d6 100644
--- a/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
+++ b/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
@@ -23,8 +23,6 @@
     />
   </div>
 
-  <!-- Pixels, not rem: the overlay location strategy runs parseFloat on
-       max-width, so "20rem" would be read as 20px. 328 matches v-date-picker. -->
   <v-dialog v-model="showDateDialog" max-width="328">
     <v-card class="date-picker-card">
       <div class="d-flex align-center px-4 py-2">

From 934084d57d8eebfb936f88a9c684753c2826adc7 Mon Sep 17 00:00:00 2001
From: Ken Lippold <klippold@Kens-MacBook-Pro-2.local>
Date: Tue, 1 Sep 2026 15:36:47 -0700
Subject: [PATCH 24/93] Update version

---
 django/contracts/openapi/auth.openapi.json | 2 +-
 django/contracts/openapi/data.openapi.json | 2 +-
 django/hydroserver/__init__.py             | 2 +-
 packages/hydroserverpy/setup.cfg           | 2 +-
 4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/django/contracts/openapi/auth.openapi.json b/django/contracts/openapi/auth.openapi.json
index 97d182d1..a35fb19a 100644
--- a/django/contracts/openapi/auth.openapi.json
+++ b/django/contracts/openapi/auth.openapi.json
@@ -678,7 +678,7 @@
   "info": {
     "description": "",
     "title": "HydroServer User Authentication API",
-    "version": "1.12.0b2"
+    "version": "1.12.0b3"
   },
   "openapi": "3.1.0",
   "paths": {
diff --git a/django/contracts/openapi/data.openapi.json b/django/contracts/openapi/data.openapi.json
index ec9e5445..fcc1b4ba 100644
--- a/django/contracts/openapi/data.openapi.json
+++ b/django/contracts/openapi/data.openapi.json
@@ -11613,7 +11613,7 @@
   "info": {
     "description": "",
     "title": "HydroServer Data Management API",
-    "version": "1.12.0b2"
+    "version": "1.12.0b3"
   },
   "openapi": "3.1.0",
   "paths": {
diff --git a/django/hydroserver/__init__.py b/django/hydroserver/__init__.py
index 490eaec2..88ceaf79 100644
--- a/django/hydroserver/__init__.py
+++ b/django/hydroserver/__init__.py
@@ -1,4 +1,4 @@
 from .celery import app as celery_app
 
-__version__ = "1.12.0b2"
+__version__ = "1.12.0b3"
 __all__ = ("celery_app", "__version__",)
diff --git a/packages/hydroserverpy/setup.cfg b/packages/hydroserverpy/setup.cfg
index 5dbe2d8b..8d018290 100644
--- a/packages/hydroserverpy/setup.cfg
+++ b/packages/hydroserverpy/setup.cfg
@@ -1,6 +1,6 @@
 [metadata]
 name = hydroserverpy
-version = 1.12.0b2
+version = 1.12.0b3
 description = A Python client for managing HydroServer data
 long_description = file: README.md
 long_description_content_type = text/markdown

From e6a1533e6ceddc1a572f0e9f0d61509165af5e07 Mon Sep 17 00:00:00 2001
From: Ken Lippold <klippold@Kens-MacBook-Pro-2.local>
Date: Tue, 1 Sep 2026 15:44:32 -0700
Subject: [PATCH 25/93] Update version

---
 django/contracts/openapi/auth.openapi.json | 2 +-
 django/contracts/openapi/data.openapi.json | 2 +-
 django/hydroserver/__init__.py             | 2 +-
 packages/hydroserverpy/setup.cfg           | 2 +-
 4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/django/contracts/openapi/auth.openapi.json b/django/contracts/openapi/auth.openapi.json
index a35fb19a..e9329604 100644
--- a/django/contracts/openapi/auth.openapi.json
+++ b/django/contracts/openapi/auth.openapi.json
@@ -678,7 +678,7 @@
   "info": {
     "description": "",
     "title": "HydroServer User Authentication API",
-    "version": "1.12.0b3"
+    "version": "1.12.0b4"
   },
   "openapi": "3.1.0",
   "paths": {
diff --git a/django/contracts/openapi/data.openapi.json b/django/contracts/openapi/data.openapi.json
index fcc1b4ba..1dd73827 100644
--- a/django/contracts/openapi/data.openapi.json
+++ b/django/contracts/openapi/data.openapi.json
@@ -11613,7 +11613,7 @@
   "info": {
     "description": "",
     "title": "HydroServer Data Management API",
-    "version": "1.12.0b3"
+    "version": "1.12.0b4"
   },
   "openapi": "3.1.0",
   "paths": {
diff --git a/django/hydroserver/__init__.py b/django/hydroserver/__init__.py
index 88ceaf79..c9a10dff 100644
--- a/django/hydroserver/__init__.py
+++ b/django/hydroserver/__init__.py
@@ -1,4 +1,4 @@
 from .celery import app as celery_app
 
-__version__ = "1.12.0b3"
+__version__ = "1.12.0b4"
 __all__ = ("celery_app", "__version__",)
diff --git a/packages/hydroserverpy/setup.cfg b/packages/hydroserverpy/setup.cfg
index 8d018290..f91dbfab 100644
--- a/packages/hydroserverpy/setup.cfg
+++ b/packages/hydroserverpy/setup.cfg
@@ -1,6 +1,6 @@
 [metadata]
 name = hydroserverpy
-version = 1.12.0b3
+version = 1.12.0b4
 description = A Python client for managing HydroServer data
 long_description = file: README.md
 long_description_content_type = text/markdown

From a8e6c00091f968b9fdbd28f7f1c5326324c74372 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 11:11:47 -0600
Subject: [PATCH 26/93] feat: choose which QC datastreams to plot from a source
 row

---
 apps/qc-app/docs/API_REFERENCE.md             |   2 +
 apps/qc-app/docs/PLOTTING.md                  |  28 +++
 apps/qc-app/docs/TESTING.md                   |   7 +
 apps/qc-app/docs/USER_GUIDE.md                |  23 +++
 apps/qc-app/e2e/plot-source-dialog.spec.ts    | 115 +++++++++++
 apps/qc-app/e2e/support/fixtures.ts           |  93 +++++++++
 apps/qc-app/e2e/support/mocks.ts              |  40 +++-
 .../EditData/StartEditingDialog.vue           |  24 +--
 apps/qc-app/src/components/VisualizeData.vue  |   8 +
 .../VisualizeData/DataVisDatasetsTable.vue    | 121 +++++++++++-
 .../VisualizeData/PlotSourceDialog.vue        | 186 ++++++++++++++++++
 .../__tests__/DataVisDatasetsTable.spec.ts    | 160 +++++++++++++++
 .../__tests__/PlotSourceDialog.spec.ts        | 127 ++++++++++++
 .../store/__tests__/dataVisualization.spec.ts | 181 +++++++++++++++++
 apps/qc-app/src/store/dataVisualization.ts    |  63 +++++-
 .../utils/__tests__/datastreamSummary.spec.ts |  60 ++++++
 apps/qc-app/src/utils/datastreamSummary.ts    |  32 +++
 17 files changed, 1236 insertions(+), 34 deletions(-)
 create mode 100644 apps/qc-app/e2e/plot-source-dialog.spec.ts
 create mode 100644 apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
 create mode 100644 apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
 create mode 100644 apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts
 create mode 100644 apps/qc-app/src/utils/__tests__/datastreamSummary.spec.ts
 create mode 100644 apps/qc-app/src/utils/datastreamSummary.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index cefd5ae7..f7e53481 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -308,6 +308,8 @@ on boot.
 | `plotDatastream`                    | action   | `(ds: Datastream) => Promise<void>`               | Add to plot; promotes to QC when nothing's there yet. |
 | `unplotDatastream`                  | action   | `(id: string) => Promise<void>`                   | Remove; promotes the previous plotted entry to QC if removing the QC target. |
 | `clearPlottedDatastreams`           | action   | `() => Promise<void>`                             | Drop the entire plotted set. |
+| `sourceGroupIds`                    | action   | `(sourceId: string) => string[]`                  | The source datastream plus every managed (QC) datastream derived from it. |
+| `plotSourceSelection`               | action   | `(sourceId: string, ids: string[]) => Promise<void>` | Apply a whole "what to plot for this source" choice at once: `ids` is the complete set wanted from that source's group. Group members absent from `ids` are unplotted, additions are appended in `ids` order, and the QC target is promoted once against the final set. One rebuild for the whole selection. |
 | `addSnapshotSeries`                 | action   | `(id: string, record: ObservationRecord, meta: SnapshotMeta) => Promise<void>` | Add a frozen history snapshot as an extra comparison line under the synthetic id `snap:<sessionId>:<opIndex>`. Never promotes to QC target; `refreshGraphSeriesArray` skips its fetch. |
 | `removeSnapshotSeries`              | action   | `(id: string) => Promise<void>`                   | Drop one snapshot line. Leaves the QC target alone. |
 | `setPlottedDatastreams`             | action   | `(items: Datastream[], qcId?: string \| null) => Promise<void>` | Wholesale replace; used by URL hydration. |
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index b4dd26c9..ada48c53 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -175,6 +175,34 @@ What differs is upstream, not here: the data never refetches (see
 `refreshGraphSeriesArray`'s `isSnapshotId` guard), and `PlottedDatastreams`
 renders the row's provenance instead of a point count.
 
+## Source groups and batched selection
+
+A source datastream and every managed (QC) datastream derived from it form a
+**source group**. Managed datastreams are filtered out of
+`filteredDatastreams`, so the group has exactly one row in the datastreams
+table, and its check box opens `PlotSourceDialog` rather than toggling.
+
+`useDataVisStore.sourceGroupIds(sourceId)` resolves the group from
+`historiesBySource`. `plotSourceSelection(sourceId, ids)` then applies the
+dialog's answer as a whole: `ids` is the complete set wanted from that group,
+so members absent from it are unplotted in the same pass.
+
+Doing it in one pass matters. Looping `plotDatastream` / `unplotDatastream`
+would promote the QC target once per change, letting it land on a datastream
+the user is in the middle of deselecting; the batched action promotes once
+against the final set, using the same "entry before the one that left" rule as
+`unplotDatastream`. It also means one `rebuildPlot`, so the coalescing lock
+never has to absorb a burst.
+
+Additions are appended in `ids` order, which the dialog builds in display
+order (raw first, then managed). That is what keeps the first-plotted-wins QC
+rule predictable: picking raw alongside a managed version from an empty plot
+leaves the raw datastream as QC target.
+
+`releaseManagedDatastream` has to account for this too: the source can already
+be plotted next to its managed datastream, so it drops the managed entry
+rather than replacing it, which would otherwise duplicate the source.
+
 ## Why `internal.ts` isn't re-exported
 
 `plotly.ts` is a barrel for everything the rest of the app needs.
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index bb67771b..8eece680 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -275,9 +275,16 @@ await installMocks(page, {
   observations: { phenomenonTime: [...], result: [...] },
   submissions: collectedSubmissions,   // accumulates bulk POSTs for assertion
   authenticated: false,                 // simulate signed-out
+  qcHistories: true,                    // give DATASTREAM_ID a managed datastream
 })
 ```
 
+`qcHistories` is off by default. When on, the catalog gains
+`managedDatastream` and the QC history linking it to `DATASTREAM_ID`, so
+that source row shows the managed-count badge and its plot check box opens
+`PlotSourceDialog`. Leaving it off keeps a catalog where every row plots
+straight from its check box, which is what the other specs assume.
+
 ### Fixtures: "now"-anchored timestamps
 
 `support/fixtures.ts` exports a `FIXTURE_OBS_START_MS` anchored to
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 1d63b8e3..f34d730c 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -98,6 +98,29 @@ The main area to the right of the window is split top/bottom:
 
 Clicking a datastream row shows the full metadata for a datastream. Clicking the plot toggle (check box) in the row plots the datastream. The first datastream selected for plotting becomes the **QC target**. You can change the QC target via the radio button column in the Plotted Datastreams list.
 
+### Datastreams with quality-controlled versions
+
+A row with a number badge next to its check box has that many **managed
+datastreams**: quality-controlled versions derived from it. Managed
+datastreams never get their own row in the table, so the check box on the
+source row opens a chooser instead of plotting straight away.
+
+The chooser lists **Raw data** first, then every managed datastream, each with
+its processing level, observation count, and session count. Tick any
+combination and press **Apply** to plot them together, which is how you compare
+a raw series against the quality-controlled version derived from it.
+
+Clicking the check box again reopens the chooser with the current selection, so
+that is also where you change or clear it. Unticking everything and applying
+unplots every series from that source. The five-series plot cap still applies:
+once it is reached, unticked rows are disabled.
+
+The row's check box reflects the whole group. It is filled when the raw
+datastream is plotted, and shows a partial mark when only managed versions are.
+
+When a managed datastream is the QC target, **Start editing** goes straight
+into its session rather than asking again which version you meant.
+
 ### Edit view
 
 The Edit View opens when you click the pencil icon in the left hand navigation rail, or when you click the **Start
diff --git a/apps/qc-app/e2e/plot-source-dialog.spec.ts b/apps/qc-app/e2e/plot-source-dialog.spec.ts
new file mode 100644
index 00000000..d5ea8e19
--- /dev/null
+++ b/apps/qc-app/e2e/plot-source-dialog.spec.ts
@@ -0,0 +1,115 @@
+/**
+ * Plotting a source datastream that has managed (QC) datastreams opens a
+ * chooser instead of toggling straight to the plot.
+ *
+ * Layout is asserted here rather than in the jsdom component test: jsdom has
+ * no layout engine, so a dialog that collapses to a one-character column
+ * still passes there.
+ */
+
+import { expect, test } from '@playwright/test'
+import { installMocks } from './support/mocks'
+import { seedWorkspaceSelection, waitForHomeReady } from './support/app'
+import { DATASTREAM_ID, MANAGED_DATASTREAM_ID } from './support/fixtures'
+
+test.describe('plot source chooser', () => {
+  test.beforeEach(async ({ page }) => {
+    await installMocks(page, { qcHistories: true })
+    await seedWorkspaceSelection(page)
+    await page.goto('/')
+    await waitForHomeReady(page)
+  })
+
+  const openChooser = async (page: any) => {
+    await page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`).click()
+    await expect(page.getByTestId('plot-source-dialog')).toBeVisible()
+  }
+
+  test('lists the raw datastream and its managed version', async ({ page }) => {
+    await openChooser(page)
+    await expect(page.getByTestId(`plot-option-${DATASTREAM_ID}`)).toBeVisible()
+    await expect(
+      page.getByTestId(`plot-option-${MANAGED_DATASTREAM_ID}`)
+    ).toBeVisible()
+    await expect(page.getByTestId('plot-source-dialog')).toContainText(
+      'Raw data'
+    )
+    await expect(page.getByTestId('plot-source-dialog')).toContainText(
+      'Streamflow Datastream (QC)'
+    )
+    await expect(page.getByTestId('plot-source-dialog')).toContainText(
+      'Level: Quality controlled'
+    )
+  })
+
+  // Regression: with `max-width` and no `width` the dialog was shrink-to-fit
+  // and the `min-width: 0` label wrappers collapsed it to a thin column.
+  test('renders at a readable width, not a collapsed column', async ({
+    page,
+  }) => {
+    await openChooser(page)
+    const box = await page.getByTestId('plot-source-dialog').boundingBox()
+    expect(box).not.toBeNull()
+    expect(box!.width).toBeGreaterThan(400)
+
+    // Each row's text must sit on a line of its own, not wrap per character.
+    const row = page.getByTestId(`plot-option-${MANAGED_DATASTREAM_ID}`)
+    const rowBox = await row.boundingBox()
+    expect(rowBox!.width).toBeGreaterThan(300)
+    expect(rowBox!.height).toBeLessThan(100)
+  })
+
+  test('hides the managed datastream from the table but counts it on the row', async ({
+    page,
+  }) => {
+    await expect(
+      page.getByTestId(`plot-checkbox-${MANAGED_DATASTREAM_ID}`)
+    ).toHaveCount(0)
+    await expect(page.getByTitle(/1 managed \(QC\) datastream/)).toBeVisible()
+  })
+
+  test('plots the picked series and leaves the raw one off', async ({
+    page,
+  }) => {
+    await openChooser(page)
+    await page
+      .getByTestId(`plot-option-${MANAGED_DATASTREAM_ID}`)
+      .locator('input')
+      .check()
+    await page.getByTestId('plot-source-apply').click()
+    await expect(page.getByTestId('plot-source-dialog')).toBeHidden()
+
+    // The row reads as partially selected: something from the group is
+    // plotted, but not the raw datastream.
+    const checkbox = page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`)
+    await expect(checkbox).toHaveAttribute('aria-pressed', 'true')
+    await expect(checkbox.locator('.mdi-checkbox-intermediate')).toBeVisible()
+  })
+
+  test('reopens with the current selection and can clear it', async ({
+    page,
+  }) => {
+    await openChooser(page)
+    await page
+      .getByTestId(`plot-option-${DATASTREAM_ID}`)
+      .locator('input')
+      .check()
+    await page.getByTestId('plot-source-apply').click()
+    await expect(page.getByTestId('plot-source-dialog')).toBeHidden()
+
+    await openChooser(page)
+    await expect(
+      page.getByTestId(`plot-option-${DATASTREAM_ID}`).locator('input')
+    ).toBeChecked()
+
+    await page
+      .getByTestId(`plot-option-${DATASTREAM_ID}`)
+      .locator('input')
+      .uncheck()
+    await page.getByTestId('plot-source-apply').click()
+    await expect(page.getByTestId('plot-source-dialog')).toBeHidden()
+    await expect(
+      page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`)
+    ).toHaveAttribute('aria-pressed', 'false')
+  })
+})
diff --git a/apps/qc-app/e2e/support/fixtures.ts b/apps/qc-app/e2e/support/fixtures.ts
index 2a380b0b..51a44a83 100644
--- a/apps/qc-app/e2e/support/fixtures.ts
+++ b/apps/qc-app/e2e/support/fixtures.ts
@@ -21,6 +21,12 @@ export const SENSOR_ID = 'sensor-adv'
 // Y-axis lights up with its own scale instead of overlaying the
 // primary axis 1:1.
 export const DATASTREAM_ID_B = 'ds-qc-e2e-b'
+// Managed (QC) datastream derived from DATASTREAM_ID, plus the history that
+// links the two. Only served when `installMocks` is given `qcHistories: true`,
+// so specs that don't care keep a catalog with no managed datastreams.
+export const MANAGED_DATASTREAM_ID = 'ds-qc-e2e-managed'
+export const QC_HISTORY_ID = 'qch-e2e'
+export const QC_PROC_LEVEL_ID = 'pl-qc'
 export const UNIT_ID_B = 'unit-celsius'
 export const OBSERVED_PROP_ID_B = 'op-water-temp'
 export const SENSOR_ID_B = 'sensor-temp'
@@ -238,6 +244,93 @@ export const datastreams = [
   },
 ]
 
+/**
+ * Only served when `installMocks` is given `qcHistories: true`. Kept out of
+ * `datastreams` so specs that don't opt in keep a catalog of plain rows.
+ */
+export const managedDatastream = {
+    id: MANAGED_DATASTREAM_ID,
+    workspaceId: WORKSPACE_ID,
+    name: 'Streamflow Datastream (QC)',
+    description: 'Quality-controlled version of the streamflow datastream',
+    observationType: 'OM_Measurement',
+    aggregationStatistic: 'Continuous',
+    timeAggregationInterval: 15,
+    timeAggregationIntervalUnit: 'minutes',
+    intendedTimeSpacing: 15,
+    intendedTimeSpacingUnit: 'minutes',
+    sampledMedium: 'Surface water',
+    resultType: 'Time Series',
+    status: 'ongoing',
+    valueCount: 118,
+    noDataValue: -9999,
+    isPrivate: false,
+    isVisible: true,
+    unitId: UNIT_ID,
+    thingId: THING_ID,
+    processingLevelId: QC_PROC_LEVEL_ID,
+    observedPropertyId: OBSERVED_PROP_ID,
+    sensorId: SENSOR_ID,
+    phenomenonBeginTime: FIXTURE_OBS_START_ISO,
+    phenomenonEndTime: FIXTURE_OBS_END_ISO,
+    resultBeginTime: FIXTURE_OBS_START_ISO,
+    resultEndTime: FIXTURE_OBS_END_ISO,
+    thing: {
+      id: THING_ID,
+      name: 'Test Stream Site',
+      samplingFeatureCode: 'STRM-E2E',
+      samplingFeatureType: 'Site',
+      siteType: 'Stream',
+    },
+    observedProperty: {
+      id: OBSERVED_PROP_ID,
+      name: 'Streamflow',
+      code: 'Q',
+      definition: 'Stream discharge',
+      type: 'Hydrology',
+    },
+    processingLevel: {
+      id: QC_PROC_LEVEL_ID,
+      code: 'QC',
+      definition: 'Quality controlled',
+      explanation: 'Reviewed readings',
+    },
+    unit: {
+      id: UNIT_ID,
+      name: 'cubic meters per second',
+      symbol: 'm³/s',
+      definition: 'SI unit of volumetric flow rate',
+      type: 'Flow',
+    },
+  }
+
+export const qcHistories = [
+  {
+    id: QC_HISTORY_ID,
+    workspaceId: WORKSPACE_ID,
+    sourceDatastreamId: DATASTREAM_ID,
+    managedDatastreamId: MANAGED_DATASTREAM_ID,
+    sourceDatastream: { id: DATASTREAM_ID, name: 'Streamflow Datastream' },
+    managedDatastream: {
+      id: MANAGED_DATASTREAM_ID,
+      name: 'Streamflow Datastream (QC)',
+    },
+  },
+]
+
+export const qcSessions = [
+  {
+    id: 'qcs-e2e-1',
+    historyId: QC_HISTORY_ID,
+    status: 'committed',
+    description: 'First pass',
+    phenomenonTimeStart: FIXTURE_OBS_START_ISO,
+    phenomenonTimeEnd: FIXTURE_OBS_END_ISO,
+    createdAt: FIXTURE_OBS_START_ISO,
+    dependencyIds: [],
+  },
+]
+
 /**
  * Generate a deterministic, well-shaped synthetic observation set:
  * `count` samples at 15-minute spacing starting at `startMs`, with
diff --git a/apps/qc-app/e2e/support/mocks.ts b/apps/qc-app/e2e/support/mocks.ts
index 2b7f3b60..fed7b352 100644
--- a/apps/qc-app/e2e/support/mocks.ts
+++ b/apps/qc-app/e2e/support/mocks.ts
@@ -24,7 +24,10 @@ import {
   buildObservations,
   datastreams,
   observedProperties,
+  managedDatastream,
   processingLevels,
+  qcHistories,
+  qcSessions,
   resultQualifiers,
   sensors,
   session,
@@ -57,6 +60,12 @@ export interface MockOptions {
    * payload contents without installing a second route handler.
    */
   submissions?: Array<{ mode: string | null; body: any }>
+  /**
+   * Serve the QC history fixture, so `DATASTREAM_ID` has a managed
+   * datastream derived from it. Off by default: most specs want a catalog
+   * where every row plots straight from its check box.
+   */
+  qcHistories?: boolean
   /** Set to false to mark the session as unauthenticated. */
   authenticated?: boolean
 }
@@ -105,6 +114,11 @@ export async function installMocks(
   const observations = options.observations ?? buildObservations()
   const observationsById = options.observationsById ?? {}
   const submissions = options.submissions ?? []
+  const withQcHistories = options.qcHistories ?? false
+  // The managed datastream only exists for specs that opted into histories.
+  const catalog = withQcHistories
+    ? [...datastreams, managedDatastream]
+    : datastreams
 
   // Match only real HydroServer API calls by pathname. A bare `**/api/**`
   // glob also catches the dev server's own source modules — the QC app
@@ -194,12 +208,34 @@ export async function installMocks(
       return json(route, { data: workspaces })
     }
 
+    // --- Quality-control histories / sessions ---
+    // Served ahead of the datastream routes: the history paths sit under
+    // the same `/api/data` prefix and would otherwise fall to the
+    // empty-list catch-all.
+    const qcSessionList = path.match(
+      /\/api\/data\/quality-control\/histories\/([^/]+)\/sessions$/
+    )
+    if (qcSessionList && method === 'GET') {
+      const historyId = qcSessionList[1]
+      return json(route, {
+        data: withQcHistories
+          ? qcSessions.filter((s) => s.historyId === historyId)
+          : [],
+      })
+    }
+    if (
+      path.endsWith('/api/data/quality-control/histories') &&
+      method === 'GET'
+    ) {
+      return json(route, { data: withQcHistories ? qcHistories : [] })
+    }
+
     // --- Things / datastreams / processing levels / observed properties ---
     if (path.endsWith('/api/data/things') && method === 'GET') {
       return json(route, { data: things })
     }
     if (path.endsWith('/api/data/datastreams') && method === 'GET') {
-      return json(route, { data: datastreams })
+      return json(route, { data: catalog })
     }
     if (path.endsWith('/api/data/processing-levels') && method === 'GET') {
       return json(route, { data: processingLevels })
@@ -218,7 +254,7 @@ export async function installMocks(
     const dsGet = path.match(/\/api\/data\/datastreams\/([^/]+)$/)
     if (dsGet && method === 'GET') {
       const id = dsGet[1]
-      const ds = datastreams.find((d) => d.id === id) ?? datastreams[0]
+      const ds = catalog.find((d) => d.id === id) ?? catalog[0]
       return json(route, { data: ds })
     }
 
diff --git a/apps/qc-app/src/components/EditData/StartEditingDialog.vue b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
index 95954382..ada96b6d 100644
--- a/apps/qc-app/src/components/EditData/StartEditingDialog.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
@@ -352,6 +352,7 @@ import type { Datastream, QualityControlSession } from '@hydroserver/client'
 import type { ManagedDatastreamOption } from '@/composables/useManagedDatastreams'
 import { formatDateRange } from '@/utils/time'
 import { collectDeletionChain } from '@/utils/sessionGraph'
+import { datastreamSummary } from '@/utils/datastreamSummary'
 
 defineProps<{
   source: Datastream
@@ -431,27 +432,8 @@ function onDeleteSession() {
   if (opt && sessionId) emit('deleteSession', opt, sessionId)
 }
 
-const NUMBER = new Intl.NumberFormat()
-
-// One-line recap so multiple managed datastreams are distinguishable:
-// processing level, observation count, and session summary.
-function summary(opt: ManagedDatastreamOption): string {
-  const parts: string[] = []
-  const level =
-    opt.managed.processingLevel?.definition ||
-    opt.managed.processingLevel?.code ||
-    null
-  if (level) parts.push(`Level: ${level}`)
-  parts.push(`${NUMBER.format(opt.managed.valueCount ?? 0)} obs`)
-  const inProgress = opt.sessions.filter(
-    (s) => s.status === 'in_progress'
-  ).length
-  const sessionText = `${opt.sessions.length} session${
-    opt.sessions.length === 1 ? '' : 's'
-  }${inProgress ? `, ${inProgress} in progress` : ''}`
-  parts.push(sessionText)
-  return parts.join(' · ')
-}
+const summary = (opt: ManagedDatastreamOption) =>
+  datastreamSummary(opt.managed, opt.sessions)
 
 const hasInProgress = (opt: ManagedDatastreamOption) =>
   opt.sessions.some((s) => s.status === 'in_progress')
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index 2e947ae9..bdae2b6b 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -577,6 +577,7 @@ const {
   plottedDatastreams,
   qcDatastream,
   datastreams,
+  managedDatastreamIds,
   processingLevels,
   things,
   beginDate,
@@ -1139,6 +1140,13 @@ onUnmounted(() => {
 async function openEditChooser() {
   const source = qcDatastream.value
   if (!source) return
+  // A managed datastream plotted directly is already the thing to edit, so
+  // there is nothing left to choose. It carries its own observations, so no
+  // `adoptManagedDatastream` working copy is needed either.
+  if (managedDatastreamIds.value.has(source.id)) {
+    await enterEdit()
+    return
+  }
   chooserSource.value = source
   chooserOptions.value = []
   chooserLoading.value = true
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
index 684842e0..28c89d72 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
@@ -1,4 +1,4 @@
-<template>
+<template>
   <div class="datasets-table d-flex flex-column">
     <v-toolbar flat density="compact" class="datasets-table__toolbar px-2">
       <div class="d-flex align-center ga-2" style="min-width: 0">
@@ -10,7 +10,7 @@
           :variant="plottedDatastreams.length ? 'tonal' : 'outlined'"
           label
         >
-          {{ plottedDatastreams.length }}/5 plotted
+          {{ plottedDatastreams.length }}/{{ PLOT_CAP }} plotted
         </v-chip>
       </div>
 
@@ -143,7 +143,7 @@
             :text="
               isQc(item)
                 ? 'QC target: first plotted datastream'
-                : 'Maximum of 5 datastreams plotted; remove one to add another'
+                : `Maximum of ${PLOT_CAP} datastreams plotted; remove one to add another`
             "
           >
             <template #activator="{ props: tooltipProps }">
@@ -161,13 +161,15 @@
                   :aria-label="
                     isChecked(item) ? 'Remove from plot' : 'Add to plot'
                   "
-                  @click.stop="!isAtCap(item) && toggleDatastream(item)"
+                  @click.stop="!isAtCap(item) && onPlotClick(item)"
                 >
                   <v-icon
                     :icon="
-                      isChecked(item)
-                        ? 'mdi-checkbox-marked'
-                        : 'mdi-checkbox-blank-outline'
+                      isPartial(item)
+                        ? 'mdi-checkbox-intermediate'
+                        : isChecked(item)
+                          ? 'mdi-checkbox-marked'
+                          : 'mdi-checkbox-blank-outline'
                     "
                     size="20"
                   />
@@ -244,6 +246,23 @@
         @close="openInfoCard = false"
       />
     </v-dialog>
+
+    <!-- Width in px, not rem/vw: Vuetify coerces these dimension props
+         through `convertToUnit`, which turns "34rem" into 34px and collapses
+         the dialog to a one-character column. Vuetify's own
+         `max-width: calc(100% - 48px)` keeps it on screen when narrow. -->
+    <v-dialog v-model="plotDialogOpen" width="560">
+      <PlotSourceDialog
+        v-if="plotDialogSource"
+        :source="plotDialogSource"
+        :options="plotDialogOptions"
+        :plotted-ids="plotDialogSelected"
+        :loading="plotDialogLoading"
+        :slots-left="plotDialogSlots"
+        @apply="onPlotApply"
+        @cancel="plotDialogSource = null"
+      />
+    </v-dialog>
   </div>
 </template>
 
@@ -252,13 +271,28 @@ import { useDataVisStore } from '@/store/dataVisualization'
 import { storeToRefs } from 'pinia'
 import { computed, reactive, ref } from 'vue'
 import DatastreamInformationCard from './DatastreamInformationCard.vue'
+import PlotSourceDialog from './PlotSourceDialog.vue'
 import { Datastream } from '@hydroserver/client'
 import type { DatastreamExtended } from '@hydroserver/client'
 import { downloadDatastreamsCsvZip } from '@/utils/csvExport'
+import {
+  useManagedDatastreams,
+  type ManagedDatastreamOption,
+} from '@/composables/useManagedDatastreams'
+import { Snackbar } from '@uwrl/qc-utils'
+
+/** Maximum series the plot holds at once. */
+const PLOT_CAP = 5
 
 const { filteredDatastreams, plottedDatastreams, qcDatastream, historiesBySource } =
   storeToRefs(useDataVisStore())
-const { toggleDatastream, clearPlottedDatastreams } = useDataVisStore()
+const {
+  toggleDatastream,
+  clearPlottedDatastreams,
+  sourceGroupIds,
+  plotSourceSelection,
+} = useDataVisStore()
+const { loadForSource } = useManagedDatastreams()
 
 const showOnlySelected = ref(false)
 const openInfoCard = ref(false)
@@ -346,8 +380,19 @@ const clearSelected = () => {
   void clearPlottedDatastreams()
 }
 
+const plottedIds = computed(
+  () => new Set(plottedDatastreams.value.map((d) => d.id))
+)
+
+// A source row reads as checked when the raw datastream or any managed
+// datastream derived from it is plotted.
 const isChecked = (item: Datastream) =>
-  plottedDatastreams.value.some((sds) => sds.id === item.id)
+  sourceGroupIds(item.id).some((id) => plottedIds.value.has(id))
+
+// Only managed series from this source are plotted, not the raw one — the
+// row shouldn't claim the raw line is on the plot.
+const isPartial = (item: Datastream) =>
+  !plottedIds.value.has(item.id) && isChecked(item)
 
 const isQc = (item: Datastream) => qcDatastream.value?.id === item.id
 
@@ -356,7 +401,63 @@ const managedCount = (item: Datastream) =>
   historiesBySource.value.get(item.id)?.length ?? 0
 
 const isAtCap = (item: Datastream) =>
-  plottedDatastreams.value.length >= 5 && !isChecked(item)
+  plottedDatastreams.value.length >= PLOT_CAP && !isChecked(item)
+
+// --- Plot selection ---------------------------------------------------------
+// Sources with managed datastreams can't be a single toggle: the click opens
+// a chooser instead, both to plot the first time and to change the selection.
+const plotDialogSource = ref<(Datastream & DatastreamExtended) | null>(null)
+const plotDialogOptions = ref<ManagedDatastreamOption[]>([])
+const plotDialogLoading = ref(false)
+
+const plotDialogOpen = computed({
+  get: () => !!plotDialogSource.value,
+  set: (open: boolean) => {
+    if (!open) plotDialogSource.value = null
+  },
+})
+
+const plotDialogSelected = computed(() => {
+  const source = plotDialogSource.value
+  if (!source) return []
+  return sourceGroupIds(source.id).filter((id) => plottedIds.value.has(id))
+})
+
+// Slots this source's group may occupy: the cap minus what other sources
+// already hold, so the dialog can swap freely within its own group.
+const plotDialogSlots = computed(() => {
+  const source = plotDialogSource.value
+  if (!source) return 0
+  const group = new Set(sourceGroupIds(source.id))
+  const others = plottedDatastreams.value.filter((d) => !group.has(d.id)).length
+  return Math.max(PLOT_CAP - others, 0)
+})
+
+async function onPlotClick(item: Datastream & DatastreamExtended) {
+  if (!managedCount(item)) {
+    await toggleDatastream(item)
+    return
+  }
+  plotDialogSource.value = item
+  plotDialogOptions.value = []
+  plotDialogLoading.value = true
+  try {
+    plotDialogOptions.value = await loadForSource(item.id)
+  } catch (e) {
+    // The raw option stays selectable, so the click isn't a dead end.
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not load QC datastreams.'
+    )
+  } finally {
+    plotDialogLoading.value = false
+  }
+}
+
+async function onPlotApply(ids: string[]) {
+  const source = plotDialogSource.value
+  plotDialogSource.value = null
+  if (source) await plotSourceSelection(source.id, ids)
+}
 
 const getRowProps = ({ item }: { item: Datastream }) => ({
   class: {
diff --git a/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue b/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
new file mode 100644
index 00000000..8dfe6bd9
--- /dev/null
+++ b/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
@@ -0,0 +1,186 @@
+<template>
+  <v-card rounded="lg" data-testid="plot-source-dialog">
+    <div class="px-4 pt-4 pb-2">
+      <div class="text-title-medium font-weight-bold">
+        Plot "{{ source.name }}"
+      </div>
+      <div class="text-body-small text-medium-emphasis mt-1">
+        This datastream has quality-controlled versions. Pick which series to
+        plot.
+      </div>
+    </div>
+
+    <v-divider />
+
+    <div class="plot-source__body pa-4">
+      <div v-if="loading" class="py-6 text-center">
+        <v-progress-circular indeterminate color="primary" size="28" />
+      </div>
+
+      <template v-else>
+        <v-card
+          v-for="row in rows"
+          :key="row.id"
+          variant="outlined"
+          class="mb-2"
+          :class="{ 'plot-source__row--disabled': isDisabled(row.id) }"
+        >
+          <v-checkbox
+            :model-value="checked.includes(row.id)"
+            :disabled="isDisabled(row.id)"
+            density="compact"
+            hide-details
+            color="primary"
+            class="px-2"
+            :data-testid="`plot-option-${row.id}`"
+            @update:model-value="toggle(row.id)"
+          >
+            <template #label>
+              <div class="d-flex align-center ga-2" style="min-width: 0">
+                <v-icon
+                  :icon="row.raw ? 'mdi-chart-line' : 'mdi-pencil-box-outline'"
+                  :color="row.raw ? undefined : 'primary'"
+                  size="18"
+                />
+                <div class="d-flex flex-column" style="min-width: 0">
+                  <span class="text-body-medium font-weight-medium">
+                    {{ row.name }}
+                  </span>
+                  <span class="text-body-small text-medium-emphasis">
+                    {{ row.summary }}
+                  </span>
+                </div>
+              </div>
+            </template>
+          </v-checkbox>
+        </v-card>
+
+        <div
+          v-if="!slotsLeftForNew"
+          class="text-body-small text-medium-emphasis"
+          data-testid="plot-source-cap"
+        >
+          Maximum of 5 datastreams plotted; remove one to add another.
+        </div>
+      </template>
+    </div>
+
+    <v-divider />
+
+    <v-card-actions class="px-4 py-2">
+      <v-spacer />
+      <v-btn variant="text" data-testid="plot-source-cancel" @click="emit('cancel')">
+        Cancel
+      </v-btn>
+      <v-btn
+        color="primary"
+        variant="flat"
+        data-testid="plot-source-apply"
+        @click="emit('apply', checked)"
+      >
+        Apply
+      </v-btn>
+    </v-card-actions>
+  </v-card>
+</template>
+
+<script setup lang="ts">
+import { computed, ref, watch } from 'vue'
+import type { Datastream, DatastreamExtended } from '@hydroserver/client'
+import type { ManagedDatastreamOption } from '@/composables/useManagedDatastreams'
+import { datastreamSummary } from '@/utils/datastreamSummary'
+
+const props = defineProps<{
+  source: Datastream & Partial<DatastreamExtended>
+  options: ManagedDatastreamOption[]
+  plottedIds: string[]
+  loading?: boolean
+  /** Free slots under the 5-series plot cap, ignoring this source's group. */
+  slotsLeft: number
+}>()
+
+const emit = defineEmits<{
+  (e: 'apply', ids: string[]): void
+  (e: 'cancel'): void
+}>()
+
+const checked = ref<string[]>([...props.plottedIds])
+
+// The dialog is reopened to edit an existing selection, so re-seed whenever
+// the caller hands over a different one.
+watch(
+  () => props.plottedIds,
+  (ids) => {
+    checked.value = [...ids]
+  }
+)
+
+interface Row {
+  id: string
+  name: string
+  summary: string
+  raw: boolean
+}
+
+// Raw first so the first-plotted-wins QC rule keeps the source as target
+// when both it and a managed datastream are picked from an empty plot.
+const rows = computed<Row[]>(() => [
+  {
+    id: props.source.id,
+    name: 'Raw data',
+    summary: datastreamSummary(props.source),
+    raw: true,
+  },
+  ...props.options.map((o) => ({
+    id: o.managed.id,
+    name: o.managed.name,
+    summary: datastreamSummary(o.managed, o.sessions),
+    raw: false,
+  })),
+])
+
+const slotsLeftForNew = computed(() => props.slotsLeft - checked.value.length)
+
+const isDisabled = (id: string) =>
+  !checked.value.includes(id) && slotsLeftForNew.value <= 0
+
+function toggle(id: string) {
+  if (checked.value.includes(id)) {
+    checked.value = checked.value.filter((x) => x !== id)
+    return
+  }
+  if (isDisabled(id)) return
+  // Re-derive from `rows` so the applied order is always display order,
+  // not the order the boxes happened to be clicked.
+  const wanted = new Set([...checked.value, id])
+  checked.value = rows.value.map((r) => r.id).filter((rid) => wanted.has(rid))
+}
+</script>
+
+<style scoped>
+.plot-source__body {
+  max-height: 60vh;
+  overflow-y: auto;
+}
+
+.plot-source__row--disabled {
+  opacity: 0.5;
+}
+
+/* Vuetify sizes the label to its content and dims it. Neither suits a row
+   whose label carries the name and summary. */
+.plot-source__body :deep(.v-selection-control .v-label) {
+  width: 100%;
+  opacity: 1;
+}
+
+.plot-source__body :deep(.v-label > div) {
+  min-width: 0;
+}
+
+.plot-source__body :deep(.v-label span) {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+</style>
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
new file mode 100644
index 00000000..79d83e8e
--- /dev/null
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
@@ -0,0 +1,160 @@
+/**
+ * Unit tests for DataVisDatasetsTable.vue — focused on the plot checkbox.
+ * A source with managed (QC) datastreams opens a chooser instead of
+ * toggling, so the row's checked state has to speak for the whole group.
+ */
+
+import { mount, flushPromises } from '@vue/test-utils'
+import { ref } from 'vue'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import PlotSourceDialog from '../PlotSourceDialog.vue'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+;(globalThis as any).visualViewport ||= {
+  addEventListener() {},
+  removeEventListener() {},
+}
+
+const raw = { id: 'src', name: 'Raw', valueCount: 10, thing: { id: 't-1' } }
+const lonely = { id: 'solo', name: 'Solo', valueCount: 5, thing: { id: 't-1' } }
+
+const filteredDatastreams = ref<any[]>([raw, lonely])
+const plottedDatastreams = ref<any[]>([])
+const qcDatastream = ref<any>(null)
+const historiesBySource = ref(
+  new Map<string, any[]>([
+    ['src', [{ id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'src' }]],
+  ])
+)
+
+const toggleDatastream = vi.fn().mockResolvedValue(undefined)
+const clearPlottedDatastreams = vi.fn().mockResolvedValue(undefined)
+const plotSourceSelection = vi.fn().mockResolvedValue(undefined)
+const sourceGroupIds = (id: string) => (id === 'src' ? ['src', 'mgd'] : [id])
+
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () => ({
+    filteredDatastreams,
+    plottedDatastreams,
+    qcDatastream,
+    historiesBySource,
+    toggleDatastream,
+    clearPlottedDatastreams,
+    sourceGroupIds,
+    plotSourceSelection,
+  }),
+}))
+
+const loadForSource = vi.fn().mockResolvedValue([
+  {
+    historyId: 'h-1',
+    managed: { id: 'mgd', name: 'Raw (QC)', valueCount: 8 },
+    sessions: [],
+  },
+])
+
+vi.mock('@/composables/useManagedDatastreams', () => ({
+  useManagedDatastreams: () => ({ loadForSource }),
+}))
+
+vi.mock('@/utils/csvExport', () => ({
+  downloadDatastreamsCsvZip: vi.fn().mockResolvedValue(undefined),
+}))
+
+const snackbarError = vi.fn()
+vi.mock('@uwrl/qc-utils', async (importOriginal) => {
+  const actual = (await importOriginal()) as Record<string, unknown>
+  return { ...actual, Snackbar: { error: snackbarError, success: vi.fn() } }
+})
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  plottedDatastreams.value = []
+  qcDatastream.value = null
+})
+
+const mountTable = async () => {
+  const DataVisDatasetsTable = (await import('../DataVisDatasetsTable.vue'))
+    .default
+  const wrapper = mount(DataVisDatasetsTable, {
+    global: { plugins: [createTestVuetify()] },
+    attachTo: document.body,
+  })
+  await flushPromises()
+  return wrapper
+}
+
+const checkbox = (wrapper: any, id: string) =>
+  wrapper.find(`[data-testid="plot-checkbox-${id}"]`)
+
+describe('DataVisDatasetsTable plot checkbox', () => {
+  it('toggles directly when the source has no managed datastreams', async () => {
+    const wrapper = await mountTable()
+    await checkbox(wrapper, 'solo').trigger('click')
+    await flushPromises()
+
+    expect(toggleDatastream).toHaveBeenCalledWith(
+      expect.objectContaining({ id: 'solo' })
+    )
+    expect(wrapper.findComponent(PlotSourceDialog).exists()).toBe(false)
+  })
+
+  it('opens the chooser when the source has managed datastreams', async () => {
+    const wrapper = await mountTable()
+    await checkbox(wrapper, 'src').trigger('click')
+    await flushPromises()
+
+    expect(toggleDatastream).not.toHaveBeenCalled()
+    expect(loadForSource).toHaveBeenCalledWith('src')
+    expect(wrapper.findComponent(PlotSourceDialog).exists()).toBe(true)
+  })
+
+  it('reports a chooser failure and still offers the raw option', async () => {
+    loadForSource.mockRejectedValueOnce(new Error('boom'))
+    const wrapper = await mountTable()
+    await checkbox(wrapper, 'src').trigger('click')
+    await flushPromises()
+
+    expect(snackbarError).toHaveBeenCalledWith('boom')
+    const dialog = wrapper.findComponent(PlotSourceDialog)
+    expect(dialog.exists()).toBe(true)
+    expect(dialog.props('options')).toEqual([])
+  })
+
+  it('applies the chooser selection through the batched store action', async () => {
+    const wrapper = await mountTable()
+    await checkbox(wrapper, 'src').trigger('click')
+    await flushPromises()
+
+    wrapper.findComponent(PlotSourceDialog).vm.$emit('apply', ['mgd'])
+    await flushPromises()
+
+    expect(plotSourceSelection).toHaveBeenCalledWith('src', ['mgd'])
+  })
+
+  it('shows a checked box when the raw datastream is plotted', async () => {
+    plottedDatastreams.value = [raw]
+    const wrapper = await mountTable()
+    expect(checkbox(wrapper, 'src').html()).toContain('mdi-checkbox-marked')
+  })
+
+  // The row must not claim the raw line is plotted when only a QC version is.
+  it('shows an indeterminate box when only a managed datastream is plotted', async () => {
+    plottedDatastreams.value = [{ id: 'mgd', name: 'Raw (QC)' }]
+    const wrapper = await mountTable()
+    const html = checkbox(wrapper, 'src').html()
+    expect(html).toContain('mdi-checkbox-intermediate')
+    expect(checkbox(wrapper, 'src').attributes('aria-pressed')).toBe('true')
+  })
+
+  it('shows an empty box when nothing from the group is plotted', async () => {
+    const wrapper = await mountTable()
+    expect(checkbox(wrapper, 'src').html()).toContain(
+      'mdi-checkbox-blank-outline'
+    )
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts
new file mode 100644
index 00000000..26fca03d
--- /dev/null
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts
@@ -0,0 +1,127 @@
+/**
+ * Unit tests for PlotSourceDialog.vue — the chooser shown when plotting a
+ * source datastream that has managed (QC) datastreams derived from it.
+ * Purely presentational, so no stores are mocked.
+ */
+
+import { mount } from '@vue/test-utils'
+import { describe, it, expect } from 'vitest'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import PlotSourceDialog from '../PlotSourceDialog.vue'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+const source = {
+  id: 'src',
+  name: 'Logan River Temp',
+  valueCount: 1000,
+  processingLevel: { definition: 'Raw' },
+} as any
+
+const options = [
+  {
+    historyId: 'h-1',
+    managed: {
+      id: 'mgd-1',
+      name: 'Logan River Temp (QC)',
+      valueCount: 900,
+      processingLevel: { definition: 'Quality controlled' },
+    },
+    sessions: [{ id: 's-1', status: 'in_progress' }],
+  },
+  {
+    historyId: 'h-2',
+    managed: { id: 'mgd-2', name: 'Logan River Temp (QC2)', valueCount: 800 },
+    sessions: [],
+  },
+] as any
+
+const mountDialog = (props: Record<string, any> = {}) =>
+  mount(PlotSourceDialog, {
+    props: {
+      source,
+      options,
+      plottedIds: [],
+      slotsLeft: 5,
+      ...props,
+    },
+    global: { plugins: [createTestVuetify()] },
+  })
+
+const optionInput = (wrapper: any, id: string) =>
+  wrapper.find(`[data-testid="plot-option-${id}"] input`)
+
+describe('PlotSourceDialog', () => {
+  it('lists the raw datastream first, then one row per managed datastream', () => {
+    const wrapper = mountDialog()
+    const labels = wrapper
+      .findAll('[data-testid^="plot-option-"]')
+      .map((el) => el.attributes('data-testid'))
+    expect(labels).toEqual([
+      'plot-option-src',
+      'plot-option-mgd-1',
+      'plot-option-mgd-2',
+    ])
+    expect(wrapper.text()).toContain('Raw data')
+    expect(wrapper.text()).toContain('Logan River Temp (QC)')
+  })
+
+  it('shows a summary line for each managed datastream', () => {
+    const wrapper = mountDialog()
+    expect(wrapper.text()).toContain('Level: Quality controlled')
+    expect(wrapper.text()).toContain('1 session, 1 in progress')
+    expect(wrapper.text()).toContain('0 sessions')
+  })
+
+  it('starts with the already-plotted series checked', () => {
+    const wrapper = mountDialog({ plottedIds: ['mgd-1'] })
+    expect(
+      (optionInput(wrapper, 'mgd-1').element as HTMLInputElement).checked
+    ).toBe(true)
+    expect(
+      (optionInput(wrapper, 'src').element as HTMLInputElement).checked
+    ).toBe(false)
+  })
+
+  it('shows a spinner and no rows while loading', () => {
+    const wrapper = mountDialog({ loading: true, options: [] })
+    expect(wrapper.findAll('[data-testid^="plot-option-"]')).toHaveLength(0)
+    expect(wrapper.find('.v-progress-circular').exists()).toBe(true)
+  })
+
+  it('emits the checked ids in display order, not click order', async () => {
+    const wrapper = mountDialog()
+    await optionInput(wrapper, 'mgd-2').setValue(true)
+    await optionInput(wrapper, 'src').setValue(true)
+    await wrapper.find('[data-testid="plot-source-apply"]').trigger('click')
+    expect(wrapper.emitted('apply')?.[0]).toEqual([['src', 'mgd-2']])
+  })
+
+  it('emits an empty selection when everything is unchecked', async () => {
+    const wrapper = mountDialog({ plottedIds: ['src'] })
+    await optionInput(wrapper, 'src').setValue(false)
+    await wrapper.find('[data-testid="plot-source-apply"]').trigger('click')
+    expect(wrapper.emitted('apply')?.[0]).toEqual([[]])
+  })
+
+  it('disables unchecked rows once the plot cap is reached', async () => {
+    const wrapper = mountDialog({ plottedIds: ['src'], slotsLeft: 1 })
+    expect(optionInput(wrapper, 'mgd-1').attributes('disabled')).toBeDefined()
+    // The checked one stays enabled so the selection can still be changed.
+    expect(optionInput(wrapper, 'src').attributes('disabled')).toBeUndefined()
+    expect(wrapper.find('[data-testid="plot-source-cap"]').exists()).toBe(true)
+
+    await optionInput(wrapper, 'src').setValue(false)
+    expect(optionInput(wrapper, 'mgd-1').attributes('disabled')).toBeUndefined()
+  })
+
+  it('emits cancel without a selection', async () => {
+    const wrapper = mountDialog()
+    await wrapper.find('[data-testid="plot-source-cancel"]').trigger('click')
+    expect(wrapper.emitted('cancel')).toHaveLength(1)
+    expect(wrapper.emitted('apply')).toBeUndefined()
+  })
+})
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index 2cd1df97..b502dd57 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -792,3 +792,184 @@ describe('useDataVisStore snapshot series', () => {
     ).toBe(false)
   })
 })
+
+describe('useDataVisStore.sourceGroupIds', () => {
+  it('returns the source plus every managed datastream derived from it', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    store.qcHistories = [
+      { id: 'h-1', managedDatastreamId: 'mgd-1', sourceDatastreamId: 'src' },
+      { id: 'h-2', managedDatastreamId: 'mgd-2', sourceDatastreamId: 'src' },
+      { id: 'h-3', managedDatastreamId: 'mgd-3', sourceDatastreamId: 'other' },
+    ] as any
+
+    expect(store.sourceGroupIds('src')).toEqual(['src', 'mgd-1', 'mgd-2'])
+    expect(store.sourceGroupIds('lonely')).toEqual(['lonely'])
+  })
+})
+
+describe('useDataVisStore.plotSourceSelection', () => {
+  const withGroup = (store: any) => {
+    store.qcHistories = [
+      { id: 'h-1', managedDatastreamId: 'mgd-1', sourceDatastreamId: 'src' },
+      { id: 'h-2', managedDatastreamId: 'mgd-2', sourceDatastreamId: 'src' },
+    ] as any
+    store.datastreams = [
+      makeDs({ id: 'src', name: 'Raw' }),
+      makeDs({ id: 'mgd-1', name: 'Raw (QC)' }),
+      makeDs({ id: 'mgd-2', name: 'Raw (QC2)' }),
+      makeDs({ id: 'other', name: 'Other' }),
+    ] as any
+  }
+
+  it('plots the chosen series and promotes the first as QC target', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withGroup(store)
+
+    await store.plotSourceSelection('src', ['src', 'mgd-1'])
+
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual([
+      'src',
+      'mgd-1',
+    ])
+    expect(store.qcDatastreamId).toBe('src')
+  })
+
+  it('makes a managed datastream the QC target when the raw one is not picked', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withGroup(store)
+
+    await store.plotSourceSelection('src', ['mgd-2'])
+
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['mgd-2'])
+    expect(store.qcDatastreamId).toBe('mgd-2')
+  })
+
+  it('adds and removes within the group in a single call', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withGroup(store)
+    store.plottedDatastreams = [
+      makeDs({ id: 'src', name: 'Raw' }),
+      makeDs({ id: 'mgd-1', name: 'Raw (QC)' }),
+    ] as any
+    store.qcDatastreamId = 'src'
+
+    await store.plotSourceSelection('src', ['mgd-1', 'mgd-2'])
+
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual([
+      'mgd-1',
+      'mgd-2',
+    ])
+  })
+
+  it('leaves datastreams from other sources untouched', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withGroup(store)
+    store.plottedDatastreams = [makeDs({ id: 'other', name: 'Other' })] as any
+    store.qcDatastreamId = 'other'
+
+    await store.plotSourceSelection('src', ['mgd-1'])
+
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual([
+      'other',
+      'mgd-1',
+    ])
+    expect(store.qcDatastreamId).toBe('other')
+  })
+
+  it('promotes a new QC target when the current one is deselected', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withGroup(store)
+    store.plottedDatastreams = [
+      makeDs({ id: 'other', name: 'Other' }),
+      makeDs({ id: 'src', name: 'Raw' }),
+    ] as any
+    store.qcDatastreamId = 'src'
+
+    await store.plotSourceSelection('src', ['mgd-1'])
+
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual([
+      'other',
+      'mgd-1',
+    ])
+    expect(store.qcDatastreamId).toBe('other')
+  })
+
+  it('clears the QC target when nothing is left plotted', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withGroup(store)
+    store.plottedDatastreams = [makeDs({ id: 'mgd-1', name: 'Raw (QC)' })] as any
+    store.qcDatastreamId = 'mgd-1'
+
+    await store.plotSourceSelection('src', [])
+
+    expect(store.plottedDatastreams).toEqual([])
+    expect(store.qcDatastreamId).toBeNull()
+  })
+
+  it('ignores ids that do not belong to the source group', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withGroup(store)
+
+    await store.plotSourceSelection('src', ['other', 'mgd-1'])
+
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['mgd-1'])
+  })
+
+  // Looping plot/unplot would rebuild once per change; the batched action
+  // settles the whole selection against one rebuild.
+  it('rebuilds the plot once for the whole selection', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withGroup(store)
+
+    await store.plotSourceSelection('src', ['src', 'mgd-1', 'mgd-2'])
+
+    expect(mockUpdateOptions).toHaveBeenCalledTimes(1)
+  })
+
+  it('does nothing when the selection already matches', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withGroup(store)
+    store.plottedDatastreams = [makeDs({ id: 'src', name: 'Raw' })] as any
+    store.qcDatastreamId = 'src'
+
+    await store.plotSourceSelection('src', ['src'])
+
+    expect(mockUpdateOptions).not.toHaveBeenCalled()
+  })
+})
+
+// Both the source and a managed datastream can be plotted at once now, so
+// leaving the editor must not re-add a source that is already there.
+describe('useDataVisStore.releaseManagedDatastream with the source plotted', () => {
+  it('drops the managed datastream instead of duplicating the source', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    store.qcHistories = [
+      { id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'src' },
+    ] as any
+    store.datastreams = [
+      makeDs({ id: 'src', name: 'Raw' }),
+      makeDs({ id: 'mgd', name: 'Raw (QC)' }),
+    ] as any
+    store.plottedDatastreams = [
+      makeDs({ id: 'src', name: 'Raw' }),
+      makeDs({ id: 'mgd', name: 'Raw (QC)' }),
+    ] as any
+    store.qcDatastreamId = 'mgd'
+
+    await store.releaseManagedDatastream()
+
+    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['src'])
+    expect(store.qcDatastreamId).toBe('src')
+  })
+})
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index 09536f2e..08e3ad23 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -245,6 +245,59 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     if (plotlyRef.value) await handleNewPlot(undefined, { preserveZoom: true })
   }
 
+  /** The source datastream plus every managed datastream derived from it. */
+  function sourceGroupIds(sourceId: string): string[] {
+    const ids = [sourceId]
+    for (const h of historiesBySource.value.get(sourceId) ?? []) {
+      const id = historyManagedId(h)
+      if (id) ids.push(id)
+    }
+    return ids
+  }
+
+  /**
+   * Apply a whole "what to plot for this source" choice at once: `ids` is
+   * the complete set wanted from that source's group. Group members absent
+   * from `ids` are unplotted; additions are appended in `ids` order, so the
+   * first-plotted-wins QC rule sees the caller's ordering.
+   *
+   * Batched rather than looping plot/unplot so the QC target is promoted
+   * once against the final set instead of drifting through each
+   * intermediate state.
+   */
+  async function plotSourceSelection(sourceId: string, ids: string[]) {
+    const group = new Set(sourceGroupIds(sourceId))
+    const wanted = ids.filter((id) => group.has(id))
+    const wantedSet = new Set(wanted)
+
+    const kept = plottedDatastreams.value.filter(
+      (d) => !group.has(d.id) || wantedSet.has(d.id)
+    )
+    const present = new Set(kept.map((d) => d.id))
+    const added = wanted
+      .filter((id) => !present.has(id))
+      .map((id) => datastreams.value.find((d) => d.id === id))
+      .filter((d): d is Datastream & DatastreamExtended => !!d)
+
+    const next = [...kept, ...added]
+    const unchanged =
+      next.length === plottedDatastreams.value.length &&
+      next.every((d, i) => d.id === plottedDatastreams.value[i]?.id)
+    if (unchanged) return
+
+    const previousIndex = plottedDatastreams.value.findIndex(
+      (d) => d.id === qcDatastreamId.value
+    )
+    plottedDatastreams.value = next
+    if (!next.some((d) => d.id === qcDatastreamId.value)) {
+      // Same promotion rule as `unplotDatastream`: the entry before the one
+      // that left, clamped into the surviving set.
+      const idx = Math.max(Math.min(previousIndex - 1, next.length - 1), 0)
+      qcDatastreamId.value = next[idx]?.id ?? null
+    }
+    await rebuildPlot()
+  }
+
   /** Clear every plotted datastream at once. */
   async function clearPlottedDatastreams() {
     if (!plottedDatastreams.value.length) return
@@ -335,7 +388,13 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
 
     const idx = plottedDatastreams.value.findIndex((d) => d.id === managedId)
     if (idx < 0) return
-    plottedDatastreams.value.splice(idx, 1, source)
+    // The source can already be plotted alongside its managed datastream
+    // (both picked in the plot chooser); replacing would duplicate it.
+    if (plottedDatastreams.value.some((d) => d.id === source.id)) {
+      plottedDatastreams.value.splice(idx, 1)
+    } else {
+      plottedDatastreams.value.splice(idx, 1, source)
+    }
     qcDatastreamId.value = source.id
 
     // The working copy holds uncommitted edits; `rebuildPlot` refetches.
@@ -761,6 +820,8 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     toggleDatastream,
     plotDatastream,
     unplotDatastream,
+    sourceGroupIds,
+    plotSourceSelection,
     clearPlottedDatastreams,
     addSnapshotSeries,
     removeSnapshotSeries,
diff --git a/apps/qc-app/src/utils/__tests__/datastreamSummary.spec.ts b/apps/qc-app/src/utils/__tests__/datastreamSummary.spec.ts
new file mode 100644
index 00000000..6efb9279
--- /dev/null
+++ b/apps/qc-app/src/utils/__tests__/datastreamSummary.spec.ts
@@ -0,0 +1,60 @@
+import { describe, it, expect } from 'vitest'
+import { datastreamSummary } from '@/utils/datastreamSummary'
+
+const ds = (overrides: Record<string, any> = {}) =>
+  ({
+    id: 'ds-1',
+    name: 'Stream',
+    valueCount: 1234,
+    processingLevel: { definition: 'Quality controlled' },
+    ...overrides,
+  }) as any
+
+const session = (status: string) => ({ id: 's', status }) as any
+
+describe('datastreamSummary', () => {
+  it('leads with the processing level definition', () => {
+    expect(datastreamSummary(ds())).toContain('Level: Quality controlled')
+  })
+
+  it('falls back to the processing level code when there is no definition', () => {
+    expect(datastreamSummary(ds({ processingLevel: { code: 'L1' } }))).toContain(
+      'Level: L1'
+    )
+  })
+
+  it('omits the level entirely when the datastream has none', () => {
+    expect(datastreamSummary(ds({ processingLevel: undefined }))).not.toContain(
+      'Level:'
+    )
+  })
+
+  it('formats the observation count and treats a missing count as zero', () => {
+    expect(datastreamSummary(ds())).toContain(
+      `${new Intl.NumberFormat().format(1234)} obs`
+    )
+    expect(datastreamSummary(ds({ valueCount: undefined }))).toContain('0 obs')
+  })
+
+  it('omits sessions when none are supplied', () => {
+    expect(datastreamSummary(ds())).not.toContain('session')
+  })
+
+  it('pluralizes the session count', () => {
+    expect(datastreamSummary(ds(), [])).toContain('0 sessions')
+    expect(datastreamSummary(ds(), [session('committed')])).toContain(
+      '1 session'
+    )
+    expect(
+      datastreamSummary(ds(), [session('committed'), session('committed')])
+    ).toContain('2 sessions')
+  })
+
+  it('calls out in-progress sessions', () => {
+    const summary = datastreamSummary(ds(), [
+      session('committed'),
+      session('in_progress'),
+    ])
+    expect(summary).toContain('2 sessions, 1 in progress')
+  })
+})
diff --git a/apps/qc-app/src/utils/datastreamSummary.ts b/apps/qc-app/src/utils/datastreamSummary.ts
new file mode 100644
index 00000000..cbdedc0c
--- /dev/null
+++ b/apps/qc-app/src/utils/datastreamSummary.ts
@@ -0,0 +1,32 @@
+import type {
+  Datastream,
+  DatastreamExtended,
+  QualityControlSession,
+} from '@hydroserver/client'
+
+const NUMBER = new Intl.NumberFormat()
+
+/**
+ * One-line recap of a datastream, so several derived from the same source
+ * are distinguishable in a chooser: processing level, observation count,
+ * and — when sessions are supplied — how many QC sessions it carries.
+ */
+export function datastreamSummary(
+  datastream: Datastream & Partial<DatastreamExtended>,
+  sessions?: QualityControlSession[]
+): string {
+  const parts: string[] = []
+  const level =
+    datastream.processingLevel?.definition || datastream.processingLevel?.code
+  if (level) parts.push(`Level: ${level}`)
+  parts.push(`${NUMBER.format(datastream.valueCount ?? 0)} obs`)
+  if (sessions) {
+    const inProgress = sessions.filter((s) => s.status === 'in_progress').length
+    parts.push(
+      `${sessions.length} session${sessions.length === 1 ? '' : 's'}${
+        inProgress ? `, ${inProgress} in progress` : ''
+      }`
+    )
+  }
+  return parts.join(' · ')
+}

From 8502b55a41220825ba5f9a8eca1dae7911d100ec Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 11:34:00 -0600
Subject: [PATCH 27/93] refactor: add time range preset resolver

---
 apps/qc-app/docs/ARCHITECTURE.md              |   1 +
 .../utils/__tests__/timeRangePresets.spec.ts  | 104 ++++++++++++++++++
 apps/qc-app/src/utils/timeRangePresets.ts     |  75 +++++++++++++
 3 files changed, 180 insertions(+)
 create mode 100644 apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts
 create mode 100644 apps/qc-app/src/utils/timeRangePresets.ts

diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 0960e853..ba3b5503 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -107,6 +107,7 @@ src/
 ├─ utils/
 │  ├─ plotting/                 Plotly integration (trace builders, event handlers, selection, staging).
 │  ├─ dateMath.ts               Time-range arithmetic for presets ("1w", "1m", "All", …).
+│  ├─ timeRangePresets.ts       Preset definitions, resolved against the plotted data's extent.
 │  ├─ observations.ts           Observation fetch helpers (paged columnar fetch).
 │  ├─ rules.ts                  Vuetify form validation rules.
 │  └─ time.ts                   Time unit conversions.
diff --git a/apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts b/apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts
new file mode 100644
index 00000000..22feb112
--- /dev/null
+++ b/apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts
@@ -0,0 +1,104 @@
+import { describe, it, expect } from 'vitest'
+import {
+  ALL_PRESET_ID,
+  DEFAULT_PRESET_ID,
+  TIME_RANGE_PRESETS,
+  dataExtent,
+  presetWindow,
+} from '@/utils/timeRangePresets'
+
+const DAY = 24 * 60 * 60 * 1000
+
+describe('TIME_RANGE_PRESETS', () => {
+  it('keeps the stable preset ids and defaults to 1m', () => {
+    expect(TIME_RANGE_PRESETS.map((p) => [p.id, p.label])).toEqual([
+      [0, '1w'],
+      [1, '1m'],
+      [2, '6m'],
+      [4, '1y'],
+      [3, 'YTD'],
+      [5, 'All'],
+    ])
+    expect(DEFAULT_PRESET_ID).toBe(1)
+    expect(ALL_PRESET_ID).toBe(5)
+  })
+})
+
+describe('dataExtent', () => {
+  it('spans the earliest begin to the latest end', () => {
+    const extent = dataExtent([
+      {
+        phenomenonBeginTime: '2020-01-01T00:00:00Z',
+        phenomenonEndTime: '2021-06-30T12:00:00Z',
+      },
+      {
+        phenomenonBeginTime: '2019-05-01T00:00:00Z',
+        phenomenonEndTime: '2020-02-01T00:00:00Z',
+      },
+    ])
+    expect(extent?.begin.toISOString()).toBe('2019-05-01T00:00:00.000Z')
+    expect(extent?.end.toISOString()).toBe('2021-06-30T12:00:00.000Z')
+  })
+
+  it('skips datastreams without observations', () => {
+    const extent = dataExtent([
+      { phenomenonBeginTime: null, phenomenonEndTime: null },
+      {},
+      {
+        phenomenonBeginTime: '2020-01-01T00:00:00Z',
+        phenomenonEndTime: '2020-02-01T00:00:00Z',
+      },
+    ])
+    expect(extent?.end.toISOString()).toBe('2020-02-01T00:00:00.000Z')
+  })
+
+  it('is null when nothing has observations', () => {
+    expect(dataExtent([])).toBeNull()
+    expect(dataExtent([{ phenomenonEndTime: null }])).toBeNull()
+  })
+})
+
+describe('presetWindow', () => {
+  const extent = {
+    begin: new Date(2019, 4, 1),
+    end: new Date(2021, 2, 31, 12),
+  }
+
+  it('1w counts back seven days from the data end', () => {
+    const w = presetWindow(0, { begin: extent.begin, end: new Date(2021, 5, 30, 12) })
+    expect(w?.end.getTime()).toBe(new Date(2021, 5, 30, 12).getTime())
+    expect(w!.end.getTime() - w!.begin.getTime()).toBe(7 * DAY)
+  })
+
+  it('1m clamps to the end of a shorter month', () => {
+    const w = presetWindow(1, extent)
+    expect(w?.begin.getTime()).toBe(new Date(2021, 1, 28, 12).getTime())
+    expect(w?.end.getTime()).toBe(extent.end.getTime())
+  })
+
+  it('6m and 1y count back from the data end', () => {
+    expect(presetWindow(2, extent)?.begin.getTime()).toBe(
+      new Date(2020, 8, 30, 12).getTime()
+    )
+    expect(presetWindow(4, extent)?.begin.getTime()).toBe(
+      new Date(2020, 2, 31, 12).getTime()
+    )
+  })
+
+  it('YTD starts on Jan 1 of the data end year', () => {
+    expect(presetWindow(3, extent)?.begin.getTime()).toBe(
+      new Date(2021, 0, 1).getTime()
+    )
+  })
+
+  it('All spans the whole extent', () => {
+    const w = presetWindow(5, extent)
+    expect(w?.begin.getTime()).toBe(extent.begin.getTime())
+    expect(w?.end.getTime()).toBe(extent.end.getTime())
+  })
+
+  it('is null for an unknown id', () => {
+    expect(presetWindow(999, extent)).toBeNull()
+    expect(presetWindow(-1, extent)).toBeNull()
+  })
+})
diff --git a/apps/qc-app/src/utils/timeRangePresets.ts b/apps/qc-app/src/utils/timeRangePresets.ts
new file mode 100644
index 00000000..835649c2
--- /dev/null
+++ b/apps/qc-app/src/utils/timeRangePresets.ts
@@ -0,0 +1,75 @@
+/**
+ * Time range presets. A preset resolves against a data extent rather than
+ * the wall clock, so it always lands on observations.
+ */
+
+import { subtractDays, subtractMonths, subtractYears } from '@/utils/dateMath'
+
+export interface TimeRangePreset {
+  id: number
+  icon: string
+  label: '1w' | '1m' | '6m' | '1y' | 'YTD' | 'All'
+  title: string
+}
+
+export interface TimeWindow {
+  begin: Date
+  end: Date
+}
+
+interface PhenomenonTimes {
+  phenomenonBeginTime?: string | null
+  phenomenonEndTime?: string | null
+}
+
+export const TIME_RANGE_PRESETS: readonly TimeRangePreset[] = [
+  { id: 0, icon: 'mdi-calendar-week', label: '1w', title: 'Last week of data' },
+  { id: 1, icon: 'mdi-calendar-month', label: '1m', title: 'Last month of data' },
+  { id: 2, icon: 'mdi-calendar-range', label: '6m', title: 'Last 6 months of data' },
+  { id: 4, icon: 'mdi-calendar', label: '1y', title: 'Last year of data' },
+  { id: 3, icon: 'mdi-calendar-today', label: 'YTD', title: 'Year to date' },
+  { id: 5, icon: 'mdi-infinity', label: 'All', title: 'All data' },
+]
+
+export const DEFAULT_PRESET_ID = 1
+export const ALL_PRESET_ID = 5
+export const CUSTOM_PRESET_ID = -1
+
+export const findPreset = (id: number) =>
+  TIME_RANGE_PRESETS.find((p) => p.id === id)
+
+/** Earliest first to latest last observation; null when none has data. */
+export function dataExtent(
+  datastreams: readonly PhenomenonTimes[]
+): TimeWindow | null {
+  let begin = Infinity
+  let end = -Infinity
+  for (const ds of datastreams) {
+    if (!ds.phenomenonBeginTime || !ds.phenomenonEndTime) continue
+    begin = Math.min(begin, Date.parse(ds.phenomenonBeginTime))
+    end = Math.max(end, Date.parse(ds.phenomenonEndTime))
+  }
+  if (!Number.isFinite(begin) || !Number.isFinite(end)) return null
+  return { begin: new Date(begin), end: new Date(end) }
+}
+
+/** The window a preset covers, counting back from the extent's end. */
+export function presetWindow(id: number, extent: TimeWindow): TimeWindow | null {
+  const preset = findPreset(id)
+  if (!preset) return null
+  const end = new Date(extent.end)
+  switch (preset.label) {
+    case '1w':
+      return { begin: subtractDays(end, 7), end }
+    case '1m':
+      return { begin: subtractMonths(end, 1), end }
+    case '6m':
+      return { begin: subtractMonths(end, 6), end }
+    case '1y':
+      return { begin: subtractYears(end, 1), end }
+    case 'YTD':
+      return { begin: new Date(end.getFullYear(), 0, 1), end }
+    case 'All':
+      return { begin: new Date(extent.begin), end }
+  }
+}

From a1364577f5f5d0cd60125fd11ba819bbc2e685c3 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 11:46:12 -0600
Subject: [PATCH 28/93] fix: anchor time range presets to the plotted data

---
 apps/qc-app/docs/API_REFERENCE.md             |   8 +-
 apps/qc-app/docs/USER_GUIDE.md                |  14 +-
 apps/qc-app/e2e/all-operations.spec.ts        |   2 +-
 apps/qc-app/e2e/edit-fill-gaps.spec.ts        |   2 +-
 apps/qc-app/e2e/filter-gaps.spec.ts           |   2 +-
 apps/qc-app/e2e/filter-persistence.spec.ts    |   2 +-
 apps/qc-app/e2e/support/fixtures.ts           |  15 +-
 apps/qc-app/src/components/VisualizeData.vue  |  13 +-
 .../VisualizeData/DataVisTimeFilters.vue      |  21 +-
 .../src/components/VisualizeData/Plot.vue     |  87 ++------
 .../store/__tests__/dataVisualization.spec.ts | 207 +++++++++---------
 apps/qc-app/src/store/dataVisualization.ts    | 141 +++---------
 apps/qc-app/src/utils/share.ts                |  10 +-
 13 files changed, 190 insertions(+), 334 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index f7e53481..c9630df5 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -293,15 +293,13 @@ on boot.
 | `selectedData`                      | state    | `number[] \| null`                                | Index list of the active selection (lasso, box, click). |
 | `hasSelectionShape`                 | state    | `boolean`                                         | True while a box/lasso shape exists, even when it captured zero points. |
 | `loadingStates`                     | state    | `Map<string, boolean>`                            | Per-datastream in-flight observation fetches. |
-| `beginDate` / `endDate`             | state    | `Date`                                            | Active loaded time-range window (the date pickers' source of truth). |
-| `dateOptions`                       | state    | `Array<{ id, icon, label, title, calculateBeginDate }>` | Preset definitions (1w, 1m, 6m, 1y, YTD, All). |
-| `selectedDateBtnId`                 | state    | `number`                                          | Active preset id; `-1` when the user picked dates manually. |
+| `beginDate` / `endDate`             | state    | `Date`                                            | Active loaded window. A preset re-resolves it from the plotted datastreams' phenomenon times on every plot rebuild; a custom range stays fixed. |
+| `selectedDateBtnId`                 | state    | `number`                                          | Active preset id (default `1`, 1m); `-1` (`CUSTOM_PRESET_ID`) for a manual range. Presets are defined in `utils/timeRangePresets.ts`. |
 | `matchesSelectedThing`              | action   | `(ds) => boolean`                                 | Filter predicate; exposed so the table can reuse it on row updates. |
 | `matchesSelectedObservedProperty`   | action   | `(ds) => boolean`                                 | Same shape as above. |
 | `matchesSelectedProcessingLevel`    | action   | `(ds) => boolean`                                 | Same shape as above. |
 | `setDateRange`                      | action   | `({ begin?, end?, update?, custom? }) => Promise<void>` | No-ops when neither bound moves; clears zoom history when it does. |
-| `onDateBtnClick`                    | action   | `(id: number) => void`                            | Anchors `endDate` to today, recomputes `beginDate`, applies the preset. |
-| `syncRangeToPreset`                 | action   | `() => void`                                      | Re-derives `beginDate`/`endDate` from `selectedDateBtnId`. Run on `afterHydrate` so the restored preset's window applies to the first load. |
+| `onDateBtnClick`                    | action   | `(id: number) => Promise<void>`                   | Selects the preset and applies its window over the plotted data's extent. With nothing plotted, only the selection changes. |
 | `refreshGraphSeriesArray`           | action   | `() => Promise<unknown[]>`                        | Reconciles `graphSeriesArray` against `plottedDatastreams` (fetch deltas + reorder + recolor). |
 | `resetState`                        | action   | `() => void`                                      | Clears filters + plotted set on a workspace swap; preserves the preset preference. |
 | `toggleDatastream`                  | action   | `(ds: Datastream) => Promise<void>`               | Plot if absent, unplot if present. |
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index f34d730c..b1fb97ad 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -79,13 +79,7 @@ The select view is the default landing surface after picking a workspace. The le
 
 The filter drawer has two collapsible sections:
 
-- **Time range**: the loaded time window for the selected datastreams. The two date pickers (`From` / `To`) are the source of truth. They will respond when you make a selection in the row of preset chips below them:
-  **1w**, **1m**, **6m**, **1y**, **YTD**, **All**. Picking a preset re-fetches observations from the server. A `Custom` chip appears when the dates were edited manually in the `From` and `To` data pickers.
-
-  > **Tip:** when previewing a brand-new datastream whose observations
-  > might be years old, click **All** first. The default `1w` preset
-  > can show an empty window for old data and make the plot look
-  > broken.
+- **Time range**: the loaded time window for the plotted datastreams. The two date pickers (`From` / `To`) show the window. The preset chips below them, **1w**, **1m**, **6m**, **1y**, **YTD**, **All**, count back from the last observation of the plotted datastreams (the newest one when several are plotted), so a preset always lands on data, even for datastreams whose observations are years old. **1m** is the default, and the last preset you pick is remembered. Adding or removing a datastream re-anchors the preset. A `Custom` chip appears when you change a date in `From` or `To`; a custom window stays fixed while you add or remove datastreams.
 
 - **Datastream filters**: These filters allow you control the list of datastreams shown in the datastreams table by picking the site, observed property, and/or processing level. The list of matching datastreams updates live in the datastreams table.
 
@@ -200,7 +194,7 @@ The URL encodes everything needed to reproduce what the sender is looking at - q
 - **View** (Select vs Edit; `m=e` for Edit)
 - **Active center-column tab** (`tab=t` for Table)
 - **Plotted datastreams** (`ds`), in order. The first id is the QC target.
-- **Time window**: either a preset id (`r=0..5`) which the recipient re-anchors to *their* "now", or an explicit `from` / `to` pair as base36 second-epochs when the sender used a custom range.
+- **Time window**: either a preset id (`r=0..5`) which the recipient resolves against the plotted data, or an explicit `from` / `to` pair as base36 second-epochs when the sender used a custom range.
 - **Per-trace eye-toggle visibility** (`h`) and **per-axis visibility** (`ya`) as hex bitmasks over the `ds` order.
 - **Plot zoom**: X zoom (`z`) plus optional per-Y-axis zoom (`yz`) for axes that aren't at their default fit.
 - **Data points mode** (`dp`) and **threshold** (`th`) when they differ from the app defaults.
@@ -587,7 +581,7 @@ See [PERFORMANCE.md](./PERFORMANCE.md) for the envelope details.
 
 1. Pick a workspace, then pick the datastream you want to QC.
 2. Plot it (it becomes the QC target).
-3. Click **All** in the Time range so you load the full series.
+3. Click **All** in the Time range to load the full series (plotting loads the last month of data).
 4. Click the pencil icon → expand **Value thresholds**, set `Greater than: 1000`, press Enter.
 5. Expand **Delete points**, click Delete.
 6. Click **Save** at the bottom of the Edit history panel, then **Commit** and confirm in the dialog.
@@ -616,7 +610,7 @@ Click the grid icon in the nav rail → pick another. If you have unsaved edits,
 | Symptom | Likely cause | Fix |
 |---------|--------------|-----|
 | Blank page on load | Wrong API URL or `localhost` vs `127.0.0.1` mismatch. | See [DEPLOYMENT.md](./DEPLOYMENT.md). |
-| Plot stays empty after picking a datastream | Time range falls outside the datastream's observations. The plotted row shows a database-off icon and the subtitle reads `0 pts loaded`. | Click **All** in Time range. |
+| Plot stays empty after picking a datastream | A `Custom` time range that doesn't overlap the datastream's observations, or the datastream has no observations. The plotted row shows a database-off icon and the subtitle reads `0 pts loaded`. | Click a preset such as **1m** or **All** in Time range. |
 | Pencil ("Edit") icon is greyed out | No QC datastream selected. | Plot at least one datastream. The first becomes the QC target. |
 | Big edits freeze the page | `SharedArrayBuffer` not available; running inline. | Have your admin re-enable COOP/COEP headers, or accept the slower fallback. |
 | Save fails with a backend error | Permissions / workspace issue / network. | The Snackbar shows the backend message verbatim. Share that with your admin. |
diff --git a/apps/qc-app/e2e/all-operations.spec.ts b/apps/qc-app/e2e/all-operations.spec.ts
index 763fa86d..0c75c467 100644
--- a/apps/qc-app/e2e/all-operations.spec.ts
+++ b/apps/qc-app/e2e/all-operations.spec.ts
@@ -30,7 +30,7 @@ import { FIXTURE_OBS_START_MS } from './support/fixtures'
  */
 function observationsWithGap() {
   // Anchor to FIXTURE_OBS_START_MS (relative to "now") so the series
-  // falls inside the QC app's default 1w window — a hard-coded literal
+  // falls inside the QC app's default time range — a hard-coded literal
   // would slide out of range as the calendar moves and leave the main
   // plot empty.
   const startMs = FIXTURE_OBS_START_MS
diff --git a/apps/qc-app/e2e/edit-fill-gaps.spec.ts b/apps/qc-app/e2e/edit-fill-gaps.spec.ts
index cea6e030..1a707779 100644
--- a/apps/qc-app/e2e/edit-fill-gaps.spec.ts
+++ b/apps/qc-app/e2e/edit-fill-gaps.spec.ts
@@ -12,7 +12,7 @@ import { FIXTURE_OBS_START_MS } from './support/fixtures'
 
 function observationsWithGap() {
   // See FIXTURE_OBS_START_MS — anchored to "now" so the series sits
-  // inside the QC app's default 1w window.
+  // inside the QC app's default time range.
   const startMs = FIXTURE_OBS_START_MS
   const spacingMs = 15 * 60 * 1000
   const phenomenonTime: string[] = []
diff --git a/apps/qc-app/e2e/filter-gaps.spec.ts b/apps/qc-app/e2e/filter-gaps.spec.ts
index 56c1aa01..7d3fc919 100644
--- a/apps/qc-app/e2e/filter-gaps.spec.ts
+++ b/apps/qc-app/e2e/filter-gaps.spec.ts
@@ -15,7 +15,7 @@ import { FIXTURE_OBS_START_MS } from './support/fixtures'
 
 function observationsWithGap() {
   // See FIXTURE_OBS_START_MS — anchored to "now" so the series sits
-  // inside the QC app's default 1w window.
+  // inside the QC app's default time range.
   const startMs = FIXTURE_OBS_START_MS
   const spacingMs = 15 * 60 * 1000
   const phenomenonTime: string[] = []
diff --git a/apps/qc-app/e2e/filter-persistence.spec.ts b/apps/qc-app/e2e/filter-persistence.spec.ts
index 060aba23..a03da722 100644
--- a/apps/qc-app/e2e/filter-persistence.spec.ts
+++ b/apps/qc-app/e2e/filter-persistence.spec.ts
@@ -12,7 +12,7 @@ import { FIXTURE_OBS_START_MS } from './support/fixtures'
 
 function observationsWithPlateau() {
   // See FIXTURE_OBS_START_MS — anchored to "now" so the series sits
-  // inside the QC app's default 1w window.
+  // inside the QC app's default time range.
   const startMs = FIXTURE_OBS_START_MS
   const spacingMs = 15 * 60 * 1000
   const result: number[] = []
diff --git a/apps/qc-app/e2e/support/fixtures.ts b/apps/qc-app/e2e/support/fixtures.ts
index 51a44a83..38d03d87 100644
--- a/apps/qc-app/e2e/support/fixtures.ts
+++ b/apps/qc-app/e2e/support/fixtures.ts
@@ -32,15 +32,9 @@ export const OBSERVED_PROP_ID_B = 'op-water-temp'
 export const SENSOR_ID_B = 'sensor-temp'
 
 /**
- * Synthetic-observation timing constants, anchored to "now" at module
- * load so the generated series always falls inside the QC app's default
- * `1w` time-range window. Hard-coding a literal anchor (e.g.
- * `2024-01-01`) was a footgun: as the calendar moved past the fixture,
- * the main plot — which slices by `[beginDate, endDate]` — silently
- * collapsed to an empty window even though the ContextPlot (which uses
- * all observations un-windowed) kept looking correct. Specs that build
- * custom observation series should import `FIXTURE_OBS_START_MS` and
- * derive their timestamps from it for the same reason.
+ * Synthetic-observation timing constants. The catalog's phenomenon times are
+ * derived from these, and the app's presets anchor to them, so specs that
+ * build custom series should derive timestamps from `FIXTURE_OBS_START_MS`.
  */
 export const FIXTURE_OBS_SPACING_MS = 15 * 60 * 1000
 export const FIXTURE_OBS_COUNT = 120
@@ -336,8 +330,7 @@ export const qcSessions = [
  * `count` samples at 15-minute spacing starting at `startMs`, with
  * values following `y = 10 + 5 * sin(i / 5)` so every filter op has
  * interesting but predictable points to select. The default anchor is
- * `FIXTURE_OBS_START_MS` (relative to "now") so the series always lies
- * inside the QC app's default `1w` time-range window.
+ * `FIXTURE_OBS_START_MS`, matching the catalog's phenomenon times.
  */
 export function buildObservations(
   count = FIXTURE_OBS_COUNT,
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index bdae2b6b..c97bf14f 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -566,6 +566,7 @@ import {
   encodeShareState,
   type ShareState,
 } from '@/utils/share'
+import { CUSTOM_PRESET_ID } from '@/utils/timeRangePresets'
 import { isSnapshotId, parseSnapshotId } from '@/utils/snapshotId'
 import { useHistorySnapshots } from '@/composables/useHistorySnapshots'
 import { useWorkspaceStore } from '@/store/workspaces'
@@ -583,7 +584,6 @@ const {
   beginDate,
   endDate,
   selectedDateBtnId,
-  dateOptions,
   selectedThings,
   selectedObservedPropertyNames,
   selectedProcessingLevelNames,
@@ -965,17 +965,12 @@ const hydrateFromUrl = () => {
   // QC target is the first id by convention.
   const qcId = resolved[0]?.id ?? null
 
-  // Apply the date window BEFORE loading datastreams so the first
-  // fetch uses the correct range.
+  // Set the window BEFORE loading datastreams so the first fetch uses it.
+  // A preset resolves against the plotted data during that load.
   if (state.datePresetId != null && state.datePresetId >= 0) {
     selectedDateBtnId.value = state.datePresetId
-    const option = dateOptions.value.find((o) => o.id === state.datePresetId)
-    if (option) {
-      endDate.value = new Date()
-      beginDate.value = option.calculateBeginDate()
-    }
   } else if (state.beginMs != null || state.endMs != null) {
-    selectedDateBtnId.value = -1
+    selectedDateBtnId.value = CUSTOM_PRESET_ID
     if (state.beginMs != null) beginDate.value = new Date(state.beginMs)
     if (state.endMs != null) endDate.value = new Date(state.endMs)
   }
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue b/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
index 689b6862..53cac21c 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
@@ -1,9 +1,9 @@
-<template>
+<template>
   <div class="d-flex flex-column ga-2 w-100">
     <div>
       <div class="text-body-small text-medium-emphasis">
         <strong>Loaded time window.</strong>
-        Changing the range re-fetches observations from the server.
+        Presets count back from the plotted data's last observation.
       </div>
     </div>
 
@@ -11,6 +11,7 @@
       <div>
         <div class="text-body-small text-medium-emphasis mb-1">From</div>
         <DatePickerField
+          data-testid="date-range-from"
           :model-value="beginDate"
           placeholder="Start date"
           @update:model-value="setDateRange({ begin: $event })"
@@ -19,6 +20,7 @@
       <div>
         <div class="text-body-small text-medium-emphasis mb-1">To</div>
         <DatePickerField
+          data-testid="date-range-to"
           :model-value="endDate"
           placeholder="End date"
           @update:model-value="setDateRange({ end: $event })"
@@ -28,19 +30,21 @@
 
     <div class="time-filters__presets">
       <v-chip
-        v-for="option in dateOptions"
+        v-for="option in TIME_RANGE_PRESETS"
         :key="option.id"
+        :data-testid="`date-preset-${option.label}`"
         :color="selectedDateBtnId === option.id ? 'primary' : undefined"
         :variant="selectedDateBtnId === option.id ? 'tonal' : 'outlined'"
         size="small"
-        :title="(option as any).title ?? option.label"
+        :title="option.title"
         class="time-filters__preset-chip justify-center"
         @click="onDateBtnClick(option.id)"
       >
         {{ option.label }}
       </v-chip>
       <v-chip
-        v-if="selectedDateBtnId === -1"
+        v-if="selectedDateBtnId === CUSTOM_PRESET_ID"
+        data-testid="date-preset-custom"
         color="secondary"
         variant="tonal"
         size="small"
@@ -56,11 +60,14 @@
 <script setup lang="ts">
 import DatePickerField from '@/components/VisualizeData/DatePickerField.vue'
 import { useDataVisStore } from '@/store/dataVisualization'
+import {
+  CUSTOM_PRESET_ID,
+  TIME_RANGE_PRESETS,
+} from '@/utils/timeRangePresets'
 import { storeToRefs } from 'pinia'
 
 const { setDateRange, onDateBtnClick } = useDataVisStore()
-const { dateOptions, beginDate, endDate, selectedDateBtnId } =
-  storeToRefs(useDataVisStore())
+const { beginDate, endDate, selectedDateBtnId } = storeToRefs(useDataVisStore())
 </script>
 
 <style scoped>
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index ec7df040..19cb06a8 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -288,7 +288,7 @@
             color="surface"
             variant="flat"
             size="small"
-            :title="option.editorLabel"
+            :title="option.title"
             class="text-none text-body-small font-weight-medium border elevation-2"
             @click="selectRangePreset(option.id)"
           >
@@ -411,7 +411,11 @@ import {
   handleRelayout,
   zoomXaxisTo,
 } from '@/utils/plotting/plotly'
-import { subtractDays, subtractMonths, subtractYears } from '@/utils/dateMath'
+import {
+  ALL_PRESET_ID,
+  TIME_RANGE_PRESETS,
+  presetWindow,
+} from '@/utils/timeRangePresets'
 import DataTable from '@/components/VisualizeData/DataTable.vue'
 import ContextPlot from '@/components/VisualizeData/ContextPlot.vue'
 import { useDataSelection } from '@/composables/useDataSelection'
@@ -443,13 +447,9 @@ const {
   plotlyRef,
   activeTab,
 } = storeToRefs(usePlotlyStore())
-const { selectedData, hasSelectionShape, qcDatastream, dateOptions } =
+const { selectedData, hasSelectionShape, qcDatastream } =
   storeToRefs(useDataVisStore())
 
-const allPresetId = computed(
-  () => dateOptions.value.find((o) => o.label === 'All')?.id ?? null
-)
-
 const tooltipsAutoDisabled = computed(
   () =>
     tooltipsMode.value === 'auto' &&
@@ -593,19 +593,7 @@ const latestDataX = computed<number | null>(() => {
   return Number.isFinite(max) ? max : null
 })
 
-const EDITOR_LABELS: Record<string, string> = {
-  '1w': 'Last week of data',
-  '1m': 'Last month of data',
-  '6m': 'Last 6 months of data',
-  '1y': 'Last year of data',
-  All: 'All data',
-}
-
-const editorDateOptions = computed(() =>
-  dateOptions.value
-    .filter((o) => o.label !== 'YTD')
-    .map((o) => ({ ...o, editorLabel: EDITOR_LABELS[o.label] ?? o.label }))
-)
+const editorDateOptions = TIME_RANGE_PRESETS.filter((o) => o.label !== 'YTD')
 
 const rangeDialOpen = ref(false)
 
@@ -614,53 +602,18 @@ function selectRangePreset(id: number) {
   rangeDialOpen.value = false
 }
 
-// Editor presets are a pure x-axis zoom (no refetch) so they don't
-// blow away the edit history. Relative presets (1w/1m/6m/1y) anchor
-// to the loaded data's end. "All" snaps to the data extent.
+// Editor presets are a pure x-axis zoom (no refetch) so they don't blow away
+// the edit history. They resolve against the loaded data.
 function onEditorDatePreset(id: number) {
-  const option = dateOptions.value.find((o) => o.id === id)
-  if (!option) return
-
-  const dataEndMs = latestDataX.value
-  const dataEnd = dataEndMs != null ? new Date(dataEndMs) : null
-
-  let begin: Date | null = null
-  let end: Date | null = null
-
-  switch (option.label) {
-    case '1w':
-      if (!dataEnd) return
-      end = dataEnd
-      begin = subtractDays(dataEnd, 7)
-      break
-    case '1m':
-      if (!dataEnd) return
-      end = dataEnd
-      begin = subtractMonths(dataEnd, 1)
-      break
-    case '6m':
-      if (!dataEnd) return
-      end = dataEnd
-      begin = subtractMonths(dataEnd, 6)
-      break
-    case '1y':
-      if (!dataEnd) return
-      end = dataEnd
-      begin = subtractYears(dataEnd, 1)
-      break
-    case 'All':
-      if (earliestDataX.value == null || dataEndMs == null) return
-      begin = new Date(earliestDataX.value)
-      end = new Date(dataEndMs)
-      break
-    default:
-      return
-  }
-
-  if (!begin || !end) return
-  zoomXaxisTo(plotlyRef.value, begin.getTime(), end.getTime())
+  if (earliestDataX.value == null || latestDataX.value == null) return
+  const range = presetWindow(id, {
+    begin: new Date(earliestDataX.value),
+    end: new Date(latestDataX.value),
+  })
+  if (!range) return
+  zoomXaxisTo(plotlyRef.value, range.begin.getTime(), range.end.getTime())
   // Keep the table in sync: scroll it so the range's first row is on top.
-  requestTableScroll(begin.getTime())
+  requestTableScroll(range.begin.getTime())
 }
 
 function toggleTooltips() {
@@ -747,9 +700,7 @@ onMounted(async () => {
   setTimeout(() => {
     updateOptions()
     handleNewPlot(plot.value)
-    if (!props.preview && allPresetId.value != null) {
-      onEditorDatePreset(allPresetId.value)
-    }
+    if (!props.preview) onEditorDatePreset(ALL_PRESET_ID)
 
     const target = plot.value
     if (target && typeof ResizeObserver !== 'undefined') {
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index b502dd57..10148eb6 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -2,6 +2,7 @@ import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
 import { createPinia, setActivePinia } from 'pinia'
 import piniaPluginPersistedstate from 'pinia-plugin-persistedstate'
 import { createApp, ref } from 'vue'
+import { subtractMonths } from '@/utils/dateMath'
 
 // Shared mutable stub state so each test can reset between runs.
 const mockPlotlyRef = ref<any>(null)
@@ -380,56 +381,6 @@ describe('useDataVisStore.setDateRange', () => {
   })
 })
 
-describe('useDataVisStore.onDateBtnClick', () => {
-  beforeEach(() => {
-    vi.useFakeTimers()
-    vi.setSystemTime(new Date('2026-01-15T12:00:00Z'))
-  })
-
-  it('updates selectedDateBtnId and sets a 1-week window for id=0', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.onDateBtnClick(0)
-    expect(store.selectedDateBtnId).toBe(0)
-    const spanMs = store.endDate.getTime() - store.beginDate.getTime()
-    expect(spanMs).toBe(7 * 24 * 60 * 60 * 1000)
-  })
-
-  it('sets a ~1-month window for id=1', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.onDateBtnClick(1)
-    expect(store.selectedDateBtnId).toBe(1)
-    expect(store.beginDate.getUTCMonth()).toBe(11) // December
-    expect(store.beginDate.getUTCFullYear()).toBe(2025)
-  })
-
-  it('sets a YTD window for id=3 (Jan 1 local)', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.onDateBtnClick(3)
-    expect(store.selectedDateBtnId).toBe(3)
-    expect(store.beginDate.getMonth()).toBe(0)
-    expect(store.beginDate.getDate()).toBe(1)
-  })
-
-  it('sets beginDate to epoch 0 for id=5 (All)', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.onDateBtnClick(5)
-    expect(store.selectedDateBtnId).toBe(5)
-    expect(store.beginDate.getTime()).toBe(0)
-  })
-
-  it('is a no-op when the id does not match any option', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    const prev = store.selectedDateBtnId
-    store.onDateBtnClick(999)
-    expect(store.selectedDateBtnId).toBe(prev)
-  })
-})
-
 describe('useDataVisStore.resetState', () => {
   it('resets filter arrays, plotted state, and qc id', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
@@ -463,97 +414,137 @@ describe('useDataVisStore.resetState', () => {
     store.resetState()
     expect(store.selectedDateBtnId).toBe(2)
   })
+})
+
+describe('useDataVisStore time range presets', () => {
+  const OLD_END = '2021-06-30T12:00:00Z'
+  const oldDs = (overrides: Record<string, any> = {}) =>
+    makeDs({
+      id: 'old',
+      phenomenonBeginTime: '2019-01-01T00:00:00Z',
+      phenomenonEndTime: OLD_END,
+      ...overrides,
+    })
+  const newerDs = () =>
+    makeDs({
+      id: 'newer',
+      phenomenonBeginTime: '2020-01-01T00:00:00Z',
+      phenomenonEndTime: '2022-03-01T00:00:00Z',
+    })
 
-  it('reapplies the persisted preset to beginDate', async () => {
+  it('defaults to the 1m preset', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.selectedDateBtnId = 0 // "1w" preset
-    store.resetState()
-    const spanMs = store.endDate.getTime() - store.beginDate.getTime()
-    expect(spanMs).toBe(7 * 24 * 60 * 60 * 1000)
+    expect(useDataVisStore().selectedDateBtnId).toBe(1)
   })
-})
 
-describe('useDataVisStore.syncRangeToPreset', () => {
-  it('derives the "All" window (epoch 0) from the persisted preset', async () => {
+  it('anchors a preset click to the plotted data end, not now', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
-    store.selectedDateBtnId = 5 // "All"
-    store.syncRangeToPreset()
-    expect(store.beginDate.getTime()).toBe(0)
+    store.plottedDatastreams = [oldDs()] as any
+    await store.onDateBtnClick(0)
+    expect(store.selectedDateBtnId).toBe(0)
+    expect(store.endDate.toISOString()).toBe('2021-06-30T12:00:00.000Z')
+    expect(store.endDate.getTime() - store.beginDate.getTime()).toBe(
+      7 * 24 * 60 * 60 * 1000
+    )
   })
 
-  it('derives a relative window (1w) from the persisted preset', async () => {
+  it('All spans every plotted datastream', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
-    store.selectedDateBtnId = 0 // "1w"
-    store.syncRangeToPreset()
-    const spanMs = store.endDate.getTime() - store.beginDate.getTime()
-    expect(spanMs).toBe(7 * 24 * 60 * 60 * 1000)
+    store.plottedDatastreams = [oldDs(), newerDs()] as any
+    await store.onDateBtnClick(5)
+    expect(store.beginDate.toISOString()).toBe('2019-01-01T00:00:00.000Z')
+    expect(store.endDate.toISOString()).toBe('2022-03-01T00:00:00.000Z')
   })
 
-  it('falls back to a 1-week window when the id matches no preset (custom)', async () => {
+  it('ignores datastreams without observations when anchoring', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
-    store.selectedDateBtnId = -1 // custom: no preset
-    store.syncRangeToPreset()
-    const spanMs = store.endDate.getTime() - store.beginDate.getTime()
-    expect(spanMs).toBe(7 * 24 * 60 * 60 * 1000)
+    store.plottedDatastreams = [makeDs({ id: 'empty' }), oldDs()] as any
+    await store.onDateBtnClick(0)
+    expect(store.endDate.toISOString()).toBe('2021-06-30T12:00:00.000Z')
   })
-})
 
-// Regression: the persisted preset must actually drive the loaded window
-// on first load. Only `selectedDateBtnId` is persisted, so without the
-// afterHydrate sync the highlighted preset (e.g. "All") wouldn't take
-// effect until the chip was clicked a second time. The persistence plugin
-// only activates once the pinia is installed on an app, so these tests go
-// through `app.use(pinia)` rather than a bare `createPinia()`.
-describe('useDataVisStore persisted-preset hydration', () => {
-  beforeEach(() => {
-    localStorage.clear()
+  it('selects the preset without moving the window when nothing is plotted', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    const begin = store.beginDate.getTime()
+    await store.onDateBtnClick(5)
+    expect(store.selectedDateBtnId).toBe(5)
+    expect(store.beginDate.getTime()).toBe(begin)
+    expect(mockRedraw).not.toHaveBeenCalled()
   })
 
-  afterEach(() => {
-    localStorage.clear()
+  it('is a no-op for an unknown preset id', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    await store.onDateBtnClick(999)
+    expect(store.selectedDateBtnId).toBe(1)
   })
 
-  function hydrateWith(persisted: Record<string, unknown>) {
-    localStorage.setItem('dataVisualization', JSON.stringify(persisted))
-    const pinia = createPinia()
-    pinia.use(piniaPluginPersistedstate)
-    // app.use(pinia) runs the plugin install that activates persistence
-    // and sets the active pinia.
-    createApp({ render: () => null }).use(pinia)
-  }
+  it('re-anchors the active preset when a datastream is plotted', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    await store.plotDatastream(oldDs() as any)
+    const end = new Date(OLD_END)
+    expect(store.endDate.getTime()).toBe(end.getTime())
+    expect(store.beginDate.getTime()).toBe(subtractMonths(end, 1).getTime())
+    expect(mockFetchGraphSeries).toHaveBeenCalledWith(
+      expect.objectContaining({ id: 'old' }),
+      store.beginDate,
+      store.endDate
+    )
+  })
 
-  it('applies a persisted "All" preset to beginDate on hydration', async () => {
-    hydrateWith({ selectedDateBtnId: 5 })
+  it('re-anchors when the newest datastream is unplotted', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
-    expect(store.selectedDateBtnId).toBe(5)
-    // "All" → epoch 0, proving afterHydrate recomputed the window.
-    expect(store.beginDate.getTime()).toBe(0)
+    store.plottedDatastreams = [oldDs(), newerDs()] as any
+    store.qcDatastreamId = 'old'
+    await store.unplotDatastream('newer')
+    expect(store.endDate.toISOString()).toBe('2021-06-30T12:00:00.000Z')
   })
 
-  it('applies a persisted relative preset (6m) to the loaded window', async () => {
-    hydrateWith({ selectedDateBtnId: 2 })
+  it('keeps a custom range when the plotted set changes', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
-    expect(store.selectedDateBtnId).toBe(2)
-    // 6 months back is well beyond the default 1-week window.
-    const spanMs = store.endDate.getTime() - store.beginDate.getTime()
-    expect(spanMs).toBeGreaterThan(7 * 24 * 60 * 60 * 1000)
+    const begin = new Date('2025-01-01T00:00:00Z')
+    const end = new Date('2025-02-01T00:00:00Z')
+    await store.setDateRange({ begin, end })
+    await store.plotDatastream(oldDs() as any)
+    expect(store.selectedDateBtnId).toBe(-1)
+    expect(store.beginDate.getTime()).toBe(begin.getTime())
+    expect(store.endDate.getTime()).toBe(end.getTime())
   })
+})
+
+// The persistence plugin only activates once the pinia is installed on an
+// app, so these go through `app.use(pinia)`.
+describe('useDataVisStore persisted preset', () => {
+  beforeEach(() => localStorage.clear())
+  afterEach(() => localStorage.clear())
 
-  it('leaves the default 1-week window when nothing is persisted', async () => {
+  const installPinia = () => {
     const pinia = createPinia()
     pinia.use(piniaPluginPersistedstate)
     createApp({ render: () => null }).use(pinia)
+  }
+
+  it('restores the persisted preset id', async () => {
+    localStorage.setItem(
+      'dataVisualization',
+      JSON.stringify({ selectedDateBtnId: 5 })
+    )
+    installPinia()
     const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    expect(store.selectedDateBtnId).toBe(0)
-    const spanMs = store.endDate.getTime() - store.beginDate.getTime()
-    expect(spanMs).toBe(7 * 24 * 60 * 60 * 1000)
+    expect(useDataVisStore().selectedDateBtnId).toBe(5)
+  })
+
+  it('defaults to 1m when nothing is persisted', async () => {
+    installPinia()
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    expect(useDataVisStore().selectedDateBtnId).toBe(1)
   })
 })
 
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index 08e3ad23..e361e589 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -4,7 +4,14 @@ import { usePlotlyStore } from './plotly'
 import { useObservationStore } from './observations'
 import { Snackbar } from '@uwrl/qc-utils'
 import { handleNewPlot } from '@/utils/plotting/plotly'
-import { subtractDays, subtractMonths, subtractYears } from '@/utils/dateMath'
+import { subtractMonths } from '@/utils/dateMath'
+import {
+  CUSTOM_PRESET_ID,
+  DEFAULT_PRESET_ID,
+  dataExtent,
+  findPreset,
+  presetWindow,
+} from '@/utils/timeRangePresets'
 import { isSnapshotId } from '@/utils/snapshotId'
 import type { SnapshotMeta } from '@/types'
 import type { ObservationRecord } from '@uwrl/qc-utils'
@@ -123,25 +130,18 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
    * Set to true when we get a response from the API. Keyed by datastream id. */
   const loadingStates = ref(new Map<string, boolean>())
 
-  // Time range
+  // Time range. Until something with observations is plotted there is no
+  // data to anchor a preset to, so the window is a placeholder.
   const endDate = ref<Date>(new Date())
-  const oneWeek = 7 * 24 * 60 * 60 * 1000
-  const beginDate = ref<Date>(new Date(endDate.value.getTime() - oneWeek))
-  const selectedDateBtnId = ref(0)
-
-  // Re-derive the loaded window [beginDate, endDate] from the active
-  // preset. Only `selectedDateBtnId` is persisted; beginDate/endDate keep
-  // their fixed 1-week defaults until this runs. Without it, a restored
-  // preset (e.g. "All") is highlighted but the first datastream load
-  // fetches the stale default window until the chip is clicked again.
-  function syncRangeToPreset() {
-    endDate.value = new Date()
-    const option = dateOptions.value.find(
-      (o) => o.id === selectedDateBtnId.value
-    )
-    beginDate.value = option
-      ? option.calculateBeginDate()
-      : new Date(endDate.value.getTime() - oneWeek)
+  const beginDate = ref<Date>(subtractMonths(endDate.value, 1))
+  const selectedDateBtnId = ref(DEFAULT_PRESET_ID)
+
+  /** The active preset's window over the plotted data; null for a custom
+   *  range or when nothing plotted has observations. */
+  function resolvePresetWindow() {
+    if (selectedDateBtnId.value === CUSTOM_PRESET_ID) return null
+    const extent = dataExtent(plottedDatastreams.value)
+    return extent ? presetWindow(selectedDateBtnId.value, extent) : null
   }
 
   function resetState() {
@@ -150,11 +150,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     qcDatastreamId.value = null
     selectedObservedPropertyNames.value = []
     selectedProcessingLevelNames.value = []
-    // Re-apply the persisted preset so the chosen time range survives
-    // workspace switches. selectedDateBtnId is intentionally NOT reset here —
-    // it's a user preference, not workspace-specific state, and resetting it
-    // would overwrite the persisted localStorage value.
-    syncRangeToPreset()
+    // selectedDateBtnId is a user preference, not workspace state.
     // Old watcher used to call clearChartState when plottedDatastreams
     // emptied; with the watcher gone, do it here explicitly.
     clearChartState()
@@ -468,6 +464,12 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     }
     const { clearZoomHistory } = usePlotlyStore()
     clearZoomHistory()
+    // The plotted set changed, and with it the data a preset anchors to.
+    const presetRange = resolvePresetWindow()
+    if (presetRange) {
+      beginDate.value = presetRange.begin
+      endDate.value = presetRange.end
+    }
     await refreshGraphSeriesArray()
     updateOptions()
     const { plotlyRef } = storeToRefs(usePlotlyStore())
@@ -522,61 +524,6 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     )
   })
 
-  // All relative presets (1w/1m/6m/1y) anchor to the working window's
-  // end so the range always lands on data. The subtractors clamp
-  // day-of-month correctly across month/year boundaries (see
-  // `src/utils/dateMath.ts`) — the old `new Date(y, m - N, d)`
-  // constructor silently overflowed on `Aug 31 - 6 months` and friends.
-  const dateOptions = ref([
-    {
-      id: 0,
-      icon: 'mdi-calendar-week',
-      label: '1w',
-      title: 'Last week',
-      calculateBeginDate: () => subtractDays(endDate.value, 7),
-    },
-    {
-      id: 1,
-      icon: 'mdi-calendar-month',
-      label: '1m',
-      title: 'Last month',
-      calculateBeginDate: () => subtractMonths(endDate.value, 1),
-    },
-    {
-      id: 2,
-      icon: 'mdi-calendar-range',
-      label: '6m',
-      title: 'Last 6 months',
-      calculateBeginDate: () => subtractMonths(endDate.value, 6),
-    },
-    {
-      id: 4,
-      icon: 'mdi-calendar',
-      label: '1y',
-      title: 'Last year',
-      calculateBeginDate: () => subtractYears(endDate.value, 1),
-    },
-    {
-      id: 3,
-      icon: 'mdi-calendar-today',
-      label: 'YTD',
-      title: 'Year to date',
-      // Jan 1 of the year containing the working window's end. The
-      // editor overrides this to real calendar year in
-      // `Plot.vue#onEditorDatePreset` since zoom-only semantics want
-      // "this calendar year so far" regardless of the loaded window.
-      calculateBeginDate: () =>
-        new Date(endDate.value.getFullYear(), 0, 1),
-    },
-    {
-      id: 5,
-      icon: 'mdi-infinity',
-      label: 'All',
-      title: 'Full history',
-      calculateBeginDate: () => new Date(0),
-    },
-  ])
-
   interface SetDateRangeParams {
     begin?: Date
     end?: Date
@@ -604,7 +551,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
 
     if (begin) beginDate.value = begin
     if (end) endDate.value = end
-    if (custom) selectedDateBtnId.value = -1
+    if (custom) selectedDateBtnId.value = CUSTOM_PRESET_ID
 
     if (
       update &&
@@ -626,17 +573,12 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     }
   }
 
-  const onDateBtnClick = (selectedId: number) => {
-    const selectedOption = dateOptions.value.find(
-      (option) => option.id === selectedId
-    )
-    if (selectedOption) {
-      // Presets always anchor end to today so the window is relative to now.
-      // Set endDate first so calculateBeginDate closures read the new value.
-      endDate.value = new Date()
-      const newBeginDate = selectedOption.calculateBeginDate()
-      selectedDateBtnId.value = selectedId
-      setDateRange({ begin: newBeginDate, end: endDate.value, custom: false })
+  const onDateBtnClick = async (selectedId: number) => {
+    if (!findPreset(selectedId)) return
+    selectedDateBtnId.value = selectedId
+    const range = resolvePresetWindow()
+    if (range) {
+      await setDateRange({ begin: range.begin, end: range.end, custom: false })
     }
   }
 
@@ -800,7 +742,6 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     plottedDatastreams,
     beginDate,
     endDate,
-    dateOptions,
     loadingStates,
     selectedDateBtnId,
     qcDatastream,
@@ -814,7 +755,6 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     matchesSelectedThing,
     setDateRange,
     onDateBtnClick,
-    syncRangeToPreset,
     refreshGraphSeriesArray,
     resetState,
     toggleDatastream,
@@ -833,20 +773,9 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     // updateOrFetchGraphSeries,
   }
 }, {
-  // Persist only the user's time-range preset choice. Everything else
-  // in this store (server catalogs, in-flight loading maps, live
-  // filters) should refetch cleanly on every load — persisting them
-  // would surface stale data after catalog or role changes. `dateOptions`
-  // in particular can't be serialized because it carries closure
-  // functions (`calculateBeginDate`).
+  // Persist only the user's preset choice. Catalogs, loading maps and
+  // filters refetch cleanly on every load; the window resolves from the data.
   persist: {
     pick: ['selectedDateBtnId'],
-    // Only the preset id is persisted, so the loaded window must be
-    // recomputed from it after hydration — otherwise beginDate/endDate
-    // keep their fixed 1-week defaults and the first datastream load
-    // ignores the restored preset until the chip is clicked again.
-    afterHydrate: (ctx) => {
-      ;(ctx.store as unknown as { syncRangeToPreset: () => void }).syncRangeToPreset()
-    },
   },
 })
diff --git a/apps/qc-app/src/utils/share.ts b/apps/qc-app/src/utils/share.ts
index fc1784ad..85f8f303 100644
--- a/apps/qc-app/src/utils/share.ts
+++ b/apps/qc-app/src/utils/share.ts
@@ -49,8 +49,8 @@ export interface ShareState {
    *  QC-target-is-first rule and the `h` / `ya` bitmask indices still hold. */
   snapshots?: { sessionId: string; opIndex: number }[]
   /** Date range preset id (`0..5`). When set, `begin`/`end` are
-   *  omitted from the URL and the receiver recomputes the window
-   *  from "now". */
+   *  omitted from the URL and the receiver resolves the window
+   *  from the plotted data. */
   datePresetId?: number | null
   /** Custom-window begin (epoch ms). Only emitted when no preset is
    *  active. */
@@ -138,10 +138,8 @@ export function encodeShareState(state: ShareState): Record<string, string> {
     q.snap = state.snapshots.map((s) => `${s.sessionId}:${s.opIndex}`).join(',')
   }
 
-  // Preset wins over begin/end. If a preset is active, dropping the
-  // explicit dates lets the recipient pick up the same preset window
-  // anchored to *their* "now" — which is the sender's intent when
-  // they clicked the preset.
+  // Preset wins over begin/end: the recipient resolves the same preset
+  // against the plotted data, which is what the sender clicked.
   if (
     state.datePresetId != null &&
     Number.isFinite(state.datePresetId) &&

From 1475d35df7bce1d36d7e7ccfdfb0ec6661712c43 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 11:54:16 -0600
Subject: [PATCH 29/93] fix: refresh the managed datastream after a commit

---
 apps/qc-app/docs/API_REFERENCE.md             |  1 +
 .../__tests__/useEditSession.spec.ts          | 19 ++++++++++++++++-
 apps/qc-app/src/composables/useEditSession.ts |  8 ++++++-
 .../store/__tests__/dataVisualization.spec.ts | 21 +++++++++++++++++++
 apps/qc-app/src/store/dataVisualization.ts    |  9 ++++++++
 5 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index c9630df5..d80808c1 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -279,6 +279,7 @@ on boot.
 | `historiesBySource`                 | computed | `Map<string, QualityControlHistory[]>`            | `sourceDatastreamId` -> its QC histories; drives the Start-editing chooser. |
 | `addQcHistory`                      | action   | `(history: QualityControlHistory) => void`        | Register a newly-created history so its managed datastream hides from the catalog and shows in the chooser without a reload. |
 | `removeManagedDatastream`           | action   | `(historyId: string, managedId: string) => void`  | Drop a deleted managed datastream + its history from local state (chooser/catalog) after deleting it server-side. |
+| `replaceDatastream`                 | action   | `(ds: Datastream & DatastreamExtended) => void`   | Swap a fresh copy into the catalog and plotted set (used after a commit moves a managed datastream's phenomenon times). |
 | `observedProperties`                | state    | `ObservedProperty[]`                              | Taxonomy for the filter chips. |
 | `processingLevels`                  | state    | `ProcessingLevel[]`                               | Taxonomy for the filter chips. |
 | `selectedThings`                    | state    | `Thing[]`                                         | Site filter selection (sidebar). |
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index ae6b604c..f2e0debc 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -5,8 +5,9 @@ import { makeQcFake } from '@/services/qualityControl/__tests__/qcServiceFake'
 import { unwrap } from '@/services/qualityControl/unwrap'
 
 const qcDatastream = ref<any>(null)
+const replaceDatastream = vi.fn()
 vi.mock('@/store/dataVisualization', () => ({
-  useDataVisStore: () => ({ qcDatastream }),
+  useDataVisStore: () => ({ qcDatastream, replaceDatastream }),
 }))
 
 const selectedSeries = ref<any>(null)
@@ -218,6 +219,22 @@ describe('useEditSession', () => {
     expect(store.inProgressSession).toBeNull()
   })
 
+  it('commit refreshes the managed datastream so its new extent is known', async () => {
+    await seedHistory()
+    const refreshed = { id: 'm-1', phenomenonEndTime: '2025-02-01T00:00:00Z' }
+    getItem.mockImplementation(async (id: string) =>
+      id === 'm-1' ? refreshed : { id: 's-1', name: 'Source' }
+    )
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    await session.commit()
+
+    expect(getItem).toHaveBeenCalledWith('m-1', { expand_related: true })
+    expect(replaceDatastream).toHaveBeenCalledWith(refreshed)
+  })
+
   it('tracks unsaved edits against the last saved snapshot', async () => {
     await seedHistory()
     const { useEditSession } = await import('@/composables/useEditSession')
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index e75fb3b9..86de8fbe 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -16,7 +16,7 @@
 import { ref, computed } from 'vue'
 import { storeToRefs } from 'pinia'
 import { serializeHistory, applyHistory } from '@uwrl/qc-utils'
-import type { Datastream, QualityControlSessionContract } from '@hydroserver/client'
+import type { Datastream, DatastreamExtended, QualityControlSessionContract } from '@hydroserver/client'
 import type { ObservationRecord, HistoryItem } from '@uwrl/qc-utils'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { usePlotlyStore } from '@/store/plotly'
@@ -70,6 +70,7 @@ function clampSpecToSource(
 
 export function useEditSession() {
   const { qcDatastream } = storeToRefs(useDataVisStore())
+  const { replaceDatastream } = useDataVisStore()
   const { selectedSeries } = storeToRefs(usePlotlyStore())
   const { redraw } = usePlotlyStore()
   const { hs } = storeToRefs(useHydroServer())
@@ -321,6 +322,11 @@ export function useEditSession() {
         })
       },
     })
+    // The push moved the managed datastream's phenomenon times.
+    const refreshed = await hs.value.datastreams.getItem(managed.id, {
+      expand_related: true,
+    })
+    if (refreshed) replaceDatastream(refreshed as Datastream & DatastreamExtended)
     await sessionStore.loadSessions(historyId)
     snapshotSavedEdits()
   }
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index 10148eb6..69a3ba33 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -964,3 +964,24 @@ describe('useDataVisStore.releaseManagedDatastream with the source plotted', ()
     expect(store.qcDatastreamId).toBe('src')
   })
 })
+
+describe('useDataVisStore.replaceDatastream', () => {
+  it('swaps the datastream in the catalog and the plotted set', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    store.datastreams = [
+      makeDs({ id: 'a' }),
+      makeDs({ id: 'm', phenomenonEndTime: null }),
+    ] as any
+    store.plottedDatastreams = [makeDs({ id: 'm', phenomenonEndTime: null })] as any
+    store.qcDatastreamId = 'm'
+
+    store.replaceDatastream(
+      makeDs({ id: 'm', phenomenonEndTime: '2021-06-30T12:00:00Z' }) as any
+    )
+
+    expect(store.datastreams.map((d) => d.id)).toEqual(['a', 'm'])
+    expect(store.datastreams[1]?.phenomenonEndTime).toBe('2021-06-30T12:00:00Z')
+    expect(store.qcDatastream?.phenomenonEndTime).toBe('2021-06-30T12:00:00Z')
+  })
+})
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index e361e589..a21e4fea 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -77,6 +77,14 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     datastreams.value = datastreams.value.filter((d) => d.id !== managedId)
   }
 
+  /** Swap in a fresh copy of a datastream wherever it is referenced. */
+  function replaceDatastream(ds: Datastream & DatastreamExtended) {
+    datastreams.value = datastreams.value.map((d) => (d.id === ds.id ? ds : d))
+    plottedDatastreams.value = plottedDatastreams.value.map((d) =>
+      d.id === ds.id ? ds : d
+    )
+  }
+
   /** sourceDatastreamId -> the histories linking its managed datastreams. */
   const historiesBySource = computed(() => {
     const map = new Map<string, QualityControlHistory[]>()
@@ -733,6 +741,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     historiesBySource,
     addQcHistory,
     removeManagedDatastream,
+    replaceDatastream,
     processingLevels,
     observedProperties,
     selectedThings,

From 0ca132920cd5bd7547e5af31f2d5f8919ed2cf69 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 12:03:01 -0600
Subject: [PATCH 30/93] fix: keep a commit successful when the datastream
 refresh fails

---
 .../__tests__/useEditSession.spec.ts          | 46 ++++++++++++++++++-
 apps/qc-app/src/composables/useEditSession.ts | 26 ++++++++---
 2 files changed, 65 insertions(+), 7 deletions(-)

diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index f2e0debc..26bf4ef9 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -29,7 +29,8 @@ vi.mock('@/store/observations', () => ({
 }))
 
 // qc-utils is only used at runtime by the composable (the service layer
-// imports types only), so stub serializeHistory/applyHistory here.
+// imports types only), so stub serializeHistory/applyHistory/Snackbar here.
+const snackbarWarn = vi.fn()
 vi.mock('@uwrl/qc-utils', () => ({
   serializeHistory: vi.fn((record: any, window: any) => ({
     version: '1',
@@ -41,6 +42,7 @@ vi.mock('@uwrl/qc-utils', () => ({
     })),
   })),
   applyHistory: vi.fn(async () => ({ applied: 0, failed: [] })),
+  Snackbar: { warn: snackbarWarn },
 }))
 
 import { useQcSessionStore } from '@/store/qcSession'
@@ -235,6 +237,48 @@ describe('useEditSession', () => {
     expect(replaceDatastream).toHaveBeenCalledWith(refreshed)
   })
 
+  it('commit keeps the session locked even when the datastream refresh throws', async () => {
+    await seedHistory()
+    getItem.mockImplementation(async (id: string) => {
+      if (id === 'm-1') throw new Error('Network error')
+      return { id: 's-1', name: 'Source' }
+    })
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    await session.commit()
+
+    const store = useQcSessionStore()
+    expect(store.committedSessions.length).toBe(1)
+    expect(store.inProgressSession).toBeNull()
+    expect(replaceDatastream).not.toHaveBeenCalled()
+    expect(snackbarWarn).toHaveBeenCalledWith(
+      'Session committed, but the datastream details could not be refreshed. Reload to see its updated time range.'
+    )
+  })
+
+  it('commit keeps the session locked when the datastream refresh returns null', async () => {
+    await seedHistory()
+    getItem.mockImplementation(async (id: string) => {
+      if (id === 'm-1') return null
+      return { id: 's-1', name: 'Source' }
+    })
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    await session.commit()
+
+    const store = useQcSessionStore()
+    expect(store.committedSessions.length).toBe(1)
+    expect(store.inProgressSession).toBeNull()
+    expect(replaceDatastream).not.toHaveBeenCalled()
+    expect(snackbarWarn).toHaveBeenCalledWith(
+      'Session committed, but the datastream details could not be refreshed. Reload to see its updated time range.'
+    )
+  })
+
   it('tracks unsaved edits against the last saved snapshot', async () => {
     await seedHistory()
     const { useEditSession } = await import('@/composables/useEditSession')
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index 86de8fbe..7139baf8 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -15,7 +15,7 @@
 
 import { ref, computed } from 'vue'
 import { storeToRefs } from 'pinia'
-import { serializeHistory, applyHistory } from '@uwrl/qc-utils'
+import { serializeHistory, applyHistory, Snackbar } from '@uwrl/qc-utils'
 import type { Datastream, DatastreamExtended, QualityControlSessionContract } from '@hydroserver/client'
 import type { ObservationRecord, HistoryItem } from '@uwrl/qc-utils'
 import { useDataVisStore } from '@/store/dataVisualization'
@@ -322,13 +322,27 @@ export function useEditSession() {
         })
       },
     })
-    // The push moved the managed datastream's phenomenon times.
-    const refreshed = await hs.value.datastreams.getItem(managed.id, {
-      expand_related: true,
-    })
-    if (refreshed) replaceDatastream(refreshed as Datastream & DatastreamExtended)
     await sessionStore.loadSessions(historyId)
     snapshotSavedEdits()
+    // The push moved the managed datastream's phenomenon times. Refresh last so
+    // a network failure does not abort the commit (session is already locked).
+    try {
+      const refreshed = await hs.value.datastreams.getItem(managed.id, {
+        expand_related: true,
+      })
+      if (!refreshed) {
+        Snackbar.warn(
+          'Session committed, but the datastream details could not be refreshed. Reload to see its updated time range.'
+        )
+      } else {
+        replaceDatastream(refreshed as Datastream & DatastreamExtended)
+      }
+    } catch (error) {
+      console.error('Failed to refresh managed datastream after commit:', error)
+      Snackbar.warn(
+        'Session committed, but the datastream details could not be refreshed. Reload to see its updated time range.'
+      )
+    }
   }
 
   return {

From 19a112ceb2fa8052ed4b5700d3409c17c48aa7bf Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 12:05:58 -0600
Subject: [PATCH 31/93] fix: keep the time range preset when a date field is
 left unchanged

---
 .../VisualizeData/DatePickerField.vue         |  7 +++
 .../__tests__/DatePickerField.spec.ts         | 55 +++++++++++++++++++
 2 files changed, 62 insertions(+)
 create mode 100644 apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts

diff --git a/apps/qc-app/src/components/VisualizeData/DatePickerField.vue b/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
index 4d1935d6..333675ae 100644
--- a/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
+++ b/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
@@ -145,6 +145,13 @@ const emitCombined = (date: Date, time: string) => {
   const s = props.seconds ? (parts[2] || 0) : 0
   const result = new Date(date)
   result.setHours(h, m, s, 0)
+  // Unchanged at the field's resolution, even if the model is finer.
+  if (
+    formatDateStr(result) === formatDateStr(props.modelValue) &&
+    formatTimeStr(result) === formatTimeStr(props.modelValue)
+  ) {
+    return
+  }
   emit('update:modelValue', result)
 }
 
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts
new file mode 100644
index 00000000..429b4f25
--- /dev/null
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts
@@ -0,0 +1,55 @@
+/**
+ * DatePickerField only reports a change when the value it shows changes.
+ * The model can carry seconds the field doesn't display.
+ */
+
+import { mount } from '@vue/test-utils'
+import { describe, it, expect } from 'vitest'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import DatePickerField from '../DatePickerField.vue'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+const mountField = (modelValue: Date) =>
+  mount(DatePickerField, {
+    props: { modelValue, placeholder: 'Start date' },
+    global: {
+      plugins: [createTestVuetify()],
+      // Render the calendar inline so the picker can be driven directly.
+      stubs: { VDialog: { template: '<div><slot /></div>' } },
+    },
+    attachTo: document.body,
+  })
+
+describe('DatePickerField', () => {
+  it('does not emit when the date field is left unchanged', async () => {
+    const wrapper = mountField(new Date(2021, 5, 30, 12, 34, 56, 789))
+    await wrapper.findAll('input')[0]!.trigger('blur')
+    expect(wrapper.emitted('update:modelValue')).toBeUndefined()
+    wrapper.unmount()
+  })
+
+  it('does not emit when the time field is left unchanged', async () => {
+    const wrapper = mountField(new Date(2021, 5, 30, 12, 34, 56, 789))
+    await wrapper.findAll('input')[1]!.trigger('blur')
+    expect(wrapper.emitted('update:modelValue')).toBeUndefined()
+    wrapper.unmount()
+  })
+
+  it('emits the new value when a different day is picked', async () => {
+    const wrapper = mountField(new Date(2021, 5, 30, 12, 34, 56, 789))
+    wrapper
+      .findComponent({ name: 'VDatePicker' })
+      .vm.$emit('update:modelValue', new Date(2021, 6, 4))
+    await wrapper.vm.$nextTick()
+    const emitted = wrapper.emitted('update:modelValue')
+    expect(emitted).toHaveLength(1)
+    expect((emitted![0]![0] as Date).getTime()).toBe(
+      new Date(2021, 6, 4, 12, 34).getTime()
+    )
+    wrapper.unmount()
+  })
+})

From 0e56c951ceab2b1d3911d106379804b2c9dcecae Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 12:11:01 -0600
Subject: [PATCH 32/93] feat: continue with the current workspace from the
 picker

---
 apps/qc-app/docs/USER_GUIDE.md                |  6 +-
 apps/qc-app/e2e/navigation.spec.ts            | 17 ++++
 .../components/Navigation/NavigationRail.vue  |  1 +
 apps/qc-app/src/pages/Workspaces.vue          | 34 +++++--
 .../src/pages/__tests__/Workspaces.spec.ts    | 94 +++++++++++++++++++
 .../src/router/__tests__/nextLocation.spec.ts | 18 ++++
 apps/qc-app/src/router/guards.ts              |  4 +-
 apps/qc-app/src/router/nextLocation.ts        |  7 ++
 8 files changed, 166 insertions(+), 15 deletions(-)
 create mode 100644 apps/qc-app/src/pages/__tests__/Workspaces.spec.ts
 create mode 100644 apps/qc-app/src/router/__tests__/nextLocation.spec.ts
 create mode 100644 apps/qc-app/src/router/nextLocation.ts

diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index b1fb97ad..0ddcec2e 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -44,7 +44,7 @@ Everything runs in your web browser. The backend never sees your edit history un
    expires, QC redirects back to the Data Management login page and returns
    you to the QC page after authentication.
 
-3. On the **Workspaces** page, pick the workspace you want to work in. The choice is remembered locally, so next time you sign in you'll land directly on Home.
+3. On the **Workspaces** page, click **Select** on the workspace you want to work in. The choice is remembered locally, so next time you sign in you'll land directly on Home. When you come back to this page later, the current workspace shows **Continue** to return to the app.
 
    ![Workspaces picker](./images/workspaces.png)
 
@@ -66,7 +66,7 @@ A thin, always-visible column of icons.
 | Cursor (Select) | Top left. Show the datastream Select drawer + plot. |
 | Pencil (Edit) | Top left. Open the Edit view. **Disabled** until you've picked a QC datastream for editing; the tooltip explains why. |
 | Stopwatch (Performance) | Bottom left. Open the Performance Calibration dialog. See "Performance" below. |
-| Grid (Workspace) | Bottom left. Switch workspace. |
+| Briefcase (Workspaces) | Bottom left. Switch workspace, or Continue in the current one. |
 | Logout | Bottom left. Sign out. |
 
 If you click any of these while you have unsaved edits in the Edit view, the app shows an "Unsaved edits" dialog with **Save & continue** / **Discard** / **Cancel**. Discarded edits cannot be recovered.
@@ -603,7 +603,7 @@ See [PERFORMANCE.md](./PERFORMANCE.md) for the envelope details.
 
 ### "I picked the wrong workspace."
 
-Click the grid icon in the nav rail → pick another. If you have unsaved edits, the app asks first.
+Click **Workspaces** in the nav rail and **Select** another, or **Continue** to stay in the current one. If you have unsaved edits, the app asks first.
 
 ## Troubleshooting
 
diff --git a/apps/qc-app/e2e/navigation.spec.ts b/apps/qc-app/e2e/navigation.spec.ts
index eda2d464..db6650c7 100644
--- a/apps/qc-app/e2e/navigation.spec.ts
+++ b/apps/qc-app/e2e/navigation.spec.ts
@@ -16,6 +16,7 @@ import {
   setupEditView,
 } from './support/app'
 import { selectAllPoints } from './support/ops'
+import { WORKSPACE_ID } from './support/fixtures'
 
 test.describe('navigation', () => {
   test.beforeEach(async ({ page }) => {
@@ -51,6 +52,22 @@ test.describe('navigation', () => {
     })
   })
 
+  test('the current workspace offers Continue back to Home', async ({
+    page,
+  }) => {
+    await gotoHome(page)
+    await page.getByTestId('nav-rail-workspaces').click()
+    await expect(page.getByTestId('workspace-current-hint')).toContainText(
+      'E2E Test Workspace'
+    )
+    const button = page.getByTestId(`workspace-pick-${WORKSPACE_ID}`)
+    await expect(button).toHaveText(/Continue/)
+    await button.click({ force: true })
+    await expect(page.getByTestId('datastreams-table')).toBeVisible({
+      timeout: 30_000,
+    })
+  })
+
   test('Edit rail item is disabled until a datastream is plotted', async ({
     page,
   }) => {
diff --git a/apps/qc-app/src/components/Navigation/NavigationRail.vue b/apps/qc-app/src/components/Navigation/NavigationRail.vue
index 027da2d9..40eefea1 100644
--- a/apps/qc-app/src/components/Navigation/NavigationRail.vue
+++ b/apps/qc-app/src/components/Navigation/NavigationRail.vue
@@ -81,6 +81,7 @@
           <button
             v-bind="tipProps"
             class="rail-btn rail-btn-secondary"
+            data-testid="nav-rail-workspaces"
             @click.prevent="guardExit(onSwitchWorkspace)"
           >
             <span class="rail-pill rail-pill-secondary">
diff --git a/apps/qc-app/src/pages/Workspaces.vue b/apps/qc-app/src/pages/Workspaces.vue
index 9bade159..f023fad8 100644
--- a/apps/qc-app/src/pages/Workspaces.vue
+++ b/apps/qc-app/src/pages/Workspaces.vue
@@ -11,6 +11,18 @@
       </div>
     </div>
 
+    <v-alert
+      v-if="selectedWorkspace?.name"
+      data-testid="workspace-current-hint"
+      type="info"
+      variant="tonal"
+      density="compact"
+      class="mb-4"
+    >
+      You're working in <strong>{{ selectedWorkspace.name }}</strong>.
+      Continue, or pick another workspace.
+    </v-alert>
+
     <v-card v-if="isLoading" class="pa-6 text-center">
       <v-progress-circular indeterminate color="primary" size="32" />
       <div class="text-body-small text-medium-emphasis mt-3">
@@ -33,8 +45,8 @@
       <template v-for="(ws, idx) in availableWorkspaces" :key="ws.id">
         <v-list-item
           :title="ws.name"
-          :active="selectedWorkspace?.id === ws.id"
-          :class="{ 'workspace-picker__item--current': selectedWorkspace?.id === ws.id }"
+          :active="isCurrent(ws)"
+          :class="{ 'workspace-picker__item--current': isCurrent(ws) }"
           @click="onPick(ws.id)"
         >
           <template #prepend>
@@ -42,7 +54,7 @@
               :icon="
                 ws.isPrivate ? 'mdi-lock-outline' : 'mdi-earth'
               "
-              :color="selectedWorkspace?.id === ws.id ? 'primary' : undefined"
+              :color="isCurrent(ws) ? 'primary' : undefined"
             />
           </template>
 
@@ -122,13 +134,14 @@
                 </template>
               </v-tooltip>
               <v-btn
+                :data-testid="`workspace-pick-${ws.id}`"
                 size="small"
                 variant="flat"
                 color="primary"
-                :disabled="selectedWorkspace?.id === ws.id"
+                :append-icon="isCurrent(ws) ? 'mdi-arrow-right' : undefined"
                 @click.stop="onPick(ws.id)"
               >
-                {{ selectedWorkspace?.id === ws.id ? 'Selected' : 'Select' }}
+                {{ isCurrent(ws) ? 'Continue' : 'Select' }}
               </v-btn>
             </div>
           </template>
@@ -143,10 +156,11 @@
 import { onMounted, ref } from 'vue'
 import { storeToRefs } from 'pinia'
 import { useRouter, useRoute } from 'vue-router'
-import { Datastream, ResultQualifier } from '@hydroserver/client'
+import { Datastream, ResultQualifier, type Workspace } from '@hydroserver/client'
 import { useWorkspaceStore } from '@/store/workspaces'
 import { useHydroServer } from '@/store/hydroserver'
 import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
+import { nextLocation } from '@/router/nextLocation'
 
 const router = useRouter()
 const route = useRoute()
@@ -155,6 +169,8 @@ const { availableWorkspaces, selectedWorkspace, isLoading } = storeToRefs(store)
 const { hs } = storeToRefs(useHydroServer())
 const { roleName, canEdit } = useWorkspacePermissions()
 
+const isCurrent = (ws: Workspace) => selectedWorkspace.value?.id === ws.id
+
 // One unscoped listing bucketed by workspaceId is cheaper than N
 // scoped listings: server RBAC already filters to visible datastreams.
 const datastreamCounts = ref<Record<string, number>>({})
@@ -237,10 +253,8 @@ onMounted(async () => {
 })
 
 function onPick(id: string) {
-  const picked = store.selectWorkspace(id)
-  if (!picked) return
-  const next = typeof route.query.next === 'string' ? route.query.next : 'Home'
-  router.push({ name: next })
+  if (!store.selectWorkspace(id)) return
+  router.push(nextLocation(route.query.next))
 }
 </script>
 
diff --git a/apps/qc-app/src/pages/__tests__/Workspaces.spec.ts b/apps/qc-app/src/pages/__tests__/Workspaces.spec.ts
new file mode 100644
index 00000000..67e6e201
--- /dev/null
+++ b/apps/qc-app/src/pages/__tests__/Workspaces.spec.ts
@@ -0,0 +1,94 @@
+import { mount, flushPromises } from '@vue/test-utils'
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { createPinia, setActivePinia } from 'pinia'
+import { reactive, ref } from 'vue'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import { useWorkspaceStore } from '@/store/workspaces'
+import Workspaces from '../Workspaces.vue'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+const push = vi.fn()
+const route = reactive<{ query: Record<string, unknown> }>({ query: {} })
+vi.mock('vue-router', () => ({
+  useRouter: () => ({ push }),
+  useRoute: () => route,
+}))
+
+const WS_A = { id: 'ws-a', name: 'Alpha', isPrivate: false, collaboratorRole: null }
+const WS_B = { id: 'ws-b', name: 'Beta', isPrivate: true, collaboratorRole: null }
+const hs = ref<any>({
+  workspaces: { list: vi.fn(async () => ({ ok: true, data: [WS_A, WS_B] })) },
+  datastreams: { list: vi.fn(async () => ({ ok: true, data: [] })) },
+  resultQualifiers: { list: vi.fn(async () => ({ ok: true, data: [] })) },
+})
+vi.mock('@/store/hydroserver', () => ({ useHydroServer: () => ({ hs }) }))
+vi.mock('@/composables/useWorkspacePermissions', () => ({
+  useWorkspacePermissions: () => ({ roleName: () => 'owner', canEdit: () => true }),
+}))
+
+let pinia: ReturnType<typeof createPinia>
+
+beforeEach(() => {
+  pinia = createPinia()
+  setActivePinia(pinia)
+  push.mockClear()
+  route.query = {}
+})
+
+const mountPage = async () => {
+  const wrapper = mount(Workspaces, {
+    global: { plugins: [createTestVuetify(), pinia] },
+  })
+  await flushPromises()
+  return wrapper
+}
+
+const pickButton = (wrapper: any, id: string) =>
+  wrapper.find(`[data-testid="workspace-pick-${id}"]`)
+
+describe('Workspaces page', () => {
+  it('offers Continue on the current workspace and returns to the app', async () => {
+    const store = useWorkspaceStore()
+    store.selectedWorkspace = WS_A as any
+    const wrapper = await mountPage()
+
+    const button = pickButton(wrapper, 'ws-a')
+    expect(button.text()).toContain('Continue')
+    expect(button.attributes('disabled')).toBeUndefined()
+    await button.trigger('click')
+
+    expect(push).toHaveBeenCalledWith({ name: 'Home' })
+    expect(store.selectedWorkspace?.id).toBe('ws-a')
+  })
+
+  it('labels the other workspaces Select', async () => {
+    useWorkspaceStore().selectedWorkspace = WS_A as any
+    const wrapper = await mountPage()
+    expect(pickButton(wrapper, 'ws-b').text()).toBe('Select')
+  })
+
+  it('names the current workspace in a hint', async () => {
+    useWorkspaceStore().selectedWorkspace = WS_A as any
+    const wrapper = await mountPage()
+    expect(
+      wrapper.find('[data-testid="workspace-current-hint"]').text()
+    ).toContain('Alpha')
+  })
+
+  it('shows no hint and no Continue without a selection', async () => {
+    const wrapper = await mountPage()
+    expect(wrapper.find('[data-testid="workspace-current-hint"]').exists()).toBe(false)
+    expect(pickButton(wrapper, 'ws-a').text()).toBe('Select')
+  })
+
+  it('returns to a path given in next', async () => {
+    route.query = { next: '/some/page' }
+    const wrapper = await mountPage()
+    await pickButton(wrapper, 'ws-b').trigger('click')
+    expect(push).toHaveBeenCalledWith({ path: '/some/page' })
+  })
+})
diff --git a/apps/qc-app/src/router/__tests__/nextLocation.spec.ts b/apps/qc-app/src/router/__tests__/nextLocation.spec.ts
new file mode 100644
index 00000000..28b81fbf
--- /dev/null
+++ b/apps/qc-app/src/router/__tests__/nextLocation.spec.ts
@@ -0,0 +1,18 @@
+import { describe, it, expect } from 'vitest'
+import { nextLocation } from '@/router/nextLocation'
+
+describe('nextLocation', () => {
+  it('treats a plain value as a route name', () => {
+    expect(nextLocation('Home')).toEqual({ name: 'Home' })
+  })
+
+  it('treats a leading slash as a path', () => {
+    expect(nextLocation('/some/page')).toEqual({ path: '/some/page' })
+  })
+
+  it('falls back to Home', () => {
+    expect(nextLocation(undefined)).toEqual({ name: 'Home' })
+    expect(nextLocation('')).toEqual({ name: 'Home' })
+    expect(nextLocation(['Home'])).toEqual({ name: 'Home' })
+  })
+})
diff --git a/apps/qc-app/src/router/guards.ts b/apps/qc-app/src/router/guards.ts
index 5131458e..a24bb21b 100644
--- a/apps/qc-app/src/router/guards.ts
+++ b/apps/qc-app/src/router/guards.ts
@@ -4,6 +4,7 @@ import {
   RouteLocationNormalized,
   RouteLocationRaw,
 } from 'vue-router'
+import { nextLocation } from './nextLocation'
 
 type RouteGuardResult = RouteLocationRaw | false | null | undefined | void
 
@@ -62,8 +63,7 @@ export const guards: RouteGuard[] = [
     if (to.query.switch === '1') return null
     const { hasSelection } = useWorkspaceStore()
     if (!hasSelection) return null
-    const next = typeof to.query.next === 'string' ? to.query.next : 'Home'
-    return next.startsWith('/') ? { path: next } : { name: next }
+    return nextLocation(to.query.next)
   },
 
   // hasWorkspaceGuard — every data-bearing route needs an active
diff --git a/apps/qc-app/src/router/nextLocation.ts b/apps/qc-app/src/router/nextLocation.ts
new file mode 100644
index 00000000..55618c49
--- /dev/null
+++ b/apps/qc-app/src/router/nextLocation.ts
@@ -0,0 +1,7 @@
+import type { RouteLocationRaw } from 'vue-router'
+
+/** Where to go after the workspace picker: `next` is a route name or a path. */
+export function nextLocation(next: unknown): RouteLocationRaw {
+  const target = typeof next === 'string' && next ? next : 'Home'
+  return target.startsWith('/') ? { path: target } : { name: target }
+}

From 4965803f1be91f8f89e179626c5279d15b2273fd Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 12:16:23 -0600
Subject: [PATCH 33/93] test: cover plotting old data with the default preset

---
 apps/qc-app/docs/TESTING.md         |  4 +--
 apps/qc-app/e2e/support/fixtures.ts |  3 +-
 apps/qc-app/e2e/support/mocks.ts    | 12 +++++--
 apps/qc-app/e2e/time-range.spec.ts  | 53 +++++++++++++++++++++++++++++
 4 files changed, 66 insertions(+), 6 deletions(-)
 create mode 100644 apps/qc-app/e2e/time-range.spec.ts

diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 8eece680..11ae2ddb 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -12,8 +12,8 @@ the operator's manual; QUALITY.md is the policy.
 
 | Layer      | Runner                          | Where                        | Count    |
 | ---------- | ------------------------------- | ---------------------------- | -------- |
-| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 24 files |
-| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 22 files |
+| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 58 files |
+| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 28 files |
 
 There is no separate "integration" tier — component tests live in the
 unit tier and mount real Vue components with the Vue Test Utils
diff --git a/apps/qc-app/e2e/support/fixtures.ts b/apps/qc-app/e2e/support/fixtures.ts
index 38d03d87..87e2c96d 100644
--- a/apps/qc-app/e2e/support/fixtures.ts
+++ b/apps/qc-app/e2e/support/fixtures.ts
@@ -34,7 +34,8 @@ export const SENSOR_ID_B = 'sensor-temp'
 /**
  * Synthetic-observation timing constants. The catalog's phenomenon times are
  * derived from these, and the app's presets anchor to them, so specs that
- * build custom series should derive timestamps from `FIXTURE_OBS_START_MS`.
+ * build custom series should derive timestamps from `FIXTURE_OBS_START_MS`
+ * or override the catalog times to match (`catalogOverrides`).
  */
 export const FIXTURE_OBS_SPACING_MS = 15 * 60 * 1000
 export const FIXTURE_OBS_COUNT = 120
diff --git a/apps/qc-app/e2e/support/mocks.ts b/apps/qc-app/e2e/support/mocks.ts
index fed7b352..5946ab19 100644
--- a/apps/qc-app/e2e/support/mocks.ts
+++ b/apps/qc-app/e2e/support/mocks.ts
@@ -54,6 +54,11 @@ export interface MockOptions {
     string,
     { phenomenonTime: string[]; result: number[] }
   >
+  /**
+   * Field overrides for catalog datastreams, keyed by id. Lets a spec move a
+   * datastream's phenomenon times to match a custom observation series.
+   */
+  catalogOverrides?: Record<string, Record<string, unknown>>
   /**
    * Accumulates every bulk-create submission the app makes while the
    * mocks are active. Consumers can assert on request ordering /
@@ -116,9 +121,10 @@ export async function installMocks(
   const submissions = options.submissions ?? []
   const withQcHistories = options.qcHistories ?? false
   // The managed datastream only exists for specs that opted into histories.
-  const catalog = withQcHistories
-    ? [...datastreams, managedDatastream]
-    : datastreams
+  const overrides = options.catalogOverrides ?? {}
+  const catalog = (
+    withQcHistories ? [...datastreams, managedDatastream] : datastreams
+  ).map((ds) => ({ ...ds, ...overrides[ds.id] }))
 
   // Match only real HydroServer API calls by pathname. A bare `**/api/**`
   // glob also catches the dev server's own source modules — the QC app
diff --git a/apps/qc-app/e2e/time-range.spec.ts b/apps/qc-app/e2e/time-range.spec.ts
new file mode 100644
index 00000000..4801e7a6
--- /dev/null
+++ b/apps/qc-app/e2e/time-range.spec.ts
@@ -0,0 +1,53 @@
+/**
+ * Time range presets anchor to the plotted data, so a datastream whose
+ * observations are years old plots without widening the range first.
+ */
+
+import { expect, test } from '@playwright/test'
+import { installMocks } from './support/mocks'
+import { gotoHome, plotFirstDatastream } from './support/app'
+import {
+  DATASTREAM_ID,
+  FIXTURE_OBS_COUNT,
+  FIXTURE_OBS_SPACING_MS,
+  buildObservations,
+} from './support/fixtures'
+
+const OLD_START_MS = Date.UTC(2021, 5, 1)
+const OLD_END_MS = OLD_START_MS + (FIXTURE_OBS_COUNT - 1) * FIXTURE_OBS_SPACING_MS
+
+test.describe('time range presets', () => {
+  test.beforeEach(async ({ page }) => {
+    await installMocks(page, {
+      observations: buildObservations(FIXTURE_OBS_COUNT, OLD_START_MS),
+      catalogOverrides: {
+        [DATASTREAM_ID]: {
+          phenomenonBeginTime: new Date(OLD_START_MS).toISOString(),
+          phenomenonEndTime: new Date(OLD_END_MS).toISOString(),
+        },
+      },
+    })
+    await gotoHome(page)
+  })
+
+  test('plots years-old data with the default preset', async ({ page }) => {
+    await plotFirstDatastream(page)
+    await expect(page.locator('.plotted-item__subtitle').first()).toHaveText(
+      `${FIXTURE_OBS_COUNT} pts loaded`
+    )
+    await expect(page.getByTestId('date-preset-1m')).toHaveClass(
+      /v-chip--variant-tonal/
+    )
+    await expect(page.getByTestId('date-preset-custom')).toHaveCount(0)
+  })
+
+  test('leaving a date field untouched keeps the preset', async ({ page }) => {
+    await plotFirstDatastream(page)
+    await page.getByTestId('date-range-from').locator('input').first().click()
+    await page.keyboard.press('Tab')
+    await expect(page.getByTestId('date-preset-custom')).toHaveCount(0)
+    await expect(page.getByTestId('date-preset-1m')).toHaveClass(
+      /v-chip--variant-tonal/
+    )
+  })
+})

From fc9aa6095325c0bca255780979f02a5a498ffd6a Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 12:46:32 -0600
Subject: [PATCH 34/93] fix: freeze the time range while editing and restore
 only real presets

---
 apps/qc-app/docs/API_REFERENCE.md             |  2 +-
 apps/qc-app/docs/USER_GUIDE.md                |  2 +-
 apps/qc-app/e2e/all-operations.spec.ts        |  2 +-
 apps/qc-app/e2e/time-range.spec.ts            |  8 +-
 apps/qc-app/src/components/VisualizeData.vue  |  8 +-
 apps/qc-app/src/composables/useEditSession.ts |  9 +--
 .../store/__tests__/dataVisualization.spec.ts | 81 +++++++++++++++++++
 apps/qc-app/src/store/dataVisualization.ts    | 28 +++++--
 apps/qc-app/src/utils/timeRangePresets.ts     | 13 ++-
 9 files changed, 127 insertions(+), 26 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index d80808c1..769c63f0 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -294,7 +294,7 @@ on boot.
 | `selectedData`                      | state    | `number[] \| null`                                | Index list of the active selection (lasso, box, click). |
 | `hasSelectionShape`                 | state    | `boolean`                                         | True while a box/lasso shape exists, even when it captured zero points. |
 | `loadingStates`                     | state    | `Map<string, boolean>`                            | Per-datastream in-flight observation fetches. |
-| `beginDate` / `endDate`             | state    | `Date`                                            | Active loaded window. A preset re-resolves it from the plotted datastreams' phenomenon times on every plot rebuild; a custom range stays fixed. |
+| `beginDate` / `endDate`             | state    | `Date`                                            | Active loaded window. A preset re-resolves it from the plotted datastreams' phenomenon times on every plot rebuild; a custom range stays fixed. Presets do not re-resolve while the Edit view is active, so plotting/unplotting a datastream there leaves the window alone. |
 | `selectedDateBtnId`                 | state    | `number`                                          | Active preset id (default `1`, 1m); `-1` (`CUSTOM_PRESET_ID`) for a manual range. Presets are defined in `utils/timeRangePresets.ts`. |
 | `matchesSelectedThing`              | action   | `(ds) => boolean`                                 | Filter predicate; exposed so the table can reuse it on row updates. |
 | `matchesSelectedObservedProperty`   | action   | `(ds) => boolean`                                 | Same shape as above. |
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 0ddcec2e..aeb73a81 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -79,7 +79,7 @@ The select view is the default landing surface after picking a workspace. The le
 
 The filter drawer has two collapsible sections:
 
-- **Time range**: the loaded time window for the plotted datastreams. The two date pickers (`From` / `To`) show the window. The preset chips below them, **1w**, **1m**, **6m**, **1y**, **YTD**, **All**, count back from the last observation of the plotted datastreams (the newest one when several are plotted), so a preset always lands on data, even for datastreams whose observations are years old. **1m** is the default, and the last preset you pick is remembered. Adding or removing a datastream re-anchors the preset. A `Custom` chip appears when you change a date in `From` or `To`; a custom window stays fixed while you add or remove datastreams.
+- **Time range**: the loaded time window for the plotted datastreams. The two date pickers (`From` / `To`) show the window. The preset chips below them, **1w**, **1m**, **6m**, **1y**, **YTD**, **All**, count back from the last observation of the plotted datastreams (the newest one when several are plotted), so a preset always lands on data, even for datastreams whose observations are years old. That last observation is the one known when the workspace loaded, so data that arrives later only shows up after a reload. **1m** is the default, and the last preset you pick is remembered. Adding or removing a datastream re-anchors the preset. A `Custom` chip appears when you change a date in `From` or `To`; a custom window stays fixed while you add or remove datastreams. In the Edit view the loaded window is frozen: plotting or removing a datastream there never moves it.
 
 - **Datastream filters**: These filters allow you control the list of datastreams shown in the datastreams table by picking the site, observed property, and/or processing level. The list of matching datastreams updates live in the datastreams table.
 
diff --git a/apps/qc-app/e2e/all-operations.spec.ts b/apps/qc-app/e2e/all-operations.spec.ts
index 0c75c467..ec7db4e2 100644
--- a/apps/qc-app/e2e/all-operations.spec.ts
+++ b/apps/qc-app/e2e/all-operations.spec.ts
@@ -30,7 +30,7 @@ import { FIXTURE_OBS_START_MS } from './support/fixtures'
  */
 function observationsWithGap() {
   // Anchor to FIXTURE_OBS_START_MS (relative to "now") so the series
-  // falls inside the QC app's default time range — a hard-coded literal
+  // falls inside the QC app's default time range. A hard-coded literal
   // would slide out of range as the calendar moves and leave the main
   // plot empty.
   const startMs = FIXTURE_OBS_START_MS
diff --git a/apps/qc-app/e2e/time-range.spec.ts b/apps/qc-app/e2e/time-range.spec.ts
index 4801e7a6..56cf45f7 100644
--- a/apps/qc-app/e2e/time-range.spec.ts
+++ b/apps/qc-app/e2e/time-range.spec.ts
@@ -15,6 +15,12 @@ import {
 
 const OLD_START_MS = Date.UTC(2021, 5, 1)
 const OLD_END_MS = OLD_START_MS + (FIXTURE_OBS_COUNT - 1) * FIXTURE_OBS_SPACING_MS
+// Off a whole-minute boundary: DatePickerField has no seconds field for
+// this control, so a blur reconstructs the date at minute resolution. On
+// a whole-minute end time that reconstruction already matches the raw
+// model value, so the untouched-field test below would pass even without
+// the field's own unchanged-value guard.
+const OLD_END_ISO = new Date(OLD_END_MS + 37_500).toISOString()
 
 test.describe('time range presets', () => {
   test.beforeEach(async ({ page }) => {
@@ -23,7 +29,7 @@ test.describe('time range presets', () => {
       catalogOverrides: {
         [DATASTREAM_ID]: {
           phenomenonBeginTime: new Date(OLD_START_MS).toISOString(),
-          phenomenonEndTime: new Date(OLD_END_MS).toISOString(),
+          phenomenonEndTime: OLD_END_ISO,
         },
       },
     })
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index c97bf14f..1bb8b19c 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -566,7 +566,7 @@ import {
   encodeShareState,
   type ShareState,
 } from '@/utils/share'
-import { CUSTOM_PRESET_ID } from '@/utils/timeRangePresets'
+import { CUSTOM_PRESET_ID, findPreset } from '@/utils/timeRangePresets'
 import { isSnapshotId, parseSnapshotId } from '@/utils/snapshotId'
 import { useHistorySnapshots } from '@/composables/useHistorySnapshots'
 import { useWorkspaceStore } from '@/store/workspaces'
@@ -966,8 +966,10 @@ const hydrateFromUrl = () => {
   const qcId = resolved[0]?.id ?? null
 
   // Set the window BEFORE loading datastreams so the first fetch uses it.
-  // A preset resolves against the plotted data during that load.
-  if (state.datePresetId != null && state.datePresetId >= 0) {
+  // A preset resolves against the plotted data during that load. An
+  // unknown id (a share link built by a newer version, or hand-edited)
+  // is not accepted as a preset.
+  if (state.datePresetId != null && findPreset(state.datePresetId)) {
     selectedDateBtnId.value = state.datePresetId
   } else if (state.beginMs != null || state.endMs != null) {
     selectedDateBtnId.value = CUSTOM_PRESET_ID
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index 7139baf8..f05ec448 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -330,13 +330,8 @@ export function useEditSession() {
       const refreshed = await hs.value.datastreams.getItem(managed.id, {
         expand_related: true,
       })
-      if (!refreshed) {
-        Snackbar.warn(
-          'Session committed, but the datastream details could not be refreshed. Reload to see its updated time range.'
-        )
-      } else {
-        replaceDatastream(refreshed as Datastream & DatastreamExtended)
-      }
+      if (!refreshed) throw new Error('Datastream refresh returned no item')
+      replaceDatastream(refreshed as Datastream & DatastreamExtended)
     } catch (error) {
       console.error('Failed to refresh managed datastream after commit:', error)
       Snackbar.warn(
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index 69a3ba33..129c94e1 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -36,6 +36,14 @@ vi.mock('@/store/observations', () => ({
   }),
 }))
 
+// Only `currentView` matters to the store under test; the real store also
+// wires up qcDatastream/operationParams watchers that don't belong here.
+const mockCurrentView = ref<'Edit' | 'Select'>('Select')
+vi.mock('@/store/userInterface', () => ({
+  useUIStore: () => ({ currentView: mockCurrentView.value }),
+  DrawerType: { Edit: 'Edit', Select: 'Select', None: '' },
+}))
+
 // handleNewPlot touches DOM / Plotly; stub everything the store imports.
 vi.mock('@/utils/plotting/plotly', () => ({
   handleNewPlot: vi.fn().mockResolvedValue(undefined),
@@ -63,6 +71,7 @@ beforeEach(() => {
   vi.clearAllMocks()
   mockPlotlyRef.value = null
   mockGraphSeriesArray.value = []
+  mockCurrentView.value = 'Select'
 })
 
 afterEach(() => {
@@ -519,6 +528,58 @@ describe('useDataVisStore time range presets', () => {
   })
 })
 
+// Plotting/unplotting in the Edit view must not move the edit session's
+// loaded window (finding: preset re-anchoring was reachable from
+// DatastreamInformationCard / PlottedDatastreams in the Edit view too).
+describe('useDataVisStore time range freeze in the Edit view', () => {
+  const OLD_END = '2021-06-30T12:00:00Z'
+  const oldDs = (overrides: Record<string, any> = {}) =>
+    makeDs({
+      id: 'old',
+      phenomenonBeginTime: '2019-01-01T00:00:00Z',
+      phenomenonEndTime: OLD_END,
+      ...overrides,
+    })
+  const newerDs = () =>
+    makeDs({
+      id: 'newer',
+      phenomenonBeginTime: '2020-01-01T00:00:00Z',
+      phenomenonEndTime: '2022-03-01T00:00:00Z',
+    })
+
+  it('leaves beginDate/endDate unchanged when plotting a datastream', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    mockCurrentView.value = 'Edit'
+    const begin = store.beginDate.getTime()
+    const end = store.endDate.getTime()
+    await store.plotDatastream(oldDs() as any)
+    expect(store.beginDate.getTime()).toBe(begin)
+    expect(store.endDate.getTime()).toBe(end)
+  })
+
+  it('leaves beginDate/endDate unchanged when unplotting a datastream', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    store.plottedDatastreams = [oldDs(), newerDs()] as any
+    store.qcDatastreamId = 'old'
+    mockCurrentView.value = 'Edit'
+    const begin = store.beginDate.getTime()
+    const end = store.endDate.getTime()
+    await store.unplotDatastream('newer')
+    expect(store.beginDate.getTime()).toBe(begin)
+    expect(store.endDate.getTime()).toBe(end)
+  })
+
+  it('still re-anchors in the Select view (unaffected by the freeze)', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    mockCurrentView.value = 'Select'
+    await store.plotDatastream(oldDs() as any)
+    expect(store.endDate.toISOString()).toBe('2021-06-30T12:00:00.000Z')
+  })
+})
+
 // The persistence plugin only activates once the pinia is installed on an
 // app, so these go through `app.use(pinia)`.
 describe('useDataVisStore persisted preset', () => {
@@ -546,6 +607,26 @@ describe('useDataVisStore persisted preset', () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     expect(useDataVisStore().selectedDateBtnId).toBe(1)
   })
+
+  it('restores a persisted Custom id (-1) as the default preset', async () => {
+    localStorage.setItem(
+      'dataVisualization',
+      JSON.stringify({ selectedDateBtnId: -1 })
+    )
+    installPinia()
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    expect(useDataVisStore().selectedDateBtnId).toBe(1)
+  })
+
+  it('restores an unknown persisted id as the default preset', async () => {
+    localStorage.setItem(
+      'dataVisualization',
+      JSON.stringify({ selectedDateBtnId: 9 })
+    )
+    installPinia()
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    expect(useDataVisStore().selectedDateBtnId).toBe(1)
+  })
 })
 
 describe('useDataVisStore.setQcDatastream', () => {
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index a21e4fea..f9dfd6e6 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -13,6 +13,7 @@ import {
   presetWindow,
 } from '@/utils/timeRangePresets'
 import { isSnapshotId } from '@/utils/snapshotId'
+import { DrawerType, useUIStore } from '@/store/userInterface'
 import type { SnapshotMeta } from '@/types'
 import type { ObservationRecord } from '@uwrl/qc-utils'
 import {
@@ -472,11 +473,19 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     }
     const { clearZoomHistory } = usePlotlyStore()
     clearZoomHistory()
-    // The plotted set changed, and with it the data a preset anchors to.
-    const presetRange = resolvePresetWindow()
-    if (presetRange) {
-      beginDate.value = presetRange.begin
-      endDate.value = presetRange.end
+    // Presets re-anchor only in the Select view. In the Edit view the
+    // loaded window is the edit session's window; moving it here would
+    // refetch the working copy over a different range and corrupt what
+    // gets committed. `useUIStore()` is called lazily here, not at this
+    // store's setup top level, since userInterface.ts imports this store
+    // back and calling it during setup would recurse into a store that
+    // isn't finished constructing.
+    if (useUIStore().currentView !== DrawerType.Edit) {
+      const presetRange = resolvePresetWindow()
+      if (presetRange) {
+        beginDate.value = presetRange.begin
+        endDate.value = presetRange.end
+      }
     }
     await refreshGraphSeriesArray()
     updateOptions()
@@ -786,5 +795,14 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   // filters refetch cleanly on every load; the window resolves from the data.
   persist: {
     pick: ['selectedDateBtnId'],
+    // A persisted Custom id (or a stale/unknown one) comes back with no
+    // window to resolve against, so the placeholder range would apply
+    // instead. Only a real preset survives hydration.
+    afterHydrate: (ctx) => {
+      const store = ctx.store as unknown as { selectedDateBtnId: number }
+      if (!findPreset(store.selectedDateBtnId)) {
+        store.selectedDateBtnId = DEFAULT_PRESET_ID
+      }
+    },
   },
 })
diff --git a/apps/qc-app/src/utils/timeRangePresets.ts b/apps/qc-app/src/utils/timeRangePresets.ts
index 835649c2..65e3be31 100644
--- a/apps/qc-app/src/utils/timeRangePresets.ts
+++ b/apps/qc-app/src/utils/timeRangePresets.ts
@@ -7,7 +7,6 @@ import { subtractDays, subtractMonths, subtractYears } from '@/utils/dateMath'
 
 export interface TimeRangePreset {
   id: number
-  icon: string
   label: '1w' | '1m' | '6m' | '1y' | 'YTD' | 'All'
   title: string
 }
@@ -23,12 +22,12 @@ interface PhenomenonTimes {
 }
 
 export const TIME_RANGE_PRESETS: readonly TimeRangePreset[] = [
-  { id: 0, icon: 'mdi-calendar-week', label: '1w', title: 'Last week of data' },
-  { id: 1, icon: 'mdi-calendar-month', label: '1m', title: 'Last month of data' },
-  { id: 2, icon: 'mdi-calendar-range', label: '6m', title: 'Last 6 months of data' },
-  { id: 4, icon: 'mdi-calendar', label: '1y', title: 'Last year of data' },
-  { id: 3, icon: 'mdi-calendar-today', label: 'YTD', title: 'Year to date' },
-  { id: 5, icon: 'mdi-infinity', label: 'All', title: 'All data' },
+  { id: 0, label: '1w', title: 'Last week of data' },
+  { id: 1, label: '1m', title: 'Last month of data' },
+  { id: 2, label: '6m', title: 'Last 6 months of data' },
+  { id: 4, label: '1y', title: 'Last year of data' },
+  { id: 3, label: 'YTD', title: 'Year of the last observation' },
+  { id: 5, label: 'All', title: 'All data' },
 ]
 
 export const DEFAULT_PRESET_ID = 1

From 2a1ebc0fa4f23cc41eb2853105f10c5b87366e16 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 13:20:35 -0600
Subject: [PATCH 35/93] fix: stop nesting history row controls inside a button

---
 apps/qc-app/e2e/history.spec.ts               |  4 +-
 .../src/components/EditData/EditHistory.vue   | 60 +++++++++++++------
 .../EditData/__tests__/EditHistory.spec.ts    | 54 ++++++++++++++++-
 3 files changed, 97 insertions(+), 21 deletions(-)

diff --git a/apps/qc-app/e2e/history.spec.ts b/apps/qc-app/e2e/history.spec.ts
index 7392b1ee..4fab41f4 100644
--- a/apps/qc-app/e2e/history.spec.ts
+++ b/apps/qc-app/e2e/history.spec.ts
@@ -55,7 +55,9 @@ test.describe('edit history toolbar', () => {
     page,
   }) => {
     const row = page.getByTestId('history-item-1')
-    await row.getByRole('button', { name: /expand arguments/i }).click()
+    await row
+      .getByRole('button', { name: 'Expand arguments', exact: true })
+      .click()
     await expect(row.getByText('Arguments')).toBeVisible()
   })
 
diff --git a/apps/qc-app/src/components/EditData/EditHistory.vue b/apps/qc-app/src/components/EditData/EditHistory.vue
index 5e8a7a54..41347467 100644
--- a/apps/qc-app/src/components/EditData/EditHistory.vue
+++ b/apps/qc-app/src/components/EditData/EditHistory.vue
@@ -125,16 +125,14 @@
       <SessionList @view="emit('view-session', $event)">
         <template #operations>
           <div class="rounded border bg-surface overflow-hidden">
+      <!-- Row clicks are a mouse shortcut. The step button is the real control,
+           so the row's other buttons are not nested inside a button. -->
       <div
         class="edit-history__row edit-history__row--baseline px-3 py-2 d-flex align-center"
         :class="{ 'edit-history__row--clickable': canStepTo }"
-        :role="canStepTo ? 'button' : undefined"
-        :tabindex="canStepTo ? 0 : undefined"
         data-testid="history-reload-step-baseline"
         :title="canStepTo ? `Reload the session's starting state` : undefined"
         @click="onRowReload(SNAPSHOT_BASELINE_INDEX)"
-        @keydown.enter.prevent="onRowReload(SNAPSHOT_BASELINE_INDEX)"
-        @keydown.space.prevent="onRowReload(SNAPSHOT_BASELINE_INDEX)"
       >
         <v-icon
           :icon="
@@ -146,9 +144,16 @@
           :color="selectedSeries?.data.isLoading ? 'grey' : 'success'"
           class="mr-2"
         />
-        <span class="text-body-small font-weight-medium flex-grow-1 text-truncate">
+        <button
+          type="button"
+          class="edit-history__step text-body-small font-weight-medium flex-grow-1 text-truncate"
+          data-testid="history-step-btn-baseline"
+          :disabled="!canStepTo"
+          :title="canStepTo ? `Reload the session's starting state` : undefined"
+          @click.stop="onRowReload(SNAPSHOT_BASELINE_INDEX)"
+        >
           {{ selectedSeries?.data.isLoading ? 'Loading data…' : 'Data loaded' }}
-        </span>
+        </button>
         <v-chip
           v-if="shownStepIndex === SNAPSHOT_BASELINE_INDEX"
           size="x-small"
@@ -263,16 +268,8 @@
               'edit-history__row--unapplied': !isApplied(index),
               'edit-history__row--clickable': canStepTo,
             }"
-            :role="canStepTo ? 'button' : undefined"
-            :tabindex="canStepTo ? 0 : undefined"
-            :title="
-              isApplied(index)
-                ? 'Reload from this step'
-                : 'Not applied in the step currently shown. Click to reload from here.'
-            "
+            :title="stepTitle(index)"
             @click="onRowReload(index)"
-            @keydown.enter.prevent="onRowReload(index)"
-            @keydown.space.prevent="onRowReload(index)"
           >
             <button
               type="button"
@@ -304,7 +301,14 @@
             />
 
             <!-- Grows as one unit so the badge sits against the title text. -->
-            <div class="edit-history__title flex-grow-1 d-flex align-center ga-1">
+            <button
+              type="button"
+              class="edit-history__step edit-history__title flex-grow-1 d-flex align-center ga-1"
+              :data-testid="`history-step-btn-${index}`"
+              :disabled="!canStepTo || entry.execution?.inFlight"
+              :title="stepTitle(index)"
+              @click.stop="onRowReload(index)"
+            >
               <span class="edit-history__method text-truncate font-weight-medium">
                 {{ formatMethod(entry.method) }}
               </span>
@@ -321,7 +325,7 @@
                   />
                 </template>
               </v-tooltip>
-            </div>
+            </button>
 
             <v-chip
               v-if="shownStepIndex === index"
@@ -646,6 +650,11 @@ function toggle(index: number) {
 const isApplied = (index: number) =>
   shownStepIndex.value === null || index <= shownStepIndex.value
 
+const stepTitle = (index: number) =>
+  isApplied(index)
+    ? 'Reload from this step'
+    : 'Not applied in the step currently shown. Click to reload from here.'
+
 function formatMethod(method: string) {
   if (!method) return ''
   return method
@@ -878,9 +887,21 @@ onBeforeUnmount(() => window.removeEventListener('keydown', onKeydown))
   cursor: pointer;
 }
 
-.edit-history__row--clickable:focus-visible {
+/* Size and weight come from the typography utility classes. */
+.edit-history__step {
+  min-width: 0;
+  padding: 0;
+  border: 0;
+  background: none;
+  color: inherit;
+  font-family: inherit;
+  text-align: start;
+  cursor: inherit;
+}
+
+.edit-history__step:focus-visible {
   outline: 2px solid rgb(var(--v-theme-primary));
-  outline-offset: -2px;
+  outline-offset: 2px;
 }
 
 .edit-history__row--loaded {
@@ -921,6 +942,7 @@ onBeforeUnmount(() => window.removeEventListener('keydown', onKeydown))
 
 .edit-history__title {
   min-width: 0;
+  line-height: inherit;
 }
 
 .edit-history__method {
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
index 8aaad177..4a30240e 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
@@ -658,7 +658,9 @@ describe('EditHistory.vue actions', () => {
 
       const baseline = w.find('[data-testid="history-reload-step-baseline"]')
       expect(baseline.classes()).not.toContain('edit-history__row--clickable')
-      expect(baseline.attributes('role')).toBeUndefined()
+      expect(
+        w.find('[data-testid="history-step-btn-baseline"]').attributes('disabled')
+      ).toBeDefined()
       await baseline.trigger('click')
       expect(reloadHistory).not.toHaveBeenCalled()
     })
@@ -795,6 +797,56 @@ describe('EditHistory.vue actions', () => {
     })
   })
 
+  // A button can't contain other controls, so the row stays a plain click
+  // target and the step name is the control keyboards and screen readers use.
+  describe('step button', () => {
+    it('keeps the row out of the button role', async () => {
+      editHistory.value = [makeEntry('SELECTION')]
+      const w = createWrapper()
+      await flushPromises()
+
+      const row = w.find('[data-testid="history-item-0"] .edit-history__row')
+      expect(row.attributes('role')).toBeUndefined()
+      expect(row.attributes('tabindex')).toBeUndefined()
+      const baseline = w.find('[data-testid="history-reload-step-baseline"]')
+      expect(baseline.attributes('role')).toBeUndefined()
+
+      const step = w.find('[data-testid="history-step-btn-0"]')
+      expect(step.element.tagName).toBe('BUTTON')
+      expect(step.find('button').exists()).toBe(false)
+      expect(step.text()).toBe('Selection')
+    })
+
+    it('replays to its step once', async () => {
+      const history = [makeEntry('SELECTION'), makeEntry('DELETE_POINTS')]
+      editHistory.value = history
+      const reloadHistory = vi.fn(async () => [])
+      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const w = createWrapper()
+      await flushPromises()
+
+      await w.find('[data-testid="history-step-btn-0"]').trigger('click')
+      await vi.waitFor(() => expect(isUpdating.value).toBe(false))
+      await flushPromises()
+      expect(reloadHistory).toHaveBeenCalledTimes(1)
+      expect(reloadHistory).toHaveBeenCalledWith(0)
+    })
+
+    it('is disabled while a dispatch is running', async () => {
+      editHistory.value = [makeEntry('SELECTION')]
+      isUpdating.value = true
+      const w = createWrapper()
+      await flushPromises()
+
+      expect(
+        w.find('[data-testid="history-step-btn-0"]').attributes('disabled')
+      ).toBeDefined()
+      expect(
+        w.find('[data-testid="history-step-btn-baseline"]').attributes('disabled')
+      ).toBeDefined()
+    })
+  })
+
   describe('row click does not swallow its buttons', () => {
     it('expanding the args drawer does not step back', async () => {
       const history = [makeEntry('SELECTION'), makeEntry('DELETE_POINTS')]

From f04141cb5d13f8ee95208701c63967fb0ed7e0b1 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 13:20:35 -0600
Subject: [PATCH 36/93] test: cover QC session save and commit in the submit
 spec

---
 apps/qc-app/docs/TESTING.md         |  13 ++-
 apps/qc-app/e2e/submit.spec.ts      |  74 ++++++++------
 apps/qc-app/e2e/support/app.ts      |  31 +++++-
 apps/qc-app/e2e/support/fixtures.ts |  13 ++-
 apps/qc-app/e2e/support/mocks.ts    | 153 ++++++++++++++++++++++++++--
 5 files changed, 241 insertions(+), 43 deletions(-)

diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 11ae2ddb..70cf6a62 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -228,7 +228,7 @@ The CI gate prints uncovered line numbers per file. Common causes:
 ```
 e2e/
 ├── support/
-│   ├── app.ts        — flow helpers (gotoHome, setupEditView, openOp, waitForSelection)
+│   ├── app.ts        — flow helpers (gotoHome, setupEditView, setupSessionEditView, openOp, waitForSelection)
 │   ├── fixtures.ts   — workspace / datastream / observation fixtures
 │   ├── mocks.ts      — page.route() handlers that stand in for HydroServer
 │   └── ops.ts        — op-specific preambles (selectAllPoints, expectHistoryContains)
@@ -276,6 +276,7 @@ await installMocks(page, {
   submissions: collectedSubmissions,   // accumulates bulk POSTs for assertion
   authenticated: false,                 // simulate signed-out
   qcHistories: true,                    // give DATASTREAM_ID a managed datastream
+  qcSessionState: sessions,             // live QC sessions, for assertions
 })
 ```
 
@@ -285,6 +286,16 @@ that source row shows the managed-count badge and its plot check box opens
 `PlotSourceDialog`. Leaving it off keeps a catalog where every row plots
 straight from its check box, which is what the other specs assume.
 
+The QC session and operation routes are stateful. They start from the
+session fixtures and apply the app's creates, saves, commits and deletes, so
+a spec can pass a `qcSessionState` array and assert on what was persisted
+(see `submit.spec.ts`).
+
+Save and Commit only appear once a session is open. `setupEditView` switches
+to the Edit view through the rail, which opens no session, so those specs see
+New session in the history footer. Use `setupSessionEditView` (with
+`qcHistories: true`) to go through Start editing and start a session first.
+
 ### Fixtures: "now"-anchored timestamps
 
 `support/fixtures.ts` exports a `FIXTURE_OBS_START_MS` anchored to
diff --git a/apps/qc-app/e2e/submit.spec.ts b/apps/qc-app/e2e/submit.spec.ts
index 11162d49..55c2e8f9 100644
--- a/apps/qc-app/e2e/submit.spec.ts
+++ b/apps/qc-app/e2e/submit.spec.ts
@@ -1,47 +1,63 @@
 /**
- * Save + submit flow:
- *   - Save triggers the "Submit QC observations?" confirmation
- *   - Confirming posts to `bulk-create?mode=replace` — the mocks log
- *     the submission so we can assert on its shape
- *   - Success snackbar appears and editHistory is cleared
+ * Committing a QC session:
+ *   - Save persists the session's operations
+ *   - Commit posts the edited observations to the managed datastream with
+ *     `bulk-create?mode=replace` and locks the session, which swaps the
+ *     footer's Save and Commit for New session
  */
 
 import { expect, test } from '@playwright/test'
-import { installMocks } from './support/mocks'
-import { openOp, setupEditView } from './support/app'
+import { installMocks, type MockQcSession } from './support/mocks'
+import { openOp, setupSessionEditView } from './support/app'
 import { expectHistoryContains, selectAllPoints } from './support/ops'
 
-test.describe('submit quality-controlled observations', () => {
-  test('save → confirm → server receives replace-mode payload', async ({
-    page,
-  }) => {
+test.describe('commit a QC session', () => {
+  test('save then commit pushes a replace-mode payload', async ({ page }) => {
     const submissions: Array<{ mode: string | null; body: any }> = []
-    await installMocks(page, { submissions })
-    await setupEditView(page)
+    const sessions: MockQcSession[] = []
+    await installMocks(page, {
+      qcHistories: true,
+      submissions,
+      qcSessionState: sessions,
+    })
+    await setupSessionEditView(page)
+
     await selectAllPoints(page)
     await openOp(page, 'changeValues')
-    await page.getByLabel('Value').fill('1')
-    await page.getByRole('button', { name: 'Apply' }).click()
+    const value = page.getByLabel('Value')
+    await value.fill('1')
+    await value.press('Enter')
     await expectHistoryContains(page, 'Change Values')
 
+    const session = sessions.find((s) => s.status === 'in_progress')
+    expect(session).toBeDefined()
+
     await page.getByTestId('exit-save-btn').click()
-    await expect(page.getByText('Submit QC observations?')).toBeVisible()
-    await page.getByRole('button', { name: 'Submit' }).click()
+    await expect(page.getByText('Draft saved.')).toBeVisible()
+    expect(session!.operations.map((o) => o.operationType)).toContain(
+      'CHANGE_VALUES'
+    )
 
-    await expect(
-      page.getByText('Quality-controlled observations submitted')
-    ).toBeVisible({ timeout: 30_000 })
+    await page.getByTestId('exit-commit-btn').click()
+    const dialog = page
+      .locator('.v-overlay__content')
+      .filter({ hasText: 'Commit session to datastream?' })
+    await expect(dialog).toBeVisible()
+    await dialog.getByLabel('Session description (optional)').fill('E2E pass')
+    await dialog.getByRole('button', { name: 'Commit', exact: true }).click()
 
-    // History is cleared after a successful submit.
-    await expect(page.getByTestId('history-item-0')).toHaveCount(0, {
-      timeout: 15_000,
+    await expect(page.getByText('Session committed.')).toBeVisible({
+      timeout: 30_000,
     })
+    await expect(page.getByTestId('exit-new-session-btn')).toBeVisible()
+
+    expect(submissions).toHaveLength(1)
+    const [submission] = submissions
+    expect(submission!.mode).toBe('replace')
+    expect(submission!.body?.fields).toEqual(['phenomenonTime', 'result'])
+    expect(submission!.body?.data.length).toBeGreaterThan(0)
 
-    // The mocks recorded exactly one bulk-create call in replace mode.
-    expect(submissions.length).toBeGreaterThanOrEqual(1)
-    const first = submissions[0]
-    expect(first.mode).toBe('replace')
-    expect(first.body?.fields).toEqual(['phenomenonTime', 'result'])
-    expect(Array.isArray(first.body?.data)).toBe(true)
+    expect(session!.status).toBe('committed')
+    expect(session!.description).toBe('E2E pass')
   })
 })
diff --git a/apps/qc-app/e2e/support/app.ts b/apps/qc-app/e2e/support/app.ts
index 6e4b2760..d293542e 100644
--- a/apps/qc-app/e2e/support/app.ts
+++ b/apps/qc-app/e2e/support/app.ts
@@ -8,7 +8,11 @@
  */
 
 import { expect, type Page } from '@playwright/test'
-import { DATASTREAM_ID, WORKSPACE_ID } from './fixtures'
+import {
+  DATASTREAM_ID,
+  MANAGED_DATASTREAM_ID,
+  WORKSPACE_ID,
+} from './fixtures'
 
 /** Wait for the datastreams table to become visible on Home. */
 export async function waitForHomeReady(page: Page): Promise<void> {
@@ -128,6 +132,31 @@ export async function setupEditView(page: Page): Promise<void> {
   await expect(page.getByText('Filter Data')).toBeVisible()
 }
 
+/**
+ * Enter the editor through a QC session, which is what puts Save and Commit
+ * in the history footer: plot the raw source from its chooser, Start editing,
+ * and start a new session on the managed datastream. `setupEditView` only
+ * switches views and never opens a session. Needs
+ * `installMocks(page, { qcHistories: true })`.
+ */
+export async function setupSessionEditView(page: Page): Promise<void> {
+  await gotoHome(page)
+  await page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`).click()
+  await page
+    .getByTestId(`plot-option-${DATASTREAM_ID}`)
+    .locator('input')
+    .check()
+  await page.getByTestId('plot-source-apply').click()
+  await page
+    .getByTestId('data-loading-indicator')
+    .waitFor({ state: 'hidden', timeout: 30_000 })
+  await page.getByRole('button', { name: 'Start editing' }).click()
+  await page.getByTestId(`edit-managed-${MANAGED_DATASTREAM_ID}`).click()
+  await expect(page.getByTestId('exit-save-btn')).toBeVisible({
+    timeout: 30_000,
+  })
+}
+
 /** Open an operation panel in the edit drawer by id (see operations.ts). */
 export async function openOp(page: Page, id: string): Promise<void> {
   await page.getByTestId(`op-${id}`).click()
diff --git a/apps/qc-app/e2e/support/fixtures.ts b/apps/qc-app/e2e/support/fixtures.ts
index 87e2c96d..df748256 100644
--- a/apps/qc-app/e2e/support/fixtures.ts
+++ b/apps/qc-app/e2e/support/fixtures.ts
@@ -313,16 +313,25 @@ export const qcHistories = [
   },
 ]
 
+export const QC_SESSION_AUTHOR = {
+  name: 'Test User',
+  email: 'test@example.com',
+}
+export const QC_SOURCE_CHECKSUM = 'e2e-source-checksum'
+
 export const qcSessions = [
   {
     id: 'qcs-e2e-1',
     historyId: QC_HISTORY_ID,
-    status: 'committed',
+    status: 'committed' as const,
     description: 'First pass',
     phenomenonTimeStart: FIXTURE_OBS_START_ISO,
     phenomenonTimeEnd: FIXTURE_OBS_END_ISO,
+    sourceChecksum: QC_SOURCE_CHECKSUM,
     createdAt: FIXTURE_OBS_START_ISO,
-    dependencyIds: [],
+    committedAt: FIXTURE_OBS_START_ISO,
+    createdBy: QC_SESSION_AUTHOR,
+    dependencyIds: [] as string[],
   },
 ]
 
diff --git a/apps/qc-app/e2e/support/mocks.ts b/apps/qc-app/e2e/support/mocks.ts
index 5946ab19..aa972ecb 100644
--- a/apps/qc-app/e2e/support/mocks.ts
+++ b/apps/qc-app/e2e/support/mocks.ts
@@ -26,6 +26,8 @@ import {
   observedProperties,
   managedDatastream,
   processingLevels,
+  QC_SESSION_AUTHOR,
+  QC_SOURCE_CHECKSUM,
   qcHistories,
   qcSessions,
   resultQualifiers,
@@ -71,10 +73,40 @@ export interface MockOptions {
    * where every row plots straight from its check box.
    */
   qcHistories?: boolean
+  /**
+   * Live QC session state, seeded from the session fixtures when
+   * `qcHistories` is on. The handlers create, save, and commit sessions in
+   * it, so pass an array to assert on what the app persisted.
+   */
+  qcSessionState?: MockQcSession[]
   /** Set to false to mark the session as unauthenticated. */
   authenticated?: boolean
 }
 
+export interface MockQcOperation {
+  id: string
+  operationType: string
+  arguments: unknown
+  comment?: string | null
+  order: number
+  createdAt: string
+}
+
+export interface MockQcSession {
+  id: string
+  historyId: string
+  status: 'in_progress' | 'committed'
+  description: string | null
+  phenomenonTimeStart: string
+  phenomenonTimeEnd: string
+  sourceChecksum: string
+  createdAt: string
+  committedAt: string | null
+  createdBy: { name: string; email: string }
+  dependencyIds: string[]
+  operations: MockQcOperation[]
+}
+
 function corsHeaders(route: Route): Record<string, string> {
   // The HydroServer client sends requests with `credentials: 'include'`.
   // Browsers reject `Access-Control-Allow-Origin: *` for credentialed
@@ -120,6 +152,10 @@ export async function installMocks(
   const observationsById = options.observationsById ?? {}
   const submissions = options.submissions ?? []
   const withQcHistories = options.qcHistories ?? false
+  const sessionState = options.qcSessionState ?? []
+  if (withQcHistories) {
+    sessionState.push(...qcSessions.map((s) => ({ ...s, operations: [] })))
+  }
   // The managed datastream only exists for specs that opted into histories.
   const overrides = options.catalogOverrides ?? {}
   const catalog = (
@@ -214,20 +250,15 @@ export async function installMocks(
       return json(route, { data: workspaces })
     }
 
-    // --- Quality-control histories / sessions ---
+    // --- Quality-control histories / sessions / operations ---
     // Served ahead of the datastream routes: the history paths sit under
     // the same `/api/data` prefix and would otherwise fall to the
     // empty-list catch-all.
-    const qcSessionList = path.match(
-      /\/api\/data\/quality-control\/histories\/([^/]+)\/sessions$/
+    const qcSessionRoute = path.match(
+      /\/api\/data\/quality-control\/histories\/([^/]+)\/sessions(?:\/([^/]+)(?:\/(commit|operations)(?:\/([^/]+))?)?)?$/
     )
-    if (qcSessionList && method === 'GET') {
-      const historyId = qcSessionList[1]
-      return json(route, {
-        data: withQcHistories
-          ? qcSessions.filter((s) => s.historyId === historyId)
-          : [],
-      })
+    if (qcSessionRoute) {
+      return handleQcSessions(route, sessionState, qcSessionRoute)
     }
     if (
       path.endsWith('/api/data/quality-control/histories') &&
@@ -277,6 +308,108 @@ export async function installMocks(
   })
 }
 
+/**
+ * Stateful stand-in for the QC session and operation endpoints, covering
+ * what the editor calls to start, save, commit, and delete a session.
+ */
+async function handleQcSessions(
+  route: Route,
+  state: MockQcSession[],
+  [, historyId, sessionId, action, operationId]: RegExpMatchArray
+): Promise<void> {
+  const request = route.request()
+  const method = request.method()
+  const now = new Date().toISOString()
+  const noContent = () =>
+    route.fulfill({ status: 204, headers: corsHeaders(route) })
+
+  if (!sessionId) {
+    if (method === 'GET') {
+      const status = new URL(request.url()).searchParams.get('status')
+      return json(route, {
+        data: state.filter(
+          (s) => s.historyId === historyId && (!status || s.status === status)
+        ),
+      })
+    }
+    if (method === 'POST') {
+      const body = await safeJson(request)
+      const session: MockQcSession = {
+        id: `qcs-e2e-new-${state.length + 1}`,
+        historyId: historyId!,
+        status: 'in_progress',
+        description: body?.description ?? null,
+        phenomenonTimeStart: body?.phenomenonTimeStart,
+        phenomenonTimeEnd: body?.phenomenonTimeEnd,
+        sourceChecksum: QC_SOURCE_CHECKSUM,
+        createdAt: now,
+        committedAt: null,
+        createdBy: QC_SESSION_AUTHOR,
+        dependencyIds: [],
+        operations: [],
+      }
+      state.push(session)
+      return json(route, { data: session }, 201)
+    }
+  }
+
+  const session = state.find(
+    (s) => s.id === sessionId && s.historyId === historyId
+  )
+  if (!session) return json(route, { detail: 'Session not found.' }, 404)
+
+  if (action === 'commit' && method === 'POST') {
+    session.status = 'committed'
+    session.committedAt = now
+    return json(route, { data: session })
+  }
+
+  if (action === 'operations' && !operationId) {
+    if (method === 'GET') return json(route, { data: session.operations })
+    if (method === 'POST') {
+      const bodies = ((await safeJson(request)) ?? []) as Array<
+        Omit<MockQcOperation, 'id' | 'createdAt'>
+      >
+      const created = bodies.map((b) => ({
+        ...b,
+        id: `${session.id}-op-${b.order}`,
+        createdAt: now,
+      }))
+      session.operations.push(...created)
+      return json(route, { data: created }, 201)
+    }
+  }
+
+  if (action === 'operations' && operationId) {
+    const index = session.operations.findIndex((o) => o.id === operationId)
+    if (index < 0) return json(route, { detail: 'Operation not found.' }, 404)
+    if (method === 'PATCH') {
+      const body = await safeJson(request)
+      session.operations[index]!.comment = body?.comment ?? null
+      return json(route, { data: session.operations[index] })
+    }
+    if (method === 'DELETE') {
+      session.operations.splice(index, 1)
+      return noContent()
+    }
+  }
+
+  if (!action) {
+    if (method === 'GET') return json(route, { data: session })
+    if (method === 'PATCH') {
+      const body = await safeJson(request)
+      if (body && 'description' in body) session.description = body.description
+      return json(route, { data: session })
+    }
+    if (method === 'DELETE') {
+      state.splice(state.indexOf(session), 1)
+      return noContent()
+    }
+  }
+
+  return json(route, { detail: `Unmocked ${method} ${pathOf(request.url())}` }, 405)
+}
+
 async function safeJson(request: ReturnType<Page['request']> | any): Promise<any> {
   try {
     return JSON.parse(request.postData() ?? 'null')

From d7ce519aef143a838bf3bfb85a33c070e08d6212 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 13:22:32 -0600
Subject: [PATCH 37/93] fix: edit a copy of the base data instead of the cached
 record

---
 .../__tests__/useEditSession.spec.ts          |  32 ++++-
 .../__tests__/buildSnapshot.spec.ts           |   1 +
 .../__tests__/cloneRecord.spec.ts             |  23 ++++
 .../__tests__/reconstructSession.spec.ts      |  47 ++++++-
 .../qualityControl/__tests__/session.spec.ts  | 123 ++++--------------
 .../services/qualityControl/buildSnapshot.ts  |   3 +-
 .../services/qualityControl/cloneRecord.ts    |  18 +++
 .../src/services/qualityControl/index.ts      |   1 +
 .../qualityControl/reconstructSession.ts      |  13 +-
 .../src/services/qualityControl/session.ts    |  31 ++---
 10 files changed, 168 insertions(+), 124 deletions(-)
 create mode 100644 apps/qc-app/src/services/qualityControl/__tests__/cloneRecord.spec.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/cloneRecord.ts

diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 26bf4ef9..8363bce3 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -30,7 +30,31 @@ vi.mock('@/store/observations', () => ({
 
 // qc-utils is only used at runtime by the composable (the service layer
 // imports types only), so stub serializeHistory/applyHistory/Snackbar here.
-const snackbarWarn = vi.fn()
+// `vi.hoisted` keeps these safe to reference from the hoisted `vi.mock`
+// factory below regardless of how it's shaped.
+const { snackbarWarn, ObservationRecordDouble } = vi.hoisted(() => {
+  // `loadLatestBase`'s default clone constructs a real `ObservationRecord`.
+  // This test double exposes just enough surface for the session layer.
+  class ObservationRecordDouble {
+    dataX: number[]
+    dataY: number[]
+    history: any[] = []
+    redoStack: any[] = []
+    reload = vi.fn(async () => {})
+    // `discardUnsavedEdits` calls this on the working copy; mirror the
+    // truncate-in-place semantics the other test double in this file uses.
+    reloadHistory = vi.fn(async function (this: ObservationRecordDouble, index: number) {
+      this.history.splice(index + 1)
+      return []
+    })
+    constructor({ datetimes, dataValues }: { datetimes: number[]; dataValues: number[] }) {
+      this.dataX = datetimes
+      this.dataY = dataValues
+    }
+  }
+  return { snackbarWarn: vi.fn(), ObservationRecordDouble }
+})
+
 vi.mock('@uwrl/qc-utils', () => ({
   serializeHistory: vi.fn((record: any, window: any) => ({
     version: '1',
@@ -43,6 +67,7 @@ vi.mock('@uwrl/qc-utils', () => ({
   })),
   applyHistory: vi.fn(async () => ({ applied: 0, failed: [] })),
   Snackbar: { warn: snackbarWarn },
+  ObservationRecord: ObservationRecordDouble,
 }))
 
 import { useQcSessionStore } from '@/store/qcSession'
@@ -154,7 +179,10 @@ describe('useEditSession', () => {
     expect(useQcSessionStore().inProgressSession?.description).toBe('Jan')
     // Working copy comes from the managed datastream (latest committed state).
     expect(fetchObservationsInRange.mock.calls[0]?.[0].id).toBe('m-1')
-    expect(selectedSeries.value.data).toEqual(managedBase)
+    expect(Array.from(selectedSeries.value.data.dataX)).toEqual(
+      Array.from(managedBase.dataX)
+    )
+    expect(selectedSeries.value.data).not.toBe(managedBase)
   })
 
   it('startSession clamps the window to the source datastream extent', async () => {
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
index 54d61251..e20734f9 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
@@ -60,6 +60,7 @@ function makeDeps(base = rec()) {
       captured = h
       return { applied: h.operations.length, failed: [] }
     }),
+    cloneRecord: async (r: ObservationRecord) => r,
   } as unknown as ReconstructSessionDeps
 
   return { base, captured: () => captured, deps, fetchInRange }
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/cloneRecord.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/cloneRecord.spec.ts
new file mode 100644
index 00000000..657a789a
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/__tests__/cloneRecord.spec.ts
@@ -0,0 +1,23 @@
+import { describe, it, expect } from 'vitest'
+import { ObservationRecord } from '@uwrl/qc-utils'
+import { cloneRecord } from '../cloneRecord'
+
+describe('cloneRecord', () => {
+  it('copies the data into an independent record', async () => {
+    const original = new ObservationRecord({
+      datetimes: [1, 2, 3],
+      dataValues: [10, 20, 30],
+    })
+    await original.reload()
+
+    const copy = await cloneRecord(original)
+
+    expect(copy).not.toBe(original)
+    expect(Array.from(copy.dataX)).toEqual([1, 2, 3])
+    expect(Array.from(copy.dataY)).toEqual([10, 20, 30])
+    expect(copy.history).toHaveLength(0)
+
+    copy.dataY[0] = 99
+    expect(original.dataY[0]).toBe(10)
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
index 337bfad3..9ef0c0d2 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
@@ -54,7 +54,13 @@ describe('reconstructSession', () => {
     )
 
     const result = await reconstructSession(
-      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      {
+        qcSessions: qc.sessions,
+        qcOperations: qc.operations,
+        fetchInRange,
+        applyHistory,
+        cloneRecord: async (r: ObservationRecord) => r,
+      },
       managed,
       source,
       historyId,
@@ -99,7 +105,13 @@ describe('reconstructSession', () => {
     const applyHistory = vi.fn(async () => ({ applied: 0, failed: [] }))
 
     const result = await reconstructSession(
-      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      {
+        qcSessions: qc.sessions,
+        qcOperations: qc.operations,
+        fetchInRange,
+        applyHistory,
+        cloneRecord: async (r: ObservationRecord) => r,
+      },
       managed,
       source,
       historyId,
@@ -111,6 +123,37 @@ describe('reconstructSession', () => {
     expect(fetchInRange.mock.calls[1][0]).toBe(source)
     expect(result.record).toBe(sourceRec)
   })
+
+  it('replays onto the cloned base, not the fetched record', async () => {
+    const qc = makeQcFake()
+    const historyId = await newHistory(qc)
+    const s = unwrap(
+      await qc.sessions.create(historyId, win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'))
+    )
+    const fetched = rec([Date.UTC(2025, 0, 1)])
+    const copy = rec([Date.UTC(2025, 0, 1)])
+    const applyHistory = vi.fn(async (_record: ObservationRecord, _history: QcHistory) => ({
+      applied: 0,
+      failed: [],
+    }))
+
+    const result = await reconstructSession(
+      {
+        qcSessions: qc.sessions,
+        qcOperations: qc.operations,
+        fetchInRange: vi.fn().mockResolvedValue(fetched),
+        applyHistory,
+        cloneRecord: async () => copy,
+      },
+      managed,
+      source,
+      historyId,
+      s.id
+    )
+
+    expect(result.record).toBe(copy)
+    expect(applyHistory.mock.calls[0]?.[0]).toBe(copy)
+  })
 })
 
 describe('reconstructCommittedSession', () => {
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
index 68d03ff6..a34b06e3 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
@@ -90,119 +90,50 @@ describe('loadLatestBase', () => {
   const source = { id: 's-1' } as unknown as Datastream
   const start = new Date('2025-01-01T00:00:00Z')
   const end = new Date('2025-02-01T00:00:00Z')
-  // Real records always carry `history`/`redoStack`; `loadLatestBase` clears
-  // them, since the store hands back a cached instance.
   const recWith = (n: number) =>
-    ({
-      dataX: Array(n).fill(0),
-      dataY: Array(n).fill(0),
-      history: [],
-      redoStack: [],
-      reload: vi.fn(async () => {}),
-    }) as unknown as ObservationRecord
-
-  it('uses the managed datastream when it has committed data', async () => {
+    ({ dataX: Array(n).fill(0), dataY: Array(n).fill(0), history: [] }) as unknown as ObservationRecord
+  const copyOf = (r: ObservationRecord) => ({ copyOf: r }) as unknown as ObservationRecord
+
+  it('copies the managed datastream window when it has committed data', async () => {
     const managedRec = recWith(3)
     const fetchInRange = vi.fn().mockResolvedValue(managedRec)
-    const result = await loadLatestBase(fetchInRange, managed, source, start, end)
-    expect(result).toBe(managedRec)
+    const clone = vi.fn(async (r: ObservationRecord) => copyOf(r))
+
+    const result = await loadLatestBase(fetchInRange, managed, source, start, end, clone)
+
     expect(fetchInRange).toHaveBeenCalledTimes(1)
     expect(fetchInRange.mock.calls[0][0]).toBe(managed)
+    expect(clone).toHaveBeenCalledWith(managedRec)
+    expect((result as any).copyOf).toBe(managedRec)
   })
 
-  it('falls back to the source when the managed datastream is empty', async () => {
+  it('copies the source window when the managed datastream is empty', async () => {
     const sourceRec = recWith(2)
     const fetchInRange = vi
       .fn()
       .mockResolvedValueOnce(recWith(0))
       .mockResolvedValueOnce(sourceRec)
-    const result = await loadLatestBase(fetchInRange, managed, source, start, end)
-    expect(result).toBe(sourceRec)
-    expect(fetchInRange).toHaveBeenCalledTimes(2)
-    expect(fetchInRange.mock.calls[1][0]).toBe(source)
-  })
-})
-
-describe('loadLatestBase — cached record', () => {
-  // The observation store keeps one record per datastream and returns the
-  // same instance, so a new session's base can arrive carrying the previous
-  // session's replayed operations.
-  const makeRecord = (history: any[], dataX = [1, 2, 3]) => {
-    const reload = vi.fn(async () => {})
-    return {
-      dataX,
-      history,
-      redoStack: [] as any[],
-      reload,
-    } as any
-  }
-
-  it('clears operations left on the cached record and reloads from raw', async () => {
-    const cached = makeRecord([{ method: 'SELECTION' }, { method: 'DELETE_POINTS' }])
-    const fetchInRange = vi.fn().mockResolvedValue(cached)
-
-    const base = await loadLatestBase(
-      fetchInRange,
-      { id: 'm-1' } as any,
-      { id: 's-1' } as any,
-      new Date('2025-01-01T00:00:00Z'),
-      new Date('2025-02-01T00:00:00Z')
-    )
-
-    expect(base.history).toHaveLength(0)
-    expect(base.redoStack).toHaveLength(0)
-    // Data was mutated by the previous session's replay, so raw is restored.
-    expect(cached.reload).toHaveBeenCalled()
-  })
+    const clone = vi.fn(async (r: ObservationRecord) => copyOf(r))
 
-  it('keeps the array reference so bound consumers stay connected', async () => {
-    const history: any[] = [{ method: 'SELECTION' }]
-    const cached = makeRecord(history)
-    const fetchInRange = vi.fn().mockResolvedValue(cached)
+    const result = await loadLatestBase(fetchInRange, managed, source, start, end, clone)
 
-    const base = await loadLatestBase(
-      fetchInRange,
-      { id: 'm-1' } as any,
-      { id: 's-1' } as any,
-      new Date('2025-01-01T00:00:00Z'),
-      new Date('2025-02-01T00:00:00Z')
-    )
-
-    expect(base.history).toBe(history)
-  })
-
-  it('leaves a clean record untouched', async () => {
-    const cached = makeRecord([])
-    const fetchInRange = vi.fn().mockResolvedValue(cached)
-
-    await loadLatestBase(
-      fetchInRange,
-      { id: 'm-1' } as any,
-      { id: 's-1' } as any,
-      new Date('2025-01-01T00:00:00Z'),
-      new Date('2025-02-01T00:00:00Z')
-    )
-
-    expect(cached.reload).not.toHaveBeenCalled()
+    expect(fetchInRange).toHaveBeenCalledTimes(2)
+    expect(fetchInRange.mock.calls[1][0]).toBe(source)
+    expect((result as any).copyOf).toBe(sourceRec)
   })
 
-  it('falls back to the source when the managed datastream is empty', async () => {
-    const empty = makeRecord([], [])
-    const sourceRecord = makeRecord([{ method: 'SELECTION' }])
+  // The store hands back its cached record; editing a copy is what keeps a
+  // plotted raw line free of draft edits.
+  it('never returns the fetched record itself', async () => {
+    const sourceRec = recWith(2)
     const fetchInRange = vi
       .fn()
-      .mockResolvedValueOnce(empty)
-      .mockResolvedValueOnce(sourceRecord)
-
-    const base = await loadLatestBase(
-      fetchInRange,
-      { id: 'm-1' } as any,
-      { id: 's-1' } as any,
-      new Date('2025-01-01T00:00:00Z'),
-      new Date('2025-02-01T00:00:00Z')
-    )
-
-    expect(base).toBe(sourceRecord)
-    expect(base.history).toHaveLength(0)
+      .mockResolvedValueOnce(recWith(0))
+      .mockResolvedValueOnce(sourceRec)
+    const clone = vi.fn(async (r: ObservationRecord) => copyOf(r))
+
+    const result = await loadLatestBase(fetchInRange, managed, source, start, end, clone)
+
+    expect(result).not.toBe(sourceRec)
   })
 })
diff --git a/apps/qc-app/src/services/qualityControl/buildSnapshot.ts b/apps/qc-app/src/services/qualityControl/buildSnapshot.ts
index d03f44b3..032f62cc 100644
--- a/apps/qc-app/src/services/qualityControl/buildSnapshot.ts
+++ b/apps/qc-app/src/services/qualityControl/buildSnapshot.ts
@@ -61,7 +61,8 @@ export async function buildSnapshotRecord(
     managed,
     source,
     new Date(session.phenomenonTimeStart),
-    new Date(session.phenomenonTimeEnd)
+    new Date(session.phenomenonTimeEnd),
+    deps.cloneRecord
   )
 
   const operations =
diff --git a/apps/qc-app/src/services/qualityControl/cloneRecord.ts b/apps/qc-app/src/services/qualityControl/cloneRecord.ts
new file mode 100644
index 00000000..a040eef3
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/cloneRecord.ts
@@ -0,0 +1,18 @@
+import { ObservationRecord } from '@uwrl/qc-utils'
+
+export type CloneRecord = (record: ObservationRecord) => Promise<ObservationRecord>
+
+/**
+ * A standalone record holding a copy of `record`'s current window. The
+ * observation store shares one record per datastream, so edits must happen
+ * on a copy. `dataX`/`dataY` are views over a shared buffer, hence the copy.
+ */
+export const cloneRecord: CloneRecord = async (record) => {
+  const copy = new ObservationRecord({
+    datetimes: Float64Array.from(record.dataX),
+    dataValues: Float32Array.from(record.dataY),
+  })
+  // The constructor starts loading without awaiting it.
+  await copy.reload()
+  return copy
+}
diff --git a/apps/qc-app/src/services/qualityControl/index.ts b/apps/qc-app/src/services/qualityControl/index.ts
index 44146cdd..acd7376f 100644
--- a/apps/qc-app/src/services/qualityControl/index.ts
+++ b/apps/qc-app/src/services/qualityControl/index.ts
@@ -17,3 +17,4 @@ export * from './reconstructSession'
 export * from './findHistory'
 export * from './observationsBody'
 export * from './unwrap'
+export * from './cloneRecord'
diff --git a/apps/qc-app/src/services/qualityControl/reconstructSession.ts b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
index a374c9eb..44776224 100644
--- a/apps/qc-app/src/services/qualityControl/reconstructSession.ts
+++ b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
@@ -26,6 +26,7 @@ import type {
 } from '@uwrl/qc-utils'
 import { unwrap } from './unwrap'
 import { loadLatestBase, type FetchObservationsInRange } from './session'
+import type { CloneRecord } from './cloneRecord'
 
 /** API operation -> qc-utils replayable operation (rename operationType/arguments). */
 const toSerialized = (op: QualityControlOperation): QcHistoryOperation => {
@@ -52,6 +53,7 @@ export interface ReconstructSessionDeps {
     record: ObservationRecord,
     history: QcHistory
   ) => Promise<ApplyHistoryReport>
+  cloneRecord?: CloneRecord
 }
 
 export interface ReconstructSessionResult {
@@ -66,13 +68,20 @@ export async function reconstructSession(
   historyId: string,
   sessionId: string
 ): Promise<ReconstructSessionResult> {
-  const { qcSessions, qcOperations, fetchInRange, applyHistory } = deps
+  const { qcSessions, qcOperations, fetchInRange, applyHistory, cloneRecord } = deps
 
   const session = unwrap(await qcSessions.get(historyId, sessionId))
   const start = new Date(session.phenomenonTimeStart)
   const end = new Date(session.phenomenonTimeEnd)
 
-  const record = await loadLatestBase(fetchInRange, managed, source, start, end)
+  const record = await loadLatestBase(
+    fetchInRange,
+    managed,
+    source,
+    start,
+    end,
+    cloneRecord
+  )
   const ops = unwrap(
     await qcOperations.list(historyId, sessionId, { fetch_all: true })
   )
diff --git a/apps/qc-app/src/services/qualityControl/session.ts b/apps/qc-app/src/services/qualityControl/session.ts
index 67e55180..a0c22314 100644
--- a/apps/qc-app/src/services/qualityControl/session.ts
+++ b/apps/qc-app/src/services/qualityControl/session.ts
@@ -14,6 +14,7 @@ import type {
 } from '@hydroserver/client'
 import type { ObservationRecord } from '@uwrl/qc-utils'
 import { unwrap } from './unwrap'
+import { cloneRecord, type CloneRecord } from './cloneRecord'
 
 type QcSessionDetail = QualityControlSessionContract.DetailResponse
 type QcSessionSummary = QualityControlSessionContract.SummaryResponse
@@ -73,35 +74,23 @@ export async function loadSourceWindow(
 }
 
 /**
- * The latest committed state for a window. Every commit replays its session
- * into the managed datastream (in-range replace), so the managed datastream's
- * observations already carry all previously-committed sessions. Falls back to
- * the raw source when nothing has been committed yet (the first session).
+ * The latest committed state for a window, as a standalone copy. Every commit
+ * replays its session into the managed datastream (in-range replace), so the
+ * managed datastream's observations already carry all previously-committed
+ * sessions. Falls back to the raw source when nothing has been committed yet
+ * (the first session). A copy, so editing it never touches the store's cached
+ * record for either datastream.
  */
 export async function loadLatestBase(
   fetchInRange: FetchObservationsInRange,
   managed: Datastream,
   source: Datastream,
   start: Date,
-  end: Date
+  end: Date,
+  clone: CloneRecord = cloneRecord
 ): Promise<ObservationRecord> {
   const base = await fetchInRange(managed, start, end)
   const record =
     (base.dataX?.length ?? 0) > 0 ? base : await fetchInRange(source, start, end)
-  return resetToStoredState(record)
-}
-
-/**
- * The observation store keeps one `ObservationRecord` per datastream and
- * hands back the same instance, so this may be the one a previous session
- * was editing. Truncated in place to keep bound references.
- */
-async function resetToStoredState(
-  record: ObservationRecord
-): Promise<ObservationRecord> {
-  if (!record.history.length && !record.redoStack.length) return record
-  record.history.length = 0
-  record.redoStack.length = 0
-  await record.reload()
-  return record
+  return clone(record)
 }

From 6d738ee53e1e48d70589a07540479a2cc56f536e Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 13:27:13 -0600
Subject: [PATCH 38/93] feat: add a working copy store for managed datastreams

---
 apps/qc-app/docs/API_REFERENCE.md             |  16 ++
 apps/qc-app/docs/ARCHITECTURE.md              |   1 +
 .../src/store/__tests__/workingCopies.spec.ts | 143 ++++++++++++++++++
 apps/qc-app/src/store/workingCopies.ts        | 116 ++++++++++++++
 4 files changed, 276 insertions(+)
 create mode 100644 apps/qc-app/src/store/__tests__/workingCopies.spec.ts
 create mode 100644 apps/qc-app/src/store/workingCopies.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 769c63f0..7a993856 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -317,6 +317,22 @@ on boot.
 | `releaseManagedDatastream`          | action   | `() => Promise<void>`                             | Inverse of `adoptManagedDatastream`, for leaving the editor: swap the managed datastream back to its source (resolved through `qcHistories`), drop the editor's working copy and rebuild, so the plot shows the source as stored rather than the session's uncommitted edits. Managed datastreams are hidden from the catalog table, so without this the Select view shows a plot with nothing selected. No-op when the QC target isn't managed or its source isn't in the catalog. |
 | `rebuildPlot`                       | action   | `() => Promise<void>`                             | Serialized rebuild (drop zoom history, refresh series, regenerate options, render). Coalesces concurrent callers. |
 
+### `useWorkingCopiesStore()` — `src/store/workingCopies.ts`
+
+One working copy per managed datastream, keyed by its in-progress
+session: the Select-view plot and the editor share it so the preview
+shows exactly what editing opens. Not persisted, not reactive (records
+hold large typed arrays that must not be proxied).
+
+| Name         | Kind   | Type / signature                                                                                     | Notes |
+|--------------|--------|-------------------------------------------------------------------------------------------------------|-------|
+| `get`        | action | `(managedId: string) => WorkingCopy \| undefined`                                                     | Current cached copy for a managed datastream, if any. |
+| `load`       | action | `(managed: Datastream, source: Datastream, historyId: string) => Promise<WorkingCopy \| null>`        | Returns the cached copy when it matches the history's in-progress session; rebuilds and caches otherwise; `null` (and evicts any stale cache entry) when the history has no in-progress session. |
+| `rebuild`    | action | `(managed: Datastream, source: Datastream, historyId: string, session: SessionWindow) => Promise<WorkingCopy>` | Always reconstructs from the session window and replays its operations, even when a cached copy already matches; overwrites the cache. |
+| `set`        | action | `(managedId: string, sessionId: string, record: ObservationRecord, begin: Date, end: Date) => void`   | Insert or replace a cache entry directly (used by the editor after a local edit). |
+| `invalidate` | action | `(managedId: string) => void`                                                                          | Drop a managed datastream's cached copy. |
+| `extents`    | action | `(managedIds: string[]) => { phenomenonBeginTime: string; phenomenonEndTime: string }[]`               | The cached copies' windows for the given managed ids, in ISO form; ids with no cached copy are omitted. |
+
 ### `usePlotlyStore()` — `src/store/plotly.ts`
 
 Owns the Plotly DOM ref, the per-series array driving the chart,
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index ba3b5503..e08b8a12 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -134,6 +134,7 @@ goes one direction.
 | `userInterface.ts`    | Drawer state (Select/Edit), persisted prefs, current view.            |
 | `operationParams.ts`  | Per-operation form inputs, persisted so they survive panel re-opens.  |
 | `uiLayout.ts`         | Drawer widths, table heights — persisted UI geometry.                 |
+| `workingCopies.ts`    | Working copy per managed datastream, keyed by its in-progress session.|
 
 The persisted stores use `pinia-plugin-persistedstate` with explicit
 `storage: localStorage` and an explicit `paths` list. **Never** persist
diff --git a/apps/qc-app/src/store/__tests__/workingCopies.spec.ts b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
new file mode 100644
index 00000000..ea00d7ee
--- /dev/null
+++ b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
@@ -0,0 +1,143 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { createPinia, setActivePinia } from 'pinia'
+import { ref } from 'vue'
+import { makeQcFake } from '@/services/qualityControl/__tests__/qcServiceFake'
+import { unwrap } from '@/services/qualityControl/unwrap'
+
+const hs = ref<any>(null)
+vi.mock('@/store/hydroserver', () => ({ useHydroServer: () => ({ hs }) }))
+
+const fetchObservationsInRange = vi.fn()
+vi.mock('@/store/observations', () => ({
+  useObservationStore: () => ({ fetchObservationsInRange }),
+}))
+
+const applyHistory = vi.fn(async () => ({ applied: 0, failed: [] }))
+vi.mock('@uwrl/qc-utils', () => ({
+  applyHistory: (...args: any[]) => applyHistory(...(args as [])),
+  ObservationRecord: class {
+    dataX: number[]
+    dataY: number[]
+    history: any[] = []
+    redoStack: any[] = []
+    constructor(d: { datetimes: ArrayLike<number>; dataValues: ArrayLike<number> }) {
+      this.dataX = Array.from(d.datetimes)
+      this.dataY = Array.from(d.dataValues)
+    }
+    async reload() {}
+  },
+}))
+
+const managed = { id: 'm-1' } as any
+const source = { id: 's-1' } as any
+const rec = (xs: number[]) => ({ dataX: xs, dataY: xs.map(() => 1), history: [] })
+
+let qc: ReturnType<typeof makeQcFake>
+let historyId: string
+
+beforeEach(async () => {
+  setActivePinia(createPinia())
+  vi.clearAllMocks()
+  qc = makeQcFake()
+  hs.value = {
+    qualityControlSessions: qc.sessions,
+    qualityControlOperations: qc.operations,
+  }
+  historyId = unwrap(
+    await qc.histories.create({ managedDatastreamId: 'm-1', sourceDatastreamId: 's-1' })
+  ).id
+  // Managed datastream empty, source has data.
+  fetchObservationsInRange.mockImplementation(async (ds: any) =>
+    ds.id === 'm-1' ? rec([]) : rec([1, 2, 3])
+  )
+})
+
+const startSession = () =>
+  qc.sessions.create(historyId, {
+    phenomenonTimeStart: '2025-01-01T00:00:00Z',
+    phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+  })
+
+describe('useWorkingCopiesStore', () => {
+  it('returns null and caches nothing without an in-progress session', async () => {
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+    expect(await store.load(managed, source, historyId)).toBeNull()
+    expect(store.get('m-1')).toBeUndefined()
+  })
+
+  it('builds the working copy from the session window and replays its operations', async () => {
+    const session = unwrap(await startSession())
+    await qc.operations.create(historyId, session.id, [
+      { operationType: 'SELECTION' as any, order: 0 },
+    ])
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const copy = await store.load(managed, source, historyId)
+
+    expect(copy?.sessionId).toBe(session.id)
+    expect(copy?.begin.toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect(copy?.end.toISOString()).toBe('2025-02-01T00:00:00.000Z')
+    expect(Array.from(copy!.record.dataX)).toEqual([1, 2, 3])
+    expect(applyHistory).toHaveBeenCalledTimes(1)
+    expect((applyHistory.mock.calls[0] as any[])[0]).toBe(copy!.record)
+    expect(store.get('m-1')).toBe(copy)
+  })
+
+  it('reuses the cached copy for the same session', async () => {
+    await startSession()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const first = await store.load(managed, source, historyId)
+    const second = await store.load(managed, source, historyId)
+
+    expect(second).toBe(first)
+    expect(applyHistory).toHaveBeenCalledTimes(1)
+  })
+
+  it('drops a cached copy once its session is no longer in progress', async () => {
+    const session = unwrap(await startSession())
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+    await store.load(managed, source, historyId)
+
+    await qc.sessions.commit(historyId, session.id)
+
+    expect(await store.load(managed, source, historyId)).toBeNull()
+    expect(store.get('m-1')).toBeUndefined()
+  })
+
+  it('rebuild always reconstructs, even for a cached session', async () => {
+    const session = unwrap(await startSession())
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+    const first = await store.load(managed, source, historyId)
+
+    const rebuilt = await store.rebuild(managed, source, historyId, session)
+
+    expect(rebuilt).not.toBe(first)
+    expect(store.get('m-1')).toBe(rebuilt)
+    expect(applyHistory).toHaveBeenCalledTimes(2)
+  })
+
+  it('set, invalidate and extents manage entries by managed id', async () => {
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+    const record = rec([1]) as any
+    store.set('m-1', 's-9', record, new Date('2024-03-01T00:00:00Z'), new Date('2024-04-01T00:00:00Z'))
+
+    expect(store.get('m-1')?.record).toBe(record)
+    expect(store.extents(['m-1', 'other'])).toEqual([
+      {
+        phenomenonBeginTime: '2024-03-01T00:00:00.000Z',
+        phenomenonEndTime: '2024-04-01T00:00:00.000Z',
+      },
+    ])
+
+    store.invalidate('m-1')
+    expect(store.get('m-1')).toBeUndefined()
+    expect(store.extents(['m-1'])).toEqual([])
+  })
+})
diff --git a/apps/qc-app/src/store/workingCopies.ts b/apps/qc-app/src/store/workingCopies.ts
new file mode 100644
index 00000000..cef406df
--- /dev/null
+++ b/apps/qc-app/src/store/workingCopies.ts
@@ -0,0 +1,116 @@
+/**
+ * One editable working copy per managed datastream: its in-progress
+ * session's latest committed base (or raw source) with the saved draft
+ * operations replayed. The Select-view plot and the editor share it, so the
+ * preview shows exactly what editing opens.
+ */
+
+import { defineStore, storeToRefs } from 'pinia'
+import { applyHistory } from '@uwrl/qc-utils'
+import type { ObservationRecord } from '@uwrl/qc-utils'
+import type { Datastream } from '@hydroserver/client'
+import { useHydroServer } from '@/store/hydroserver'
+import { useObservationStore } from '@/store/observations'
+import {
+  getInProgressSession,
+  reconstructSession,
+} from '@/services/qualityControl'
+
+export interface WorkingCopy {
+  sessionId: string
+  record: ObservationRecord
+  begin: Date
+  end: Date
+}
+
+export type SessionWindow = {
+  id: string
+  phenomenonTimeStart: string
+  phenomenonTimeEnd: string
+}
+
+export const useWorkingCopiesStore = defineStore('workingCopies', () => {
+  // Not reactive: records hold large typed arrays that must not be proxied.
+  const copies = new Map<string, WorkingCopy>()
+
+  const get = (managedId: string) => copies.get(managedId)
+
+  function set(
+    managedId: string,
+    sessionId: string,
+    record: ObservationRecord,
+    begin: Date,
+    end: Date
+  ) {
+    copies.set(managedId, { sessionId, record, begin, end })
+  }
+
+  function invalidate(managedId: string) {
+    copies.delete(managedId)
+  }
+
+  async function rebuild(
+    managed: Datastream,
+    source: Datastream,
+    historyId: string,
+    session: SessionWindow
+  ): Promise<WorkingCopy> {
+    const { hs } = storeToRefs(useHydroServer())
+    const { fetchObservationsInRange } = useObservationStore()
+    const { record } = await reconstructSession(
+      {
+        qcSessions: hs.value.qualityControlSessions,
+        qcOperations: hs.value.qualityControlOperations,
+        fetchInRange: fetchObservationsInRange,
+        applyHistory,
+      },
+      managed,
+      source,
+      historyId,
+      session.id
+    )
+    const copy: WorkingCopy = {
+      sessionId: session.id,
+      record,
+      begin: new Date(session.phenomenonTimeStart),
+      end: new Date(session.phenomenonTimeEnd),
+    }
+    copies.set(managed.id, copy)
+    return copy
+  }
+
+  async function load(
+    managed: Datastream,
+    source: Datastream,
+    historyId: string
+  ): Promise<WorkingCopy | null> {
+    const { hs } = storeToRefs(useHydroServer())
+    const session = await getInProgressSession(
+      hs.value.qualityControlSessions,
+      historyId
+    )
+    if (!session) {
+      copies.delete(managed.id)
+      return null
+    }
+    const cached = copies.get(managed.id)
+    if (cached?.sessionId === session.id) return cached
+    return rebuild(managed, source, historyId, session)
+  }
+
+  function extents(managedIds: string[]) {
+    return managedIds.flatMap((id) => {
+      const copy = copies.get(id)
+      return copy
+        ? [
+            {
+              phenomenonBeginTime: copy.begin.toISOString(),
+              phenomenonEndTime: copy.end.toISOString(),
+            },
+          ]
+        : []
+    })
+  }
+
+  return { get, set, invalidate, load, rebuild, extents }
+})

From c145dfb5c3d6acbe5a16893ad9b2287af0ef631c Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 13:33:59 -0600
Subject: [PATCH 39/93] fix: replay committed sessions onto a copy and keep the
 resume test green

---
 apps/qc-app/docs/ARCHITECTURE.md              |  6 +++-
 .../__tests__/useEditSession.spec.ts          | 30 +++++++++++++-----
 .../__tests__/reconstructSession.spec.ts      | 31 +++++++++++++++++++
 .../qualityControl/reconstructSession.ts      | 15 +++++++--
 4 files changed, 71 insertions(+), 11 deletions(-)

diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index e08b8a12..0a077ded 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -270,7 +270,11 @@ Two contract notes worth keeping in mind:
   committed session. `startSession`/`reconstructSession` therefore load the
   managed datastream as the working base (via `loadLatestBase`, falling back to
   the source only when nothing has been committed yet) and replay just the
-  current session's own draft operations on top.
+  current session's own draft operations on top. `loadLatestBase` hands back a
+  standalone copy (`cloneRecord`), not the observation store's cached record,
+  so editing, resuming, and viewing a committed session never modify the
+  store's cached records. The raw line stays unedited regardless of what a
+  session does to its working copy.
 - **A reload resumes from the last save, not from memory.** Only
   `qcSession.resumeDatastreamId` is persisted; on load the editor replots that
   datastream and re-runs `beginEditing`, which rebuilds the working copy from
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 8363bce3..90e1d7d5 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest'
-import { ref } from 'vue'
+import { ref, toRaw } from 'vue'
 import { createPinia, setActivePinia } from 'pinia'
 import { makeQcFake } from '@/services/qualityControl/__tests__/qcServiceFake'
 import { unwrap } from '@/services/qualityControl/unwrap'
@@ -65,7 +65,10 @@ vi.mock('@uwrl/qc-utils', () => ({
       args: h.args ?? [],
     })),
   })),
-  applyHistory: vi.fn(async () => ({ applied: 0, failed: [] })),
+  applyHistory: vi.fn(async (_record?: unknown, _history?: unknown) => ({
+    applied: 0,
+    failed: [],
+  })),
   Snackbar: { warn: snackbarWarn },
   ObservationRecord: ObservationRecordDouble,
 }))
@@ -158,10 +161,19 @@ describe('useEditSession', () => {
     const { beginEditing, needsSession } = useEditSession()
     await beginEditing()
     expect(needsSession.value).toBe(false)
-    expect(qcUtils.applyHistory).toHaveBeenCalled()
-    // The QC-target series now shows the reconstructed session, not the
-    // empty managed datastream.
-    expect(selectedSeries.value.data).toEqual(reconstructed)
+    // The QC-target series shows the reconstructed session's data, but on
+    // its own copy: resume never hands out the fetched record itself, so
+    // editing it can't mutate the store's cached one.
+    const seriesRecord = selectedSeries.value.data
+    expect(Array.from(seriesRecord.dataX)).toEqual(Array.from(reconstructed.dataX))
+    expect(seriesRecord).not.toBe(reconstructed)
+    // applyHistory replayed onto that same copy, not the fetched record.
+    // `toRaw` unwraps the reactive proxy Vue puts on the assigned object,
+    // since applyHistory was called with the pre-proxy instance.
+    const applyHistoryMock = qcUtils.applyHistory as unknown as {
+      mock: { calls: unknown[][] }
+    }
+    expect(applyHistoryMock.mock.calls[0]?.[0]).toBe(toRaw(seriesRecord))
     // Nothing watches for a swapped-in record, so resume has to rebuild the
     // plot itself or it keeps rendering the pre-reconstruction trace.
     expect(redraw).toHaveBeenCalled()
@@ -457,7 +469,11 @@ describe('useEditSession.viewSession', () => {
 
     await session.viewSession(committed.id)
 
-    expect(selectedSeries.value.data).toEqual(viewed)
+    // The panel shows the viewed session's data, but on its own copy: a
+    // read-only view must not replay onto the source's cached record.
+    const seriesRecord = selectedSeries.value.data
+    expect(Array.from(seriesRecord.dataX)).toEqual(Array.from(viewed.dataX))
+    expect(toRaw(seriesRecord)).not.toBe(viewed)
     expect(store.viewedSessionId).toBe(committed.id)
     expect(store.isReadOnly).toBe(true)
     expect(redraw).toHaveBeenCalled()
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
index 9ef0c0d2..135469e0 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
@@ -342,6 +342,37 @@ describe('reconstructCommittedSession', () => {
       'INTERPOLATE',
     ])
   })
+
+  it('replays onto a copy of the fetched record, not the record itself', async () => {
+    const qc = makeQcFake()
+    const { historyId, second } = await chainOf(qc)
+    const fetched = rec([Date.UTC(2025, 0, 1)])
+    const copy = rec([Date.UTC(2025, 0, 1)])
+    const fetchInRange = vi.fn().mockResolvedValue(fetched)
+    const applyHistory = vi.fn(async (_record: ObservationRecord, _history: QcHistory) => ({
+      applied: 0,
+      failed: [],
+    }))
+    const { reconstructCommittedSession } = await import('../reconstructSession')
+
+    const { record } = await reconstructCommittedSession(
+      {
+        qcSessions: qc.sessions,
+        qcOperations: qc.operations,
+        fetchInRange,
+        applyHistory,
+        cloneRecord: async () => copy,
+      },
+      source,
+      historyId,
+      second.id
+    )
+
+    expect(record).toBe(copy)
+    expect(applyHistory.mock.calls[0]?.[0]).toBe(copy)
+    expect(record).not.toBe(fetched)
+    expect(fetched.history).toHaveLength(0)
+  })
 })
 
 describe('reconstructCommittedSession — chain order', () => {
diff --git a/apps/qc-app/src/services/qualityControl/reconstructSession.ts b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
index 44776224..14a8cbb9 100644
--- a/apps/qc-app/src/services/qualityControl/reconstructSession.ts
+++ b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
@@ -26,7 +26,7 @@ import type {
 } from '@uwrl/qc-utils'
 import { unwrap } from './unwrap'
 import { loadLatestBase, type FetchObservationsInRange } from './session'
-import type { CloneRecord } from './cloneRecord'
+import { cloneRecord as defaultCloneRecord, type CloneRecord } from './cloneRecord'
 
 /** API operation -> qc-utils replayable operation (rename operationType/arguments). */
 const toSerialized = (op: QualityControlOperation): QcHistoryOperation => {
@@ -108,6 +108,8 @@ export async function reconstructSession(
  * Ordered by `committedAt`, since commit order is what a later session
  * built on. Loaded over the union of the chain's windows: operations replay
  * against array indices, so a narrower base misaligns a wider ancestor.
+ * Replays onto a copy of the fetched record, so viewing a committed session
+ * never mutates the source datastream's cached record.
  */
 export async function reconstructCommittedSession(
   deps: ReconstructSessionDeps,
@@ -116,7 +118,13 @@ export async function reconstructCommittedSession(
   sessionId: string,
   opLimit?: number
 ): Promise<ReconstructSessionResult> {
-  const { qcSessions, qcOperations, fetchInRange, applyHistory } = deps
+  const {
+    qcSessions,
+    qcOperations,
+    fetchInRange,
+    applyHistory,
+    cloneRecord = defaultCloneRecord,
+  } = deps
 
   const session = unwrap(await qcSessions.get(historyId, sessionId))
   const ancestors = unwrap(
@@ -138,7 +146,8 @@ export async function reconstructCommittedSession(
     ...chain.map((s) => new Date(s.phenomenonTimeEnd).getTime())
   )
 
-  const record = await fetchInRange(source, new Date(startMs), new Date(endMs))
+  const fetched = await fetchInRange(source, new Date(startMs), new Date(endMs))
+  const record = await cloneRecord(fetched)
 
   const perSession = await Promise.all(
     chain.map(async (s) =>

From a65996f694c034f84488e1bd9d0325973b98a46b Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 13:39:26 -0600
Subject: [PATCH 40/93] test: drain undo timer and auto-unmount in EditHistory
 spec

---
 apps/qc-app/docs/TESTING.md                      | 16 +++++++++++++++-
 .../EditData/__tests__/EditHistory.spec.ts       | 12 ++++++++++--
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 70cf6a62..9ea77297 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -146,6 +146,17 @@ undefined (reading 'value')` from inside a `setTimeout` (the
    via a `resetStoreState` helper. Drift across describe blocks is
    the most common cause of order-dependent failures.
 
+6. **Don't let a test outlive its own async work.** Components that
+   defer through `setTimeout` (EditHistory's undo, redo and reload)
+   read the shared mock refs when the timer fires. If a test ends
+   before that, the timer runs in the next test against its mocks and
+   surfaces as an unhandled rejection blamed on the wrong test. Wait
+   for the settle signal (`isUpdating` back to `false`) or drain with
+   `vi.runAllTimersAsync()` under fake timers. Specs for components
+   that listen on `window` should also call
+   `enableAutoUnmount(afterEach)`, or wrappers from earlier tests keep
+   handling the events later tests dispatch.
+
 ### Coverage thresholds
 
 Configured in [`vite.config.ts:130-140`](../vite.config.ts). The
@@ -258,7 +269,10 @@ test.describe('edit: delete points', () => {
 Everything except `qc-golden-path.spec.ts` happens against the mocked
 backend. The live golden-path spec is gated by `E2E_LIVE=1`, expects both
 frontends to be running, and enters QC through the Data Management
-same-origin entrypoint.
+same-origin entrypoint. It edits through a QC session, so the workspace
+needs a source datastream with a managed datastream and QC history: the
+spec plots the first source showing the managed-count badge, clicks Start
+editing, and ends by saving and committing the session.
 
 ### Mocks
 
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
index 4a30240e..29ed5ced 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
@@ -1,8 +1,12 @@
-import { mount, flushPromises } from '@vue/test-utils'
+import { mount, flushPromises, enableAutoUnmount } from '@vue/test-utils'
 import { ref } from 'vue'
-import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
 import { createTestPinia } from '@/utils/test/pinia'
 import { createTestVuetify } from '@/utils/test/vuetify'
+
+// EditHistory listens for Ctrl+Z/Y on window. A wrapper left mounted keeps
+// handling shortcuts dispatched by later tests.
+enableAutoUnmount(afterEach)
 // The per-operation comment textarea (Vuetify auto-grow) observes resizes.
 ;(globalThis as any).ResizeObserver ||= class {
   observe() {}
@@ -602,6 +606,10 @@ describe('EditHistory.vue actions', () => {
         expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(false)
       )
       expect(w.find('[data-testid="history-loaded-1"]').exists()).toBe(true)
+      // The marker clears before undo's deferred replay runs. Drain it here or
+      // it fires in the next test against a series with no `undo`.
+      await vi.waitFor(() => expect(isUpdating.value).toBe(false))
+      expect(undo).toHaveBeenCalled()
     })
 
     // The baseline row is step -1: the state the session started from,

From b552f13c6b6a311624d72e204e7a84e812f71541 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 13:41:20 -0600
Subject: [PATCH 41/93] fix: guard working copy builds against stale and
 concurrent loads

---
 apps/qc-app/docs/API_REFERENCE.md             | 15 +++-
 .../src/store/__tests__/workingCopies.spec.ts | 79 +++++++++++++++++++
 apps/qc-app/src/store/workingCopies.ts        | 77 ++++++++++++++++--
 3 files changed, 161 insertions(+), 10 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 7a993856..b6b3d7f6 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -324,13 +324,20 @@ session: the Select-view plot and the editor share it so the preview
 shows exactly what editing opens. Not persisted, not reactive (records
 hold large typed arrays that must not be proxied).
 
+Concurrency: a per-managed-id generation counter, bumped by `invalidate`,
+`set`, and every new build, guards each build's cache write — a build
+whose generation is no longer current (superseded by a later
+`invalidate`/`set`/`rebuild` while it awaited) is discarded rather than
+resurrecting a stale copy. Concurrent `load()` calls for the same managed
+id share one in-flight build and resolve to the same record.
+
 | Name         | Kind   | Type / signature                                                                                     | Notes |
 |--------------|--------|-------------------------------------------------------------------------------------------------------|-------|
 | `get`        | action | `(managedId: string) => WorkingCopy \| undefined`                                                     | Current cached copy for a managed datastream, if any. |
-| `load`       | action | `(managed: Datastream, source: Datastream, historyId: string) => Promise<WorkingCopy \| null>`        | Returns the cached copy when it matches the history's in-progress session; rebuilds and caches otherwise; `null` (and evicts any stale cache entry) when the history has no in-progress session. |
-| `rebuild`    | action | `(managed: Datastream, source: Datastream, historyId: string, session: SessionWindow) => Promise<WorkingCopy>` | Always reconstructs from the session window and replays its operations, even when a cached copy already matches; overwrites the cache. |
-| `set`        | action | `(managedId: string, sessionId: string, record: ObservationRecord, begin: Date, end: Date) => void`   | Insert or replace a cache entry directly (used by the editor after a local edit). |
-| `invalidate` | action | `(managedId: string) => void`                                                                          | Drop a managed datastream's cached copy. |
+| `load`       | action | `(managed: Datastream, source: Datastream, historyId: string) => Promise<WorkingCopy \| null>`        | Returns the cached copy when it matches the history's in-progress session; rebuilds and caches otherwise; `null` (and evicts any stale cache entry) when the history has no in-progress session. Concurrent calls for the same managed id dedupe onto one build. If superseded mid-build by `invalidate`/`set`/`rebuild`, resolves to the now-current cache entry (or `null`) instead of the discarded build. |
+| `rebuild`    | action | `(managed: Datastream, source: Datastream, historyId: string, session: SessionWindow) => Promise<WorkingCopy>` | Always reconstructs from the session window and replays its operations, even when a cached copy already matches; supersedes any in-flight `load()` build for this managed id and overwrites the cache (unless itself superseded while awaiting, in which case the cache is left alone). |
+| `set`        | action | `(managedId: string, sessionId: string, record: ObservationRecord, begin: Date, end: Date) => void`   | Insert or replace a cache entry directly (used by the editor after a local edit); supersedes any in-flight build for this managed id. |
+| `invalidate` | action | `(managedId: string) => void`                                                                          | Drop a managed datastream's cached copy; supersedes any in-flight build for this managed id. |
 | `extents`    | action | `(managedIds: string[]) => { phenomenonBeginTime: string; phenomenonEndTime: string }[]`               | The cached copies' windows for the given managed ids, in ISO form; ids with no cached copy are omitted. |
 
 ### `usePlotlyStore()` — `src/store/plotly.ts`
diff --git a/apps/qc-app/src/store/__tests__/workingCopies.spec.ts b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
index ea00d7ee..21c560b9 100644
--- a/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
+++ b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
@@ -32,6 +32,31 @@ const managed = { id: 'm-1' } as any
 const source = { id: 's-1' } as any
 const rec = (xs: number[]) => ({ dataX: xs, dataY: xs.map(() => 1), history: [] })
 
+function deferred<T>() {
+  let resolve!: (value: T) => void
+  const promise = new Promise<T>((res) => {
+    resolve = res
+  })
+  return { promise, resolve }
+}
+
+/**
+ * Gates every `fetchObservationsInRange` call behind `gate`, and resolves
+ * `started` the first time a call is made — so a test can wait for a
+ * build to have actually begun (past its generation bump) before racing
+ * another store call against it.
+ */
+function gateFetch() {
+  const gate = deferred<void>()
+  const started = deferred<void>()
+  fetchObservationsInRange.mockImplementation(async (ds: any) => {
+    started.resolve()
+    await gate.promise
+    return ds.id === 'm-1' ? rec([]) : rec([1, 2, 3])
+  })
+  return { gate, started }
+}
+
 let qc: ReturnType<typeof makeQcFake>
 let historyId: string
 
@@ -140,4 +165,58 @@ describe('useWorkingCopiesStore', () => {
     expect(store.get('m-1')).toBeUndefined()
     expect(store.extents(['m-1'])).toEqual([])
   })
+
+  it('dedupes concurrent load() calls into a single reconstruction', async () => {
+    await startSession()
+    const { gate } = gateFetch()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const first = store.load(managed, source, historyId)
+    const second = store.load(managed, source, historyId)
+    gate.resolve()
+
+    const [a, b] = await Promise.all([first, second])
+    expect(a).toBe(b)
+    expect(applyHistory).toHaveBeenCalledTimes(1)
+    expect(store.get('m-1')).toBe(a)
+  })
+
+  it('invalidate() during an in-flight load() leaves no cached entry and the load does not resurrect it', async () => {
+    await startSession()
+    const { gate, started } = gateFetch()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const pending = store.load(managed, source, historyId)
+    await started.promise
+    store.invalidate('m-1')
+    gate.resolve()
+
+    expect(await pending).toBeNull()
+    expect(store.get('m-1')).toBeUndefined()
+  })
+
+  it('set() during an in-flight load() keeps the set record', async () => {
+    await startSession()
+    const { gate, started } = gateFetch()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+    const setRecord = rec([9]) as any
+    const setBegin = new Date('2024-03-01T00:00:00Z')
+    const setEnd = new Date('2024-04-01T00:00:00Z')
+
+    const pending = store.load(managed, source, historyId)
+    await started.promise
+    store.set('m-1', 's-9', setRecord, setBegin, setEnd)
+    gate.resolve()
+
+    expect(await pending).toEqual({
+      sessionId: 's-9',
+      record: setRecord,
+      begin: setBegin,
+      end: setEnd,
+    })
+    expect(store.get('m-1')?.record).toBe(setRecord)
+  })
 })
diff --git a/apps/qc-app/src/store/workingCopies.ts b/apps/qc-app/src/store/workingCopies.ts
index cef406df..f284499a 100644
--- a/apps/qc-app/src/store/workingCopies.ts
+++ b/apps/qc-app/src/store/workingCopies.ts
@@ -33,6 +33,27 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
   // Not reactive: records hold large typed arrays that must not be proxied.
   const copies = new Map<string, WorkingCopy>()
 
+  // Per-managed-id generation, bumped by invalidate/set and by every new
+  // build (load's own build, and rebuild). A build only writes its result
+  // to `copies` if its generation is still the latest once its await
+  // resolves; otherwise a later invalidate/set/build has already
+  // superseded it and the late write is silently dropped. This is what
+  // keeps a stale in-flight reconstruction from resurrecting a copy after
+  // the session it belongs to was committed or deleted.
+  const generations = new Map<string, number>()
+
+  // One in-flight load() build per managed id, so concurrent load() calls
+  // for the same datastream share a single reconstruction and resolve to
+  // the same record instead of each producing their own (the editor
+  // mutates records in place, so two instances would diverge).
+  const inFlightLoads = new Map<string, Promise<WorkingCopy | null>>()
+
+  function bump(managedId: string): number {
+    const next = (generations.get(managedId) ?? 0) + 1
+    generations.set(managedId, next)
+    return next
+  }
+
   const get = (managedId: string) => copies.get(managedId)
 
   function set(
@@ -42,18 +63,21 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
     begin: Date,
     end: Date
   ) {
+    bump(managedId)
     copies.set(managedId, { sessionId, record, begin, end })
   }
 
   function invalidate(managedId: string) {
+    bump(managedId)
     copies.delete(managedId)
   }
 
-  async function rebuild(
+  async function build(
     managed: Datastream,
     source: Datastream,
     historyId: string,
-    session: SessionWindow
+    session: SessionWindow,
+    generation: number
   ): Promise<WorkingCopy> {
     const { hs } = storeToRefs(useHydroServer())
     const { fetchObservationsInRange } = useObservationStore()
@@ -75,11 +99,26 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
       begin: new Date(session.phenomenonTimeStart),
       end: new Date(session.phenomenonTimeEnd),
     }
-    copies.set(managed.id, copy)
+    if (generations.get(managed.id) === generation) {
+      copies.set(managed.id, copy)
+    }
     return copy
   }
 
-  async function load(
+  async function rebuild(
+    managed: Datastream,
+    source: Datastream,
+    historyId: string,
+    session: SessionWindow
+  ): Promise<WorkingCopy> {
+    // A rebuild always forces a fresh reconstruction, and supersedes any
+    // load() build already in flight for this managed id.
+    const generation = bump(managed.id)
+    inFlightLoads.delete(managed.id)
+    return build(managed, source, historyId, session, generation)
+  }
+
+  async function performLoad(
     managed: Datastream,
     source: Datastream,
     historyId: string
@@ -90,12 +129,38 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
       historyId
     )
     if (!session) {
-      copies.delete(managed.id)
+      invalidate(managed.id)
       return null
     }
     const cached = copies.get(managed.id)
     if (cached?.sessionId === session.id) return cached
-    return rebuild(managed, source, historyId, session)
+
+    const generation = bump(managed.id)
+    const built = await build(managed, source, historyId, session, generation)
+    // Superseded while awaiting (invalidate/set/rebuild landed first):
+    // don't hand the caller a record that isn't (and won't become) the
+    // cached entry. Report whatever is current instead.
+    if (generations.get(managed.id) !== generation) {
+      return copies.get(managed.id) ?? null
+    }
+    return built
+  }
+
+  function load(
+    managed: Datastream,
+    source: Datastream,
+    historyId: string
+  ): Promise<WorkingCopy | null> {
+    const existing = inFlightLoads.get(managed.id)
+    if (existing) return existing
+
+    const promise = performLoad(managed, source, historyId).finally(() => {
+      if (inFlightLoads.get(managed.id) === promise) {
+        inFlightLoads.delete(managed.id)
+      }
+    })
+    inFlightLoads.set(managed.id, promise)
+    return promise
   }
 
   function extents(managedIds: string[]) {

From fcc8c04aa8db7e05c5b5c27f3fd7a3bfd0de7796 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 13:43:25 -0600
Subject: [PATCH 42/93] test: assert the resume test checks the raw record
 identity

---
 apps/qc-app/src/composables/__tests__/useEditSession.spec.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 90e1d7d5..ce7d48b5 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -166,7 +166,7 @@ describe('useEditSession', () => {
     // editing it can't mutate the store's cached one.
     const seriesRecord = selectedSeries.value.data
     expect(Array.from(seriesRecord.dataX)).toEqual(Array.from(reconstructed.dataX))
-    expect(seriesRecord).not.toBe(reconstructed)
+    expect(toRaw(seriesRecord)).not.toBe(reconstructed)
     // applyHistory replayed onto that same copy, not the fetched record.
     // `toRaw` unwraps the reactive proxy Vue puts on the assigned object,
     // since applyHistory was called with the pre-proxy instance.

From 6edbb27682f0ba1a3d9e900f35c8f224c40d7cb9 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 13:58:56 -0600
Subject: [PATCH 43/93] fix: return only the cached working copy from a
 superseded rebuild

---
 apps/qc-app/docs/API_REFERENCE.md             |  8 ++-
 .../src/store/__tests__/workingCopies.spec.ts | 72 +++++++++++++++++++
 apps/qc-app/src/store/workingCopies.ts        | 51 +++++++++----
 3 files changed, 117 insertions(+), 14 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index b6b3d7f6..f9dd6c4f 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -329,13 +329,17 @@ Concurrency: a per-managed-id generation counter, bumped by `invalidate`,
 whose generation is no longer current (superseded by a later
 `invalidate`/`set`/`rebuild` while it awaited) is discarded rather than
 resurrecting a stale copy. Concurrent `load()` calls for the same managed
-id share one in-flight build and resolve to the same record.
+id share one in-flight build and resolve to the same record. A `rebuild()`
+superseded by a later `rebuild()` chains onto it and resolves to the same
+copy regardless of which finishes its fetch first; superseded by
+`invalidate`/`set` instead, it resolves to whatever is now cached (`null`
+after an `invalidate`) — never to the record it built but discarded.
 
 | Name         | Kind   | Type / signature                                                                                     | Notes |
 |--------------|--------|-------------------------------------------------------------------------------------------------------|-------|
 | `get`        | action | `(managedId: string) => WorkingCopy \| undefined`                                                     | Current cached copy for a managed datastream, if any. |
 | `load`       | action | `(managed: Datastream, source: Datastream, historyId: string) => Promise<WorkingCopy \| null>`        | Returns the cached copy when it matches the history's in-progress session; rebuilds and caches otherwise; `null` (and evicts any stale cache entry) when the history has no in-progress session. Concurrent calls for the same managed id dedupe onto one build. If superseded mid-build by `invalidate`/`set`/`rebuild`, resolves to the now-current cache entry (or `null`) instead of the discarded build. |
-| `rebuild`    | action | `(managed: Datastream, source: Datastream, historyId: string, session: SessionWindow) => Promise<WorkingCopy>` | Always reconstructs from the session window and replays its operations, even when a cached copy already matches; supersedes any in-flight `load()` build for this managed id and overwrites the cache (unless itself superseded while awaiting, in which case the cache is left alone). |
+| `rebuild`    | action | `(managed: Datastream, source: Datastream, historyId: string, session: SessionWindow) => Promise<WorkingCopy \| null>` | Always reconstructs from the session window and replays its operations, even when a cached copy already matches; supersedes any in-flight `load()` build or earlier `rebuild()` for this managed id and overwrites the cache. If superseded itself while awaiting, never returns the discarded build: resolves to the copy that superseded it (chaining onto a later in-flight `rebuild()`, or reading the cache after a synchronous `invalidate`/`set`), which is `null` when nothing ended up cached. |
 | `set`        | action | `(managedId: string, sessionId: string, record: ObservationRecord, begin: Date, end: Date) => void`   | Insert or replace a cache entry directly (used by the editor after a local edit); supersedes any in-flight build for this managed id. |
 | `invalidate` | action | `(managedId: string) => void`                                                                          | Drop a managed datastream's cached copy; supersedes any in-flight build for this managed id. |
 | `extents`    | action | `(managedIds: string[]) => { phenomenonBeginTime: string; phenomenonEndTime: string }[]`               | The cached copies' windows for the given managed ids, in ISO form; ids with no cached copy are omitted. |
diff --git a/apps/qc-app/src/store/__tests__/workingCopies.spec.ts b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
index 21c560b9..33963a4b 100644
--- a/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
+++ b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
@@ -57,6 +57,17 @@ function gateFetch() {
   return { gate, started }
 }
 
+/**
+ * Gates one `applyHistory` call (the last step of a rebuild's
+ * reconstruction) so a test can hold a specific rebuild() call in flight
+ * without gating the others sharing the same mocked fetch.
+ */
+function gateOneApplyHistory() {
+  const gate = deferred<{ applied: number; failed: never[] }>()
+  applyHistory.mockImplementationOnce(() => gate.promise)
+  return gate
+}
+
 let qc: ReturnType<typeof makeQcFake>
 let historyId: string
 
@@ -219,4 +230,65 @@ describe('useWorkingCopiesStore', () => {
     })
     expect(store.get('m-1')?.record).toBe(setRecord)
   })
+
+  it('invalidate() during an in-flight rebuild() resolves it to null and leaves nothing cached', async () => {
+    const session = unwrap(await startSession())
+    const { gate, started } = gateFetch()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const pending = store.rebuild(managed, source, historyId, session)
+    await started.promise
+    store.invalidate('m-1')
+    gate.resolve()
+
+    expect(await pending).toBeNull()
+    expect(store.get('m-1')).toBeUndefined()
+  })
+
+  it('set() during an in-flight rebuild() resolves it to the set record', async () => {
+    const session = unwrap(await startSession())
+    const { gate, started } = gateFetch()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+    const setRecord = rec([9]) as any
+    const setBegin = new Date('2024-03-01T00:00:00Z')
+    const setEnd = new Date('2024-04-01T00:00:00Z')
+
+    const pending = store.rebuild(managed, source, historyId, session)
+    await started.promise
+    store.set('m-1', 's-9', setRecord, setBegin, setEnd)
+    gate.resolve()
+
+    expect(await pending).toEqual({
+      sessionId: 's-9',
+      record: setRecord,
+      begin: setBegin,
+      end: setEnd,
+    })
+    expect(store.get('m-1')?.record).toBe(setRecord)
+  })
+
+  it('a second rebuild() supersedes the first, which resolves to the same cached copy regardless of which fetch finishes first', async () => {
+    const session = unwrap(await startSession())
+    const gate1 = gateOneApplyHistory()
+    const gate2 = gateOneApplyHistory()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const first = store.rebuild(managed, source, historyId, session)
+    const second = store.rebuild(managed, source, historyId, session)
+
+    // The first call's own fetch settles before the second's, even
+    // though the second call is the one that supersedes it: `first`
+    // must still chain onto `second`'s result rather than returning its
+    // own (discarded) build.
+    gate1.resolve({ applied: 0, failed: [] })
+    gate2.resolve({ applied: 0, failed: [] })
+
+    const [firstResult, secondResult] = await Promise.all([first, second])
+    expect(firstResult).toBe(secondResult)
+    expect(store.get('m-1')).toBe(secondResult)
+    expect(applyHistory).toHaveBeenCalledTimes(2)
+  })
 })
diff --git a/apps/qc-app/src/store/workingCopies.ts b/apps/qc-app/src/store/workingCopies.ts
index f284499a..43e6eb43 100644
--- a/apps/qc-app/src/store/workingCopies.ts
+++ b/apps/qc-app/src/store/workingCopies.ts
@@ -48,9 +48,18 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
   // mutates records in place, so two instances would diverge).
   const inFlightLoads = new Map<string, Promise<WorkingCopy | null>>()
 
+  // The most recently started rebuild() for a managed id, if still in
+  // flight. A rebuild superseded by a later one chains onto this instead
+  // of guessing at the cache, so it resolves to the same result as the
+  // rebuild that actually wins the race, regardless of which finishes
+  // its fetch first. Cleared by bump() (any invalidate/set/new build
+  // supersedes it) and re-registered by rebuild() right after.
+  const activeRebuild = new Map<string, Promise<WorkingCopy | null>>()
+
   function bump(managedId: string): number {
     const next = (generations.get(managedId) ?? 0) + 1
     generations.set(managedId, next)
+    activeRebuild.delete(managedId)
     return next
   }
 
@@ -72,13 +81,19 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
     copies.delete(managedId)
   }
 
+  /**
+   * Reconstruct and, if still current, cache the result. Returns null
+   * (and leaves the cache untouched) when superseded while awaiting, so
+   * `load`/`rebuild` can tell the two cases apart and never hand a caller
+   * a record that isn't the cached one.
+   */
   async function build(
     managed: Datastream,
     source: Datastream,
     historyId: string,
     session: SessionWindow,
     generation: number
-  ): Promise<WorkingCopy> {
+  ): Promise<WorkingCopy | null> {
     const { hs } = storeToRefs(useHydroServer())
     const { fetchObservationsInRange } = useObservationStore()
     const { record } = await reconstructSession(
@@ -93,15 +108,14 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
       historyId,
       session.id
     )
+    if (generations.get(managed.id) !== generation) return null
     const copy: WorkingCopy = {
       sessionId: session.id,
       record,
       begin: new Date(session.phenomenonTimeStart),
       end: new Date(session.phenomenonTimeEnd),
     }
-    if (generations.get(managed.id) === generation) {
-      copies.set(managed.id, copy)
-    }
+    copies.set(managed.id, copy)
     return copy
   }
 
@@ -110,12 +124,28 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
     source: Datastream,
     historyId: string,
     session: SessionWindow
-  ): Promise<WorkingCopy> {
-    // A rebuild always forces a fresh reconstruction, and supersedes any
-    // load() build already in flight for this managed id.
+  ): Promise<WorkingCopy | null> {
+    // Always forces a fresh reconstruction, superseding any load() build
+    // or earlier rebuild() in flight for this managed id.
     const generation = bump(managed.id)
     inFlightLoads.delete(managed.id)
-    return build(managed, source, historyId, session, generation)
+    const pending = build(managed, source, historyId, session, generation)
+    activeRebuild.set(managed.id, pending)
+    try {
+      const built = await pending
+      if (built) return built
+      // Superseded while awaiting. A later rebuild is now the
+      // authoritative one: chain onto it so both resolve together,
+      // whichever fetch actually finishes first. Otherwise (a
+      // synchronous invalidate/set) the cache already holds the answer.
+      const winner = activeRebuild.get(managed.id)
+      if (winner && winner !== pending) return await winner
+      return copies.get(managed.id) ?? null
+    } finally {
+      if (activeRebuild.get(managed.id) === pending) {
+        activeRebuild.delete(managed.id)
+      }
+    }
   }
 
   async function performLoad(
@@ -140,10 +170,7 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
     // Superseded while awaiting (invalidate/set/rebuild landed first):
     // don't hand the caller a record that isn't (and won't become) the
     // cached entry. Report whatever is current instead.
-    if (generations.get(managed.id) !== generation) {
-      return copies.get(managed.id) ?? null
-    }
-    return built
+    return built ?? copies.get(managed.id) ?? null
   }
 
   function load(

From 6d624d1c102f6db1b56d44fb627caed936879a7a Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 13:59:42 -0600
Subject: [PATCH 44/93] test: capture the commit dialog and commit a session in
 the golden path

---
 apps/qc-app/docs/USER_GUIDE.md            |   8 +-
 apps/qc-app/docs/images/commit-dialog.png | Bin 0 -> 22410 bytes
 apps/qc-app/docs/images/submit-dialog.png | Bin 16364 -> 0 bytes
 apps/qc-app/e2e/qc-golden-path.spec.ts    |  87 +++++++++++++---------
 apps/qc-app/e2e/screenshots.spec.ts       |  18 +++--
 5 files changed, 68 insertions(+), 45 deletions(-)
 create mode 100644 apps/qc-app/docs/images/commit-dialog.png
 delete mode 100644 apps/qc-app/docs/images/submit-dialog.png

diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index aeb73a81..36507dd3 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -556,12 +556,12 @@ When you're satisfied with the edits, hit one of the action buttons at the botto
 
 Clicking Commit opens a confirmation dialog so a misclick won't push data to the server.
 
-![Submit confirmation dialog](./images/submit-dialog.png)
+![Commit confirmation dialog](./images/commit-dialog.png)
 
-Once you confirm:
+The dialog lets you add an optional session description. Once you confirm:
 
-1. The app POSTs the cleaned observations to HydroServer, which either overwrites the existing observations in the plotted window or saves the cleaned observations to a different datastream, depending on your data versioning scheme.
-2. On success, the Snackbar shows "Quality-controlled observations submitted" and the local history is cleared.
+1. The app saves the session's operations, then POSTs the cleaned observations to the managed datastream in `replace` mode, overwriting its observations over the session's time range.
+2. The session is committed and becomes read-only. The Snackbar shows "Session committed." and the footer swaps Save and Commit for **New session**.
 3. On failure, the Snackbar shows the backend's error message verbatim. Show that to your administrator if you need help.
 
 ## Performance and big datastreams
diff --git a/apps/qc-app/docs/images/commit-dialog.png b/apps/qc-app/docs/images/commit-dialog.png
new file mode 100644
index 0000000000000000000000000000000000000000..bb2cffcc751dc8590a83a926c5c040f4e0ac31e3
GIT binary patch
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diff --git a/apps/qc-app/e2e/qc-golden-path.spec.ts b/apps/qc-app/e2e/qc-golden-path.spec.ts
index 4ab5d2e3..92a17423 100644
--- a/apps/qc-app/e2e/qc-golden-path.spec.ts
+++ b/apps/qc-app/e2e/qc-golden-path.spec.ts
@@ -43,12 +43,15 @@ async function loginThroughDataManagementIfNeeded(page: Page) {
 //      selection, so the router's workspace guard redirects to /workspaces.
 //   2. Select "Test Workspace #2" from the picker.
 //   3. Wait for datastreams table (init-complete signal on Home).
-//   4. Select the first datastream and wait for data-loading-indicator to hide.
-//   5. Switch to Edit view via the navigation rail.
+//   4. Plot the first source with a managed datastream, load the All range.
+//   5. Start editing on its first managed datastream, which opens a session.
 //   6. Open "Value thresholds", add a filter, close dialog.
-//   7. Wait for selection to populate, open "Change values", commit edit.
+//   7. Wait for selection to populate, open "Change values", apply edit.
 //   8. Assert CHANGE_VALUES appears in EditHistory.
-//   9. Click Save Changes, confirm, assert success snackbar.
+//   9. Save the draft, then Commit the session through the dialog.
+//
+// Needs a workspace source datastream with a managed datastream and QC
+// history; without one there is no session to Save or Commit.
 //
 // Live smoke — talks to the backend configured behind Data Management's
 // /api proxy. Opt in with
@@ -61,7 +64,7 @@ test.describe('QC golden path (live same-origin)', () => {
     'Set E2E_LIVE=1 to run the live same-origin smoke.'
   )
 
-  test('qc golden path: filter, edit, submit', async ({ page }) => {
+  test('qc golden path: filter, edit, save and commit', async ({ page }) => {
     // First-run Vite + createHydroServer + qc-utils calibration + observation
     // fetch can exceed the default 30s test timeout. 180s gives the whole
     // flow room to breathe on a cold dev server.
@@ -92,26 +95,37 @@ test.describe('QC golden path (live same-origin)', () => {
       timeout: 60_000,
     })
 
-    // Step 4 — Select the first available datastream. The table is
-    // virtualized (`v-data-table-virtual`), so rows outside the
-    // initial viewport aren't in the DOM. We don't care which
-    // datastream we pick — the filter/edit/submit flow is the same
-    // for any — so target the first rendered `plot-checkbox-*` row
-    // instead of pinning to a specific id. After checking, wait
-    // for the data-loading-indicator to hide — observations have
-    // finished loading into Plotly.
-    const firstPlotButton = page
+    // Step 4: Plot the first rendered source that has a managed datastream
+    // (managed-count badge). The table is virtualized, so the workspace needs
+    // such a source near the top. Its check box opens the plot-source chooser;
+    // plot the raw source, then load the All range so the session window
+    // covers real observations.
+    const sourceCheckbox = page
+      .locator('tr')
+      .filter({ has: page.locator('.managed-count') })
       .locator('[data-testid^="plot-checkbox-"]')
       .first()
-    await expect(firstPlotButton).toBeVisible({ timeout: 30_000 })
-    await firstPlotButton.click()
-    await page
-      .getByTestId('data-loading-indicator')
-      .waitFor({ state: 'hidden', timeout: 90_000 })
-
-    // Step 5 — Switch to the Edit view. Edit rail item becomes enabled once
-    // qcDatastream is set (happens when a datastream is plotted).
-    await page.getByTestId('nav-rail-item-edit').click()
+    await expect(sourceCheckbox).toBeVisible({ timeout: 30_000 })
+    const sourceId = (await sourceCheckbox.getAttribute('data-testid'))!.replace(
+      'plot-checkbox-',
+      ''
+    )
+    await sourceCheckbox.click()
+    await page.getByTestId(`plot-option-${sourceId}`).locator('input').check()
+    await page.getByTestId('plot-source-apply').click()
+    const loading = page.getByTestId('data-loading-indicator')
+    await loading.waitFor({ state: 'hidden', timeout: 90_000 })
+    await page.getByTestId('date-preset-All').click()
+    await loading.waitFor({ state: 'hidden', timeout: 90_000 })
+
+    // Step 5: Start editing and pick the first managed datastream. This
+    // resumes its in-progress session or starts one, which is what renders
+    // Save and Commit in the history footer.
+    await page.getByRole('button', { name: 'Start editing' }).click()
+    await page.locator('[data-testid^="edit-managed-"]').first().click()
+    await expect(page.getByTestId('exit-save-btn')).toBeVisible({
+      timeout: 60_000,
+    })
     await expect(page.getByText('Filter Data')).toBeVisible()
 
     // Step 6 — Open "Value thresholds" operation panel, add a filter.
@@ -167,18 +181,23 @@ test.describe('QC golden path (live same-origin)', () => {
     await expect(modeChip).toBeVisible()
     await expect(modeChip).toHaveText(/inline|worker/)
 
-    // Step 9 — Save Changes → confirm → assert success snackbar.
+    // Step 9: Save the draft, then Commit through the confirmation dialog.
+    // Committing locks the session, so the footer swaps to New session.
     await page.getByTestId('exit-save-btn').click()
-    await expect(page.getByText('Submit QC observations?')).toBeVisible()
-    await page.getByRole('button', { name: 'Submit' }).click()
-    // ⚠ REQUIRES LIVE VALIDATION: success snackbar text — confirmed against
-    // Snackbar.success call sites but not yet observed in a live run.
-    await expect(
-      page.getByText('Quality-controlled observations submitted')
-    ).toBeVisible({ timeout: 60_000 })
-    // MH-17: after submit the store clears editHistory → history-item-0 must be gone.
-    await expect(page.getByTestId('history-item-0')).toHaveCount(0, {
-      timeout: 15_000,
+    await expect(page.getByText('Draft saved.')).toBeVisible({
+      timeout: 60_000,
+    })
+    await page.getByTestId('exit-commit-btn').click()
+    const commitDialog = page
+      .locator('.v-overlay__content')
+      .filter({ hasText: 'Commit session to datastream?' })
+    await expect(commitDialog).toBeVisible()
+    await commitDialog
+      .getByRole('button', { name: 'Commit', exact: true })
+      .click()
+    await expect(page.getByText('Session committed.')).toBeVisible({
+      timeout: 60_000,
     })
+    await expect(page.getByTestId('exit-new-session-btn')).toBeVisible()
   })
 })
diff --git a/apps/qc-app/e2e/screenshots.spec.ts b/apps/qc-app/e2e/screenshots.spec.ts
index 015b9a7e..fd27cab4 100644
--- a/apps/qc-app/e2e/screenshots.spec.ts
+++ b/apps/qc-app/e2e/screenshots.spec.ts
@@ -3,7 +3,7 @@
  *
  * Drives the mocked-backend e2e harness through every notable UI state
  * (workspaces, home/select, plot, edit view, each filter/edit/add
- * panel, edit history, submit dialog) and writes one PNG per state to
+ * panel, edit history, commit dialog) and writes one PNG per state to
  * `docs/images/`.
  *
  * Skipped by default so a normal `npx playwright test` run doesn't
@@ -27,6 +27,7 @@ import { installMocks } from './support/mocks'
 import {
   gotoHome,
   setupEditView,
+  setupSessionEditView,
   openOp,
   plotFirstDatastream,
   plotDatastreamById,
@@ -334,20 +335,23 @@ test.describe('docs screenshots', () => {
     await snapEl(history, 'edit-history.png')
   })
 
-  test('submit confirmation dialog', async ({ page }) => {
+  test('commit confirmation dialog', async ({ page }) => {
     await page.setViewportSize(STD_VIEWPORT)
-    await installMocks(page)
-    await setupEditView(page)
+    await installMocks(page, { qcHistories: true })
+    await setupSessionEditView(page)
     await selectAllPoints(page)
     await openOp(page, 'changeValues')
     const opPanel = page.getByTestId('operation-panel-changeValues')
     await opPanel.getByLabel('Value').fill('1')
     await opPanel.getByRole('button', { name: /^apply$/i }).click()
-    await page.getByTestId('exit-save-btn').click()
+    await expect(
+      page.locator('[data-testid^="history-item-"]').first()
+    ).toBeVisible({ timeout: 10_000 })
+    await page.getByTestId('exit-commit-btn').click()
     const dialog = page
       .locator('.v-overlay__content')
-      .filter({ hasText: 'Submit QC observations?' })
+      .filter({ hasText: 'Commit session to datastream?' })
     await expect(dialog).toBeVisible({ timeout: 5_000 })
-    await snapEl(dialog, 'submit-dialog.png')
+    await snapEl(dialog, 'commit-dialog.png')
   })
 })

From 49b4ec20dbcbf4b62d5ef344a6fcd4edc7af5b10 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 13:59:56 -0600
Subject: [PATCH 45/93] fix: resize the editor plot when it is drawn before its
 container settles

---
 apps/qc-app/docs/TESTING.md                   |  2 +-
 apps/qc-app/e2e/plot-layout.spec.ts           | 49 +++++++++++++++++++
 .../src/components/VisualizeData/Plot.vue     | 34 +++++++------
 3 files changed, 69 insertions(+), 16 deletions(-)
 create mode 100644 apps/qc-app/e2e/plot-layout.spec.ts

diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 9ea77297..671665e8 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -13,7 +13,7 @@ the operator's manual; QUALITY.md is the policy.
 | Layer      | Runner                          | Where                        | Count    |
 | ---------- | ------------------------------- | ---------------------------- | -------- |
 | Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 58 files |
-| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 28 files |
+| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 29 files |
 
 There is no separate "integration" tier — component tests live in the
 unit tier and mount real Vue components with the Vue Test Utils
diff --git a/apps/qc-app/e2e/plot-layout.spec.ts b/apps/qc-app/e2e/plot-layout.spec.ts
new file mode 100644
index 00000000..19330c39
--- /dev/null
+++ b/apps/qc-app/e2e/plot-layout.spec.ts
@@ -0,0 +1,49 @@
+/**
+ * The editor plot fills its container once the Edit view opens, whether it
+ * was entered through the nav rail or through Start editing. Plotly can lay
+ * the plot out while the view is still switching, so the resize observer has
+ * to catch up to the container's final size.
+ */
+
+import { expect, test, type Page } from '@playwright/test'
+import { installMocks } from './support/mocks'
+import { setupEditView, setupSessionEditView } from './support/app'
+
+/** Largest gap in px between Plotly's laid-out size and the plot div. */
+function layoutMismatch(page: Page): Promise<number> {
+  return page.evaluate(() => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as
+      | (HTMLElement & { _fullLayout?: { width: number; height: number } })
+      | null
+    const layout = gd?._fullLayout
+    if (!gd || !layout) return Number.MAX_SAFE_INTEGER
+    return Math.max(
+      Math.abs(layout.width - gd.offsetWidth),
+      Math.abs(layout.height - gd.offsetHeight)
+    )
+  })
+}
+
+test.describe('editor plot layout', () => {
+  test.beforeEach(async ({ page }) => {
+    await page.setViewportSize({ width: 1440, height: 900 })
+  })
+
+  test('fills its container after entering through the rail', async ({
+    page,
+  }) => {
+    await installMocks(page)
+    await setupEditView(page)
+    await expect
+      .poll(() => layoutMismatch(page), { timeout: 5_000 })
+      .toBeLessThanOrEqual(1)
+  })
+
+  test('fills its container after starting a session', async ({ page }) => {
+    await installMocks(page, { qcHistories: true })
+    await setupSessionEditView(page)
+    await expect
+      .poll(() => layoutMismatch(page), { timeout: 5_000 })
+      .toBeLessThanOrEqual(1)
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index 19cb06a8..76948e66 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -697,30 +697,34 @@ onMounted(async () => {
   updateOptions()
 
   // Wait for the view-switch animation to expand the container.
-  setTimeout(() => {
+  setTimeout(async () => {
     updateOptions()
-    handleNewPlot(plot.value)
+    const drawn = handleNewPlot(plot.value)
     if (!props.preview) onEditorDatePreset(ALL_PRESET_ID)
+    await drawn
 
+    // Unmounted while drawing.
     const target = plot.value
     if (target && typeof ResizeObserver !== 'undefined') {
-      // Skip the initial "observe started" notification so we don't
-      // resize on top of the freshly-built plot.
-      let initialFired = false
+      // The plot can be laid out before the container reaches its final
+      // size, so resize whenever the two differ, including on the first
+      // notification.
       plotResizeObserver = new ResizeObserver(() => {
-        if (!initialFired) {
-          initialFired = true
-          return
-        }
         if (pendingResizeFrame != null) return
         pendingResizeFrame = requestAnimationFrame(() => {
           pendingResizeFrame = null
-          const gd = plot.value
-          if (!gd) return
-          // Plotly.Plots.resize throws when gd has no _fullLayout
-          // (observer can fire before handleNewPlot finishes).
-          const anyGd = gd as unknown as { _fullLayout?: unknown }
-          if (!anyGd._fullLayout) return
+          const gd = plot.value as
+            | (HTMLElement & { _fullLayout?: { width: number; height: number } })
+            | null
+          const layout = gd?._fullLayout
+          // Plotly.Plots.resize rejects a hidden div (Table tab).
+          if (!gd || !layout || !gd.offsetWidth || !gd.offsetHeight) return
+          if (
+            Math.abs(layout.width - gd.offsetWidth) < 1 &&
+            Math.abs(layout.height - gd.offsetHeight) < 1
+          ) {
+            return
+          }
           void Plotly.Plots.resize(gd as unknown as Plotly.Root)
         })
       })

From edd13e044ae8ff614e67d179d54d838d747fc0a2 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 14:17:59 -0600
Subject: [PATCH 46/93] fix: save the session draft from the nav rail exit
 guard

---
 apps/qc-app/README.md                         |   4 +-
 apps/qc-app/docs/API_REFERENCE.md             |  31 ++--
 apps/qc-app/docs/ARCHITECTURE.md              |  10 +-
 apps/qc-app/docs/ONBOARDING.md                |  12 +-
 apps/qc-app/docs/QUALITY.md                   |   9 +-
 apps/qc-app/docs/USER_GUIDE.md                |   4 +-
 apps/qc-app/e2e/navigation.spec.ts            | 106 ++++++++++++--
 .../components/Navigation/NavigationRail.vue  |  79 ++++++-----
 .../__tests__/useEditSession.spec.ts          |  16 +++
 .../__tests__/useQcSubmission.spec.ts         | 133 ------------------
 apps/qc-app/src/composables/useEditSession.ts |  11 +-
 .../qc-app/src/composables/useQcSubmission.ts |  60 --------
 apps/qc-app/src/store/plotly.ts               |   2 -
 apps/qc-app/src/store/qcSession.ts            |  11 ++
 14 files changed, 202 insertions(+), 286 deletions(-)
 delete mode 100644 apps/qc-app/src/composables/__tests__/useQcSubmission.spec.ts
 delete mode 100644 apps/qc-app/src/composables/useQcSubmission.ts

diff --git a/apps/qc-app/README.md b/apps/qc-app/README.md
index cb004262..2882285e 100644
--- a/apps/qc-app/README.md
+++ b/apps/qc-app/README.md
@@ -18,7 +18,7 @@ The app is the operator's view of HydroServer's QC pipeline:
 2. **QC one stream at a time** — the first plotted stream is the QC target. The other plotted traces are read-only context.
 3. **Filter / edit / add** — every operation (Value Threshold, Find Gaps, Persistence, Interpolate, Drift Correction, Fill Gaps, Add Points, etc.) commits a `HistoryItem` to a replayable edit history backed by [`@uwrl/qc-utils`](https://www.npmjs.com/package/@uwrl/qc-utils).
 4. **Save / load a QC History** — export the history as a JSON document, replay it on the same datastream a week later, or templatize across stations.
-5. **Submit** — push the quality-controlled observations back to HydroServer in `replace` mode.
+5. **Save and commit** — save the edits to a QC session as a draft, then commit the session to push the quality-controlled observations to its managed datastream in `replace` mode.
 
 The heavy lifting (worker-parallelized typed-array kernels, calibration, history replay, save / load wire format) lives in `@uwrl/qc-utils`. This repo is the Vue / Vuetify / Pinia / Plotly UI plus the orchestration around it.
 
@@ -100,7 +100,7 @@ Specs live next to source under `src/**/__tests__/`. Plotly is mocked at the mod
 
 ### End-to-end tests (Playwright)
 
-End-to-end specs cover the QC golden path: load a datastream, apply a filter, apply an edit, submit. Browser matrix: **chromium** and **firefox**. WebKit is intentionally excluded — `SharedArrayBuffer` + COOP / COEP behaviour differs in Safari and needs separate validation.
+End-to-end specs cover the QC golden path: load a datastream, apply a filter, apply an edit, save and commit the session. Browser matrix: **chromium** and **firefox**. WebKit is intentionally excluded — `SharedArrayBuffer` + COOP / COEP behaviour differs in Safari and needs separate validation.
 
 One-time setup:
 
diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index f9dd6c4f..4594422b 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -82,8 +82,8 @@ incremental submission is not implemented today.
 
 Listed via `fetchWorkspaceResultQualifiers`. The QC App tracks selected
 qualifiers in `store/qualifiers.ts`, but **does not yet serialize them on
-submit** (see [QUALITY.md](./QUALITY.md) tech-debt section and the TODO
-in `useQcSubmission.ts:42`).
+commit** (see [QUALITY.md](./QUALITY.md) tech-debt section and the note
+in `services/qualityControl/observationsBody.ts`).
 
 ## Internal: composables
 
@@ -156,26 +156,16 @@ const report = await importHistory(file)
 into the active datastream **before** replay (selection-coupled ops
 reference indices against this windowed dataset).
 
-### `useQcSubmission()`
-
-```ts
-const { submitQcEdits } = useQcSubmission()
-await submitQcEdits()    // POST observations with mode=replace, clear history
-```
-
-Single-shot: guards on (selectedSeries + qcDatastream + non-empty
-history), serializes `[phenomenonTime, result]` rows, POSTs with
-`mode: 'replace'`, surfaces a Snackbar, and clears the history in place
-on success.
-
 ### `useEditSession()`
 
 Orchestrates the server-backed QC session workflow against the
 `services/qualityControl/` glue:
 
 ```ts
-const { beginEditing, startSession, saveDraft, commit, needsSession, needsHistory } =
-  useEditSession()
+const {
+  beginEditing, startSession, saveDraft, discardUnsavedEdits, commit,
+  needsSession, needsHistory, hasUnsavedChanges, unsavedEditCount,
+} = useEditSession()
 ```
 
 - `beginEditing()` — resolves the QC history for the QC datastream, loads
@@ -184,8 +174,14 @@ const { beginEditing, startSession, saveDraft, commit, needsSession, needsHistor
 - `startSession(spec)` — creates a session and copies the source window in.
 - `saveDraft()` — persists the record's edit operations to the session
   (append-only reconcile).
+- `discardUnsavedEdits()` — drops edits made since the last save and restores
+  edited comments.
 - `commit()` — saves, verifies checksum C, pushes observations
   (`mode: 'replace'`), then locks the session.
+- `hasUnsavedChanges` / `unsavedEditCount` — the working copy compared with
+  the saved-edits baseline (`qcSession.savedEdits` / `savedComments`). The
+  baseline lives in the store, so every caller agrees: the editor footer and
+  the nav rail's exit guard both read it.
 
 ### `useCreateManagedDatastream()`
 
@@ -366,7 +362,6 @@ handles, live chart caches).
 | `editHistory`              | state    | `HistoryItem[]`                                   | Mirrors `selectedSeries.data.history` (mutated in place — never reassign). |
 | `suppressedEchoSelection`  | state    | `number[] \| null`                                | Sentinel armed by programmatic Plotly writes to suppress the echo SELECTION dispatch. |
 | `isUpdating`               | state    | `boolean`                                         | Surfaced in the nav rail while a redraw runs. |
-| `isSubmitting`             | state    | `boolean`                                         | True during a QC submit POST. |
 | `showLegend`               | state    | `boolean`                                         | Drives Plotly's legend visibility. |
 | `showTooltip`              | state    | `boolean`                                         | Legacy flag; tooltip control routes through the auto/manual mode below. |
 | `tooltipsMaxDataPoints`    | state    | `number`                                          | Auto-mode cutoff (default 10 000); user-tunable from the data-points menu. |
@@ -556,6 +551,8 @@ puts the editor in read-only mode.
 | `viewedSessionId`   | state    | `string \| null`                        | The session currently being viewed. |
 | `isLoading`         | state    | `boolean`                               | True while `loadSessions` is in flight. |
 | `isSwitchingSession`| state    | `boolean`                               | True while another session's data and operations load. The operations panel renders a loading state instead of the outgoing session's entries, which would otherwise linger and read as the incoming session's. |
+| `savedEdits`        | state    | `HistoryItem[]`                         | Edit history entries (by reference) at the last load or save. `useEditSession` compares the working copy against it for `hasUnsavedChanges`; kept in the store so the editor footer and the nav rail's exit guard agree. |
+| `savedComments`     | state    | `string[]`                              | Comment text of `savedEdits`, since comments are edited in place. |
 | `isReadOnly`        | computed | `boolean`                               | True when sessions exist and the viewed one isn't the in-progress session. Guarded on `sessions.length` so plain editing outside the session workflow isn't treated as read-only. |
 | `inProgressSession` | computed | `QualityControlSession \| null`         | The editable session, if any. |
 | `committedSessions` | computed | `QualityControlSession[]`               | Sessions with status `committed`. |
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 0a077ded..38cc0a72 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -100,7 +100,6 @@ src/
 │  ├─ useDataSelection.ts       Bridges Plotly's selectedpoints into the Pinia store.
 │  ├─ useFilterDispatch.ts      Shared "open panel → dispatch op → highlight result" flow.
 │  ├─ useQcHistory.ts            Save / load QC Historys (calls qc-utils' serializeHistory / applyHistory).
-│  ├─ useQcSubmission.ts        Submit the QC'd observations back to HydroServer (replace mode).
 │  ├─ useResizable.ts           Generic drag-to-resize hook used by drawers + the plot.
 │  └─ useBufferedNumber.ts      Debounced numeric input wrapper for filter panels.
 ├─ store/                       Pinia stores — see "State stores" below.
@@ -376,9 +375,12 @@ every navigation:
 - **`hasWorkspaceGuard`** — redirects users without a selected workspace
   to `/workspaces`.
 
-The nav rail's "Edit" entry is gated behind a selected QC datastream and
-runs the "unsaved edits" confirmation dialog before navigating away from
-an in-progress QC session (see `NavigationRail.vue` + `useQcSubmission.ts`).
+The nav rail's "Edit" entry is gated behind a selected QC datastream. Leaving
+the Edit view with edits not yet saved to the session runs the "Unsaved
+edits" dialog: Save & continue saves a draft to the in-progress session and
+Discard returns to the last save. The unsaved state comes from
+`useEditSession`, whose saved-edits baseline lives in the `qcSession` store so
+the rail and the editor footer agree (see `NavigationRail.vue`).
 
 Auth itself is delegated to HydroServer's Django AllAuth setup; the app
 keeps no credentials of its own. The browser holds a session cookie.
diff --git a/apps/qc-app/docs/ONBOARDING.md b/apps/qc-app/docs/ONBOARDING.md
index 0ecbf648..2588b1dc 100644
--- a/apps/qc-app/docs/ONBOARDING.md
+++ b/apps/qc-app/docs/ONBOARDING.md
@@ -57,7 +57,7 @@ Read these files in order:
    landing experience.
 5. `src/store/dataVisualization.ts` and `src/store/plotly.ts` — the two
    stores that hold "what the user sees on the plot."
-6. `src/composables/useFilterDispatch.ts` and `useQcSubmission.ts` —
+6. `src/composables/useFilterDispatch.ts` and `useEditSession.ts` —
    the two end-to-end flows worth tracing.
 7. `packages/qc-utils/src/utils/plotting/observation-record.ts` — the QC engine's
    dispatch surface. You don't need to read the kernels; the dispatcher
@@ -113,11 +113,11 @@ you need to verify the published package artifacts.
 ## Documentation gaps
 
 1. **Result qualifiers are partial.** The `QualifyingComments` op panel
-   exists and writes to the in-memory history, but the submit path
-   (`useQcSubmission.ts`) currently serializes only `phenomenonTime` and
-   `result` — qualifier codes are deferred pending the HydroServer API
-   adding a workable columnar response. There's a TODO in
-   `useQcSubmission.ts:42` that points at this.
+   exists and writes to the in-memory history, but the commit path
+   (`services/qualityControl/observationsBody.ts`) currently serializes only
+   `phenomenonTime` and `result` — qualifier codes are deferred pending the
+   HydroServer API adding a workable columnar response. The note in
+   `observationsBody.ts` points at this.
 2. **No load-testing artifacts.** "How big a datastream can you QC in
    one session?" is answered empirically per browser via the calibration
    pass, but there is no published "this is the supported envelope"
diff --git a/apps/qc-app/docs/QUALITY.md b/apps/qc-app/docs/QUALITY.md
index 3f9401ac..81d646ed 100644
--- a/apps/qc-app/docs/QUALITY.md
+++ b/apps/qc-app/docs/QUALITY.md
@@ -18,8 +18,7 @@ than discovering.
   aid solutions; do things the right way" is the standing rule. Inline
   comments are concentrated on the few invariants that resist being
   read off the code (e.g. the `suppressedEchoSelection` sentinel in
-  `plotly.ts`, the `<` vs `<=` fix in `observations.ts`, the in-place
-  clear in `useQcSubmission.ts`).
+  `plotly.ts`, the `<` vs `<=` fix in `observations.ts`).
 - **Prettier** for formatting (`.prettierrc`) plus **ESLint** (`eslint.config.js`)
   for typescript-eslint + eslint-plugin-vue. `npm run lint` runs the
   configured ruleset across `src/` and `e2e/`; `npm run lint:fix`
@@ -79,12 +78,12 @@ entrypoint so QC and Data Management share the same session.
 These are real, named, worth flagging up front. Not exhaustive — but
 the items most likely to bite a new team in the first three months.
 
-### 1. Result-qualifier submit path is partial
+### 1. Result-qualifier commit path is partial
 
 `store/qualifiers.ts` and `components/EditData/QualifyingComments.vue`
 collect qualifier codes per selection, but
-`composables/useQcSubmission.ts:42` only serializes
-`['phenomenonTime', 'result']` on the bulk POST. The row format would
+`services/qualityControl/observationsBody.ts` only serializes
+`['phenomenonTime', 'result']` on the bulk POST a commit sends. The row format would
 carry qualifiers, but it times out on >35k-point fetches today (see
 `src/utils/observations.ts:24`). Resolution is blocked on the
 HydroServer API team either making the columnar response carry
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 36507dd3..2cbb9eee 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -69,7 +69,7 @@ A thin, always-visible column of icons.
 | Briefcase (Workspaces) | Bottom left. Switch workspace, or Continue in the current one. |
 | Logout | Bottom left. Sign out. |
 
-If you click any of these while you have unsaved edits in the Edit view, the app shows an "Unsaved edits" dialog with **Save & continue** / **Discard** / **Cancel**. Discarded edits cannot be recovered.
+If you click any of these while the Edit view has edits not yet saved to the session, the app shows an "Unsaved edits" dialog with **Save & continue** / **Discard** / **Cancel**. **Save & continue** saves a draft to the session before leaving; it is unavailable when no session is open. **Discard** drops the edits made since the last save, and they cannot be recovered.
 
 ### Select view
 
@@ -552,7 +552,7 @@ When you're satisfied with the edits, hit one of the action buttons at the botto
 - **Commit**: materializes the session into the managed datastream and locks it into the history.
 - **Discard**: drops every edit made since the last save, returning the session to its last saved state. Edits already saved to the session stay. Disabled when there is nothing unsaved, and it asks for confirmation first.
 - **New session**: replaces Save and Commit once the session is committed. Opens a new session over the current time range, starting from the state the last commit left behind.
-- **Close**: leaves the editor. If you have unsaved edits, the Unsaved-edits dialog intercepts you.
+- **Close**: leaves the editor. If you have unsaved edits, a "Save before closing?" dialog lets you save first or close without saving.
 
 Clicking Commit opens a confirmation dialog so a misclick won't push data to the server.
 
diff --git a/apps/qc-app/e2e/navigation.spec.ts b/apps/qc-app/e2e/navigation.spec.ts
index db6650c7..41136d86 100644
--- a/apps/qc-app/e2e/navigation.spec.ts
+++ b/apps/qc-app/e2e/navigation.spec.ts
@@ -4,20 +4,33 @@
  *   - picking a workspace navigates to Home
  *   - the nav-rail workspace-switch button offers to revisit the picker
  *   - the edit rail item is disabled until a datastream is plotted
- *   - unsaved-edits dialog appears when leaving Edit with history
+ *   - leaving Edit with edits not saved to the session asks first
  */
 
-import { expect, test } from '@playwright/test'
-import { installMocks } from './support/mocks'
+import { expect, test, type Page } from '@playwright/test'
+import { installMocks, type MockQcSession } from './support/mocks'
 import {
   gotoHome,
   openOp,
   plotFirstDatastream,
   setupEditView,
+  setupSessionEditView,
 } from './support/app'
-import { selectAllPoints } from './support/ops'
+import { expectHistoryContains, selectAllPoints } from './support/ops'
 import { WORKSPACE_ID } from './support/fixtures'
 
+async function applyChangeValues(page: Page) {
+  await selectAllPoints(page)
+  await openOp(page, 'changeValues')
+  await page.getByLabel('Value').fill('1')
+  await page.getByRole('button', { name: 'Apply' }).click()
+  await expectHistoryContains(page, 'Change Values')
+}
+
+function exitDialog(page: Page) {
+  return page.getByRole('dialog').filter({ hasText: 'Unsaved edits' })
+}
+
 test.describe('navigation', () => {
   test.beforeEach(async ({ page }) => {
     await installMocks(page)
@@ -78,20 +91,83 @@ test.describe('navigation', () => {
     await expect(editRail).toHaveAttribute('aria-disabled', 'false')
   })
 
-  test('unsaved-edits dialog warns before navigating away from Edit', async ({
-    page,
-  }) => {
+  test('without a session the exit dialog cannot save', async ({ page }) => {
     await setupEditView(page)
-    await selectAllPoints(page)
-    await openOp(page, 'changeValues')
-    await page.getByLabel('Value').fill('1')
-    await page.getByRole('button', { name: 'Apply' }).click()
+    await applyChangeValues(page)
 
-    // Click back to the select view via the nav-rail Select item.
     await page.getByTestId('nav-rail-item-select').click()
-    await expect(page.getByText(/unsaved edits/i)).toBeVisible()
+    const dialog = exitDialog(page)
+    await expect(dialog).toBeVisible()
     await expect(
-      page.getByRole('dialog').getByRole('button', { name: /discard/i })
-    ).toBeVisible()
+      dialog.getByRole('button', { name: /save & continue/i })
+    ).toBeDisabled()
+    await expect(dialog.getByRole('button', { name: /discard/i })).toBeEnabled()
+  })
+})
+
+test.describe('navigation: leaving a QC session', () => {
+  let submissions: Array<{ mode: string | null; body: any }>
+  let sessions: MockQcSession[]
+
+  test.beforeEach(async ({ page }) => {
+    // Entering through Start editing and starting a session is slow enough to
+    // outrun the default budget when these run in parallel.
+    test.slow()
+    submissions = []
+    sessions = []
+    await installMocks(page, {
+      qcHistories: true,
+      submissions,
+      qcSessionState: sessions,
+    })
+    await setupSessionEditView(page)
+  })
+
+  test('Save & continue saves the draft without posting observations', async ({
+    page,
+  }) => {
+    await applyChangeValues(page)
+
+    await page.getByTestId('nav-rail-item-select').click()
+    const dialog = exitDialog(page)
+    await expect(dialog).toBeVisible()
+    await dialog.getByRole('button', { name: /save & continue/i }).click()
+
+    await expect(page.getByTestId('datastreams-table')).toBeVisible({
+      timeout: 30_000,
+    })
+    const session = sessions.find((s) => s.status === 'in_progress')
+    expect(session!.operations.map((o) => o.operationType)).toContain(
+      'CHANGE_VALUES'
+    )
+    expect(submissions).toHaveLength(0)
+  })
+
+  test('Discard continues without saving', async ({ page }) => {
+    await applyChangeValues(page)
+
+    await page.getByTestId('nav-rail-item-select').click()
+    const dialog = exitDialog(page)
+    await expect(dialog).toBeVisible()
+    await dialog.getByRole('button', { name: /discard/i }).click()
+
+    await expect(page.getByTestId('datastreams-table')).toBeVisible({
+      timeout: 30_000,
+    })
+    const session = sessions.find((s) => s.status === 'in_progress')
+    expect(session!.operations).toHaveLength(0)
+    expect(submissions).toHaveLength(0)
+  })
+
+  test('saved edits leave without asking', async ({ page }) => {
+    await applyChangeValues(page)
+    await page.getByTestId('exit-save-btn').click()
+    await expect(page.getByText('Draft saved.')).toBeVisible()
+
+    await page.getByTestId('nav-rail-item-select').click()
+    await expect(page.getByTestId('datastreams-table')).toBeVisible({
+      timeout: 30_000,
+    })
+    await expect(exitDialog(page)).toHaveCount(0)
   })
 })
diff --git a/apps/qc-app/src/components/Navigation/NavigationRail.vue b/apps/qc-app/src/components/Navigation/NavigationRail.vue
index 40eefea1..5c0b53cb 100644
--- a/apps/qc-app/src/components/Navigation/NavigationRail.vue
+++ b/apps/qc-app/src/components/Navigation/NavigationRail.vue
@@ -126,14 +126,23 @@
         <div class="d-flex flex-column">
           <div class="text-title-large font-weight-bold">Unsaved edits</div>
           <div class="text-body-small text-medium-emphasis">
-            {{ editCount }} pending change{{ editCount === 1 ? '' : 's' }} in
-            the editor
+            <template v-if="unsavedEditCount > 0">
+              {{ unsavedEditCount }} edit{{ unsavedEditCount === 1 ? '' : 's' }}
+              not yet saved to the session
+            </template>
+            <template v-else>You have unsaved changes</template>
           </div>
         </div>
       </div>
       <v-card-text class="text-body-medium pt-2 pb-4 px-6">
-        Save your edits before leaving, or discard them to continue. Discarded
-        edits cannot be recovered.
+        <template v-if="canSave">
+          Save your edits to the session before leaving, or discard the edits
+          made since the last save. Discarded edits cannot be recovered.
+        </template>
+        <template v-else>
+          No session is open, so these edits cannot be saved. Discard them to
+          continue. Discarded edits cannot be recovered.
+        </template>
       </v-card-text>
       <v-divider />
       <v-card-actions class="d-flex align-center ga-2 px-4 py-3">
@@ -155,7 +164,7 @@
           color="primary"
           variant="flat"
           prepend-icon="mdi-content-save-outline"
-          :disabled="isBusy"
+          :disabled="isBusy || !canSave"
           :loading="isBusy && exitAction === 'save'"
           @click="saveAndContinue"
         >
@@ -179,26 +188,26 @@ import router from '@/router/router'
 import { useHydroServer } from '@/store/hydroserver'
 import { useWorkspaceStore } from '@/store/workspaces'
 import { usePlotlyStore } from '@/store/plotly'
-import { useQcSubmission } from '@/composables/useQcSubmission'
-import { useDataSelection } from '@/composables/useDataSelection'
+import { useQcSessionStore } from '@/store/qcSession'
+import { useEditSession } from '@/composables/useEditSession'
 
 const { onRailItemClicked } = useUIStore()
 const { selectedDrawer, isDrawerOpen, currentView } = storeToRefs(useUIStore())
-const { resetState, refreshGraphSeriesArray } = useDataVisStore()
+const { resetState } = useDataVisStore()
 const { qcDatastream, qcDatastreamId } = storeToRefs(useDataVisStore())
 const { hs } = storeToRefs(useHydroServer())
 const workspaceStore = useWorkspaceStore()
 const { selectedWorkspace } = storeToRefs(workspaceStore)
-const { editHistory, isUpdating, selectedSeries } =
-  storeToRefs(usePlotlyStore())
 const { redraw } = usePlotlyStore()
-const { submitQcEdits } = useQcSubmission()
-const { clearSelected } = useDataSelection()
+const { inProgressSession } = storeToRefs(useQcSessionStore())
+const { hasUnsavedChanges, unsavedEditCount, saveDraft, discardUnsavedEdits } =
+  useEditSession()
 
-const editCount = computed(() => editHistory.value?.length ?? 0)
-const hasUnsavedEdits = computed(
-  () => currentView.value === DrawerType.Edit && editCount.value > 0
+const needsExitConfirm = computed(
+  () => currentView.value === DrawerType.Edit && hasUnsavedChanges.value
 )
+// Entering Edit through the rail opens no session, so there is nothing to save to.
+const canSave = computed(() => !!inProgressSession.value)
 
 const showExitConfirm = ref(false)
 const exitAction = ref<'save' | 'discard' | null>(null)
@@ -206,7 +215,7 @@ const isBusy = ref(false)
 let pendingAction: (() => void | Promise<void>) | null = null
 
 function guardExit(action: () => void | Promise<void>) {
-  if (hasUnsavedEdits.value) {
+  if (needsExitConfirm.value) {
     pendingAction = action
     showExitConfirm.value = true
   } else {
@@ -220,43 +229,47 @@ function cancelExit() {
   showExitConfirm.value = false
 }
 
+async function continueExit() {
+  const next = pendingAction
+  pendingAction = null
+  showExitConfirm.value = false
+  await next?.()
+}
+
 async function saveAndContinue() {
-  if (isBusy.value) return
+  if (isBusy.value || !canSave.value) return
   isBusy.value = true
   exitAction.value = 'save'
   try {
-    await submitQcEdits()
-    const next = pendingAction
-    pendingAction = null
-    showExitConfirm.value = false
-    await next?.()
+    await saveDraft()
+    Snackbar.success('Draft saved.')
+  } catch (e) {
+    Snackbar.error(e instanceof Error ? e.message : 'Could not save the draft.')
+    return
   } finally {
     isBusy.value = false
     exitAction.value = null
   }
+  await continueExit()
 }
 
 async function discardAndContinue() {
   if (isBusy.value) return
   isBusy.value = true
   exitAction.value = 'discard'
-  isUpdating.value = true
   try {
-    // In-place clear: reassigning `history = []` detaches the editHistory ref.
-    if (selectedSeries.value) selectedSeries.value.data.history.length = 0
-    await refreshGraphSeriesArray()
-    await selectedSeries.value?.data.reload()
-    await clearSelected({ recordHistory: false })
+    await discardUnsavedEdits()
     await redraw()
-    const next = pendingAction
-    pendingAction = null
-    showExitConfirm.value = false
-    await next?.()
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not discard the edits.'
+    )
+    return
   } finally {
-    isUpdating.value = false
     isBusy.value = false
     exitAction.value = null
   }
+  await continueExit()
 }
 
 function goHome() {
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index ce7d48b5..ee67c00d 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -338,6 +338,22 @@ describe('useEditSession', () => {
     expect(session.unsavedEditCount.value).toBe(0)
   })
 
+  it('shares the unsaved state across callers', async () => {
+    await seedHistory()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const editor = useEditSession()
+    const rail = useEditSession()
+    await editor.beginEditing()
+    await editor.startSession(WIN)
+
+    selectedSeries.value.data.history.push({ method: 'DELETE_POINTS', args: [] })
+    expect(rail.hasUnsavedChanges.value).toBe(true)
+    expect(rail.unsavedEditCount.value).toBe(1)
+
+    await rail.saveDraft()
+    expect(editor.hasUnsavedChanges.value).toBe(false)
+  })
+
   // A committed session cannot be edited, so nothing about it can be
   // unsaved. Stepping through its history replaces entries as it replays,
   // which an identity comparison would otherwise read as pending edits.
diff --git a/apps/qc-app/src/composables/__tests__/useQcSubmission.spec.ts b/apps/qc-app/src/composables/__tests__/useQcSubmission.spec.ts
deleted file mode 100644
index 9f8f7ce4..00000000
--- a/apps/qc-app/src/composables/__tests__/useQcSubmission.spec.ts
+++ /dev/null
@@ -1,133 +0,0 @@
-import { describe, it, expect, vi, beforeEach } from 'vitest'
-import { createPinia, setActivePinia } from 'pinia'
-import { ref } from 'vue'
-
-// Snackbar stub on the @uwrl/qc-utils seam.
-vi.mock('@uwrl/qc-utils', () => ({
-  Snackbar: {
-    success: vi.fn(),
-    error: vi.fn(),
-    info: vi.fn(),
-  },
-}))
-
-// Stub the HydroServer store with a resolving createObservations.
-// Individual tests override the mock implementation as needed.
-const createObservations = vi.fn().mockResolvedValue(undefined)
-vi.mock('@/store/hydroserver', () => {
-  const hs = ref({
-    datastreams: {
-      createObservations: (...args: unknown[]) => createObservations(...args),
-    },
-  } as any)
-  return {
-    useHydroServer: () => ({ hs }),
-  }
-})
-
-// Shared mutable history array — the SAME reference is handed to both
-// `selectedSeries.data.history` and `editHistory.value`. This mirrors the
-// production sync at src/utils/plotting/plotly.ts:207/:387 and is what makes
-// the in-place `length = 0` fix in useQcSubmission observable from both seams.
-const historyArr: any[] = [
-  { method: 'delete', indices: [0], values: [1] } as any,
-]
-
-const selectedSeries = ref({
-  id: 'qc',
-  name: 'qc',
-  data: {
-    history: historyArr,
-    dataX: [1700000000000],
-    dataY: [42],
-  },
-} as any)
-const editHistory = ref(historyArr)
-const isSubmitting = ref(false)
-
-vi.mock('@/store/plotly', () => ({
-  usePlotlyStore: () => ({
-    selectedSeries,
-    editHistory,
-    isSubmitting,
-  }),
-}))
-
-vi.mock('@/store/dataVisualization', () => {
-  const qcDatastream = ref({ id: 'datastream-x' } as any)
-  return {
-    useDataVisStore: () => ({ qcDatastream }),
-  }
-})
-
-describe('useQcSubmission.submitQcEdits', () => {
-  beforeEach(() => {
-    setActivePinia(createPinia())
-    vi.clearAllMocks()
-
-    // Reset shared state to a single pending edit, preserving the
-    // shared-array-reference invariant between editHistory and data.history.
-    historyArr.length = 0
-    historyArr.push({ method: 'delete', indices: [0], values: [1] } as any)
-    selectedSeries.value.data.history = historyArr
-    editHistory.value = historyArr
-    isSubmitting.value = false
-    createObservations.mockReset()
-    createObservations.mockResolvedValue(undefined)
-  })
-
-  it('clears editHistory in-place on success', async () => {
-    const { useQcSubmission } = await import('@/composables/useQcSubmission')
-    const { Snackbar } = await import('@uwrl/qc-utils')
-
-    // Pre-condition: shared reference between data.history and editHistory.
-    expect(selectedSeries.value.data.history).toBe(editHistory.value)
-    expect(editHistory.value.length).toBe(1)
-
-    const { submitQcEdits } = useQcSubmission()
-    await submitQcEdits()
-
-    expect(createObservations).toHaveBeenCalledTimes(1)
-    expect(createObservations).toHaveBeenCalledWith(
-      'datastream-x',
-      expect.objectContaining({
-        fields: ['phenomenonTime', 'result'],
-      }),
-      { mode: 'replace' }
-    )
-    expect(Snackbar.success).toHaveBeenCalledWith(
-      'Quality-controlled observations submitted'
-    )
-    expect(editHistory.value.length).toBe(0)
-    expect(selectedSeries.value.data.history.length).toBe(0)
-    expect(isSubmitting.value).toBe(false)
-  })
-
-  it('leaves editHistory untouched on error', async () => {
-    createObservations.mockRejectedValueOnce(new Error('boom'))
-
-    const { useQcSubmission } = await import('@/composables/useQcSubmission')
-    const { Snackbar } = await import('@uwrl/qc-utils')
-
-    const { submitQcEdits } = useQcSubmission()
-    await submitQcEdits()
-
-    expect(Snackbar.error).toHaveBeenCalledTimes(1)
-    expect(editHistory.value.length).toBe(1)
-    expect(selectedSeries.value.data.history.length).toBe(1)
-    expect(isSubmitting.value).toBe(false)
-  })
-
-  it('fires Snackbar.info and skips network when there are no edits', async () => {
-    historyArr.length = 0
-
-    const { useQcSubmission } = await import('@/composables/useQcSubmission')
-    const { Snackbar } = await import('@uwrl/qc-utils')
-
-    const { submitQcEdits } = useQcSubmission()
-    await submitQcEdits()
-
-    expect(Snackbar.info).toHaveBeenCalledWith('No edits to submit')
-    expect(createObservations).not.toHaveBeenCalled()
-  })
-})
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index f05ec448..24d58cf2 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -77,18 +77,15 @@ export function useEditSession() {
   const { fetchObservationsInRange } = useObservationStore()
   const sessionStore = useQcSessionStore()
 
-  const { sourceDatastream } = storeToRefs(sessionStore)
+  // The saved-edits snapshot lives in the store so every caller (the editor
+  // footer and the nav rail's exit guard) sees the same unsaved state.
+  const { sourceDatastream, savedEdits, savedComments } =
+    storeToRefs(sessionStore)
   /** True when the managed datastream has no in-progress session to resume. */
   const needsSession = ref(false)
   /** True when the selected datastream has no QC history (not a managed datastream). */
   const needsHistory = ref(false)
 
-  // Snapshot (by item reference) of the operation history at the last
-  // load/save, so the editor knows whether there are unsaved edits.
-  const savedEdits = ref<HistoryItem[]>([])
-  // Comments are edited in place on the existing history entry, so the
-  // reference comparison below can't see them; snapshot their text too.
-  const savedComments = ref<string[]>([])
   const currentEdits = (): HistoryItem[] =>
     (selectedSeries.value?.data as ObservationRecord | undefined)?.history ?? []
   const commentOf = (item: HistoryItem) => item.comment?.trim() ?? ''
diff --git a/apps/qc-app/src/composables/useQcSubmission.ts b/apps/qc-app/src/composables/useQcSubmission.ts
deleted file mode 100644
index ddec4021..00000000
--- a/apps/qc-app/src/composables/useQcSubmission.ts
+++ /dev/null
@@ -1,60 +0,0 @@
-import { useDataVisStore } from '@/store/dataVisualization'
-import { usePlotlyStore } from '@/store/plotly'
-import { useHydroServer } from '@/store/hydroserver'
-import { observationsBulkBody } from '@/services/qualityControl/observationsBody'
-import { Snackbar } from '@uwrl/qc-utils'
-import { storeToRefs } from 'pinia'
-
-/**
- * Encapsulates the QC submission flow: guard checks, serialization of the
- * edited ObservationRecord into ObservationBulkPostBody, calling
- * hs.datastreams.createObservations with mode 'replace', user-facing
- * Snackbar feedback, and clearing the edit history on a committed signal.
- */
-export function useQcSubmission() {
-  const { hs } = storeToRefs(useHydroServer())
-  const { selectedSeries, isSubmitting } = storeToRefs(usePlotlyStore())
-  const { qcDatastream } = storeToRefs(useDataVisStore())
-
-  const submitQcEdits = async () => {
-    // Guard: need a selected series, a QC datastream id, and at least one edit
-    if (
-      !selectedSeries.value ||
-      !qcDatastream.value?.id ||
-      !selectedSeries.value.data.history.length
-    ) {
-      Snackbar.info('No edits to submit')
-      return
-    }
-
-    // TODO: resultQualifierCodes serialization is deferred — qualifier
-    // tracking in useDataVisStore is still stubbed. Submit only
-    // phenomenonTime + result for now.
-    const body = observationsBulkBody(selectedSeries.value.data)
-
-    isSubmitting.value = true
-    try {
-      await hs.value.datastreams.createObservations(
-        qcDatastream.value.id,
-        body,
-        { mode: 'replace' }
-      )
-      Snackbar.success('Quality-controlled observations submitted')
-      // Clear history in-place so the Pinia editHistory ref (which points
-      // to the same underlying array via storeToRefs sync in plotly.ts:207
-      // and :387) reflects the cleared state without needing a re-sync.
-      // Reassigning (history = []) would desync the reactive ref and leave
-      // EditHistory.vue rendering stale entries (MH-17 E2E regression).
-      selectedSeries.value.data.history.length = 0
-    } catch (err: any) {
-      Snackbar.error(
-        'Failed to submit observations: ' + (err?.message ?? 'unknown error')
-      )
-      // Swallow — UI does not need the error propagated.
-    } finally {
-      isSubmitting.value = false
-    }
-  }
-
-  return { submitQcEdits }
-}
diff --git a/apps/qc-app/src/store/plotly.ts b/apps/qc-app/src/store/plotly.ts
index 4374aef3..c1bb5f34 100644
--- a/apps/qc-app/src/store/plotly.ts
+++ b/apps/qc-app/src/store/plotly.ts
@@ -33,7 +33,6 @@ export const usePlotlyStore = defineStore('Plotly', () => {
   const showLegend = ref(true)
   const showTooltip = ref(false)
   const isUpdating = ref(false)
-  const isSubmitting = ref(false)
   // Persisted as a user preference — large plots are cheap on fast machines
   // and expensive on slow ones, so let the user pick. Bounded in the UI
   // but not hard-clamped here so power users can override via storage.
@@ -536,7 +535,6 @@ export const usePlotlyStore = defineStore('Plotly', () => {
     plotlyRef,
     mainPlotEpoch,
     isUpdating,
-    isSubmitting,
     tooltipsMaxDataPoints,
     visiblePoints,
     tooltipsMode,
diff --git a/apps/qc-app/src/store/qcSession.ts b/apps/qc-app/src/store/qcSession.ts
index 783426c4..c16d4a93 100644
--- a/apps/qc-app/src/store/qcSession.ts
+++ b/apps/qc-app/src/store/qcSession.ts
@@ -12,6 +12,7 @@ import { computed, ref } from 'vue'
 import { useHydroServer } from '@/store/hydroserver'
 import { unwrap } from '@/services/qualityControl'
 import type { Datastream, QualityControlSession } from '@hydroserver/client'
+import type { HistoryItem } from '@uwrl/qc-utils'
 
 export const useQcSessionStore = defineStore('qcSession', () => {
   const historyId = ref<string | null>(null)
@@ -27,6 +28,12 @@ export const useQcSessionStore = defineStore('qcSession', () => {
   const viewedSessionId = ref<string | null>(null)
   const isLoading = ref(false)
   const isSwitchingSession = ref(false)
+  /** Edit history entries (by reference) at the last load or save, the
+   *  baseline `useEditSession` compares against for unsaved edits. Kept here
+   *  so the editor and the nav rail's exit guard agree. */
+  const savedEdits = ref<HistoryItem[]>([])
+  /** Comment text of `savedEdits`, since comments are edited in place. */
+  const savedComments = ref<string[]>([])
 
   /** Editing is allowed only while viewing the in-progress session. */
   // TODO(backend): ask for a way to keep editing the most recent session after
@@ -127,6 +134,8 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     currentSessionId.value = null
     viewedSessionId.value = null
     isLoading.value = false
+    savedEdits.value = []
+    savedComments.value = []
   }
 
   return {
@@ -138,6 +147,8 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     viewedSessionId,
     isLoading,
     isSwitchingSession,
+    savedEdits,
+    savedComments,
     isReadOnly,
     inProgressSession,
     committedSessions,

From 745e6ebc766140e4155a97755fbc3572806245c6 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 14:24:02 -0600
Subject: [PATCH 47/93] feat: plot the session working copy for managed
 datastreams

---
 apps/qc-app/docs/API_REFERENCE.md             |   5 +-
 apps/qc-app/docs/USER_GUIDE.md                |   2 +
 .../store/__tests__/dataVisualization.spec.ts | 111 +++++++++++++++++-
 apps/qc-app/src/store/dataVisualization.ts    |  47 +++++++-
 apps/qc-app/src/store/plotly.ts               |  30 +++--
 5 files changed, 178 insertions(+), 17 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 4594422b..25991eff 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -297,7 +297,7 @@ on boot.
 | `matchesSelectedProcessingLevel`    | action   | `(ds) => boolean`                                 | Same shape as above. |
 | `setDateRange`                      | action   | `({ begin?, end?, update?, custom? }) => Promise<void>` | No-ops when neither bound moves; clears zoom history when it does. |
 | `onDateBtnClick`                    | action   | `(id: number) => Promise<void>`                   | Selects the preset and applies its window over the plotted data's extent. With nothing plotted, only the selection changes. |
-| `refreshGraphSeriesArray`           | action   | `() => Promise<unknown[]>`                        | Reconciles `graphSeriesArray` against `plottedDatastreams` (fetch deltas + reorder + recolor). |
+| `refreshGraphSeriesArray`           | action   | `() => Promise<unknown[]>`                        | Reconciles `graphSeriesArray` against `plottedDatastreams` (fetch deltas + reorder + recolor). A managed datastream with a loaded working copy (`useWorkingCopiesStore`) uses it instead of fetching. |
 | `resetState`                        | action   | `() => void`                                      | Clears filters + plotted set on a workspace swap; preserves the preset preference. |
 | `toggleDatastream`                  | action   | `(ds: Datastream) => Promise<void>`               | Plot if absent, unplot if present. |
 | `plotDatastream`                    | action   | `(ds: Datastream) => Promise<void>`               | Add to plot; promotes to QC when nothing's there yet. |
@@ -389,7 +389,8 @@ handles, live chart caches).
 | `updateOptions`            | action   | `() => void`                                      | Rebuild `plotlyOptions` from `graphSeriesArray`. |
 | `redraw`                   | action   | `(recomputeXaxisRange?: boolean, preserveZoom?: boolean) => Promise<void>` | Push typed-array updates + restyle; preserves the user's zoom by default. |
 | `clearChartState`          | action   | `() => void`                                      | Drop all series + zoom history (used on workspace swap). |
-| `fetchGraphSeries`         | action   | `(ds, start: Date, end: Date) => Promise<GraphSeries>` | Build a fresh `GraphSeries` from observations; colour is filled later by `assignSeriesColors`. |
+| `fetchGraphSeries`         | action   | `(ds, start: Date, end: Date) => Promise<GraphSeries>` | Fetch observations for `ds` over `[start, end]` and build a `GraphSeries` via `buildGraphSeries`. |
+| `buildGraphSeries`         | action   | `(ds: Datastream, data: ObservationRecord) => GraphSeries` | Build a `GraphSeries` from an already-loaded record, no fetch. Used directly for a managed datastream's working copy. |
 | `assignSeriesColors`       | action   | `(orderedIds: string[]) => void`                  | Stable per-id colour assignment over the legend order. |
 | `colorForDatastream`       | action   | `(id?: string) => string`                         | Resolve the line colour for a datastream (QC is always black). |
 | `labelColorForDatastream`  | action   | `(id?: string) => string`                         | Darker companion for legend text. |
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 2cbb9eee..55d156df 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -215,6 +215,8 @@ The right-hand list (visible on both Select and Edit views) is the roster of cur
 
 If a plotted datastream has no observations in the current window (either because the dataset is empty there or because the chosen time range doesn't cover its data), the row title shows a small warning-tinted database icon. Hover it for the tooltip "No observations in the current time window". Widening the time range (or clicking **All** in the Time range section) usually clears it.
 
+A QC datastream whose edit session is in progress plots that session's working data instead — the same data the editor opens: committed data where it exists, otherwise the raw datastream, over the session's time window, with saved draft edits applied. Its line always covers the whole session window, and presets anchor to that window rather than the (empty) stored managed datastream. With no session in progress it shows its committed observations as usual.
+
 ![Plotted datastreams list with two rows](./images/plotted-datastreams-list.png)
 
 ### Plotting multiple datastreams
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index 129c94e1..d28957f2 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -1,16 +1,19 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
 import { createPinia, setActivePinia } from 'pinia'
 import piniaPluginPersistedstate from 'pinia-plugin-persistedstate'
-import { createApp, ref } from 'vue'
+import { createApp, ref, shallowRef } from 'vue'
 import { subtractMonths } from '@/utils/dateMath'
 
 // Shared mutable stub state so each test can reset between runs.
 const mockPlotlyRef = ref<any>(null)
-const mockGraphSeriesArray = ref<any[]>([])
+// shallowRef: pushed objects keep their identity (not deep-reactive-wrapped),
+// so tests can assert `data` is the exact working-copy record reference.
+const mockGraphSeriesArray = shallowRef<any[]>([])
 const mockUpdateOptions = vi.fn()
 const mockClearChartState = vi.fn()
 const mockClearZoomHistory = vi.fn()
 const mockFetchGraphSeries = vi.fn().mockResolvedValue({ id: 'stub', data: {} })
+const mockBuildGraphSeries = vi.fn((ds: any, data: any) => ({ id: ds.id, name: ds.name, data }))
 const mockAssignSeriesColors = vi.fn()
 const mockRedraw = vi.fn()
 const mockFetchObservationsInRange = vi
@@ -25,6 +28,7 @@ vi.mock('@/store/plotly', () => ({
     clearChartState: mockClearChartState,
     clearZoomHistory: mockClearZoomHistory,
     fetchGraphSeries: mockFetchGraphSeries,
+    buildGraphSeries: mockBuildGraphSeries,
     assignSeriesColors: mockAssignSeriesColors,
     redraw: mockRedraw,
   }),
@@ -36,6 +40,22 @@ vi.mock('@/store/observations', () => ({
   }),
 }))
 
+const mockWorkingCopies = new Map<string, any>()
+const mockLoadWorkingCopy = vi.fn(async (managed: any) => mockWorkingCopies.get(managed.id) ?? null)
+vi.mock('@/store/workingCopies', () => ({
+  useWorkingCopiesStore: () => ({
+    get: (id: string) => mockWorkingCopies.get(id),
+    load: mockLoadWorkingCopy,
+    extents: (ids: string[]) =>
+      ids.flatMap((id) => {
+        const c = mockWorkingCopies.get(id)
+        return c
+          ? [{ phenomenonBeginTime: c.begin.toISOString(), phenomenonEndTime: c.end.toISOString() }]
+          : []
+      }),
+  }),
+}))
+
 // Only `currentView` matters to the store under test; the real store also
 // wires up qcDatastream/operationParams watchers that don't belong here.
 const mockCurrentView = ref<'Edit' | 'Select'>('Select')
@@ -72,6 +92,7 @@ beforeEach(() => {
   mockPlotlyRef.value = null
   mockGraphSeriesArray.value = []
   mockCurrentView.value = 'Select'
+  mockWorkingCopies.clear()
 })
 
 afterEach(() => {
@@ -1046,6 +1067,92 @@ describe('useDataVisStore.releaseManagedDatastream with the source plotted', ()
   })
 })
 
+describe('useDataVisStore managed datastream working copy', () => {
+  const withManaged = (store: any) => {
+    store.qcHistories = [
+      { id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'src' },
+    ] as any
+    store.datastreams = [
+      makeDs({ id: 'src', name: 'Raw', phenomenonBeginTime: '2020-01-01T00:00:00Z', phenomenonEndTime: '2022-01-01T00:00:00Z' }),
+      makeDs({ id: 'mgd', name: 'Raw (QC)', phenomenonBeginTime: null, phenomenonEndTime: null }),
+    ] as any
+  }
+  const copy = { sessionId: 's-1', record: { dataX: [1], history: [] }, begin: new Date('2021-03-08T00:00:00Z'), end: new Date('2021-08-18T00:00:00Z') }
+
+  it('plots the working copy instead of fetching the empty managed datastream', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withManaged(store)
+    mockWorkingCopies.set('mgd', copy)
+
+    await store.plotDatastream(store.datastreams[1] as any)
+
+    expect(mockLoadWorkingCopy).toHaveBeenCalledWith(
+      expect.objectContaining({ id: 'mgd' }),
+      expect.objectContaining({ id: 'src' }),
+      'h-1'
+    )
+    expect(mockFetchGraphSeries).not.toHaveBeenCalled()
+    expect(mockGraphSeriesArray.value.find((s: any) => s.id === 'mgd')?.data).toBe(copy.record)
+  })
+
+  it('anchors presets to the working copy session window', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withManaged(store)
+    mockWorkingCopies.set('mgd', copy)
+
+    await store.plotDatastream(store.datastreams[1] as any)
+
+    expect(store.endDate.toISOString()).toBe('2021-08-18T00:00:00.000Z')
+  })
+
+  it('fetches as before when there is no session in progress', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withManaged(store)
+
+    await store.plotDatastream(store.datastreams[1] as any)
+
+    expect(mockFetchGraphSeries).toHaveBeenCalledWith(
+      expect.objectContaining({ id: 'mgd' }),
+      expect.any(Date),
+      expect.any(Date)
+    )
+  })
+
+  it('keeps the working copy record on a refresh of an existing series', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withManaged(store)
+    mockWorkingCopies.set('mgd', copy)
+    await store.plotDatastream(store.datastreams[1] as any)
+
+    await store.setDateRange({
+      begin: new Date('2021-01-01T00:00:00Z'),
+      end: new Date('2021-02-01T00:00:00Z'),
+    })
+
+    expect(mockFetchObservationsInRange).not.toHaveBeenCalledWith(
+      expect.objectContaining({ id: 'mgd' }),
+      expect.anything(),
+      expect.anything()
+    )
+    expect(mockGraphSeriesArray.value.find((s: any) => s.id === 'mgd')?.data).toBe(copy.record)
+  })
+
+  it('falls back to fetching when loading the working copy fails', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withManaged(store)
+    mockLoadWorkingCopy.mockRejectedValueOnce(new Error('network'))
+
+    await store.plotDatastream(store.datastreams[1] as any)
+
+    expect(mockFetchGraphSeries).toHaveBeenCalled()
+  })
+})
+
 describe('useDataVisStore.replaceDatastream', () => {
   it('swaps the datastream in the catalog and the plotted set', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index f9dfd6e6..5d5f6ea2 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -14,6 +14,7 @@ import {
 } from '@/utils/timeRangePresets'
 import { isSnapshotId } from '@/utils/snapshotId'
 import { DrawerType, useUIStore } from '@/store/userInterface'
+import { useWorkingCopiesStore } from '@/store/workingCopies'
 import type { SnapshotMeta } from '@/types'
 import type { ObservationRecord } from '@uwrl/qc-utils'
 import {
@@ -31,6 +32,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     updateOptions,
     clearChartState,
     fetchGraphSeries,
+    buildGraphSeries,
     assignSeriesColors,
   } = usePlotlyStore()
   const { fetchObservationsInRange } = useObservationStore()
@@ -50,6 +52,33 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   const historySourceId = (h: QualityControlHistory): string | undefined =>
     (h as any).sourceDatastreamId ?? (h as any).sourceDatastream?.id
 
+  /** History and catalog source for a managed datastream, if both are known. */
+  function managedContext(managedId: string) {
+    const history = qcHistories.value.find((h) => historyManagedId(h) === managedId)
+    const sourceId = history ? historySourceId(history) : undefined
+    const source = sourceId ? datastreams.value.find((d) => d.id === sourceId) : undefined
+    return history && source ? { historyId: (history as any).id as string, source } : null
+  }
+
+  /** Load or reuse the working copy of every plotted managed datastream. */
+  async function loadWorkingCopies() {
+    const workingCopies = useWorkingCopiesStore()
+    await Promise.all(
+      plottedDatastreams.value
+        .filter((ds) => managedDatastreamIds.value.has(ds.id))
+        .map(async (ds) => {
+          const context = managedContext(ds.id)
+          if (!context) return
+          try {
+            await workingCopies.load(ds, context.source, context.historyId)
+          } catch (error) {
+            // Plotting still works from the committed observations.
+            console.error(`Failed to load the working copy for ${ds.id}:`, error)
+          }
+        })
+    )
+  }
+
   /** Ids of every managed datastream; these are hidden from the catalog. */
   const managedDatastreamIds = computed(() => {
     const ids = new Set<string>()
@@ -149,7 +178,11 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
    *  range or when nothing plotted has observations. */
   function resolvePresetWindow() {
     if (selectedDateBtnId.value === CUSTOM_PRESET_ID) return null
-    const extent = dataExtent(plottedDatastreams.value)
+    const workingCopies = useWorkingCopiesStore()
+    const extent = dataExtent([
+      ...plottedDatastreams.value,
+      ...workingCopies.extents(plottedDatastreams.value.map((d) => d.id)),
+    ])
     return extent ? presetWindow(selectedDateBtnId.value, extent) : null
   }
 
@@ -473,6 +506,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     }
     const { clearZoomHistory } = usePlotlyStore()
     clearZoomHistory()
+    await loadWorkingCopies()
     // Presets re-anchor only in the Select view. In the Edit view the
     // loaded window is the edit session's window; moving it here would
     // refetch the working copy over a different range and corrupt what
@@ -605,6 +639,17 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     end: Date
   ) => {
     try {
+      // A managed datastream with a session in progress plots its working
+      // copy: it spans the session window and is never re-windowed, since a
+      // window change would reload it and drop the replayed edits.
+      const workingCopy = useWorkingCopiesStore().get(datastream.id)
+      if (workingCopy) {
+        const existing = graphSeriesArray.value.find((s) => s.id === datastream.id)
+        if (existing) existing.data = workingCopy.record
+        else graphSeriesArray.value.push(buildGraphSeries(datastream, workingCopy.record))
+        return
+      }
+
       const seriesIndex = graphSeriesArray.value.findIndex(
         (series) => series.id === datastream.id
       )
diff --git a/apps/qc-app/src/store/plotly.ts b/apps/qc-app/src/store/plotly.ts
index c1bb5f34..63c9d474 100644
--- a/apps/qc-app/src/store/plotly.ts
+++ b/apps/qc-app/src/store/plotly.ts
@@ -2,6 +2,7 @@ import { GraphSeries } from '@/types'
 import { defineStore, storeToRefs } from 'pinia'
 import { computed, Ref, ref } from 'vue'
 import { HistoryItem } from "@uwrl/qc-utils"
+import type { ObservationRecord } from "@uwrl/qc-utils"
 import type { LayoutAxis, PlotData } from 'plotly.js-dist'
 import { useDataVisStore } from './dataVisualization'
 
@@ -362,19 +363,10 @@ export const usePlotlyStore = defineStore('Plotly', () => {
     }
   }
 
-  const fetchGraphSeries = async (
+  const buildGraphSeries = (
     datastream: Datastream,
-    start: Date,
-    end: Date
-  ): Promise<GraphSeries> => {
-    const { fetchObservationsInRange } = useObservationStore()
-
-    const data = await fetchObservationsInRange(datastream, start, end)
-
-    if (!data.dataset.source.x) {
-      await data.reload()
-    }
-
+    data: ObservationRecord
+  ): GraphSeries => {
     // HydroServer returns full `observedProperty` / `unit` objects on
     // the wire even though the published `Datastream` type only carries
     // their ids. The catalog endpoint enriches the response, so we
@@ -410,6 +402,19 @@ export const usePlotlyStore = defineStore('Plotly', () => {
     } as GraphSeries
   }
 
+  const fetchGraphSeries = async (
+    datastream: Datastream,
+    start: Date,
+    end: Date
+  ): Promise<GraphSeries> => {
+    const { fetchObservationsInRange } = useObservationStore()
+    const data = await fetchObservationsInRange(datastream, start, end)
+    if (!data.dataset.source.x) {
+      await data.reload()
+    }
+    return buildGraphSeries(datastream, data)
+  }
+
   /**
    * Convert a datastream's declared `intendedTimeSpacing` (+ unit) into
    * milliseconds. Returns null when either field is missing or unusable
@@ -531,6 +536,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
     labelColorForDatastream,
     assignSeriesColors,
     fetchGraphSeries,
+    buildGraphSeries,
     plotlyOptions,
     plotlyRef,
     mainPlotEpoch,

From 8c9cf533226573572ce49ea896b9b74cc27d8d27 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 14:36:43 -0600
Subject: [PATCH 48/93] refactor: share the session working copy between the
 plot and the editor

---
 apps/qc-app/docs/ARCHITECTURE.md              |  12 +-
 .../components/Navigation/NavigationRail.vue  |   6 +
 apps/qc-app/src/components/VisualizeData.vue  |   7 ++
 .../__tests__/useEditSession.spec.ts          | 108 +++++++++++++++---
 apps/qc-app/src/composables/useEditSession.ts |  39 ++++---
 5 files changed, 137 insertions(+), 35 deletions(-)

diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 38cc0a72..0519b781 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -277,10 +277,18 @@ Two contract notes worth keeping in mind:
 - **A reload resumes from the last save, not from memory.** Only
   `qcSession.resumeDatastreamId` is persisted; on load the editor replots that
   datastream and re-runs `beginEditing`, which rebuilds the working copy from
-  the server via `reconstructSession`. Edits made since the last save are not
-  recoverable, so `useUnsavedChangesWarning` raises the browser's native
+  the server via `workingCopies.rebuild`. Edits made since the last save are
+  not recoverable, so `useUnsavedChangesWarning` raises the browser's native
   confirmation while `hasUnsavedChanges` is true. A deliberate exit clears the
   pointer, so only an interrupted session reopens.
+- **The editor and the Select-view plot share one working copy per managed
+  datastream**, cached in `workingCopies` (`store/workingCopies.ts`). It is
+  built on a copy of the base (`reconstructSession`/`loadLatestBase`'s
+  `cloneRecord`), never the observation store's cached record, so the raw
+  datastream's cached observations are never edited. It is dropped
+  (`invalidate`) on commit, on leaving the editor, and when its session or
+  managed datastream is deleted, so the next preview or resume rebuilds it
+  from what was actually saved.
 - **Viewing a past session replays its ancestor chain from the source.**
   The managed datastream carries every commit, so it cannot be the base for a
   historical view: replaying an older session's operations on top of it would
diff --git a/apps/qc-app/src/components/Navigation/NavigationRail.vue b/apps/qc-app/src/components/Navigation/NavigationRail.vue
index 5c0b53cb..4dd9416d 100644
--- a/apps/qc-app/src/components/Navigation/NavigationRail.vue
+++ b/apps/qc-app/src/components/Navigation/NavigationRail.vue
@@ -190,11 +190,13 @@ import { useWorkspaceStore } from '@/store/workspaces'
 import { usePlotlyStore } from '@/store/plotly'
 import { useQcSessionStore } from '@/store/qcSession'
 import { useEditSession } from '@/composables/useEditSession'
+import { useWorkingCopiesStore } from '@/store/workingCopies'
 
 const { onRailItemClicked } = useUIStore()
 const { selectedDrawer, isDrawerOpen, currentView } = storeToRefs(useUIStore())
 const { resetState } = useDataVisStore()
 const { qcDatastream, qcDatastreamId } = storeToRefs(useDataVisStore())
+const workingCopies = useWorkingCopiesStore()
 const { hs } = storeToRefs(useHydroServer())
 const workspaceStore = useWorkspaceStore()
 const { selectedWorkspace } = storeToRefs(workspaceStore)
@@ -303,6 +305,10 @@ async function onSwitchWorkspace() {
   // filters to router.replace, racing our push and stranding the user.
   // `switch=1` prevents the Workspaces picker from auto-redirecting back.
   await router.push({ name: 'Workspaces', query: { switch: '1' } })
+  // Deselecting the datastream leaves the editor without releasing it via
+  // exitToSelect; drop its working copy here so a later preview of it (back
+  // in this workspace) rebuilds from what was actually saved.
+  if (qcDatastream.value) workingCopies.invalidate(qcDatastream.value.id)
   qcDatastreamId.value = null
   currentView.value = DrawerType.Select
   selectedDrawer.value = DrawerType.Select
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index 1bb8b19c..ca021692 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -571,8 +571,10 @@ import { isSnapshotId, parseSnapshotId } from '@/utils/snapshotId'
 import { useHistorySnapshots } from '@/composables/useHistorySnapshots'
 import { useWorkspaceStore } from '@/store/workspaces'
 import { useResizable, usePersistedFlag } from '@/composables/useResizable'
+import { useWorkingCopiesStore } from '@/store/workingCopies'
 
 const { resetState } = useDataVisStore()
+const workingCopies = useWorkingCopiesStore()
 const { toggleSnapshot } = useHistorySnapshots()
 const {
   plottedDatastreams,
@@ -787,6 +789,9 @@ function exitToSelect() {
   isDrawerOpen.value = true
   // Leaving the editor deliberately, so a later reload shouldn't reopen it.
   resumeDatastreamId.value = null
+  // Unsaved edits live only on the working copy; the next preview rebuilds
+  // it from what was saved.
+  if (qcDatastream.value) workingCopies.invalidate(qcDatastream.value.id)
   // Put the source back in the plot so the catalog table shows the row the
   // user had selected; the managed datastream it was swapped for is hidden
   // from that table.
@@ -1190,6 +1195,7 @@ async function onChooserDelete(option: ManagedDatastreamOption) {
   try {
     await deleteManaged(option.historyId, option.managed.id)
     removeManagedDatastream(option.historyId, option.managed.id)
+    workingCopies.invalidate(option.managed.id)
     chooserOptions.value = chooserOptions.value.filter(
       (o) => o.historyId !== option.historyId
     )
@@ -1213,6 +1219,7 @@ async function onChooserDeleteSession(
       option.historyId,
       collectDeletionChain(option.sessions, sessionId)
     )
+    workingCopies.invalidate(option.managed.id)
     const gone = new Set(deleted)
     chooserOptions.value = chooserOptions.value.map((o) =>
       o.historyId === option.historyId
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index ee67c00d..cc52dcf5 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -28,6 +28,17 @@ vi.mock('@/store/observations', () => ({
   useObservationStore: () => ({ fetchObservationsInRange }),
 }))
 
+const wcRebuild = vi.fn()
+const wcSet = vi.fn()
+const wcInvalidate = vi.fn()
+vi.mock('@/store/workingCopies', () => ({
+  useWorkingCopiesStore: () => ({
+    rebuild: wcRebuild,
+    set: wcSet,
+    invalidate: wcInvalidate,
+  }),
+}))
+
 // qc-utils is only used at runtime by the composable (the service layer
 // imports types only), so stub serializeHistory/applyHistory/Snackbar here.
 // `vi.hoisted` keeps these safe to reference from the hoisted `vi.mock`
@@ -117,6 +128,12 @@ beforeEach(() => {
   getItem.mockResolvedValue({ id: 's-1', name: 'Source' })
   createObservations.mockResolvedValue(undefined)
   fetchObservationsInRange.mockResolvedValue(makeRecord())
+  wcRebuild.mockResolvedValue({
+    sessionId: 'x',
+    record: makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }]),
+    begin: new Date(0),
+    end: new Date(1),
+  })
 })
 
 const seedHistory = async () => {
@@ -145,40 +162,65 @@ describe('useEditSession', () => {
     expect(needsSession.value).toBe(false)
   })
 
-  it('beginEditing resumes an in-progress session (replays via applyHistory)', async () => {
+  it('beginEditing resumes an in-progress session from the shared working copy', async () => {
     const h = unwrap(
       await qc.histories.create({
         managedDatastreamId: 'm-1',
         sourceDatastreamId: 's-1',
       })
     )
-    await qc.sessions.create(h.id, WIN)
-    // The reconstructed working copy that resume should wire into the series.
-    const reconstructed = makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }])
-    fetchObservationsInRange.mockResolvedValue(reconstructed)
+    const created = unwrap(await qc.sessions.create(h.id, WIN))
+    // The working copy resume should wire into the series, shared with the
+    // Select-view plot via the workingCopies store.
+    const sharedRecord = makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }])
+    wcRebuild.mockResolvedValue({
+      sessionId: created.id,
+      record: sharedRecord,
+      begin: new Date(0),
+      end: new Date(1),
+    })
     const { useEditSession } = await import('@/composables/useEditSession')
-    const qcUtils = await import('@uwrl/qc-utils')
     const { beginEditing, needsSession } = useEditSession()
     await beginEditing()
     expect(needsSession.value).toBe(false)
-    // The QC-target series shows the reconstructed session's data, but on
-    // its own copy: resume never hands out the fetched record itself, so
-    // editing it can't mutate the store's cached one.
-    const seriesRecord = selectedSeries.value.data
-    expect(Array.from(seriesRecord.dataX)).toEqual(Array.from(reconstructed.dataX))
-    expect(toRaw(seriesRecord)).not.toBe(reconstructed)
-    // applyHistory replayed onto that same copy, not the fetched record.
-    // `toRaw` unwraps the reactive proxy Vue puts on the assigned object,
-    // since applyHistory was called with the pre-proxy instance.
-    const applyHistoryMock = qcUtils.applyHistory as unknown as {
-      mock: { calls: unknown[][] }
-    }
-    expect(applyHistoryMock.mock.calls[0]?.[0]).toBe(toRaw(seriesRecord))
+    expect(wcRebuild).toHaveBeenCalledWith(
+      expect.objectContaining({ id: 'm-1' }),
+      expect.objectContaining({ id: 's-1' }),
+      h.id,
+      expect.objectContaining({ id: created.id })
+    )
+    // The QC-target series shows exactly the record the working copy store
+    // resolved, so the plot and the editor never diverge. `toRaw` unwraps
+    // the reactive proxy Vue puts on the assigned object.
+    expect(toRaw(selectedSeries.value.data)).toBe(sharedRecord)
     // Nothing watches for a swapped-in record, so resume has to rebuild the
     // plot itself or it keeps rendering the pre-reconstruction trace.
     expect(redraw).toHaveBeenCalled()
   })
 
+  it('beginEditing leaves the session unresumed when the working copy is invalidated mid-resume', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    await qc.sessions.create(h.id, WIN)
+    // Superseded by an invalidate while resuming (e.g. the user already left
+    // Edit, or the session was deleted) leaves nothing cached to show.
+    wcRebuild.mockResolvedValueOnce(null)
+    const original = selectedSeries.value.data
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const { beginEditing, needsSession } = useEditSession()
+    await beginEditing()
+    // Falls back to "needs a session": entering the editor via `enterEdit`
+    // will auto-start one, and `startOrResumeSession` is idempotent, so it
+    // resumes the same in-progress session rather than creating a new one.
+    expect(needsSession.value).toBe(true)
+    expect(selectedSeries.value.data).toBe(original)
+    expect(redraw).not.toHaveBeenCalled()
+  })
+
   it('startSession loads the managed datastream as the working base', async () => {
     await seedHistory()
     const managedBase = makeRecord()
@@ -412,6 +454,34 @@ describe('useEditSession', () => {
     const store = useQcSessionStore()
     expect(store.committedSessions[0]?.description).toBe('Reviewed January spike')
   })
+
+  it('startSession stores its base as the working copy', async () => {
+    await seedHistory()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+
+    const inProgress = useQcSessionStore().inProgressSession!
+    expect(wcSet).toHaveBeenCalledWith(
+      'm-1',
+      inProgress.id,
+      selectedSeries.value.data,
+      new Date(inProgress.phenomenonTimeStart),
+      new Date(inProgress.phenomenonTimeEnd)
+    )
+  })
+
+  it('commit drops the working copy once the session is committed', async () => {
+    await seedHistory()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    await session.commit()
+
+    expect(wcInvalidate).toHaveBeenCalledWith('m-1')
+  })
 })
 
 describe('useEditSession.discardUnsavedEdits', () => {
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index 24d58cf2..4356f1cc 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -23,13 +23,13 @@ import { usePlotlyStore } from '@/store/plotly'
 import { useHydroServer } from '@/store/hydroserver'
 import { useObservationStore } from '@/store/observations'
 import { useQcSessionStore } from '@/store/qcSession'
+import { useWorkingCopiesStore } from '@/store/workingCopies'
 import {
   findHistoryForDatastream,
   startOrResumeSession,
   loadLatestBase,
   persistSessionOperations,
   commitQcSession,
-  reconstructSession,
   reconstructCommittedSession,
   observationsBulkBody,
 } from '@/services/qualityControl'
@@ -76,6 +76,7 @@ export function useEditSession() {
   const { hs } = storeToRefs(useHydroServer())
   const { fetchObservationsInRange } = useObservationStore()
   const sessionStore = useQcSessionStore()
+  const workingCopies = useWorkingCopiesStore()
 
   // The saved-edits snapshot lives in the store so every caller (the editor
   // footer and the nav rail's exit guard) sees the same unsaved state.
@@ -138,23 +139,25 @@ export function useEditSession() {
     const inProgress = sessionStore.inProgressSession
     const record = selectedSeries.value?.data
     if (inProgress && sourceDatastream.value && record) {
-      // Resume: reconstruct the in-progress session's working state (source
-      // window + replayed draft operations) and wire it into the QC-target
-      // series so the editor shows the saved edits instead of the empty,
-      // uncommitted managed datastream.
-      const { record: reconstructed } = await reconstructSession(
-        {
-          qcSessions: hs.value.qualityControlSessions,
-          qcOperations: hs.value.qualityControlOperations,
-          fetchInRange: fetchObservationsInRange,
-          applyHistory,
-        },
+      // Resume: rebuild the same working copy the Select-view plot shows,
+      // from the server, so it holds exactly the saved draft operations.
+      const built = await workingCopies.rebuild(
         managed,
         sourceDatastream.value,
         history.id,
-        inProgress.id
+        inProgress
       )
-      if (selectedSeries.value) selectedSeries.value.data = reconstructed
+      if (!built) {
+        // The rebuild was superseded by an invalidate while in flight (the
+        // user already left Edit, or the session/datastream was deleted
+        // mid-resume): there is nothing to wire in. Leave `needsSession` true
+        // rather than fake a resume; `enterEdit` treats that as "start a
+        // session", and `startOrResumeSession` is idempotent, so it resumes
+        // this same in-progress session instead of creating a duplicate.
+        needsSession.value = true
+        return
+      }
+      if (selectedSeries.value) selectedSeries.value.data = built.record
       // Nothing watches for a swapped-in record; resume has no caller that
       // redraws, unlike the start-session path.
       await redraw()
@@ -240,6 +243,13 @@ export function useEditSession() {
       new Date(session.phenomenonTimeEnd)
     )
     if (selectedSeries.value) selectedSeries.value.data = base
+    workingCopies.set(
+      managed.id,
+      session.id,
+      base,
+      new Date(session.phenomenonTimeStart),
+      new Date(session.phenomenonTimeEnd)
+    )
     needsSession.value = false
     // Fresh session: the loaded working copy is the saved baseline.
     snapshotSavedEdits()
@@ -319,6 +329,7 @@ export function useEditSession() {
         })
       },
     })
+    workingCopies.invalidate(managed.id)
     await sessionStore.loadSessions(historyId)
     snapshotSavedEdits()
     // The push moved the managed datastream's phenomenon times. Refresh last so

From e2bec2a433579c29f3421342708b6913d116dddd Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 14:42:50 -0600
Subject: [PATCH 49/93] test: cover the managed datastream working copy preview

---
 apps/qc-app/docs/TESTING.md             |  2 +-
 apps/qc-app/e2e/managed-preview.spec.ts | 79 +++++++++++++++++++++++++
 2 files changed, 80 insertions(+), 1 deletion(-)
 create mode 100644 apps/qc-app/e2e/managed-preview.spec.ts

diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 671665e8..303bd0d1 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -13,7 +13,7 @@ the operator's manual; QUALITY.md is the policy.
 | Layer      | Runner                          | Where                        | Count    |
 | ---------- | ------------------------------- | ---------------------------- | -------- |
 | Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 58 files |
-| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 29 files |
+| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 30 files |
 
 There is no separate "integration" tier — component tests live in the
 unit tier and mount real Vue components with the Vue Test Utils
diff --git a/apps/qc-app/e2e/managed-preview.spec.ts b/apps/qc-app/e2e/managed-preview.spec.ts
new file mode 100644
index 00000000..1f781754
--- /dev/null
+++ b/apps/qc-app/e2e/managed-preview.spec.ts
@@ -0,0 +1,79 @@
+/**
+ * A managed datastream with nothing committed and a session in progress plots
+ * the session's working copy, the same data the editor opens.
+ */
+
+import { expect, test } from '@playwright/test'
+import { installMocks, type MockQcSession } from './support/mocks'
+import { gotoHome } from './support/app'
+import {
+  DATASTREAM_ID,
+  FIXTURE_OBS_COUNT,
+  FIXTURE_OBS_END_ISO,
+  FIXTURE_OBS_START_ISO,
+  MANAGED_DATASTREAM_ID,
+  QC_HISTORY_ID,
+} from './support/fixtures'
+
+/** A fresh in-progress session over the fixture window, no saved operations. */
+function inProgressSession(): MockQcSession {
+  return {
+    id: 'qcs-e2e-draft',
+    historyId: QC_HISTORY_ID,
+    status: 'in_progress',
+    description: 'Draft',
+    phenomenonTimeStart: FIXTURE_OBS_START_ISO,
+    phenomenonTimeEnd: FIXTURE_OBS_END_ISO,
+    sourceChecksum: 'e2e-draft-checksum',
+    createdAt: FIXTURE_OBS_START_ISO,
+    committedAt: null,
+    createdBy: { name: 'Test User', email: 'test@example.com' },
+    dependencyIds: [],
+    operations: [],
+  }
+}
+
+test.describe('managed datastream preview', () => {
+  test.beforeEach(async ({ page }) => {
+    await installMocks(page, {
+      qcHistories: true,
+      qcSessionState: [inProgressSession()],
+      observationsById: {
+        [MANAGED_DATASTREAM_ID]: { phenomenonTime: [], result: [] },
+      },
+      catalogOverrides: {
+        [MANAGED_DATASTREAM_ID]: {
+          valueCount: 0,
+          phenomenonBeginTime: null,
+          phenomenonEndTime: null,
+        },
+      },
+    })
+    await gotoHome(page)
+  })
+
+  const plotOptions = async (page: any, ids: string[]) => {
+    await page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`).click()
+    await expect(page.getByTestId('plot-source-dialog')).toBeVisible()
+    for (const id of ids) {
+      await page.getByTestId(`plot-option-${id}`).locator('input').check()
+    }
+    await page.getByTestId('plot-source-apply').click()
+    await expect(page.getByTestId('plot-source-dialog')).toBeHidden()
+  }
+
+  test('plots the session working copy instead of an empty line', async ({ page }) => {
+    await plotOptions(page, [MANAGED_DATASTREAM_ID])
+    await expect(page.locator('.plotted-item__subtitle').first()).toHaveText(
+      `${FIXTURE_OBS_COUNT} pts loaded`
+    )
+  })
+
+  test('plots raw and working copy side by side', async ({ page }) => {
+    await plotOptions(page, [DATASTREAM_ID, MANAGED_DATASTREAM_ID])
+    const subtitles = page.locator('.plotted-item__subtitle')
+    await expect(subtitles).toHaveCount(2)
+    await expect(subtitles.nth(0)).toHaveText(`${FIXTURE_OBS_COUNT} pts loaded`)
+    await expect(subtitles.nth(1)).toHaveText(`${FIXTURE_OBS_COUNT} pts loaded`)
+  })
+})

From 0afd4838e5951bcf53d61f5c986eaaa3b8932241 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 14:48:15 -0600
Subject: [PATCH 50/93] fix: keep the editor consistent when resuming loses its
 working copy

---
 .../__tests__/useEditSession.spec.ts          | 39 +++++++++++++++++++
 apps/qc-app/src/composables/useEditSession.ts | 31 ++++++++++++---
 2 files changed, 64 insertions(+), 6 deletions(-)

diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index cc52dcf5..b386d649 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -621,6 +621,45 @@ describe('useEditSession.viewSession', () => {
     expect(store.isSwitchingSession).toBe(false)
   })
 
+  it('does not report editable over a committed snapshot when the return-to-current rebuild is invalidated', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const committed = unwrap(await qc.sessions.create(h.id, WIN))
+    await qc.sessions.commit(h.id, committed.id)
+    const inProgress = unwrap(await qc.sessions.create(h.id, WIN))
+
+    const store = useQcSessionStore()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    expect(store.viewedSessionId).toBe(inProgress.id)
+    expect(store.isReadOnly).toBe(false)
+
+    // View the committed session read-only.
+    const viewed = makeRecord([])
+    fetchObservationsInRange.mockResolvedValue(viewed)
+    await session.viewSession(committed.id)
+    expect(store.viewedSessionId).toBe(committed.id)
+    expect(store.isReadOnly).toBe(true)
+    const plottedBeforeReturn = selectedSeries.value.data
+
+    // Returning to the in-progress session is superseded mid-flight (e.g.
+    // the working copy was invalidated), so its rebuild resolves null.
+    wcRebuild.mockResolvedValueOnce(null)
+    await session.viewSession(inProgress.id)
+
+    // Stays on the committed session: still read-only, still showing the
+    // committed snapshot, not silently editable over it.
+    expect(store.viewedSessionId).toBe(committed.id)
+    expect(store.isReadOnly).toBe(true)
+    expect(selectedSeries.value.data).toBe(plottedBeforeReturn)
+    expect(session.needsSession.value).toBe(false)
+  })
+
   it('rejects before a managed datastream is loaded', async () => {
     const { useEditSession } = await import('@/composables/useEditSession')
     await expect(useEditSession().viewSession('s-1')).rejects.toThrow(
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index 4356f1cc..76feb1c8 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -3,7 +3,9 @@
  * wiring the QC service layer to the app's stores:
  *   - beginEditing: resolve the managed datastream's history, load its
  *     sessions, and resume the in-progress one (or signal that a session
- *     must be started),
+ *     must be started); reports whether it actually resumed, so a caller
+ *     that changed view state beforehand (viewSession) can undo it on
+ *     failure,
  *   - startSession: create a session and copy the source window in,
  *   - saveDraft: persist the record's edit operations to the session
  *     (append-only, so each user's operations keep their creator),
@@ -117,9 +119,11 @@ export function useEditSession() {
       .length
   })
 
-  async function beginEditing(): Promise<void> {
+  /** Returns true when an in-progress session's working copy was wired into
+   *  the plot, i.e. the editor is now genuinely editable over it. */
+  async function beginEditing(): Promise<boolean> {
     const managed = qcDatastream.value
-    if (!managed) return
+    if (!managed) return false
     needsHistory.value = false
 
     const history = await findHistoryForDatastream(
@@ -130,7 +134,7 @@ export function useEditSession() {
       // Not a managed datastream: the caller should offer to create one
       // from it (with this datastream as the source).
       needsHistory.value = true
-      return
+      return false
     }
     sourceDatastream.value =
       (await hs.value.datastreams.getItem(history.sourceDatastream.id)) ?? null
@@ -154,8 +158,11 @@ export function useEditSession() {
         // rather than fake a resume; `enterEdit` treats that as "start a
         // session", and `startOrResumeSession` is idempotent, so it resumes
         // this same in-progress session instead of creating a duplicate.
+        // Report failure so a caller that already flipped view state (e.g.
+        // `viewSession`) knows to undo it instead of leaving the editor
+        // reporting editable over whatever was plotted before.
         needsSession.value = true
-        return
+        return false
       }
       if (selectedSeries.value) selectedSeries.value.data = built.record
       // Nothing watches for a swapped-in record; resume has no caller that
@@ -164,8 +171,10 @@ export function useEditSession() {
       // The replayed draft operations are the saved baseline.
       snapshotSavedEdits()
       needsSession.value = false
+      return true
     } else {
       needsSession.value = true
+      return false
     }
   }
 
@@ -181,10 +190,20 @@ export function useEditSession() {
       throw new Error('Load a managed datastream for editing first.')
     }
     if (sessionId === sessionStore.currentSessionId) {
+      // `returnToCurrent` flips `isReadOnly` to false immediately, before
+      // the in-progress working copy is actually (re)built. If that build
+      // fails (superseded by an invalidate), undo the flip and go back to
+      // what is still on the plot, instead of reporting editable over a
+      // committed session's snapshot.
+      const previousSessionId = sessionStore.viewedSessionId
       sessionStore.returnToCurrent()
       sessionStore.isSwitchingSession = true
       try {
-        await beginEditing()
+        const resumed = await beginEditing()
+        if (!resumed && previousSessionId) {
+          sessionStore.viewSession(previousSessionId)
+          needsSession.value = false
+        }
       } finally {
         sessionStore.isSwitchingSession = false
       }

From 47308f766a41f6106cedc088a6c27a9cd722e313 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 21:29:18 -0600
Subject: [PATCH 51/93] test: pin raw and working copy rows by name in the
 preview spec

---
 apps/qc-app/e2e/managed-preview.spec.ts | 30 +++++++++++++++++++------
 1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/apps/qc-app/e2e/managed-preview.spec.ts b/apps/qc-app/e2e/managed-preview.spec.ts
index 1f781754..13639876 100644
--- a/apps/qc-app/e2e/managed-preview.spec.ts
+++ b/apps/qc-app/e2e/managed-preview.spec.ts
@@ -3,7 +3,7 @@
  * the session's working copy, the same data the editor opens.
  */
 
-import { expect, test } from '@playwright/test'
+import { expect, test, type Page } from '@playwright/test'
 import { installMocks, type MockQcSession } from './support/mocks'
 import { gotoHome } from './support/app'
 import {
@@ -13,8 +13,12 @@ import {
   FIXTURE_OBS_START_ISO,
   MANAGED_DATASTREAM_ID,
   QC_HISTORY_ID,
+  QC_SOURCE_CHECKSUM,
 } from './support/fixtures'
 
+const RAW_NAME = 'Streamflow Datastream'
+const MANAGED_NAME = 'Streamflow Datastream (QC)'
+
 /** A fresh in-progress session over the fixture window, no saved operations. */
 function inProgressSession(): MockQcSession {
   return {
@@ -24,7 +28,7 @@ function inProgressSession(): MockQcSession {
     description: 'Draft',
     phenomenonTimeStart: FIXTURE_OBS_START_ISO,
     phenomenonTimeEnd: FIXTURE_OBS_END_ISO,
-    sourceChecksum: 'e2e-draft-checksum',
+    sourceChecksum: QC_SOURCE_CHECKSUM,
     createdAt: FIXTURE_OBS_START_ISO,
     committedAt: null,
     createdBy: { name: 'Test User', email: 'test@example.com' },
@@ -52,7 +56,7 @@ test.describe('managed datastream preview', () => {
     await gotoHome(page)
   })
 
-  const plotOptions = async (page: any, ids: string[]) => {
+  const plotOptions = async (page: Page, ids: string[]) => {
     await page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`).click()
     await expect(page.getByTestId('plot-source-dialog')).toBeVisible()
     for (const id of ids) {
@@ -62,6 +66,15 @@ test.describe('managed datastream preview', () => {
     await expect(page.getByTestId('plot-source-dialog')).toBeHidden()
   }
 
+  // The raw name is a substring of the managed name ("Streamflow Datastream"
+  // vs "Streamflow Datastream (QC)"), so pin the row by its title text using
+  // exact matching rather than a loose text filter.
+  const subtitleForRow = (page: Page, name: string) =>
+    page
+      .locator('.plotted-item')
+      .filter({ has: page.getByText(name, { exact: true }) })
+      .locator('.plotted-item__subtitle')
+
   test('plots the session working copy instead of an empty line', async ({ page }) => {
     await plotOptions(page, [MANAGED_DATASTREAM_ID])
     await expect(page.locator('.plotted-item__subtitle').first()).toHaveText(
@@ -71,9 +84,12 @@ test.describe('managed datastream preview', () => {
 
   test('plots raw and working copy side by side', async ({ page }) => {
     await plotOptions(page, [DATASTREAM_ID, MANAGED_DATASTREAM_ID])
-    const subtitles = page.locator('.plotted-item__subtitle')
-    await expect(subtitles).toHaveCount(2)
-    await expect(subtitles.nth(0)).toHaveText(`${FIXTURE_OBS_COUNT} pts loaded`)
-    await expect(subtitles.nth(1)).toHaveText(`${FIXTURE_OBS_COUNT} pts loaded`)
+    await expect(page.locator('.plotted-item__subtitle')).toHaveCount(2)
+    await expect(subtitleForRow(page, RAW_NAME)).toHaveText(
+      `${FIXTURE_OBS_COUNT} pts loaded`
+    )
+    await expect(subtitleForRow(page, MANAGED_NAME)).toHaveText(
+      `${FIXTURE_OBS_COUNT} pts loaded`
+    )
   })
 })

From 85e8215dabc637dd8100bf4a9063819324389f57 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 14 Sep 2026 22:03:40 -0600
Subject: [PATCH 52/93] fix: never resume over saved drafts without replaying
 them and clear working copies on reset

---
 apps/qc-app/docs/API_REFERENCE.md             |  41 ++++---
 apps/qc-app/docs/ARCHITECTURE.md              |  19 ++-
 apps/qc-app/docs/TESTING.md                   |   2 +-
 apps/qc-app/docs/USER_GUIDE.md                |   2 +-
 apps/qc-app/e2e/managed-preview.spec.ts       |  39 +++++--
 .../components/Navigation/NavigationRail.vue  |   6 -
 apps/qc-app/src/components/VisualizeData.vue  |  23 +++-
 .../__tests__/useEditSession.spec.ts          | 106 +++++++++++++++--
 apps/qc-app/src/composables/useEditSession.ts |  77 ++++++------
 .../__tests__/reconstructSession.spec.ts      |  49 ++++++++
 .../qualityControl/__tests__/session.spec.ts  |  10 +-
 .../src/services/qualityControl/session.ts    |  11 +-
 .../store/__tests__/dataVisualization.spec.ts |  51 ++++++++
 .../src/store/__tests__/workingCopies.spec.ts | 110 +++++++++++++++++-
 apps/qc-app/src/store/dataVisualization.ts    |  15 +++
 apps/qc-app/src/store/workingCopies.ts        | 103 ++++++++--------
 16 files changed, 521 insertions(+), 143 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 25991eff..bc303832 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -170,8 +170,14 @@ const {
 
 - `beginEditing()` — resolves the QC history for the QC datastream, loads
   its sessions, resumes the in-progress one (or sets `needsSession`); sets
-  `needsHistory` when the datastream isn't QC-managed yet.
+  `needsHistory` when the datastream isn't QC-managed yet. Resuming rebuilds
+  the shared working copy; if that rebuild is superseded it throws
+  `ResumeSupersededError`, leaves `needsSession` false, and `enterEdit`
+  returns to the Select view.
 - `startSession(spec)` — creates a session and copies the source window in.
+  When the history already has an in-progress session it resumes that one
+  through the same replay instead (throwing `ResumeSupersededError` the same
+  way), so saved draft operations are never dropped.
 - `saveDraft()` — persists the record's edit operations to the session
   (append-only reconcile).
 - `discardUnsavedEdits()` — drops edits made since the last save and restores
@@ -297,8 +303,8 @@ on boot.
 | `matchesSelectedProcessingLevel`    | action   | `(ds) => boolean`                                 | Same shape as above. |
 | `setDateRange`                      | action   | `({ begin?, end?, update?, custom? }) => Promise<void>` | No-ops when neither bound moves; clears zoom history when it does. |
 | `onDateBtnClick`                    | action   | `(id: number) => Promise<void>`                   | Selects the preset and applies its window over the plotted data's extent. With nothing plotted, only the selection changes. |
-| `refreshGraphSeriesArray`           | action   | `() => Promise<unknown[]>`                        | Reconciles `graphSeriesArray` against `plottedDatastreams` (fetch deltas + reorder + recolor). A managed datastream with a loaded working copy (`useWorkingCopiesStore`) uses it instead of fetching. |
-| `resetState`                        | action   | `() => void`                                      | Clears filters + plotted set on a workspace swap; preserves the preset preference. |
+| `refreshGraphSeriesArray`           | action   | `() => Promise<unknown[]>`                        | Reconciles `graphSeriesArray` against `plottedDatastreams` (fetch deltas + reorder + recolor). A managed datastream with a loaded working copy (`useWorkingCopiesStore`) uses it instead of fetching. Invalidates the working copy of any managed datastream no longer plotted, except the QC target. |
+| `resetState`                        | action   | `() => void`                                      | Clears filters, the plotted set, and every working copy on a workspace swap; preserves the preset preference. |
 | `toggleDatastream`                  | action   | `(ds: Datastream) => Promise<void>`               | Plot if absent, unplot if present. |
 | `plotDatastream`                    | action   | `(ds: Datastream) => Promise<void>`               | Add to plot; promotes to QC when nothing's there yet. |
 | `unplotDatastream`                  | action   | `(id: string) => Promise<void>`                   | Remove; promotes the previous plotted entry to QC if removing the QC target. |
@@ -313,7 +319,7 @@ on boot.
 | `releaseManagedDatastream`          | action   | `() => Promise<void>`                             | Inverse of `adoptManagedDatastream`, for leaving the editor: swap the managed datastream back to its source (resolved through `qcHistories`), drop the editor's working copy and rebuild, so the plot shows the source as stored rather than the session's uncommitted edits. Managed datastreams are hidden from the catalog table, so without this the Select view shows a plot with nothing selected. No-op when the QC target isn't managed or its source isn't in the catalog. |
 | `rebuildPlot`                       | action   | `() => Promise<void>`                             | Serialized rebuild (drop zoom history, refresh series, regenerate options, render). Coalesces concurrent callers. |
 
-### `useWorkingCopiesStore()` — `src/store/workingCopies.ts`
+### `useWorkingCopiesStore()` (`src/store/workingCopies.ts`)
 
 One working copy per managed datastream, keyed by its in-progress
 session: the Select-view plot and the editor share it so the preview
@@ -321,23 +327,26 @@ shows exactly what editing opens. Not persisted, not reactive (records
 hold large typed arrays that must not be proxied).
 
 Concurrency: a per-managed-id generation counter, bumped by `invalidate`,
-`set`, and every new build, guards each build's cache write — a build
-whose generation is no longer current (superseded by a later
-`invalidate`/`set`/`rebuild` while it awaited) is discarded rather than
+`set`, `clear`, and every new build, guards each build's cache write: a
+build whose generation is no longer current is discarded rather than
 resurrecting a stale copy. Concurrent `load()` calls for the same managed
-id share one in-flight build and resolve to the same record. A `rebuild()`
-superseded by a later `rebuild()` chains onto it and resolves to the same
-copy regardless of which finishes its fetch first; superseded by
-`invalidate`/`set` instead, it resolves to whatever is now cached (`null`
-after an `invalidate`) — never to the record it built but discarded.
+id share one in-flight build and resolve to the same record, and a `load()`
+that meets an in-flight `rebuild()` joins it instead of starting its own
+build. A superseded `rebuild()` or `load()` never returns the record it
+discarded: it resolves to the newer in-flight `rebuild()`'s result
+(whichever fetch finishes first), otherwise to whatever is now cached
+(`null` after `invalidate` or `clear`). `invalidate`, `set`, and `clear`
+also drop the in-flight load they supersede, so the next `load()` starts
+fresh.
 
 | Name         | Kind   | Type / signature                                                                                     | Notes |
 |--------------|--------|-------------------------------------------------------------------------------------------------------|-------|
 | `get`        | action | `(managedId: string) => WorkingCopy \| undefined`                                                     | Current cached copy for a managed datastream, if any. |
-| `load`       | action | `(managed: Datastream, source: Datastream, historyId: string) => Promise<WorkingCopy \| null>`        | Returns the cached copy when it matches the history's in-progress session; rebuilds and caches otherwise; `null` (and evicts any stale cache entry) when the history has no in-progress session. Concurrent calls for the same managed id dedupe onto one build. If superseded mid-build by `invalidate`/`set`/`rebuild`, resolves to the now-current cache entry (or `null`) instead of the discarded build. |
-| `rebuild`    | action | `(managed: Datastream, source: Datastream, historyId: string, session: SessionWindow) => Promise<WorkingCopy \| null>` | Always reconstructs from the session window and replays its operations, even when a cached copy already matches; supersedes any in-flight `load()` build or earlier `rebuild()` for this managed id and overwrites the cache. If superseded itself while awaiting, never returns the discarded build: resolves to the copy that superseded it (chaining onto a later in-flight `rebuild()`, or reading the cache after a synchronous `invalidate`/`set`), which is `null` when nothing ended up cached. |
-| `set`        | action | `(managedId: string, sessionId: string, record: ObservationRecord, begin: Date, end: Date) => void`   | Insert or replace a cache entry directly (used by the editor after a local edit); supersedes any in-flight build for this managed id. |
-| `invalidate` | action | `(managedId: string) => void`                                                                          | Drop a managed datastream's cached copy; supersedes any in-flight build for this managed id. |
+| `load`       | action | `(managed: Datastream, source: Datastream, historyId: string) => Promise<WorkingCopy \| null>`        | Returns the cached copy when it matches the history's in-progress session; rebuilds and caches otherwise; `null` (and evicts any stale cache entry) when the history has no in-progress session. Concurrent calls for the same managed id dedupe onto one build, and a call made while a `rebuild()` is in flight joins it. If superseded mid-build, resolves to the superseding `rebuild()`'s result or the now-current cache entry (or `null`) instead of the discarded build. |
+| `rebuild`    | action | `(managed: Datastream, source: Datastream, historyId: string, session: SessionWindow) => Promise<WorkingCopy \| null>` | Always reconstructs from the session window and replays its operations, even when a cached copy already matches; supersedes any in-flight `load()` build or earlier `rebuild()` for this managed id and overwrites the cache. If superseded itself while awaiting, never returns the discarded build: resolves to a later in-flight `rebuild()`'s result, or reads the cache after a synchronous `invalidate`/`set`/`clear`, which is `null` when nothing ended up cached. |
+| `set`        | action | `(managedId: string, sessionId: string, record: ObservationRecord, begin: Date, end: Date) => void`   | Insert or replace a cache entry directly (used by `startSession` to store a new session's fresh base); supersedes any in-flight build or load for this managed id. |
+| `invalidate` | action | `(managedId: string) => void`                                                                          | Drop a managed datastream's cached copy; supersedes any in-flight build or load for this managed id. |
+| `clear`      | action | `() => void`                                                                                           | Drop every cached copy and supersede every in-flight build and load; called by `useDataVisStore().resetState()` on a workspace reset. |
 | `extents`    | action | `(managedIds: string[]) => { phenomenonBeginTime: string; phenomenonEndTime: string }[]`               | The cached copies' windows for the given managed ids, in ISO form; ids with no cached copy are omitted. |
 
 ### `usePlotlyStore()` — `src/store/plotly.ts`
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 0519b781..a3eb04a1 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -285,10 +285,21 @@ Two contract notes worth keeping in mind:
   datastream**, cached in `workingCopies` (`store/workingCopies.ts`). It is
   built on a copy of the base (`reconstructSession`/`loadLatestBase`'s
   `cloneRecord`), never the observation store's cached record, so the raw
-  datastream's cached observations are never edited. It is dropped
-  (`invalidate`) on commit, on leaving the editor, and when its session or
-  managed datastream is deleted, so the next preview or resume rebuilds it
-  from what was actually saved.
+  datastream's cached observations are never edited. It is invalidated when
+  the editor closes through `exitToSelect` (Close, Save and close, or close
+  without saving), on commit, when its session or managed datastream is
+  deleted, and when a managed datastream other than the QC target stops being
+  plotted; `resetState` clears every copy on a workspace reset. A nav rail
+  switch to Select keeps the copy, which then equals the saved state because
+  the exit guard saves or discards first. The next preview or resume rebuilds
+  it from what was actually saved.
+- **Editing never starts over saved draft operations without replaying
+  them.** The save reconciles operations by position, so a base without the
+  replayed draft would delete the server's operations on the next save.
+  `startSession` resumes an existing in-progress session through
+  `workingCopies.rebuild` like `beginEditing` does, and a resume whose rebuild
+  is superseded (`null`) throws `ResumeSupersededError` instead of falling
+  back to starting a session; the editor then returns to the Select view.
 - **Viewing a past session replays its ancestor chain from the source.**
   The managed datastream carries every commit, so it cannot be the base for a
   historical view: replaying an older session's operations on top of it would
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 303bd0d1..f7e8f3a1 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -12,7 +12,7 @@ the operator's manual; QUALITY.md is the policy.
 
 | Layer      | Runner                          | Where                        | Count    |
 | ---------- | ------------------------------- | ---------------------------- | -------- |
-| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 58 files |
+| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 59 files |
 | End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 30 files |
 
 There is no separate "integration" tier — component tests live in the
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 55d156df..c2037aca 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -215,7 +215,7 @@ The right-hand list (visible on both Select and Edit views) is the roster of cur
 
 If a plotted datastream has no observations in the current window (either because the dataset is empty there or because the chosen time range doesn't cover its data), the row title shows a small warning-tinted database icon. Hover it for the tooltip "No observations in the current time window". Widening the time range (or clicking **All** in the Time range section) usually clears it.
 
-A QC datastream whose edit session is in progress plots that session's working data instead — the same data the editor opens: committed data where it exists, otherwise the raw datastream, over the session's time window, with saved draft edits applied. Its line always covers the whole session window, and presets anchor to that window rather than the (empty) stored managed datastream. With no session in progress it shows its committed observations as usual.
+A QC datastream whose edit session is in progress plots that session's working data instead, the same data the editor opens: committed data where it exists, otherwise the raw datastream, over the session's time window, with saved draft edits applied. Its line always covers the whole session window, and presets anchor to that window rather than the (empty) stored managed datastream. With no session in progress it shows its committed observations as usual.
 
 ![Plotted datastreams list with two rows](./images/plotted-datastreams-list.png)
 
diff --git a/apps/qc-app/e2e/managed-preview.spec.ts b/apps/qc-app/e2e/managed-preview.spec.ts
index 13639876..66aafa0d 100644
--- a/apps/qc-app/e2e/managed-preview.spec.ts
+++ b/apps/qc-app/e2e/managed-preview.spec.ts
@@ -1,6 +1,7 @@
 /**
  * A managed datastream with nothing committed and a session in progress plots
- * the session's working copy, the same data the editor opens.
+ * the session's working copy (its saved draft replayed), the same data the
+ * editor opens, while the raw line keeps every point.
  */
 
 import { expect, test, type Page } from '@playwright/test'
@@ -18,11 +19,18 @@ import {
 
 const RAW_NAME = 'Streamflow Datastream'
 const MANAGED_NAME = 'Streamflow Datastream (QC)'
+const SESSION_ID = 'qcs-e2e-draft'
+const DELETED_INDICES = [10, 11, 12]
 
-/** A fresh in-progress session over the fixture window, no saved operations. */
+/** An in-progress session whose saved draft deletes `DELETED_INDICES`. */
 function inProgressSession(): MockQcSession {
+  // Persisted as the app saves a delete: the SELECTION it consumes, then DELETE_POINTS.
+  const draft = [
+    { operationType: 'SELECTION', arguments: [DELETED_INDICES] },
+    { operationType: 'DELETE_POINTS', arguments: [] },
+  ]
   return {
-    id: 'qcs-e2e-draft',
+    id: SESSION_ID,
     historyId: QC_HISTORY_ID,
     status: 'in_progress',
     description: 'Draft',
@@ -33,7 +41,13 @@ function inProgressSession(): MockQcSession {
     committedAt: null,
     createdBy: { name: 'Test User', email: 'test@example.com' },
     dependencyIds: [],
-    operations: [],
+    operations: draft.map((op, order) => ({
+      ...op,
+      id: `${SESSION_ID}-op-${order}`,
+      order,
+      comment: null,
+      createdAt: FIXTURE_OBS_START_ISO,
+    })),
   }
 }
 
@@ -75,20 +89,23 @@ test.describe('managed datastream preview', () => {
       .filter({ has: page.getByText(name, { exact: true }) })
       .locator('.plotted-item__subtitle')
 
-  test('plots the session working copy instead of an empty line', async ({ page }) => {
+  const replayedCount = FIXTURE_OBS_COUNT - DELETED_INDICES.length
+
+  test('plots the session working copy with its saved draft replayed', async ({ page }) => {
     await plotOptions(page, [MANAGED_DATASTREAM_ID])
-    await expect(page.locator('.plotted-item__subtitle').first()).toHaveText(
-      `${FIXTURE_OBS_COUNT} pts loaded`
+    await expect(page.locator('.plotted-item__subtitle')).toHaveCount(1)
+    await expect(subtitleForRow(page, MANAGED_NAME)).toHaveText(
+      `${replayedCount} pts loaded`
     )
   })
 
-  test('plots raw and working copy side by side', async ({ page }) => {
+  test('plots raw and working copy side by side without editing the raw line', async ({ page }) => {
     await plotOptions(page, [DATASTREAM_ID, MANAGED_DATASTREAM_ID])
     await expect(page.locator('.plotted-item__subtitle')).toHaveCount(2)
-    await expect(subtitleForRow(page, RAW_NAME)).toHaveText(
-      `${FIXTURE_OBS_COUNT} pts loaded`
-    )
     await expect(subtitleForRow(page, MANAGED_NAME)).toHaveText(
+      `${replayedCount} pts loaded`
+    )
+    await expect(subtitleForRow(page, RAW_NAME)).toHaveText(
       `${FIXTURE_OBS_COUNT} pts loaded`
     )
   })
diff --git a/apps/qc-app/src/components/Navigation/NavigationRail.vue b/apps/qc-app/src/components/Navigation/NavigationRail.vue
index 4dd9416d..5c0b53cb 100644
--- a/apps/qc-app/src/components/Navigation/NavigationRail.vue
+++ b/apps/qc-app/src/components/Navigation/NavigationRail.vue
@@ -190,13 +190,11 @@ import { useWorkspaceStore } from '@/store/workspaces'
 import { usePlotlyStore } from '@/store/plotly'
 import { useQcSessionStore } from '@/store/qcSession'
 import { useEditSession } from '@/composables/useEditSession'
-import { useWorkingCopiesStore } from '@/store/workingCopies'
 
 const { onRailItemClicked } = useUIStore()
 const { selectedDrawer, isDrawerOpen, currentView } = storeToRefs(useUIStore())
 const { resetState } = useDataVisStore()
 const { qcDatastream, qcDatastreamId } = storeToRefs(useDataVisStore())
-const workingCopies = useWorkingCopiesStore()
 const { hs } = storeToRefs(useHydroServer())
 const workspaceStore = useWorkspaceStore()
 const { selectedWorkspace } = storeToRefs(workspaceStore)
@@ -305,10 +303,6 @@ async function onSwitchWorkspace() {
   // filters to router.replace, racing our push and stranding the user.
   // `switch=1` prevents the Workspaces picker from auto-redirecting back.
   await router.push({ name: 'Workspaces', query: { switch: '1' } })
-  // Deselecting the datastream leaves the editor without releasing it via
-  // exitToSelect; drop its working copy here so a later preview of it (back
-  // in this workspace) rebuilds from what was actually saved.
-  if (qcDatastream.value) workingCopies.invalidate(qcDatastream.value.id)
   qcDatastreamId.value = null
   currentView.value = DrawerType.Select
   selectedDrawer.value = DrawerType.Select
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index ca021692..fc1d902f 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -541,7 +541,10 @@ import { computed, onUnmounted, ref, useTemplateRef, watch } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import PlottedDatastreams from './VisualizeData/PlottedDatastreams.vue'
 import { usePlotlyStore } from '@/store/plotly'
-import { useEditSession } from '@/composables/useEditSession'
+import {
+  ResumeSupersededError,
+  useEditSession,
+} from '@/composables/useEditSession'
 import { useUnsavedChangesWarning } from '@/composables/useUnsavedChangesWarning'
 import { useResumeEditSession } from '@/composables/useResumeEditSession'
 import { useQcSessionStore } from '@/store/qcSession'
@@ -826,12 +829,20 @@ async function startSessionForWindow() {
     await redraw()
     Snackbar.success('Edit session started.')
   } catch (e) {
-    Snackbar.error(e instanceof Error ? e.message : 'Could not start the session.')
+    if (e instanceof ResumeSupersededError) abandonSupersededResume(e)
+    else Snackbar.error(e instanceof Error ? e.message : 'Could not start the session.')
   } finally {
     isStartingSession.value = false
   }
 }
 
+// Nothing replayed is wired in and the footer would still offer Save, so
+// leave for Select rather than let edits save over the saved draft.
+function abandonSupersededResume(e: ResumeSupersededError) {
+  exitToSelect()
+  Snackbar.error(e.message)
+}
+
 async function onCreateProcessingLevel(input: {
   code: string
   definition?: string
@@ -1245,7 +1256,13 @@ async function onChooserDeleteSession(
 // Enter the editor on the current QC-target managed datastream: resume its
 // in-progress session, or start a new one over the selected time range.
 async function enterEdit() {
-  await beginEditing()
+  try {
+    await beginEditing()
+  } catch (e) {
+    if (!(e instanceof ResumeSupersededError)) throw e
+    abandonSupersededResume(e)
+    return
+  }
   if (needsHistory.value) {
     Snackbar.error('This datastream is not set up for QC editing.')
     return
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index b386d649..4b8668a0 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -198,7 +198,7 @@ describe('useEditSession', () => {
     expect(redraw).toHaveBeenCalled()
   })
 
-  it('beginEditing leaves the session unresumed when the working copy is invalidated mid-resume', async () => {
+  it('beginEditing rejects when the working copy is superseded mid-resume, without asking to start a session', async () => {
     const h = unwrap(
       await qc.histories.create({
         managedDatastreamId: 'm-1',
@@ -206,18 +206,22 @@ describe('useEditSession', () => {
       })
     )
     await qc.sessions.create(h.id, WIN)
-    // Superseded by an invalidate while resuming (e.g. the user already left
-    // Edit, or the session was deleted) leaves nothing cached to show.
+    // Superseded while resuming (e.g. the session was deleted, or the
+    // workspace was reset), so nothing replayed is cached.
     wcRebuild.mockResolvedValueOnce(null)
     const original = selectedSeries.value.data
-    const { useEditSession } = await import('@/composables/useEditSession')
+    const { useEditSession, ResumeSupersededError } = await import(
+      '@/composables/useEditSession'
+    )
     const { beginEditing, needsSession } = useEditSession()
-    await beginEditing()
-    // Falls back to "needs a session": entering the editor via `enterEdit`
-    // will auto-start one, and `startOrResumeSession` is idempotent, so it
-    // resumes the same in-progress session rather than creating a new one.
-    expect(needsSession.value).toBe(true)
+    needsSession.value = true
+
+    await expect(beginEditing()).rejects.toBeInstanceOf(ResumeSupersededError)
+
+    // Starting a session here would edit a bare base under the saved draft.
+    expect(needsSession.value).toBe(false)
     expect(selectedSeries.value.data).toBe(original)
+    expect(useQcSessionStore().savedEdits).toEqual([])
     expect(redraw).not.toHaveBeenCalled()
   })
 
@@ -472,6 +476,86 @@ describe('useEditSession', () => {
     )
   })
 
+  it('startSession over an existing in-progress session replays its saved operations as the saved baseline', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    expect(session.needsSession.value).toBe(true)
+
+    // Someone else starts the session and saves a draft first.
+    const existing = unwrap(await qc.sessions.create(h.id, WIN))
+    await qc.operations.create(h.id, existing.id, [
+      { operationType: 'SELECTION' as any, arguments: [[0]], order: 0 },
+      { operationType: 'DELETE_POINTS' as any, arguments: [], order: 1 },
+    ])
+    const replayed = makeRecord([
+      { method: 'SELECTION', args: [[0]] },
+      { method: 'DELETE_POINTS', args: [] },
+    ])
+    wcRebuild.mockResolvedValue({
+      sessionId: existing.id,
+      record: replayed,
+      begin: new Date(0),
+      end: new Date(1),
+    })
+    const savedIds = unwrap(await qc.operations.list(h.id, existing.id)).map((o) => o.id)
+
+    await session.startSession(WIN)
+
+    expect(wcRebuild).toHaveBeenCalledWith(
+      expect.objectContaining({ id: 'm-1' }),
+      expect.objectContaining({ id: 's-1' }),
+      h.id,
+      expect.objectContaining({ id: existing.id })
+    )
+    expect(wcSet).not.toHaveBeenCalled()
+    expect(toRaw(selectedSeries.value.data)).toBe(replayed)
+    expect(useQcSessionStore().savedEdits).toHaveLength(2)
+    expect(session.hasUnsavedChanges.value).toBe(false)
+    expect(session.needsSession.value).toBe(false)
+
+    // Saving keeps the server's operations instead of reconciling them away.
+    await session.saveDraft()
+    const afterSave = unwrap(await qc.operations.list(h.id, existing.id))
+    expect(afterSave.map((o) => o.id)).toEqual(savedIds)
+    expect(unwrap(await qc.sessions.list(h.id))).toHaveLength(1)
+  })
+
+  it('startSession rejects when resuming an existing session is superseded, without editing a bare base', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const { useEditSession, ResumeSupersededError } = await import(
+      '@/composables/useEditSession'
+    )
+    const session = useEditSession()
+    await session.beginEditing()
+    const existing = unwrap(await qc.sessions.create(h.id, WIN))
+    await qc.operations.create(h.id, existing.id, [
+      { operationType: 'SELECTION' as any, arguments: [[0]], order: 0 },
+      { operationType: 'DELETE_POINTS' as any, arguments: [], order: 1 },
+    ])
+    wcRebuild.mockResolvedValueOnce(null)
+    const original = selectedSeries.value.data
+
+    await expect(session.startSession(WIN)).rejects.toBeInstanceOf(
+      ResumeSupersededError
+    )
+
+    expect(wcSet).not.toHaveBeenCalled()
+    expect(selectedSeries.value.data).toBe(original)
+    expect(unwrap(await qc.operations.list(h.id, existing.id))).toHaveLength(2)
+  })
+
   it('commit drops the working copy once the session is committed', async () => {
     await seedHistory()
     const { useEditSession } = await import('@/composables/useEditSession')
@@ -650,7 +734,9 @@ describe('useEditSession.viewSession', () => {
     // Returning to the in-progress session is superseded mid-flight (e.g.
     // the working copy was invalidated), so its rebuild resolves null.
     wcRebuild.mockResolvedValueOnce(null)
-    await session.viewSession(inProgress.id)
+    await expect(session.viewSession(inProgress.id)).rejects.toThrow(
+      /changed while it was opening/
+    )
 
     // Stays on the committed session: still read-only, still showing the
     // committed snapshot, not silently editable over it.
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index 76feb1c8..f4fad9ea 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -3,10 +3,9 @@
  * wiring the QC service layer to the app's stores:
  *   - beginEditing: resolve the managed datastream's history, load its
  *     sessions, and resume the in-progress one (or signal that a session
- *     must be started); reports whether it actually resumed, so a caller
- *     that changed view state beforehand (viewSession) can undo it on
- *     failure,
- *   - startSession: create a session and copy the source window in,
+ *     must be started); reports whether it actually resumed,
+ *   - startSession: create a session and copy the source window in, or
+ *     resume the in-progress one if it already exists,
  *   - saveDraft: persist the record's edit operations to the session
  *     (append-only, so each user's operations keep their creator),
  *   - commit: push the final observations (replace) and lock the session.
@@ -25,7 +24,7 @@ import { usePlotlyStore } from '@/store/plotly'
 import { useHydroServer } from '@/store/hydroserver'
 import { useObservationStore } from '@/store/observations'
 import { useQcSessionStore } from '@/store/qcSession'
-import { useWorkingCopiesStore } from '@/store/workingCopies'
+import { useWorkingCopiesStore, type SessionWindow } from '@/store/workingCopies'
 import {
   findHistoryForDatastream,
   startOrResumeSession,
@@ -70,6 +69,15 @@ function clampSpecToSource(
   }
 }
 
+/** A resume whose working copy was superseded mid-build, so nothing replayed
+ *  could be wired in. Editing must not continue over it. */
+export class ResumeSupersededError extends Error {
+  constructor() {
+    super('The edit session changed while it was opening. Open it again to keep editing.')
+    this.name = 'ResumeSupersededError'
+  }
+}
+
 export function useEditSession() {
   const { qcDatastream } = storeToRefs(useDataVisStore())
   const { replaceDatastream } = useDataVisStore()
@@ -119,6 +127,22 @@ export function useEditSession() {
       .length
   })
 
+  /** Wire in the session's working copy with its saved operations replayed.
+   *  Throws if superseded: a bare base would save over those operations. */
+  async function resumeWorkingCopy(
+    managed: Datastream,
+    source: Datastream,
+    historyId: string,
+    session: SessionWindow
+  ): Promise<void> {
+    const built = await workingCopies.rebuild(managed, source, historyId, session)
+    if (!built) throw new ResumeSupersededError()
+    if (selectedSeries.value) selectedSeries.value.data = built.record
+    // The replayed draft operations are the saved baseline.
+    snapshotSavedEdits()
+    needsSession.value = false
+  }
+
   /** Returns true when an in-progress session's working copy was wired into
    *  the plot, i.e. the editor is now genuinely editable over it. */
   async function beginEditing(): Promise<boolean> {
@@ -143,34 +167,16 @@ export function useEditSession() {
     const inProgress = sessionStore.inProgressSession
     const record = selectedSeries.value?.data
     if (inProgress && sourceDatastream.value && record) {
-      // Resume: rebuild the same working copy the Select-view plot shows,
-      // from the server, so it holds exactly the saved draft operations.
-      const built = await workingCopies.rebuild(
+      // A session exists, so a failed resume must never ask to start one.
+      needsSession.value = false
+      await resumeWorkingCopy(
         managed,
         sourceDatastream.value,
         history.id,
         inProgress
       )
-      if (!built) {
-        // The rebuild was superseded by an invalidate while in flight (the
-        // user already left Edit, or the session/datastream was deleted
-        // mid-resume): there is nothing to wire in. Leave `needsSession` true
-        // rather than fake a resume; `enterEdit` treats that as "start a
-        // session", and `startOrResumeSession` is idempotent, so it resumes
-        // this same in-progress session instead of creating a duplicate.
-        // Report failure so a caller that already flipped view state (e.g.
-        // `viewSession`) knows to undo it instead of leaving the editor
-        // reporting editable over whatever was plotted before.
-        needsSession.value = true
-        return false
-      }
-      if (selectedSeries.value) selectedSeries.value.data = built.record
-      // Nothing watches for a swapped-in record; resume has no caller that
-      // redraws, unlike the start-session path.
+      // Nothing watches for a swapped-in record, and no caller redraws here.
       await redraw()
-      // The replayed draft operations are the saved baseline.
-      snapshotSavedEdits()
-      needsSession.value = false
       return true
     } else {
       needsSession.value = true
@@ -190,11 +196,8 @@ export function useEditSession() {
       throw new Error('Load a managed datastream for editing first.')
     }
     if (sessionId === sessionStore.currentSessionId) {
-      // `returnToCurrent` flips `isReadOnly` to false immediately, before
-      // the in-progress working copy is actually (re)built. If that build
-      // fails (superseded by an invalidate), undo the flip and go back to
-      // what is still on the plot, instead of reporting editable over a
-      // committed session's snapshot.
+      // `returnToCurrent` flips to editable before the rebuild; undo that
+      // unless the resume actually wires in the working copy.
       const previousSessionId = sessionStore.viewedSessionId
       sessionStore.returnToCurrent()
       sessionStore.isSwitchingSession = true
@@ -204,6 +207,9 @@ export function useEditSession() {
           sessionStore.viewSession(previousSessionId)
           needsSession.value = false
         }
+      } catch (e) {
+        if (previousSessionId) sessionStore.viewSession(previousSessionId)
+        throw e
       } finally {
         sessionStore.isSwitchingSession = false
       }
@@ -246,12 +252,17 @@ export function useEditSession() {
     if (!historyId || !source || !managed) {
       throw new Error('Load a managed datastream for editing first.')
     }
-    const session = await startOrResumeSession(
+    const { session, resumed } = await startOrResumeSession(
       hs.value.qualityControlSessions,
       historyId,
       clampSpecToSource(spec, source)
     )
     await sessionStore.loadSessions(historyId)
+    if (resumed) {
+      // It may already hold saved operations; edit their replay, not a bare base.
+      await resumeWorkingCopy(managed, source, historyId, session)
+      return
+    }
     // Start from the latest committed state (the managed datastream), or the
     // raw source when nothing has been committed yet.
     const base = await loadLatestBase(
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
index 135469e0..d17fb0f0 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
@@ -534,3 +534,52 @@ describe('reconstructCommittedSession — attribution', () => {
     ])
   })
 })
+
+describe('reconstructSession with a real ObservationRecord', () => {
+  // Pins the persisted shape of a delete (also seeded by
+  // e2e/managed-preview.spec.ts): the SELECTION it consumes, then DELETE_POINTS.
+  it('replays a saved SELECTION and DELETE_POINTS onto a copy of the base', async () => {
+    const { ObservationRecord: Record, applyHistory, serializeHistory } =
+      await import('@uwrl/qc-utils')
+    const { sessionOperationsFromSerialized } = await import('../persistOperations')
+    const qc = makeQcFake()
+    const historyId = await newHistory(qc)
+    const range = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
+    const s = unwrap(await qc.sessions.create(historyId, range))
+    const persisted = [
+      { operationType: 'SELECTION', arguments: [[1, 2, 3]], order: 0 },
+      { operationType: 'DELETE_POINTS', arguments: [], order: 1 },
+    ]
+    await qc.operations.create(historyId, s.id, persisted as any)
+
+    const sourceRec = new Record({
+      datetimes: [1, 2, 3, 4, 5, 6],
+      dataValues: [10, 20, 30, 40, 50, 60],
+    })
+    await sourceRec.reload()
+    const fetchInRange = vi.fn(async (ds: Datastream) =>
+      ds.id === managed.id ? ({ dataX: [] } as unknown as ObservationRecord) : sourceRec
+    )
+
+    const { record, report } = await reconstructSession(
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+      managed,
+      source,
+      historyId,
+      s.id
+    )
+
+    expect(report).toEqual({ applied: 2, failed: [] })
+    expect(Array.from(record.dataX)).toEqual([1, 5, 6])
+    expect(Array.from(record.dataY)).toEqual([10, 50, 60])
+    expect(Array.from(sourceRec.dataX)).toEqual([1, 2, 3, 4, 5, 6])
+    // Saving the replayed history yields the same operations back.
+    const resaved = sessionOperationsFromSerialized(
+      serializeHistory(record, {
+        startDate: range.phenomenonTimeStart,
+        endDate: range.phenomenonTimeEnd,
+      }).operations
+    )
+    expect(resaved).toEqual(persisted)
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
index a34b06e3..1415579d 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
@@ -45,10 +45,11 @@ describe('startOrResumeSession', () => {
   it('starts a new session when none is in progress', async () => {
     const qc = makeQcFake()
     const historyId = await historyWith(qc)
-    const session = await startOrResumeSession(qc.sessions, historyId, {
+    const { session, resumed } = await startOrResumeSession(qc.sessions, historyId, {
       ...WIN,
       description: 'Jan QC',
     })
+    expect(resumed).toBe(false)
     expect(session.status).toBe('in_progress')
     expect(session.description).toBe('Jan QC')
     expect(unwrap(await qc.sessions.list(historyId))).toHaveLength(1)
@@ -58,12 +59,13 @@ describe('startOrResumeSession', () => {
     const qc = makeQcFake()
     const historyId = await historyWith(qc)
     const first = unwrap(await qc.sessions.create(historyId, WIN))
-    const resumed = await startOrResumeSession(qc.sessions, historyId, {
+    const { session, resumed } = await startOrResumeSession(qc.sessions, historyId, {
       phenomenonTimeStart: '2030-01-01T00:00:00Z',
       phenomenonTimeEnd: '2030-02-01T00:00:00Z',
     })
-    expect(resumed.id).toBe(first.id)
-    expect(resumed.phenomenonTimeStart).toBe(WIN.phenomenonTimeStart)
+    expect(resumed).toBe(true)
+    expect(session.id).toBe(first.id)
+    expect(session.phenomenonTimeStart).toBe(WIN.phenomenonTimeStart)
     expect(unwrap(await qc.sessions.list(historyId))).toHaveLength(1)
   })
 })
diff --git a/apps/qc-app/src/services/qualityControl/session.ts b/apps/qc-app/src/services/qualityControl/session.ts
index a0c22314..fd232259 100644
--- a/apps/qc-app/src/services/qualityControl/session.ts
+++ b/apps/qc-app/src/services/qualityControl/session.ts
@@ -32,20 +32,21 @@ export async function getInProgressSession(
 /**
  * Resume the history's in-progress session if one exists, otherwise start
  * a new one with the given window/description. Returns the session detail
- * (with its operations) either way. On resume the spec is ignored — the
- * existing session keeps its own window.
+ * (with its operations) and whether it already existed. On resume the spec
+ * is ignored and the existing session keeps its own window.
  */
 export async function startOrResumeSession(
   qcSessions: QualityControlSessionService,
   historyId: string,
   spec: QcSessionPostBody
-): Promise<QcSessionDetail> {
+): Promise<{ session: QcSessionDetail; resumed: boolean }> {
   const existing = await getInProgressSession(qcSessions, historyId)
   if (existing) {
     // A single-resource GET returns the detail shape (with operations).
-    return unwrap(await qcSessions.get(historyId, existing.id)) as QcSessionDetail
+    const session = unwrap(await qcSessions.get(historyId, existing.id)) as QcSessionDetail
+    return { session, resumed: true }
   }
-  return unwrap(await qcSessions.create(historyId, spec))
+  return { session: unwrap(await qcSessions.create(historyId, spec)), resumed: false }
 }
 
 /** Fetch signature matching `useObservationStore.fetchObservationsInRange`. */
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index d28957f2..640fa329 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -42,10 +42,14 @@ vi.mock('@/store/observations', () => ({
 
 const mockWorkingCopies = new Map<string, any>()
 const mockLoadWorkingCopy = vi.fn(async (managed: any) => mockWorkingCopies.get(managed.id) ?? null)
+const mockInvalidateWorkingCopy = vi.fn((id: string) => mockWorkingCopies.delete(id))
+const mockClearWorkingCopies = vi.fn(() => mockWorkingCopies.clear())
 vi.mock('@/store/workingCopies', () => ({
   useWorkingCopiesStore: () => ({
     get: (id: string) => mockWorkingCopies.get(id),
     load: mockLoadWorkingCopy,
+    invalidate: mockInvalidateWorkingCopy,
+    clear: mockClearWorkingCopies,
     extents: (ids: string[]) =>
       ids.flatMap((id) => {
         const c = mockWorkingCopies.get(id)
@@ -437,6 +441,13 @@ describe('useDataVisStore.resetState', () => {
     expect(mockClearChartState).toHaveBeenCalled()
   })
 
+  it('clears every working copy, which belongs to the workspace being left', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    store.resetState()
+    expect(mockClearWorkingCopies).toHaveBeenCalled()
+  })
+
   it('preserves selectedDateBtnId (user preference, not workspace state)', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
@@ -1151,6 +1162,46 @@ describe('useDataVisStore managed datastream working copy', () => {
 
     expect(mockFetchGraphSeries).toHaveBeenCalled()
   })
+
+  it('drops the working copy of a managed datastream that is unplotted', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withManaged(store)
+    mockWorkingCopies.set('mgd', copy)
+    await store.plotDatastream(store.datastreams[0] as any)
+    await store.plotDatastream(store.datastreams[1] as any)
+    expect(mockInvalidateWorkingCopy).not.toHaveBeenCalled()
+
+    await store.unplotDatastream('mgd')
+
+    expect(mockInvalidateWorkingCopy).toHaveBeenCalledWith('mgd')
+  })
+
+  it('drops working copies when the whole plot is cleared', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withManaged(store)
+    mockWorkingCopies.set('mgd', copy)
+    await store.plotDatastream(store.datastreams[1] as any)
+
+    await store.clearPlottedDatastreams()
+
+    expect(mockInvalidateWorkingCopy).toHaveBeenCalledWith('mgd')
+  })
+
+  it('keeps the QC target working copy even when it is not plotted', async () => {
+    const { useDataVisStore } = await import('@/store/dataVisualization')
+    const store = useDataVisStore()
+    withManaged(store)
+    mockWorkingCopies.set('mgd', copy)
+    await store.plotDatastream(store.datastreams[1] as any)
+
+    store.plottedDatastreams = [store.datastreams[0] as any]
+    await store.refreshGraphSeriesArray()
+
+    expect(store.qcDatastreamId).toBe('mgd')
+    expect(mockInvalidateWorkingCopy).not.toHaveBeenCalled()
+  })
 })
 
 describe('useDataVisStore.replaceDatastream', () => {
diff --git a/apps/qc-app/src/store/__tests__/workingCopies.spec.ts b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
index 33963a4b..0b5face4 100644
--- a/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
+++ b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
@@ -42,7 +42,7 @@ function deferred<T>() {
 
 /**
  * Gates every `fetchObservationsInRange` call behind `gate`, and resolves
- * `started` the first time a call is made — so a test can wait for a
+ * `started` the first time a call is made, so a test can wait for a
  * build to have actually begun (past its generation bump) before racing
  * another store call against it.
  */
@@ -291,4 +291,112 @@ describe('useWorkingCopiesStore', () => {
     expect(store.get('m-1')).toBe(secondResult)
     expect(applyHistory).toHaveBeenCalledTimes(2)
   })
+
+  it('load() during an in-flight rebuild() joins it, so both resolve to the same cached copy', async () => {
+    const session = unwrap(await startSession())
+    const { gate, started } = gateFetch()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const rebuilding = store.rebuild(managed, source, historyId, session)
+    await started.promise
+    const loading = store.load(managed, source, historyId)
+    gate.resolve()
+
+    const [rebuilt, loaded] = await Promise.all([rebuilding, loading])
+    expect(rebuilt).not.toBeNull()
+    expect(loaded).toBe(rebuilt)
+    expect(store.get('m-1')).toBe(rebuilt)
+    expect(applyHistory).toHaveBeenCalledTimes(1)
+  })
+
+  it('a load() still listing sessions when a rebuild() starts joins that rebuild', async () => {
+    const session = unwrap(await startSession())
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const loading = store.load(managed, source, historyId)
+    const rebuilding = store.rebuild(managed, source, historyId, session)
+
+    const [loaded, rebuilt] = await Promise.all([loading, rebuilding])
+    expect(rebuilt).not.toBeNull()
+    expect(loaded).toBe(rebuilt)
+    expect(applyHistory).toHaveBeenCalledTimes(1)
+  })
+
+  it('invalidate() drops an in-flight load(), so the next load() builds afresh', async () => {
+    await startSession()
+    const { gate, started } = gateFetch()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const stale = store.load(managed, source, historyId)
+    await started.promise
+    store.invalidate('m-1')
+    const fresh = store.load(managed, source, historyId)
+    gate.resolve()
+    await stale
+
+    const copy = await fresh
+    expect(copy).not.toBeNull()
+    expect(store.get('m-1')).toBe(copy)
+  })
+
+  it('clear() drops every cached copy', async () => {
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+    const begin = new Date('2024-03-01T00:00:00Z')
+    const end = new Date('2024-04-01T00:00:00Z')
+    store.set('m-1', 's-1', rec([1]) as any, begin, end)
+    store.set('m-2', 's-2', rec([2]) as any, begin, end)
+
+    store.clear()
+
+    expect(store.get('m-1')).toBeUndefined()
+    expect(store.get('m-2')).toBeUndefined()
+    expect(store.extents(['m-1', 'm-2'])).toEqual([])
+  })
+
+  it('clear() during an in-flight rebuild() resolves it to null and caches nothing', async () => {
+    const session = unwrap(await startSession())
+    const { gate, started } = gateFetch()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const pending = store.rebuild(managed, source, historyId, session)
+    await started.promise
+    store.clear()
+    gate.resolve()
+
+    expect(await pending).toBeNull()
+    expect(store.get('m-1')).toBeUndefined()
+  })
+
+  it('clear() during an in-flight load() build resolves it to null and caches nothing', async () => {
+    await startSession()
+    const { gate, started } = gateFetch()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const pending = store.load(managed, source, historyId)
+    await started.promise
+    store.clear()
+    gate.resolve()
+
+    expect(await pending).toBeNull()
+    expect(store.get('m-1')).toBeUndefined()
+  })
+
+  it('clear() while a load() is still listing sessions stops it from building', async () => {
+    await startSession()
+    const { useWorkingCopiesStore } = await import('@/store/workingCopies')
+    const store = useWorkingCopiesStore()
+
+    const pending = store.load(managed, source, historyId)
+    store.clear()
+
+    expect(await pending).toBeNull()
+    expect(store.get('m-1')).toBeUndefined()
+    expect(applyHistory).not.toHaveBeenCalled()
+  })
 })
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index 5d5f6ea2..fc39fdbf 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -79,6 +79,17 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     )
   }
 
+  /** Drop copies no longer plotted, which would hide drafts saved since;
+   *  the QC target keeps its copy for the editor. */
+  function invalidateUnplottedWorkingCopies() {
+    const workingCopies = useWorkingCopiesStore()
+    const plotted = new Set(plottedDatastreams.value.map((d) => d.id))
+    for (const { id } of graphSeriesArray.value) {
+      if (plotted.has(id) || id === qcDatastreamId.value) continue
+      if (managedDatastreamIds.value.has(id)) workingCopies.invalidate(id)
+    }
+  }
+
   /** Ids of every managed datastream; these are hidden from the catalog. */
   const managedDatastreamIds = computed(() => {
     const ids = new Set<string>()
@@ -192,6 +203,8 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     qcDatastreamId.value = null
     selectedObservedPropertyNames.value = []
     selectedProcessingLevelNames.value = []
+    // Working copies belong to the workspace being left.
+    useWorkingCopiesStore().clear()
     // selectedDateBtnId is a user preference, not workspace state.
     // Old watcher used to call clearChartState when plottedDatastreams
     // emptied; with the watcher gone, do it here explicitly.
@@ -501,6 +514,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   async function doRebuildPlot() {
     hasSelectionShape.value = false
     if (!plottedDatastreams.value.length) {
+      invalidateUnplottedWorkingCopies()
       clearChartState()
       return
     }
@@ -696,6 +710,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   /** Refreshes the graphSeriesArray based on the current selection of datastreams */
   const refreshGraphSeriesArray = async () => {
     // Remove graphSeries that are no longer selected
+    invalidateUnplottedWorkingCopies()
     const currentIds = new Set(plottedDatastreams.value.map((ds) => ds.id))
     graphSeriesArray.value = graphSeriesArray.value.filter((s) =>
       currentIds.has(s.id)
diff --git a/apps/qc-app/src/store/workingCopies.ts b/apps/qc-app/src/store/workingCopies.ts
index 43e6eb43..454cbcc1 100644
--- a/apps/qc-app/src/store/workingCopies.ts
+++ b/apps/qc-app/src/store/workingCopies.ts
@@ -33,27 +33,14 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
   // Not reactive: records hold large typed arrays that must not be proxied.
   const copies = new Map<string, WorkingCopy>()
 
-  // Per-managed-id generation, bumped by invalidate/set and by every new
-  // build (load's own build, and rebuild). A build only writes its result
-  // to `copies` if its generation is still the latest once its await
-  // resolves; otherwise a later invalidate/set/build has already
-  // superseded it and the late write is silently dropped. This is what
-  // keeps a stale in-flight reconstruction from resurrecting a copy after
-  // the session it belongs to was committed or deleted.
+  // Bumped by every invalidate/set/clear and new build; a build caches its
+  // result only if its generation is still current.
   const generations = new Map<string, number>()
 
-  // One in-flight load() build per managed id, so concurrent load() calls
-  // for the same datastream share a single reconstruction and resolve to
-  // the same record instead of each producing their own (the editor
-  // mutates records in place, so two instances would diverge).
+  // Shared so concurrent loads resolve to one record (edits mutate in place).
   const inFlightLoads = new Map<string, Promise<WorkingCopy | null>>()
 
-  // The most recently started rebuild() for a managed id, if still in
-  // flight. A rebuild superseded by a later one chains onto this instead
-  // of guessing at the cache, so it resolves to the same result as the
-  // rebuild that actually wins the race, regardless of which finishes
-  // its fetch first. Cleared by bump() (any invalidate/set/new build
-  // supersedes it) and re-registered by rebuild() right after.
+  // The latest in-flight rebuild; loads and superseded rebuilds join it.
   const activeRebuild = new Map<string, Promise<WorkingCopy | null>>()
 
   function bump(managedId: string): number {
@@ -73,20 +60,27 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
     end: Date
   ) {
     bump(managedId)
+    inFlightLoads.delete(managedId)
     copies.set(managedId, { sessionId, record, begin, end })
   }
 
   function invalidate(managedId: string) {
     bump(managedId)
+    inFlightLoads.delete(managedId)
     copies.delete(managedId)
   }
 
-  /**
-   * Reconstruct and, if still current, cache the result. Returns null
-   * (and leaves the cache untouched) when superseded while awaiting, so
-   * `load`/`rebuild` can tell the two cases apart and never hand a caller
-   * a record that isn't the cached one.
-   */
+  /** Drop every copy and supersede every in-flight build. */
+  function clear() {
+    // A load still listing sessions has no generation yet; bump it too.
+    const ids = new Set([...generations.keys(), ...inFlightLoads.keys()])
+    for (const managedId of ids) bump(managedId)
+    copies.clear()
+    inFlightLoads.clear()
+    activeRebuild.clear()
+  }
+
+  /** Reconstruct and cache the result; null when superseded while awaiting. */
   async function build(
     managed: Datastream,
     source: Datastream,
@@ -119,33 +113,42 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
     return copy
   }
 
-  async function rebuild(
+  /** What a superseded build resolves to: a newer rebuild's result, else the cache. */
+  async function current(managedId: string): Promise<WorkingCopy | null> {
+    const rebuilding = activeRebuild.get(managedId)
+    if (rebuilding) return rebuilding
+    return copies.get(managedId) ?? null
+  }
+
+  /** Always reconstructs, superseding any in-flight load or earlier rebuild. */
+  function rebuild(
     managed: Datastream,
     source: Datastream,
     historyId: string,
     session: SessionWindow
   ): Promise<WorkingCopy | null> {
-    // Always forces a fresh reconstruction, superseding any load() build
-    // or earlier rebuild() in flight for this managed id.
     const generation = bump(managed.id)
     inFlightLoads.delete(managed.id)
-    const pending = build(managed, source, historyId, session, generation)
-    activeRebuild.set(managed.id, pending)
-    try {
-      const built = await pending
-      if (built) return built
-      // Superseded while awaiting. A later rebuild is now the
-      // authoritative one: chain onto it so both resolve together,
-      // whichever fetch actually finishes first. Otherwise (a
-      // synchronous invalidate/set) the cache already holds the answer.
-      const winner = activeRebuild.get(managed.id)
-      if (winner && winner !== pending) return await winner
-      return copies.get(managed.id) ?? null
-    } finally {
-      if (activeRebuild.get(managed.id) === pending) {
-        activeRebuild.delete(managed.id)
-      }
-    }
+    const promise: Promise<WorkingCopy | null> = build(
+      managed,
+      source,
+      historyId,
+      session,
+      generation
+    )
+      .then(async (built) => {
+        if (built) return built
+        const winner = activeRebuild.get(managed.id)
+        if (winner && winner !== promise) return winner
+        return copies.get(managed.id) ?? null
+      })
+      .finally(() => {
+        if (activeRebuild.get(managed.id) === promise) {
+          activeRebuild.delete(managed.id)
+        }
+      })
+    activeRebuild.set(managed.id, promise)
+    return promise
   }
 
   async function performLoad(
@@ -153,11 +156,16 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
     source: Datastream,
     historyId: string
   ): Promise<WorkingCopy | null> {
+    const startGeneration = generations.get(managed.id)
     const { hs } = storeToRefs(useHydroServer())
     const session = await getInProgressSession(
       hs.value.qualityControlSessions,
       historyId
     )
+    // Superseded while listing sessions: never bump over what replaced us.
+    if (generations.get(managed.id) !== startGeneration) {
+      return current(managed.id)
+    }
     if (!session) {
       invalidate(managed.id)
       return null
@@ -167,10 +175,7 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
 
     const generation = bump(managed.id)
     const built = await build(managed, source, historyId, session, generation)
-    // Superseded while awaiting (invalidate/set/rebuild landed first):
-    // don't hand the caller a record that isn't (and won't become) the
-    // cached entry. Report whatever is current instead.
-    return built ?? copies.get(managed.id) ?? null
+    return built ?? current(managed.id)
   }
 
   function load(
@@ -178,6 +183,8 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
     source: Datastream,
     historyId: string
   ): Promise<WorkingCopy | null> {
+    const rebuilding = activeRebuild.get(managed.id)
+    if (rebuilding) return rebuilding
     const existing = inFlightLoads.get(managed.id)
     if (existing) return existing
 
@@ -204,5 +211,5 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
     })
   }
 
-  return { get, set, invalidate, load, rebuild, extents }
+  return { get, set, invalidate, clear, load, rebuild, extents }
 })

From 2542fbce4c2df82eb7e740abe6b60ac5f2ed334b Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:00:56 -0600
Subject: [PATCH 53/93] feat: edit datastreams from a row button with the
 source as context

Plot checkboxes only plot now. A row Edit button picks the edit target,
a new session starts with a window step, and the editor draws the raw
source in grey plus a Context range that reloads only the context series.
---
 apps/qc-app/docs/API_REFERENCE.md             | 129 ++--
 apps/qc-app/docs/ARCHITECTURE.md              |  33 +-
 apps/qc-app/docs/PERFORMANCE.md               |   2 +-
 apps/qc-app/docs/PLOTTING.md                  |  71 ++-
 apps/qc-app/docs/TESTING.md                   |  16 +-
 apps/qc-app/docs/USER_GUIDE.md                | 119 ++--
 apps/qc-app/e2e/all-operations.spec.ts        |   2 +-
 apps/qc-app/e2e/date-picker-dialog.spec.ts    |   2 +-
 apps/qc-app/e2e/edit-add-points.spec.ts       |   2 +-
 apps/qc-app/e2e/edit-change-values.spec.ts    |   2 +-
 apps/qc-app/e2e/edit-delete-points.spec.ts    |   2 +-
 apps/qc-app/e2e/edit-drift-correction.spec.ts |   2 +-
 apps/qc-app/e2e/edit-fill-gaps.spec.ts        |   2 +-
 apps/qc-app/e2e/edit-interpolate.spec.ts      |   2 +-
 apps/qc-app/e2e/edit-selection.spec.ts        | 338 ++++++++++
 apps/qc-app/e2e/edit-shift-datetimes.spec.ts  |   2 +-
 apps/qc-app/e2e/filter-change.spec.ts         |   2 +-
 apps/qc-app/e2e/filter-datetime-range.spec.ts |   2 +-
 apps/qc-app/e2e/filter-gaps.spec.ts           |   2 +-
 apps/qc-app/e2e/filter-persistence.spec.ts    |   2 +-
 apps/qc-app/e2e/filter-range-toggle.spec.ts   |   2 +-
 apps/qc-app/e2e/filter-rate-of-change.spec.ts |   2 +-
 .../qc-app/e2e/filter-value-threshold.spec.ts |   2 +-
 apps/qc-app/e2e/history.spec.ts               |   2 +-
 apps/qc-app/e2e/navigation.spec.ts            |  39 +-
 apps/qc-app/e2e/plot-layout.spec.ts           |  27 +-
 .../e2e/plot-no-intended-spacing.spec.ts      |  25 +-
 apps/qc-app/e2e/qc-golden-path.spec.ts        |  47 +-
 apps/qc-app/e2e/qc-history.spec.ts            |   2 +-
 apps/qc-app/e2e/screenshots.spec.ts           |  30 +-
 apps/qc-app/e2e/share-url.spec.ts             | 108 ++--
 apps/qc-app/e2e/smoke.spec.ts                 |   2 +-
 apps/qc-app/e2e/submit.spec.ts                |   4 +-
 apps/qc-app/e2e/support/app.ts                |  33 +-
 apps/qc-app/e2e/tooltip-threshold.spec.ts     |   2 +-
 .../EditData/SessionWindowDialog.vue          | 132 ++++
 .../components/EditData/StartEditingFlow.vue  | 341 ++++++++++
 .../__tests__/SessionWindowDialog.spec.ts     | 105 ++++
 .../__tests__/StartEditingFlow.spec.ts        | 443 +++++++++++++
 .../components/Navigation/NavigationRail.vue  |  25 +-
 .../__tests__/NavigationRail.spec.ts          | 211 +++++++
 apps/qc-app/src/components/VisualizeData.vue  | 496 +++------------
 .../components/VisualizeData/ContextPlot.vue  |  11 +-
 .../VisualizeData/DataVisDatasetsTable.vue    | 109 ++--
 .../VisualizeData/DataVisTimeFilters.vue      |   9 +-
 .../VisualizeData/DataVisualization.vue       |  98 ++-
 .../DatastreamInformationCard.vue             |   2 +-
 .../src/components/VisualizeData/Plot.vue     | 135 ++--
 .../VisualizeData/PlottedDatastreams.vue      | 176 +++---
 .../__tests__/DataVisDatasetsTable.spec.ts    |  87 ++-
 .../__tests__/DataVisTimeFilters.spec.ts      |  52 ++
 .../__tests__/DataVisualization.spec.ts       | 129 ++++
 .../VisualizeData/__tests__/Plot.spec.ts      | 146 +++++
 .../__tests__/PlottedDatastreams.spec.ts      | 160 ++++-
 .../__tests__/useEditEntry.spec.ts            | 273 ++++++++
 .../__tests__/useEditSession.spec.ts          | 138 ++++-
 .../__tests__/useQcHistory.spec.ts            | 110 ++++
 .../__tests__/useResumeEditSession.spec.ts    |  39 +-
 apps/qc-app/src/composables/useEditEntry.ts   | 114 ++++
 apps/qc-app/src/composables/useEditSession.ts |  49 +-
 apps/qc-app/src/composables/useQcHistory.ts   |  30 +-
 .../src/composables/useResumeEditSession.ts   |  27 +-
 .../composables/useWorkspacePermissions.ts    |   2 +-
 .../store/__tests__/dataVisualization.spec.ts | 581 +++++++-----------
 .../qc-app/src/store/__tests__/plotly.spec.ts |  84 ++-
 apps/qc-app/src/store/dataVisualization.ts    | 323 ++++------
 apps/qc-app/src/store/plotly.ts               |  22 +-
 .../src/utils/__tests__/sessionWindow.spec.ts | 113 ++++
 apps/qc-app/src/utils/__tests__/share.spec.ts |  41 +-
 .../utils/plotting/__tests__/options.spec.ts  | 109 +++-
 .../utils/plotting/__tests__/shapes.spec.ts   |  41 ++
 .../utils/plotting/__tests__/staging.spec.ts  |  86 +++
 apps/qc-app/src/utils/plotting/operations.ts  |   6 +-
 apps/qc-app/src/utils/plotting/options.ts     | 105 +++-
 apps/qc-app/src/utils/plotting/shapes.ts      |  41 ++
 apps/qc-app/src/utils/plotting/staging.ts     |  41 +-
 apps/qc-app/src/utils/sessionWindow.ts        |  72 +++
 apps/qc-app/src/utils/share.ts                |  58 +-
 78 files changed, 4719 insertions(+), 1714 deletions(-)
 create mode 100644 apps/qc-app/e2e/edit-selection.spec.ts
 create mode 100644 apps/qc-app/src/components/EditData/SessionWindowDialog.vue
 create mode 100644 apps/qc-app/src/components/EditData/StartEditingFlow.vue
 create mode 100644 apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
 create mode 100644 apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
 create mode 100644 apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
 create mode 100644 apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
 create mode 100644 apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts
 create mode 100644 apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
 create mode 100644 apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
 create mode 100644 apps/qc-app/src/composables/__tests__/useQcHistory.spec.ts
 create mode 100644 apps/qc-app/src/composables/useEditEntry.ts
 create mode 100644 apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
 create mode 100644 apps/qc-app/src/utils/plotting/__tests__/shapes.spec.ts
 create mode 100644 apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts
 create mode 100644 apps/qc-app/src/utils/plotting/shapes.ts
 create mode 100644 apps/qc-app/src/utils/sessionWindow.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index bc303832..91a7e0df 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -112,18 +112,55 @@ await setSelected([0, 1, 2, 5])      // dispatches SELECTION
 await clearSelected({ recordHistory: false })  // skip history append on cleanup
 ```
 
+### `useEditEntry()`
+
+```ts
+const { enterEdit, startSessionOver, leaveEdit } = useEditEntry()
+
+const result = await enterEdit(managedId, window)
+// result: 'editing' | 'needs-window' | 'not-managed' | 'superseded'
+```
+
+Sets the edit target and switches to the Edit view. Shared by the row Edit
+flow (`StartEditingFlow.vue`), reload resume (`useResumeEditSession()`
+below), and share-link hydration (the `ed` query param).
+
+- `enterEdit(managedId, window?)`: sets the edit target
+  (`setEditTarget`), resolves its QC history via `beginEditing()`, and
+  resumes an in-progress session. With no session and no `window`, returns
+  `'needs-window'` so the caller opens `SessionWindowDialog.vue` (the
+  session-window step) and calls back in with the chosen window, or calls
+  `startSessionOver` instead; with a `window`, starts the session
+  immediately.
+- `startSessionOver(window)`: starts a new session on the current edit
+  target over `window` (a `utils/timeRangePresets.ts` `TimeWindow`); backs
+  **Start new session** and the editor footer's **New session**.
+- `leaveEdit()`: returns to the Select view, clears the resume pointer and
+  the edit target (`clearEditTarget`). Every exit uses it, including the nav
+  rail's Home and Workspaces buttons, so a reload never reopens an editor the
+  user left.
+
+Entries can overlap (a reload resume and a click land close together);
+whichever call sets the edit target last owns the view: a call that finds
+the target changed after an `await` stands down without touching view
+state, the target, or the resume pointer.
+
 ### `useResumeEditSession()`
 
 ```ts
-const { resume } = useResumeEditSession(enterEdit)
+const { resume } = useResumeEditSession(async (id) => {
+  await startEditing.value?.resume(id) // StartEditingFlow
+})
 ```
 
 Reopens the editor after a page reload, using the persisted
-`qcSession.resumeDatastreamId`: replots that datastream, makes it the QC
-target, then calls the supplied `enterEdit`. The workspace catalog loads
-asynchronously and is empty at mount, so it waits for the catalog to arrive
-and resumes at most once. A pointer to a datastream missing from the catalog
-(deleted, or another workspace) is dropped rather than retried.
+`qcSession.resumeDatastreamId`: waits for the workspace catalog to arrive
+(it's empty at mount), then calls the supplied callback with the managed
+datastream id (normally wired to `StartEditingFlow`'s exposed `resume`, which
+enters through `useEditEntry()` and opens the session-window step when there
+is no session to continue). Resumes at
+most once. A pointer to a datastream missing from the catalog (deleted, or
+another workspace) is dropped rather than retried.
 
 Note the watcher must not use Vue's `once` together with `immediate`: the
 immediate call fires on the initial empty catalog and stops the watcher, so
@@ -213,9 +250,9 @@ const options = await loadForSource(sourceDatastreamId)
 ```
 
 Resolves a source datastream's managed (QC) datastreams from the loaded QC
-histories and fetches each one's sessions — feeds the "Start editing"
-chooser, which lists managed datastreams with their in-progress/committed
-sessions.
+histories and fetches each one's sessions. Feeds the row Edit button's
+chooser (`StartEditingFlow.vue`), which lists managed datastreams with their
+in-progress/committed sessions.
 
 ### `useWorkspacePermissions()`
 
@@ -232,8 +269,9 @@ canCreateDatastream(ws)   // can create the managed datastream here?
 roleName(ws)              // 'Owner' | <collaborator role> | 'Admin' | 'Read-only'
 ```
 
-Used to disable the editor's Start editing / Save / Commit / Create
-controls and to mark each workspace's role on the picker.
+Used to disable the row Edit button, the editor's Save / Commit controls,
+and the create-datastream form, and to mark each workspace's role on the
+picker.
 
 ### `useResizable()`
 
@@ -265,7 +303,7 @@ specific slice picked. Stores with no Persistence line are session-only.
 ### `useDataVisStore()` — `src/store/dataVisualization.ts`
 
 Catalog data (sites, datastreams, taxonomy), sidebar filters, plotted
-set + QC target, time-range window. The orchestrator for everything in
+set + edit target, time-range window. The orchestrator for everything in
 the Select drawer and the rebuild pipeline that owns `rebuildPlot()`.
 
 Persistence: `selectedDateBtnId` only (so the user's preset choice
@@ -277,8 +315,8 @@ on boot.
 | `things`                            | state    | `Thing[]`                                         | Sites in the active workspace; fetched once on workspace mount. |
 | `datastreams`                       | state    | `(Datastream & DatastreamExtended)[]`             | All visible datastreams (with `expand_related` nested objects). |
 | `qcHistories`                       | state    | `QualityControlHistory[]`                         | Workspace QC histories (each links a managed datastream to its source); loaded with the catalog. |
-| `managedDatastreamIds`              | computed | `Set<string>`                                     | Ids of every managed (QC) datastream; hidden from the catalog (reached via the Start-editing chooser). |
-| `historiesBySource`                 | computed | `Map<string, QualityControlHistory[]>`            | `sourceDatastreamId` -> its QC histories; drives the Start-editing chooser. |
+| `managedDatastreamIds`              | computed | `Set<string>`                                     | Ids of every managed (QC) datastream; hidden from the catalog (reached via the row Edit chooser). |
+| `historiesBySource`                 | computed | `Map<string, QualityControlHistory[]>`            | `sourceDatastreamId` -> its QC histories; drives the row Edit chooser. |
 | `addQcHistory`                      | action   | `(history: QualityControlHistory) => void`        | Register a newly-created history so its managed datastream hides from the catalog and shows in the chooser without a reload. |
 | `removeManagedDatastream`           | action   | `(historyId: string, managedId: string) => void`  | Drop a deleted managed datastream + its history from local state (chooser/catalog) after deleting it server-side. |
 | `replaceDatastream`                 | action   | `(ds: Datastream & DatastreamExtended) => void`   | Swap a fresh copy into the catalog and plotted set (used after a commit moves a managed datastream's phenomenon times). |
@@ -288,36 +326,38 @@ on boot.
 | `selectedObservedPropertyNames`     | state    | `string[]`                                        | Observed-property filter selection. |
 | `selectedProcessingLevelNames`      | state    | `string[]`                                        | Processing-level filter selection. |
 | `filteredDatastreams`               | computed | `(Datastream & DatastreamExtended)[]`             | `datastreams` narrowed by the three filter selections, with managed (QC) datastreams excluded. |
-| `plottedDatastreams`                | state    | `Datastream[]`                                    | Up to 5 streams currently on the chart. |
-| `qcDatastreamId`                    | state    | `string \| null`                                  | Storage form of the QC target; survives plotted-list mutations. |
-| `qcDatastream`                      | computed | `Datastream \| null`                              | Live lookup of `qcDatastreamId` in `plottedDatastreams`. |
-| `qualifierSet`                      | state    | `Set<string>`                                     | Qualifier codes seen on the QC target's loaded points. |
+| `plottedDatastreams`                | state    | `Datastream[]`                                    | Up to 5 streams the user chose to plot. Editing never adds to or removes from it (snapshots are the exception, dropped on leave); the 5-stream cap doesn't count the edit target or its source. |
+| `qcDatastreamId`                    | state    | `string \| null`                                  | The edit target's id. Set only by the edit flow (`setEditTarget` / `clearEditTarget`); null in the Select view. |
+| `qcDatastream`                      | computed | `Datastream \| null`                              | Live catalog lookup of `qcDatastreamId` in `datastreams`, not `plottedDatastreams`, since the edit target isn't a plotted entry. |
+| `sourceContextDatastream`           | computed | `Datastream \| null`                              | The edit target's catalog source, resolved through `qcHistories`; null without an edit target. Drawn behind the edit target as context. |
+| `seriesDatastreams`                 | computed | `Datastream[]`                                    | What the plot actually draws, in order: `[edit target, its source, ...plotted minus those]` while editing, otherwise `plottedDatastreams` unchanged. Refresh, colour assignment, series ordering, working-copy invalidation, `PlottedDatastreams`, the share watcher, and snapshots all iterate this instead of `plottedDatastreams`. |
+| `qualifierSet`                      | state    | `Set<string>`                                     | Qualifier codes seen on the edit target's loaded points. |
 | `selectedQualifier`                 | state    | `string`                                          | Active qualifier in the picker. |
 | `selectedData`                      | state    | `number[] \| null`                                | Index list of the active selection (lasso, box, click). |
 | `hasSelectionShape`                 | state    | `boolean`                                         | True while a box/lasso shape exists, even when it captured zero points. |
 | `loadingStates`                     | state    | `Map<string, boolean>`                            | Per-datastream in-flight observation fetches. |
-| `beginDate` / `endDate`             | state    | `Date`                                            | Active loaded window. A preset re-resolves it from the plotted datastreams' phenomenon times on every plot rebuild; a custom range stays fixed. Presets do not re-resolve while the Edit view is active, so plotting/unplotting a datastream there leaves the window alone. |
+| `beginDate` / `endDate`             | state    | `Date`                                            | Active loaded window. A preset re-resolves it from the context data's (`seriesDatastreams` minus the edit target) phenomenon times on every plot rebuild; a custom range stays fixed. |
 | `selectedDateBtnId`                 | state    | `number`                                          | Active preset id (default `1`, 1m); `-1` (`CUSTOM_PRESET_ID`) for a manual range. Presets are defined in `utils/timeRangePresets.ts`. |
 | `matchesSelectedThing`              | action   | `(ds) => boolean`                                 | Filter predicate; exposed so the table can reuse it on row updates. |
 | `matchesSelectedObservedProperty`   | action   | `(ds) => boolean`                                 | Same shape as above. |
 | `matchesSelectedProcessingLevel`    | action   | `(ds) => boolean`                                 | Same shape as above. |
-| `setDateRange`                      | action   | `({ begin?, end?, update?, custom? }) => Promise<void>` | No-ops when neither bound moves; clears zoom history when it does. |
-| `onDateBtnClick`                    | action   | `(id: number) => Promise<void>`                   | Selects the preset and applies its window over the plotted data's extent. With nothing plotted, only the selection changes. |
-| `refreshGraphSeriesArray`           | action   | `() => Promise<unknown[]>`                        | Reconciles `graphSeriesArray` against `plottedDatastreams` (fetch deltas + reorder + recolor). A managed datastream with a loaded working copy (`useWorkingCopiesStore`) uses it instead of fetching. Invalidates the working copy of any managed datastream no longer plotted, except the QC target. |
-| `resetState`                        | action   | `() => void`                                      | Clears filters, the plotted set, and every working copy on a workspace swap; preserves the preset preference. |
+| `setDateRange`                      | action   | `({ begin?, end?, update?, custom? }) => Promise<void>` | No-ops when neither bound moves. Otherwise refreshes `seriesDatastreams`; while editing this reloads context only and preserves the zoom (`redraw(false, true)`), since the edit target's data isn't fetched here. Otherwise it clears zoom history and applies the new window. |
+| `onDateBtnClick`                    | action   | `(id: number) => Promise<void>`                   | Selects the preset and applies its window over the context data's extent. With nothing plotted, only the selection changes. |
+| `refreshGraphSeriesArray`           | action   | `() => Promise<unknown[]>`                        | Reconciles `graphSeriesArray` against `seriesDatastreams` (fetch deltas + reorder + recolor), skipping the edit target (its data is owned by the edit session, not this refresh). A managed datastream with a loaded working copy (`useWorkingCopiesStore`) uses it instead of fetching. Invalidates the working copy of any managed datastream no longer in `seriesDatastreams`, except the edit target. |
+| `resetState`                        | action   | `() => void`                                      | Clears filters, the plotted set, and the edit target, and drops every working copy, on a workspace swap; preserves the preset preference. |
 | `toggleDatastream`                  | action   | `(ds: Datastream) => Promise<void>`               | Plot if absent, unplot if present. |
-| `plotDatastream`                    | action   | `(ds: Datastream) => Promise<void>`               | Add to plot; promotes to QC when nothing's there yet. |
-| `unplotDatastream`                  | action   | `(id: string) => Promise<void>`                   | Remove; promotes the previous plotted entry to QC if removing the QC target. |
+| `plotDatastream`                    | action   | `(ds: Datastream) => Promise<void>`               | Add to plot. Plotting never picks or changes the edit target. |
+| `unplotDatastream`                  | action   | `(id: string) => Promise<void>`                   | Remove from the plotted set. Never touches the edit target. |
 | `clearPlottedDatastreams`           | action   | `() => Promise<void>`                             | Drop the entire plotted set. |
 | `sourceGroupIds`                    | action   | `(sourceId: string) => string[]`                  | The source datastream plus every managed (QC) datastream derived from it. |
-| `plotSourceSelection`               | action   | `(sourceId: string, ids: string[]) => Promise<void>` | Apply a whole "what to plot for this source" choice at once: `ids` is the complete set wanted from that source's group. Group members absent from `ids` are unplotted, additions are appended in `ids` order, and the QC target is promoted once against the final set. One rebuild for the whole selection. |
-| `addSnapshotSeries`                 | action   | `(id: string, record: ObservationRecord, meta: SnapshotMeta) => Promise<void>` | Add a frozen history snapshot as an extra comparison line under the synthetic id `snap:<sessionId>:<opIndex>`. Never promotes to QC target; `refreshGraphSeriesArray` skips its fetch. |
-| `removeSnapshotSeries`              | action   | `(id: string) => Promise<void>`                   | Drop one snapshot line. Leaves the QC target alone. |
-| `setPlottedDatastreams`             | action   | `(items: Datastream[], qcId?: string \| null) => Promise<void>` | Wholesale replace; used by URL hydration. |
-| `setQcDatastream`                   | action   | `(id: string \| null) => Promise<void>`           | Change QC target; preserves the current zoom. |
-| `adoptManagedDatastream`            | action   | `(managed: Datastream, sourceId: string) => Promise<void>` | Enter editing on a freshly-created managed datastream: replace the source in the plot and re-key its already-loaded series as the managed datastream's working copy (no second, empty item; no re-fetch). |
-| `releaseManagedDatastream`          | action   | `() => Promise<void>`                             | Inverse of `adoptManagedDatastream`, for leaving the editor: swap the managed datastream back to its source (resolved through `qcHistories`), drop the editor's working copy and rebuild, so the plot shows the source as stored rather than the session's uncommitted edits. Managed datastreams are hidden from the catalog table, so without this the Select view shows a plot with nothing selected. No-op when the QC target isn't managed or its source isn't in the catalog. |
-| `rebuildPlot`                       | action   | `() => Promise<void>`                             | Serialized rebuild (drop zoom history, refresh series, regenerate options, render). Coalesces concurrent callers. |
+| `plotSourceSelection`               | action   | `(sourceId: string, ids: string[]) => Promise<void>` | Apply a whole "what to plot for this source" choice at once: `ids` is the complete set wanted from that source's group. Group members absent from `ids` are unplotted, additions are appended in `ids` order. One rebuild for the whole selection; never touches the edit target. |
+| `addSnapshotSeries`                 | action   | `(id: string, record: ObservationRecord, meta: SnapshotMeta) => Promise<void>` | Add a frozen history snapshot as an extra comparison line under the synthetic id `snap:<sessionId>:<opIndex>`. Never touches the edit target; `refreshGraphSeriesArray` skips its fetch. |
+| `removeSnapshotSeries`              | action   | `(id: string) => Promise<void>`                   | Drop one snapshot line. Leaves the edit target alone. |
+| `setPlottedDatastreams`             | action   | `(items: Datastream[]) => Promise<void>`          | Replace the plotted set wholesale (used by URL hydration). Doesn't touch the edit target. Hydrate that separately with `setEditTarget`. |
+| `setEditTarget`                     | action   | `(managedId: string) => Promise<void>`            | Begin editing `managedId`: drops snapshots, resets the `qcSession` store when the target changes (not `resumeDatastreamId`), sets `qcDatastreamId`, rebuilds the plot. Its data arrives later via `setEditRecord`. This action doesn't fetch it. |
+| `setEditRecord`                     | action   | `(record: ObservationRecord) => Promise<void>`    | Put the edit target's record on the plot: updates its graph series in place if one exists, otherwise adds it (`buildGraphSeries`) and redraws. The only path by which the edit target's data reaches the plot. |
+| `clearEditTarget`                   | action   | `() => Promise<void>`                             | Stop editing: drops snapshots, invalidates the edit target's working copy, clears `qcDatastreamId`, removes its graph series, and rebuilds. Plotted datastreams are left exactly as they were. |
+| `rebuildPlot`                       | action   | `() => Promise<void>`                             | Serialized rebuild (refresh series, regenerate options, render). Drops zoom history in the Select view; keeps the zoom while an edit target is set. Coalesces concurrent callers. |
 
 ### `useWorkingCopiesStore()` (`src/store/workingCopies.ts`)
 
@@ -366,7 +406,7 @@ handles, live chart caches).
 | `plotlyOptions`            | state    | `PlotlyChartOptions`                              | Cached `createPlotlyOption` output; seeded empty so consumers can read without null-guards. |
 | `plotlyRef`                | state    | `AppPlotlyHTMLElement \| null`                    | Live Plotly DOM element; populated by `handleNewPlot`. |
 | `mainPlotEpoch`            | state    | `number`                                          | Monotonic counter — bumped per `handleNewPlot` so listeners can re-attach. |
-| `selectedSeriesIndex`      | computed | `number`                                          | Index of the QC target in `graphSeriesArray` (`-1` when none). |
+| `selectedSeriesIndex`      | computed | `number`                                          | Index of the edit target in `graphSeriesArray` (`-1` when none). |
 | `selectedSeries`           | computed | `GraphSeries`                                     | Convenience for `graphSeriesArray[selectedSeriesIndex]`. |
 | `editHistory`              | state    | `HistoryItem[]`                                   | Mirrors `selectedSeries.data.history` (mutated in place — never reassign). |
 | `suppressedEchoSelection`  | state    | `number[] \| null`                                | Sentinel armed by programmatic Plotly writes to suppress the echo SELECTION dispatch. |
@@ -384,7 +424,7 @@ handles, live chart caches).
 | `hiddenAxisIds`            | state    | `Set<string>`                                     | Datastream ids whose right-side y-axis chrome is hidden. |
 | `hiddenTraceIds`           | state    | `Set<string>`                                     | Datastream ids whose trace is fully hidden (eye toggle). |
 | `activeTab`                | state    | `'plot' \| 'table'`                               | Center-column tab; captured by the share URL. |
-| `tableScrollRequest`       | state    | `{ time: number; seq: number } \| null`           | Signal from the "zoom to range" presets; `DataTable` scrolls to the first row at/after `time`. `seq` re-triggers on repeats. |
+| `tableScrollRequest`       | state    | `{ time: number; seq: number } \| null`           | Set when the plot zooms to the session window; `DataTable` scrolls to the first row at/after `time`. `seq` re-triggers on repeats. |
 | `requestTableScroll`       | action   | `(time: number) => void`                          | Publish a `tableScrollRequest` for the given epoch-ms range start (bumps `seq`). |
 | `axisChips`                | state    | `AxisChip[]`                                      | Horizontal axis title chips (replaces Plotly's rotated titles). |
 | `previewMode`              | state    | `boolean`                                         | Strips select/lasso/etc when the chart is rendered in the Select view's preview slot. |
@@ -396,7 +436,7 @@ handles, live chart caches).
 | `canRedoZoom`              | computed | `boolean`                                         | `zoomRedoStack.length > 0`. |
 | `currentZoom`              | computed | `ZoomState \| null`                               | Top of the undo stack; what the share URL writer subscribes to. |
 | `updateOptions`            | action   | `() => void`                                      | Rebuild `plotlyOptions` from `graphSeriesArray`. |
-| `redraw`                   | action   | `(recomputeXaxisRange?: boolean, preserveZoom?: boolean) => Promise<void>` | Push typed-array updates + restyle; preserves the user's zoom by default. |
+| `redraw`                   | action   | `(recomputeXaxisRange?: boolean, preserveZoom?: boolean) => Promise<void>` | Push typed-array updates + restyle; preserves the user's zoom by default. Keeps shapes other writers own (the staging band) and replaces `edit-window`. |
 | `clearChartState`          | action   | `() => void`                                      | Drop all series + zoom history (used on workspace swap). |
 | `fetchGraphSeries`         | action   | `(ds, start: Date, end: Date) => Promise<GraphSeries>` | Fetch observations for `ds` over `[start, end]` and build a `GraphSeries` via `buildGraphSeries`. |
 | `buildGraphSeries`         | action   | `(ds: Datastream, data: ObservationRecord) => GraphSeries` | Build a `GraphSeries` from an already-loaded record, no fetch. Used directly for a managed datastream's working copy. |
@@ -605,6 +645,23 @@ columnar fetch, returns `{ datetimes: number[]; dataValues: number[] }`.
 `subtractDays`, `subtractMonths`, `subtractYears` for the time-range
 preset buttons.
 
+### `src/utils/sessionWindow.ts`
+
+Pure validation for the session-window step (`SessionWindowDialog.vue`,
+opened from the row Edit chooser or the editor footer's **New session**):
+
+- `defaultSessionWindow(source, sessions)`. The step's default window:
+  starts where the committed history ends, or at the source's own start if
+  nothing is committed yet; runs to the end of the source data. `null` when
+  the source has no observations.
+- `validateSessionWindow(window, source, sessions)`: `null` when `window`
+  is valid, otherwise an error string. The window must lie inside the
+  source's extent, and it can't leave a gap before or after the committed
+  history (history spec 7.2.3 / 7.2.4). Touching an edge or overlapping is
+  fine.
+- `committedExtent(sessions)`: the earliest committed start to the latest
+  committed end, or `null` with nothing committed.
+
 ### `src/utils/rules.ts`
 
 Vuetify validation rules used across forms (required, numeric, range).
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index a3eb04a1..01d05646 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -181,8 +181,11 @@ Invariants:
   `plotly_selected` events both for user gestures and for our programmatic
   `Plotly.restyle({ selectedpoints })` calls. The sentinel in `plotly.ts`
   drops the echo so we don't append a duplicate `SELECTION` entry.
-- **The first plotted stream is the QC target.** The others are read-only
-  context traces. Dispatch only ever runs against `selectedSeries.data`.
+- **The plot draws `seriesDatastreams`, not `plottedDatastreams` directly.**
+  While editing it's `[edit target, its source, ...plotted]`; otherwise it's
+  `plottedDatastreams` as-is. Dispatch only ever runs against
+  `selectedSeries.data`, the edit target's series; without an edit target
+  `selectedSeries` is undefined.
 
 ## Plotly integration
 
@@ -196,6 +199,7 @@ in tests.
 | `relayout.ts`    | Debounced viewport recomputation, tick alignment.                      |
 | `selected.ts`    | Translates Plotly selection events into a `SELECTION` dispatch.        |
 | `staging.ts`     | Ghost-fill markers + drag-resizable stage shape (Add Points, Fill Gaps). |
+| `shapes.ts`      | Composes `layout.shapes` by name so each writer keeps the others' shapes. |
 | `interaction.ts` | High-level "user clicked a point" / "user dragged the stage" wiring.   |
 | `operations.ts`  | Per-op redraw concerns (highlighting after a filter, fading after delete). |
 | `zoom.ts`        | Synchronized multi-axis zoom + scroll-zoom handling.                   |
@@ -243,6 +247,18 @@ the `observationsBulkBody` serializer. `unwrap` bridges `ApiResponse` to the
 thrown errors this glue surfaces. None of it is a transport — swapping the QC
 client out is a `@hydroserver/client` change, not an app one.
 
+**Edit entry.** The row Edit button on a source (`StartEditingFlow.vue`)
+picks the managed datastream to edit and, through `useEditEntry()`, sets it
+as the edit target and switches to the Edit view, resuming an in-progress
+session, or opening the session-window step when one is needed. Session
+window validity (inside the source's extent, no gap before or after
+committed history) is a pure util, `utils/sessionWindow.ts`, used by the
+session-window dialog. `startSession` only clamps the chosen window to the
+source's extent (`clampSpecToSource`) as a backstop. Once editing, context refresh
+(`refreshGraphSeriesArray`) never fetches the edit target: its data is
+owned by the session (working copy, `startSession`, `viewSession`,
+`resumeWorkingCopy`).
+
 Two contract notes worth keeping in mind:
 
 - **The backend stores the operation DAG as metadata only — it never replays
@@ -288,7 +304,7 @@ Two contract notes worth keeping in mind:
   datastream's cached observations are never edited. It is invalidated when
   the editor closes through `exitToSelect` (Close, Save and close, or close
   without saving), on commit, when its session or managed datastream is
-  deleted, and when a managed datastream other than the QC target stops being
+  deleted, and when a managed datastream other than the edit target stops being
   plotted; `resetState` clears every copy on a workspace reset. A nav rail
   switch to Select keeps the copy, which then equals the saved state because
   the exit guard saves or discards first. The next preview or resume rebuilds
@@ -352,8 +368,8 @@ package, not the app.
 **Permission gating.** QC editing writes to the source datastream's workspace
 (creates the managed datastream, pushes observations), so the editor's entry
 points are gated on the signed-in user's workspace role via
-`useWorkspacePermissions()` — a read-only collaborator sees disabled Start
-editing / Save / Commit controls and an explanation instead of a mid-flow 403,
+`useWorkspacePermissions()`: a read-only collaborator sees a disabled row
+Edit button, and disabled Save / Commit controls, with an explanation instead of a mid-flow 403,
 and each workspace's role is marked on the picker. The role rides along on the
 `Workspace` object (`collaboratorRole.permissions`; owners have a null role;
 admins override), so no extra request is needed.
@@ -379,9 +395,9 @@ Three constraints shape the implementation:
   `snap:<sessionId>:<opIndex>` so legend rendering, colour assignment,
   visibility and reorder work unchanged. `isSnapshotId()` guards the paths
   that would otherwise treat one as real: `refreshGraphSeriesArray` skips its
-  fetch, `releaseManagedDatastream` drops them when the editor closes, and
+  fetch, `clearEditTarget` drops them when the editor closes, and
   the share encoder keeps them out of `ds` (they use their own `snap` key, so
-  the QC-target-is-first rule and the `h`/`ya` bitmask indices still hold).
+  the `h`/`ya` bitmask indices over the plotted list still hold).
 
 ## Routing and auth
 
@@ -394,7 +410,8 @@ every navigation:
 - **`hasWorkspaceGuard`** — redirects users without a selected workspace
   to `/workspaces`.
 
-The nav rail's "Edit" entry is gated behind a selected QC datastream. Leaving
+The nav rail's "Edit" entry is enabled only while an edit target is set: it
+returns to the open editor and never picks one itself. Leaving
 the Edit view with edits not yet saved to the session runs the "Unsaved
 edits" dialog: Save & continue saves a draft to the in-progress session and
 Discard returns to the last save. The unsaved state comes from
diff --git a/apps/qc-app/docs/PERFORMANCE.md b/apps/qc-app/docs/PERFORMANCE.md
index 5b76bdb6..8b426a07 100644
--- a/apps/qc-app/docs/PERFORMANCE.md
+++ b/apps/qc-app/docs/PERFORMANCE.md
@@ -128,7 +128,7 @@ Plotly handles downsampling internally on zoom, but multi-axis
 synchronized plotting at high point density is the dominant interactive
 cost. Practical guidance:
 
-- **One QC target + one or two context streams** is the sweet spot.
+- **One edit target, its source, and one or two context streams** is the sweet spot.
   Five plotted streams is the hard cap; in practice three is comfortable
   at multi-hundred-thousand points.
 - **Tick alignment and viewport recompute are debounced.** See
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index ada48c53..6d83d58e 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -22,6 +22,7 @@ contributor can read the modules in the order the code runs.
 | `interaction.ts`  | DOM-level handlers: throttled mousemove crosshair, wheel-zoom, axis-chip placement, y-axis drag-rect widening.         |
 | `operations.ts`   | Imperative helpers callable from components: `zoomXaxisTo`, `toggleTraceVisibility`, `setSelectedPoints`, etc.         |
 | `staging.ts`      | Visual-only overlays while a Find-Gaps / Fill-Gaps operation is staged. Ghost-fill trace + drag-resizable shape.       |
+| `shapes.ts`       | `composeShapes` / `withLiveShapes`: each `layout.shapes` writer replaces only the shapes it names.                     |
 
 The split is not by file size, it's by the Plotly API each module
 talks to. `options.ts` owns trace + layout construction. `events.ts`
@@ -168,8 +169,8 @@ A `GraphSeries` carrying a `snapshot` field is a frozen replay of a QC
 session at one operation, not a live datastream. To `createPlotlyOption` it
 is an ordinary non-QC series: it gets its own overlaying right-side axis and
 its own colour from the shared assigner. That is deliberate. Being able to
-shift a snapshot on its own axis is how the user lines it up against the QC
-target, which is the point of plotting it.
+shift a snapshot on its own axis is how the user lines it up against the
+edit target, which is the point of plotting it.
 
 What differs is upstream, not here: the data never refetches (see
 `refreshGraphSeriesArray`'s `isSnapshotId` guard), and `PlottedDatastreams`
@@ -187,21 +188,61 @@ table, and its check box opens `PlotSourceDialog` rather than toggling.
 dialog's answer as a whole: `ids` is the complete set wanted from that group,
 so members absent from it are unplotted in the same pass.
 
-Doing it in one pass matters. Looping `plotDatastream` / `unplotDatastream`
-would promote the QC target once per change, letting it land on a datastream
-the user is in the middle of deselecting; the batched action promotes once
-against the final set, using the same "entry before the one that left" rule as
-`unplotDatastream`. It also means one `rebuildPlot`, so the coalescing lock
-never has to absorb a burst.
+Doing it in one pass matters even with nothing to promote: looping
+`plotDatastream` / `unplotDatastream` would trigger a `rebuildPlot` per
+change, where the batched action means exactly one, against the final set.
+The coalescing lock never has to absorb a burst.
 
 Additions are appended in `ids` order, which the dialog builds in display
-order (raw first, then managed). That is what keeps the first-plotted-wins QC
-rule predictable: picking raw alongside a managed version from an empty plot
-leaves the raw datastream as QC target.
-
-`releaseManagedDatastream` has to account for this too: the source can already
-be plotted next to its managed datastream, so it drops the managed entry
-rather than replacing it, which would otherwise duplicate the source.
+order (raw first, then managed). Plotting never singles one out as an edit
+target, so this order only matters for the legend and axis stacking.
+
+## Series roles
+
+`createPlotlyOption` reads roles off the stores, not a field on `GraphSeries`:
+
+- **Edit target** (`useDataVisStore().qcDatastream`): primary axis `y`,
+  black, selectable.
+- **Source context** (`sourceContextDatastream`): shares `y` with the edit
+  target, grey, drawn underneath it, read-only (no selection styling, no
+  own axis).
+- **Everything else** (context traces, and history snapshots): overlaying
+  right-side axes (`y2`, `y3`, …), same as today.
+- **No edit target** (Select view): the first entry in `seriesDatastreams`
+  (the first plotted datastream) takes the primary axis `y`, styled like any
+  other context trace but without an axis chip.
+
+The edit target's data reaches the plot through one store action,
+`setEditRecord(record)`, which upserts its graph series. `useEditSession`
+calls it instead of assigning `selectedSeries.value.data` directly.
+
+## Context range
+
+The editor toolbar's **Context** menu reuses `DataVisTimeFilters` (the same
+preset chips and From / To pickers as the Select view) bound to the same
+`beginDate` / `endDate` store range. Picking a preset or a custom date calls
+`setDateRange`, which, while an edit target is set, reloads only the
+context series (`refreshGraphSeriesArray`, which never fetches the edit
+target) and redraws with `redraw(false, true)` to keep the user's current
+zoom. The edit target's working copy is never re-windowed by this control.
+
+The session window itself is shaded as a layout shape (`name: 'edit-window'`
+in `options.ts`), drawn from `useQcSessionStore().viewedSession` (falling
+back to `inProgressSession`), only while an edit target is set.
+`setEditTarget` resets the session store when the target changes, so the
+previous target's window is never drawn over the new one.
+
+`layout.shapes` has more than one writer: `createPlotlyOption` owns
+`edit-window` and `staging.ts` owns `stage`. `Plotly.update` and
+`Plotly.relayout` replace the whole array, so every write goes through
+`utils/plotting/shapes.ts`: `redraw` and `cropXaxisRange` use
+`withLiveShapes` to swap in the fresh `edit-window` and keep the live
+`stage` band, and the staging flush uses `composeShapes` to do the reverse
+(stage stays first so drag events still address `shapes[0]`).
+
+Plot rebuilds (`rebuildPlot`, run when plotted datastreams change) keep
+the user's zoom while an edit target is set and drop it in the Select
+view.
 
 ## Why `internal.ts` isn't re-exported
 
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index f7e8f3a1..44ce43c1 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -239,7 +239,7 @@ The CI gate prints uncovered line numbers per file. Common causes:
 ```
 e2e/
 ├── support/
-│   ├── app.ts        — flow helpers (gotoHome, setupEditView, setupSessionEditView, openOp, waitForSelection)
+│   ├── app.ts        — flow helpers (gotoHome, setupEditView, startSessionFromRow, openOp, waitForSelection)
 │   ├── fixtures.ts   — workspace / datastream / observation fixtures
 │   ├── mocks.ts      — page.route() handlers that stand in for HydroServer
 │   └── ops.ts        — op-specific preambles (selectAllPoints, expectHistoryContains)
@@ -271,8 +271,8 @@ backend. The live golden-path spec is gated by `E2E_LIVE=1`, expects both
 frontends to be running, and enters QC through the Data Management
 same-origin entrypoint. It edits through a QC session, so the workspace
 needs a source datastream with a managed datastream and QC history: the
-spec plots the first source showing the managed-count badge, clicks Start
-editing, and ends by saving and committing the session.
+spec plots the first source showing the managed-count badge, clicks its
+row's pencil (Edit) button, and ends by saving and committing the session.
 
 ### Mocks
 
@@ -305,10 +305,12 @@ session fixtures and apply the app's creates, saves, commits and deletes, so
 a spec can pass a `qcSessionState` array and assert on what was persisted
 (see `submit.spec.ts`).
 
-Save and Commit only appear once a session is open. `setupEditView` switches
-to the Edit view through the rail, which opens no session, so those specs see
-New session in the history footer. Use `setupSessionEditView` (with
-`qcHistories: true`) to go through Start editing and start a session first.
+Save and Commit only appear once a session is open. `setupEditView` enters
+through a row's pencil (Edit) button rather than the nav rail, so it needs
+`installMocks(page, { qcHistories: true })` to give the source a managed
+datastream for the chooser to pick. Use `startSessionFromRow` (also with
+`qcHistories: true`) when a spec needs to plot or pick a range first and
+then enter from the row; `setupEditView` is `gotoHome` plus that call.
 
 ### Fixtures: "now"-anchored timestamps
 
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index c2037aca..104aec3b 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -14,7 +14,7 @@ The QC App is the operator's view of HydroServer's quality control pipeline. Wit
 
 1. **Browse** monitoring sites and datastreams in a HydroServer workspace.
 2. **Plot up to five datastreams** on a synchronized multi-axis chart for visual context.
-3. **Pick one datastream as the QC target.** The rest are read-only reference traces.
+3. **Pick one datastream to edit with its row's pencil button.** Plotted datastreams are read-only context.
 4. **Filter** suspicious points using value thresholds, time windows, change detection, rate-of-change limits, gap detection, or persistence runs.
 5. **Edit** the selected points: change values, interpolate, drift-correct, shift datetimes, delete, fill gaps, add points, attach qualifier flags.
 6. **Save your edits as a QC history** (a JSON file you can replay on the same datastream later).
@@ -30,8 +30,8 @@ Everything runs in your web browser. The backend never sees your edit history un
 | **Thing / Site** | A physical monitoring site or sampling location. |
 | **Datastream** | A single time-series at a site: one variable, one sensor, one processing level. |
 | **Observation** | A single (timestamp, value) measurement. |
-| **QC target** | The datastream you are editing. There is always exactly one QC target when the Edit view is open. |
-| **Context traces** | The other plotted datastreams. Visible but read-only. They exist to give you context for the QC target. |
+| **Edit target** | The datastream you are editing, chosen with a row's Edit button. |
+| **Context traces** | The raw source and any plotted datastreams shown around it. |
 | **History** | The ordered list of filters + selections + edits you've applied in the current edit session. Undo / redo / save / load all operate on this list. |
 | **Selection** | The set of point indices a filter (or your click / lasso) produced. Edits operate on the current selection. |
 | **QC history** | A JSON file holding your editing history for a datastream. This is the canonical save format. |
@@ -64,7 +64,7 @@ A thin, always-visible column of icons.
 |------|--------|
 | HydroServer logo | Top left. Go home. Resets the current view. Prompts before discarding unsaved edits. |
 | Cursor (Select) | Top left. Show the datastream Select drawer + plot. |
-| Pencil (Edit) | Top left. Open the Edit view. **Disabled** until you've picked a QC datastream for editing; the tooltip explains why. |
+| Pencil (Edit) | Top left. Return to the Edit view. **Disabled** until you're editing: pick a datastream to edit with its row's pencil button first. |
 | Stopwatch (Performance) | Bottom left. Open the Performance Calibration dialog. See "Performance" below. |
 | Briefcase (Workspaces) | Bottom left. Switch workspace, or Continue in the current one. |
 | Logout | Bottom left. Sign out. |
@@ -85,12 +85,12 @@ The filter drawer has two collapsible sections:
 
 The main area to the right of the window is split top/bottom:
 
-- **Top card** carries the preview plot, the current QC target's name (or "No datastream plotted"), and the **Start editing** button that jumps to the Edit view. The right pane of the card lists currently plotted datastreams.
-- **Bottom card** is the **Datastreams selection table**: listing every datastream the filters match. Each row is a datastream and has a plot toggle (check box); the first datastream you toggle on becomes the QC target (radio button column in the Plotted Datastreams List).
+- **Top card** carries the preview plot and a header showing how many datastreams are plotted ("N datastreams plotted" or "No datastream plotted"). The right pane of the card lists currently plotted datastreams.
+- **Bottom card** is the **Datastreams selection table**: listing every datastream the filters match. Each row has a **Plot** toggle (check box) and an **Edit** column (pencil button). See [Datastreams with quality-controlled versions](#datastreams-with-quality-controlled-versions) below for what the Edit button does.
 
 ![Select view with one datastream plotted](./images/home-plotted.png)
 
-Clicking a datastream row shows the full metadata for a datastream. Clicking the plot toggle (check box) in the row plots the datastream. The first datastream selected for plotting becomes the **QC target**. You can change the QC target via the radio button column in the Plotted Datastreams list.
+Clicking a datastream row shows the full metadata for a datastream. Clicking the plot toggle (check box) in the row plots the datastream as read-only context: plotting never picks what you edit, and the first plotted datastream has no special role.
 
 ### Datastreams with quality-controlled versions
 
@@ -112,22 +112,51 @@ once it is reached, unticked rows are disabled.
 The row's check box reflects the whole group. It is filled when the raw
 datastream is plotted, and shows a partial mark when only managed versions are.
 
-When a managed datastream is the QC target, **Start editing** goes straight
-into its session rather than asking again which version you meant.
+The row's pencil (**Edit**) button is separate from plotting: it opens a
+chooser of that source's managed datastreams, or, the first time, skips
+straight to the create-datastream form. Pick a managed datastream to
+continue its in-progress session, or **Start new session** to open the
+session window step.
+
+#### The session window
+
+The session window step shows the source's data extent and, if the managed
+datastream already has committed history, that history's extent too. The
+default window starts where the committed history ends (or at the start of
+the source data, if nothing is committed yet) and runs to the end of the
+source data.
+
+You can adjust the From / To pickers, but two rules apply:
+
+- The window has to stay inside the source's own data.
+- It can't leave a gap before or after the committed history: it has to
+  touch or overlap it. Starting after the committed history ends, or ending
+  before it starts, is rejected. Overlapping the committed history, or
+  landing exactly on its edge, is fine.
+
+The **Start session** button stays disabled, with an explanation, while the
+window is invalid.
 
 ### Edit view
 
-The Edit View opens when you click the pencil icon in the left hand navigation rail, or when you click the **Start
-editing** button on the Select view. The Edit View is available only after you've picked a QC datastream.
+The Edit View opens once you finish the row Edit button's chooser and session
+window step (see [Datastreams with quality-controlled versions](#datastreams-with-quality-controlled-versions)
+above), or when you click the pencil icon in the navigation rail to return to
+a session already open.
 
 ![Edit view (full layout)](./images/edit-view.png)
 
+On the plot, the datastream you're editing draws in black on the primary
+axis; its raw source draws underneath in grey as read-only context. Any
+other plotted datastreams render on their own axes to the right, same as the
+Select view. The edit session's time window is shaded.
+
 The Edit View consists of three columns, each independently resizable / collapsible:
 
 | Column | Contents |
 |--------|----------|
 | **Left** ("Operations") | The Edit drawer with three sections: Filter Data, Edit Data, Add Data. |
-| **Center** | The Plotly chart + a tab-switched data table (upper left) for the QC target. |
+| **Center** | The Plotly chart + a tab-switched data table (upper left) for the edit target. |
 | **Right** ("Aux") | The list of Plotted Datastreams, Edit history, and the currently staged Operation Panel. The **Save** / **Commit** / **Close** actions sit at the bottom of the Edit history panel. |
 
 The chart at the top of the center column has a Plotly toolbar with:
@@ -143,7 +172,14 @@ See the [Pan and zoom across axes](#pan-and-zoom-across-axes) section below for
 
 Hover any toolbar icon to see its name. The left side of the toolbar flips between **Plot** and **Table** views (see below); the right side carries the **data points toggle**, a share-link button, and the `?` help menu.
 
-Between the help menu and the right edge is a drop down box with pre-selected zoom levels (**All Data**, **Last Week of Data**, **Last Month of Data**, **Last 6 months of data**, **Last Year of Data**). Clicking one of these options zooms the X axis to that window, *without* refetching observations, unlike the sidebar's identical-looking chips on the Select view. The active chip stays highlighted as a reminder of the last preset applied; clicking it again re-applies it, which is the easiest way to "snap back" after a manual pan or wheel-zoom. Selecting a preset also scrolls the **Table** view so the first row is the first observation in the chosen window, keeping the two tabs in sync.
+Between the help menu and the right edge is the **Context** menu. It holds
+the same preset chips and From / To pickers as the Select view's Time range
+filter, but here they control how much of the raw source and any plotted
+datastreams load around your edit, not what you're editing. Changing it
+reloads that context data and keeps your current zoom. Your edits are
+never reloaded or re-windowed. The editor opens zoomed to the edit session's
+own window; open the Context menu (or click **All**) to see more of the
+surrounding data.
 
 Clicking a single point on the plot selects just that point. Clicking the empty plot area clears the selection.
 
@@ -151,9 +187,9 @@ Clicking a single point on the plot selects just that point. Clicking the empty
 
 The segmented control at the top-left of the plot toolbar (`Plot` / `Table`) switches the center column between the Plotly chart and an editable observation table. Both tabs operate on the same QC datastream and share the selection state; flipping between them preserves edits, history, and the currently selected points.
 
-The **Table** tab opens a virtualized list of the QC target's observations:
+The **Table** tab opens a virtualized list of the edit target's observations:
 
-![Table view of the QC target](./images/table-view.png)
+![Table view of the edit target](./images/table-view.png)
 
 What you can do here:
 
@@ -163,7 +199,7 @@ What you can do here:
 - **Track pending edits.** The toolbar chip `N unsaved` lights up whenever the table has uncommitted edits. **Discard** rolls every pending edit back; **Save changes** flushes them through the same history machinery as a Change-values or Shift-datetimes operation (one history entry per cell type).
 - **Inspect qualifiers.** The `Qualifiers` column shows the qualifier codes attached to each observation (set via the
   Qualifying comments panel). Hover a chip for its description.
-- **Jump to a range.** Picking a zoom-to-range preset from the toolbar drop down scrolls the table to the first observation in that window, so you land on the same data the plot just zoomed to.
+- **Jump to a range.** The editor opens with the table scrolled to the first observation of the edit session's window, matching the plot's initial zoom.
 
 The table is the fastest path for a small number of targeted edits. For anything wider (a hundred points, a window of bad values, a drift correction) the plot's box-select gestures + Edit drawer are faster.
 
@@ -193,7 +229,8 @@ The URL encodes everything needed to reproduce what the sender is looking at - q
 - **Workspace** (`ws`)
 - **View** (Select vs Edit; `m=e` for Edit)
 - **Active center-column tab** (`tab=t` for Table)
-- **Plotted datastreams** (`ds`), in order. The first id is the QC target.
+- **Plotted datastreams** (`ds`), in order.
+- **Edit target** (`ed`), the datastream you're editing, present only in the Edit view.
 - **Time window**: either a preset id (`r=0..5`) which the recipient resolves against the plotted data, or an explicit `from` / `to` pair as base36 second-epochs when the sender used a custom range.
 - **Per-trace eye-toggle visibility** (`h`) and **per-axis visibility** (`ya`) as hex bitmasks over the `ds` order.
 - **Plot zoom**: X zoom (`z`) plus optional per-Y-axis zoom (`yz`) for axes that aren't at their default fit.
@@ -204,40 +241,38 @@ URL only on the Select view, because they drive the datastreams table rather tha
 
 ### Plotted datastreams list
 
-The right-hand list (visible on both Select and Edit views) is the roster of currently plotted datastreams. Each row carries:
+The right-hand list (visible on both Select and Edit views) is the roster of everything currently on the plot. Each row carries:
 
-- A drag handle to reorder the list (drag the QC target's row to promote / demote it; the line colors track the order).
-- A colored radio dot that picks the **QC target**. The active row is tinted; the other rows render in their reference color.
+- A drag handle to reorder the list (the line colors track the order). While editing, the **edit target** and its **raw source** are pinned at the top and can't be reordered or unplotted; only the other plotted datastreams can be dragged.
 - An **eye** toggle that hides the trace from the plot without unplotting it. Hidden rows render with a strikethrough.
-- A **Y-axis** toggle (non-QC rows only) that collapses that datastream's secondary axis to provide more horizontal space for the plot.
-- The datastream name and a subtitle showing the number of points loaded **in the current time window**, e.g. `1,248 pts loaded`. While the fetch is still in flight, the subtitle reads `loading…`.
-  - An `×` button to unplot the row.
+- A **Y-axis** toggle (non-primary rows only) that collapses that datastream's secondary axis to provide more horizontal space for the plot. The edit target and its source share the primary axis, so neither row has this toggle.
+- The datastream name and a subtitle showing the number of points loaded **in the current time window**, e.g. `1,248 pts loaded`. While the fetch is still in flight, the subtitle reads `loading…`. The source row carries a `raw source` chip.
+- An `×` button to unplot the row. The edit target and its source can't be unplotted this way. Leave the editor to drop them.
 
-If a plotted datastream has no observations in the current window (either because the dataset is empty there or because the chosen time range doesn't cover its data), the row title shows a small warning-tinted database icon. Hover it for the tooltip "No observations in the current time window". Widening the time range (or clicking **All** in the Time range section) usually clears it.
+If a plotted datastream has no observations in the current window (either because the dataset is empty there or because the chosen time range doesn't cover its data), the row title shows a small warning-tinted database icon. Hover it for the tooltip "No observations in the current time window". Widening the time range (or clicking **All** in the Time range / Context section) usually clears it.
 
-A QC datastream whose edit session is in progress plots that session's working data instead, the same data the editor opens: committed data where it exists, otherwise the raw datastream, over the session's time window, with saved draft edits applied. Its line always covers the whole session window, and presets anchor to that window rather than the (empty) stored managed datastream. With no session in progress it shows its committed observations as usual.
+While editing, the edit target's row shows the session's working data: committed data where it exists, otherwise the raw datastream, over the session's window, with saved draft edits applied. Its line always covers the whole session window.
 
 ![Plotted datastreams list with two rows](./images/plotted-datastreams-list.png)
 
 ### Plotting multiple datastreams
 
-You can plot up to **5 datastreams at a time**. The plotted count and cap are surfaced in the Datastreams table toolbar as a chip ("`N/5 plotted`"). Once you hit the cap, the unchecked rows disable their plot toggles and a tooltip explains why. Unplot a row from either the table or the list to free up a slot.
+You can plot up to **5 datastreams at a time**. The plotted count and cap are surfaced in the Datastreams table toolbar as a chip ("`N/5 plotted`"). Once you hit the cap, the unchecked rows disable their plot toggles and a tooltip explains why. Unplot a row from either the table or the list to free up a slot. While editing, the edit target and its raw source are shown in addition to the 5 plotted datastreams. They don't count against the cap.
 
 ![Two datastreams on independent y-axes](./images/home-multi-datastreams.png)
 
 When more than one datastream is on the plot:
 
-- The **first** datastream you plot becomes the **QC target** and uses the primary (left) Y axis. Its line draws in black with a point marker on every observation.
-- Each additional datastream gets its **own Y axis** on the right side of the plot. The chip at the top of each axis carries the datastream's display name plus its unit (e.g. `Water Temperature (°C)`). Up to four secondary axes stack side by side.
+- In the Select view, the **first** datastream you plot uses the primary (left) Y axis; in the Edit view, the edit target and its raw source share it instead. In the Select view that line keeps its own series colour; in the Edit view the edit target draws in black with a point marker on every observation, and the raw source draws underneath it in grey.
+- Every other datastream gets its **own Y axis** on the right side of the plot. The chip at the top of each axis carries the datastream's display name plus its unit (e.g. `Water Temperature (°C)`). Up to four secondary axes stack side by side.
 - Axis chips are colored to match their line so you can tell at a glance which trace goes with which axis.
 
 Once a datastream is on the plot, the **plotted datastreams list** on the right is where you manage it (see the previous section for the row anatomy). The interactions most relevant to multi-series work:
 
-- **QC target picker**: clicking the colored radio dot on a non-QC row promotes that datastream to the QC target. The primary Y axis rebinds to its scale and the previous QC datastream demotes to a secondary axis. The Edit view shows only the QC target's points for selection. Context traces are read-only.
 - **Eye toggle**: hide / show a trace on the plot without unplotting it. Useful when one series is visually crowding the others. Hidden rows render with a strikethrough; their axis stays on the plot so the scale doesn't jump.
-- **Y-axis toggle** (non-QC rows only): collapse just the secondary axis without removing the trace. Reach for it when the extra axes start eating horizontal room and you don't actually need the numeric scale.
-- **Drag handle**: drag a row up or down to reorder the legend. The plot redraws so the trace colors track the new order.
-- **× button**: unplot the row entirely. Removing the QC target promotes the next plotted row to QC.
+- **Y-axis toggle** (non-primary rows only): collapse just the secondary axis without removing the trace. Reach for it when the extra axes start eating horizontal room and you don't actually need the numeric scale.
+- **Drag handle**: drag a row up or down to reorder the legend. The plot redraws so the trace colors track the new order. The edit target and its source are pinned and can't be reordered.
+- **× button**: unplot the row entirely.
 
 ### Pan and zoom across axes
 
@@ -461,7 +496,7 @@ managed datastream while you were editing, or the source is still ingesting
 data and you want the new points. The memory replay can never surface either,
 because it faithfully restores the copy you started with.
 
-Below the body sits the session action bar (see [Submit](#submit-save--commit--close)). It stays put when the panel is collapsed, so the actions are always one click away. While a session is open it carries **Save**, **Discard**, and **Commit**; once you commit, those are replaced by **New session**, which opens the next one over the current time range without a trip back to the Select view. **Close** is always there.
+Below the body sits the session action bar (see [Submit](#submit-save--commit--close)). It stays put when the panel is collapsed, so the actions are always one click away. While a session is open it carries **Save**, **Discard**, and **Commit**; once you commit, those are replaced by **New session**, which opens the session window step for a new one without a trip back to the Select view. **Close** is always there.
 
 Clicking the chevron at the very top of the panel collapses the body; the pop-out icon opens the same panel inside a wider modal so you can scan a long history without losing the rest of the sidebar. The modal shows the history only, not the session actions.
 
@@ -480,7 +515,7 @@ Things worth knowing:
   `snapshot` chip, and a provenance line such as
   `step 3 of 7: Fill Gaps - by Alice - Mar 14, 2026`.
 - Each snapshot gets its own Y axis, so you can shift it to line it up
-  against the QC target, exactly like any other plotted datastream.
+  against the edit target, exactly like any other plotted datastream.
 - A snapshot is **frozen**. It is computed once, over its own session's
   window, and changing the plot's time range never refetches or recomputes
   it. Zoom outside that window and the line simply stops.
@@ -492,7 +527,7 @@ Things worth knowing:
 
 ## Deleting a session
 
-The **Start editing** chooser shows every session on a managed datastream as a
+The **Edit** chooser shows every session on a managed datastream as a
 timeline, oldest first, each with a trash icon. Both in-progress and committed
 sessions can be deleted. When no session is open, the timeline is headed by a
 **Start new session** node.
@@ -553,7 +588,7 @@ When you're satisfied with the edits, hit one of the action buttons at the botto
 - **Save**: writes the session's operations to the backend as a draft and keeps you in the Edit view.
 - **Commit**: materializes the session into the managed datastream and locks it into the history.
 - **Discard**: drops every edit made since the last save, returning the session to its last saved state. Edits already saved to the session stay. Disabled when there is nothing unsaved, and it asks for confirmation first.
-- **New session**: replaces Save and Commit once the session is committed. Opens a new session over the current time range, starting from the state the last commit left behind.
+- **New session**: replaces Save and Commit once the session is committed. Opens the session window step, starting from the state the last commit left behind.
 - **Close**: leaves the editor. If you have unsaved edits, a "Save before closing?" dialog lets you save first or close without saving.
 
 Clicking Commit opens a confirmation dialog so a misclick won't push data to the server.
@@ -581,12 +616,10 @@ See [PERFORMANCE.md](./PERFORMANCE.md) for the envelope details.
 
 ### "I just want to drop everything above 1000 and re-submit."
 
-1. Pick a workspace, then pick the datastream you want to QC.
-2. Plot it (it becomes the QC target).
-3. Click **All** in the Time range to load the full series (plotting loads the last month of data).
-4. Click the pencil icon → expand **Value thresholds**, set `Greater than: 1000`, press Enter.
-5. Expand **Delete points**, click Delete.
-6. Click **Save** at the bottom of the Edit history panel, then **Commit** and confirm in the dialog.
+1. Pick a workspace, then click the pencil button on the row of the datastream you want to QC.
+2. Expand **Value thresholds**, set `Greater than: 1000`, press Enter.
+3. Expand **Delete points**, click Delete.
+4. Click **Save** at the bottom of the Edit history panel, then **Commit** and confirm in the dialog.
 
 ### "I want to drift-correct a known-bad interval."
 
@@ -613,7 +646,7 @@ Click **Workspaces** in the nav rail and **Select** another, or **Continue** to
 |---------|--------------|-----|
 | Blank page on load | Wrong API URL or `localhost` vs `127.0.0.1` mismatch. | See [DEPLOYMENT.md](./DEPLOYMENT.md). |
 | Plot stays empty after picking a datastream | A `Custom` time range that doesn't overlap the datastream's observations, or the datastream has no observations. The plotted row shows a database-off icon and the subtitle reads `0 pts loaded`. | Click a preset such as **1m** or **All** in Time range. |
-| Pencil ("Edit") icon is greyed out | No QC datastream selected. | Plot at least one datastream. The first becomes the QC target. |
+| The Edit rail item is greyed out | You are not editing. | Click a row's pencil button. |
 | Big edits freeze the page | `SharedArrayBuffer` not available; running inline. | Have your admin re-enable COOP/COEP headers, or accept the slower fallback. |
 | Save fails with a backend error | Permissions / workspace issue / network. | The Snackbar shows the backend message verbatim. Share that with your admin. |
 | The history shows a red failed entry after Load | The QC history referenced something missing in this datastream. | The rest of the QC history still ran. Click the chevron on the row to see its arguments. |
diff --git a/apps/qc-app/e2e/all-operations.spec.ts b/apps/qc-app/e2e/all-operations.spec.ts
index ec7db4e2..7ca779f0 100644
--- a/apps/qc-app/e2e/all-operations.spec.ts
+++ b/apps/qc-app/e2e/all-operations.spec.ts
@@ -61,7 +61,7 @@ async function seedWideSelection(page: Page) {
 
 test.describe('all operations: single-session walkthrough', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page, { observations: observationsWithGap() })
+    await installMocks(page, { observations: observationsWithGap(), qcHistories: true })
     await setupEditView(page)
   })
 
diff --git a/apps/qc-app/e2e/date-picker-dialog.spec.ts b/apps/qc-app/e2e/date-picker-dialog.spec.ts
index 18fb52b8..1b829855 100644
--- a/apps/qc-app/e2e/date-picker-dialog.spec.ts
+++ b/apps/qc-app/e2e/date-picker-dialog.spec.ts
@@ -11,7 +11,7 @@ import { openOp, setupEditView, waitForSelection } from './support/app'
 
 test.describe('date picker dialog', () => {
   test('opens at the full width of the date picker', async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await openOp(page, 'datetimeRange')
     await waitForSelection(page, 1)
diff --git a/apps/qc-app/e2e/edit-add-points.spec.ts b/apps/qc-app/e2e/edit-add-points.spec.ts
index 1cd3811e..fd8a15ab 100644
--- a/apps/qc-app/e2e/edit-add-points.spec.ts
+++ b/apps/qc-app/e2e/edit-add-points.spec.ts
@@ -11,7 +11,7 @@ import { expectHistoryContains } from './support/ops'
 
 test.describe('edit: add points', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
   })
 
diff --git a/apps/qc-app/e2e/edit-change-values.spec.ts b/apps/qc-app/e2e/edit-change-values.spec.ts
index aa0156e3..ea0fbf2d 100644
--- a/apps/qc-app/e2e/edit-change-values.spec.ts
+++ b/apps/qc-app/e2e/edit-change-values.spec.ts
@@ -12,7 +12,7 @@ import { expectHistoryContains, selectAllPoints } from './support/ops'
 
 test.describe('edit: change values', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await selectAllPoints(page)
   })
diff --git a/apps/qc-app/e2e/edit-delete-points.spec.ts b/apps/qc-app/e2e/edit-delete-points.spec.ts
index dce02435..5c7e28a4 100644
--- a/apps/qc-app/e2e/edit-delete-points.spec.ts
+++ b/apps/qc-app/e2e/edit-delete-points.spec.ts
@@ -11,7 +11,7 @@ import { expectHistoryContains, selectAllPoints } from './support/ops'
 
 test.describe('edit: delete points', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await selectAllPoints(page)
   })
diff --git a/apps/qc-app/e2e/edit-drift-correction.spec.ts b/apps/qc-app/e2e/edit-drift-correction.spec.ts
index 413d0905..2c780b7e 100644
--- a/apps/qc-app/e2e/edit-drift-correction.spec.ts
+++ b/apps/qc-app/e2e/edit-drift-correction.spec.ts
@@ -12,7 +12,7 @@ import { expectHistoryContains, selectAllPoints } from './support/ops'
 
 test.describe('edit: drift correction', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await selectAllPoints(page)
   })
diff --git a/apps/qc-app/e2e/edit-fill-gaps.spec.ts b/apps/qc-app/e2e/edit-fill-gaps.spec.ts
index 1a707779..400469cd 100644
--- a/apps/qc-app/e2e/edit-fill-gaps.spec.ts
+++ b/apps/qc-app/e2e/edit-fill-gaps.spec.ts
@@ -31,7 +31,7 @@ function observationsWithGap() {
 
 test.describe('edit: fill gaps', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page, { observations: observationsWithGap() })
+    await installMocks(page, { observations: observationsWithGap(), qcHistories: true })
     await setupEditView(page)
   })
 
diff --git a/apps/qc-app/e2e/edit-interpolate.spec.ts b/apps/qc-app/e2e/edit-interpolate.spec.ts
index 3bca6838..3264f17c 100644
--- a/apps/qc-app/e2e/edit-interpolate.spec.ts
+++ b/apps/qc-app/e2e/edit-interpolate.spec.ts
@@ -10,7 +10,7 @@ import { expectHistoryContains, selectAllPoints } from './support/ops'
 
 test.describe('edit: interpolate', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await selectAllPoints(page)
   })
diff --git a/apps/qc-app/e2e/edit-selection.spec.ts b/apps/qc-app/e2e/edit-selection.spec.ts
new file mode 100644
index 00000000..24825230
--- /dev/null
+++ b/apps/qc-app/e2e/edit-selection.spec.ts
@@ -0,0 +1,338 @@
+/**
+ * Picking what to edit: the row Edit button, the window step, and what the
+ * editor draws around the session.
+ *   - Edit on a row does not plot it
+ *   - the window step prefills a valid window and blocks an invalid one
+ *   - the editor draws the edit target, its source, and plotted datastreams
+ *   - closing the editor keeps the plotted datastreams
+ *   - the session window band survives staging shapes
+ *   - the editor opens zoomed to the session window
+ *   - changing the Context range keeps the user zoom
+ */
+
+import { expect, test, type Locator, type Page } from '@playwright/test'
+import { installMocks } from './support/mocks'
+import {
+  gotoHome,
+  openOp,
+  plotDatastreamById,
+  setupEditView,
+  startSessionFromRow,
+} from './support/app'
+import {
+  DATASTREAM_ID,
+  DATASTREAM_ID_B,
+  FIXTURE_OBS_END_MS,
+  FIXTURE_OBS_START_MS,
+  MANAGED_DATASTREAM_ID,
+} from './support/fixtures'
+
+type PlotRoot = HTMLElement & {
+  data?: Array<{ id?: string }>
+  layout?: {
+    shapes?: Array<{ name?: string }>
+    xaxis?: { range?: Array<number | string> }
+  }
+}
+
+function shapeNames(page: Page): Promise<string[]> {
+  return page.evaluate(() => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as PlotRoot
+    return (gd?.layout?.shapes ?? []).map((s) => s.name ?? '')
+  })
+}
+
+type ReloadProbe = { __contextStart: number; __afterplots: number[] }
+
+function traceIds(page: Page): Promise<string[]> {
+  return page.evaluate(() => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as PlotRoot
+    return (gd?.data ?? []).map((t) => t.id ?? '').filter(Boolean)
+  })
+}
+
+/** Live x-axis range in epoch ms. Plotly date strings are UTC. */
+function xRange(page: Page): Promise<[number, number] | null> {
+  return page.evaluate(() => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as PlotRoot
+    const range = gd?.layout?.xaxis?.range
+    if (!range || range.length !== 2) return null
+    const toMs = (v: number | string) =>
+      typeof v === 'number' ? v : Date.parse(`${v.replace(' ', 'T')}Z`)
+    return [toMs(range[0]!), toMs(range[1]!)] as [number, number]
+  })
+}
+
+/**
+ * Type a local date and time into a `DatePickerField`. Its inputs mask
+ * digit by digit, so select the whole value and type digits only.
+ */
+async function typeDateTime(field: Locator, when: Date) {
+  const pad = (n: number) => String(n).padStart(2, '0')
+  const date = `${pad(when.getMonth() + 1)}${pad(when.getDate())}${when.getFullYear()}`
+  const time = `${pad(when.getHours())}${pad(when.getMinutes())}`
+  const inputs = field.locator('input')
+  for (const [input, digits] of [
+    [inputs.nth(0), date],
+    [inputs.nth(1), time],
+  ] as const) {
+    await input.click()
+    await input.evaluate((el: HTMLInputElement) =>
+      el.setSelectionRange(0, el.value.length)
+    )
+    await input.pressSequentially(digits)
+    await input.blur()
+  }
+}
+
+async function openWindowStep(page: Page) {
+  await page.getByTestId(`edit-datastream-${DATASTREAM_ID}`).click()
+  await page.getByTestId(`edit-managed-${MANAGED_DATASTREAM_ID}`).click()
+  await expect(page.getByTestId('session-window-start')).toBeVisible()
+}
+
+test.describe('edit selection', () => {
+  test.beforeEach(async ({ page }) => {
+    test.slow()
+    await installMocks(page, { qcHistories: true })
+  })
+
+  test('Edit on a row does not plot it', async ({ page }) => {
+    await gotoHome(page)
+    await page.getByTestId(`edit-datastream-${DATASTREAM_ID}`).click()
+    await expect(
+      page.getByTestId(`edit-managed-${MANAGED_DATASTREAM_ID}`)
+    ).toBeVisible()
+    await page.getByTestId('chooser-cancel').click()
+    await expect(
+      page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`)
+    ).toHaveAttribute('aria-pressed', 'false')
+  })
+
+  test('the window step prefills a valid window and blocks an invalid one', async ({
+    page,
+  }) => {
+    await gotoHome(page)
+    await openWindowStep(page)
+    await expect(page.getByTestId('session-window-start')).toBeEnabled()
+    await expect(page.getByTestId('session-window-error')).toBeHidden()
+    await expect(page.getByTestId('session-window-committed')).not.toContainText(
+      'Nothing committed'
+    )
+
+    // A From after the source ends is outside the source data.
+    await typeDateTime(
+      page.getByTestId('session-window-from'),
+      new Date(FIXTURE_OBS_END_MS + 2 * 24 * 60 * 60 * 1000)
+    )
+    await expect(page.getByTestId('session-window-error')).toBeVisible()
+    await expect(page.getByTestId('session-window-start')).toBeDisabled()
+  })
+
+  test('the editor draws the source and plotted datastreams around the session', async ({
+    page,
+  }) => {
+    await gotoHome(page)
+    await plotDatastreamById(page, DATASTREAM_ID_B)
+    await startSessionFromRow(page)
+
+    await expect
+      .poll(() => traceIds(page))
+      .toEqual(
+        expect.arrayContaining([
+          MANAGED_DATASTREAM_ID,
+          DATASTREAM_ID,
+          DATASTREAM_ID_B,
+        ])
+      )
+
+    await page.getByTestId('context-range-btn').click()
+    await page
+      .getByTestId('context-range-menu')
+      .getByTestId('date-preset-All')
+      .click()
+    await expect(page.getByTestId('exit-save-btn')).toBeVisible()
+  })
+
+  test('closing the editor keeps the plotted datastreams', async ({ page }) => {
+    await gotoHome(page)
+    await plotDatastreamById(page, DATASTREAM_ID_B)
+    await startSessionFromRow(page)
+    await page.getByTestId('exit-close-btn').click()
+    await expect(page.getByTestId('datastreams-table')).toBeVisible({
+      timeout: 30_000,
+    })
+    await expect(
+      page.getByTestId(`plot-checkbox-${DATASTREAM_ID_B}`)
+    ).toHaveAttribute('aria-pressed', 'true')
+    await expect(
+      page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`)
+    ).toHaveAttribute('aria-pressed', 'false')
+  })
+
+  test('the session window band survives staging shapes', async ({ page }) => {
+    await setupEditView(page)
+    await expect.poll(() => shapeNames(page)).toContain('edit-window')
+
+    await openOp(page, 'gaps')
+    await page.getByTestId('filter-range-enable-btn').click()
+    await expect.poll(() => shapeNames(page)).toContain('stage')
+    expect(await shapeNames(page)).toContain('edit-window')
+
+    // A context reload redraws the plot; the stage band must survive it.
+    await page.evaluate(() => {
+      const gd = document.querySelector('[data-testid="main-plot"]') as
+        | (PlotRoot & { on: (event: string, cb: () => void) => void })
+      const w = window as unknown as { __updates: number }
+      w.__updates = 0
+      gd.on('plotly_update', () => w.__updates++)
+    })
+    await page.getByTestId('context-range-btn').click()
+    const menu = page.getByTestId('context-range-menu')
+    const allActive = /v-chip--variant-tonal/.test(
+      (await menu.getByTestId('date-preset-All').getAttribute('class')) ?? ''
+    )
+    await menu.getByTestId(allActive ? 'date-preset-1w' : 'date-preset-All').click()
+    await page.waitForFunction(
+      () => (window as unknown as { __updates: number }).__updates > 0
+    )
+    await page.keyboard.press('Escape')
+    expect(await shapeNames(page)).toEqual(
+      expect.arrayContaining(['stage', 'edit-window'])
+    )
+
+    await page.getByTestId('filter-range-disable-btn').click()
+    await expect.poll(() => shapeNames(page)).not.toContain('stage')
+    expect(await shapeNames(page)).toContain('edit-window')
+  })
+
+  test('the editor opens zoomed to the session window', async ({ page }) => {
+    await gotoHome(page)
+    await plotDatastreamById(page, DATASTREAM_ID_B)
+    await page.getByTestId('date-preset-All').click()
+
+    // Start the session halfway through the source, so the window is
+    // narrower than the context around it.
+    const mid = new Date((FIXTURE_OBS_START_MS + FIXTURE_OBS_END_MS) / 2)
+    mid.setSeconds(0, 0)
+    await openWindowStep(page)
+    await typeDateTime(page.getByTestId('session-window-from'), mid)
+    await expect(page.getByTestId('session-window-start')).toBeEnabled()
+    await page.getByTestId('session-window-start').click()
+    await expect(page.getByTestId('exit-save-btn')).toBeVisible({
+      timeout: 30_000,
+    })
+
+    const tolerance = 60_000
+    await expect
+      .poll(async () => {
+        const r = await xRange(page)
+        return (
+          !!r &&
+          Math.abs(r[0] - mid.getTime()) <= tolerance &&
+          Math.abs(r[1] - FIXTURE_OBS_END_MS) <= tolerance
+        )
+      })
+      .toBe(true)
+  })
+
+  test('changing the Context range keeps the user zoom', async ({ page }) => {
+    await gotoHome(page)
+    await plotDatastreamById(page, DATASTREAM_ID_B)
+    await page.getByTestId('date-preset-All').click()
+    await startSessionFromRow(page)
+    await expect.poll(() => xRange(page)).not.toBeNull()
+    const opened = (await xRange(page))!
+
+    // Zoom in with the wheel so the view differs from the session window.
+    const box = (await page.getByTestId('main-plot').boundingBox())!
+    await page.mouse.move(box.x + box.width * 0.4, box.y + box.height / 2)
+    for (let i = 0; i < 4; i++) await page.mouse.wheel(0, -200)
+    await expect
+      .poll(async () => {
+        const r = await xRange(page)
+        return !!r && r[1] - r[0] < (opened[1] - opened[0]) * 0.9
+      })
+      .toBe(true)
+    const zoomed = (await xRange(page))!
+
+    await page.getByTestId('context-range-btn').click()
+    const menu = page.getByTestId('context-range-menu')
+
+    // Record every redraw from here on, in page time, so none is missed.
+    await page.evaluate(() => {
+      const gd = document.querySelector('[data-testid="main-plot"]') as
+        | (PlotRoot & { on: (event: string, cb: () => void) => void })
+      const w = window as unknown as ReloadProbe
+      // Dev-server module loads fill the default resource timing buffer.
+      performance.clearResourceTimings()
+      performance.setResourceTimingBufferSize(1_000)
+      w.__contextStart = performance.now()
+      w.__afterplots = []
+      gd.on('plotly_afterplot', () => w.__afterplots.push(performance.now()))
+    })
+    // The reload refetches the source and plotted datastreams, never the edit target.
+    const reloaded = Promise.all(
+      [DATASTREAM_ID, DATASTREAM_ID_B].map((id) =>
+        page.waitForResponse(
+          (r) =>
+            r.url().includes(`/api/data/datastreams/${id}/observations`) &&
+            r.request().method() === 'GET'
+        )
+      )
+    )
+    // 1w, not All: All is already the context, so picking it changes nothing.
+    await menu.getByTestId('date-preset-1w').click()
+    await expect(menu.getByTestId('date-preset-1w')).toHaveClass(
+      /v-chip--variant-tonal/
+    )
+    await expect(menu.getByTestId('date-preset-All')).not.toHaveClass(
+      /v-chip--variant-tonal/
+    )
+    await reloaded
+
+    // Redrawn after both refetches landed: a plotly_afterplot later than the
+    // last observations response in the page's resource timeline.
+    await page.waitForFunction((ids) => {
+      const w = window as unknown as ReloadProbe
+      const fetched = performance
+        .getEntriesByType('resource')
+        .filter((e) => e.startTime >= w.__contextStart)
+      const ends = ids.map((id) =>
+        Math.max(
+          ...fetched
+            .filter((e) => e.name.includes(`/datastreams/${id}/observations`))
+            .map((e) => (e as PerformanceResourceTiming).responseEnd)
+        )
+      )
+      if (ends.some((t) => !Number.isFinite(t))) return false
+      const landed = Math.max(...ends)
+      return w.__afterplots.some((t) => t > landed)
+    }, [DATASTREAM_ID, DATASTREAM_ID_B])
+    expect(await traceIds(page)).toContain(MANAGED_DATASTREAM_ID)
+
+    // Sample the range over several frames so a late redraw would show up.
+    const drift = await page.evaluate(async ([lo, hi]) => {
+      const toMs = (v: number | string) =>
+        typeof v === 'number' ? v : Date.parse(`${v.replace(' ', 'T')}Z`)
+      let worst = 0
+      const end = performance.now() + 1_500
+      while (performance.now() < end) {
+        const gd = document.querySelector('[data-testid="main-plot"]') as
+          | (HTMLElement & { layout?: { xaxis?: { range?: Array<number | string> } } })
+          | null
+        const r = gd?.layout?.xaxis?.range
+        if (!r) return Infinity
+        worst = Math.max(
+          worst,
+          Math.abs(toMs(r[0]!) - lo!),
+          Math.abs(toMs(r[1]!) - hi!)
+        )
+        await new Promise((res) => setTimeout(res, 100))
+      }
+      return worst
+    }, zoomed)
+    expect(drift).toBeLessThanOrEqual(1_000)
+    expect(await traceIds(page)).toContain(MANAGED_DATASTREAM_ID)
+  })
+})
diff --git a/apps/qc-app/e2e/edit-shift-datetimes.spec.ts b/apps/qc-app/e2e/edit-shift-datetimes.spec.ts
index c9574b41..abf5bc5b 100644
--- a/apps/qc-app/e2e/edit-shift-datetimes.spec.ts
+++ b/apps/qc-app/e2e/edit-shift-datetimes.spec.ts
@@ -12,7 +12,7 @@ import { expectHistoryContains, selectAllPoints } from './support/ops'
 
 test.describe('edit: shift datetimes', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await selectAllPoints(page)
   })
diff --git a/apps/qc-app/e2e/filter-change.spec.ts b/apps/qc-app/e2e/filter-change.spec.ts
index 4ce6df8b..d47c21fc 100644
--- a/apps/qc-app/e2e/filter-change.spec.ts
+++ b/apps/qc-app/e2e/filter-change.spec.ts
@@ -10,7 +10,7 @@ import { openOp, setupEditView, waitForSelection } from './support/app'
 
 test.describe('filter: change threshold', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
   })
 
diff --git a/apps/qc-app/e2e/filter-datetime-range.spec.ts b/apps/qc-app/e2e/filter-datetime-range.spec.ts
index 339de2df..c6f71aaa 100644
--- a/apps/qc-app/e2e/filter-datetime-range.spec.ts
+++ b/apps/qc-app/e2e/filter-datetime-range.spec.ts
@@ -11,7 +11,7 @@ import { openOp, setupEditView, waitForSelection } from './support/app'
 
 test.describe('filter: datetime range', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
   })
 
diff --git a/apps/qc-app/e2e/filter-gaps.spec.ts b/apps/qc-app/e2e/filter-gaps.spec.ts
index 7d3fc919..6fe22bba 100644
--- a/apps/qc-app/e2e/filter-gaps.spec.ts
+++ b/apps/qc-app/e2e/filter-gaps.spec.ts
@@ -35,7 +35,7 @@ function observationsWithGap() {
 
 test.describe('filter: find gaps', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page, { observations: observationsWithGap() })
+    await installMocks(page, { observations: observationsWithGap(), qcHistories: true })
     await setupEditView(page)
   })
 
diff --git a/apps/qc-app/e2e/filter-persistence.spec.ts b/apps/qc-app/e2e/filter-persistence.spec.ts
index a03da722..4a17b71a 100644
--- a/apps/qc-app/e2e/filter-persistence.spec.ts
+++ b/apps/qc-app/e2e/filter-persistence.spec.ts
@@ -27,7 +27,7 @@ function observationsWithPlateau() {
 
 test.describe('filter: persistence', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page, { observations: observationsWithPlateau() })
+    await installMocks(page, { observations: observationsWithPlateau(), qcHistories: true })
     await setupEditView(page)
   })
 
diff --git a/apps/qc-app/e2e/filter-range-toggle.spec.ts b/apps/qc-app/e2e/filter-range-toggle.spec.ts
index 88942b9a..4ae77182 100644
--- a/apps/qc-app/e2e/filter-range-toggle.spec.ts
+++ b/apps/qc-app/e2e/filter-range-toggle.spec.ts
@@ -19,7 +19,7 @@ const rangePanel = '[data-testid="filter-range-panel"]'
 
 test.describe('filter range toggle', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
   })
 
diff --git a/apps/qc-app/e2e/filter-rate-of-change.spec.ts b/apps/qc-app/e2e/filter-rate-of-change.spec.ts
index 8995e931..47677f84 100644
--- a/apps/qc-app/e2e/filter-rate-of-change.spec.ts
+++ b/apps/qc-app/e2e/filter-rate-of-change.spec.ts
@@ -10,7 +10,7 @@ import { openOp, setupEditView, waitForSelection } from './support/app'
 
 test.describe('filter: rate of change', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
   })
 
diff --git a/apps/qc-app/e2e/filter-value-threshold.spec.ts b/apps/qc-app/e2e/filter-value-threshold.spec.ts
index c4dcb710..0751dad1 100644
--- a/apps/qc-app/e2e/filter-value-threshold.spec.ts
+++ b/apps/qc-app/e2e/filter-value-threshold.spec.ts
@@ -13,7 +13,7 @@ import { openOp, setupEditView, waitForSelection } from './support/app'
 
 test.describe('filter: value threshold', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
   })
 
diff --git a/apps/qc-app/e2e/history.spec.ts b/apps/qc-app/e2e/history.spec.ts
index 4fab41f4..40161659 100644
--- a/apps/qc-app/e2e/history.spec.ts
+++ b/apps/qc-app/e2e/history.spec.ts
@@ -16,7 +16,7 @@ import { expectHistoryContains, selectAllPoints } from './support/ops'
 
 test.describe('edit history toolbar', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await selectAllPoints(page)
     await openOp(page, 'changeValues')
diff --git a/apps/qc-app/e2e/navigation.spec.ts b/apps/qc-app/e2e/navigation.spec.ts
index 41136d86..f3eefae4 100644
--- a/apps/qc-app/e2e/navigation.spec.ts
+++ b/apps/qc-app/e2e/navigation.spec.ts
@@ -3,7 +3,7 @@
  *   - fresh browser redirects to /workspaces
  *   - picking a workspace navigates to Home
  *   - the nav-rail workspace-switch button offers to revisit the picker
- *   - the edit rail item is disabled until a datastream is plotted
+ *   - the edit rail item is enabled only while editing
  *   - leaving Edit with edits not saved to the session asks first
  */
 
@@ -12,12 +12,12 @@ import { installMocks, type MockQcSession } from './support/mocks'
 import {
   gotoHome,
   openOp,
-  plotFirstDatastream,
+  plotDatastreamById,
   setupEditView,
-  setupSessionEditView,
+  startSessionFromRow,
 } from './support/app'
 import { expectHistoryContains, selectAllPoints } from './support/ops'
-import { WORKSPACE_ID } from './support/fixtures'
+import { DATASTREAM_ID_B, WORKSPACE_ID } from './support/fixtures'
 
 async function applyChangeValues(page: Page) {
   await selectAllPoints(page)
@@ -33,7 +33,7 @@ function exitDialog(page: Page) {
 
 test.describe('navigation', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
   })
 
   test('fresh browser redirects to the workspace picker', async ({ page }) => {
@@ -81,28 +81,17 @@ test.describe('navigation', () => {
     })
   })
 
-  test('Edit rail item is disabled until a datastream is plotted', async ({
-    page,
-  }) => {
+  test('Edit rail item is disabled outside the editor', async ({ page }) => {
+    test.slow()
     await gotoHome(page)
     const editRail = page.getByTestId('nav-rail-item-edit')
     await expect(editRail).toHaveAttribute('aria-disabled', 'true')
-    await plotFirstDatastream(page)
+    // Plotting never picks an edit target.
+    await plotDatastreamById(page, DATASTREAM_ID_B)
+    await expect(editRail).toHaveAttribute('aria-disabled', 'true')
+    await startSessionFromRow(page)
     await expect(editRail).toHaveAttribute('aria-disabled', 'false')
   })
-
-  test('without a session the exit dialog cannot save', async ({ page }) => {
-    await setupEditView(page)
-    await applyChangeValues(page)
-
-    await page.getByTestId('nav-rail-item-select').click()
-    const dialog = exitDialog(page)
-    await expect(dialog).toBeVisible()
-    await expect(
-      dialog.getByRole('button', { name: /save & continue/i })
-    ).toBeDisabled()
-    await expect(dialog.getByRole('button', { name: /discard/i })).toBeEnabled()
-  })
 })
 
 test.describe('navigation: leaving a QC session', () => {
@@ -110,8 +99,8 @@ test.describe('navigation: leaving a QC session', () => {
   let sessions: MockQcSession[]
 
   test.beforeEach(async ({ page }) => {
-    // Entering through Start editing and starting a session is slow enough to
-    // outrun the default budget when these run in parallel.
+    // Entering through the row Edit flow and starting a session is slow
+    // enough to outrun the default budget when these run in parallel.
     test.slow()
     submissions = []
     sessions = []
@@ -120,7 +109,7 @@ test.describe('navigation: leaving a QC session', () => {
       submissions,
       qcSessionState: sessions,
     })
-    await setupSessionEditView(page)
+    await setupEditView(page)
   })
 
   test('Save & continue saves the draft without posting observations', async ({
diff --git a/apps/qc-app/e2e/plot-layout.spec.ts b/apps/qc-app/e2e/plot-layout.spec.ts
index 19330c39..07f2b84f 100644
--- a/apps/qc-app/e2e/plot-layout.spec.ts
+++ b/apps/qc-app/e2e/plot-layout.spec.ts
@@ -1,13 +1,13 @@
 /**
- * The editor plot fills its container once the Edit view opens, whether it
- * was entered through the nav rail or through Start editing. Plotly can lay
- * the plot out while the view is still switching, so the resize observer has
- * to catch up to the container's final size.
+ * The editor plot fills its container once the Edit view opens through the
+ * row Edit flow. Plotly can lay the plot out while the view is still
+ * switching, so the resize observer has to catch up to the container's final
+ * size.
  */
 
 import { expect, test, type Page } from '@playwright/test'
 import { installMocks } from './support/mocks'
-import { setupEditView, setupSessionEditView } from './support/app'
+import { setupEditView } from './support/app'
 
 /** Largest gap in px between Plotly's laid-out size and the plot div. */
 function layoutMismatch(page: Page): Promise<number> {
@@ -25,23 +25,10 @@ function layoutMismatch(page: Page): Promise<number> {
 }
 
 test.describe('editor plot layout', () => {
-  test.beforeEach(async ({ page }) => {
-    await page.setViewportSize({ width: 1440, height: 900 })
-  })
-
-  test('fills its container after entering through the rail', async ({
-    page,
-  }) => {
-    await installMocks(page)
-    await setupEditView(page)
-    await expect
-      .poll(() => layoutMismatch(page), { timeout: 5_000 })
-      .toBeLessThanOrEqual(1)
-  })
-
   test('fills its container after starting a session', async ({ page }) => {
+    await page.setViewportSize({ width: 1440, height: 900 })
     await installMocks(page, { qcHistories: true })
-    await setupSessionEditView(page)
+    await setupEditView(page)
     await expect
       .poll(() => layoutMismatch(page), { timeout: 5_000 })
       .toBeLessThanOrEqual(1)
diff --git a/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts b/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts
index 4d82a6c9..8402c5d0 100644
--- a/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts
+++ b/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts
@@ -18,10 +18,14 @@
 
 import { expect, test, type Page, type Route } from '@playwright/test'
 import { installMocks } from './support/mocks'
-import { datastreams, DATASTREAM_ID } from './support/fixtures'
+import {
+  datastreams,
+  managedDatastream,
+  MANAGED_DATASTREAM_ID,
+} from './support/fixtures'
 import { setupEditView } from './support/app'
 
-type DatastreamRecord = (typeof datastreams)[number]
+type DatastreamRecord = (typeof datastreams)[number] | typeof managedDatastream
 type RoutedTrace = {
   id?: string
   mode?: string
@@ -58,6 +62,9 @@ function withoutIntendedSpacing(ds: DatastreamRecord): DatastreamRecord {
   return { ...ds, intendedTimeSpacing: null, intendedTimeSpacingUnit: null }
 }
 
+// The managed datastream is served too, so the row Edit flow can enter the editor.
+const catalog: DatastreamRecord[] = [...datastreams, managedDatastream]
+
 async function patchDatastreamFixture(route: Route): Promise<void> {
   const request = route.request()
   if (request.method() !== 'GET') return route.fallback()
@@ -80,13 +87,13 @@ async function patchDatastreamFixture(route: Route): Promise<void> {
       status: 200,
       contentType: 'application/json',
       headers,
-      body: JSON.stringify({ data: datastreams.map(withoutIntendedSpacing) }),
+      body: JSON.stringify({ data: catalog.map(withoutIntendedSpacing) }),
     })
   }
   const single = path.match(/\/api\/data\/datastreams\/([^/]+)$/)
   if (single) {
     const id = single[1]
-    const ds = datastreams.find((d) => d.id === id) ?? datastreams[0]!
+    const ds = catalog.find((d) => d.id === id) ?? catalog[0]!
     return route.fulfill({
       status: 200,
       contentType: 'application/json',
@@ -99,7 +106,7 @@ async function patchDatastreamFixture(route: Route): Promise<void> {
 
 test.describe('plot: datastream without intendedTimeSpacing', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     // Routes added with `page.route` are invoked LIFO, so this stack
     // sits on top of the catch-all installed by `installMocks` and
     // wins for both the list and single-get endpoints. The sub-path
@@ -114,7 +121,7 @@ test.describe('plot: datastream without intendedTimeSpacing', () => {
     page,
   }) => {
     await setupEditView(page)
-    await waitForTraceRendered(page, DATASTREAM_ID)
+    await waitForTraceRendered(page, MANAGED_DATASTREAM_ID)
 
     const trace = await page.evaluate((id) => {
       const gd = document.querySelector('[data-testid="main-plot"]') as
@@ -133,7 +140,7 @@ test.describe('plot: datastream without intendedTimeSpacing', () => {
         // so its absence is the precise signal.
         anyGapOverlay: traces.some((t) => t._gapOverlayFor === id),
       }
-    }, DATASTREAM_ID)
+    }, MANAGED_DATASTREAM_ID)
 
     expect(trace.mainMode).toBe('markers')
     expect(trace.overlayExists).toBe(false)
@@ -147,7 +154,7 @@ test.describe('plot: datastream without intendedTimeSpacing', () => {
     page,
   }) => {
     await setupEditView(page)
-    await waitForTraceRendered(page, DATASTREAM_ID)
+    await waitForTraceRendered(page, MANAGED_DATASTREAM_ID)
 
     // The toggle button only renders in manual mode; default is auto,
     // so open the dropdown and pick manual first. Picking manual also
@@ -176,7 +183,7 @@ test.describe('plot: datastream without intendedTimeSpacing', () => {
         | null
       const t = gd?.data?.find((tr) => tr.id === id)
       return t ? { opacity: t.marker?.opacity ?? null } : null
-    }, DATASTREAM_ID)
+    }, MANAGED_DATASTREAM_ID)
 
     expect(main).not.toBeNull()
     // Scatter-only series ignore the toggle: markers stay fully opaque.
diff --git a/apps/qc-app/e2e/qc-golden-path.spec.ts b/apps/qc-app/e2e/qc-golden-path.spec.ts
index 92a17423..6ac8e116 100644
--- a/apps/qc-app/e2e/qc-golden-path.spec.ts
+++ b/apps/qc-app/e2e/qc-golden-path.spec.ts
@@ -43,8 +43,9 @@ async function loginThroughDataManagementIfNeeded(page: Page) {
 //      selection, so the router's workspace guard redirects to /workspaces.
 //   2. Select "Test Workspace #2" from the picker.
 //   3. Wait for datastreams table (init-complete signal on Home).
-//   4. Plot the first source with a managed datastream, load the All range.
-//   5. Start editing on its first managed datastream, which opens a session.
+//   4. Find the first source with a managed datastream.
+//   5. Its row Edit button, then its first managed datastream; resume the
+//      session in progress or Start one in the window step.
 //   6. Open "Value thresholds", add a filter, close dialog.
 //   7. Wait for selection to populate, open "Change values", apply edit.
 //   8. Assert CHANGE_VALUES appears in EditHistory.
@@ -95,37 +96,31 @@ test.describe('QC golden path (live same-origin)', () => {
       timeout: 60_000,
     })
 
-    // Step 4: Plot the first rendered source that has a managed datastream
+    // Step 4: Find the first rendered source that has a managed datastream
     // (managed-count badge). The table is virtualized, so the workspace needs
-    // such a source near the top. Its check box opens the plot-source chooser;
-    // plot the raw source, then load the All range so the session window
-    // covers real observations.
-    const sourceCheckbox = page
+    // such a source near the top.
+    const sourceEdit = page
       .locator('tr')
       .filter({ has: page.locator('.managed-count') })
-      .locator('[data-testid^="plot-checkbox-"]')
+      .locator('[data-testid^="edit-datastream-"]')
       .first()
-    await expect(sourceCheckbox).toBeVisible({ timeout: 30_000 })
-    const sourceId = (await sourceCheckbox.getAttribute('data-testid'))!.replace(
-      'plot-checkbox-',
+    await expect(sourceEdit).toBeVisible({ timeout: 30_000 })
+    const sourceId = (await sourceEdit.getAttribute('data-testid'))!.replace(
+      'edit-datastream-',
       ''
     )
-    await sourceCheckbox.click()
-    await page.getByTestId(`plot-option-${sourceId}`).locator('input').check()
-    await page.getByTestId('plot-source-apply').click()
-    const loading = page.getByTestId('data-loading-indicator')
-    await loading.waitFor({ state: 'hidden', timeout: 90_000 })
-    await page.getByTestId('date-preset-All').click()
-    await loading.waitFor({ state: 'hidden', timeout: 90_000 })
-
-    // Step 5: Start editing and pick the first managed datastream. This
-    // resumes its in-progress session or starts one, which is what renders
-    // Save and Commit in the history footer.
-    await page.getByRole('button', { name: 'Start editing' }).click()
+
+    // Step 5: The row's Edit button, then the first managed datastream. A
+    // managed datastream with a session in progress resumes it directly;
+    // otherwise the window step opens and Start begins a session over the
+    // default window. Either way Save and Commit render in the footer.
+    await page.getByTestId(`edit-datastream-${sourceId}`).click()
     await page.locator('[data-testid^="edit-managed-"]').first().click()
-    await expect(page.getByTestId('exit-save-btn')).toBeVisible({
-      timeout: 60_000,
-    })
+    const windowStart = page.getByTestId('session-window-start')
+    const saveBtn = page.getByTestId('exit-save-btn')
+    await expect(windowStart.or(saveBtn)).toBeVisible({ timeout: 60_000 })
+    if (await windowStart.isVisible()) await windowStart.click()
+    await expect(saveBtn).toBeVisible({ timeout: 60_000 })
     await expect(page.getByText('Filter Data')).toBeVisible()
 
     // Step 6 — Open "Value thresholds" operation panel, add a filter.
diff --git a/apps/qc-app/e2e/qc-history.spec.ts b/apps/qc-app/e2e/qc-history.spec.ts
index d93704fb..e801a057 100644
--- a/apps/qc-app/e2e/qc-history.spec.ts
+++ b/apps/qc-app/e2e/qc-history.spec.ts
@@ -19,7 +19,7 @@ import { expectHistoryContains, selectAllPoints } from './support/ops'
 
 test.describe('QC history: save / load round-trip', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
   })
 
diff --git a/apps/qc-app/e2e/screenshots.spec.ts b/apps/qc-app/e2e/screenshots.spec.ts
index fd27cab4..0e735df0 100644
--- a/apps/qc-app/e2e/screenshots.spec.ts
+++ b/apps/qc-app/e2e/screenshots.spec.ts
@@ -27,10 +27,10 @@ import { installMocks } from './support/mocks'
 import {
   gotoHome,
   setupEditView,
-  setupSessionEditView,
   openOp,
   plotFirstDatastream,
   plotDatastreamById,
+  startSessionFromRow,
   waitForHomeReady,
 } from './support/app'
 import { selectAllPoints } from './support/ops'
@@ -147,7 +147,7 @@ test.describe('docs screenshots', () => {
 
   test('edit view (full layout)', async ({ page }) => {
     await page.setViewportSize(STD_VIEWPORT)
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await snapPage(page, 'edit-view.png')
   })
@@ -174,15 +174,15 @@ test.describe('docs screenshots', () => {
   }) => {
     await page.setViewportSize(STD_VIEWPORT)
     await installMocks(page, {
+      qcHistories: true,
       observationsById: {
         [DATASTREAM_ID]: buildObservations(),
         [DATASTREAM_ID_B]: buildTemperatureObservations(),
       },
     })
     await gotoHome(page)
-    await plotDatastreamById(page, DATASTREAM_ID)
     await plotDatastreamById(page, DATASTREAM_ID_B)
-    await page.getByTestId('nav-rail-item-edit').click()
+    await startSessionFromRow(page)
     await expect(page.getByText('Filter Data')).toBeVisible()
     await page.waitForTimeout(800)
     await snapPage(page, 'edit-view-multi.png')
@@ -190,7 +190,7 @@ test.describe('docs screenshots', () => {
 
   test('data points mode menu (auto, threshold visible)', async ({ page }) => {
     await page.setViewportSize(STD_VIEWPORT)
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await page.getByTestId('tooltips-mode-btn').click()
     const menu = page.getByTestId('tooltips-mode-menu')
@@ -200,7 +200,7 @@ test.describe('docs screenshots', () => {
 
   test('table view (observations editable rows)', async ({ page }) => {
     await page.setViewportSize(STD_VIEWPORT)
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     // Plot tab is the default; switch to Table.
     await page.getByRole('button', { name: /^Table$/ }).click()
@@ -211,7 +211,7 @@ test.describe('docs screenshots', () => {
 
   test('plot help menu (open)', async ({ page }) => {
     await page.setViewportSize(STD_VIEWPORT)
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     // The help button is the only `?` icon in the toolbar.
     await page.getByRole('button', { name: /plot controls/i }).click()
@@ -248,7 +248,7 @@ test.describe('docs screenshots', () => {
   for (const id of filterOps) {
     test(`filter panel — ${id}`, async ({ page }) => {
       await page.setViewportSize(TALL_VIEWPORT)
-      await installMocks(page)
+      await installMocks(page, { qcHistories: true })
       await setupEditView(page)
       await openOp(page, id)
       const panel = page.getByTestId(`operation-panel-${id}`)
@@ -267,7 +267,7 @@ test.describe('docs screenshots', () => {
   for (const id of editOps) {
     test(`edit panel — ${id}`, async ({ page }) => {
       await page.setViewportSize(TALL_VIEWPORT)
-      await installMocks(page)
+      await installMocks(page, { qcHistories: true })
       await setupEditView(page)
       // Edit ops require a selection; seed one with a wide value
       // threshold so the panel renders its real body.
@@ -280,7 +280,7 @@ test.describe('docs screenshots', () => {
 
   test('date range mask (filter panel, mask enabled)', async ({ page }) => {
     await page.setViewportSize(TALL_VIEWPORT)
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await openOp(page, 'valueThreshold')
     const panel = page.getByTestId('operation-panel-valueThreshold')
@@ -292,7 +292,7 @@ test.describe('docs screenshots', () => {
 
   test('add panel — qualifyingComments', async ({ page }) => {
     await page.setViewportSize(TALL_VIEWPORT)
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await selectAllPoints(page)
     await openOp(page, 'qualifyingComments')
@@ -302,7 +302,7 @@ test.describe('docs screenshots', () => {
 
   test('add panel — addPoints', async ({ page }) => {
     await page.setViewportSize(TALL_VIEWPORT)
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await openOp(page, 'addPoints')
     const panel = page.getByTestId('operation-panel-addPoints')
@@ -311,7 +311,7 @@ test.describe('docs screenshots', () => {
 
   test('add panel — fillGaps', async ({ page }) => {
     await page.setViewportSize(TALL_VIEWPORT)
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await openOp(page, 'fillGaps')
     const panel = page.getByTestId('operation-panel-fillGaps')
@@ -320,7 +320,7 @@ test.describe('docs screenshots', () => {
 
   test('edit history with entries', async ({ page }) => {
     await page.setViewportSize(STD_VIEWPORT)
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     await selectAllPoints(page)
     await openOp(page, 'changeValues')
@@ -338,7 +338,7 @@ test.describe('docs screenshots', () => {
   test('commit confirmation dialog', async ({ page }) => {
     await page.setViewportSize(STD_VIEWPORT)
     await installMocks(page, { qcHistories: true })
-    await setupSessionEditView(page)
+    await setupEditView(page)
     await selectAllPoints(page)
     await openOp(page, 'changeValues')
     const opPanel = page.getByTestId('operation-panel-changeValues')
diff --git a/apps/qc-app/e2e/share-url.spec.ts b/apps/qc-app/e2e/share-url.spec.ts
index 314df9a2..a5b66cfa 100644
--- a/apps/qc-app/e2e/share-url.spec.ts
+++ b/apps/qc-app/e2e/share-url.spec.ts
@@ -3,17 +3,26 @@
  *
  * Two complementary halves:
  *   - **Emit**: drive the UI, then read `window.location.search` and
- *     assert the new compact-shape keys appear (and the legacy ones
- *     are gone).
- *   - **Hydrate**: open a deep-linked URL carrying the new keys, then
+ *     assert the compact-shape keys appear (and the legacy ones are
+ *     gone).
+ *   - **Hydrate**: open a deep-linked URL carrying the keys, then
  *     assert the app lands in the expected state (plotted ids, tab,
  *     edit view, data-points threshold, hidden traces).
  */
 
-import { expect, test } from '@playwright/test'
+import { expect, test, type Page } from '@playwright/test'
 import { installMocks } from './support/mocks'
-import type { Page } from '@playwright/test'
 import { setupEditView } from './support/app'
+import {
+  DATASTREAM_ID,
+  DATASTREAM_ID_B,
+  MANAGED_DATASTREAM_ID,
+  buildObservations,
+  buildTemperatureObservations,
+  FIXTURE_OBS_END_MS,
+  FIXTURE_OBS_START_MS,
+  WORKSPACE_ID,
+} from './support/fixtures'
 
 /**
  * Poll the live plot for: the current X range (as `[loMs, hiMs]`)
@@ -90,19 +99,42 @@ async function clickResetAxes(page: Page) {
     button.click()
   }, selector)
 }
-import {
-  DATASTREAM_ID,
-  DATASTREAM_ID_B,
-  buildObservations,
-  buildTemperatureObservations,
-  FIXTURE_OBS_END_MS,
-  FIXTURE_OBS_START_MS,
-  WORKSPACE_ID,
-} from './support/fixtures'
+
+/** Wait until the live plot draws a trace for every id in `ids`. */
+async function waitForTraces(page: Page, ids: string[]) {
+  await page.waitForFunction(
+    (wanted) => {
+      const gd = document.querySelector('[data-testid="main-plot"]') as
+        | (HTMLElement & { data?: Array<{ id?: string }> })
+        | null
+      const drawn = new Set((gd?.data ?? []).map((t) => t.id))
+      return wanted.every((id) => drawn.has(id))
+    },
+    ids,
+    { timeout: 30_000 }
+  )
+}
+
+/**
+ * Open an Edit-view link on the managed datastream. No session is in
+ * progress in the fixture, so the window step opens; Start begins one.
+ */
+async function openEditLink(page: Page, extraQuery = '') {
+  await page.goto(
+    `/?ws=${WORKSPACE_ID}&m=e&ed=${MANAGED_DATASTREAM_ID}${extraQuery}`
+  )
+  await page.getByTestId('session-window-start').click({ timeout: 30_000 })
+  await expect(page.getByTestId('exit-save-btn')).toBeVisible({
+    timeout: 30_000,
+  })
+  await expect(page.getByText('Filter Data')).toBeVisible()
+}
 
 test.describe('share URL', () => {
   test.beforeEach(async ({ page }) => {
+    test.slow()
     await installMocks(page, {
+      qcHistories: true,
       observationsById: {
         [DATASTREAM_ID]: buildObservations(),
         [DATASTREAM_ID_B]: buildTemperatureObservations(),
@@ -110,9 +142,7 @@ test.describe('share URL', () => {
     })
   })
 
-  test('writer emits the new short keys after plotting + entering Edit', async ({
-    page,
-  }) => {
+  test('writer emits the short keys after entering Edit', async ({ page }) => {
     await setupEditView(page)
     // Snapshot the query string the app has written.
     const search = await page.evaluate(() => window.location.search)
@@ -120,7 +150,9 @@ test.describe('share URL', () => {
 
     expect(params.get('ws')).toBe(WORKSPACE_ID)
     expect(params.get('m')).toBe('e')
-    expect(params.get('ds')).toBe(DATASTREAM_ID)
+    // Nothing was plotted; the edit target travels in `ed`, not `ds`.
+    expect(params.has('ds')).toBe(false)
+    expect(params.get('ed')).toBe(MANAGED_DATASTREAM_ID)
 
     // Legacy keys should be gone.
     expect(params.has('workspace')).toBe(false)
@@ -198,14 +230,10 @@ test.describe('share URL', () => {
     expect(params.has('pl')).toBe(false)
   })
 
-  test('hydrator restores Edit view + datastream + Table tab from URL', async ({
+  test('hydrator restores Edit view + edit target + Table tab from URL', async ({
     page,
   }) => {
-    const url = `/?ws=${WORKSPACE_ID}&m=e&ds=${DATASTREAM_ID}&tab=t`
-    await page.goto(url)
-
-    // The Edit drawer shows "Filter Data" once Edit view is mounted.
-    await expect(page.getByText('Filter Data')).toBeVisible({ timeout: 30_000 })
+    await openEditLink(page, '&tab=t')
 
     // Table tab is the active tab segment. v-btn-toggle marks the
     // chosen child with `v-btn--active`.
@@ -217,9 +245,7 @@ test.describe('share URL', () => {
   test('hydrator applies the data-points threshold from the URL', async ({
     page,
   }) => {
-    const url = `/?ws=${WORKSPACE_ID}&m=e&ds=${DATASTREAM_ID}&th=42000`
-    await page.goto(url)
-    await expect(page.getByText('Filter Data')).toBeVisible({ timeout: 30_000 })
+    await openEditLink(page, '&th=42000')
 
     // The auto-mode counter renders the threshold after the slash.
     const counter = page.getByTestId('tooltips-counter')
@@ -239,9 +265,7 @@ test.describe('share URL', () => {
     const toS36 = (ms: number) => Math.floor(ms / 1000).toString(36)
     const z = `${toS36(xLo)}.${toS36(xHi)}`
 
-    const url = `/?ws=${WORKSPACE_ID}&m=e&ds=${DATASTREAM_ID}&z=${z}`
-    await page.goto(url)
-    await expect(page.getByText('Filter Data')).toBeVisible({ timeout: 30_000 })
+    await openEditLink(page, `&z=${z}`)
 
     // Probe Plotly's live x-axis range. Poll because the rebuild
     // can run a hair after `handleNewPlot` from the mount hook;
@@ -293,22 +317,16 @@ test.describe('share URL', () => {
     page,
   }) => {
     // Two datastreams plotted, with bit 1 of the hidden-trace mask
-    // set → the second (non-QC) trace is hidden. The fix this guards
+    // set → the second trace is hidden. The fix this guards
     // against: `createPlotlyOption` honours `hiddenTraceIds`, so a
     // fresh load that hydrates the set from `h=` produces a plot
     // with the hidden traces already invisible. Pre-fix, the
     // sidebar correctly showed the row as hidden but the plot
     // continued to draw the trace until the user manually toggled
-    // the eye again.
-    await installMocks(page, {
-      observationsById: {
-        [DATASTREAM_ID]: buildObservations(),
-        [DATASTREAM_ID_B]: buildTemperatureObservations(),
-      },
-    })
-    const url = `/?ws=${WORKSPACE_ID}&m=e&ds=${DATASTREAM_ID},${DATASTREAM_ID_B}&h=2`
+    // the eye again. The mask indexes `ds` only, so no Edit view.
+    const url = `/?ws=${WORKSPACE_ID}&ds=${DATASTREAM_ID},${DATASTREAM_ID_B}&h=2`
     await page.goto(url)
-    await expect(page.getByText('Filter Data')).toBeVisible({ timeout: 30_000 })
+    await waitForTraces(page, [DATASTREAM_ID, DATASTREAM_ID_B])
     // Let the rebuild settle.
     await page.waitForTimeout(800)
 
@@ -344,8 +362,8 @@ test.describe('share URL', () => {
 
     expect(visibility.a).not.toBeNull()
     expect(visibility.b).not.toBeNull()
-    // A (QC, bit 0 unset) renders normally. Plotly normalises an
-    // unset `visible` to `true`.
+    // A (bit 0 unset) renders normally. Plotly normalises an unset
+    // `visible` to `true`.
     expect(visibility.a!.visible).not.toBe(false)
     // B (bit 1 set in the URL mask) is hidden on the live plot,
     // along with its gap overlay.
@@ -362,14 +380,14 @@ test.describe('share URL', () => {
     // pins `_rangeInitial0/1` to the URL view — stock Reset bounces
     // back there instead of the data extent. A cold goto to a zoomed
     // URL reproduces the same condition as the user-reported
-    // zoom→reload flow.
+    // zoom→reload flow. Only a plotted datastream is needed.
     const fullSpan = FIXTURE_OBS_END_MS - FIXTURE_OBS_START_MS
     const xHi = FIXTURE_OBS_END_MS - fullSpan * 0.1
     const xLo = xHi - fullSpan * 0.2
     const toS36 = (ms: number) => Math.floor(ms / 1000).toString(36)
     const z = `${toS36(xLo)}.${toS36(xHi)}`
-    await page.goto(`/?ws=${WORKSPACE_ID}&m=e&ds=${DATASTREAM_ID}&z=${z}`)
-    await expect(page.getByText('Filter Data')).toBeVisible({ timeout: 30_000 })
+    await page.goto(`/?ws=${WORKSPACE_ID}&ds=${DATASTREAM_ID}&z=${z}`)
+    await waitForTraces(page, [DATASTREAM_ID])
 
     const afterHydrate = await waitForPlotMeta(page)
     expect(afterHydrate).not.toBeNull()
diff --git a/apps/qc-app/e2e/smoke.spec.ts b/apps/qc-app/e2e/smoke.spec.ts
index 0907fa7b..118c06e2 100644
--- a/apps/qc-app/e2e/smoke.spec.ts
+++ b/apps/qc-app/e2e/smoke.spec.ts
@@ -11,7 +11,7 @@ import { setupEditView } from './support/app'
 
 test.describe('smoke (mocked backend)', () => {
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
   })
 
   test('boots into edit view without live network', async ({ page }) => {
diff --git a/apps/qc-app/e2e/submit.spec.ts b/apps/qc-app/e2e/submit.spec.ts
index 55c2e8f9..1f01fc1a 100644
--- a/apps/qc-app/e2e/submit.spec.ts
+++ b/apps/qc-app/e2e/submit.spec.ts
@@ -8,7 +8,7 @@
 
 import { expect, test } from '@playwright/test'
 import { installMocks, type MockQcSession } from './support/mocks'
-import { openOp, setupSessionEditView } from './support/app'
+import { openOp, setupEditView } from './support/app'
 import { expectHistoryContains, selectAllPoints } from './support/ops'
 
 test.describe('commit a QC session', () => {
@@ -20,7 +20,7 @@ test.describe('commit a QC session', () => {
       submissions,
       qcSessionState: sessions,
     })
-    await setupSessionEditView(page)
+    await setupEditView(page)
 
     await selectAllPoints(page)
     await openOp(page, 'changeValues')
diff --git a/apps/qc-app/e2e/support/app.ts b/apps/qc-app/e2e/support/app.ts
index d293542e..e9cb2253 100644
--- a/apps/qc-app/e2e/support/app.ts
+++ b/apps/qc-app/e2e/support/app.ts
@@ -2,7 +2,7 @@
  * Flow helpers shared by every mocked e2e spec.
  *
  * Specs should usually go through these so the "boot → pick workspace
- * → plot datastream → open edit view" preamble doesn't get copy-pasted
+ * → Edit a row → start a session" preamble doesn't get copy-pasted
  * across 15 files. The helpers assume `installMocks()` has been
  * called; they do not install mocks themselves.
  */
@@ -121,37 +121,24 @@ export async function plotDatastreamById(
 }
 
 /**
- * Full "ready to edit" preamble: home → plot → switch to edit view.
- * After this returns the EditDrawer is visible and operation panels
- * can be opened via `op-<id>` testids.
+ * Enter the editor the way a user does: the row's Edit button, the managed
+ * datastream's "Start new session", then Start in the window step. Needs
+ * `installMocks(page, { qcHistories: true })`.
  */
 export async function setupEditView(page: Page): Promise<void> {
   await gotoHome(page)
-  await plotFirstDatastream(page)
-  await page.getByTestId('nav-rail-item-edit').click()
+  await startSessionFromRow(page)
   await expect(page.getByText('Filter Data')).toBeVisible()
 }
 
 /**
- * Enter the editor through a QC session, which is what puts Save and Commit
- * in the history footer: plot the raw source from its chooser, Start editing,
- * and start a new session on the managed datastream. `setupEditView` only
- * switches views and never opens a session. Needs
- * `installMocks(page, { qcHistories: true })`.
+ * From Home: the source row's Edit button, "Start new session" on the managed
+ * datastream, then Start with the default window. Waits for the editor footer.
  */
-export async function setupSessionEditView(page: Page): Promise<void> {
-  await gotoHome(page)
-  await page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`).click()
-  await page
-    .getByTestId(`plot-option-${DATASTREAM_ID}`)
-    .locator('input')
-    .check()
-  await page.getByTestId('plot-source-apply').click()
-  await page
-    .getByTestId('data-loading-indicator')
-    .waitFor({ state: 'hidden', timeout: 30_000 })
-  await page.getByRole('button', { name: 'Start editing' }).click()
+export async function startSessionFromRow(page: Page): Promise<void> {
+  await page.getByTestId(`edit-datastream-${DATASTREAM_ID}`).click()
   await page.getByTestId(`edit-managed-${MANAGED_DATASTREAM_ID}`).click()
+  await page.getByTestId('session-window-start').click()
   await expect(page.getByTestId('exit-save-btn')).toBeVisible({
     timeout: 30_000,
   })
diff --git a/apps/qc-app/e2e/tooltip-threshold.spec.ts b/apps/qc-app/e2e/tooltip-threshold.spec.ts
index 5560e822..a14ac575 100644
--- a/apps/qc-app/e2e/tooltip-threshold.spec.ts
+++ b/apps/qc-app/e2e/tooltip-threshold.spec.ts
@@ -64,7 +64,7 @@ test.describe('data-points combobox', () => {
   test.setTimeout(60_000)
 
   test.beforeEach(async ({ page }) => {
-    await installMocks(page)
+    await installMocks(page, { qcHistories: true })
     await setupEditView(page)
     // Reset the persisted preference bag so each test starts in auto
     // mode at the default threshold. The store persists `tooltipsMode`,
diff --git a/apps/qc-app/src/components/EditData/SessionWindowDialog.vue b/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
new file mode 100644
index 00000000..f464204b
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
@@ -0,0 +1,132 @@
+<template>
+  <v-card rounded="lg">
+    <div class="px-4 pt-4 pb-2">
+      <div class="text-title-medium font-weight-bold">
+        New session on "{{ managedName }}"
+      </div>
+      <div class="text-body-small text-medium-emphasis mt-1">
+        Pick the period to edit. It can overlap committed sessions but cannot
+        leave a gap before or after them.
+      </div>
+    </div>
+
+    <v-divider />
+
+    <div class="pa-4 d-flex flex-column ga-3">
+      <div class="text-body-small">
+        <div>
+          <span class="text-medium-emphasis">Source data:</span>
+          {{ formatDateRange(source.phenomenonBeginTime, source.phenomenonEndTime) }}
+        </div>
+        <div data-testid="session-window-committed">
+          <span class="text-medium-emphasis">Committed history:</span>
+          <template v-if="history">
+            {{ formatDateRange(history.begin.toISOString(), history.end.toISOString()) }}
+          </template>
+          <template v-else>Nothing committed yet</template>
+        </div>
+      </div>
+
+      <template v-if="begin && end">
+        <div>
+          <div class="text-body-small text-medium-emphasis mb-1">From</div>
+          <DatePickerField
+            v-model="begin"
+            data-testid="session-window-from"
+            placeholder="Start date"
+          />
+        </div>
+        <div>
+          <div class="text-body-small text-medium-emphasis mb-1">To</div>
+          <DatePickerField
+            v-model="end"
+            data-testid="session-window-to"
+            placeholder="End date"
+          />
+        </div>
+      </template>
+
+      <v-alert
+        v-if="error"
+        type="warning"
+        variant="tonal"
+        density="compact"
+        data-testid="session-window-error"
+      >
+        <span class="text-body-small">{{ error }}</span>
+      </v-alert>
+    </div>
+
+    <v-divider />
+
+    <v-card-actions class="px-4 py-2">
+      <v-spacer />
+      <v-btn
+        variant="text"
+        data-testid="session-window-cancel"
+        :disabled="loading"
+        @click="emit('cancel')"
+      >
+        Cancel
+      </v-btn>
+      <v-btn
+        color="primary"
+        variant="flat"
+        data-testid="session-window-start"
+        :disabled="!!error"
+        :loading="loading"
+        @click="onStart"
+      >
+        Start session
+      </v-btn>
+    </v-card-actions>
+  </v-card>
+</template>
+
+<script setup lang="ts">
+import { computed, ref } from 'vue'
+import DatePickerField from '@/components/VisualizeData/DatePickerField.vue'
+import { formatDateRange } from '@/utils/time'
+import type { TimeWindow } from '@/utils/timeRangePresets'
+import {
+  committedExtent,
+  defaultSessionWindow,
+  validateSessionWindow,
+  type SessionRange,
+  type SourceExtent,
+} from '@/utils/sessionWindow'
+
+const props = defineProps<{
+  managedName: string
+  source: SourceExtent
+  sessions: readonly SessionRange[]
+  loading?: boolean
+}>()
+
+const emit = defineEmits<{
+  (e: 'confirm', window: TimeWindow): void
+  (e: 'cancel'): void
+}>()
+
+const initial = defaultSessionWindow(props.source, props.sessions)
+const begin = ref<Date | null>(initial?.begin ?? null)
+const end = ref<Date | null>(initial?.end ?? null)
+
+const history = computed(() => committedExtent(props.sessions))
+
+const error = computed(() => {
+  if (!begin.value || !end.value) {
+    return 'The source datastream has no observations to edit.'
+  }
+  return validateSessionWindow(
+    { begin: begin.value, end: end.value },
+    props.source,
+    props.sessions
+  )
+})
+
+function onStart() {
+  if (error.value || !begin.value || !end.value) return
+  emit('confirm', { begin: begin.value, end: end.value })
+}
+</script>
diff --git a/apps/qc-app/src/components/EditData/StartEditingFlow.vue b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
new file mode 100644
index 00000000..516238ee
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
@@ -0,0 +1,341 @@
+<template>
+  <v-dialog v-model="showChooser" max-width="640">
+    <StartEditingDialog
+      v-if="chooserSource"
+      :source="chooserSource"
+      :options="chooserOptions"
+      :loading="chooserLoading"
+      @edit="onChooserEdit"
+      @delete="onChooserDelete"
+      @delete-session="onChooserDeleteSession"
+      @create="onChooserCreate"
+      @cancel="showChooser = false"
+    />
+  </v-dialog>
+
+  <v-dialog v-model="showCreateDatastream" max-width="560" persistent>
+    <v-card v-if="chooserSource" rounded="lg">
+      <CreateDatastreamForm
+        :source="chooserSource"
+        :processing-levels="processingLevels"
+        :default-processing-level-id="qcPreferences.processingLevelId"
+        :on-create-processing-level="onCreateProcessingLevel"
+        :permission-error="createPermissionError"
+        @cancel="onCreateCancel"
+        @confirm="onCreateDatastream"
+      />
+    </v-card>
+  </v-dialog>
+
+  <v-dialog v-model="showWindow" max-width="520" :persistent="isStartingSession">
+    <SessionWindowDialog
+      v-if="windowTarget"
+      :key="windowTarget.key"
+      :managed-name="windowTarget.name"
+      :source="windowTarget.source"
+      :sessions="windowTarget.sessions"
+      :loading="isStartingSession"
+      @confirm="onWindowConfirm"
+      @cancel="onWindowCancel"
+    />
+  </v-dialog>
+</template>
+
+<script setup lang="ts">
+/**
+ * Row Edit flow: choose a managed datastream of a source (or create the
+ * first one), pick a session window when there is no session to continue,
+ * then enter the editor.
+ */
+
+import { computed, ref } from 'vue'
+import { storeToRefs } from 'pinia'
+import { Snackbar } from '@uwrl/qc-utils'
+import type { Datastream, QualityControlSession } from '@hydroserver/client'
+import CreateDatastreamForm from '@/components/EditData/CreateDatastreamForm.vue'
+import SessionWindowDialog from '@/components/EditData/SessionWindowDialog.vue'
+import StartEditingDialog from '@/components/EditData/StartEditingDialog.vue'
+import { useDataVisStore } from '@/store/dataVisualization'
+import { useQcPreferencesStore } from '@/store/qcPreferences'
+import { useQcSessionStore } from '@/store/qcSession'
+import { useWorkingCopiesStore } from '@/store/workingCopies'
+import {
+  useCreateManagedDatastream,
+  type CreateManagedDatastreamSpec,
+} from '@/composables/useCreateManagedDatastream'
+import { useEditEntry } from '@/composables/useEditEntry'
+import {
+  useManagedDatastreams,
+  type ManagedDatastreamOption,
+} from '@/composables/useManagedDatastreams'
+import { useProcessingLevels } from '@/composables/useProcessingLevels'
+import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
+import { collectDeletionChain } from '@/utils/sessionGraph'
+import type { TimeWindow } from '@/utils/timeRangePresets'
+
+// 'entering': the editor is not open yet. 'resume': it opened with no session.
+// 'footer': it is open on committed history.
+type WindowOrigin = 'entering' | 'resume' | 'footer'
+
+interface WindowTarget {
+  /** New for every target, so the dialog never keeps a previous one's defaults. */
+  key: number
+  managedId: string
+  name: string
+  source: Datastream
+  sessions: QualityControlSession[]
+  origin: WindowOrigin
+}
+
+const { qcDatastream, datastreams, processingLevels } =
+  storeToRefs(useDataVisStore())
+const { addQcHistory, removeManagedDatastream } = useDataVisStore()
+const { sourceDatastream, sessions, inProgressSession } =
+  storeToRefs(useQcSessionStore())
+const qcPreferences = useQcPreferencesStore()
+const workingCopies = useWorkingCopiesStore()
+const { loadForSource, deleteManaged, deleteSessionChain } =
+  useManagedDatastreams()
+const { create: createManaged } = useCreateManagedDatastream()
+const { createProcessingLevel } = useProcessingLevels()
+const { canCreateDatastream, roleName } = useWorkspacePermissions()
+const { enterEdit, startSessionOver, leaveEdit } = useEditEntry()
+
+const createPermissionError = computed(() =>
+  canCreateDatastream()
+    ? ''
+    : `Your role on this workspace (${roleName()}) can't create datastreams. Ask a workspace owner for an editor role.`
+)
+
+const showChooser = ref(false)
+const chooserLoading = ref(false)
+const chooserOptions = ref<ManagedDatastreamOption[]>([])
+const chooserSource = ref<Datastream | null>(null)
+const showCreateDatastream = ref(false)
+
+const windowTarget = ref<WindowTarget | null>(null)
+let windowTargetCount = 0
+function openWindow(target: Omit<WindowTarget, 'key'>) {
+  windowTarget.value = { ...target, key: ++windowTargetCount }
+}
+const isStartingSession = ref(false)
+const showWindow = computed({
+  get: () => !!windowTarget.value,
+  // Esc and scrim closes take the same path as Cancel.
+  set: (open) => {
+    if (!open && !isStartingSession.value) void onWindowCancel()
+  },
+})
+
+/** Row Edit button: choose a managed datastream, or create the first one. */
+async function openFor(source: Datastream) {
+  chooserSource.value = source
+  chooserOptions.value = []
+  chooserLoading.value = true
+  try {
+    chooserOptions.value = await loadForSource(source.id)
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not load QC datastreams.'
+    )
+    return
+  } finally {
+    chooserLoading.value = false
+  }
+  if (chooserOptions.value.length) showChooser.value = true
+  else showCreateDatastream.value = true
+}
+
+async function onChooserEdit(option: ManagedDatastreamOption) {
+  showChooser.value = false
+  const source = chooserSource.value
+  if (!source) return
+  if (option.sessions.some((s) => s.status === 'in_progress')) {
+    await runEnter(option.managed.id)
+    return
+  }
+  openWindow({
+    managedId: option.managed.id,
+    name: option.managed.name,
+    source,
+    sessions: option.sessions,
+    origin: 'entering',
+  })
+}
+
+/** Window step for the open editor's target. False when it cannot open. */
+function openSessionWindow(origin: 'resume' | 'footer'): boolean {
+  const managed = qcDatastream.value
+  const source = sourceDatastream.value
+  if (!managed || !source) return false
+  openWindow({
+    managedId: managed.id,
+    name: managed.name,
+    source,
+    sessions: sessions.value,
+    origin,
+  })
+  return true
+}
+
+/** Editor footer "New session". */
+function openNewSession() {
+  openSessionWindow('footer')
+}
+
+async function resume(managedId: string) {
+  await runEnter(managedId)
+}
+
+async function runEnter(managedId: string, window?: TimeWindow) {
+  try {
+    const result = await enterEdit(managedId, window)
+    if (result === 'needs-window' && !openSessionWindow('resume')) {
+      // No source to window, so the open editor has nothing to edit.
+      await leaveEdit()
+      Snackbar.error('Could not load the source datastream for a new session.')
+    }
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not open the datastream for editing.'
+    )
+  }
+}
+
+async function onWindowConfirm(window: TimeWindow) {
+  const target = windowTarget.value
+  if (!target) return
+  isStartingSession.value = true
+  let pickAgain = false
+  try {
+    if (target.origin === 'entering') {
+      await runEnter(target.managedId, window)
+    } else {
+      const started = await startSessionOver(window)
+      // A plain failure keeps the editor on this target; a superseded one left.
+      pickAgain = !started && qcDatastream.value?.id === target.managedId
+    }
+  } finally {
+    isStartingSession.value = false
+    // A failed entering start reopens the step with a new target; keep that.
+    if (!pickAgain && windowTarget.value === target) windowTarget.value = null
+  }
+}
+
+async function onWindowCancel() {
+  const target = windowTarget.value
+  if (!target) return
+  windowTarget.value = null
+  // A resume without an in-progress session leaves nothing to edit.
+  if (target.origin === 'resume' && !inProgressSession.value) {
+    await leaveEdit()
+  }
+}
+
+function onChooserCreate() {
+  showChooser.value = false
+  showCreateDatastream.value = true
+}
+
+// Cancelling create returns to the chooser it was opened from, if any.
+function onCreateCancel() {
+  showCreateDatastream.value = false
+  if (chooserOptions.value.length) showChooser.value = true
+}
+
+async function onCreateProcessingLevel(input: {
+  code: string
+  definition?: string
+  explanation?: string
+}) {
+  try {
+    const level = await createProcessingLevel(input)
+    // Add to the catalog so it shows in the picker and is immediately valid.
+    processingLevels.value = [...processingLevels.value, level]
+    Snackbar.success('Processing level added.')
+    return level
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not add the processing level.'
+    )
+    return null
+  }
+}
+
+async function onCreateDatastream(spec: CreateManagedDatastreamSpec) {
+  showCreateDatastream.value = false
+  qcPreferences.processingLevelId = spec.processingLevelId
+  try {
+    const { managedDatastream, history } = await createManaged(spec)
+    // Register the new history and datastream so it's hidden from the
+    // catalog and resolvable as an edit target without a reload.
+    addQcHistory(history)
+    datastreams.value = [...datastreams.value, managedDatastream]
+    Snackbar.success('Managed datastream created.')
+    openWindow({
+      managedId: managedDatastream.id,
+      name: managedDatastream.name,
+      source: spec.source,
+      sessions: [],
+      origin: 'entering',
+    })
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not create the datastream.'
+    )
+  }
+}
+
+async function onChooserDelete(option: ManagedDatastreamOption) {
+  try {
+    await deleteManaged(option.historyId, option.managed.id)
+    removeManagedDatastream(option.historyId, option.managed.id)
+    workingCopies.invalidate(option.managed.id)
+    chooserOptions.value = chooserOptions.value.filter(
+      (o) => o.historyId !== option.historyId
+    )
+    Snackbar.success('Managed datastream deleted.')
+  } catch (e) {
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not delete the managed datastream.'
+    )
+  }
+}
+
+// Only the chosen session and those built on it go; the managed datastream
+// and its other commits remain.
+async function onChooserDeleteSession(
+  option: ManagedDatastreamOption,
+  sessionId: string
+) {
+  try {
+    // Same input the confirmation previewed, so the two cannot disagree.
+    const deleted = await deleteSessionChain(
+      option.historyId,
+      collectDeletionChain(option.sessions, sessionId)
+    )
+    workingCopies.invalidate(option.managed.id)
+    const gone = new Set(deleted)
+    chooserOptions.value = chooserOptions.value.map((o) =>
+      o.historyId === option.historyId
+        ? { ...o, sessions: o.sessions.filter((s) => !gone.has(s.id)) }
+        : o
+    )
+    Snackbar.success(
+      deleted.length === 1
+        ? 'Session deleted.'
+        : `${deleted.length} sessions deleted.`
+    )
+  } catch (e) {
+    // Refresh regardless: a partial cascade leaves the chooser's copy wrong.
+    if (chooserSource.value) {
+      chooserOptions.value = await loadForSource(chooserSource.value.id)
+    }
+    Snackbar.error(
+      e instanceof Error ? e.message : 'Could not delete the session.'
+    )
+  }
+}
+
+defineExpose({ openFor, openNewSession, resume })
+</script>
diff --git a/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
new file mode 100644
index 00000000..03e0b7ab
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
@@ -0,0 +1,105 @@
+import { mount, flushPromises } from '@vue/test-utils'
+import { describe, it, expect, afterEach } from 'vitest'
+import { defineComponent, h } from 'vue'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import SessionWindowDialog from '@/components/EditData/SessionWindowDialog.vue'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+;(globalThis as any).visualViewport ||= {
+  addEventListener() {},
+  removeEventListener() {},
+  offsetLeft: 0,
+  offsetTop: 0,
+  width: 1024,
+  height: 768,
+  scale: 1,
+}
+
+const DatePickerStub = defineComponent({
+  props: { modelValue: { type: Date, required: true } },
+  emits: ['update:modelValue'],
+  setup(props, { attrs }) {
+    return () =>
+      h('input', {
+        ...attrs,
+        value: props.modelValue.toISOString(),
+      })
+  },
+})
+
+const source = {
+  phenomenonBeginTime: '2025-01-01T00:00:00Z',
+  phenomenonEndTime: '2025-12-31T00:00:00Z',
+}
+const sessions = [
+  {
+    status: 'committed',
+    phenomenonTimeStart: '2025-01-01T00:00:00Z',
+    phenomenonTimeEnd: '2025-05-01T00:00:00Z',
+  },
+]
+
+let wrapper: ReturnType<typeof mount> | null = null
+afterEach(() => {
+  wrapper?.unmount()
+  wrapper = null
+})
+
+const mountDialog = (props: Record<string, unknown> = {}) => {
+  wrapper = mount(SessionWindowDialog, {
+    props: { managedName: 'Temp (QC)', source, sessions, ...props },
+    global: {
+      plugins: [createTestVuetify()],
+      stubs: { DatePickerField: DatePickerStub },
+    },
+    attachTo: document.body,
+  })
+  return wrapper
+}
+
+const pickers = (w: ReturnType<typeof mount>) =>
+  w.findAllComponents(DatePickerStub)
+
+describe('SessionWindowDialog', () => {
+  it('prefills the window from the end of the committed history', () => {
+    const w = mountDialog()
+    const [from, to] = pickers(w)
+    expect((from!.props('modelValue') as Date).toISOString()).toBe('2025-05-01T00:00:00.000Z')
+    expect((to!.props('modelValue') as Date).toISOString()).toBe('2025-12-31T00:00:00.000Z')
+  })
+
+  it('shows the committed history range', () => {
+    const w = mountDialog()
+    expect(w.find('[data-testid="session-window-committed"]').text()).not.toMatch(/Nothing committed/)
+  })
+
+  it('says when nothing is committed yet', () => {
+    const w = mountDialog({ sessions: [] })
+    expect(w.find('[data-testid="session-window-committed"]').text()).toMatch(/Nothing committed yet/)
+  })
+
+  it('emits the window on Start', async () => {
+    const w = mountDialog()
+    await w.find('[data-testid="session-window-start"]').trigger('click')
+    const [window] = w.emitted('confirm')![0] as [{ begin: Date; end: Date }]
+    expect(window.begin.toISOString()).toBe('2025-05-01T00:00:00.000Z')
+    expect(window.end.toISOString()).toBe('2025-12-31T00:00:00.000Z')
+  })
+
+  it('blocks Start and explains a gap after the history', async () => {
+    const w = mountDialog()
+    pickers(w)[0]!.vm.$emit('update:modelValue', new Date('2025-07-01T00:00:00Z'))
+    await flushPromises()
+    expect(w.find('[data-testid="session-window-error"]').text()).toMatch(/gap after/)
+    expect(w.find('[data-testid="session-window-start"]').attributes('disabled')).toBeDefined()
+  })
+
+  it('emits cancel', async () => {
+    const w = mountDialog()
+    await w.find('[data-testid="session-window-cancel"]').trigger('click')
+    expect(w.emitted('cancel')).toHaveLength(1)
+  })
+})
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
new file mode 100644
index 00000000..84a8b57e
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
@@ -0,0 +1,443 @@
+import { mount, flushPromises, enableAutoUnmount } from '@vue/test-utils'
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
+import { defineComponent, h } from 'vue'
+import { VDialog } from 'vuetify/components'
+import { createTestPinia } from '@/utils/test/pinia'
+import { createTestVuetify } from '@/utils/test/vuetify'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+// jsdom has no visualViewport; Vuetify's overlay positioning reads it.
+;(globalThis as any).visualViewport ||= {
+  addEventListener() {},
+  removeEventListener() {},
+  offsetLeft: 0,
+  offsetTop: 0,
+  width: 1024,
+  height: 768,
+  scale: 1,
+}
+
+const {
+  loadForSource,
+  deleteManaged,
+  deleteSessionChain,
+  enterEdit,
+  startSessionOver,
+  leaveEdit,
+  createManaged,
+  addQcHistory,
+  removeManagedDatastream,
+  invalidate,
+  qcDatastream,
+  datastreams,
+  processingLevels,
+  sourceDatastream,
+  sessions,
+  success,
+  error,
+} = vi.hoisted(() => {
+  const { ref: r } = require('vue') as typeof import('vue')
+  return {
+    loadForSource: vi.fn(),
+    deleteManaged: vi.fn(),
+    deleteSessionChain: vi.fn(),
+    enterEdit: vi.fn(),
+    startSessionOver: vi.fn(),
+    leaveEdit: vi.fn(),
+    createManaged: vi.fn(),
+    addQcHistory: vi.fn(),
+    removeManagedDatastream: vi.fn(),
+    invalidate: vi.fn(),
+    qcDatastream: r<any>(null),
+    datastreams: r<any[]>([]),
+    processingLevels: r<any[]>([]),
+    sourceDatastream: r<any>(null),
+    sessions: r<any[]>([]),
+    success: vi.fn(),
+    error: vi.fn(),
+  }
+})
+
+vi.mock('@/store/dataVisualization', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useDataVisStore: defineStore('dataVisualization', () => ({
+      qcDatastream,
+      datastreams,
+      processingLevels,
+      addQcHistory,
+      removeManagedDatastream,
+    })),
+  }
+})
+
+vi.mock('@/store/qcSession', async () => {
+  const { defineStore } = await import('pinia')
+  const { computed } = await import('vue')
+  return {
+    useQcSessionStore: defineStore('qcSession', () => ({
+      sourceDatastream,
+      sessions,
+      inProgressSession: computed(
+        () => sessions.value.find((s) => s.status === 'in_progress') ?? null
+      ),
+    })),
+  }
+})
+
+vi.mock('@/store/workingCopies', () => ({
+  useWorkingCopiesStore: () => ({ invalidate }),
+}))
+
+vi.mock('@/composables/useManagedDatastreams', () => ({
+  useManagedDatastreams: () => ({
+    loadForSource,
+    deleteManaged,
+    deleteSessionChain,
+  }),
+}))
+
+vi.mock('@/composables/useEditEntry', () => ({
+  useEditEntry: () => ({ enterEdit, startSessionOver, leaveEdit }),
+}))
+
+vi.mock('@/composables/useCreateManagedDatastream', () => ({
+  useCreateManagedDatastream: () => ({ create: createManaged }),
+}))
+
+vi.mock('@/composables/useProcessingLevels', () => ({
+  useProcessingLevels: () => ({ createProcessingLevel: vi.fn() }),
+}))
+
+vi.mock('@/composables/useWorkspacePermissions', () => ({
+  useWorkspacePermissions: () => ({
+    canCreateDatastream: () => true,
+    roleName: () => 'Owner',
+  }),
+}))
+
+vi.mock('@uwrl/qc-utils', () => ({ Snackbar: { success, error } }))
+
+import StartEditingFlow from '@/components/EditData/StartEditingFlow.vue'
+
+const stub = (name: string, testId: string, emits: string[]) =>
+  defineComponent({
+    name,
+    emits,
+    setup: () => () => h('div', { 'data-testid': testId }),
+  })
+
+const ChooserStub = stub('StartEditingDialog', 'chooser-stub', [
+  'edit',
+  'delete',
+  'deleteSession',
+  'create',
+  'cancel',
+])
+const CreateStub = stub('CreateDatastreamForm', 'create-stub', [
+  'cancel',
+  'confirm',
+])
+const WindowStub = stub('SessionWindowDialog', 'window-stub', [
+  'confirm',
+  'cancel',
+])
+
+interface FlowApi {
+  openFor(source: unknown): Promise<void>
+  openNewSession(): void
+  resume(managedId: string): Promise<void>
+}
+
+const source = {
+  id: 'src',
+  name: 'Raw Temp',
+  phenomenonBeginTime: '2025-01-01T00:00:00Z',
+  phenomenonEndTime: '2025-12-31T00:00:00Z',
+}
+const committed = {
+  id: 's-0',
+  status: 'committed',
+  phenomenonTimeStart: '2025-01-01T00:00:00Z',
+  phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+}
+const inProgress = { ...committed, id: 's-1', status: 'in_progress' }
+const optionWith = (list: unknown[]) => ({
+  historyId: 'h',
+  managed: { id: 'mgd', name: 'Temp (QC)' },
+  sessions: list,
+})
+const window = {
+  begin: new Date('2025-02-01T00:00:00Z'),
+  end: new Date('2025-03-01T00:00:00Z'),
+}
+
+const mountFlow = () => {
+  const wrapper = mount(StartEditingFlow, {
+    global: {
+      plugins: [createTestPinia(), createTestVuetify()],
+      stubs: {
+        StartEditingDialog: ChooserStub,
+        CreateDatastreamForm: CreateStub,
+        SessionWindowDialog: WindowStub,
+      },
+    },
+    attachTo: document.body,
+  })
+  return { wrapper, flow: wrapper.vm as unknown as FlowApi }
+}
+
+// v-dialog teleports its content out of the wrapper.
+const present = (testId: string) =>
+  !!document.querySelector(`[data-testid="${testId}"]`)
+
+// Dialog order: chooser, create form, session window.
+const dialogOpen = (wrapper: ReturnType<typeof mountFlow>['wrapper'], i: number) =>
+  wrapper.findAllComponents(VDialog)[i].props('modelValue')
+
+enableAutoUnmount(afterEach)
+afterEach(() => {
+  document.body.innerHTML = ''
+})
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  qcDatastream.value = null
+  datastreams.value = []
+  processingLevels.value = []
+  sourceDatastream.value = null
+  sessions.value = []
+  enterEdit.mockResolvedValue('editing')
+  startSessionOver.mockResolvedValue(true)
+  leaveEdit.mockResolvedValue(undefined)
+})
+
+describe('StartEditingFlow', () => {
+  it('opens the chooser when the source has managed datastreams', async () => {
+    loadForSource.mockResolvedValue([optionWith([committed])])
+    const { flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    expect(loadForSource).toHaveBeenCalledWith('src')
+    expect(present('chooser-stub')).toBe(true)
+    expect(present('create-stub')).toBe(false)
+  })
+
+  it('goes straight to the create form when the source has none', async () => {
+    loadForSource.mockResolvedValue([])
+    const { flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    expect(present('create-stub')).toBe(true)
+    expect(present('chooser-stub')).toBe(false)
+  })
+
+  it('continues an in-progress session without asking for a window', async () => {
+    const option = optionWith([committed, inProgress])
+    loadForSource.mockResolvedValue([option])
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    wrapper.findComponent(ChooserStub).vm.$emit('edit', option)
+    await flushPromises()
+    expect(enterEdit).toHaveBeenCalledTimes(1)
+    expect(enterEdit.mock.calls[0]).toEqual(['mgd', undefined])
+    expect(present('window-stub')).toBe(false)
+  })
+
+  it('asks for a window before starting a new session', async () => {
+    const option = optionWith([committed])
+    loadForSource.mockResolvedValue([option])
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    wrapper.findComponent(ChooserStub).vm.$emit('edit', option)
+    await flushPromises()
+    expect(present('window-stub')).toBe(true)
+    expect(enterEdit).not.toHaveBeenCalled()
+
+    wrapper.findComponent(WindowStub).vm.$emit('confirm', window)
+    await flushPromises()
+    expect(enterEdit).toHaveBeenCalledWith('mgd', window)
+    expect(present('window-stub')).toBe(false)
+  })
+
+  it('opens the window step when a resume finds no session', async () => {
+    enterEdit.mockResolvedValue('needs-window')
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    sourceDatastream.value = source
+    const { flow } = mountFlow()
+    await flow.resume('mgd')
+    await flushPromises()
+    expect(enterEdit.mock.calls[0]).toEqual(['mgd', undefined])
+    expect(present('window-stub')).toBe(true)
+  })
+
+  it('leaves the editor when the window is cancelled with no sessions at all', async () => {
+    enterEdit.mockResolvedValue('needs-window')
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    sourceDatastream.value = source
+    const { wrapper, flow } = mountFlow()
+    await flow.resume('mgd')
+    await flushPromises()
+    wrapper.findComponent(WindowStub).vm.$emit('cancel')
+    await flushPromises()
+    expect(leaveEdit).toHaveBeenCalled()
+    expect(present('window-stub')).toBe(false)
+  })
+
+  it('leaves the editor when the window is closed with Esc or the scrim after an empty resume', async () => {
+    enterEdit.mockResolvedValue('needs-window')
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    sourceDatastream.value = source
+    const { wrapper, flow } = mountFlow()
+    await flow.resume('mgd')
+    await flushPromises()
+    wrapper.findAllComponents(VDialog)[2].vm.$emit('update:modelValue', false)
+    await flushPromises()
+    expect(leaveEdit).toHaveBeenCalled()
+    expect(present('window-stub')).toBe(false)
+  })
+
+  it('leaves the editor when the window is cancelled after a resume over committed-only sessions', async () => {
+    enterEdit.mockResolvedValue('needs-window')
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    sourceDatastream.value = source
+    sessions.value = [committed]
+    const { wrapper, flow } = mountFlow()
+    await flow.resume('mgd')
+    await flushPromises()
+    wrapper.findComponent(WindowStub).vm.$emit('cancel')
+    await flushPromises()
+    expect(leaveEdit).toHaveBeenCalled()
+  })
+
+  it('stays in the editor when the footer window is cancelled', async () => {
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    sourceDatastream.value = source
+    const { wrapper, flow } = mountFlow()
+    flow.openNewSession()
+    await flushPromises()
+    wrapper.findComponent(WindowStub).vm.$emit('cancel')
+    await flushPromises()
+    expect(leaveEdit).not.toHaveBeenCalled()
+    expect(present('window-stub')).toBe(false)
+  })
+
+  it('starts the session in the open editor when a resume window is confirmed', async () => {
+    enterEdit.mockResolvedValue('needs-window')
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    sourceDatastream.value = source
+    const { wrapper, flow } = mountFlow()
+    await flow.resume('mgd')
+    await flushPromises()
+    wrapper.findComponent(WindowStub).vm.$emit('confirm', window)
+    await flushPromises()
+    expect(startSessionOver).toHaveBeenCalledWith(window)
+    expect(enterEdit).toHaveBeenCalledTimes(1)
+  })
+
+  it('keeps the window open to pick again when starting the session fails', async () => {
+    startSessionOver.mockResolvedValue(false)
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    sourceDatastream.value = source
+    const { wrapper, flow } = mountFlow()
+    flow.openNewSession()
+    await flushPromises()
+    wrapper.findComponent(WindowStub).vm.$emit('confirm', window)
+    await flushPromises()
+    expect(present('window-stub')).toBe(true)
+  })
+
+  it('remounts the window step for a new target, so its defaults are fresh', async () => {
+    const option = optionWith([committed])
+    loadForSource.mockResolvedValue([option])
+    // The entering start fails and reopens the step on the open editor.
+    enterEdit.mockResolvedValue('needs-window')
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    sourceDatastream.value = source
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    wrapper.findComponent(ChooserStub).vm.$emit('edit', option)
+    await flushPromises()
+    const first = wrapper.findComponent(WindowStub).vm
+
+    wrapper.findComponent(WindowStub).vm.$emit('confirm', window)
+    await flushPromises()
+
+    expect(present('window-stub')).toBe(true)
+    expect(wrapper.findComponent(WindowStub).vm).not.toBe(first)
+  })
+
+  it('closes the window when the start left the editor', async () => {
+    startSessionOver.mockImplementation(async () => {
+      qcDatastream.value = null
+      return false
+    })
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    sourceDatastream.value = source
+    const { wrapper, flow } = mountFlow()
+    flow.openNewSession()
+    await flushPromises()
+    wrapper.findComponent(WindowStub).vm.$emit('confirm', window)
+    await flushPromises()
+    expect(present('window-stub')).toBe(false)
+  })
+
+  it('leaves the editor when a resume needs a window but the source is missing', async () => {
+    enterEdit.mockResolvedValue('needs-window')
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    sourceDatastream.value = null
+    const { flow } = mountFlow()
+    await flow.resume('mgd')
+    await flushPromises()
+    expect(leaveEdit).toHaveBeenCalled()
+    expect(error).toHaveBeenCalled()
+    expect(present('window-stub')).toBe(false)
+  })
+
+  it('starts a new session in the open editor from the footer', async () => {
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    sourceDatastream.value = source
+    sessions.value = [committed]
+    const { wrapper, flow } = mountFlow()
+    flow.openNewSession()
+    await flushPromises()
+    wrapper.findComponent(WindowStub).vm.$emit('confirm', window)
+    await flushPromises()
+    expect(startSessionOver).toHaveBeenCalledWith(window)
+    expect(enterEdit).not.toHaveBeenCalled()
+    expect(present('window-stub')).toBe(false)
+  })
+
+  it('asks for a window for a newly created managed datastream', async () => {
+    loadForSource.mockResolvedValue([])
+    const managed = { id: 'mgd-new', name: 'Temp (QC new)' }
+    createManaged.mockResolvedValue({
+      managedDatastream: managed,
+      history: { id: 'h-new' },
+    })
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    wrapper
+      .findComponent(CreateStub)
+      .vm.$emit('confirm', { source, processingLevelId: 'pl' })
+    await flushPromises()
+
+    expect(addQcHistory).toHaveBeenCalledWith({ id: 'h-new' })
+    expect(datastreams.value).toContainEqual(managed)
+    expect(success).toHaveBeenCalledWith('Managed datastream created.')
+    // Closed dialogs keep their content mounted, so check the dialog itself.
+    expect(dialogOpen(wrapper, 1)).toBe(false)
+    expect(present('window-stub')).toBe(true)
+
+    wrapper.findComponent(WindowStub).vm.$emit('confirm', window)
+    await flushPromises()
+    expect(enterEdit).toHaveBeenCalledWith('mgd-new', window)
+  })
+})
diff --git a/apps/qc-app/src/components/Navigation/NavigationRail.vue b/apps/qc-app/src/components/Navigation/NavigationRail.vue
index 5c0b53cb..30ec1947 100644
--- a/apps/qc-app/src/components/Navigation/NavigationRail.vue
+++ b/apps/qc-app/src/components/Navigation/NavigationRail.vue
@@ -68,7 +68,7 @@
           </button>
         </template>
         <span v-if="item.title === 'Edit' && !qcDatastream">
-          Edit: select a datastream for quality control before navigating here.
+          Edit a datastream from its row first.
         </span>
         <span v-else>{{ item.title }}</span>
       </v-tooltip>
@@ -190,11 +190,12 @@ import { useWorkspaceStore } from '@/store/workspaces'
 import { usePlotlyStore } from '@/store/plotly'
 import { useQcSessionStore } from '@/store/qcSession'
 import { useEditSession } from '@/composables/useEditSession'
+import { useEditEntry } from '@/composables/useEditEntry'
 
 const { onRailItemClicked } = useUIStore()
 const { selectedDrawer, isDrawerOpen, currentView } = storeToRefs(useUIStore())
 const { resetState } = useDataVisStore()
-const { qcDatastream, qcDatastreamId } = storeToRefs(useDataVisStore())
+const { qcDatastream } = storeToRefs(useDataVisStore())
 const { hs } = storeToRefs(useHydroServer())
 const workspaceStore = useWorkspaceStore()
 const { selectedWorkspace } = storeToRefs(workspaceStore)
@@ -202,11 +203,12 @@ const { redraw } = usePlotlyStore()
 const { inProgressSession } = storeToRefs(useQcSessionStore())
 const { hasUnsavedChanges, unsavedEditCount, saveDraft, discardUnsavedEdits } =
   useEditSession()
+const { leaveEdit } = useEditEntry()
 
 const needsExitConfirm = computed(
   () => currentView.value === DrawerType.Edit && hasUnsavedChanges.value
 )
-// Entering Edit through the rail opens no session, so there is nothing to save to.
+// With no in-progress session there is nothing to save to.
 const canSave = computed(() => !!inProgressSession.value)
 
 const showExitConfirm = ref(false)
@@ -272,12 +274,10 @@ async function discardAndContinue() {
   await continueExit()
 }
 
-function goHome() {
+async function goHome() {
   resetState()
-  qcDatastreamId.value = null
-  currentView.value = DrawerType.Select
-  selectedDrawer.value = DrawerType.Select
-  isDrawerOpen.value = true
+  // Clears the resume pointer, or the reload would reopen the editor.
+  await leaveEdit()
   window.location.assign('/')
 }
 
@@ -288,6 +288,10 @@ const items = ref([
 
 function onMainRailItemClicked(item: DrawerType) {
   if (item === DrawerType.Edit && !qcDatastream.value) return
+  if (item === DrawerType.Select && currentView.value === DrawerType.Edit) {
+    guardExit(() => leaveEdit())
+    return
+  }
   guardExit(() => onRailItemClicked(item))
 }
 
@@ -303,10 +307,7 @@ async function onSwitchWorkspace() {
   // filters to router.replace, racing our push and stranding the user.
   // `switch=1` prevents the Workspaces picker from auto-redirecting back.
   await router.push({ name: 'Workspaces', query: { switch: '1' } })
-  qcDatastreamId.value = null
-  currentView.value = DrawerType.Select
-  selectedDrawer.value = DrawerType.Select
-  isDrawerOpen.value = true
+  await leaveEdit()
 }
 </script>
 
diff --git a/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts b/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
new file mode 100644
index 00000000..efee9def
--- /dev/null
+++ b/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
@@ -0,0 +1,211 @@
+import { mount, flushPromises } from '@vue/test-utils'
+import { defineComponent, h } from 'vue'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import { VLayout } from 'vuetify/components'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+const {
+  qcDatastream,
+  qcDatastreamId,
+  resetState,
+  clearEditTarget,
+  currentView,
+  selectedDrawer,
+  isDrawerOpen,
+  resumeDatastreamId,
+  hasUnsavedChanges,
+  push,
+  assign,
+} = vi.hoisted(() => {
+  const { ref: r, computed: c } = require('vue') as typeof import('vue')
+  const qcDatastreamId = r<string | null>(null)
+  return {
+    qcDatastreamId,
+    qcDatastream: c(() =>
+      qcDatastreamId.value ? { id: qcDatastreamId.value } : null
+    ),
+    resetState: vi.fn(() => {
+      qcDatastreamId.value = null
+    }),
+    clearEditTarget: vi.fn(async () => {
+      qcDatastreamId.value = null
+    }),
+    currentView: r('Edit'),
+    selectedDrawer: r('Edit'),
+    isDrawerOpen: r(true),
+    resumeDatastreamId: r<string | null>(null),
+    hasUnsavedChanges: r(false),
+    push: vi.fn(async () => {}),
+    assign: vi.fn(),
+  }
+})
+
+vi.mock('@/store/dataVisualization', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useDataVisStore: defineStore('dataVisualization', () => ({
+      qcDatastream,
+      qcDatastreamId,
+      resetState,
+      setEditTarget: vi.fn(),
+      clearEditTarget,
+    })),
+  }
+})
+
+vi.mock('@/store/userInterface', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    DrawerType: { Edit: 'Edit', Select: 'Select', None: '' },
+    useUIStore: defineStore('userInterface', () => ({
+      currentView,
+      selectedDrawer,
+      isDrawerOpen,
+      onRailItemClicked: vi.fn(),
+    })),
+  }
+})
+
+vi.mock('@/store/qcSession', async () => {
+  const { defineStore } = await import('pinia')
+  const { ref } = await import('vue')
+  return {
+    useQcSessionStore: defineStore('qcSession', () => ({
+      resumeDatastreamId,
+      inProgressSession: ref(null),
+    })),
+  }
+})
+
+vi.mock('@/store/hydroserver', async () => {
+  const { defineStore } = await import('pinia')
+  const { ref } = await import('vue')
+  return {
+    useHydroServer: defineStore('hydroserver', () => ({ hs: ref({}) })),
+  }
+})
+
+vi.mock('@/store/workspaces', async () => {
+  const { defineStore } = await import('pinia')
+  const { ref } = await import('vue')
+  return {
+    useWorkspaceStore: defineStore('workspaces', () => ({
+      selectedWorkspace: ref(null),
+      clearSelection: vi.fn(),
+    })),
+  }
+})
+
+vi.mock('@/store/plotly', () => ({
+  usePlotlyStore: () => ({ redraw: vi.fn() }),
+}))
+
+vi.mock('@/composables/useEditSession', async () => {
+  const { ref } = await import('vue')
+  return {
+    ResumeSupersededError: class extends Error {},
+    useEditSession: () => ({
+      hasUnsavedChanges,
+      unsavedEditCount: ref(0),
+      saveDraft: vi.fn(),
+      discardUnsavedEdits: vi.fn(),
+      beginEditing: vi.fn(),
+      startSession: vi.fn(),
+      needsSession: ref(false),
+      needsHistory: ref(false),
+    }),
+  }
+})
+
+vi.mock('@/router/router', () => ({ default: { push } }))
+
+vi.mock('@uwrl/qc-utils', () => ({
+  Snackbar: { success: vi.fn(), error: vi.fn(), info: vi.fn() },
+}))
+
+vi.mock('@/components/Navigation/SelectDrawer.vue', () => ({
+  default: { name: 'SelectDrawer', render: () => null },
+}))
+vi.mock('@/components/Navigation/PerformanceCalibration.vue', () => ({
+  default: { name: 'PerformanceCalibration', render: () => null },
+}))
+
+import NavigationRail from '@/components/Navigation/NavigationRail.vue'
+
+function mountRail() {
+  return mount(
+    { render: () => h(VLayout, () => [h(NavigationRail)]) },
+    {
+      global: {
+        plugins: [createTestPinia(), createTestVuetify()],
+        stubs: {
+          VTooltip: defineComponent({
+            setup: (_, { slots }) => () => slots.activator?.({ props: {} }),
+          }),
+          VDialog: true,
+        },
+      },
+    }
+  )
+}
+
+describe('NavigationRail leaving the editor', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    qcDatastreamId.value = 'm-1'
+    resumeDatastreamId.value = 'm-1'
+    currentView.value = 'Edit'
+    selectedDrawer.value = 'Edit'
+    hasUnsavedChanges.value = false
+    vi.stubGlobal('location', { ...window.location, assign })
+  })
+
+  it('Home clears the resume pointer before reloading', async () => {
+    const wrapper = mountRail()
+    await wrapper.find('button[aria-label="Home"]').trigger('click')
+    await flushPromises()
+
+    expect(resumeDatastreamId.value).toBeNull()
+    expect(qcDatastreamId.value).toBeNull()
+    expect(currentView.value).toBe('Select')
+    expect(assign).toHaveBeenCalledWith('/')
+    wrapper.unmount()
+  })
+
+  it('switching workspace navigates first, then clears the resume pointer', async () => {
+    let pointerAtPush: string | null = null
+    push.mockImplementationOnce(async () => {
+      pointerAtPush = resumeDatastreamId.value
+    })
+    const wrapper = mountRail()
+    await wrapper.find('[data-testid="nav-rail-workspaces"]').trigger('click')
+    await flushPromises()
+
+    expect(push).toHaveBeenCalledWith({
+      name: 'Workspaces',
+      query: { switch: '1' },
+    })
+    expect(pointerAtPush).toBe('m-1')
+    expect(resumeDatastreamId.value).toBeNull()
+    expect(clearEditTarget).toHaveBeenCalled()
+    expect(currentView.value).toBe('Select')
+    wrapper.unmount()
+  })
+
+  it('asks before leaving with unsaved edits', async () => {
+    hasUnsavedChanges.value = true
+    const wrapper = mountRail()
+    await wrapper.find('[data-testid="nav-rail-workspaces"]').trigger('click')
+    await flushPromises()
+
+    expect(push).not.toHaveBeenCalled()
+    expect(resumeDatastreamId.value).toBe('m-1')
+    wrapper.unmount()
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index fc1d902f..3395bab8 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -9,76 +9,23 @@
       >
         <v-icon icon="mdi-chart-line" color="primary" size="24" class="mr-1" />
         <div class="d-flex flex-column flex-1-1-auto" style="min-width: 0">
-          <span v-if="qcDatastream" class="text-title-medium font-weight-bold">
-            {{ qcDatastream.name }}
-          </span>
-          <span v-else class="text-title-medium font-weight-bold">
-            No datastream plotted
+          <span class="text-title-medium font-weight-bold">
+            <template v-if="plottedDatastreams.length">
+              {{ plottedDatastreams.length }}
+              datastream{{ plottedDatastreams.length === 1 ? '' : 's' }}
+              plotted
+            </template>
+            <template v-else>No datastream plotted</template>
           </span>
           <span class="text-body-small text-medium-emphasis">
-            <template v-if="qcDatastream">
-              Quality-control target: preview ready
+            <template v-if="plottedDatastreams.length">
+              Use the pencil button on a row to edit a datastream
             </template>
             <template v-else>
-              Select one from the table below to preview its data here
+              Select datastreams from the table below to preview their data
             </template>
           </span>
         </div>
-
-        <v-chip
-          v-if="qcDatastream"
-          size="small"
-          variant="tonal"
-          :color="canEditWorkspace ? 'primary' : 'grey'"
-          :prepend-icon="canEditWorkspace ? 'mdi-pencil' : 'mdi-eye-outline'"
-          class="mr-2 align-self-center"
-          :title="`Your role on this workspace: ${workspaceRole}`"
-        >
-          {{ workspaceRole }}
-        </v-chip>
-
-        <v-tooltip
-          v-if="!qcDatastream"
-          location="start"
-          text="Pick a datastream below (radio button) to enable editing"
-        >
-          <template #activator="{ props: tooltipProps }">
-            <div v-bind="tooltipProps" class="ml-auto">
-              <v-btn
-                color="primary"
-                variant="flat"
-                prepend-icon="mdi-pencil"
-                append-icon="mdi-arrow-right"
-                disabled
-              >
-                Start editing
-              </v-btn>
-            </div>
-          </template>
-        </v-tooltip>
-
-        <v-tooltip
-          v-else
-          location="start"
-          :disabled="canEditWorkspace"
-          :text="`Your role on this workspace (${workspaceRole}) is read-only. Ask a workspace owner for an editor role to make edits.`"
-        >
-          <template #activator="{ props: tooltipProps }">
-            <div v-bind="tooltipProps" class="ml-auto">
-              <v-btn
-                color="primary"
-                variant="flat"
-                prepend-icon="mdi-pencil"
-                append-icon="mdi-arrow-right"
-                :loading="isCreating"
-                :disabled="!canEditWorkspace"
-                @click="openEditChooser"
-              >
-                Start editing
-              </v-btn>
-            </div>
-          </template>
-        </v-tooltip>
       </div>
 
       <v-divider />
@@ -101,40 +48,11 @@
     </v-card>
 
     <v-card class="select-view__table d-flex flex-column flex-1-1-0 overflow-hidden">
-      <DataVisDatasetsTable class="fill-height" />
-    </v-card>
-
-    <v-dialog v-model="showChooser" max-width="640">
-      <StartEditingDialog
-        v-if="chooserSource"
-        :source="chooserSource"
-        :options="chooserOptions"
-        :loading="chooserLoading"
-        @edit="editManaged"
-        @delete="onChooserDelete"
-        @delete-session="onChooserDeleteSession"
-        @create="onChooserCreate"
-        @cancel="showChooser = false"
+      <DataVisDatasetsTable
+        class="fill-height"
+        @edit="startEditing?.openFor($event)"
       />
-    </v-dialog>
-
-    <v-dialog v-model="showCreateDatastream" max-width="560" persistent>
-      <v-card v-if="qcDatastream" rounded="lg">
-        <CreateDatastreamForm
-          :source="qcDatastream"
-          :processing-levels="processingLevels"
-          :default-processing-level-id="qcPreferences.processingLevelId"
-          :on-create-processing-level="onCreateProcessingLevel"
-          :permission-error="
-            canCreateDatastreamHere
-              ? ''
-              : `Your role on this workspace (${workspaceRole}) can't create datastreams. Ask a workspace owner for an editor role.`
-          "
-          @cancel="onCreateCancel"
-          @confirm="onCreateDatastream"
-        />
-      </v-card>
-    </v-dialog>
+    </v-card>
   </div>
 
   <div
@@ -258,7 +176,7 @@
             :style="{ height: plottedHeight + 'px' }"
           >
             <div class="rounded border bg-surface overflow-hidden">
-              <PlottedDatastreams lock-qc />
+              <PlottedDatastreams />
             </div>
           </div>
         </section>
@@ -341,14 +259,13 @@
                   variant="flat"
                   color="primary"
                   prepend-icon="mdi-plus"
-                  :disabled="!canEditWorkspace || isStartingSession"
-                  :loading="isStartingSession"
+                  :disabled="!canEditWorkspace"
                   :title="
                     canEditWorkspace
-                      ? 'Start a new session over the current time range'
+                      ? 'Start a new session'
                       : `Your role on this workspace (${workspaceRole}) is read-only.`
                   "
-                  @click="startSessionForWindow"
+                  @click="startEditing?.openNewSession()"
                 >
                   New session
                 </v-btn>
@@ -358,7 +275,7 @@
                   size="small"
                   variant="tonal"
                   prepend-icon="mdi-close"
-                  :disabled="isSavingDraft || isCommitting || isStartingSession"
+                  :disabled="isSavingDraft || isCommitting"
                   @click="requestClose"
                 >
                   Close
@@ -525,6 +442,8 @@
       </v-card>
     </v-dialog>
 </div>
+
+  <StartEditingFlow ref="startEditing" />
 </template>
 
 <script setup lang="ts">
@@ -533,37 +452,29 @@ import DataVisualization from '@/components/VisualizeData/DataVisualization.vue'
 import EditHistory from '@/components/EditData/EditHistory.vue'
 import OperationPanel from '@/components/EditData/OperationPanel.vue'
 import EditDrawer from '@/components/Navigation/EditDrawer.vue'
+import StartEditingFlow from '@/components/EditData/StartEditingFlow.vue'
 
 import { useDataVisStore } from '@/store/dataVisualization'
 import { storeToRefs } from 'pinia'
 import { useUIStore, DrawerType } from '@/store/userInterface'
-import { computed, onUnmounted, ref, useTemplateRef, watch } from 'vue'
+import {
+  computed,
+  onMounted,
+  onUnmounted,
+  ref,
+  useTemplateRef,
+  watch,
+} from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import PlottedDatastreams from './VisualizeData/PlottedDatastreams.vue'
 import { usePlotlyStore } from '@/store/plotly'
-import {
-  ResumeSupersededError,
-  useEditSession,
-} from '@/composables/useEditSession'
+import { useEditSession } from '@/composables/useEditSession'
+import { useEditEntry } from '@/composables/useEditEntry'
 import { useUnsavedChangesWarning } from '@/composables/useUnsavedChangesWarning'
 import { useResumeEditSession } from '@/composables/useResumeEditSession'
 import { useQcSessionStore } from '@/store/qcSession'
-import { useQcPreferencesStore } from '@/store/qcPreferences'
-import CreateDatastreamForm from '@/components/EditData/CreateDatastreamForm.vue'
-import StartEditingDialog from '@/components/EditData/StartEditingDialog.vue'
-import {
-  useCreateManagedDatastream,
-  type CreateManagedDatastreamSpec,
-} from '@/composables/useCreateManagedDatastream'
-import {
-  useManagedDatastreams,
-  type ManagedDatastreamOption,
-} from '@/composables/useManagedDatastreams'
 import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
-import { useProcessingLevels } from '@/composables/useProcessingLevels'
-import type { Datastream } from '@hydroserver/client'
 import { Snackbar } from '@uwrl/qc-utils'
-import { collectDeletionChain } from '@/utils/sessionGraph'
 import {
   decodeShareState,
   encodeShareState,
@@ -574,17 +485,13 @@ import { isSnapshotId, parseSnapshotId } from '@/utils/snapshotId'
 import { useHistorySnapshots } from '@/composables/useHistorySnapshots'
 import { useWorkspaceStore } from '@/store/workspaces'
 import { useResizable, usePersistedFlag } from '@/composables/useResizable'
-import { useWorkingCopiesStore } from '@/store/workingCopies'
 
 const { resetState } = useDataVisStore()
-const workingCopies = useWorkingCopiesStore()
 const { toggleSnapshot } = useHistorySnapshots()
 const {
   plottedDatastreams,
-  qcDatastream,
+  qcDatastreamId,
   datastreams,
-  managedDatastreamIds,
-  processingLevels,
   things,
   beginDate,
   endDate,
@@ -593,7 +500,7 @@ const {
   selectedObservedPropertyNames,
   selectedProcessingLevelNames,
 } = storeToRefs(useDataVisStore())
-const { currentView, selectedDrawer, isDrawerOpen, selectedOperation } =
+const { currentView, selectedDrawer, selectedOperation } =
   storeToRefs(useUIStore())
 const { selectedWorkspaceId } = storeToRefs(useWorkspaceStore())
 const {
@@ -609,22 +516,12 @@ const {
   tooltipsMaxDataPoints,
 } = storeToRefs(usePlotlyStore())
 const { redraw } = usePlotlyStore()
-const {
-  setPlottedDatastreams,
-  adoptManagedDatastream,
-  releaseManagedDatastream,
-  addQcHistory,
-  removeManagedDatastream,
-} = useDataVisStore()
+const { setPlottedDatastreams } = useDataVisStore()
 
 const {
-  beginEditing,
-  startSession,
   saveDraft,
   discardUnsavedEdits,
   commit,
-  needsSession,
-  needsHistory,
   hasUnsavedChanges,
   unsavedEditCount,
   viewSession,
@@ -655,37 +552,44 @@ async function onViewSession(sessionId: string) {
     isViewingSession.value = false
   }
 }
-const qcSessionStore = useQcSessionStore()
-const { isReadOnly, inProgressSession, resumeDatastreamId } =
-  storeToRefs(qcSessionStore)
-const { create: createManaged } = useCreateManagedDatastream()
-const qcPreferences = useQcPreferencesStore()
-const { canEdit, canCreateDatastream, roleName } = useWorkspacePermissions()
-const { createProcessingLevel } = useProcessingLevels()
-const { loadForSource, deleteManaged, deleteSessionChain } =
-  useManagedDatastreams()
-
-// Permission gating: QC editing writes to the selected workspace (creates
-// the managed datastream, pushes observations). Gate the editor entry
-// points so a read-only collaborator gets a clear disabled state and an
-// explanation instead of a 403 mid-flow.
+const { isReadOnly, inProgressSession, viewedSession, resumeDatastreamId } =
+  storeToRefs(useQcSessionStore())
+const { canEdit, roleName } = useWorkspacePermissions()
+const { leaveEdit } = useEditEntry()
+
+// Gate the editor footer so a read-only collaborator sees a disabled state
+// instead of a 403 mid-flow.
 const canEditWorkspace = computed(() => canEdit())
-const canCreateDatastreamHere = computed(() => canCreateDatastream())
 const workspaceRole = computed(() => roleName())
 
+const startEditing =
+  useTemplateRef<InstanceType<typeof StartEditingFlow>>('startEditing')
+
+// The resume watcher can fire during setup, before the flow's ref is set.
+const flowMounted = new Promise<void>((resolve) => onMounted(resolve))
+
+const route = useRoute()
+const router = useRouter()
+
+// A share link's `ed` becomes the resume pointer. Set it before registering
+// the resume hook, which checks the pointer only once.
+const initialShareState = decodeShareState(route.query as Record<string, unknown>)
+if (initialShareState.editView && initialShareState.editDatastreamId) {
+  resumeDatastreamId.value = initialShareState.editDatastreamId
+}
+
+// Reopens the editor after a page reload; waits for the catalog to land.
+useResumeEditSession(async (id) => {
+  await flowMounted
+  await startEditing.value?.resume(id)
+})
+
 const editCount = computed(() => editHistory.value?.length ?? 0)
 const showCommitConfirm = ref(false)
 const showCloseConfirm = ref(false)
-const showCreateDatastream = ref(false)
 const commitDescription = ref('')
-const showChooser = ref(false)
-const chooserLoading = ref(false)
-const chooserOptions = ref<ManagedDatastreamOption[]>([])
-const chooserSource = ref<Datastream | null>(null)
 const isSavingDraft = ref(false)
 const isCommitting = ref(false)
-const isCreating = ref(false)
-const isStartingSession = ref(false)
 const isDiscarding = ref(false)
 const showDiscardConfirm = ref(false)
 
@@ -787,22 +691,9 @@ const historyPaneStyle = computed(() => {
 })
 
 function exitToSelect() {
-  currentView.value = DrawerType.Select
-  selectedDrawer.value = DrawerType.Select
-  isDrawerOpen.value = true
-  // Leaving the editor deliberately, so a later reload shouldn't reopen it.
-  resumeDatastreamId.value = null
-  // Unsaved edits live only on the working copy; the next preview rebuilds
-  // it from what was saved.
-  if (qcDatastream.value) workingCopies.invalidate(qcDatastream.value.id)
-  // Put the source back in the plot so the catalog table shows the row the
-  // user had selected; the managed datastream it was swapped for is hidden
-  // from that table.
-  void releaseManagedDatastream()
+  void leaveEdit()
 }
 
-// No in-progress session yet: open one over the window already chosen by the
-// time-range controls in the Select view, rather than prompting for it again.
 async function onDiscardUnsaved() {
   showDiscardConfirm.value = false
   isDiscarding.value = true
@@ -819,73 +710,6 @@ async function onDiscardUnsaved() {
   }
 }
 
-async function startSessionForWindow() {
-  isStartingSession.value = true
-  try {
-    await startSession({
-      phenomenonTimeStart: beginDate.value.toISOString(),
-      phenomenonTimeEnd: endDate.value.toISOString(),
-    })
-    await redraw()
-    Snackbar.success('Edit session started.')
-  } catch (e) {
-    if (e instanceof ResumeSupersededError) abandonSupersededResume(e)
-    else Snackbar.error(e instanceof Error ? e.message : 'Could not start the session.')
-  } finally {
-    isStartingSession.value = false
-  }
-}
-
-// Nothing replayed is wired in and the footer would still offer Save, so
-// leave for Select rather than let edits save over the saved draft.
-function abandonSupersededResume(e: ResumeSupersededError) {
-  exitToSelect()
-  Snackbar.error(e.message)
-}
-
-async function onCreateProcessingLevel(input: {
-  code: string
-  definition?: string
-  explanation?: string
-}) {
-  try {
-    const level = await createProcessingLevel(input)
-    // Add to the catalog so it shows in the picker and is immediately valid.
-    processingLevels.value = [...processingLevels.value, level]
-    Snackbar.success('Processing level added.')
-    return level
-  } catch (e) {
-    Snackbar.error(
-      e instanceof Error ? e.message : 'Could not add the processing level.'
-    )
-    return null
-  }
-}
-
-async function onCreateDatastream(spec: CreateManagedDatastreamSpec) {
-  showCreateDatastream.value = false
-  qcPreferences.processingLevelId = spec.processingLevelId
-  isCreating.value = true
-  try {
-    const { managedDatastream, history } = await createManaged(spec)
-    // Register the new history + datastream so it's hidden from the catalog
-    // and the chooser can resolve it (by its name, not id) without a reload.
-    addQcHistory(history)
-    datastreams.value = [...datastreams.value, managedDatastream]
-    // Reuse the source's already-loaded series as the managed datastream's
-    // working copy instead of adding a second, empty plotted item.
-    await adoptManagedDatastream(managedDatastream, spec.source.id)
-    Snackbar.success('Managed datastream created.')
-    await enterEdit()
-  } catch (e) {
-    Snackbar.error(
-      e instanceof Error ? e.message : 'Could not create the datastream.'
-    )
-  } finally {
-    isCreating.value = false
-  }
-}
-
 async function onSaveDraft(): Promise<boolean> {
   isSavingDraft.value = true
   try {
@@ -945,21 +769,12 @@ function closeWithoutSaving() {
   exitToSelect()
 }
 
-const route = useRoute()
-const router = useRouter()
-
 // Hydrate state from the URL once datastream metadata is available.
 const hydrateFromUrl = () => {
   const state = decodeShareState(route.query as Record<string, unknown>)
 
-  if (state.editView) {
-    currentView.value = DrawerType.Edit
-    selectedDrawer.value = DrawerType.Edit
-    isDrawerOpen.value = true
-  } else {
-    currentView.value = DrawerType.Select
-    selectedDrawer.value = DrawerType.Select
-  }
+  currentView.value = DrawerType.Select
+  selectedDrawer.value = DrawerType.Select
 
   if (state.tableTab) activeTab.value = 'table'
   else activeTab.value = 'plot'
@@ -978,8 +793,6 @@ const hydrateFromUrl = () => {
   const resolved = ids
     .map((id) => datastreams.value.find((ds) => ds.id === id))
     .filter((ds): ds is NonNullable<typeof ds> => !!ds)
-  // QC target is the first id by convention.
-  const qcId = resolved[0]?.id ?? null
 
   // Set the window BEFORE loading datastreams so the first fetch uses it.
   // A preset resolves against the plotted data during that load. An
@@ -1037,13 +850,35 @@ const hydrateFromUrl = () => {
       }
     : null
 
-  // Snapshots replay against the session store, so they wait for the plot
-  // (and with it the editor's session load) to settle.
-  void setPlottedDatastreams(resolved, qcId).then(async () => {
-    for (const s of state.snapshots ?? []) {
-      await toggleSnapshot(s.sessionId, s.opIndex)
-    }
-  })
+  // `resumeDatastreamId` is already set from `ed` by the early block above,
+  // before the resume hook is registered. Nothing to do here for it.
+  void setPlottedDatastreams(resolved)
+
+  // Snapshots replay against the session store, which the resume hook loads
+  // asynchronously and independently of the plot. Wait for the editor to
+  // actually be open on the `ed` target (an in-progress or viewed session
+  // means `loadSessions` has landed) rather than chaining off
+  // `setPlottedDatastreams`, which only settles the plot.
+  const snapshots = state.snapshots ?? []
+  const editTargetId = state.editView ? state.editDatastreamId : undefined
+  // No `immediate`: resuming the editor always takes at least one await, so
+  // the ready condition cannot already hold when this watch is created.
+  if (snapshots.length && editTargetId) {
+    const stopSnapshotWatch = watch(
+      () =>
+        qcDatastreamId.value === editTargetId &&
+        !!(inProgressSession.value || viewedSession.value),
+      (ready) => {
+        if (!ready) return
+        stopSnapshotWatch()
+        void (async () => {
+          for (const s of snapshots) {
+            await toggleSnapshot(s.sessionId, s.opIndex)
+          }
+        })()
+      }
+    )
+  }
 }
 
 if (datastreams.value.length) {
@@ -1066,14 +901,14 @@ if (datastreams.value.length) {
 // lives in `share.ts` so this watcher reads as a plain assembly of
 // inputs.
 const SHARE_KEYS = [
-  'ws', 'm', 'tab', 'ds', 'snap', 'r', 'from', 'to',
+  'ws', 'm', 'ed', 'tab', 'ds', 'snap', 'r', 'from', 'to',
   't', 'op', 'pl', 'h', 'ya', 'z', 'yz', 'dp', 'th',
 ] as const
 
 watch(
   [
     plottedDatastreams,
-    qcDatastream,
+    qcDatastreamId,
     currentView,
     activeTab,
     beginDate,
@@ -1091,8 +926,8 @@ watch(
     tooltipsMaxDataPoints,
   ],
   () => {
-    // Snapshots travel in their own key; `ids` stays real datastreams so the
-    // QC-target-is-first rule and the visibility bitmasks still line up.
+    // Snapshots travel in their own key; `ds` stays real plotted datastreams
+    // so the visibility bitmasks line up.
     const plotted = plottedDatastreams.value
     const ids = plotted.filter((ds) => !isSnapshotId(ds.id)).map((ds) => ds.id)
     const snapshots = plotted
@@ -1104,6 +939,7 @@ watch(
     const state: ShareState = {
       workspaceId: selectedWorkspaceId.value || null,
       editView: isEdit,
+      editDatastreamId: isEdit ? (qcDatastreamId.value ?? undefined) : undefined,
       tableTab: activeTab.value === 'table',
       datastreamIds: ids,
       snapshots,
@@ -1147,136 +983,6 @@ watch(
 onUnmounted(() => {
   resetState()
 })
-
-// "Start editing" opens a chooser of the source's managed datastreams and
-// their sessions, rather than editing the raw datastream directly.
-async function openEditChooser() {
-  const source = qcDatastream.value
-  if (!source) return
-  // A managed datastream plotted directly is already the thing to edit, so
-  // there is nothing left to choose. It carries its own observations, so no
-  // `adoptManagedDatastream` working copy is needed either.
-  if (managedDatastreamIds.value.has(source.id)) {
-    await enterEdit()
-    return
-  }
-  chooserSource.value = source
-  chooserOptions.value = []
-  chooserLoading.value = true
-  showChooser.value = true
-  try {
-    chooserOptions.value = await loadForSource(source.id)
-  } catch (e) {
-    Snackbar.error(
-      e instanceof Error ? e.message : 'Could not load QC datastreams.'
-    )
-  } finally {
-    chooserLoading.value = false
-  }
-}
-
-// Edit a chosen managed datastream: reuse the source's loaded series as its
-// working copy, then resume its in-progress session (or start a new one).
-async function editManaged(option: ManagedDatastreamOption) {
-  showChooser.value = false
-  const source = chooserSource.value
-  if (!source) return
-  try {
-    await adoptManagedDatastream(option.managed, source.id)
-    await enterEdit()
-  } catch (e) {
-    Snackbar.error(
-      e instanceof Error ? e.message : 'Could not open the datastream for editing.'
-    )
-  }
-}
-
-function onChooserCreate() {
-  showChooser.value = false
-  showCreateDatastream.value = true
-}
-
-// Cancelling create returns to the chooser it was opened from.
-function onCreateCancel() {
-  showCreateDatastream.value = false
-  if (chooserSource.value) showChooser.value = true
-}
-
-async function onChooserDelete(option: ManagedDatastreamOption) {
-  try {
-    await deleteManaged(option.historyId, option.managed.id)
-    removeManagedDatastream(option.historyId, option.managed.id)
-    workingCopies.invalidate(option.managed.id)
-    chooserOptions.value = chooserOptions.value.filter(
-      (o) => o.historyId !== option.historyId
-    )
-    Snackbar.success('Managed datastream deleted.')
-  } catch (e) {
-    Snackbar.error(
-      e instanceof Error ? e.message : 'Could not delete the managed datastream.'
-    )
-  }
-}
-
-// Discard an in-progress session from the chooser, dropping its draft edits.
-// Only that session goes; the managed datastream and its commits remain.
-async function onChooserDeleteSession(
-  option: ManagedDatastreamOption,
-  sessionId: string
-) {
-  try {
-    // Same input the confirmation previewed, so the two cannot disagree.
-    const deleted = await deleteSessionChain(
-      option.historyId,
-      collectDeletionChain(option.sessions, sessionId)
-    )
-    workingCopies.invalidate(option.managed.id)
-    const gone = new Set(deleted)
-    chooserOptions.value = chooserOptions.value.map((o) =>
-      o.historyId === option.historyId
-        ? { ...o, sessions: o.sessions.filter((s) => !gone.has(s.id)) }
-        : o
-    )
-    Snackbar.success(
-      deleted.length === 1
-        ? 'Session deleted.'
-        : `${deleted.length} sessions deleted.`
-    )
-  } catch (e) {
-    // Refresh regardless: a partial cascade leaves the chooser's copy wrong.
-    if (chooserSource.value) {
-      chooserOptions.value = await loadForSource(chooserSource.value.id)
-    }
-    Snackbar.error(
-      e instanceof Error ? e.message : 'Could not delete the session.'
-    )
-  }
-}
-
-// Enter the editor on the current QC-target managed datastream: resume its
-// in-progress session, or start a new one over the selected time range.
-async function enterEdit() {
-  try {
-    await beginEditing()
-  } catch (e) {
-    if (!(e instanceof ResumeSupersededError)) throw e
-    abandonSupersededResume(e)
-    return
-  }
-  if (needsHistory.value) {
-    Snackbar.error('This datastream is not set up for QC editing.')
-    return
-  }
-  currentView.value = DrawerType.Edit
-  selectedDrawer.value = DrawerType.Edit
-  isDrawerOpen.value = true
-  // Remember the target so a page reload can come back to this session.
-  resumeDatastreamId.value = qcDatastream.value?.id ?? null
-  if (needsSession.value) await startSessionForWindow()
-}
-
-// Reopens the editor after a page reload; waits for the catalog to land.
-useResumeEditSession(enterEdit)
 </script>
 
 <style scoped>
diff --git a/apps/qc-app/src/components/VisualizeData/ContextPlot.vue b/apps/qc-app/src/components/VisualizeData/ContextPlot.vue
index c64ca3b7..d95b385a 100644
--- a/apps/qc-app/src/components/VisualizeData/ContextPlot.vue
+++ b/apps/qc-app/src/components/VisualizeData/ContextPlot.vue
@@ -40,7 +40,7 @@ import Plotly from 'plotly.js-dist'
 import type { Layout, LayoutAxis, PlotlyHTMLElement } from 'plotly.js-dist'
 import { usePlotlyStore } from '@/store/plotly'
 import { useDataVisStore } from '@/store/dataVisualization'
-import { COLORS, type AppPlotlyTrace } from '@/utils/plotting/plotly'
+import { COLORS, SOURCE_CONTEXT_COLOR, type AppPlotlyTrace } from '@/utils/plotting/plotly'
 import type { GraphSeries } from '@/types'
 
 const TARGET_POINTS = 2000
@@ -55,7 +55,7 @@ type PlotlyEventEmitter = {
 
 const { plotlyRef, graphSeriesArray, mainPlotEpoch } =
   storeToRefs(usePlotlyStore())
-const { qcDatastream } = storeToRefs(useDataVisStore())
+const { qcDatastream, sourceContextDatastream } = storeToRefs(useDataVisStore())
 
 const rootEl = ref<HTMLDivElement>()
 const plotEl = ref<HTMLDivElement>()
@@ -119,6 +119,7 @@ function buildContextTraces(series: GraphSeries[]): {
   extent: [number, number] | null
 } {
   const qcId = qcDatastream.value?.id
+  const sourceId = sourceContextDatastream.value?.id
   const traces: AppPlotlyTrace[] = []
   let xMin = Infinity
   let xMax = -Infinity
@@ -133,7 +134,8 @@ function buildContextTraces(series: GraphSeries[]): {
     const sampled = strideSample(xs, ys, TARGET_POINTS)
     const yNorm = normalize(sampled.y)
     const isQc = !!qcId && s.id === qcId
-    const color = isQc ? COLORS[0] : (s.color ?? COLORS[1])
+    const isSource = !!sourceId && s.id === sourceId
+    const color = isQc ? COLORS[0] : isSource ? SOURCE_CONTEXT_COLOR : (s.color ?? COLORS[1])
     traces.push({
       x: sampled.x,
       y: yNorm,
@@ -417,11 +419,12 @@ async function buildOrUpdate() {
 // per-cell y-edits stay cheap; overall shape barely changes anyway.
 const rebuildSignature = computed(() => {
   const qid = qcDatastream.value?.id ?? ''
+  const sid = sourceContextDatastream.value?.id ?? ''
   const parts = graphSeriesArray.value.map(
     (s) => `${s.id}:${s.data?.dataX?.length ?? 0}:${s.color}`
   )
   const hidden = [...hiddenIds.value].sort().join(',')
-  return parts.join('|') + '#' + qid + '!' + hidden
+  return parts.join('|') + '#' + qid + '~' + sid + '!' + hidden
 })
 
 watch(rebuildSignature, () => {
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
index 28c89d72..0c332923 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
@@ -100,17 +100,6 @@
 
     <v-divider />
 
-    <div
-      v-if="!plottedDatastreams.length"
-      class="datasets-table__hint d-flex align-center px-3 py-1"
-    >
-      <v-icon icon="mdi-information-outline" size="14" class="mr-2" />
-      <span class="text-body-small">
-        First plotted datastream becomes the
-        <b>QC target</b>. Click a row to see its details.
-      </span>
-    </div>
-
     <div class="datasets-table__body flex-grow-1 d-flex flex-column">
       <v-data-table-virtual
         data-testid="datastreams-table"
@@ -136,15 +125,20 @@
           />
         </template>
 
+        <template #header.edit>
+          <v-icon
+            icon="mdi-pencil"
+            size="18"
+            title="Edit"
+            aria-label="Edit"
+          />
+        </template>
+
         <template v-slot:item.plot="{ item }">
           <v-tooltip
-            :disabled="!isAtCap(item) && !isQc(item)"
+            :disabled="!isAtCap(item)"
             location="top"
-            :text="
-              isQc(item)
-                ? 'QC target: first plotted datastream'
-                : `Maximum of ${PLOT_CAP} datastreams plotted; remove one to add another`
-            "
+            :text="`Maximum of ${PLOT_CAP} datastreams plotted; remove one to add another`"
           >
             <template #activator="{ props: tooltipProps }">
               <div class="d-flex align-center" v-bind="tooltipProps">
@@ -174,12 +168,6 @@
                     size="20"
                   />
                 </button>
-                <span
-                  v-if="isQc(item)"
-                  class="qc-pill ml-1 d-inline-flex align-center justify-center text-white"
-                >
-                  QC
-                </span>
                 <span
                   v-if="managedCount(item) > 0"
                   class="managed-count ml-1 d-inline-flex align-center justify-center"
@@ -194,6 +182,33 @@
           </v-tooltip>
         </template>
 
+        <template #item.edit="{ item }">
+          <v-tooltip
+            location="top"
+            :text="
+              canEditWorkspace
+                ? 'Edit'
+                : `Your role on this workspace (${workspaceRole}) is read-only`
+            "
+          >
+            <template #activator="{ props: tooltipProps }">
+              <!-- A disabled button passes clicks to this wrapper; keep them off the row. -->
+              <span v-bind="tooltipProps" @click.stop>
+                <v-btn
+                  icon="mdi-pencil"
+                  size="small"
+                  variant="text"
+                  density="comfortable"
+                  :data-testid="`edit-datastream-${item.id}`"
+                  :aria-label="`Edit ${item.name}`"
+                  :disabled="!canEditWorkspace"
+                  @click.stop="emit('edit', item)"
+                />
+              </span>
+            </template>
+          </v-tooltip>
+        </template>
+
         <template #item.name="{ item }">
           <span class="name-cell" :title="item.name">{{ item.name || '-' }}</span>
         </template>
@@ -280,11 +295,20 @@ import {
   type ManagedDatastreamOption,
 } from '@/composables/useManagedDatastreams'
 import { Snackbar } from '@uwrl/qc-utils'
+import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
 
 /** Maximum series the plot holds at once. */
 const PLOT_CAP = 5
 
-const { filteredDatastreams, plottedDatastreams, qcDatastream, historiesBySource } =
+const emit = defineEmits<{
+  (e: 'edit', datastream: Datastream & DatastreamExtended): void
+}>()
+
+const { canEdit, roleName } = useWorkspacePermissions()
+const canEditWorkspace = computed(() => canEdit())
+const workspaceRole = computed(() => roleName())
+
+const { filteredDatastreams, plottedDatastreams, historiesBySource } =
   storeToRefs(useDataVisStore())
 const {
   toggleDatastream,
@@ -394,8 +418,6 @@ const isChecked = (item: Datastream) =>
 const isPartial = (item: Datastream) =>
   !plottedIds.value.has(item.id) && isChecked(item)
 
-const isQc = (item: Datastream) => qcDatastream.value?.id === item.id
-
 // How many managed (QC) datastreams exist for this source datastream.
 const managedCount = (item: Datastream) =>
   historiesBySource.value.get(item.id)?.length ?? 0
@@ -463,13 +485,13 @@ const getRowProps = ({ item }: { item: Datastream }) => ({
   class: {
     'datasets-table__row--at-cap': isAtCap(item),
     'datasets-table__row--plotted': isChecked(item),
-    'datasets-table__row--qc': isQc(item),
   },
 })
 
 const search = ref()
 const headers = reactive([
   { title: 'Plot', key: 'plot', visible: true, width: 96, sortable: false },
+  { title: 'Edit', key: 'edit', visible: true, width: 56, sortable: false },
   {
     title: 'Name',
     key: 'name',
@@ -507,7 +529,7 @@ const headers = reactive([
 const visibleHeaders = computed(() => headers.filter((h) => h.visible))
 
 const selectableHeaders = computed(() =>
-  headers.filter((h) => !['plot'].includes(h.key))
+  headers.filter((h) => !['plot', 'edit'].includes(h.key))
 )
 
 // Single-sort default. Multi-sort was previously enabled but the
@@ -539,21 +561,12 @@ const resetSort = () => {
   min-height: 0;
 }
 
-/* Inline tip strip below the toolbar; only rendered while no
-   datastreams are plotted (see template). Quiet primary tint so it
-   reads as guidance, not an alert. */
-.datasets-table__hint {
-  background-color: rgba(var(--v-theme-primary), 0.06);
-  color: rgba(var(--v-theme-on-surface), 0.75);
-  border-bottom: 1px solid rgba(var(--v-theme-primary), 0.12);
-}
-
 :deep(.v-table .v-data-table__tr:nth-child(even) td) {
   background: #f7f7f7;
 }
 
 /* Tint + primary leading bar so a plotted row reads even when the
-   checkbox column is scrolled away. QC row gets a saturated bar. */
+   checkbox column is scrolled away. */
 :deep(tbody tr.datasets-table__row--plotted > td) {
   background-color: rgba(var(--v-theme-primary), 0.05);
 }
@@ -562,14 +575,6 @@ const resetSort = () => {
   box-shadow: inset 3px 0 0 rgba(var(--v-theme-primary), 0.45);
 }
 
-:deep(tbody tr.datasets-table__row--qc > td) {
-  background-color: rgba(var(--v-theme-primary), 0.09);
-}
-
-:deep(tbody tr.datasets-table__row--qc > td:first-child) {
-  box-shadow: inset 3px 0 0 rgb(var(--v-theme-primary));
-}
-
 :deep(tbody tr:hover > td) {
   background-color: rgba(var(--v-theme-primary), 0.08) !important;
   cursor: pointer;
@@ -681,16 +686,4 @@ const resetSort = () => {
   background-color: rgba(var(--v-theme-primary), 0.14);
   border-radius: 9px;
 }
-
-/* Compact "QC" pill rendered next to the plot checkbox on the QC row.
-   Marks the quality-control target without occupying its own column. */
-.qc-pill {
-  height: 18px;
-  padding: 0 6px;
-  font-size: 0.65rem;
-  font-weight: 700;
-  letter-spacing: 0.5px;
-  background-color: rgb(var(--v-theme-primary));
-  border-radius: 4px;
-}
 </style>
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue b/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
index 53cac21c..9f77dffc 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
@@ -2,8 +2,11 @@
   <div class="d-flex flex-column ga-2 w-100">
     <div>
       <div class="text-body-small text-medium-emphasis">
-        <strong>Loaded time window.</strong>
-        Presets count back from the plotted data's last observation.
+        <template v-if="description">{{ description }}</template>
+        <template v-else>
+          <strong>Loaded time window.</strong>
+          Presets count back from the plotted data's last observation.
+        </template>
       </div>
     </div>
 
@@ -66,6 +69,8 @@ import {
 } from '@/utils/timeRangePresets'
 import { storeToRefs } from 'pinia'
 
+defineProps<{ description?: string }>()
+
 const { setDateRange, onDateBtnClick } = useDataVisStore()
 const { beginDate, endDate, selectedDateBtnId } = storeToRefs(useDataVisStore())
 </script>
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisualization.vue b/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
index 10df3bf7..d7e6695b 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
@@ -1,32 +1,36 @@
-<template>
+<template>
+  <!-- Stays mounted while loading so a refresh keeps the plot and its zoom. -->
   <div
-    v-if="isUpdating"
-    class="data-vis-state fill-height d-flex flex-column align-center justify-center pa-6 text-center"
-    data-testid="data-loading-indicator"
+    v-if="isUpdating || isDataAvailable"
+    class="fill-height position-relative"
   >
-    <v-progress-circular
-      color="primary"
-      :size="56"
-      :width="4"
-      indeterminate
-      class="mb-4"
-    />
-    <div class="text-title-medium font-weight-bold mb-1">
-      Loading observations…
-    </div>
-    <div class="text-body-small text-medium-emphasis">
-      Fetching data for
-      {{ plottedDatastreams.length }}
-      datastream{{ plottedDatastreams.length === 1 ? '' : 's' }}
+    <Plot class="data-vis-plot fill-height" :preview="preview" />
+
+    <div
+      v-if="isUpdating"
+      class="data-vis-loading position-absolute d-flex flex-column align-center justify-center pa-6 text-center"
+      data-testid="data-loading-indicator"
+    >
+      <v-progress-circular
+        color="primary"
+        :size="56"
+        :width="4"
+        indeterminate
+        class="mb-4"
+      />
+      <div class="text-title-medium font-weight-bold mb-1">
+        Loading observations…
+      </div>
+      <div class="text-body-small text-medium-emphasis">
+        Fetching data for
+        {{ loadingCount }}
+        datastream{{ loadingCount === 1 ? '' : 's' }}
+      </div>
     </div>
   </div>
 
-  <div v-else-if="isDataAvailable" class="fill-height">
-    <Plot class="fill-height" :preview="preview" />
-  </div>
-
   <div
-    v-else-if="plottedDatastreams.length"
+    v-else-if="seriesDatastreams.length"
     class="data-vis-state fill-height d-flex flex-column align-center justify-center pa-6 text-center"
   >
     <v-icon
@@ -39,7 +43,7 @@
       No observations in this range
     </div>
     <div class="text-body-medium text-medium-emphasis" style="max-width: 360px">
-      The selected datastream{{ plottedDatastreams.length === 1 ? '' : 's' }}
+      The selected datastream{{ seriesDatastreams.length === 1 ? '' : 's' }}
       returned no data for the current time window. Try a different range from
       the drawer on the left.
     </div>
@@ -70,16 +74,15 @@
       <div class="data-vis-state__step">
         <div class="data-vis-state__step-num d-inline-flex align-center justify-center rounded-pill text-white">2</div>
         <v-icon
-          icon="mdi-radiobox-marked"
+          icon="mdi-checkbox-marked-outline"
           size="24"
           color="primary"
           class="data-vis-state__step-icon"
         />
         <div class="data-vis-state__step-body">
-          <div class="text-title-small font-weight-bold">Pick the QC target</div>
+          <div class="text-title-small font-weight-bold">Plot datastreams</div>
           <div class="text-body-small text-medium-emphasis">
-            Click the <b>Plot</b> checkbox on a row. The first one becomes the
-            quality-control target, shown in primary blue.
+            Click the <b>Plot</b> checkbox on rows to preview them together.
           </div>
         </div>
       </div>
@@ -100,6 +103,23 @@
           </div>
         </div>
       </div>
+
+      <div class="data-vis-state__step">
+        <div class="data-vis-state__step-num d-inline-flex align-center justify-center rounded-pill text-white">4</div>
+        <v-icon
+          icon="mdi-pencil"
+          size="24"
+          color="primary"
+          class="data-vis-state__step-icon"
+        />
+        <div class="data-vis-state__step-body">
+          <div class="text-title-small font-weight-bold">Edit one</div>
+          <div class="text-body-small text-medium-emphasis">
+            Click the <b>pencil</b> on a row to pick or create its QC datastream
+            and start a session.
+          </div>
+        </div>
+      </div>
     </div>
   </div>
 </template>
@@ -117,18 +137,32 @@ defineProps<{
 
 const { plotlyOptions } = storeToRefs(usePlotlyStore())
 
-const { loadingStates, plottedDatastreams } = storeToRefs(useDataVisStore())
+const { loadingStates, seriesDatastreams } = storeToRefs(useDataVisStore())
 
-const isUpdating = computed(() =>
-  Array.from(loadingStates.value.values()).some((isLoading) => isLoading)
+// Only what is fetching: the edit target never is.
+const loadingCount = computed(
+  () => Array.from(loadingStates.value.values()).filter(Boolean).length
 )
+const isUpdating = computed(() => loadingCount.value > 0)
 
+// The editor draws its target and source even with nothing else plotted.
 const isDataAvailable = computed(() => {
-  return plotlyOptions.value.traces?.length && plottedDatastreams.value?.length
+  return plotlyOptions.value.traces?.length && seriesDatastreams.value?.length
 })
 </script>
 
 <style scoped>
+/* Contains Plotly's own z-indexes so the overlay covers them. */
+.data-vis-plot {
+  isolation: isolate;
+}
+
+.data-vis-loading {
+  inset: 0;
+  z-index: 1;
+  background-color: rgba(var(--v-theme-surface), 0.75);
+}
+
 .data-vis-state {
   background-color: rgba(var(--v-theme-primary), 0.02);
 }
diff --git a/apps/qc-app/src/components/VisualizeData/DatastreamInformationCard.vue b/apps/qc-app/src/components/VisualizeData/DatastreamInformationCard.vue
index 0ca0e877..7b140f86 100644
--- a/apps/qc-app/src/components/VisualizeData/DatastreamInformationCard.vue
+++ b/apps/qc-app/src/components/VisualizeData/DatastreamInformationCard.vue
@@ -300,7 +300,7 @@ const addToPlot = async (datastream: Datastream) => {
 
 const clearAndPlot = async (datastream: Datastream) => {
   emit('close')
-  await setPlottedDatastreams([datastream], datastream.id)
+  await setPlottedDatastreams([datastream])
 }
 
 const d = computed(() => props.datastream)
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index 76948e66..427ca528 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -265,36 +265,29 @@
           </v-card>
         </v-menu>
 
-        <v-speed-dial
+        <v-menu
           v-if="!preview"
-          v-model="rangeDialOpen"
-          location="bottom center"
-          transition="fade-transition"
+          v-model="contextRangeOpen"
+          :close-on-content-click="false"
+          location="bottom end"
+          offset="6"
         >
-          <template #activator="{ props: activatorProps }">
+          <template #activator="{ props: menuProps }">
             <v-btn
-              v-bind="activatorProps"
+              v-bind="menuProps"
               size="small"
               variant="text"
-              icon="mdi-magnify-scan"
-              title="Zoom to range (does not reload data)"
-              aria-label="Zoom to range"
-            />
+              prepend-icon="mdi-calendar-range"
+              data-testid="context-range-btn"
+              title="Context range"
+            >
+              Context
+            </v-btn>
           </template>
-
-          <v-btn
-            v-for="option in editorDateOptions"
-            :key="option.id"
-            color="surface"
-            variant="flat"
-            size="small"
-            :title="option.title"
-            class="text-none text-body-small font-weight-medium border elevation-2"
-            @click="selectRangePreset(option.id)"
-          >
-            {{ option.label }}
-          </v-btn>
-        </v-speed-dial>
+          <v-card width="300" class="pa-3" data-testid="context-range-menu">
+            <DataVisTimeFilters description="How much of the source and plotted datastreams to show around the session. Your edits are not reloaded." />
+          </v-card>
+        </v-menu>
       </div>
     </div>
 
@@ -411,18 +404,15 @@ import {
   handleRelayout,
   zoomXaxisTo,
 } from '@/utils/plotting/plotly'
-import {
-  ALL_PRESET_ID,
-  TIME_RANGE_PRESETS,
-  presetWindow,
-} from '@/utils/timeRangePresets'
 import DataTable from '@/components/VisualizeData/DataTable.vue'
 import ContextPlot from '@/components/VisualizeData/ContextPlot.vue'
+import DataVisTimeFilters from '@/components/VisualizeData/DataVisTimeFilters.vue'
 import { useDataSelection } from '@/composables/useDataSelection'
 import { useBufferedNumber } from '@/composables/useBufferedNumber'
 import { usePersistedFlag } from '@/composables/useResizable'
 import { formatDate, Snackbar } from '@uwrl/qc-utils'
 import { useDataVisStore } from '@/store/dataVisualization'
+import { useQcSessionStore } from '@/store/qcSession'
 
 // Preview strips the in-plot chrome for the Select view.
 const props = defineProps<{
@@ -446,9 +436,11 @@ const {
   previewMode,
   plotlyRef,
   activeTab,
+  pendingShareZoom,
 } = storeToRefs(usePlotlyStore())
 const { selectedData, hasSelectionShape, qcDatastream } =
   storeToRefs(useDataVisStore())
+const { viewedSession, inProgressSession } = storeToRefs(useQcSessionStore())
 
 const tooltipsAutoDisabled = computed(
   () =>
@@ -511,8 +503,6 @@ const yReadoutUnit = computed(() => {
   return symbol ? ` ${symbol}` : ''
 })
 
-const { graphSeriesArray } = storeToRefs(usePlotlyStore())
-
 // Default left placement; promote to right when the measured chip
 // fits between the axis line and the plot's right edge.
 const chipEls = new Map<string, HTMLElement>()
@@ -571,50 +561,32 @@ async function copyShareableLink() {
   }
 }
 
-const earliestDataX = computed<number | null>(() => {
-  let min = Infinity
-  for (const s of graphSeriesArray.value) {
-    const xs = s.data?.dataX
-    if (!xs?.length) continue
-    const first = xs[0] as number
-    if (first < min) min = first
-  }
-  return Number.isFinite(min) ? min : null
-})
+const contextRangeOpen = ref(false)
 
-const latestDataX = computed<number | null>(() => {
-  let max = -Infinity
-  for (const s of graphSeriesArray.value) {
-    const xs = s.data?.dataX
-    if (!xs?.length) continue
-    const last = xs[xs.length - 1] as number
-    if (last > max) max = last
-  }
-  return Number.isFinite(max) ? max : null
+const editWindow = computed(() => {
+  const s = viewedSession.value ?? inProgressSession.value
+  return s
+    ? {
+        begin: Date.parse(s.phenomenonTimeStart),
+        end: Date.parse(s.phenomenonTimeEnd),
+      }
+    : null
 })
 
-const editorDateOptions = TIME_RANGE_PRESETS.filter((o) => o.label !== 'YTD')
-
-const rangeDialOpen = ref(false)
-
-function selectRangePreset(id: number) {
-  onEditorDatePreset(id)
-  rangeDialOpen.value = false
+// The editor opens on the session window; context stays a zoom-out away.
+function zoomToEditWindow() {
+  const w = editWindow.value
+  if (!w || !plotlyRef.value) return
+  zoomXaxisTo(plotlyRef.value, w.begin, w.end)
+  requestTableScroll(w.begin)
 }
 
-// Editor presets are a pure x-axis zoom (no refetch) so they don't blow away
-// the edit history. They resolve against the loaded data.
-function onEditorDatePreset(id: number) {
-  if (earliestDataX.value == null || latestDataX.value == null) return
-  const range = presetWindow(id, {
-    begin: new Date(earliestDataX.value),
-    end: new Date(latestDataX.value),
-  })
-  if (!range) return
-  zoomXaxisTo(plotlyRef.value, range.begin.getTime(), range.end.getTime())
-  // Keep the table in sync: scroll it so the range's first row is on top.
-  requestTableScroll(range.begin.getTime())
-}
+watch(
+  () => editWindow.value && `${editWindow.value.begin}-${editWindow.value.end}`,
+  () => {
+    if (!props.preview) zoomToEditWindow()
+  }
+)
 
 function toggleTooltips() {
   // Manual override; switching back to auto must go through the
@@ -689,6 +661,9 @@ const keyboardShortcuts = [
 
 let plotResizeObserver: ResizeObserver | null = null
 let pendingResizeFrame: number | null = null
+let mountDrawTimer: ReturnType<typeof setTimeout> | null = null
+// Deferred work checks this so it never touches a detached plot.
+let isUnmounted = false
 
 onMounted(async () => {
   // Flip before handleNewPlot so createPlotlyOption emits the
@@ -697,13 +672,17 @@ onMounted(async () => {
   updateOptions()
 
   // Wait for the view-switch animation to expand the container.
-  setTimeout(async () => {
+  mountDrawTimer = setTimeout(async () => {
+    mountDrawTimer = null
     updateOptions()
-    const drawn = handleNewPlot(plot.value)
-    if (!props.preview) onEditorDatePreset(ALL_PRESET_ID)
-    await drawn
+    // A share-URL zoom, if any, is applied and cleared inside
+    // handleNewPlot itself, so capture it before that happens: it
+    // should win over the session-window zoom below.
+    const hadPendingShareZoom = !!pendingShareZoom.value
+    await handleNewPlot(plot.value)
+    if (isUnmounted) return
+    if (!props.preview && !hadPendingShareZoom) zoomToEditWindow()
 
-    // Unmounted while drawing.
     const target = plot.value
     if (target && typeof ResizeObserver !== 'undefined') {
       // The plot can be laid out before the container reaches its final
@@ -734,8 +713,13 @@ onMounted(async () => {
 })
 
 onBeforeUnmount(() => {
+  isUnmounted = true
   // Reset so the next Plot mount in Edit view doesn't inherit preview.
   if (previewMode.value) previewMode.value = false
+  if (mountDrawTimer != null) {
+    clearTimeout(mountDrawTimer)
+    mountDrawTimer = null
+  }
   if (pendingResizeFrame != null) {
     cancelAnimationFrame(pendingResizeFrame)
     pendingResizeFrame = null
@@ -749,6 +733,7 @@ onBeforeUnmount(() => {
 const onTabChange = () => {
   if (tab.value === 'plot') {
     setTimeout(() => {
+      if (isUnmounted) return
       setPlotSelection(selectedData.value || [])
     })
   }
diff --git a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
index ba31a89b..fb8a51dd 100644
--- a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
+++ b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
@@ -1,6 +1,9 @@
 <template>
   <div class="plotted-wrapper d-flex flex-column">
-    <div v-if="!lockQc" class="plotted-toolbar d-flex align-center px-3 py-2">
+    <div
+      v-if="!qcDatastream"
+      class="plotted-toolbar d-flex align-center px-3 py-2"
+    >
       <v-spacer></v-spacer>
       <v-btn
         :disabled="!plottedDatastreams.length"
@@ -15,12 +18,12 @@
     <v-divider />
     <ul class="plotted-list pa-0 ma-0">
       <li
-        v-for="(datastream, index) of plottedDatastreams"
+        v-for="(datastream, index) of seriesDatastreams"
         :key="datastream.id"
         class="plotted-item"
         :class="{
-          'plotted-item--qc': qcDatastream === datastream,
-          'plotted-item--locked': lockQc,
+          'plotted-item--qc': isEdit(datastream),
+          'plotted-item--source': isSource(datastream),
           'plotted-item--hidden': visibleDict[datastream.id] === false,
           'plotted-item--drop-before':
             dragIndex !== null &&
@@ -33,7 +36,8 @@
             dropIndex !== dragIndex &&
             (dragIndex as number) < index,
         }"
-        draggable="true"
+        :data-testid="`plotted-item-${datastream.id}`"
+        :draggable="!isPinned(datastream)"
         @dragstart="onDragStart(index, $event)"
         @dragover.prevent="onDragOver(index, $event)"
         @dragleave="onDragLeave(index)"
@@ -41,30 +45,14 @@
         @dragend="onDragEnd"
       >
         <v-icon
+          v-if="!isPinned(datastream)"
           class="plotted-item__drag cursor-grab"
           icon="mdi-drag-vertical"
           size="16"
-          :color="plottedDatastreams.length > 1 ? 'grey' : 'grey-lighten-2'"
+          :color="contextCount > 1 ? 'grey' : 'grey-lighten-2'"
           title="Drag to reorder"
         />
-
-        <button
-          v-if="!lockQc"
-          type="button"
-          class="plotted-item__dot cursor-pointer rounded-circle"
-          :class="{ 'plotted-item__dot--active': qcDatastream === datastream }"
-          :style="{
-            color: colorForDatastream(datastream.id),
-          }"
-          :disabled="isUpdating"
-          :title="
-            qcDatastream === datastream
-              ? 'Current QC target'
-              : 'Set as QC target'
-          "
-          :aria-pressed="qcDatastream === datastream"
-          @click="setQcDatastream(datastream)"
-        />
+        <span v-else />
 
         <button
           type="button"
@@ -90,10 +78,9 @@
           />
         </button>
 
-        <!-- QC rows sit on the always-rendered primary left axis, so
-             the Y-axis toggle is only for non-QC rows. -->
+        <!-- Primary rows sit on the always-rendered left axis. -->
         <button
-          v-if="qcDatastream !== datastream"
+          v-if="!isPrimary(datastream, index)"
           type="button"
           class="plotted-item__axis-toggle d-inline-flex align-center justify-center cursor-pointer rounded-sm"
           :title="
@@ -135,6 +122,15 @@
               class="flex-shrink-0"
             />
             <span>{{ datastream.name }}</span>
+            <v-chip
+              v-if="isSource(datastream)"
+              size="x-small"
+              variant="tonal"
+              label
+              class="flex-shrink-0"
+            >
+              raw source
+            </v-chip>
             <v-chip
               v-if="snapshotFor(datastream.id)"
               size="x-small"
@@ -182,7 +178,7 @@
         </div>
 
         <button
-          v-if="!(lockQc && qcDatastream === datastream)"
+          v-if="!isPinned(datastream)"
           type="button"
           class="plotted-item__close d-inline-flex align-center justify-center cursor-pointer rounded-sm"
           :title="`Remove ${datastream.name} from plot`"
@@ -198,8 +194,6 @@
 </template>
 
 <script setup lang="ts">
-defineProps<{ sectionTitle?: string; lockQc?: boolean }>()
-
 import { storeToRefs } from 'pinia'
 import { useDataVisStore } from '@/store/dataVisualization'
 import {
@@ -210,8 +204,11 @@ import {
 import type { AppPlotlyTrace } from '@/utils/plotting/plotly'
 import type { GraphSeries } from '@/types'
 import { usePlotlyStore } from '@/store/plotly'
-const { updateOptions, colorForDatastream, labelColorForDatastream } =
-  usePlotlyStore()
+import { ref, computed } from 'vue'
+import { Datastream } from '@hydroserver/client'
+import { formatDayStamp } from '@/utils/time'
+
+const { updateOptions, labelColorForDatastream } = usePlotlyStore()
 const {
   plotlyRef,
   graphSeriesArray,
@@ -219,32 +216,38 @@ const {
   hiddenTraceIds,
   plotlyOptions,
 } = storeToRefs(usePlotlyStore())
-import { ref, computed } from 'vue'
-import { Datastream } from '@hydroserver/client'
-import { formatDayStamp } from '@/utils/time'
 
-const { plottedDatastreams, qcDatastream, loadingStates } =
-  storeToRefs(useDataVisStore())
 const {
-  toggleDatastream,
-  setQcDatastream: setQcInStore,
-  clearPlottedDatastreams,
-} = useDataVisStore()
+  plottedDatastreams,
+  seriesDatastreams,
+  qcDatastream,
+  sourceContextDatastream,
+  loadingStates,
+} = storeToRefs(useDataVisStore())
+const { toggleDatastream, clearPlottedDatastreams } = useDataVisStore()
+
+const isEdit = (ds: Datastream) => qcDatastream.value?.id === ds.id
+const isSource = (ds: Datastream) =>
+  sourceContextDatastream.value?.id === ds.id
+/** Edit target and its source: always shown, never removed or reordered. */
+const isPinned = (ds: Datastream) => isEdit(ds) || isSource(ds)
+const isPrimary = (ds: Datastream, index: number) =>
+  isPinned(ds) || (!qcDatastream.value && index === 0)
+
+const contextCount = computed(
+  () => seriesDatastreams.value.filter((ds) => !isPinned(ds)).length
+)
 
 // Template treats `visibleDict[id] === false` as "hidden"; source of
 // truth lives in the store so the share URL can read it.
 const visibleDict = computed<Record<string, boolean>>(() => {
   const out: Record<string, boolean> = {}
-  for (const ds of plottedDatastreams.value) {
+  for (const ds of seriesDatastreams.value) {
     out[ds.id] = !hiddenTraceIds.value.has(ds.id)
   }
   return out
 })
 
-const setQcDatastream = async (datastream: Datastream) => {
-  await setQcInStore(datastream.id)
-}
-
 const isUpdating = computed(() =>
   Array.from(loadingStates.value.values()).some((isLoading) => isLoading)
 )
@@ -324,18 +327,25 @@ const toggleVisibility = async (datastream: Datastream) => {
   }
 }
 
-// Firefox needs `setData` for a drag to actually start, hence the payload.
+// Drag indices are row positions in `seriesDatastreams`. Firefox needs
+// `setData` for a drag to actually start, hence the payload.
 const dragIndex = ref<number | null>(null)
 const dropIndex = ref<number | null>(null)
 
+const isPinnedAt = (index: number) => {
+  const ds = seriesDatastreams.value[index]
+  return !ds || isPinned(ds)
+}
+
 function onDragStart(index: number, ev: DragEvent) {
+  if (isPinnedAt(index)) return
   dragIndex.value = index
   ev.dataTransfer?.setData('text/plain', String(index))
   if (ev.dataTransfer) ev.dataTransfer.effectAllowed = 'move'
 }
 
 function onDragOver(index: number, ev: DragEvent) {
-  if (dragIndex.value === null) return
+  if (dragIndex.value === null || isPinnedAt(index)) return
   dropIndex.value = index
   if (ev.dataTransfer) ev.dataTransfer.dropEffect = 'move'
 }
@@ -348,8 +358,8 @@ async function onDrop(index: number) {
   const from = dragIndex.value
   dragIndex.value = null
   dropIndex.value = null
-  if (from === null || from === index) return
-  reorder(from, index)
+  if (from === null || from === index || isPinnedAt(index)) return
+  if (!reorder(from, index)) return
   updateOptions()
   await handleNewPlot(undefined, { preserveZoom: true })
 }
@@ -359,23 +369,24 @@ function onDragEnd() {
   dropIndex.value = null
 }
 
-// Mirrors the reorder onto `graphSeriesArray` so plot trace order (and
-// the colour assignments derived from it) stays in sync.
-function reorder(from: number, to: number) {
+// Moves the dragged row to the target row's place in `plottedDatastreams`,
+// then re-sorts `graphSeriesArray` so trace order (and the colours derived
+// from it) follows `seriesDatastreams`.
+function reorder(from: number, to: number): boolean {
+  const movedId = seriesDatastreams.value[from]?.id
+  const targetId = seriesDatastreams.value[to]?.id
   const list = plottedDatastreams.value
-  const moved = list.splice(from, 1)[0]
-  if (!moved) return
-  list.splice(to, 0, moved)
-
-  const series = graphSeriesArray.value as GraphSeries[]
-  const fromSeries = series.findIndex((s) => s.id === moved.id)
-  if (fromSeries >= 0) {
-    const movedSeries = series.splice(fromSeries, 1)[0]
-    if (!movedSeries) return
-    const ids = list.map((d) => d.id)
-    const newIdx = ids.indexOf(moved.id)
-    series.splice(Math.max(0, newIdx), 0, movedSeries)
-  }
+  const fromPos = list.findIndex((d) => d.id === movedId)
+  const toPos = list.findIndex((d) => d.id === targetId)
+  if (fromPos < 0 || toPos < 0) return false
+  const moved = list.splice(fromPos, 1)[0]!
+  list.splice(toPos, 0, moved)
+
+  const order = new Map(seriesDatastreams.value.map((d, i) => [d.id, i]))
+  ;(graphSeriesArray.value as GraphSeries[]).sort(
+    (a, b) => (order.get(a.id) ?? 0) - (order.get(b.id) ?? 0)
+  )
+  return true
 }
 </script>
 
@@ -394,7 +405,7 @@ function reorder(from: number, to: number) {
 
 .plotted-item {
   display: grid;
-  grid-template-columns: 16px 18px 22px 22px 1fr 22px;
+  grid-template-columns: 16px 22px 22px 1fr 22px;
   align-items: center;
   gap: 6px;
   padding: 6px 8px;
@@ -403,10 +414,6 @@ function reorder(from: number, to: number) {
   position: relative;
 }
 
-.plotted-item--locked {
-  grid-template-columns: 16px 22px 22px 1fr 22px;
-}
-
 .plotted-item:last-child {
   border-bottom: none;
 }
@@ -459,33 +466,6 @@ function reorder(from: number, to: number) {
   cursor: grabbing;
 }
 
-/* Compact QC-target radio. currentColor picks up the inline series
-   colour per row; v-radio was too tall for the rail. */
-.plotted-item__dot {
-  width: 14px;
-  height: 14px;
-  padding: 0;
-  border: 2px solid currentColor;
-  background: transparent;
-  transition:
-    background-color 120ms ease,
-    box-shadow 120ms ease;
-}
-
-.plotted-item__dot:hover:not(:disabled) {
-  box-shadow: 0 0 0 3px rgba(0, 0, 0, 0.08);
-}
-
-.plotted-item__dot--active {
-  background: currentColor;
-  box-shadow: inset 0 0 0 2px #fff;
-}
-
-.plotted-item__dot:disabled {
-  cursor: default;
-  opacity: 0.5;
-}
-
 .plotted-item__visibility,
 .plotted-item__axis-toggle,
 .plotted-item__close {
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
index 79d83e8e..ded898c3 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
@@ -1,7 +1,7 @@
 /**
- * Unit tests for DataVisDatasetsTable.vue — focused on the plot checkbox.
- * A source with managed (QC) datastreams opens a chooser instead of
- * toggling, so the row's checked state has to speak for the whole group.
+ * Unit tests for DataVisDatasetsTable.vue: the plot checkbox and the row
+ * Edit button. A source with managed (QC) datastreams opens a chooser
+ * instead of toggling, so the row's checked state speaks for the group.
  */
 
 import { mount, flushPromises } from '@vue/test-utils'
@@ -9,6 +9,7 @@ import { ref } from 'vue'
 import { describe, it, expect, beforeEach, vi } from 'vitest'
 import { createTestVuetify } from '@/utils/test/vuetify'
 import PlotSourceDialog from '../PlotSourceDialog.vue'
+import DatastreamInformationCard from '../DatastreamInformationCard.vue'
 ;(globalThis as any).ResizeObserver ||= class {
   observe() {}
   unobserve() {}
@@ -24,7 +25,6 @@ const lonely = { id: 'solo', name: 'Solo', valueCount: 5, thing: { id: 't-1' } }
 
 const filteredDatastreams = ref<any[]>([raw, lonely])
 const plottedDatastreams = ref<any[]>([])
-const qcDatastream = ref<any>(null)
 const historiesBySource = ref(
   new Map<string, any[]>([
     ['src', [{ id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'src' }]],
@@ -40,7 +40,6 @@ vi.mock('@/store/dataVisualization', () => ({
   useDataVisStore: () => ({
     filteredDatastreams,
     plottedDatastreams,
-    qcDatastream,
     historiesBySource,
     toggleDatastream,
     clearPlottedDatastreams,
@@ -49,6 +48,11 @@ vi.mock('@/store/dataVisualization', () => ({
   }),
 }))
 
+const canEdit = vi.fn(() => true)
+vi.mock('@/composables/useWorkspacePermissions', () => ({
+  useWorkspacePermissions: () => ({ canEdit, roleName: () => 'owner' }),
+}))
+
 const loadForSource = vi.fn().mockResolvedValue([
   {
     historyId: 'h-1',
@@ -73,15 +77,18 @@ vi.mock('@uwrl/qc-utils', async (importOriginal) => {
 
 beforeEach(() => {
   vi.clearAllMocks()
+  canEdit.mockReturnValue(true)
   plottedDatastreams.value = []
-  qcDatastream.value = null
 })
 
 const mountTable = async () => {
   const DataVisDatasetsTable = (await import('../DataVisDatasetsTable.vue'))
     .default
   const wrapper = mount(DataVisDatasetsTable, {
-    global: { plugins: [createTestVuetify()] },
+    global: {
+      plugins: [createTestVuetify()],
+      stubs: { DatastreamInformationCard: true },
+    },
     attachTo: document.body,
   })
   await flushPromises()
@@ -158,3 +165,69 @@ describe('DataVisDatasetsTable plot checkbox', () => {
     )
   })
 })
+
+describe('DataVisDatasetsTable edit button', () => {
+  const editButton = (wrapper: any, id: string) =>
+    wrapper.find(`[data-testid="edit-datastream-${id}"]`)
+
+  it('emits edit with the row datastream when its Edit button is clicked', async () => {
+    const wrapper = await mountTable()
+    await editButton(wrapper, 'solo').trigger('click')
+
+    const emitted = wrapper.emitted('edit') as any[][]
+    expect(emitted).toHaveLength(1)
+    expect(emitted[0]![0].id).toBe('solo')
+  })
+
+  it('does not plot the row or open its details when Edit is clicked', async () => {
+    const wrapper = await mountTable()
+    await editButton(wrapper, 'solo').trigger('click')
+    await flushPromises()
+
+    expect(toggleDatastream).not.toHaveBeenCalled()
+    expect(loadForSource).not.toHaveBeenCalled()
+    expect(wrapper.findComponent(DatastreamInformationCard).exists()).toBe(
+      false
+    )
+  })
+
+  // Control for the test above: a plain row click does open the details.
+  it('opens the details when the row itself is clicked', async () => {
+    const wrapper = await mountTable()
+    await wrapper.find('.name-cell').trigger('click')
+    await flushPromises()
+
+    expect(wrapper.findComponent(DatastreamInformationCard).exists()).toBe(true)
+  })
+
+  it('disables Edit without workspace edit rights', async () => {
+    canEdit.mockReturnValue(false)
+    const wrapper = await mountTable()
+
+    expect(editButton(wrapper, 'solo').attributes('disabled')).toBeDefined()
+  })
+
+  // A disabled v-btn has `pointer-events: none`, so real clicks land on its
+  // tooltip wrapper. jsdom skips that CSS, hence the click on the wrapper.
+  it('does not open details when a disabled Edit wrapper is clicked', async () => {
+    canEdit.mockReturnValue(false)
+    const wrapper = await mountTable()
+    const tooltipWrapper = editButton(wrapper, 'solo').element.parentElement!
+    tooltipWrapper.dispatchEvent(new MouseEvent('click', { bubbles: true }))
+    await flushPromises()
+
+    expect(wrapper.emitted('edit')).toBeUndefined()
+    expect(wrapper.findComponent(DatastreamInformationCard).exists()).toBe(
+      false
+    )
+  })
+
+  it('no longer marks a QC target row', async () => {
+    plottedDatastreams.value = [lonely]
+    const wrapper = await mountTable()
+
+    expect(wrapper.find('.qc-pill').exists()).toBe(false)
+    expect(wrapper.find('.datasets-table__row--qc').exists()).toBe(false)
+    expect(wrapper.text()).not.toContain('QC target')
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
new file mode 100644
index 00000000..14f06485
--- /dev/null
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
@@ -0,0 +1,52 @@
+import { mount } from '@vue/test-utils'
+import { ref } from 'vue'
+import { describe, it, expect, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+import { createTestVuetify } from '@/utils/test/vuetify'
+
+const beginDate = ref<string | null>(null)
+const endDate = ref<string | null>(null)
+const selectedDateBtnId = ref<number | null>(null)
+const setDateRange = () => {}
+const onDateBtnClick = () => {}
+
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () => ({
+    beginDate,
+    endDate,
+    selectedDateBtnId,
+    setDateRange,
+    onDateBtnClick,
+  }),
+}))
+
+vi.mock('@/components/VisualizeData/DatePickerField.vue', () => ({
+  default: {
+    name: 'DatePickerField',
+    props: ['modelValue', 'placeholder'],
+    emits: ['update:modelValue'],
+    template: '<div class="date-picker-field-stub" />',
+  },
+}))
+
+import DataVisTimeFilters from '@/components/VisualizeData/DataVisTimeFilters.vue'
+
+function mountIt(props: Record<string, unknown> = {}) {
+  return mount(DataVisTimeFilters, {
+    props,
+    global: { plugins: [createTestPinia(), createTestVuetify()] },
+  })
+}
+
+describe('DataVisTimeFilters.vue', () => {
+  it('renders the default copy without the prop', () => {
+    const w = mountIt()
+    expect(w.text()).toContain('Loaded time window')
+  })
+
+  it('shows the given description instead of the default', () => {
+    const w = mountIt({ description: 'Context around the session.' })
+    expect(w.text()).toContain('Context around the session.')
+    expect(w.text()).not.toContain('Loaded time window')
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts
new file mode 100644
index 00000000..c3dde438
--- /dev/null
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts
@@ -0,0 +1,129 @@
+import { mount } from '@vue/test-utils'
+import {
+  defineComponent,
+  h,
+  nextTick,
+  onBeforeUnmount,
+  onMounted,
+  reactive,
+  ref,
+} from 'vue'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+import { createTestVuetify } from '@/utils/test/vuetify'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+const loadingStates = ref(new Map<string, boolean>())
+const seriesDatastreams = ref<any[]>([])
+const plotlyOptions = ref<any>({ traces: [] })
+
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () => reactive({ loadingStates, seriesDatastreams }),
+}))
+
+vi.mock('@/store/plotly', () => ({
+  usePlotlyStore: () => reactive({ plotlyOptions }),
+}))
+
+const lifecycle = vi.hoisted(() => ({ mounts: 0, unmounts: 0 }))
+
+vi.mock('@/components/VisualizeData/Plot.vue', () => ({
+  default: defineComponent({
+    name: 'PlotStub',
+    setup() {
+      onMounted(() => lifecycle.mounts++)
+      onBeforeUnmount(() => lifecycle.unmounts++)
+      return () => h('div', { 'data-testid': 'plot-stub' })
+    },
+  }),
+}))
+
+import DataVisualization from '@/components/VisualizeData/DataVisualization.vue'
+
+function mountIt() {
+  return mount(DataVisualization, {
+    global: { plugins: [createTestPinia(), createTestVuetify()] },
+  })
+}
+
+const setLoading = async (loading: boolean) => {
+  loadingStates.value = new Map([['ds-a', loading]])
+  await nextTick()
+}
+
+describe('DataVisualization.vue', () => {
+  beforeEach(() => {
+    lifecycle.mounts = 0
+    lifecycle.unmounts = 0
+    loadingStates.value = new Map()
+    seriesDatastreams.value = [{ id: 'ds-a' }]
+    plotlyOptions.value = { traces: [{ id: 'ds-a' }] }
+  })
+
+  it('keeps the same Plot instance mounted across a data update', async () => {
+    const wrapper = mountIt()
+    const plot = wrapper.findComponent({ name: 'PlotStub' })
+    expect(plot.exists()).toBe(true)
+
+    await setLoading(true)
+    expect(
+      wrapper.find('[data-testid="data-loading-indicator"]').exists()
+    ).toBe(true)
+    expect(wrapper.findComponent({ name: 'PlotStub' }).vm).toBe(plot.vm)
+
+    await setLoading(false)
+    expect(
+      wrapper.find('[data-testid="data-loading-indicator"]').exists()
+    ).toBe(false)
+    expect(wrapper.findComponent({ name: 'PlotStub' }).vm).toBe(plot.vm)
+    expect(lifecycle.mounts).toBe(1)
+    expect(lifecycle.unmounts).toBe(0)
+  })
+
+  it('mounts the plot under the loading overlay on first load', async () => {
+    plotlyOptions.value = { traces: [] }
+    const wrapper = mountIt()
+    await setLoading(true)
+    expect(
+      wrapper.find('[data-testid="data-loading-indicator"]').exists()
+    ).toBe(true)
+    expect(wrapper.findComponent({ name: 'PlotStub' }).exists()).toBe(true)
+
+    plotlyOptions.value = { traces: [{ id: 'ds-a' }] }
+    await setLoading(false)
+    expect(wrapper.findComponent({ name: 'PlotStub' }).exists()).toBe(true)
+    expect(lifecycle.mounts).toBe(1)
+  })
+
+  it('counts only the datastreams being fetched, not the edit target', async () => {
+    seriesDatastreams.value = [{ id: 'edit' }, { id: 'src' }, { id: 'ds-a' }]
+    const wrapper = mountIt()
+    loadingStates.value = new Map([
+      ['src', true],
+      ['ds-a', true],
+    ])
+    await nextTick()
+    expect(
+      wrapper.find('[data-testid="data-loading-indicator"]').text()
+    ).toContain('Fetching data for 2 datastreams')
+  })
+
+  it('shows the no-observations state when the load returns no traces', async () => {
+    plotlyOptions.value = { traces: [] }
+    const wrapper = mountIt()
+    expect(wrapper.text()).toContain('No observations in this range')
+    expect(wrapper.findComponent({ name: 'PlotStub' }).exists()).toBe(false)
+  })
+
+  it('shows the getting-started steps when nothing is plotted', () => {
+    seriesDatastreams.value = []
+    plotlyOptions.value = { traces: [] }
+    const wrapper = mountIt()
+    expect(wrapper.text()).toContain('Find a datastream')
+    expect(wrapper.findComponent({ name: 'PlotStub' }).exists()).toBe(false)
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
new file mode 100644
index 00000000..45139df8
--- /dev/null
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
@@ -0,0 +1,146 @@
+import { flushPromises, mount } from '@vue/test-utils'
+import { reactive, ref } from 'vue'
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+import { createTestVuetify } from '@/utils/test/vuetify'
+
+const plotly = {
+  isUpdating: ref(false),
+  areTooltipsEnabled: ref(true),
+  visiblePoints: ref(0),
+  tooltipsMaxDataPoints: ref(1000),
+  tooltipsMode: ref<'auto' | 'manual'>('auto'),
+  tooltipsManualEnabled: ref(true),
+  hover: ref({ x: 0, y: 0 }),
+  showCoordinates: ref(false),
+  crosshair: ref({ visible: false }),
+  axisChips: ref<any[]>([]),
+  previewMode: ref(false),
+  plotlyRef: ref<any>(null),
+  activeTab: ref('plot'),
+  pendingShareZoom: ref<any>(null),
+}
+const updateOptions = vi.fn()
+const requestTableScroll = vi.fn()
+
+vi.mock('@/store/plotly', () => ({
+  usePlotlyStore: () =>
+    reactive({ ...plotly, updateOptions, requestTableScroll }),
+}))
+
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () =>
+    reactive({
+      selectedData: ref(null),
+      hasSelectionShape: ref(false),
+      qcDatastream: ref(null),
+    }),
+}))
+
+const viewedSession = ref<any>(null)
+
+vi.mock('@/store/qcSession', () => ({
+  useQcSessionStore: () =>
+    reactive({ viewedSession, inProgressSession: ref(null) }),
+}))
+
+vi.mock('@/composables/useDataSelection', () => ({
+  useDataSelection: () => ({
+    setPlotSelection: vi.fn(),
+    clearSelected: vi.fn(),
+  }),
+}))
+
+vi.mock('@/composables/useResizable', () => ({
+  usePersistedFlag: (_key: string, initial: boolean) => ref(initial),
+}))
+
+const { handleNewPlot, zoomXaxisTo } = vi.hoisted(() => ({
+  handleNewPlot: vi.fn(),
+  zoomXaxisTo: vi.fn(),
+}))
+
+vi.mock('@/utils/plotting/plotly', () => ({
+  handleNewPlot,
+  handleRelayout: vi.fn(),
+  zoomXaxisTo,
+}))
+
+vi.mock('plotly.js-dist', () => ({ default: { Plots: { resize: vi.fn() } } }))
+
+import Plot from '@/components/VisualizeData/Plot.vue'
+
+function mountIt(preview = true) {
+  return mount(Plot, {
+    props: { preview },
+    global: {
+      plugins: [createTestPinia(), createTestVuetify()],
+      stubs: { ContextPlot: true, DataTable: true, DataVisTimeFilters: true },
+    },
+  })
+}
+
+describe('Plot.vue delayed mount', () => {
+  beforeEach(() => {
+    vi.useFakeTimers()
+    handleNewPlot.mockReset().mockResolvedValue(undefined)
+    zoomXaxisTo.mockClear()
+    plotly.plotlyRef.value = null
+    viewedSession.value = null
+  })
+
+  afterEach(() => {
+    vi.useRealTimers()
+  })
+
+  it('draws into the plot element after the mount delay', async () => {
+    const wrapper = mountIt()
+    await vi.advanceTimersByTimeAsync(200)
+    expect(handleNewPlot).toHaveBeenCalledTimes(1)
+    expect(handleNewPlot).toHaveBeenCalledWith(
+      wrapper.find('[data-testid="main-plot"]').element
+    )
+    wrapper.unmount()
+  })
+
+  it('does not draw when unmounted before the delay fires', async () => {
+    const wrapper = mountIt()
+    wrapper.unmount()
+    await vi.advanceTimersByTimeAsync(200)
+    expect(handleNewPlot).not.toHaveBeenCalled()
+  })
+
+  describe('editor zoom to the session window', () => {
+    let resolveDraw: () => void
+
+    beforeEach(() => {
+      plotly.plotlyRef.value = {}
+      viewedSession.value = {
+        phenomenonTimeStart: '2020-01-01T00:00:00Z',
+        phenomenonTimeEnd: '2020-01-02T00:00:00Z',
+      }
+      handleNewPlot.mockImplementation(
+        () => new Promise<void>((resolve) => (resolveDraw = resolve))
+      )
+    })
+
+    it('zooms once the mount draw finishes', async () => {
+      const wrapper = mountIt(false)
+      await vi.advanceTimersByTimeAsync(200)
+      resolveDraw()
+      await flushPromises()
+      expect(zoomXaxisTo).toHaveBeenCalledTimes(1)
+      wrapper.unmount()
+    })
+
+    it('does not zoom when unmounted while the mount draw is pending', async () => {
+      const wrapper = mountIt(false)
+      await vi.advanceTimersByTimeAsync(200)
+      expect(handleNewPlot).toHaveBeenCalledTimes(1)
+      wrapper.unmount()
+      resolveDraw()
+      await flushPromises()
+      expect(zoomXaxisTo).not.toHaveBeenCalled()
+    })
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
index d6f0f003..0a661164 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
@@ -5,8 +5,8 @@
  * we drop into `graphSeriesArray` for each case.
  */
 
-import { mount } from '@vue/test-utils'
-import { ref } from 'vue'
+import { mount, flushPromises } from '@vue/test-utils'
+import { computed, reactive, ref } from 'vue'
 import { describe, it, expect, beforeEach, vi } from 'vitest'
 import { createTestPinia } from '@/utils/test/pinia'
 import { createTestVuetify } from '@/utils/test/vuetify'
@@ -16,6 +16,18 @@ const datastreamB = { id: 'ds-b', name: 'Beta Stream', valueCount: 9999 }
 
 const plottedDatastreams = ref<any[]>([])
 const qcDatastream = ref<any>(null)
+const sourceContextDatastream = ref<any>(null)
+// Mirrors the store: edit target, its source, then the rest of plotted.
+const seriesDatastreams = computed(() => {
+  const edit = qcDatastream.value
+  if (!edit) return plottedDatastreams.value
+  const pinned = [edit, sourceContextDatastream.value].filter(Boolean)
+  const pinnedIds = new Set(pinned.map((d) => d.id))
+  return [
+    ...pinned,
+    ...plottedDatastreams.value.filter((d) => !pinnedIds.has(d.id)),
+  ]
+})
 const loadingStates = ref(new Map<string, boolean>())
 const graphSeriesArray = ref<any[]>([])
 const plotlyRef = ref<any>(null)
@@ -24,21 +36,23 @@ const hiddenAxisIds = ref<Set<string>>(new Set())
 const hiddenTraceIds = ref<Set<string>>(new Set())
 
 const toggleDatastream = vi.fn().mockResolvedValue(undefined)
-const setQcInStore = vi.fn().mockResolvedValue(undefined)
 const clearPlottedDatastreams = vi.fn().mockResolvedValue(undefined)
 const updateOptions = vi.fn()
 const colorForDatastream = vi.fn(() => '#000')
 const labelColorForDatastream = vi.fn(() => '#000')
 
+// Reactive so `storeToRefs` unwraps the computed as it does for a real store.
 vi.mock('@/store/dataVisualization', () => ({
-  useDataVisStore: () => ({
+  useDataVisStore: () =>
+    reactive({
     plottedDatastreams,
     qcDatastream,
+    sourceContextDatastream,
+    seriesDatastreams,
     loadingStates,
     toggleDatastream,
-    setQcDatastream: setQcInStore,
     clearPlottedDatastreams,
-  }),
+    }),
 }))
 
 vi.mock('@/store/plotly', () => ({
@@ -287,3 +301,137 @@ describe('PlottedDatastreams snapshot rows', () => {
     expect(rows[1]!.find('.plotted-item__axis-toggle').exists()).toBe(true)
   })
 })
+
+describe('PlottedDatastreams row kinds', () => {
+  const edit = { id: 'mgd', name: 'Edit Stream' }
+  const source = { id: 'src', name: 'Raw Stream' }
+  const ctx = { id: 'ctx', name: 'Context Stream' }
+  const ctx2 = { id: 'ctx2', name: 'Other Stream' }
+
+  beforeEach(() => {
+    vi.clearAllMocks()
+    qcDatastream.value = null
+    sourceContextDatastream.value = null
+    plottedDatastreams.value = [ctx, ctx2]
+    graphSeriesArray.value = []
+    plotlyOptions.value = { traces: [] }
+    hiddenAxisIds.value = new Set()
+    hiddenTraceIds.value = new Set()
+  })
+
+  function startEditing() {
+    qcDatastream.value = edit
+    sourceContextDatastream.value = source
+  }
+
+  const row = (wrapper: ReturnType<typeof mountIt>, id: string) =>
+    wrapper.find(`[data-testid="plotted-item-${id}"]`)
+  const plottedIds = () => plottedDatastreams.value.map((d) => d.id)
+
+  it('lists the edit target, its source, then the plotted context', () => {
+    startEditing()
+    const wrapper = mountIt()
+    const ids = wrapper
+      .findAll('.plotted-item')
+      .map((r) => r.attributes('data-testid'))
+
+    expect(ids).toEqual([
+      'plotted-item-mgd',
+      'plotted-item-src',
+      'plotted-item-ctx',
+      'plotted-item-ctx2',
+    ])
+  })
+
+  it('gives the source row no remove and no axis toggle', () => {
+    startEditing()
+    const r = row(mountIt(), 'src')
+
+    expect(r.classes()).toContain('plotted-item--source')
+    expect(r.text()).toContain('raw source')
+    expect(r.find('.plotted-item__close').exists()).toBe(false)
+    expect(r.find('.plotted-item__axis-toggle').exists()).toBe(false)
+  })
+
+  it('gives the edit row no remove and no axis toggle', () => {
+    startEditing()
+    const r = row(mountIt(), 'mgd')
+
+    expect(r.classes()).toContain('plotted-item--qc')
+    expect(r.find('.plotted-item__close').exists()).toBe(false)
+    expect(r.find('.plotted-item__axis-toggle').exists()).toBe(false)
+  })
+
+  it('keeps remove and axis toggle on context rows while editing', () => {
+    startEditing()
+    const r = row(mountIt(), 'ctx')
+
+    expect(r.find('.plotted-item__close').exists()).toBe(true)
+    expect(r.find('.plotted-item__axis-toggle').exists()).toBe(true)
+  })
+
+  it('hides the axis toggle on the first row in Select only', () => {
+    const wrapper = mountIt()
+
+    expect(row(wrapper, 'ctx').find('.plotted-item__axis-toggle').exists()).toBe(
+      false
+    )
+    expect(row(wrapper, 'ctx').find('.plotted-item__close').exists()).toBe(true)
+    expect(
+      row(wrapper, 'ctx2').find('.plotted-item__axis-toggle').exists()
+    ).toBe(true)
+  })
+
+  it('has no QC target radio', () => {
+    expect(mountIt().find('.plotted-item__dot').exists()).toBe(false)
+  })
+
+  it('shows Unplot all only when nothing is being edited', () => {
+    expect(mountIt().text()).toContain('Unplot all')
+    startEditing()
+    expect(mountIt().text()).not.toContain('Unplot all')
+  })
+
+  it('makes only context rows draggable', () => {
+    startEditing()
+    const wrapper = mountIt()
+
+    expect(row(wrapper, 'mgd').attributes('draggable')).toBe('false')
+    expect(row(wrapper, 'src').attributes('draggable')).toBe('false')
+    expect(row(wrapper, 'ctx').attributes('draggable')).toBe('true')
+  })
+
+  it('reorders plotted datastreams when a context row is dropped while editing', async () => {
+    startEditing()
+    const wrapper = mountIt()
+    await row(wrapper, 'ctx2').trigger('dragstart')
+    await row(wrapper, 'ctx').trigger('dragover')
+    await row(wrapper, 'ctx').trigger('drop')
+    await flushPromises()
+
+    expect(plottedIds()).toEqual(['ctx2', 'ctx'])
+    expect(updateOptions).toHaveBeenCalled()
+  })
+
+  it('ignores a drop onto a pinned row', async () => {
+    startEditing()
+    const wrapper = mountIt()
+    await row(wrapper, 'ctx2').trigger('dragstart')
+    await row(wrapper, 'mgd').trigger('dragover')
+    await row(wrapper, 'mgd').trigger('drop')
+    await flushPromises()
+
+    expect(plottedIds()).toEqual(['ctx', 'ctx2'])
+    expect(updateOptions).not.toHaveBeenCalled()
+  })
+
+  it('reorders plotted datastreams in Select', async () => {
+    const wrapper = mountIt()
+    await row(wrapper, 'ctx2').trigger('dragstart')
+    await row(wrapper, 'ctx').trigger('dragover')
+    await row(wrapper, 'ctx').trigger('drop')
+    await flushPromises()
+
+    expect(plottedIds()).toEqual(['ctx2', 'ctx'])
+  })
+})
diff --git a/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts b/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
new file mode 100644
index 00000000..15be4fd1
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
@@ -0,0 +1,273 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+
+// vi.mock factories are hoisted above module-level consts, so anything they
+// close over has to be created inside vi.hoisted.
+const {
+  qcDatastream,
+  setEditTarget,
+  clearEditTarget,
+  beginEditing,
+  startSession,
+  needsSession,
+  needsHistory,
+  currentView,
+  selectedDrawer,
+  isDrawerOpen,
+  resumeDatastreamId,
+  success,
+  error,
+  ResumeSupersededError,
+} = vi.hoisted(() => {
+  const { ref: r } = require('vue') as typeof import('vue')
+  class ResumeSupersededError extends Error {
+    constructor() {
+      super('superseded')
+      this.name = 'ResumeSupersededError'
+    }
+  }
+  return {
+    qcDatastream: r<{ id: string } | null>(null),
+    setEditTarget: vi.fn(),
+    clearEditTarget: vi.fn(),
+    beginEditing: vi.fn(),
+    startSession: vi.fn(),
+    needsSession: r(false),
+    needsHistory: r(false),
+    currentView: r('Select'),
+    selectedDrawer: r('Select'),
+    isDrawerOpen: r(false),
+    resumeDatastreamId: r<string | null>(null),
+    success: vi.fn(),
+    error: vi.fn(),
+    ResumeSupersededError,
+  }
+})
+
+// Real Pinia stores, so `storeToRefs` behaves as it does in production.
+vi.mock('@/store/dataVisualization', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useDataVisStore: defineStore('dataVisualization', () => ({
+      qcDatastream,
+      setEditTarget,
+      clearEditTarget,
+    })),
+  }
+})
+
+vi.mock('@/store/userInterface', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    DrawerType: { Edit: 'Edit', Select: 'Select', None: '' },
+    useUIStore: defineStore('userInterface', () => ({
+      currentView,
+      selectedDrawer,
+      isDrawerOpen,
+    })),
+  }
+})
+
+vi.mock('@/store/qcSession', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useQcSessionStore: defineStore('qcSession', () => ({ resumeDatastreamId })),
+  }
+})
+
+vi.mock('@/composables/useEditSession', () => ({
+  ResumeSupersededError,
+  useEditSession: () => ({
+    beginEditing,
+    startSession,
+    needsSession,
+    needsHistory,
+  }),
+}))
+
+vi.mock('@uwrl/qc-utils', () => ({ Snackbar: { success, error } }))
+
+import { useEditEntry } from '../useEditEntry'
+
+const window = {
+  begin: new Date('2025-01-01T00:00:00Z'),
+  end: new Date('2025-02-01T00:00:00Z'),
+}
+
+beforeEach(() => {
+  createTestPinia()
+  vi.clearAllMocks()
+  needsSession.value = false
+  needsHistory.value = false
+  currentView.value = 'Select'
+  selectedDrawer.value = 'Select'
+  isDrawerOpen.value = false
+  resumeDatastreamId.value = null
+  qcDatastream.value = null
+  setEditTarget.mockImplementation(async (id: string) => {
+    qcDatastream.value = { id }
+  })
+  clearEditTarget.mockImplementation(async () => {
+    qcDatastream.value = null
+  })
+  beginEditing.mockResolvedValue(true)
+  startSession.mockResolvedValue(undefined)
+})
+
+describe('useEditEntry', () => {
+  it('resumes into the editor when a session is in progress', async () => {
+    needsSession.value = false
+    const result = await useEditEntry().enterEdit('mgd')
+    expect(setEditTarget).toHaveBeenCalledWith('mgd')
+    expect(result).toBe('editing')
+    expect(currentView.value).toBe('Edit')
+    expect(resumeDatastreamId.value).toBe('mgd')
+  })
+
+  it('asks for a window when there is no session and none was given', async () => {
+    beginEditing.mockImplementation(async () => {
+      needsSession.value = true
+      return false
+    })
+    expect(await useEditEntry().enterEdit('mgd')).toBe('needs-window')
+    expect(startSession).not.toHaveBeenCalled()
+  })
+
+  it('starts a session over the given window', async () => {
+    beginEditing.mockImplementation(async () => {
+      needsSession.value = true
+      return false
+    })
+    expect(await useEditEntry().enterEdit('mgd', window)).toBe('editing')
+    expect(startSession).toHaveBeenCalledWith({
+      phenomenonTimeStart: '2025-01-01T00:00:00.000Z',
+      phenomenonTimeEnd: '2025-02-01T00:00:00.000Z',
+    })
+  })
+
+  it('keeps the editor open when starting the session fails', async () => {
+    beginEditing.mockImplementation(async () => {
+      needsSession.value = true
+      return false
+    })
+    startSession.mockRejectedValueOnce(new Error('bad window'))
+    expect(await useEditEntry().enterEdit('mgd', window)).toBe('needs-window')
+    expect(error).toHaveBeenCalledWith('bad window')
+    expect(currentView.value).toBe('Edit')
+    expect(clearEditTarget).not.toHaveBeenCalled()
+  })
+
+  it('leaves the editor when the new session was superseded', async () => {
+    beginEditing.mockImplementation(async () => {
+      needsSession.value = true
+      return false
+    })
+    startSession.mockRejectedValueOnce(new ResumeSupersededError())
+    expect(await useEditEntry().enterEdit('mgd', window)).toBe('superseded')
+    expect(currentView.value).toBe('Select')
+    expect(clearEditTarget).toHaveBeenCalled()
+  })
+
+  it('leaves the editor when the resume was superseded', async () => {
+    beginEditing.mockRejectedValueOnce(new ResumeSupersededError())
+    expect(await useEditEntry().enterEdit('mgd')).toBe('superseded')
+    expect(error).toHaveBeenCalledWith('superseded')
+    expect(currentView.value).toBe('Select')
+    expect(resumeDatastreamId.value).toBeNull()
+  })
+
+  it('leaves the editor and rethrows any other resume failure', async () => {
+    beginEditing.mockRejectedValueOnce(new Error('offline'))
+    await expect(useEditEntry().enterEdit('mgd')).rejects.toThrow('offline')
+    expect(clearEditTarget).toHaveBeenCalled()
+    expect(currentView.value).toBe('Select')
+  })
+
+  it('leaves the editor and rethrows when setting the target fails', async () => {
+    setEditTarget.mockImplementationOnce(async (id: string) => {
+      qcDatastream.value = { id }
+      throw new Error('rebuild failed')
+    })
+    resumeDatastreamId.value = 'mgd'
+    await expect(useEditEntry().enterEdit('mgd')).rejects.toThrow(
+      'rebuild failed'
+    )
+    expect(clearEditTarget).toHaveBeenCalled()
+    expect(resumeDatastreamId.value).toBeNull()
+    expect(currentView.value).toBe('Select')
+    expect(beginEditing).not.toHaveBeenCalled()
+  })
+
+  // A newer entry took over while this one awaited; it owns the view state.
+  const takeOverDuringBegin = (flags: () => void) =>
+    beginEditing.mockImplementation(async () => {
+      qcDatastream.value = { id: 'newer' }
+      resumeDatastreamId.value = 'newer'
+      currentView.value = 'Edit'
+      flags()
+      return false
+    })
+
+  it('stands down when the target changes before a not-managed result', async () => {
+    takeOverDuringBegin(() => {
+      needsHistory.value = true
+    })
+    expect(await useEditEntry().enterEdit('mgd')).toBe('superseded')
+    expect(clearEditTarget).not.toHaveBeenCalled()
+    expect(resumeDatastreamId.value).toBe('newer')
+    expect(currentView.value).toBe('Edit')
+    expect(error).not.toHaveBeenCalled()
+  })
+
+  it('stands down when the target changes before starting a session', async () => {
+    takeOverDuringBegin(() => {
+      needsSession.value = true
+    })
+    expect(await useEditEntry().enterEdit('mgd', window)).toBe('superseded')
+    expect(startSession).not.toHaveBeenCalled()
+    expect(clearEditTarget).not.toHaveBeenCalled()
+    expect(resumeDatastreamId.value).toBe('newer')
+  })
+
+  it('does not leave a newer target when its own start is superseded', async () => {
+    beginEditing.mockImplementation(async () => {
+      needsSession.value = true
+      return false
+    })
+    startSession.mockImplementationOnce(async () => {
+      qcDatastream.value = { id: 'newer' }
+      resumeDatastreamId.value = 'newer'
+      throw new ResumeSupersededError()
+    })
+    expect(await useEditEntry().enterEdit('mgd', window)).toBe('superseded')
+    expect(clearEditTarget).not.toHaveBeenCalled()
+    expect(resumeDatastreamId.value).toBe('newer')
+  })
+
+  it('backs out when the datastream has no QC history', async () => {
+    beginEditing.mockImplementation(async () => {
+      needsHistory.value = true
+      return false
+    })
+    expect(await useEditEntry().enterEdit('mgd')).toBe('not-managed')
+    expect(clearEditTarget).toHaveBeenCalled()
+    expect(currentView.value).toBe('Select')
+  })
+
+  it('startSessionOver reports whether the session started', async () => {
+    const { startSessionOver } = useEditEntry()
+    expect(await startSessionOver(window)).toBe(true)
+    expect(success).toHaveBeenCalledWith('Edit session started.')
+    startSession.mockRejectedValueOnce(new Error('nope'))
+    expect(await startSessionOver(window)).toBe(false)
+  })
+
+  it('leaveEdit returns to Select and forgets the resume target', async () => {
+    resumeDatastreamId.value = 'mgd'
+    currentView.value = 'Edit'
+    await useEditEntry().leaveEdit()
+    expect(currentView.value).toBe('Select')
+    expect(resumeDatastreamId.value).toBeNull()
+    expect(clearEditTarget).toHaveBeenCalled()
+  })
+})
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 4b8668a0..3e7af1b8 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -6,8 +6,15 @@ import { unwrap } from '@/services/qualityControl/unwrap'
 
 const qcDatastream = ref<any>(null)
 const replaceDatastream = vi.fn()
+const setEditRecord = vi.fn(async (record: any) => {
+  // A real await here so a dropped `await` before `setEditRecord` in the
+  // composable (leaving the update to run after the caller reads
+  // `selectedSeries`) shows up as a test failure instead of passing by luck.
+  await Promise.resolve()
+  selectedSeries.value = { data: record }
+})
 vi.mock('@/store/dataVisualization', () => ({
-  useDataVisStore: () => ({ qcDatastream, replaceDatastream }),
+  useDataVisStore: () => ({ qcDatastream, replaceDatastream, setEditRecord }),
 }))
 
 const selectedSeries = ref<any>(null)
@@ -189,13 +196,37 @@ describe('useEditSession', () => {
       h.id,
       expect.objectContaining({ id: created.id })
     )
-    // The QC-target series shows exactly the record the working copy store
-    // resolved, so the plot and the editor never diverge. `toRaw` unwraps
-    // the reactive proxy Vue puts on the assigned object.
+    // setEditRecord renders the resumed record, so the plot and the editor
+    // never diverge.
+    expect(setEditRecord).toHaveBeenCalledWith(sharedRecord)
+    // `toRaw` unwraps the reactive proxy Vue puts on the assigned object.
     expect(toRaw(selectedSeries.value.data)).toBe(sharedRecord)
-    // Nothing watches for a swapped-in record, so resume has to rebuild the
-    // plot itself or it keeps rendering the pre-reconstruction trace.
-    expect(redraw).toHaveBeenCalled()
+  })
+
+  it('resumes an in-progress session even before the edit series exists', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const created = unwrap(await qc.sessions.create(h.id, WIN))
+    // No edit series yet: `qcDatastream` isn't guaranteed to have a graph
+    // series before a record arrives, so resume must not depend on one.
+    selectedSeries.value = undefined
+    const rebuiltRecord = makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }])
+    wcRebuild.mockResolvedValue({
+      sessionId: created.id,
+      record: rebuiltRecord,
+      begin: new Date(0),
+      end: new Date(1),
+    })
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const { beginEditing } = useEditSession()
+
+    await expect(beginEditing()).resolves.toBe(true)
+
+    expect(setEditRecord).toHaveBeenCalledWith(rebuiltRecord)
   })
 
   it('beginEditing rejects when the working copy is superseded mid-resume, without asking to start a session', async () => {
@@ -222,7 +253,37 @@ describe('useEditSession', () => {
     expect(needsSession.value).toBe(false)
     expect(selectedSeries.value.data).toBe(original)
     expect(useQcSessionStore().savedEdits).toEqual([])
-    expect(redraw).not.toHaveBeenCalled()
+    expect(setEditRecord).not.toHaveBeenCalled()
+  })
+
+  it('beginEditing rejects when the edit target is cleared while the resume is in flight, without calling setEditRecord', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    await qc.sessions.create(h.id, WIN)
+    // Simulates leaving the editor mid-resume: it clears the edit target but
+    // not the session store, so the awaited rebuild resolves into a stale
+    // `managed` that no longer matches `qcDatastream`.
+    wcRebuild.mockImplementationOnce(async () => {
+      qcDatastream.value = null
+      return {
+        sessionId: 'x',
+        record: makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }]),
+        begin: new Date(0),
+        end: new Date(1),
+      }
+    })
+    const { useEditSession, ResumeSupersededError } = await import(
+      '@/composables/useEditSession'
+    )
+    const { beginEditing } = useEditSession()
+
+    await expect(beginEditing()).rejects.toBeInstanceOf(ResumeSupersededError)
+
+    expect(setEditRecord).not.toHaveBeenCalled()
   })
 
   it('startSession loads the managed datastream as the working base', async () => {
@@ -646,7 +707,7 @@ describe('useEditSession.viewSession', () => {
     expect(toRaw(seriesRecord)).not.toBe(viewed)
     expect(store.viewedSessionId).toBe(committed.id)
     expect(store.isReadOnly).toBe(true)
-    expect(redraw).toHaveBeenCalled()
+    expect(setEditRecord).toHaveBeenCalledWith(toRaw(seriesRecord))
   })
 
   it('moves the selection before loading, so the spinner sits on the new session', async () => {
@@ -746,10 +807,69 @@ describe('useEditSession.viewSession', () => {
     expect(session.needsSession.value).toBe(false)
   })
 
+  it('rejects when the edit target changes while a committed session loads, without calling setEditRecord', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const committed = unwrap(await qc.sessions.create(h.id, WIN))
+    await qc.sessions.commit(h.id, committed.id)
+    const inProgress = unwrap(await qc.sessions.create(h.id, WIN))
+
+    const store = useQcSessionStore()
+    const { useEditSession, ResumeSupersededError } = await import(
+      '@/composables/useEditSession'
+    )
+    const session = useEditSession()
+    await session.beginEditing()
+    setEditRecord.mockClear()
+
+    // The user leaves and enters another target mid-reconstruction.
+    fetchObservationsInRange.mockImplementationOnce(async () => {
+      qcDatastream.value = { id: 'm-2' }
+      return makeRecord([])
+    })
+
+    await expect(session.viewSession(committed.id)).rejects.toBeInstanceOf(
+      ResumeSupersededError
+    )
+    expect(setEditRecord).not.toHaveBeenCalled()
+    expect(store.viewedSessionId).toBe(inProgress.id)
+    expect(store.isSwitchingSession).toBe(false)
+  })
+
   it('rejects before a managed datastream is loaded', async () => {
     const { useEditSession } = await import('@/composables/useEditSession')
     await expect(useEditSession().viewSession('s-1')).rejects.toThrow(
       /Load a managed datastream/
     )
   })
+
+  it('rejects when the edit target is gone even though the session store still has state', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const committed = unwrap(await qc.sessions.create(h.id, WIN))
+    await qc.sessions.commit(h.id, committed.id)
+    await qc.sessions.create(h.id, WIN)
+
+    const store = useQcSessionStore()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    const viewedBefore = store.viewedSessionId
+
+    // Leaving the editor clears the edit target but not the session store.
+    qcDatastream.value = null
+
+    await expect(session.viewSession(committed.id)).rejects.toThrow(
+      /Load a managed datastream/
+    )
+    expect(store.viewedSessionId).toBe(viewedBefore)
+  })
 })
diff --git a/apps/qc-app/src/composables/__tests__/useQcHistory.spec.ts b/apps/qc-app/src/composables/__tests__/useQcHistory.spec.ts
new file mode 100644
index 00000000..2471b989
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useQcHistory.spec.ts
@@ -0,0 +1,110 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
+import { ref } from 'vue'
+
+const { serializeHistory } = vi.hoisted(() => ({
+  serializeHistory: vi.fn(() => ({ version: 1 })),
+}))
+
+vi.mock('@uwrl/qc-utils', () => ({
+  serializeHistory,
+  parseHistory: vi.fn(),
+  applyHistory: vi.fn(),
+}))
+
+const selectedSeries = ref<any>({ data: { id: 'record' } })
+const qcDatastream = ref<any>({ id: 'mgd', name: 'Managed' })
+const beginDate = ref(new Date('2020-01-01T00:00:00Z'))
+const endDate = ref(new Date('2020-12-31T00:00:00Z'))
+const viewedSession = ref<any>(null)
+const inProgressSession = ref<any>(null)
+
+vi.mock('@/store/plotly', () => ({
+  usePlotlyStore: () => ({ selectedSeries }),
+}))
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () => ({ qcDatastream, beginDate, endDate }),
+}))
+vi.mock('@/store/observations', () => ({
+  useObservationStore: () => ({ fetchObservationsInRange: vi.fn() }),
+}))
+vi.mock('@/store/qcSession', () => ({
+  useQcSessionStore: () => ({ viewedSession, inProgressSession }),
+}))
+
+import { useQcHistory } from '../useQcHistory'
+
+const session = (start: string, end: string) => ({
+  phenomenonTimeStart: start,
+  phenomenonTimeEnd: end,
+})
+
+describe('useQcHistory.exportHistory window', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    selectedSeries.value = { data: { id: 'record' } }
+    viewedSession.value = null
+    inProgressSession.value = null
+    URL.createObjectURL = vi.fn(() => 'blob:history')
+    URL.revokeObjectURL = vi.fn()
+    // jsdom does not implement the download navigation.
+    vi.spyOn(HTMLAnchorElement.prototype, 'click').mockImplementation(() => {})
+  })
+
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  const exportedWindow = () =>
+    (serializeHistory.mock.calls[0] as unknown as [unknown, unknown])[1]
+
+  it('uses the viewed session window while editing', async () => {
+    viewedSession.value = session('2023-03-01T00:00:00Z', '2023-04-01T00:00:00Z')
+    inProgressSession.value = session(
+      '2024-01-01T00:00:00Z',
+      '2024-02-01T00:00:00Z'
+    )
+
+    await useQcHistory().exportHistory()
+
+    expect(serializeHistory).toHaveBeenCalledWith(
+      { id: 'record' },
+      expect.anything()
+    )
+    expect(exportedWindow()).toEqual({
+      startDate: '2023-03-01T00:00:00.000Z',
+      endDate: '2023-04-01T00:00:00.000Z',
+    })
+  })
+
+  it('falls back to the in-progress session when none is viewed', async () => {
+    inProgressSession.value = session(
+      '2024-01-01T00:00:00Z',
+      '2024-02-01T00:00:00Z'
+    )
+
+    await useQcHistory().exportHistory()
+
+    expect(exportedWindow()).toEqual({
+      startDate: '2024-01-01T00:00:00.000Z',
+      endDate: '2024-02-01T00:00:00.000Z',
+    })
+  })
+
+  it('uses the loaded window when there is no session', async () => {
+    await useQcHistory().exportHistory()
+
+    expect(exportedWindow()).toEqual({
+      startDate: '2020-01-01T00:00:00.000Z',
+      endDate: '2020-12-31T00:00:00.000Z',
+    })
+  })
+
+  it('throws when no series is loaded', async () => {
+    selectedSeries.value = null
+
+    await expect(useQcHistory().exportHistory()).rejects.toThrow(
+      'No QC series loaded.'
+    )
+    expect(serializeHistory).not.toHaveBeenCalled()
+  })
+})
diff --git a/apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts
index 7e9951c3..8eae6ec3 100644
--- a/apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts
@@ -5,18 +5,10 @@ import { createPinia, setActivePinia } from 'pinia'
 
 // vi.mock factories are hoisted above module-level consts, so anything they
 // close over has to be created inside vi.hoisted.
-const {
-  datastreams,
-  plotDatastream,
-  setQcDatastream,
-  resumeDatastreamId,
-  error,
-} = vi.hoisted(() => {
+const { datastreams, resumeDatastreamId, error } = vi.hoisted(() => {
   const { ref: r } = require('vue') as typeof import('vue')
   return {
     datastreams: r<any[]>([]),
-    plotDatastream: vi.fn(),
-    setQcDatastream: vi.fn(),
     resumeDatastreamId: r<string | null>(null),
     error: vi.fn(),
   }
@@ -26,11 +18,7 @@ const {
 vi.mock('@/store/dataVisualization', async () => {
   const { defineStore } = await import('pinia')
   return {
-    useDataVisStore: defineStore('dataVisualization', () => ({
-      datastreams,
-      plotDatastream,
-      setQcDatastream,
-    })),
+    useDataVisStore: defineStore('dataVisualization', () => ({ datastreams })),
   }
 })
 
@@ -47,7 +35,7 @@ vi.mock('@uwrl/qc-utils', () => ({ Snackbar: { error } }))
 
 import { useResumeEditSession } from '../useResumeEditSession'
 
-const enterEdit = vi.fn()
+const resume = vi.fn()
 
 let pinia: ReturnType<typeof createPinia>
 // The stores are module-level refs shared across tests, so a host left
@@ -58,7 +46,7 @@ const mountHost = () => {
   const wrapper = mount(
     defineComponent({
       setup() {
-        useResumeEditSession(enterEdit)
+        useResumeEditSession(resume)
         return () => null
       },
     }),
@@ -77,6 +65,7 @@ beforeEach(() => {
   pinia = createPinia()
   setActivePinia(pinia)
   vi.clearAllMocks()
+  resume.mockResolvedValue(undefined)
   datastreams.value = []
   resumeDatastreamId.value = null
 })
@@ -88,14 +77,12 @@ describe('useResumeEditSession', () => {
   it('resumes when the catalog arrives after mount', async () => {
     resumeDatastreamId.value = 'mgd-1'
     mountHost()
-    expect(enterEdit).not.toHaveBeenCalled()
+    expect(resume).not.toHaveBeenCalled()
 
     datastreams.value = [{ id: 'mgd-1' }]
     await flushPromises()
 
-    expect(plotDatastream).toHaveBeenCalledWith({ id: 'mgd-1' })
-    expect(setQcDatastream).toHaveBeenCalledWith('mgd-1')
-    expect(enterEdit).toHaveBeenCalled()
+    expect(resume).toHaveBeenCalledWith('mgd-1')
   })
 
   it('resumes immediately when the catalog is already loaded', async () => {
@@ -103,14 +90,14 @@ describe('useResumeEditSession', () => {
     datastreams.value = [{ id: 'mgd-1' }]
     mountHost()
     await flushPromises()
-    expect(enterEdit).toHaveBeenCalled()
+    expect(resume).toHaveBeenCalledWith('mgd-1')
   })
 
   it('does nothing without a stored session', async () => {
     datastreams.value = [{ id: 'mgd-1' }]
     mountHost()
     await flushPromises()
-    expect(enterEdit).not.toHaveBeenCalled()
+    expect(resume).not.toHaveBeenCalled()
   })
 
   it('drops a pointer to a datastream missing from the catalog', async () => {
@@ -119,7 +106,7 @@ describe('useResumeEditSession', () => {
     datastreams.value = [{ id: 'mgd-1' }]
     await flushPromises()
 
-    expect(enterEdit).not.toHaveBeenCalled()
+    expect(resume).not.toHaveBeenCalled()
     expect(resumeDatastreamId.value).toBeNull()
   })
 
@@ -128,16 +115,16 @@ describe('useResumeEditSession', () => {
     mountHost()
     datastreams.value = [{ id: 'mgd-1' }]
     await flushPromises()
-    expect(enterEdit).toHaveBeenCalledTimes(1)
+    expect(resume).toHaveBeenCalledTimes(1)
 
     datastreams.value = [{ id: 'mgd-1' }, { id: 'mgd-2' }]
     await flushPromises()
-    expect(enterEdit).toHaveBeenCalledTimes(1)
+    expect(resume).toHaveBeenCalledTimes(1)
   })
 
   it('surfaces a failure instead of failing silently', async () => {
     resumeDatastreamId.value = 'mgd-1'
-    enterEdit.mockRejectedValueOnce(new Error('boom'))
+    resume.mockRejectedValueOnce(new Error('boom'))
     mountHost()
     datastreams.value = [{ id: 'mgd-1' }]
     await flushPromises()
diff --git a/apps/qc-app/src/composables/useEditEntry.ts b/apps/qc-app/src/composables/useEditEntry.ts
new file mode 100644
index 00000000..a3714c05
--- /dev/null
+++ b/apps/qc-app/src/composables/useEditEntry.ts
@@ -0,0 +1,114 @@
+/**
+ * Entering and leaving the editor for a managed datastream: set the edit
+ * target, resume its in-progress session or start one over a chosen window,
+ * and switch views. Shared by the row Edit flow, reload resume and share links.
+ *
+ * Entries can overlap (a reload resume and a click). Whichever set the target
+ * last owns the view, so an entry that finds the target changed after an await
+ * stands down without touching view state, the target or the resume pointer.
+ */
+
+import { storeToRefs } from 'pinia'
+import { Snackbar } from '@uwrl/qc-utils'
+import { useDataVisStore } from '@/store/dataVisualization'
+import { useQcSessionStore } from '@/store/qcSession'
+import { DrawerType, useUIStore } from '@/store/userInterface'
+import {
+  ResumeSupersededError,
+  useEditSession,
+} from '@/composables/useEditSession'
+import type { TimeWindow } from '@/utils/timeRangePresets'
+
+export type EnterEditResult =
+  | 'editing'
+  | 'needs-window'
+  | 'not-managed'
+  | 'superseded'
+
+type StartOutcome = 'started' | 'superseded' | 'failed'
+
+export function useEditEntry() {
+  const dataVis = useDataVisStore()
+  const { qcDatastream } = storeToRefs(dataVis)
+  const { setEditTarget, clearEditTarget } = dataVis
+  const { currentView, selectedDrawer, isDrawerOpen } = storeToRefs(useUIStore())
+  const { resumeDatastreamId } = storeToRefs(useQcSessionStore())
+  const { beginEditing, startSession, needsSession, needsHistory } =
+    useEditSession()
+
+  const owns = (id: string | undefined) => qcDatastream.value?.id === id
+  // A cleared target is not a takeover: leaving again is harmless.
+  const takenOver = (id: string | undefined) =>
+    !!qcDatastream.value && !owns(id)
+
+  async function leaveEdit() {
+    currentView.value = DrawerType.Select
+    selectedDrawer.value = DrawerType.Select
+    isDrawerOpen.value = true
+    resumeDatastreamId.value = null
+    await clearEditTarget()
+  }
+
+  // A superseded start has already left the editor (unless a newer entry
+  // owns it); a failed one leaves it open with no session.
+  async function tryStartSession(window: TimeWindow): Promise<StartOutcome> {
+    const owner = qcDatastream.value?.id
+    try {
+      await startSession({
+        phenomenonTimeStart: window.begin.toISOString(),
+        phenomenonTimeEnd: window.end.toISOString(),
+      })
+      if (!owns(owner)) return 'superseded'
+      Snackbar.success('Edit session started.')
+      return 'started'
+    } catch (e) {
+      if (takenOver(owner)) return 'superseded'
+      if (e instanceof ResumeSupersededError) {
+        await leaveEdit()
+        Snackbar.error(e.message)
+        return 'superseded'
+      }
+      Snackbar.error(
+        e instanceof Error ? e.message : 'Could not start the session.'
+      )
+      return 'failed'
+    }
+  }
+
+  async function startSessionOver(window: TimeWindow): Promise<boolean> {
+    return (await tryStartSession(window)) === 'started'
+  }
+
+  async function enterEdit(
+    managedId: string,
+    window?: TimeWindow
+  ): Promise<EnterEditResult> {
+    try {
+      await setEditTarget(managedId)
+      await beginEditing()
+    } catch (e) {
+      if (takenOver(managedId)) return 'superseded'
+      await leaveEdit()
+      if (!(e instanceof ResumeSupersededError)) throw e
+      Snackbar.error(e.message)
+      return 'superseded'
+    }
+    if (!owns(managedId)) return 'superseded'
+    if (needsHistory.value) {
+      await leaveEdit()
+      Snackbar.error('This datastream is not set up for QC editing.')
+      return 'not-managed'
+    }
+    currentView.value = DrawerType.Edit
+    selectedDrawer.value = DrawerType.Edit
+    isDrawerOpen.value = true
+    resumeDatastreamId.value = managedId
+    if (!needsSession.value) return 'editing'
+    if (!window) return 'needs-window'
+    const outcome = await tryStartSession(window)
+    if (outcome === 'started') return 'editing'
+    return outcome === 'failed' ? 'needs-window' : 'superseded'
+  }
+
+  return { enterEdit, startSessionOver, leaveEdit }
+}
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index f4fad9ea..f4fa8780 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -39,10 +39,11 @@ type QcSessionPostBody = QualityControlSessionContract.PostBody
 
 /**
  * Keep a new session's window within the source datastream's observed extent.
- * The window comes from the display time-range controls, whose end is usually
- * "now" — past the source's last observation — which the backend rejects
- * (`phenomenon_time_end cannot extend past the source datastream's current end
- * time`). Resume ignores the spec, so this only shapes freshly-created sessions.
+ * The window now comes from the session window dialog, so it is normally
+ * already valid; this clamp is a backstop for the backend's end-time rule
+ * (`phenomenon_time_end cannot extend past the source datastream's current
+ * end time`). Resume ignores the spec, so this only shapes freshly-created
+ * sessions.
  */
 function clampSpecToSource(
   spec: QcSessionPostBody,
@@ -57,10 +58,7 @@ function clampSpecToSource(
     end = Math.min(end, new Date(source.phenomenonEndTime).getTime())
   }
   if (!(start < end)) {
-    throw new Error(
-      'The selected time range has no source observations to edit. ' +
-        'Pick a range that overlaps the datastream (try the "All" preset).'
-    )
+    throw new Error('The selected window has no source observations to edit.')
   }
   return {
     ...spec,
@@ -80,9 +78,8 @@ export class ResumeSupersededError extends Error {
 
 export function useEditSession() {
   const { qcDatastream } = storeToRefs(useDataVisStore())
-  const { replaceDatastream } = useDataVisStore()
+  const { replaceDatastream, setEditRecord } = useDataVisStore()
   const { selectedSeries } = storeToRefs(usePlotlyStore())
-  const { redraw } = usePlotlyStore()
   const { hs } = storeToRefs(useHydroServer())
   const { fetchObservationsInRange } = useObservationStore()
   const sessionStore = useQcSessionStore()
@@ -137,7 +134,11 @@ export function useEditSession() {
   ): Promise<void> {
     const built = await workingCopies.rebuild(managed, source, historyId, session)
     if (!built) throw new ResumeSupersededError()
-    if (selectedSeries.value) selectedSeries.value.data = built.record
+    // The edit target may have changed (or been cleared) while the rebuild
+    // was in flight; wiring in a record for a stale target would be silent
+    // and wrong, since `setEditRecord` upserts whatever `qcDatastream` is now.
+    if (qcDatastream.value?.id !== managed.id) throw new ResumeSupersededError()
+    await setEditRecord(built.record)
     // The replayed draft operations are the saved baseline.
     snapshotSavedEdits()
     needsSession.value = false
@@ -165,8 +166,7 @@ export function useEditSession() {
     await sessionStore.loadSessions(history.id)
 
     const inProgress = sessionStore.inProgressSession
-    const record = selectedSeries.value?.data
-    if (inProgress && sourceDatastream.value && record) {
+    if (inProgress && sourceDatastream.value) {
       // A session exists, so a failed resume must never ask to start one.
       needsSession.value = false
       await resumeWorkingCopy(
@@ -175,8 +175,6 @@ export function useEditSession() {
         history.id,
         inProgress
       )
-      // Nothing watches for a swapped-in record, and no caller redraws here.
-      await redraw()
       return true
     } else {
       needsSession.value = true
@@ -190,6 +188,12 @@ export function useEditSession() {
    * in-progress session when given the editable one.
    */
   async function viewSession(sessionId: string): Promise<void> {
+    // Leaving the editor clears the edit target but not the session store,
+    // so `historyId`/`source` can outlive it; check the target itself first.
+    const managed = qcDatastream.value
+    if (!managed) {
+      throw new Error('Load a managed datastream for editing first.')
+    }
     const historyId = sessionStore.historyId
     const source = sourceDatastream.value
     if (!historyId || !source) {
@@ -232,9 +236,9 @@ export function useEditSession() {
         historyId,
         sessionId
       )
-      if (selectedSeries.value) selectedSeries.value.data = record
-      // Nothing watches for a swapped-in record; rebuild the plot explicitly.
-      await redraw()
+      // Same reasoning as the check in `resumeWorkingCopy`.
+      if (qcDatastream.value?.id !== managed.id) throw new ResumeSupersededError()
+      await setEditRecord(record)
       // Viewing is read-only, so there is nothing unsaved to track.
       snapshotSavedEdits()
     } catch (e) {
@@ -272,7 +276,10 @@ export function useEditSession() {
       new Date(session.phenomenonTimeStart),
       new Date(session.phenomenonTimeEnd)
     )
-    if (selectedSeries.value) selectedSeries.value.data = base
+    // The edit target may have changed under this await; same reasoning as
+    // the check in `resumeWorkingCopy`.
+    if (qcDatastream.value?.id !== managed.id) throw new ResumeSupersededError()
+    await setEditRecord(base)
     workingCopies.set(
       managed.id,
       session.id,
@@ -331,8 +338,8 @@ export function useEditSession() {
     }
     await saveDraft()
 
-    // The window was chosen earlier by the time-range controls; the
-    // description is captured here at commit time. Persist it if it changed.
+    // The window was chosen when the session started; the description is
+    // captured here at commit time. Persist it if it changed.
     const trimmed = description?.trim()
     if (trimmed !== undefined && trimmed !== (session.description ?? '')) {
       const res = await hs.value.qualityControlSessions.update(
diff --git a/apps/qc-app/src/composables/useQcHistory.ts b/apps/qc-app/src/composables/useQcHistory.ts
index 2aeb087a..55dc8d30 100644
--- a/apps/qc-app/src/composables/useQcHistory.ts
+++ b/apps/qc-app/src/composables/useQcHistory.ts
@@ -3,8 +3,8 @@
  *
  * Wraps qc-utils' `serializeHistory` / `parseHistory` / `applyHistory`
  * with the consumer-specific glue:
- *   - reading the active wall-clock window from the data-vis store
- *     for the save side
+ *   - using the session window (or, with no session, the loaded
+ *     window) as the saved history's window
  *   - file-picker / blob-download plumbing
  *   - fetching the QC history's window into the active datastream's
  *     `ObservationRecord` before replay (qc-utils itself is data-
@@ -23,6 +23,7 @@ import {
 import { usePlotlyStore } from '@/store/plotly'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { useObservationStore } from '@/store/observations'
+import { useQcSessionStore } from '@/store/qcSession'
 
 /** Filename for downloaded QC histories: `qc-history-<datastream>-<isoTimestamp>.json`. */
 function defaultFilename(datastreamName?: string): string {
@@ -54,6 +55,23 @@ export function useQcHistory() {
   const { selectedSeries } = storeToRefs(usePlotlyStore())
   const { qcDatastream, beginDate, endDate } = storeToRefs(useDataVisStore())
   const { fetchObservationsInRange } = useObservationStore()
+  const { viewedSession, inProgressSession } = storeToRefs(useQcSessionStore())
+
+  // While editing, `beginDate`/`endDate` follow the context range, not the
+  // window the edits were made over.
+  function historyWindow() {
+    const session = viewedSession.value ?? inProgressSession.value
+    if (!session) {
+      return {
+        startDate: beginDate.value.toISOString(),
+        endDate: endDate.value.toISOString(),
+      }
+    }
+    return {
+      startDate: new Date(session.phenomenonTimeStart).toISOString(),
+      endDate: new Date(session.phenomenonTimeEnd).toISOString(),
+    }
+  }
 
   /**
    * Serialize the current QC history to JSON and trigger a browser
@@ -68,10 +86,10 @@ export function useQcHistory() {
     // qc-utils' `ObservationRecord` exposes a deep typed shape; the
     // cast pins the value to that public type so vue-tsc doesn't try
     // to structurally re-derive it from the live worker bindings.
-    const history = serializeHistory(series as ObservationRecord, {
-      startDate: beginDate.value.toISOString(),
-      endDate: endDate.value.toISOString(),
-    })
+    const history = serializeHistory(
+      series as ObservationRecord,
+      historyWindow()
+    )
 
     const datastreamName = qcDatastream.value?.name
     downloadJson(history, defaultFilename(datastreamName))
diff --git a/apps/qc-app/src/composables/useResumeEditSession.ts b/apps/qc-app/src/composables/useResumeEditSession.ts
index f20d0523..791263b0 100644
--- a/apps/qc-app/src/composables/useResumeEditSession.ts
+++ b/apps/qc-app/src/composables/useResumeEditSession.ts
@@ -1,8 +1,9 @@
 /**
- * Reopen the editor after a page reload, from the last saved state:
- * `beginEditing` rebuilds the session and working copy from the server.
+ * Reopen the editor after a page reload on the managed datastream it was last
+ * open on. `resume` does the entering; this only waits for the catalog and
+ * drops a pointer to a datastream that no longer exists.
  *
- * The watcher must not use `once` with `immediate` — that fires on the
+ * The watcher must not use `once` with `immediate`: that fires on the
  * initial empty catalog and stops, which is the cold-reload case this
  * exists for. The flag gives once-only semantics without that trap.
  */
@@ -13,26 +14,24 @@ import { Snackbar } from '@uwrl/qc-utils'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { useQcSessionStore } from '@/store/qcSession'
 
-export function useResumeEditSession(enterEdit: () => Promise<void>) {
-  const dataVis = useDataVisStore()
-  const { datastreams } = storeToRefs(dataVis)
+export function useResumeEditSession(
+  resume: (managedId: string) => Promise<void>
+) {
+  const { datastreams } = storeToRefs(useDataVisStore())
   const { resumeDatastreamId } = storeToRefs(useQcSessionStore())
 
   let attempted = false
 
   /** Returns true when the editor was reopened. */
-  async function resume(): Promise<boolean> {
+  async function run(): Promise<boolean> {
     const id = resumeDatastreamId.value
     if (!id) return false
-    const managed = datastreams.value.find((d) => d.id === id)
-    if (!managed) {
+    if (!datastreams.value.some((d) => d.id === id)) {
       resumeDatastreamId.value = null
       return false
     }
     try {
-      await dataVis.plotDatastream(managed)
-      await dataVis.setQcDatastream(id)
-      await enterEdit()
+      await resume(id)
       return true
     } catch (e) {
       resumeDatastreamId.value = null
@@ -45,7 +44,7 @@ export function useResumeEditSession(enterEdit: () => Promise<void>) {
     (list) => {
       if (attempted || !list.length || !resumeDatastreamId.value) return
       attempted = true
-      resume().catch((e) => {
+      run().catch((e) => {
         Snackbar.error(
           e instanceof Error ? e.message : 'Could not reopen the edit session.'
         )
@@ -54,5 +53,5 @@ export function useResumeEditSession(enterEdit: () => Promise<void>) {
     { immediate: true }
   )
 
-  return { resume }
+  return { resume: run }
 }
diff --git a/apps/qc-app/src/composables/useWorkspacePermissions.ts b/apps/qc-app/src/composables/useWorkspacePermissions.ts
index da743454..9f081f87 100644
--- a/apps/qc-app/src/composables/useWorkspacePermissions.ts
+++ b/apps/qc-app/src/composables/useWorkspacePermissions.ts
@@ -71,7 +71,7 @@ export function useWorkspacePermissions() {
   /**
    * Can run the QC edit workflow in this workspace — needs to create the
    * managed datastream and/or write observations. Used to gate the editor's
-   * Start editing / Save / Commit controls.
+   * row Edit / Save / Commit controls.
    */
   function canEdit(ws?: Workspace | null): boolean {
     return (
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index 640fa329..e3037324 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -12,7 +12,7 @@ const mockGraphSeriesArray = shallowRef<any[]>([])
 const mockUpdateOptions = vi.fn()
 const mockClearChartState = vi.fn()
 const mockClearZoomHistory = vi.fn()
-const mockFetchGraphSeries = vi.fn().mockResolvedValue({ id: 'stub', data: {} })
+const mockFetchGraphSeries = vi.fn(async (ds: any) => ({ id: ds.id, name: ds.name, data: {} }))
 const mockBuildGraphSeries = vi.fn((ds: any, data: any) => ({ id: ds.id, name: ds.name, data }))
 const mockAssignSeriesColors = vi.fn()
 const mockRedraw = vi.fn()
@@ -60,14 +60,6 @@ vi.mock('@/store/workingCopies', () => ({
   }),
 }))
 
-// Only `currentView` matters to the store under test; the real store also
-// wires up qcDatastream/operationParams watchers that don't belong here.
-const mockCurrentView = ref<'Edit' | 'Select'>('Select')
-vi.mock('@/store/userInterface', () => ({
-  useUIStore: () => ({ currentView: mockCurrentView.value }),
-  DrawerType: { Edit: 'Edit', Select: 'Select', None: '' },
-}))
-
 // handleNewPlot touches DOM / Plotly; stub everything the store imports.
 vi.mock('@/utils/plotting/plotly', () => ({
   handleNewPlot: vi.fn().mockResolvedValue(undefined),
@@ -95,7 +87,6 @@ beforeEach(() => {
   vi.clearAllMocks()
   mockPlotlyRef.value = null
   mockGraphSeriesArray.value = []
-  mockCurrentView.value = 'Select'
   mockWorkingCopies.clear()
 })
 
@@ -210,47 +201,13 @@ describe('useDataVisStore.filteredDatastreams', () => {
   })
 })
 
-describe('useDataVisStore.qcDatastream', () => {
-  it('resolves the plotted datastream matching qcDatastreamId', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.plottedDatastreams = [makeDs({ id: 'a' }), makeDs({ id: 'b' })] as any
-    store.qcDatastreamId = 'b'
-    expect(store.qcDatastream?.id).toBe('b')
-  })
-
-  it('returns null when qcDatastreamId is unset', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.plottedDatastreams = [makeDs({ id: 'a' })] as any
-    expect(store.qcDatastream).toBeNull()
-  })
-
-  it('returns null when qcDatastreamId is set but not in plottedDatastreams', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.plottedDatastreams = [makeDs({ id: 'a' })] as any
-    store.qcDatastreamId = 'missing'
-    expect(store.qcDatastream).toBeNull()
-  })
-})
-
 describe('useDataVisStore.plotDatastream', () => {
-  it('adds the datastream and auto-elects it as qc when none set', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    await store.plotDatastream(makeDs({ id: 'a' }) as any)
-    expect(store.plottedDatastreams.map((d) => d.id)).toEqual(['a'])
-    expect(store.qcDatastreamId).toBe('a')
-  })
-
-  it('does not change qcDatastreamId when plotting a second datastream', async () => {
+  it('adds a second datastream alongside the first', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
     await store.plotDatastream(makeDs({ id: 'a' }) as any)
     await store.plotDatastream(makeDs({ id: 'b' }) as any)
-    expect(store.plottedDatastreams).toHaveLength(2)
-    expect(store.qcDatastreamId).toBe('a')
+    expect(store.plottedDatastreams.map((d) => d.id)).toEqual(['a', 'b'])
   })
 
   it('is idempotent when the datastream is already plotted', async () => {
@@ -264,34 +221,13 @@ describe('useDataVisStore.plotDatastream', () => {
 })
 
 describe('useDataVisStore.unplotDatastream', () => {
-  it('removes the datastream and promotes the previous entry to qc when removing qc', async () => {
+  it('removes only the given datastream', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
     await store.plotDatastream(makeDs({ id: 'a' }) as any)
     await store.plotDatastream(makeDs({ id: 'b' }) as any)
-    // qc = 'a'; removing 'a' promotes the datastream at max(0-1,0)=0, now 'b'.
     await store.unplotDatastream('a')
     expect(store.plottedDatastreams.map((d) => d.id)).toEqual(['b'])
-    expect(store.qcDatastreamId).toBe('b')
-  })
-
-  it('leaves qcDatastreamId untouched when removing a non-qc datastream', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    await store.plotDatastream(makeDs({ id: 'a' }) as any)
-    await store.plotDatastream(makeDs({ id: 'b' }) as any)
-    await store.unplotDatastream('b')
-    expect(store.plottedDatastreams.map((d) => d.id)).toEqual(['a'])
-    expect(store.qcDatastreamId).toBe('a')
-  })
-
-  it('clears qcDatastreamId when last plotted datastream is removed', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    await store.plotDatastream(makeDs({ id: 'a' }) as any)
-    await store.unplotDatastream('a')
-    expect(store.plottedDatastreams).toEqual([])
-    expect(store.qcDatastreamId).toBeNull()
   })
 
   it('is a no-op when the id is not present', async () => {
@@ -299,61 +235,191 @@ describe('useDataVisStore.unplotDatastream', () => {
     const store = useDataVisStore()
     await store.unplotDatastream('ghost')
     expect(store.plottedDatastreams).toEqual([])
-    expect(store.qcDatastreamId).toBeNull()
   })
 })
 
 describe('useDataVisStore.setPlottedDatastreams', () => {
-  it('honors qcId when present in the new list', async () => {
+  it('replaces the plotted list and rebuilds', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
-    await store.setPlottedDatastreams(
-      [makeDs({ id: 'a' }), makeDs({ id: 'b' })] as any,
-      'b'
-    )
+    await store.setPlottedDatastreams([
+      makeDs({ id: 'a' }),
+      makeDs({ id: 'b' }),
+    ] as any)
     expect(store.plottedDatastreams.map((d) => d.id)).toEqual(['a', 'b'])
-    expect(store.qcDatastreamId).toBe('b')
   })
+})
 
-  it('falls back to first item when qcId is not in the list', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    await store.setPlottedDatastreams(
-      [makeDs({ id: 'a' }), makeDs({ id: 'b' })] as any,
-      'missing'
-    )
-    expect(store.qcDatastreamId).toBe('a')
+const managedPair = async () => {
+  const { useDataVisStore } = await import('@/store/dataVisualization')
+  const store = useDataVisStore()
+  const source = makeDs({ id: 'src', phenomenonBeginTime: '2025-01-01T00:00:00Z', phenomenonEndTime: '2025-12-31T00:00:00Z' })
+  const managed = makeDs({ id: 'mgd' })
+  const other = makeDs({ id: 'other', phenomenonBeginTime: '2025-01-01T00:00:00Z', phenomenonEndTime: '2025-12-31T00:00:00Z' })
+  store.datastreams = [source, managed, other] as any
+  store.qcHistories = [
+    { id: 'h1', sourceDatastreamId: 'src', managedDatastreamId: 'mgd' },
+  ] as any
+  return { store, source, managed, other }
+}
+
+describe('useDataVisStore edit target', () => {
+  it('plotting never picks an edit target', async () => {
+    const { store, other } = await managedPair()
+    await store.plotDatastream(other as any)
+    expect(store.qcDatastreamId).toBeNull()
+    expect(store.seriesDatastreams.map((d) => d.id)).toEqual(['other'])
   })
 
-  it('clears qcDatastreamId when the list is empty', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.plottedDatastreams = [makeDs({ id: 'a' })] as any
-    store.qcDatastreamId = 'a'
-    await store.setPlottedDatastreams([], null)
+  it('resolves the edit target from the catalog without plotting it', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    expect(store.qcDatastream?.id).toBe('mgd')
     expect(store.plottedDatastreams).toEqual([])
+  })
+
+  it('orders series as edit target, source, then plotted', async () => {
+    const { store, other } = await managedPair()
+    await store.plotDatastream(other as any)
+    await store.setEditTarget('mgd')
+    expect(store.sourceContextDatastream?.id).toBe('src')
+    expect(store.seriesDatastreams.map((d) => d.id)).toEqual(['mgd', 'src', 'other'])
+  })
+
+  it('does not duplicate a source the user already plotted', async () => {
+    const { store, source, other } = await managedPair()
+    await store.plotDatastream(other as any)
+    await store.plotDatastream(source as any)
+    await store.setEditTarget('mgd')
+    expect(store.seriesDatastreams.map((d) => d.id)).toEqual(['mgd', 'src', 'other'])
+  })
+
+  it('never fetches the edit target when refreshing', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    // `setEditTarget` already loaded `src` as context, so this refresh
+    // updates it in place via `fetchObservationsInRange`, not a fresh fetch.
+    mockFetchGraphSeries.mockClear()
+    mockFetchObservationsInRange.mockClear()
+    await store.refreshGraphSeriesArray()
+    expect(mockFetchObservationsInRange.mock.calls.map((c) => c[0].id)).toEqual(['src'])
+    expect(mockFetchGraphSeries).not.toHaveBeenCalled()
+  })
+
+  it('setEditTarget with nothing plotted still fetches the source as context', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    expect(mockFetchGraphSeries.mock.calls.map((c) => c[0].id)).toEqual(['src'])
+  })
+
+  it('unplotting the last plotted datastream while editing keeps the edit series', async () => {
+    const { store, other } = await managedPair()
+    await store.plotDatastream(other as any)
+    await store.setEditTarget('mgd')
+    await store.setEditRecord({ dataX: [1], history: [] } as any)
+    mockClearChartState.mockClear()
+
+    await store.unplotDatastream('other')
+
+    expect(mockGraphSeriesArray.value.some((s) => s.id === 'mgd')).toBe(true)
+    expect(mockClearChartState).not.toHaveBeenCalled()
+  })
+
+  it('setEditRecord adds the edit series, then updates it in place', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    const first = { dataX: [1], history: [] } as any
+    const second = { dataX: [2], history: [] } as any
+    await store.setEditRecord(first)
+    expect(mockGraphSeriesArray.value.find((s) => s.id === 'mgd')?.data).toBe(first)
+    await store.setEditRecord(second)
+    const edits = mockGraphSeriesArray.value.filter((s) => s.id === 'mgd')
+    expect(edits).toHaveLength(1)
+    expect(edits[0].data).toBe(second)
+  })
+
+  it('clearEditTarget keeps the plotted datastreams and drops edit series', async () => {
+    const { store, other } = await managedPair()
+    await store.plotDatastream(other as any)
+    await store.setEditTarget('mgd')
+    await store.setEditRecord({ dataX: [1], history: [] } as any)
+    await store.clearEditTarget()
     expect(store.qcDatastreamId).toBeNull()
+    expect(store.plottedDatastreams.map((d) => d.id)).toEqual(['other'])
+    expect(store.seriesDatastreams.map((d) => d.id)).toEqual(['other'])
+    expect(mockGraphSeriesArray.value.some((s) => s.id === 'mgd')).toBe(false)
+    expect(mockInvalidateWorkingCopy).toHaveBeenCalledWith('mgd')
   })
 
-  it('preserves qcDatastreamId when it still appears in the new list (no qcId arg)', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.plottedDatastreams = [makeDs({ id: 'a' })] as any
-    store.qcDatastreamId = 'a'
-    await store.setPlottedDatastreams([
-      makeDs({ id: 'a' }),
-      makeDs({ id: 'b' }),
-    ] as any)
-    expect(store.qcDatastreamId).toBe('a')
+  it('unplotting never touches the edit target', async () => {
+    const { store, other } = await managedPair()
+    await store.plotDatastream(other as any)
+    await store.setEditTarget('mgd')
+    await store.unplotDatastream('other')
+    expect(store.qcDatastreamId).toBe('mgd')
   })
 
-  it('promotes first item when current qc is no longer in list (no qcId arg)', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.plottedDatastreams = [makeDs({ id: 'a' })] as any
-    store.qcDatastreamId = 'a'
-    await store.setPlottedDatastreams([makeDs({ id: 'c' })] as any)
-    expect(store.qcDatastreamId).toBe('c')
+  it('rebuilding while editing keeps the zoom', async () => {
+    const { store, other } = await managedPair()
+    await store.plotDatastream(other as any)
+    await store.setEditTarget('mgd')
+    mockPlotlyRef.value = {}
+    const { handleNewPlot } = await import('@/utils/plotting/plotly')
+    vi.mocked(handleNewPlot).mockClear()
+    mockClearZoomHistory.mockClear()
+
+    await store.unplotDatastream('other')
+
+    expect(handleNewPlot).toHaveBeenCalledWith(undefined, { preserveZoom: true })
+    expect(mockClearZoomHistory).not.toHaveBeenCalled()
+  })
+
+  it('rebuilding in Select drops the zoom', async () => {
+    const { store, other, source } = await managedPair()
+    await store.plotDatastream(other as any)
+    await store.plotDatastream(source as any)
+    mockPlotlyRef.value = {}
+    const { handleNewPlot } = await import('@/utils/plotting/plotly')
+    vi.mocked(handleNewPlot).mockClear()
+    mockClearZoomHistory.mockClear()
+
+    await store.unplotDatastream('other')
+
+    expect(handleNewPlot).toHaveBeenCalledWith(undefined, { preserveZoom: false })
+    expect(mockClearZoomHistory).toHaveBeenCalled()
+  })
+
+  it('a new edit target starts from an empty session store', async () => {
+    const { store } = await managedPair()
+    const { useQcSessionStore } = await import('@/store/qcSession')
+    const sessions = useQcSessionStore()
+    await store.setEditTarget('mgd')
+    sessions.historyId = 'h1'
+    sessions.sessions = [{ id: 's1', status: 'in_progress' }] as any
+    sessions.currentSessionId = 's1'
+    sessions.viewedSessionId = 's1'
+    sessions.resumeDatastreamId = 'mgd'
+    await store.clearEditTarget()
+
+    await store.setEditTarget('other')
+
+    expect(sessions.historyId).toBeNull()
+    expect(sessions.sessions).toEqual([])
+    expect(sessions.viewedSession).toBeNull()
+    // The resume pointer is the entry flow's to set.
+    expect(sessions.resumeDatastreamId).toBe('mgd')
+  })
+
+  it('setting the same edit target keeps its sessions', async () => {
+    const { store } = await managedPair()
+    const { useQcSessionStore } = await import('@/store/qcSession')
+    const sessions = useQcSessionStore()
+    await store.setEditTarget('mgd')
+    sessions.historyId = 'h1'
+
+    await store.setEditTarget('mgd')
+
+    expect(sessions.historyId).toBe('h1')
   })
 })
 
@@ -413,6 +479,25 @@ describe('useDataVisStore.setDateRange', () => {
     })
     expect(mockRedraw).not.toHaveBeenCalled()
   })
+
+  it('refreshes context when only the edit target is set, nothing plotted', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    // `setEditTarget` already loaded `src` as context, so the date change
+    // updates it in place via `fetchObservationsInRange`.
+    mockFetchObservationsInRange.mockClear()
+    mockRedraw.mockClear()
+    mockClearZoomHistory.mockClear()
+
+    await store.setDateRange({
+      begin: new Date('2025-02-01T00:00:00Z'),
+      end: new Date('2025-03-01T00:00:00Z'),
+    })
+
+    expect(mockFetchObservationsInRange.mock.calls.map((c) => c[0].id)).toContain('src')
+    expect(mockRedraw).toHaveBeenCalledWith(false, true)
+    expect(mockClearZoomHistory).not.toHaveBeenCalled()
+  })
 })
 
 describe('useDataVisStore.resetState', () => {
@@ -542,7 +627,6 @@ describe('useDataVisStore time range presets', () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
     store.plottedDatastreams = [oldDs(), newerDs()] as any
-    store.qcDatastreamId = 'old'
     await store.unplotDatastream('newer')
     expect(store.endDate.toISOString()).toBe('2021-06-30T12:00:00.000Z')
   })
@@ -560,58 +644,6 @@ describe('useDataVisStore time range presets', () => {
   })
 })
 
-// Plotting/unplotting in the Edit view must not move the edit session's
-// loaded window (finding: preset re-anchoring was reachable from
-// DatastreamInformationCard / PlottedDatastreams in the Edit view too).
-describe('useDataVisStore time range freeze in the Edit view', () => {
-  const OLD_END = '2021-06-30T12:00:00Z'
-  const oldDs = (overrides: Record<string, any> = {}) =>
-    makeDs({
-      id: 'old',
-      phenomenonBeginTime: '2019-01-01T00:00:00Z',
-      phenomenonEndTime: OLD_END,
-      ...overrides,
-    })
-  const newerDs = () =>
-    makeDs({
-      id: 'newer',
-      phenomenonBeginTime: '2020-01-01T00:00:00Z',
-      phenomenonEndTime: '2022-03-01T00:00:00Z',
-    })
-
-  it('leaves beginDate/endDate unchanged when plotting a datastream', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    mockCurrentView.value = 'Edit'
-    const begin = store.beginDate.getTime()
-    const end = store.endDate.getTime()
-    await store.plotDatastream(oldDs() as any)
-    expect(store.beginDate.getTime()).toBe(begin)
-    expect(store.endDate.getTime()).toBe(end)
-  })
-
-  it('leaves beginDate/endDate unchanged when unplotting a datastream', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.plottedDatastreams = [oldDs(), newerDs()] as any
-    store.qcDatastreamId = 'old'
-    mockCurrentView.value = 'Edit'
-    const begin = store.beginDate.getTime()
-    const end = store.endDate.getTime()
-    await store.unplotDatastream('newer')
-    expect(store.beginDate.getTime()).toBe(begin)
-    expect(store.endDate.getTime()).toBe(end)
-  })
-
-  it('still re-anchors in the Select view (unaffected by the freeze)', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    mockCurrentView.value = 'Select'
-    await store.plotDatastream(oldDs() as any)
-    expect(store.endDate.toISOString()).toBe('2021-06-30T12:00:00.000Z')
-  })
-})
-
 // The persistence plugin only activates once the pinia is installed on an
 // app, so these go through `app.use(pinia)`.
 describe('useDataVisStore persisted preset', () => {
@@ -661,35 +693,14 @@ describe('useDataVisStore persisted preset', () => {
   })
 })
 
-describe('useDataVisStore.setQcDatastream', () => {
-  it('updates qcDatastreamId and calls updateOptions', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.plottedDatastreams = [makeDs({ id: 'a' }), makeDs({ id: 'b' })] as any
-    store.qcDatastreamId = 'a'
-    await store.setQcDatastream('b')
-    expect(store.qcDatastreamId).toBe('b')
-    expect(mockUpdateOptions).toHaveBeenCalled()
-  })
-
-  it('is a no-op when id matches current qcDatastreamId', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.qcDatastreamId = 'a'
-    await store.setQcDatastream('a')
-    expect(mockUpdateOptions).not.toHaveBeenCalled()
-  })
-})
-
 describe('useDataVisStore.clearPlottedDatastreams + toggleDatastream', () => {
-  it('clearPlottedDatastreams empties the list and clears qcDatastreamId', async () => {
+  it('clearPlottedDatastreams empties the list', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
     await store.plotDatastream(makeDs({ id: 'a' }) as any)
     await store.plotDatastream(makeDs({ id: 'b' }) as any)
     await store.clearPlottedDatastreams()
     expect(store.plottedDatastreams).toEqual([])
-    expect(store.qcDatastreamId).toBeNull()
   })
 
   it('clearPlottedDatastreams is a no-op when already empty', async () => {
@@ -710,103 +721,6 @@ describe('useDataVisStore.clearPlottedDatastreams + toggleDatastream', () => {
   })
 })
 
-describe('useDataVisStore.adoptManagedDatastream', () => {
-  it('reuses the source series as the managed working copy (one item, data kept)', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    const sourceData = { tag: 'loaded-record' }
-    store.plottedDatastreams = [makeDs({ id: 'src', name: 'Raw' })] as any
-    store.qcDatastreamId = 'src'
-    mockGraphSeriesArray.value = [
-      { id: 'src', name: 'Raw', data: sourceData, color: '#1', yAxisLabel: 'T' },
-    ]
-
-    await store.adoptManagedDatastream(
-      makeDs({ id: 'mgd', name: 'Raw (QC)' }) as any,
-      'src'
-    )
-
-    // Single plotted item, now the managed datastream as the QC target.
-    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['mgd'])
-    expect(store.qcDatastreamId).toBe('mgd')
-    // The loaded series was re-keyed in place, keeping its data...
-    expect(mockGraphSeriesArray.value).toHaveLength(1)
-    expect(mockGraphSeriesArray.value[0].id).toBe('mgd')
-    expect(mockGraphSeriesArray.value[0].name).toBe('Raw (QC)')
-    expect(mockGraphSeriesArray.value[0].data).toEqual(sourceData)
-    // ...and the working copy was reused, not re-fetched (managed is empty).
-    expect(mockFetchGraphSeries).not.toHaveBeenCalled()
-    expect(mockFetchObservationsInRange).not.toHaveBeenCalled()
-  })
-})
-
-describe('useDataVisStore.releaseManagedDatastream', () => {
-  const withHistory = (store: any) => {
-    store.qcHistories = [
-      { id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'src' },
-    ] as any
-    store.datastreams = [
-      makeDs({ id: 'src', name: 'Raw' }),
-      makeDs({ id: 'mgd', name: 'Raw (QC)' }),
-    ] as any
-  }
-
-  // Managed datastreams are hidden from the catalog table, so leaving the
-  // editor with one plotted shows a plot with no row selected.
-  it('swaps the managed datastream back to its source and refetches its data', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    withHistory(store)
-    const working = { tag: 'uncommitted-edits' }
-    store.plottedDatastreams = [makeDs({ id: 'mgd', name: 'Raw (QC)' })] as any
-    store.qcDatastreamId = 'mgd'
-    mockGraphSeriesArray.value = [
-      { id: 'mgd', name: 'Raw (QC)', data: working, color: '#1', yAxisLabel: 'T' },
-    ]
-
-    await store.releaseManagedDatastream()
-
-    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['src'])
-    expect(store.qcDatastreamId).toBe('src')
-    // The editor's working copy carries uncommitted edits, so it is dropped
-    // and the source's stored data fetched instead.
-    expect(
-      mockGraphSeriesArray.value.some((s: any) => s.data === working)
-    ).toBe(false)
-    expect(mockFetchGraphSeries).toHaveBeenCalled()
-    expect(mockFetchGraphSeries.mock.calls[0][0].id).toBe('src')
-  })
-
-  it('is a no-op when the plotted datastream is not a managed one', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    withHistory(store)
-    store.plottedDatastreams = [makeDs({ id: 'src', name: 'Raw' })] as any
-    store.qcDatastreamId = 'src'
-
-    await store.releaseManagedDatastream()
-
-    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['src'])
-    expect(store.qcDatastreamId).toBe('src')
-  })
-
-  it('leaves the plot alone when the source is missing from the catalog', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.qcHistories = [
-      { id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'gone' },
-    ] as any
-    store.datastreams = [makeDs({ id: 'mgd', name: 'Raw (QC)' })] as any
-    store.plottedDatastreams = [makeDs({ id: 'mgd', name: 'Raw (QC)' })] as any
-    store.qcDatastreamId = 'mgd'
-
-    await store.releaseManagedDatastream()
-
-    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['mgd'])
-    expect(store.qcDatastreamId).toBe('mgd')
-  })
-})
-
 describe('useDataVisStore snapshot series', () => {
   const meta = {
     sessionId: 'sess-1',
@@ -861,35 +775,30 @@ describe('useDataVisStore snapshot series', () => {
     expect(store.qcDatastreamId).toBeNull()
   })
 
-  // Snapshots belong to the editor. The Select view lists real datastreams
-  // and lets the user pick a QC target, neither of which a snapshot can be.
-  it('drops snapshots when the editor releases the managed datastream', async () => {
+  // Snapshots belong to the editor. Leaving the edit target must not leave
+  // a replay line stranded on the plot.
+  it('drops snapshots when the editor clears the edit target', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
-    store.qcHistories = [
-      { id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'src' },
-    ] as any
-    store.datastreams = [makeDs({ id: 'src', name: 'Raw' })] as any
-    store.plottedDatastreams = [makeDs({ id: 'mgd', name: 'Raw (QC)' })] as any
+    store.datastreams = [makeDs({ id: 'mgd', name: 'Raw (QC)' })] as any
     store.qcDatastreamId = 'mgd'
 
     await store.addSnapshotSeries('snap:sess-1:0', { history: [] } as any, meta as any)
-    await store.releaseManagedDatastream()
+    await store.clearEditTarget()
 
-    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['src'])
+    expect(store.qcDatastreamId).toBeNull()
     expect(
       mockGraphSeriesArray.value.some((s: any) => s.id === 'snap:sess-1:0')
     ).toBe(false)
   })
 
-  it('drops snapshots even when there is no managed datastream to release', async () => {
+  it('drops snapshots even when there is no edit target set', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
     store.plottedDatastreams = [makeDs({ id: 'src', name: 'Raw' })] as any
-    store.qcDatastreamId = 'src'
 
     await store.addSnapshotSeries('snap:sess-1:0', { history: [] } as any, meta as any)
-    await store.releaseManagedDatastream()
+    await store.clearEditTarget()
 
     expect(
       store.plottedDatastreams.some((d: any) => d.id === 'snap:sess-1:0')
@@ -926,31 +835,6 @@ describe('useDataVisStore.plotSourceSelection', () => {
     ] as any
   }
 
-  it('plots the chosen series and promotes the first as QC target', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    withGroup(store)
-
-    await store.plotSourceSelection('src', ['src', 'mgd-1'])
-
-    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual([
-      'src',
-      'mgd-1',
-    ])
-    expect(store.qcDatastreamId).toBe('src')
-  })
-
-  it('makes a managed datastream the QC target when the raw one is not picked', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    withGroup(store)
-
-    await store.plotSourceSelection('src', ['mgd-2'])
-
-    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['mgd-2'])
-    expect(store.qcDatastreamId).toBe('mgd-2')
-  })
-
   it('adds and removes within the group in a single call', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
@@ -959,7 +843,6 @@ describe('useDataVisStore.plotSourceSelection', () => {
       makeDs({ id: 'src', name: 'Raw' }),
       makeDs({ id: 'mgd-1', name: 'Raw (QC)' }),
     ] as any
-    store.qcDatastreamId = 'src'
 
     await store.plotSourceSelection('src', ['mgd-1', 'mgd-2'])
 
@@ -974,7 +857,6 @@ describe('useDataVisStore.plotSourceSelection', () => {
     const store = useDataVisStore()
     withGroup(store)
     store.plottedDatastreams = [makeDs({ id: 'other', name: 'Other' })] as any
-    store.qcDatastreamId = 'other'
 
     await store.plotSourceSelection('src', ['mgd-1'])
 
@@ -982,39 +864,17 @@ describe('useDataVisStore.plotSourceSelection', () => {
       'other',
       'mgd-1',
     ])
-    expect(store.qcDatastreamId).toBe('other')
   })
 
-  it('promotes a new QC target when the current one is deselected', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    withGroup(store)
-    store.plottedDatastreams = [
-      makeDs({ id: 'other', name: 'Other' }),
-      makeDs({ id: 'src', name: 'Raw' }),
-    ] as any
-    store.qcDatastreamId = 'src'
-
-    await store.plotSourceSelection('src', ['mgd-1'])
-
-    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual([
-      'other',
-      'mgd-1',
-    ])
-    expect(store.qcDatastreamId).toBe('other')
-  })
-
-  it('clears the QC target when nothing is left plotted', async () => {
+  it('empties the group when nothing is selected', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
     const store = useDataVisStore()
     withGroup(store)
     store.plottedDatastreams = [makeDs({ id: 'mgd-1', name: 'Raw (QC)' })] as any
-    store.qcDatastreamId = 'mgd-1'
 
     await store.plotSourceSelection('src', [])
 
     expect(store.plottedDatastreams).toEqual([])
-    expect(store.qcDatastreamId).toBeNull()
   })
 
   it('ignores ids that do not belong to the source group', async () => {
@@ -1044,7 +904,6 @@ describe('useDataVisStore.plotSourceSelection', () => {
     const store = useDataVisStore()
     withGroup(store)
     store.plottedDatastreams = [makeDs({ id: 'src', name: 'Raw' })] as any
-    store.qcDatastreamId = 'src'
 
     await store.plotSourceSelection('src', ['src'])
 
@@ -1052,32 +911,6 @@ describe('useDataVisStore.plotSourceSelection', () => {
   })
 })
 
-// Both the source and a managed datastream can be plotted at once now, so
-// leaving the editor must not re-add a source that is already there.
-describe('useDataVisStore.releaseManagedDatastream with the source plotted', () => {
-  it('drops the managed datastream instead of duplicating the source', async () => {
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.qcHistories = [
-      { id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'src' },
-    ] as any
-    store.datastreams = [
-      makeDs({ id: 'src', name: 'Raw' }),
-      makeDs({ id: 'mgd', name: 'Raw (QC)' }),
-    ] as any
-    store.plottedDatastreams = [
-      makeDs({ id: 'src', name: 'Raw' }),
-      makeDs({ id: 'mgd', name: 'Raw (QC)' }),
-    ] as any
-    store.qcDatastreamId = 'mgd'
-
-    await store.releaseManagedDatastream()
-
-    expect(store.plottedDatastreams.map((d: any) => d.id)).toEqual(['src'])
-    expect(store.qcDatastreamId).toBe('src')
-  })
-})
-
 describe('useDataVisStore managed datastream working copy', () => {
   const withManaged = (store: any) => {
     store.qcHistories = [
@@ -1194,13 +1027,17 @@ describe('useDataVisStore managed datastream working copy', () => {
     const store = useDataVisStore()
     withManaged(store)
     mockWorkingCopies.set('mgd', copy)
-    await store.plotDatastream(store.datastreams[1] as any)
+    store.qcDatastreamId = 'mgd'
+    // Seed the series so `invalidateUnplottedWorkingCopies`'s loop actually
+    // has an 'mgd' entry to skip, not an empty array the skip is vacuous over.
+    mockGraphSeriesArray.value = [
+      { id: 'mgd', name: 'Raw (QC)', data: copy.record, color: '#1', yAxisLabel: 'T' },
+    ]
 
-    store.plottedDatastreams = [store.datastreams[0] as any]
     await store.refreshGraphSeriesArray()
 
-    expect(store.qcDatastreamId).toBe('mgd')
-    expect(mockInvalidateWorkingCopy).not.toHaveBeenCalled()
+    expect(mockInvalidateWorkingCopy).not.toHaveBeenCalledWith('mgd')
+    expect(mockFetchGraphSeries.mock.calls.map((c) => c[0].id)).not.toContain('mgd')
   })
 })
 
diff --git a/apps/qc-app/src/store/__tests__/plotly.spec.ts b/apps/qc-app/src/store/__tests__/plotly.spec.ts
index f0e0388d..097f0eed 100644
--- a/apps/qc-app/src/store/__tests__/plotly.spec.ts
+++ b/apps/qc-app/src/store/__tests__/plotly.spec.ts
@@ -4,9 +4,10 @@ import { describe, it, expect, beforeEach, vi } from 'vitest'
 
 // vi.hoisted vars are safe to reference inside vi.mock factories.
 // options.ts imports usePlotlyStore (circular), so we inline constants here.
-const { COLORS, LABEL_COLORS, qcId } = vi.hoisted(() => {
+const { COLORS, LABEL_COLORS, qcId, sourceId } = vi.hoisted(() => {
   const { ref } = require('vue') as typeof import('vue')
   const qcId = ref<string | null>(null)
+  const sourceId = ref<string | null>(null)
   const COLORS = [
     '#3f3f3f', '#aec7e8', '#ffbb78', '#98df8a', '#c5b0d5',
     '#c49c94', '#f7b6d2', '#dbdb8d', '#9edae5', '#ad494a',
@@ -15,7 +16,7 @@ const { COLORS, LABEL_COLORS, qcId } = vi.hoisted(() => {
     '#3f3f3f', '#1f77b4', '#c06a00', '#208020', '#7a4da3',
     '#8c564b', '#e377c2', '#bcbd22', '#17becf', '#d62728',
   ]
-  return { COLORS, LABEL_COLORS, qcId }
+  return { COLORS, LABEL_COLORS, qcId, sourceId }
 })
 
 // Mock dataVisualization as a real Pinia store so storeToRefs works naturally.
@@ -24,7 +25,10 @@ vi.mock('@/store/dataVisualization', () => {
     const qcDatastream = computed(() =>
       qcId.value ? ({ id: qcId.value } as { id: string }) : null
     )
-    return { qcDatastream }
+    const sourceContextDatastream = computed(() =>
+      sourceId.value ? ({ id: sourceId.value } as { id: string }) : null
+    )
+    return { qcDatastream, sourceContextDatastream }
   })
   return { useDataVisStore }
 })
@@ -34,9 +38,14 @@ vi.mock('@/store/observations', () => ({
 }))
 
 // Inline constants to break circular dep: options.ts → usePlotlyStore → plotly barrel.
+const SOURCE_CONTEXT_COLOR = '#9e9e9e'
+const SOURCE_CONTEXT_LABEL_COLOR = '#616161'
+
 vi.mock('@/utils/plotting/plotly', () => ({
   COLORS,
   LABEL_COLORS,
+  SOURCE_CONTEXT_COLOR,
+  SOURCE_CONTEXT_LABEL_COLOR,
   labelColorFor: (lineColor: string) => {
     const idx = COLORS.indexOf(lineColor)
     return idx >= 0 ? LABEL_COLORS[idx] : LABEL_COLORS[0]
@@ -67,6 +76,7 @@ describe('usePlotlyStore.assignSeriesColors', () => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
     qcId.value = null
+    sourceId.value = null
   })
 
   it('hands out colours in legend order for a fresh cold load', async () => {
@@ -212,6 +222,7 @@ describe('usePlotlyStore.colorForDatastream', () => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
     qcId.value = null
+    sourceId.value = null
   })
 
   it('returns the persisted series color for non-qc datastreams', async () => {
@@ -235,6 +246,14 @@ describe('usePlotlyStore.colorForDatastream', () => {
     store.graphSeriesArray.push(makeSeries('a', COLORS[2]))
     expect(store.colorForDatastream('zzz')).toBe(COLORS[1])
   })
+
+  it('returns SOURCE_CONTEXT_COLOR when id matches sourceContextDatastream', async () => {
+    const { usePlotlyStore } = await import('@/store/plotly')
+    const store = usePlotlyStore()
+    store.graphSeriesArray.push(makeSeries('src', COLORS[4]))
+    sourceId.value = 'src'
+    expect(store.colorForDatastream('src')).toBe(SOURCE_CONTEXT_COLOR)
+  })
 })
 
 describe('usePlotlyStore.labelColorForDatastream', () => {
@@ -242,6 +261,7 @@ describe('usePlotlyStore.labelColorForDatastream', () => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
     qcId.value = null
+    sourceId.value = null
   })
 
   it('returns LABEL_COLORS[1] fallback for undefined id', async () => {
@@ -270,6 +290,14 @@ describe('usePlotlyStore.labelColorForDatastream', () => {
     const store = usePlotlyStore()
     expect(store.labelColorForDatastream('missing')).toBe(LABEL_COLORS[1])
   })
+
+  it('returns SOURCE_CONTEXT_LABEL_COLOR when id matches sourceContextDatastream', async () => {
+    const { usePlotlyStore } = await import('@/store/plotly')
+    const store = usePlotlyStore()
+    store.graphSeriesArray.push(makeSeries('src', COLORS[4]))
+    sourceId.value = 'src'
+    expect(store.labelColorForDatastream('src')).toBe(SOURCE_CONTEXT_LABEL_COLOR)
+  })
 })
 
 describe('usePlotlyStore zoom history', () => {
@@ -277,6 +305,7 @@ describe('usePlotlyStore zoom history', () => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
     qcId.value = null
+    sourceId.value = null
   })
 
   it('starts with empty stacks and canUndo/canRedo false', async () => {
@@ -349,6 +378,7 @@ describe('usePlotlyStore.clearChartState', () => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
     qcId.value = null
+    sourceId.value = null
   })
 
   it('empties graphSeriesArray and zoom history', async () => {
@@ -383,6 +413,7 @@ describe('usePlotlyStore.updateOptions + selectedSeries', () => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
     qcId.value = null
+    sourceId.value = null
   })
 
   it('updateOptions rebuilds plotlyOptions via createPlotlyOption', async () => {
@@ -426,6 +457,7 @@ describe('usePlotlyStore — data-points (tooltips) mode', () => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
     qcId.value = null
+    sourceId.value = null
   })
 
   it('defaults: mode=auto, manualEnabled=true, threshold=10000', async () => {
@@ -489,6 +521,7 @@ describe('usePlotlyStore.requestTableScroll', () => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
     qcId.value = null
+    sourceId.value = null
   })
 
   it('starts with no pending scroll request', async () => {
@@ -518,6 +551,7 @@ describe('usePlotlyStore.editHistory', () => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
     qcId.value = null
+    sourceId.value = null
   })
 
   it('is empty when no series is selected for QC', async () => {
@@ -554,3 +588,47 @@ describe('usePlotlyStore.editHistory', () => {
     expect(store.editHistory).toHaveLength(2)
   })
 })
+
+describe('usePlotlyStore.redraw shapes', () => {
+  beforeEach(() => {
+    setActivePinia(createPinia())
+    vi.clearAllMocks()
+    qcId.value = null
+    sourceId.value = null
+  })
+
+  it('keeps a live stage shape and replaces the edit-window shape', async () => {
+    const mod = await import('@/utils/plotting/plotly')
+    const freshWindow = { name: 'edit-window', x0: 300, x1: 400 }
+    const { usePlotlyStore } = await import('@/store/plotly')
+    const store = usePlotlyStore()
+    ;(mod.createPlotlyOption as any).mockReturnValueOnce({
+      traces: [],
+      layout: { shapes: [freshWindow] },
+    })
+    const stage = { name: 'stage', x0: 10, x1: 20 }
+    store.plotlyRef = {
+      layout: { shapes: [stage, { name: 'edit-window', x0: 100, x1: 200 }] },
+    } as any
+
+    await store.redraw()
+
+    const layout = (mod.applyTraceUpdate as any).mock.calls.at(-1)[2]
+    expect(layout.shapes).toEqual([stage, freshWindow])
+  })
+
+  it('drops a stale edit-window shape once the layout has none', async () => {
+    const mod = await import('@/utils/plotting/plotly')
+    const { usePlotlyStore } = await import('@/store/plotly')
+    const store = usePlotlyStore()
+    const stage = { name: 'stage', x0: 10, x1: 20 }
+    store.plotlyRef = {
+      layout: { shapes: [stage, { name: 'edit-window', x0: 100, x1: 200 }] },
+    } as any
+
+    await store.redraw()
+
+    const layout = (mod.applyTraceUpdate as any).mock.calls.at(-1)[2]
+    expect(layout.shapes).toEqual([stage])
+  })
+})
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index fc39fdbf..6e9dd5d2 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -13,8 +13,8 @@ import {
   presetWindow,
 } from '@/utils/timeRangePresets'
 import { isSnapshotId } from '@/utils/snapshotId'
-import { DrawerType, useUIStore } from '@/store/userInterface'
 import { useWorkingCopiesStore } from '@/store/workingCopies'
+import { useQcSessionStore } from '@/store/qcSession'
 import type { SnapshotMeta } from '@/types'
 import type { ObservationRecord } from '@uwrl/qc-utils'
 import {
@@ -65,7 +65,11 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     const workingCopies = useWorkingCopiesStore()
     await Promise.all(
       plottedDatastreams.value
-        .filter((ds) => managedDatastreamIds.value.has(ds.id))
+        .filter(
+          (ds) =>
+            ds.id !== qcDatastreamId.value &&
+            managedDatastreamIds.value.has(ds.id)
+        )
         .map(async (ds) => {
           const context = managedContext(ds.id)
           if (!context) return
@@ -83,7 +87,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
    *  the QC target keeps its copy for the editor. */
   function invalidateUnplottedWorkingCopies() {
     const workingCopies = useWorkingCopiesStore()
-    const plotted = new Set(plottedDatastreams.value.map((d) => d.id))
+    const plotted = new Set(seriesDatastreams.value.map((d) => d.id))
     for (const { id } of graphSeriesArray.value) {
       if (plotted.has(id) || id === qcDatastreamId.value) continue
       if (managedDatastreamIds.value.has(id)) workingCopies.invalidate(id)
@@ -146,25 +150,33 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
 
   // Datasets
   const plottedDatastreams = ref<Datastream[]>([])
-  /**
-   * The datastream selected to go through the quality control process.
-   *
-   * Stored as an id (`qcDatastreamId`) and resolved through a computed
-   * (`qcDatastream`) that looks up the live object in
-   * `plottedDatastreams`. This avoids stale-object bugs where
-   * `qcDatastream` would hold a reference to a datastream no longer in
-   * the plot — in that case the old stored-object approach silently
-   * broke downstream code (notably the `QC is always black` colour
-   * override). With the computed, the QC reference automatically becomes
-   * `null` when the id is no longer plotted.
-   */
+
+  /** The datastream being edited. Set only by the edit flow, null in Select. */
   const qcDatastreamId = ref<string | null>(null)
-  const qcDatastream = computed<Datastream | null>(() => {
-    if (!qcDatastreamId.value) return null
-    return (
-      plottedDatastreams.value.find((ds) => ds.id === qcDatastreamId.value) ??
-      null
-    )
+  const qcDatastream = computed(() =>
+    qcDatastreamId.value
+      ? (datastreams.value.find((ds) => ds.id === qcDatastreamId.value) ?? null)
+      : null
+  )
+
+  /** The edit target's source, drawn behind it as context. */
+  const sourceContextDatastream = computed(() =>
+    qcDatastreamId.value
+      ? (managedContext(qcDatastreamId.value)?.source ?? null)
+      : null
+  )
+
+  /** What the plot draws, in order: edit target, its source, then plotted. */
+  const seriesDatastreams = computed<Datastream[]>(() => {
+    const edit = qcDatastream.value
+    if (!edit) return plottedDatastreams.value
+    const pinned: Datastream[] = [edit]
+    if (sourceContextDatastream.value) pinned.push(sourceContextDatastream.value)
+    const pinnedIds = new Set(pinned.map((d) => d.id))
+    return [
+      ...pinned,
+      ...plottedDatastreams.value.filter((d) => !pinnedIds.has(d.id)),
+    ]
   })
 
   // Qualifiers
@@ -190,9 +202,12 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   function resolvePresetWindow() {
     if (selectedDateBtnId.value === CUSTOM_PRESET_ID) return null
     const workingCopies = useWorkingCopiesStore()
+    const context = seriesDatastreams.value.filter(
+      (d) => d.id !== qcDatastreamId.value && !isSnapshotId(d.id)
+    )
     const extent = dataExtent([
-      ...plottedDatastreams.value,
-      ...workingCopies.extents(plottedDatastreams.value.map((d) => d.id)),
+      ...context,
+      ...workingCopies.extents(context.map((d) => d.id)),
     ])
     return extent ? presetWindow(selectedDateBtnId.value, extent) : null
   }
@@ -219,25 +234,18 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     else await plotDatastream(datastream)
   }
 
-  /** Add a datastream to the plot. Promotes it to QC if there isn't one
-   *  already. Triggers an explicit plot rebuild — no watcher. */
+  /** Add a datastream to the plot. Triggers an explicit plot rebuild, no watcher. */
   async function plotDatastream(ds: Datastream) {
     if (plottedDatastreams.value.some((d) => d.id === ds.id)) return
     plottedDatastreams.value.push(ds)
-    if (!qcDatastreamId.value) qcDatastreamId.value = ds.id
     await rebuildPlot()
   }
 
-  /** Remove a datastream from the plot. If the QC datastream is being
-   *  removed, promote the previous plotted entry (or clear QC). */
+  /** Remove a datastream from the plot. */
   async function unplotDatastream(id: string) {
     const idx = plottedDatastreams.value.findIndex((d) => d.id === id)
     if (idx === -1) return
     plottedDatastreams.value.splice(idx, 1)
-    if (qcDatastreamId.value === id) {
-      qcDatastreamId.value =
-        plottedDatastreams.value[Math.max(idx - 1, 0)]?.id ?? null
-    }
     await rebuildPlot()
   }
 
@@ -267,7 +275,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
       snapshot: meta,
     })
 
-    assignSeriesColors(plottedDatastreams.value.map((d) => d.id))
+    orderAndColorSeries()
     updateOptions()
     const { plotlyRef } = storeToRefs(usePlotlyStore())
     if (plotlyRef.value) await handleNewPlot(undefined, { preserveZoom: true })
@@ -309,12 +317,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   /**
    * Apply a whole "what to plot for this source" choice at once: `ids` is
    * the complete set wanted from that source's group. Group members absent
-   * from `ids` are unplotted; additions are appended in `ids` order, so the
-   * first-plotted-wins QC rule sees the caller's ordering.
-   *
-   * Batched rather than looping plot/unplot so the QC target is promoted
-   * once against the final set instead of drifting through each
-   * intermediate state.
+   * from `ids` are unplotted; additions are appended in `ids` order.
    */
   async function plotSourceSelection(sourceId: string, ids: string[]) {
     const group = new Set(sourceGroupIds(sourceId))
@@ -336,16 +339,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
       next.every((d, i) => d.id === plottedDatastreams.value[i]?.id)
     if (unchanged) return
 
-    const previousIndex = plottedDatastreams.value.findIndex(
-      (d) => d.id === qcDatastreamId.value
-    )
     plottedDatastreams.value = next
-    if (!next.some((d) => d.id === qcDatastreamId.value)) {
-      // Same promotion rule as `unplotDatastream`: the entry before the one
-      // that left, clamped into the surviving set.
-      const idx = Math.max(Math.min(previousIndex - 1, next.length - 1), 0)
-      qcDatastreamId.value = next[idx]?.id ?? null
-    }
     await rebuildPlot()
   }
 
@@ -353,106 +347,51 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   async function clearPlottedDatastreams() {
     if (!plottedDatastreams.value.length) return
     plottedDatastreams.value = []
-    qcDatastreamId.value = null
     await rebuildPlot()
   }
 
-  /** Replace the plotted set wholesale (used by URL hydration). Falls
-   *  back to promoting the first item as QC when `qcId` isn't supplied
-   *  or points to a datastream not in `items`. */
-  async function setPlottedDatastreams(
-    items: Datastream[],
-    qcId?: string | null
-  ) {
+  /** Replace the plotted set wholesale (used by URL hydration). */
+  async function setPlottedDatastreams(items: Datastream[]) {
     plottedDatastreams.value = items.slice()
-    if (qcId !== undefined) {
-      const valid = items.some((d) => d.id === qcId)
-      qcDatastreamId.value = valid ? (qcId ?? null) : (items[0]?.id ?? null)
-    } else if (!items.some((d) => d.id === qcDatastreamId.value)) {
-      qcDatastreamId.value = items[0]?.id ?? null
-    }
     await rebuildPlot()
   }
 
-  /** Change which datastream is under QC. Preserves the current zoom. */
-  async function setQcDatastream(id: string | null) {
-    if (qcDatastreamId.value === id) return
-    qcDatastreamId.value = id
-    updateOptions()
-    const { plotlyRef } = storeToRefs(usePlotlyStore())
-    if (plotlyRef.value) {
-      await handleNewPlot(undefined, { preserveZoom: true })
-    }
+  /** Start editing `managedId`. Its data arrives later via `setEditRecord`. */
+  async function setEditTarget(managedId: string) {
+    dropSnapshotSeries()
+    // The session store describes the previous target until `loadSessions`
+    // lands; drop it so its window is never drawn over this one.
+    if (managedId !== qcDatastreamId.value) useQcSessionStore().reset()
+    qcDatastreamId.value = managedId
+    await rebuildPlot()
   }
 
-  /**
-   * Enter editing on a freshly-created managed datastream by reusing the
-   * source's already-loaded series as the managed datastream's working copy,
-   * rather than adding a second, empty plotted item. The managed datastream
-   * is empty on the server, so re-fetching it would blank the plot; the QC
-   * editor zooms (never re-fetches), so this working copy survives until the
-   * first commit materializes the managed datastream.
-   */
-  async function adoptManagedDatastream(managed: Datastream, sourceId: string) {
-    const idx = plottedDatastreams.value.findIndex((d) => d.id === sourceId)
-    if (idx >= 0) plottedDatastreams.value.splice(idx, 1, managed)
-    else plottedDatastreams.value.push(managed)
-    qcDatastreamId.value = managed.id
-
-    // Re-key the loaded series to the managed datastream, keeping its data.
-    const series = graphSeriesArray.value.find((s) => s.id === sourceId)
-    if (series) {
-      series.id = managed.id
-      series.name = managed.name
+  /** Put the edit target's record on the plot, adding its series if needed. */
+  async function setEditRecord(record: ObservationRecord) {
+    const edit = qcDatastream.value
+    if (!edit) return
+    const existing = graphSeriesArray.value.find((s) => s.id === edit.id)
+    if (existing) {
+      existing.data = record
+      await usePlotlyStore().redraw()
+      return
     }
-    const ids = new Set(plottedDatastreams.value.map((d) => d.id))
-    graphSeriesArray.value = graphSeriesArray.value.filter((s) => ids.has(s.id))
-
-    assignSeriesColors(plottedDatastreams.value.map((d) => d.id))
+    graphSeriesArray.value.push(buildGraphSeries(edit, record))
+    orderAndColorSeries()
     updateOptions()
     const { plotlyRef } = storeToRefs(usePlotlyStore())
-    if (plotlyRef.value) {
-      await handleNewPlot(undefined, { preserveZoom: true })
-    }
+    if (plotlyRef.value) await handleNewPlot(undefined, { preserveZoom: true })
   }
 
-  /**
-   * Inverse of `adoptManagedDatastream`, for leaving the editor. Managed
-   * datastreams are hidden from the catalog, so without this the Select
-   * view shows a plot with no row selected. No-op when the QC target isn't
-   * managed or its source isn't in the catalog. Snapshots are dropped
-   * regardless: they have no place in the Select view.
-   */
-  async function releaseManagedDatastream() {
+  /** Stop editing. Plotted datastreams stay as they were. */
+  async function clearEditTarget() {
     dropSnapshotSeries()
-
-    const managedId = qcDatastreamId.value
-    if (!managedId) return
-    const history = qcHistories.value.find(
-      (h) => historyManagedId(h) === managedId
-    )
-    const sourceId = history ? historySourceId(history) : undefined
-    const source = sourceId
-      ? datastreams.value.find((d) => d.id === sourceId)
-      : undefined
-    if (!source) return
-
-    const idx = plottedDatastreams.value.findIndex((d) => d.id === managedId)
-    if (idx < 0) return
-    // The source can already be plotted alongside its managed datastream
-    // (both picked in the plot chooser); replacing would duplicate it.
-    if (plottedDatastreams.value.some((d) => d.id === source.id)) {
-      plottedDatastreams.value.splice(idx, 1)
-    } else {
-      plottedDatastreams.value.splice(idx, 1, source)
-    }
-    qcDatastreamId.value = source.id
-
-    // The working copy holds uncommitted edits; `rebuildPlot` refetches.
-    graphSeriesArray.value = graphSeriesArray.value.filter(
-      (s) => s.id !== managedId
-    )
-
+    const id = qcDatastreamId.value
+    if (!id) return
+    // Unsaved edits live only on the working copy.
+    useWorkingCopiesStore().invalidate(id)
+    qcDatastreamId.value = null
+    graphSeriesArray.value = graphSeriesArray.value.filter((s) => s.id !== id)
     await rebuildPlot()
   }
 
@@ -507,39 +446,31 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     }
   }
 
-  /** Rebuild the plot from scratch: drop zoom history, rebuild the
-   *  graph-series array from `plottedDatastreams`, regenerate Plotly
-   *  options, and re-render. Must run serialized — see the lock in
+  /** Rebuild the plot from scratch: rebuild the graph-series array from
+   *  `seriesDatastreams`, regenerate Plotly options, and re-render. Select
+   *  drops the zoom; the editor keeps it, since context changes never move
+   *  the user's view of the session. Must run serialized, see the lock in
    *  `rebuildPlot` above. */
   async function doRebuildPlot() {
     hasSelectionShape.value = false
-    if (!plottedDatastreams.value.length) {
+    if (!seriesDatastreams.value.length) {
       invalidateUnplottedWorkingCopies()
       clearChartState()
       return
     }
-    const { clearZoomHistory } = usePlotlyStore()
-    clearZoomHistory()
+    const keepZoom = !!qcDatastreamId.value
+    if (!keepZoom) usePlotlyStore().clearZoomHistory()
     await loadWorkingCopies()
-    // Presets re-anchor only in the Select view. In the Edit view the
-    // loaded window is the edit session's window; moving it here would
-    // refetch the working copy over a different range and corrupt what
-    // gets committed. `useUIStore()` is called lazily here, not at this
-    // store's setup top level, since userInterface.ts imports this store
-    // back and calling it during setup would recurse into a store that
-    // isn't finished constructing.
-    if (useUIStore().currentView !== DrawerType.Edit) {
-      const presetRange = resolvePresetWindow()
-      if (presetRange) {
-        beginDate.value = presetRange.begin
-        endDate.value = presetRange.end
-      }
+    const presetRange = resolvePresetWindow()
+    if (presetRange) {
+      beginDate.value = presetRange.begin
+      endDate.value = presetRange.end
     }
     await refreshGraphSeriesArray()
     updateOptions()
     const { plotlyRef } = storeToRefs(usePlotlyStore())
     if (plotlyRef.value) {
-      await handleNewPlot()
+      await handleNewPlot(undefined, { preserveZoom: keepZoom })
     }
   }
 
@@ -622,19 +553,24 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
       update &&
       beginDate.value &&
       endDate.value &&
-      plottedDatastreams.value.length
+      seriesDatastreams.value.length
     ) {
       const { redraw, clearZoomHistory } = usePlotlyStore()
       await refreshGraphSeriesArray()
-      // A date-filter change refetches data and drops the zoom window —
-      // the recorded zoom stack refers to the OLD time range and would
-      // be meaningless after redraw, so clear it.
-      clearZoomHistory()
-      // The user explicitly changed the date filter — they expect the
-      // new window to actually apply, so opt out of the zoom-preserving
-      // path in `redraw` (which otherwise copies the live range over
-      // the fresh layout and the plot would stay on the old window).
-      redraw(false, false)
+      if (qcDatastreamId.value) {
+        // Context reload in the editor: the user's view of the session stays put.
+        await redraw(false, true)
+      } else {
+        // A date-filter change refetches data and drops the zoom window:
+        // the recorded zoom stack refers to the OLD time range and would
+        // be meaningless after redraw, so clear it.
+        clearZoomHistory()
+        // The user explicitly changed the date filter, so they expect the
+        // new window to actually apply, so opt out of the zoom-preserving
+        // path in `redraw` (which otherwise copies the live range over
+        // the fresh layout and the plot would stay on the old window).
+        redraw(false, false)
+      }
     }
   }
 
@@ -707,61 +643,50 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     }
   }
 
+  /** Sort `graphSeriesArray` to match `seriesDatastreams` and assign colours.
+   *  Colour assignment excludes the edit target and its source so context
+   *  colours stay stable between Select and Edit. Re-sorting after every
+   *  fetch avoids the race where parallel cold fetches land out of order
+   *  and the plot's draw-layering / legend order goes out of sync. */
+  function orderAndColorSeries() {
+    const order = seriesDatastreams.value
+    const indexByDs = new Map(order.map((ds, i) => [ds.id, i]))
+    graphSeriesArray.value.sort(
+      (a, b) => (indexByDs.get(a.id) ?? 0) - (indexByDs.get(b.id) ?? 0)
+    )
+    const pinned = new Set([qcDatastreamId.value, sourceContextDatastream.value?.id])
+    assignSeriesColors(order.map((ds) => ds.id).filter((id) => !pinned.has(id)))
+  }
+
   /** Refreshes the graphSeriesArray based on the current selection of datastreams */
   const refreshGraphSeriesArray = async () => {
     // Remove graphSeries that are no longer selected
     invalidateUnplottedWorkingCopies()
-    const currentIds = new Set(plottedDatastreams.value.map((ds) => ds.id))
+    const currentIds = new Set(seriesDatastreams.value.map((ds) => ds.id))
     graphSeriesArray.value = graphSeriesArray.value.filter((s) =>
       currentIds.has(s.id)
     )
 
-    const updateOrFetchPromises = plottedDatastreams.value
-      // Frozen replays: no server-side datastream to fetch.
-      .filter((ds) => !isSnapshotId(ds.id))
+    const updateOrFetchPromises = seriesDatastreams.value
+      // Snapshots have no server datastream; the edit target's data belongs to the session.
+      .filter((ds) => !isSnapshotId(ds.id) && ds.id !== qcDatastreamId.value)
       .map(async (ds) => {
         loadingStates.value.set(ds.id, true)
         return updateOrFetchGraphSeries(ds, beginDate.value, endDate.value)
       })
 
     const results = await Promise.all(updateOrFetchPromises)
-
-    // `updateOrFetchGraphSeries` pushes new series into `graphSeriesArray`
-    // in whatever order the parallel fetches resolve. For URL preload
-    // (multiple datastream ids loading cold at once) that completion
-    // order usually doesn't match the user-facing `plottedDatastreams`
-    // order — so the plot iterated traces out of legend order, breaking
-    // the draw-layering and per-series colour mapping. Re-sort the
-    // array to mirror `plottedDatastreams` now that every fetch has
-    // landed; the iteration order downstream is the legend order again.
-    const indexByDs = new Map(
-      plottedDatastreams.value.map((ds, i) => [ds.id, i])
-    )
-    graphSeriesArray.value.sort(
-      (a, b) => (indexByDs.get(a.id) ?? 0) - (indexByDs.get(b.id) ?? 0)
-    )
-
-    // Colour assignment runs once per refresh, after every fetch
-    // has landed and the array is in legend order. Doing it here
-    // (rather than inline in `fetchGraphSeries`) eliminates the
-    // race where two parallel cold fetches both read the array
-    // before either's push had landed and ended up claiming the
-    // same colour slot — visible as two non-QC traces sharing a
-    // line colour, and as different colours rolling on each reload
-    // depending on which fetch resolved first.
-    assignSeriesColors(plottedDatastreams.value.map((ds) => ds.id))
-
+    orderAndColorSeries()
     return results
   }
 
-
   // The old watcher on `plottedDatastreams` that computed prev/next id
   // diffs and forked between "update time range" and "rebuild plot"
   // has been replaced by explicit actions:
   //   - plotDatastream / unplotDatastream / toggleDatastream
   //   - clearPlottedDatastreams
   //   - setPlottedDatastreams (URL hydration)
-  //   - setQcDatastream
+  //   - setEditTarget / setEditRecord / clearEditTarget
   // Each mutation site now calls the action so side effects (time range
   // sync, graph-series rebuild, zoom history clear, Plotly re-render)
   // happen inline and in a predictable order — no reactive cascade.
@@ -824,6 +749,8 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     selectedDateBtnId,
     qcDatastream,
     qcDatastreamId,
+    sourceContextDatastream,
+    seriesDatastreams,
     qualifierSet,
     selectedQualifier,
     selectedData,
@@ -844,9 +771,9 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     addSnapshotSeries,
     removeSnapshotSeries,
     setPlottedDatastreams,
-    setQcDatastream,
-    adoptManagedDatastream,
-    releaseManagedDatastream,
+    setEditTarget,
+    setEditRecord,
+    clearEditTarget,
     rebuildPlot,
     // updateOrFetchGraphSeries,
   }
diff --git a/apps/qc-app/src/store/plotly.ts b/apps/qc-app/src/store/plotly.ts
index 63c9d474..18336635 100644
--- a/apps/qc-app/src/store/plotly.ts
+++ b/apps/qc-app/src/store/plotly.ts
@@ -13,7 +13,10 @@ import {
   cropXaxisRange,
   labelColorFor,
   LABEL_COLORS,
+  SOURCE_CONTEXT_COLOR,
+  SOURCE_CONTEXT_LABEL_COLOR,
 } from '@/utils/plotting/plotly'
+import { withLiveShapes } from '@/utils/plotting/shapes'
 import type {
   AppPlotlyHTMLElement,
   AppPlotlyTrace,
@@ -118,11 +121,11 @@ export const usePlotlyStore = defineStore('Plotly', () => {
   const activeTab = ref<'plot' | 'table'>('plot')
 
   /**
-   * Cross-component signal for the "zoom to range" presets. Plot.vue
-   * sets `time` to the epoch-ms start of the chosen range; DataTable.vue
-   * watches `seq` and scrolls its virtual list so the first in-range row
-   * sits at the top. `seq` is bumped on every request so re-selecting the
-   * same preset still re-triggers the scroll.
+   * Cross-component scroll signal. Plot.vue sets `time` to the start of
+   * the session window when it zooms to it; DataTable.vue watches `seq`
+   * and scrolls its virtual list so the first row at or after `time` sits
+   * at the top. `seq` is bumped on every request so a repeat still
+   * re-triggers the scroll.
    */
   const tableScrollRequest = ref<{ time: number; seq: number } | null>(null)
   function requestTableScroll(time: number) {
@@ -355,7 +358,8 @@ export const usePlotlyStore = defineStore('Plotly', () => {
         x: opts.traces.map((t) => (t as AppPlotlyTrace).x),
         y: opts.traces.map((t) => (t as AppPlotlyTrace).y),
       } as unknown as Partial<PlotData>,
-      opts.layout
+      // `Plotly.update` replaces the whole shapes array; keep the stage band.
+      withLiveShapes(opts.layout, plotlyRef.value?.layout)
     )
 
     if (recomputeXaxisRange) {
@@ -496,8 +500,9 @@ export const usePlotlyStore = defineStore('Plotly', () => {
    */
   function colorForDatastream(id: string | undefined): string {
     if (!id) return COLORS[1]!
-    const { qcDatastream } = storeToRefs(useDataVisStore())
+    const { qcDatastream, sourceContextDatastream } = storeToRefs(useDataVisStore())
     if (qcDatastream.value?.id === id) return COLORS[0]!
+    if (sourceContextDatastream.value?.id === id) return SOURCE_CONTEXT_COLOR
     const series = graphSeriesArray.value.find((s) => s.id === id)
     // `||` (not `??`) so the empty-string sentinel emitted by
     // `fetchGraphSeries` before `assignSeriesColors` runs also falls
@@ -514,8 +519,9 @@ export const usePlotlyStore = defineStore('Plotly', () => {
    */
   function labelColorForDatastream(id: string | undefined): string {
     if (!id) return LABEL_COLORS[1]!
-    const { qcDatastream } = storeToRefs(useDataVisStore())
+    const { qcDatastream, sourceContextDatastream } = storeToRefs(useDataVisStore())
     if (qcDatastream.value?.id === id) return LABEL_COLORS[0]!
+    if (sourceContextDatastream.value?.id === id) return SOURCE_CONTEXT_LABEL_COLOR
     const series = graphSeriesArray.value.find((s) => s.id === id)
     if (!series || !series.color) return LABEL_COLORS[1]!
     return labelColorFor(series.color)
diff --git a/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts b/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
new file mode 100644
index 00000000..2c960d63
--- /dev/null
+++ b/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
@@ -0,0 +1,113 @@
+import { describe, it, expect } from 'vitest'
+import {
+  committedExtent,
+  defaultSessionWindow,
+  validateSessionWindow,
+} from '@/utils/sessionWindow'
+
+const source = {
+  phenomenonBeginTime: '2025-01-01T00:00:00Z',
+  phenomenonEndTime: '2025-12-31T00:00:00Z',
+}
+const committed = (start: string, end: string) => ({
+  status: 'committed',
+  phenomenonTimeStart: start,
+  phenomenonTimeEnd: end,
+})
+const win = (begin: string, end: string) => ({
+  begin: new Date(begin),
+  end: new Date(end),
+})
+
+describe('committedExtent', () => {
+  it('spans the earliest committed start to the latest committed end', () => {
+    const extent = committedExtent([
+      committed('2025-03-01T00:00:00Z', '2025-04-01T00:00:00Z'),
+      committed('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'),
+      { status: 'in_progress', phenomenonTimeStart: '2025-06-01T00:00:00Z', phenomenonTimeEnd: '2025-07-01T00:00:00Z' },
+    ])
+    expect(extent).toEqual(win('2025-01-01T00:00:00Z', '2025-04-01T00:00:00Z'))
+  })
+
+  it('is null when nothing is committed', () => {
+    expect(committedExtent([])).toBeNull()
+  })
+})
+
+describe('defaultSessionWindow', () => {
+  it('covers the whole source when nothing is committed', () => {
+    expect(defaultSessionWindow(source, [])).toEqual(
+      win('2025-01-01T00:00:00Z', '2025-12-31T00:00:00Z')
+    )
+  })
+
+  it('starts at the end of the committed history', () => {
+    const sessions = [committed('2025-01-01T00:00:00Z', '2025-05-01T00:00:00Z')]
+    expect(defaultSessionWindow(source, sessions)).toEqual(
+      win('2025-05-01T00:00:00Z', '2025-12-31T00:00:00Z')
+    )
+  })
+
+  it('covers the whole source when history already reaches its end', () => {
+    const sessions = [committed('2025-01-01T00:00:00Z', '2025-12-31T00:00:00Z')]
+    expect(defaultSessionWindow(source, sessions)).toEqual(
+      win('2025-01-01T00:00:00Z', '2025-12-31T00:00:00Z')
+    )
+  })
+
+  it('is null when the source has no observations', () => {
+    expect(defaultSessionWindow({}, [])).toBeNull()
+  })
+})
+
+describe('validateSessionWindow', () => {
+  const history = [committed('2025-03-01T00:00:00Z', '2025-06-01T00:00:00Z')]
+
+  it('accepts a window that starts exactly at the history end', () => {
+    expect(
+      validateSessionWindow(win('2025-06-01T00:00:00Z', '2025-08-01T00:00:00Z'), source, history)
+    ).toBeNull()
+  })
+
+  it('accepts a window overlapping committed sessions', () => {
+    expect(
+      validateSessionWindow(win('2025-05-01T00:00:00Z', '2025-08-01T00:00:00Z'), source, history)
+    ).toBeNull()
+  })
+
+  it('accepts a window that ends exactly at the history start', () => {
+    expect(
+      validateSessionWindow(win('2025-01-01T00:00:00Z', '2025-03-01T00:00:00Z'), source, history)
+    ).toBeNull()
+  })
+
+  it('rejects a gap after the history', () => {
+    expect(
+      validateSessionWindow(win('2025-07-01T00:00:00Z', '2025-08-01T00:00:00Z'), source, history)
+    ).toMatch(/gap after/)
+  })
+
+  it('rejects a gap before the history', () => {
+    expect(
+      validateSessionWindow(win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'), source, history)
+    ).toMatch(/gap before/)
+  })
+
+  it('rejects a window past the source end', () => {
+    expect(
+      validateSessionWindow(win('2025-06-01T00:00:00Z', '2026-02-01T00:00:00Z'), source, history)
+    ).toMatch(/source data/)
+  })
+
+  it('rejects a start that is not before the end', () => {
+    expect(
+      validateSessionWindow(win('2025-08-01T00:00:00Z', '2025-08-01T00:00:00Z'), source, [])
+    ).toMatch(/before the end/)
+  })
+
+  it('accepts any in-source window when nothing is committed', () => {
+    expect(
+      validateSessionWindow(win('2025-09-01T00:00:00Z', '2025-10-01T00:00:00Z'), source, [])
+    ).toBeNull()
+  })
+})
diff --git a/apps/qc-app/src/utils/__tests__/share.spec.ts b/apps/qc-app/src/utils/__tests__/share.spec.ts
index 71602e94..b3aac2ce 100644
--- a/apps/qc-app/src/utils/__tests__/share.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/share.spec.ts
@@ -55,9 +55,9 @@ describe('share encoding primitives', () => {
     expect(axisNameFromIndex(0)).toBe('y')
     expect(axisNameFromIndex(1)).toBe('y2')
     expect(axisNameFromIndex(4)).toBe('y5')
-    expect(axisIndexFromName('y', 5)).toBe(0)
-    expect(axisIndexFromName('y3', 5)).toBe(2)
-    expect(axisIndexFromName('bogus', 5)).toBe(-1)
+    expect(axisIndexFromName('y')).toBe(0)
+    expect(axisIndexFromName('y3')).toBe(2)
+    expect(axisIndexFromName('bogus')).toBe(-1)
   })
 })
 
@@ -160,6 +160,27 @@ describe('encodeShareState — omits defaults', () => {
     expect(q.yz).toBe('0:0~10;1:-5~5.1235')
   })
 
+  it('keeps yz entries for axes beyond the plotted count', () => {
+    const q = encodeShareState({
+      datastreamIds: ['a'],
+      zoom: { xRange: null, yRanges: { y2: [1, 2] } },
+    })
+    expect(q.yz).toBe('1:1~2')
+  })
+
+  it('writes ed only in the Edit view', () => {
+    expect(
+      encodeShareState({
+        editView: true,
+        editDatastreamId: 'mgd',
+        datastreamIds: ['a'],
+      }).ed
+    ).toBe('mgd')
+    expect(
+      encodeShareState({ editView: false, editDatastreamId: 'mgd' }).ed
+    ).toBeUndefined()
+  })
+
   it('omits data points keys when the mode is auto and threshold is default', () => {
     const q = encodeShareState({
       dataPointsMode: 'auto',
@@ -241,6 +262,16 @@ describe('decodeShareState', () => {
     expect(decoded.dataPointsMode).toBe('manualOff')
     expect(decoded.dataPointsThreshold).toBe(25000)
   })
+
+  it('reads ed back', () => {
+    expect(decodeShareState({ m: 'e', ed: 'mgd' }).editDatastreamId).toBe('mgd')
+  })
+
+  it('reads ed even without m=e, leaving editView unset', () => {
+    const decoded = decodeShareState({ ed: 'x' })
+    expect(decoded.editDatastreamId).toBe('x')
+    expect(decoded.editView).toBeUndefined()
+  })
 })
 
 describe('round-trip', () => {
@@ -250,6 +281,7 @@ describe('round-trip', () => {
     const original: ShareState = {
       workspaceId: '01a2b3c4-d5e6-7f89-0a1b-2c3d4e5f6789',
       editView: true,
+      editDatastreamId: 'ds-1',
       tableTab: true,
       datastreamIds: ['ds-1', 'ds-2', 'ds-3'],
       datePresetId: -1,
@@ -306,8 +338,7 @@ describe('history snapshots', () => {
     expect(out.snapshots).toEqual([{ sessionId: 'sess-1', opIndex: 3 }])
   })
 
-  // Snapshots stay out of `ds` so "QC target is the first id" and the
-  // h/ya bitmask indices keep holding.
+  // Snapshots stay out of `ds` so the h/ya bitmask indices keep holding.
   it('leaves snapshots out of the datastream id list', () => {
     const q = encodeShareState({
       datastreamIds: ['ds-a'],
diff --git a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
index c1d31474..f8e02968 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
@@ -20,17 +20,33 @@ vi.mock('@uwrl/qc-utils', () => ({
 }))
 
 const qcDatastream = ref<{ id: string; phenomenonBeginTime?: string } | null>(null)
+const sourceContextDatastream = ref<{ id: string } | null>(null)
 const beginDate = ref<Date | null>(null)
 const endDate = ref<Date | null>(null)
 const selectedData = ref<number[] | null>(null)
 
+// `vi.fn()`-wrapped so individual tests can override the return value for
+// one call (`mockReturnValueOnce`) to simulate `createPlotlyOption` being
+// invoked while the data-vis store is still under construction, when
+// `storeToRefs` yields an object with no properties at all.
 vi.mock('@/store/dataVisualization', () => ({
-  useDataVisStore: () => ({
+  useDataVisStore: vi.fn(() => ({
     qcDatastream,
+    sourceContextDatastream,
     beginDate,
     endDate,
     selectedData,
-  }),
+  })),
+}))
+
+const viewedSession = ref<{ phenomenonTimeStart: string; phenomenonTimeEnd: string } | null>(null)
+const inProgressSession = ref<{ phenomenonTimeStart: string; phenomenonTimeEnd: string } | null>(null)
+
+vi.mock('@/store/qcSession', () => ({
+  useQcSessionStore: vi.fn(() => ({
+    viewedSession,
+    inProgressSession,
+  })),
 }))
 
 const previewMode = ref(false)
@@ -74,12 +90,15 @@ import {
   COLORS,
   LABEL_COLORS,
   QUALIFIER_COLORS,
+  SOURCE_CONTEXT_COLOR,
   labelColorFor,
   buildQualifierBand,
   createPlotlyOption,
   findGapIndices,
   insertGapBreaks,
 } from '../options'
+import { useDataVisStore } from '@/store/dataVisualization'
+import { useQcSessionStore } from '@/store/qcSession'
 
 const makeSeries = (overrides: Partial<Record<string, unknown>> = {}) => {
   const x = new Float64Array([1, 2, 3, 4, 5])
@@ -169,6 +188,9 @@ describe('createPlotlyOption', () => {
   beforeEach(() => {
     setActivePinia(createPinia())
     qcDatastream.value = null
+    sourceContextDatastream.value = null
+    viewedSession.value = null
+    inProgressSession.value = null
     beginDate.value = null
     endDate.value = null
     previewMode.value = false
@@ -325,6 +347,89 @@ describe('createPlotlyOption', () => {
     const main = opts.traces.find((t: any) => t.id === 'qc') as any
     expect(main.marker.opacity).toBe(0)
   })
+
+  it('puts the first series on the primary axis when nothing is being edited', () => {
+    const a = makeSeries({ id: 'a' })
+    const b = makeSeries({ id: 'b', color: COLORS[2] })
+    const opts = createPlotlyOption([a, b])
+    expect((opts.traces.find((t: any) => t.id === 'a') as any).yaxis).toBe('y')
+    expect((opts.traces.find((t: any) => t.id === 'b') as any).yaxis).toBe('y2')
+  })
+
+  it('draws the source on the edit axis in grey, without its own axis', () => {
+    qcDatastream.value = { id: 'qc' }
+    sourceContextDatastream.value = { id: 'src' }
+    const opts = createPlotlyOption([
+      makeSeries({ id: 'qc' }),
+      makeSeries({ id: 'src', color: COLORS[3] }),
+      makeSeries({ id: 'other', color: COLORS[2] }),
+    ])
+    const src = opts.traces.find((t: any) => t.id === 'src') as any
+    expect(src.yaxis).toBe('y')
+    expect(src.marker.color).toBe(SOURCE_CONTEXT_COLOR)
+    expect(src.selected).toBeUndefined()
+    expect((opts.traces.find((t: any) => t.id === 'other') as any).yaxis).toBe('y2')
+    expect((opts.layout as any).yaxis3).toBeUndefined()
+  })
+
+  it('paints the source under the edit trace', () => {
+    qcDatastream.value = { id: 'qc' }
+    sourceContextDatastream.value = { id: 'src' }
+    const opts = createPlotlyOption([makeSeries({ id: 'qc' }), makeSeries({ id: 'src' })])
+    const ids = opts.traces.map((t: any) => t.id).filter(Boolean)
+    expect(ids.indexOf('src')).toBeLessThan(ids.indexOf('qc'))
+  })
+
+  it('shades the session window while editing', () => {
+    qcDatastream.value = { id: 'qc' }
+    inProgressSession.value = {
+      phenomenonTimeStart: '2025-02-01T00:00:00Z',
+      phenomenonTimeEnd: '2025-03-01T00:00:00Z',
+    }
+    const opts = createPlotlyOption([makeSeries({ id: 'qc' })])
+    const shape = ((opts.layout as any).shapes ?? []).find((s: any) => s.name === 'edit-window')
+    expect(shape.x0).toBe(Date.parse('2025-02-01T00:00:00Z'))
+    expect(shape.x1).toBe(Date.parse('2025-03-01T00:00:00Z'))
+    expect(shape.yref).toBe('paper')
+  })
+
+  it('prefers the viewed session window over the in-progress one', () => {
+    qcDatastream.value = { id: 'qc' }
+    inProgressSession.value = { phenomenonTimeStart: '2025-02-01T00:00:00Z', phenomenonTimeEnd: '2025-03-01T00:00:00Z' }
+    viewedSession.value = { phenomenonTimeStart: '2025-01-01T00:00:00Z', phenomenonTimeEnd: '2025-01-15T00:00:00Z' }
+    const opts = createPlotlyOption([makeSeries({ id: 'qc' })])
+    const shape = (opts.layout as any).shapes.find((s: any) => s.name === 'edit-window')
+    expect(shape.x0).toBe(Date.parse('2025-01-01T00:00:00Z'))
+  })
+
+  it('draws no window without an edit target', () => {
+    inProgressSession.value = { phenomenonTimeStart: '2025-02-01T00:00:00Z', phenomenonTimeEnd: '2025-03-01T00:00:00Z' }
+    const opts = createPlotlyOption([makeSeries({ id: 'a' })])
+    expect(((opts.layout as any).shapes ?? []).some((s: any) => s.name === 'edit-window')).toBe(false)
+  })
+
+  it('does not throw when called during store setup, before the data-vis and qcSession refs exist', () => {
+    // Regression: `usePlotlyStore` seeds `plotlyOptions` with
+    // `createPlotlyOption([])` while its own store setup runs, at which
+    // point `useDataVisStore()` (and `useQcSessionStore()`) can still be
+    // under construction. `storeToRefs` on that in-progress store yields
+    // an object with no properties, so every destructured ref is
+    // `undefined` rather than a ref holding `null`.
+    vi.mocked(useDataVisStore).mockReturnValueOnce({} as any)
+    vi.mocked(useQcSessionStore).mockReturnValueOnce({} as any)
+
+    let opts: ReturnType<typeof createPlotlyOption> | undefined
+    expect(() => {
+      opts = createPlotlyOption([])
+    }).not.toThrow()
+
+    expect(opts!.traces).toEqual([])
+    expect(opts!.layout).toBeDefined()
+    expect(opts!.config).toBeDefined()
+    expect(
+      ((opts!.layout as any).shapes ?? []).some((s: any) => s.name === 'edit-window')
+    ).toBe(false)
+  })
 })
 
 describe('snapshot series axes', () => {
diff --git a/apps/qc-app/src/utils/plotting/__tests__/shapes.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/shapes.spec.ts
new file mode 100644
index 00000000..202b0d97
--- /dev/null
+++ b/apps/qc-app/src/utils/plotting/__tests__/shapes.spec.ts
@@ -0,0 +1,41 @@
+import { describe, it, expect } from 'vitest'
+import { composeShapes, withLiveShapes } from '../shapes'
+
+const stage = { name: 'stage', x0: 10, x1: 20 }
+const oldWindow = { name: 'edit-window', x0: 100, x1: 200 }
+const newWindow = { name: 'edit-window', x0: 300, x1: 400 }
+
+describe('composeShapes', () => {
+  it('replaces only the named shapes, after the others by default', () => {
+    expect(composeShapes([stage, oldWindow], 'edit-window', [newWindow])).toEqual([
+      stage,
+      newWindow,
+    ])
+  })
+
+  it('puts its own shapes first when asked', () => {
+    expect(
+      composeShapes([oldWindow, stage], 'stage', [stage], { first: true })
+    ).toEqual([stage, oldWindow])
+  })
+
+  it('handles a plot with no shapes', () => {
+    expect(composeShapes(undefined, 'stage', [])).toEqual([])
+  })
+})
+
+describe('withLiveShapes', () => {
+  it('keeps the live stage shape and takes the fresh edit window', () => {
+    const layout = { dragmode: 'pan', shapes: [newWindow] } as any
+    const merged = withLiveShapes(layout, { shapes: [stage, oldWindow] } as any)
+    expect(merged.shapes).toEqual([stage, newWindow])
+    expect(merged.dragmode).toBe('pan')
+    // The stored layout is not mutated.
+    expect(layout.shapes).toEqual([newWindow])
+  })
+
+  it('removes the live edit window when the fresh layout has none', () => {
+    const merged = withLiveShapes({}, { shapes: [stage, oldWindow] } as any)
+    expect(merged.shapes).toEqual([stage])
+  })
+})
diff --git a/apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts
new file mode 100644
index 00000000..5fbfaa2b
--- /dev/null
+++ b/apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts
@@ -0,0 +1,86 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { createPinia, setActivePinia } from 'pinia'
+import { ref } from 'vue'
+
+// --- Mocks --------------------------------------------------------------
+
+const plotlyMock = vi.hoisted(() => ({
+  relayout: vi.fn(),
+  deleteTraces: vi.fn().mockResolvedValue(undefined),
+  addTraces: vi.fn().mockResolvedValue(undefined),
+}))
+vi.mock('plotly.js-dist', () => ({ default: plotlyMock }))
+
+const plotlyRef = ref<any>(null)
+
+vi.mock('@/store/plotly', () => ({
+  usePlotlyStore: () => ({ plotlyRef }),
+}))
+
+import {
+  setStageShape,
+  clearStageShape,
+  stagePanMode,
+} from '../staging'
+
+// Real Plotly mutates `gd.layout` in place on every relayout call; the
+// stub mirrors that so the next flush's "read the live shapes" step sees
+// what the previous flush wrote.
+const applyRelayout = (root: any, update: any) => {
+  root.layout = root.layout ?? {}
+  if (update && Object.prototype.hasOwnProperty.call(update, 'shapes')) {
+    root.layout.shapes = update.shapes
+  }
+  if (update && Object.prototype.hasOwnProperty.call(update, 'dragmode')) {
+    root.layout.dragmode = update.dragmode
+  }
+  return Promise.resolve()
+}
+
+describe('staging shape merging', () => {
+  beforeEach(() => {
+    setActivePinia(createPinia())
+    vi.clearAllMocks()
+    plotlyMock.relayout.mockImplementation(applyRelayout)
+    stagePanMode.value = true
+    plotlyRef.value = {
+      layout: {
+        dragmode: 'pan',
+        shapes: [{ name: 'edit-window', x0: 100, x1: 200, yref: 'paper' }],
+      },
+    }
+  })
+
+  it('keeps the edit-window shape alongside the stage shape when showing the stage band', async () => {
+    await setStageShape(10, 20)
+
+    const lastCall = plotlyMock.relayout.mock.calls.at(-1)
+    const shapes = lastCall![1].shapes as any[]
+    expect(shapes.some((s) => s.name === 'edit-window')).toBe(true)
+    expect(shapes.some((s) => s.name === 'stage')).toBe(true)
+    // Untouched by the merge.
+    const editWindow = shapes.find((s) => s.name === 'edit-window')
+    expect(editWindow.x0).toBe(100)
+    expect(editWindow.x1).toBe(200)
+  })
+
+  it('keeps the edit-window shape after clearing the stage shape', async () => {
+    await setStageShape(10, 20)
+    await clearStageShape()
+
+    const lastCall = plotlyMock.relayout.mock.calls.at(-1)
+    const shapes = lastCall![1].shapes as any[]
+    expect(shapes.some((s) => s.name === 'edit-window')).toBe(true)
+    expect(shapes.some((s) => s.name === 'stage')).toBe(false)
+  })
+
+  it('does not duplicate the stage shape across repeated setStageShape calls', async () => {
+    await setStageShape(10, 20)
+    await setStageShape(30, 40)
+
+    const lastCall = plotlyMock.relayout.mock.calls.at(-1)
+    const shapes = lastCall![1].shapes as any[]
+    expect(shapes.filter((s) => s.name === 'stage').length).toBe(1)
+    expect(shapes.find((s) => s.name === 'stage').x0).toBe(30)
+  })
+})
diff --git a/apps/qc-app/src/utils/plotting/operations.ts b/apps/qc-app/src/utils/plotting/operations.ts
index 60f68681..a8bf252c 100644
--- a/apps/qc-app/src/utils/plotting/operations.ts
+++ b/apps/qc-app/src/utils/plotting/operations.ts
@@ -11,6 +11,7 @@ import { storeToRefs } from 'pinia'
 import { findFirstGreaterOrEqual } from '@uwrl/qc-utils'
 import { handleNewPlot } from './events'
 import { traceXAsNumbers } from './internal'
+import { withLiveShapes } from './shapes'
 import type { AppPlotlyTrace } from './options'
 
 /**
@@ -161,7 +162,10 @@ export const cropXaxisRange = async () => {
 
   setTimeout(async () => {
     try {
-      const layoutUpdates: Partial<Layout> = { ...plotlyOptions.value.layout }
+      const layoutUpdates = withLiveShapes(
+        plotlyOptions.value.layout,
+        plotlyRef.value?.layout
+      )
       const xAxis = layoutUpdates.xaxis as Partial<LayoutAxis> | undefined
       const xRange = xAxis?.range as Array<string | number> | undefined
       // Plotly will rewrite timestamps as datestrings. We need to convert them back to timestamps.
diff --git a/apps/qc-app/src/utils/plotting/options.ts b/apps/qc-app/src/utils/plotting/options.ts
index a6d5a69b..5a25e09d 100644
--- a/apps/qc-app/src/utils/plotting/options.ts
+++ b/apps/qc-app/src/utils/plotting/options.ts
@@ -8,14 +8,17 @@ import type {
   LayoutAxis,
   PlotData,
   PlotlyHTMLElement,
+  Shape,
 } from 'plotly.js-dist'
 import { storeToRefs } from 'pinia'
 import { useDataVisStore } from '@/store/dataVisualization'
+import { useQcSessionStore } from '@/store/qcSession'
 import { useQualifierStore } from '@/store/qualifiers'
 import { findFirstGreaterOrEqual } from '@uwrl/qc-utils'
 import { DENSITY_HIDE_MARKERS, Y_AXIS_KEY_RE } from './internal'
 import { undoZoom, redoZoom } from './zoom'
 import { fitXaxisToVisible, fitYaxisToVisible } from './operations'
+import { EDIT_WINDOW_SHAPE_NAME } from './shapes'
 
 /**
  * Return type of `createPlotlyOption`. Plan 03-03 will reuse this shape
@@ -110,6 +113,10 @@ export const LABEL_COLORS = [
   '#117a85', // cyan
 ]
 
+// Read-only raw-source context drawn behind the edit target while editing.
+export const SOURCE_CONTEXT_COLOR = '#9e9e9e'
+export const SOURCE_CONTEXT_LABEL_COLOR = '#616161'
+
 /** Companion text colour for a `COLORS[i]` line; falls back to QC grey. */
 export const labelColorFor = (lineColor: string): string => {
   const idx = COLORS.indexOf(lineColor)
@@ -119,6 +126,22 @@ export const labelColorFor = (lineColor: string): string => {
   return idx >= 0 ? LABEL_COLORS[idx]! : LABEL_COLORS[0]!
 }
 
+/** Left-side primary-axis chrome shared by the edit target and the first
+ *  Select-view series. Title is omitted on purpose: the horizontal chip
+ *  rendered above the plot (Plot.vue) replaces Plotly's vertical title. */
+const primaryAxis = (
+  tickColor: string,
+  lineColor: string
+): Partial<LayoutAxis> => ({
+  title: undefined,
+  tickfont: { color: tickColor },
+  side: 'left',
+  showline: true,
+  linecolor: lineColor,
+  automargin: true,
+  tickformat: '~s',
+})
+
 /**
  * Indices `i` (i >= 1) where `x[i] - x[i-1] > thresholdMs` — every spot
  * where a synthetic break should be inserted to disconnect the rendered
@@ -385,10 +408,12 @@ export { buildQualifierBand }
 export const createPlotlyOption = (
   seriesArray: GraphSeries[]
 ): PlotlyChartOptions => {
-  const { qcDatastream, beginDate, endDate } = storeToRefs(useDataVisStore())
+  const { qcDatastream, sourceContextDatastream, beginDate, endDate } =
+    storeToRefs(useDataVisStore())
   const { previewMode, hiddenAxisIds, hiddenTraceIds, plotlyRef } = storeToRefs(
     usePlotlyStore()
   )
+  const { viewedSession, inProgressSession } = storeToRefs(useQcSessionStore())
   const isPreview = previewMode?.value ?? false
   const hiddenAxes = hiddenAxisIds?.value ?? new Set<string>()
   const hiddenTraces = hiddenTraceIds?.value ?? new Set<string>()
@@ -421,10 +446,14 @@ export const createPlotlyOption = (
   let minDatetime = Infinity
 
   // Axis-naming scheme:
-  //   yaxis       (no suffix, primary)        — QC series
-  //   yaxis2, yaxis3, … (overlaying, right)   — every non-QC series
+  //   yaxis       (no suffix, primary)      : the edit target while editing
+  //                                            (its raw source shares this
+  //                                            axis, drawn under it), or the
+  //                                            first series in Select
+  //   yaxis2, yaxis3, … (overlaying, right) : every other series
   let nonQcAxisCount = 0
   let visibleNonQcCount = 0
+  let primaryAssigned = false
 
   seriesArray.forEach((s) => {
     const color = s.color ?? COLORS[1]
@@ -441,8 +470,13 @@ export const createPlotlyOption = (
       minDatetime = Math.min(xDataStart, minDatetime)
     }
 
-    const isQc = s.id === qcDatastream.value?.id
-    const axisSuffix: string | number = isQc ? '' : nonQcAxisCount + 2
+    const editId = qcDatastream?.value?.id
+    const isQc = !!editId && s.id === editId
+    const isSource = !!editId && s.id === sourceContextDatastream?.value?.id
+    // Without an edit target the first series owns the primary axis.
+    const isPrimary = isQc || isSource || (!editId && !primaryAssigned)
+    if (isPrimary) primaryAssigned = true
+    const axisSuffix: string | number = isPrimary ? '' : nonQcAxisCount + 2
     const axisKey = `yaxis${axisSuffix}`
     const axisRef = `y${axisSuffix}`
 
@@ -492,17 +526,26 @@ export const createPlotlyOption = (
       // with it — no offset needed.
       trace._windowStartIdx = 0
 
-      // Title omitted on purpose — the horizontal QC chip (rendered as
-      // an HTML overlay in Plot.vue) replaces Plotly's vertical title.
-      ;(yaxis as Record<string, Partial<LayoutAxis>>)[axisKey] = {
-        title: undefined,
-        tickfont: { color: COLORS[0] },
-        side: 'left',
-        showline: true,
-        linecolor: COLORS[0],
-        automargin: true,
-        tickformat: '~s',
-      } as Partial<LayoutAxis>
+      ;(yaxis as Record<string, Partial<LayoutAxis>>)[axisKey] = primaryAxis(
+        COLORS[0]!,
+        COLORS[0]!
+      )
+
+    } else if (isSource) {
+      // Read-only raw-source context: grey, shares the edit target's axis,
+      // never gets its own axis entry or selection styling.
+      trace.marker = { ...(trace.marker ?? {}), color: SOURCE_CONTEXT_COLOR }
+      trace.unselected = { marker: { opacity: markerOpacity } }
+
+    } else if (isPrimary) {
+      // First series in Select view: same primary-axis placement as the
+      // QC branch, styled with the series' own colour.
+      trace.unselected = { marker: { opacity: markerOpacity } }
+
+      ;(yaxis as Record<string, Partial<LayoutAxis>>)[axisKey] = primaryAxis(
+        labelColorFor(color),
+        color
+      )
 
     } else {
       // Plotly applies a global selection-fade once any trace has
@@ -578,7 +621,7 @@ export const createPlotlyOption = (
         mode: 'lines',
         hoverinfo: 'skip',
         showLegend: false,
-        line: { color: isQc ? COLORS[0] : color },
+        line: { color: isQc ? COLORS[0] : isSource ? SOURCE_CONTEXT_COLOR : color },
         _isGapOverlay: true,
         _gapOverlayFor: s.id,
       }
@@ -590,7 +633,8 @@ export const createPlotlyOption = (
     }
   })
 
-  // Reverse so the top of the legend paints last.
+  // Reverse so seriesArray[0] paints last (on top). Order is
+  // [edit, source, ...], so the source already paints under the edit trace.
   traces.reverse()
 
   const counter = nonQcAxisCount
@@ -651,6 +695,31 @@ export const createPlotlyOption = (
     showlegend: false,
   }
 
+  // Shade the session window while editing: the viewed session takes
+  // precedence over the in-progress one (viewing history overrides the
+  // live session's own window).
+  const editWindow = qcDatastream?.value
+    ? (viewedSession?.value ?? inProgressSession?.value)
+    : null
+  if (editWindow) {
+    layout.shapes = [
+      {
+        name: EDIT_WINDOW_SHAPE_NAME,
+        type: 'rect',
+        xref: 'x',
+        yref: 'paper',
+        x0: Date.parse(editWindow.phenomenonTimeStart),
+        x1: Date.parse(editWindow.phenomenonTimeEnd),
+        y0: 0,
+        y1: 1,
+        fillcolor: 'rgba(25, 118, 210, 0.07)',
+        line: { width: 0 },
+        layer: 'below',
+        editable: false,
+      } as Partial<Shape>,
+    ]
+  }
+
   // Modebar buttons. `isPreview` drops select/lasso and the Fit buttons.
   const undoZoomButton = {
     name: 'Undo zoom',
diff --git a/apps/qc-app/src/utils/plotting/shapes.ts b/apps/qc-app/src/utils/plotting/shapes.ts
new file mode 100644
index 00000000..a20a43d6
--- /dev/null
+++ b/apps/qc-app/src/utils/plotting/shapes.ts
@@ -0,0 +1,41 @@
+/**
+ * `layout.shapes` has several writers, and each owns only the shapes it
+ * names: `createPlotlyOption` owns the session window band, the staging
+ * helpers own the stage band. A write replaces its own shapes and carries
+ * everything else on the live plot through unchanged.
+ */
+
+import type { Layout } from 'plotly.js-dist'
+
+export type PlotlyShape = Partial<NonNullable<Layout['shapes']>[number]> & {
+  editable?: boolean
+  name?: string
+}
+
+export const EDIT_WINDOW_SHAPE_NAME = 'edit-window'
+export const STAGE_SHAPE_NAME = 'stage'
+
+/** `live` with the shapes named `name` replaced by `own`. `first` puts
+ *  `own` ahead of the other writers' shapes instead of after them. */
+export function composeShapes(
+  live: readonly PlotlyShape[] | undefined,
+  name: string,
+  own: readonly PlotlyShape[],
+  { first = false }: { first?: boolean } = {}
+): PlotlyShape[] {
+  const others = (live ?? []).filter((shape) => shape?.name !== name)
+  return first ? [...own, ...others] : [...others, ...own]
+}
+
+/** A copy of a fresh `createPlotlyOption` layout whose shapes keep what
+ *  other writers have on the live plot. */
+export function withLiveShapes(
+  layout: Partial<Layout>,
+  liveLayout: Partial<Layout> | undefined
+): Partial<Layout> {
+  const own = ((layout.shapes ?? []) as PlotlyShape[]).filter(
+    (shape) => shape.name === EDIT_WINDOW_SHAPE_NAME
+  )
+  const live = liveLayout?.shapes as PlotlyShape[] | undefined
+  return { ...layout, shapes: composeShapes(live, EDIT_WINDOW_SHAPE_NAME, own) }
+}
diff --git a/apps/qc-app/src/utils/plotting/staging.ts b/apps/qc-app/src/utils/plotting/staging.ts
index 740f3459..ace49967 100644
--- a/apps/qc-app/src/utils/plotting/staging.ts
+++ b/apps/qc-app/src/utils/plotting/staging.ts
@@ -6,12 +6,9 @@
  * helper tolerates being called when the plot isn't mounted yet so
  * callers can fire-and-forget from setup/unmount hooks.
  *
- * We own the shape state in module-local refs rather than
- * round-tripping through `gd.layout.shapes` between writes: Plotly
- * canonicalises layout reads (e.g. date `x0` values come back as ISO
- * strings, unknown fields like `name` get stripped) which made the
- * older "read, filter by name, write" pattern lose track of its own
- * shapes on the second call.
+ * The stage band is the only shape this module owns. Every flush
+ * rebuilds the live `layout.shapes` through `composeShapes`, so shapes
+ * other writers own (the session window band) are carried through.
  *
  * Gap bands used to live here as secondary red rectangles marking
  * detected gaps. They were removed because `edits.shapePosition:
@@ -28,23 +25,20 @@ import type { Layout } from 'plotly.js-dist'
 import { ref } from 'vue'
 import { usePlotlyStore } from '@/store/plotly'
 import { storeToRefs } from 'pinia'
+import { composeShapes, STAGE_SHAPE_NAME, type PlotlyShape } from './shapes'
 
 const GHOST_TRACE_NAME = 'qc-ghost-fills'
 
-type PlotlyShape = Partial<NonNullable<Layout['shapes']>[number]> & {
-  editable?: boolean
-}
-
 let stageShape: PlotlyShape | null = null
 /** True when the plot is in pan mode, meaning the editable stage
  *  shape should be rendered. In zoom / select / lasso modes we drop
  *  the shape from the flushed array entirely so it can't swallow
- *  the mouse-down gesture or keep its grab cursor over the band —
- *  setting `editable: false` alone doesn't fully back out Plotly's
- *  shape-edit hit-testing when `edits.shapePosition` is on.
+ *  the mouse-down gesture or keep its grab cursor over the band,
+ *  since setting `editable: false` alone doesn't fully back out
+ *  Plotly's shape-edit hit-testing when `edits.shapePosition` is on.
  *
  *  Reactive so RangeStager can show a hint ("Range hidden in zoom
- *  mode — switch back to pan to resize") when the band goes away.
+ *  mode, switch back to pan to resize") when the band goes away.
  */
 export const stagePanMode = ref(true)
 
@@ -57,11 +51,14 @@ function getRoot(): HTMLElement | null {
 async function flushShapes() {
   const root = getRoot()
   if (!root) return
-  // The stage shape is the only shape we own. Drop it outside pan
-  // mode so zoom / select / lasso gestures aren't captured by the
-  // shape-edit hit-tester.
-  const shapes: PlotlyShape[] =
-    stageShape && stagePanMode.value ? [stageShape] : []
+  const live = (root as unknown as { layout?: { shapes?: PlotlyShape[] } })
+    .layout?.shapes
+  // Drop the stage shape outside pan mode so zoom / select / lasso
+  // gestures aren't captured by the shape-edit hit-tester. Stage stays
+  // at index 0 when present so `onStageDrag`'s `shapes[0].*` event keys
+  // keep pointing at it.
+  const own = stageShape && stagePanMode.value ? [stageShape] : []
+  const shapes = composeShapes(live, STAGE_SHAPE_NAME, own, { first: true })
   await Plotly.relayout(root, { shapes } as unknown as Partial<Layout>)
 }
 
@@ -103,6 +100,7 @@ export async function enterPanMode(): Promise<void> {
 export async function setStageShape(fromTs: number, toTs: number) {
   stagePanMode.value = currentDragmode() === 'pan'
   stageShape = {
+    name: STAGE_SHAPE_NAME,
     type: 'rect',
     xref: 'x',
     yref: 'paper',
@@ -221,8 +219,9 @@ export function onStageDrag(
       return
     }
 
-    // Stage shape is the only shape we flush, so its index is
-    // always 0 in the layout shapes array.
+    // The stage shape is always prepended first in `flushShapes`, so
+    // its index in the layout shapes array is always 0 regardless of
+    // how many other shapes (e.g. the edit-window band) are present.
     const stageIdx = 0
     const x0Key = `shapes[${stageIdx}].x0`
     const x1Key = `shapes[${stageIdx}].x1`
diff --git a/apps/qc-app/src/utils/sessionWindow.ts b/apps/qc-app/src/utils/sessionWindow.ts
new file mode 100644
index 00000000..859558c2
--- /dev/null
+++ b/apps/qc-app/src/utils/sessionWindow.ts
@@ -0,0 +1,72 @@
+/**
+ * Rules for a new edit session's window (history spec 7.2.3 and 7.2.4):
+ * inside the source's observed extent, and never leaving a gap before or
+ * after the committed history. Overlapping committed sessions is allowed.
+ */
+
+import { dataExtent, type TimeWindow } from '@/utils/timeRangePresets'
+
+export interface SourceExtent {
+  phenomenonBeginTime?: string | null
+  phenomenonEndTime?: string | null
+}
+
+export interface SessionRange {
+  status: string
+  phenomenonTimeStart: string
+  phenomenonTimeEnd: string
+}
+
+const formatMs = (ms: number) => new Date(ms).toLocaleString()
+
+export function committedExtent(
+  sessions: readonly SessionRange[]
+): TimeWindow | null {
+  let begin = Infinity
+  let end = -Infinity
+  for (const s of sessions) {
+    if (s.status !== 'committed') continue
+    begin = Math.min(begin, Date.parse(s.phenomenonTimeStart))
+    end = Math.max(end, Date.parse(s.phenomenonTimeEnd))
+  }
+  if (!Number.isFinite(begin) || !Number.isFinite(end)) return null
+  return { begin: new Date(begin), end: new Date(end) }
+}
+
+export function defaultSessionWindow(
+  source: SourceExtent,
+  sessions: readonly SessionRange[]
+): TimeWindow | null {
+  const extent = dataExtent([source])
+  if (!extent) return null
+  const history = committedExtent(sessions)
+  if (history && history.end.getTime() < extent.end.getTime()) {
+    return { begin: new Date(history.end), end: extent.end }
+  }
+  return extent
+}
+
+export function validateSessionWindow(
+  window: TimeWindow,
+  source: SourceExtent,
+  sessions: readonly SessionRange[]
+): string | null {
+  const extent = dataExtent([source])
+  if (!extent) return 'The source datastream has no observations to edit.'
+  const begin = window.begin.getTime()
+  const end = window.end.getTime()
+  if (!(begin < end)) return 'The start must be before the end.'
+  if (begin < extent.begin.getTime() || end > extent.end.getTime()) {
+    return `The window must be within the source data (${formatMs(extent.begin.getTime())} to ${formatMs(extent.end.getTime())}).`
+  }
+  const history = committedExtent(sessions)
+  if (history) {
+    if (begin > history.end.getTime()) {
+      return `This leaves a gap after the committed history, which ends ${formatMs(history.end.getTime())}. Start on or before that.`
+    }
+    if (end < history.begin.getTime()) {
+      return `This leaves a gap before the committed history, which starts ${formatMs(history.begin.getTime())}. End on or after that.`
+    }
+  }
+  return null
+}
diff --git a/apps/qc-app/src/utils/share.ts b/apps/qc-app/src/utils/share.ts
index 85f8f303..eb44d4e4 100644
--- a/apps/qc-app/src/utils/share.ts
+++ b/apps/qc-app/src/utils/share.ts
@@ -2,25 +2,30 @@
  * Encode / decode the QC app's share URL.
  *
  * The URL keeps every plot-relevant piece of state the sender sees so
- * a recipient lands on an identical view: plotted datastreams, QC
- * target, time range, current view (Select / Edit), Plot/Table tab,
- * per-trace visibility, per-Y-axis visibility, X/Y zoom, and the data-
- * points (markers) mode and threshold. Sidebar filters are kept only
- * on the Select view, since they drive the datastreams table — not
- * the plot.
+ * a recipient lands on an identical view: plotted datastreams (`ds`),
+ * the datastream being edited (`ed`, Edit view only), time range,
+ * current view (Select / Edit), Plot/Table tab, per-trace visibility,
+ * per-Y-axis visibility, X/Y zoom, and the data-points (markers) mode
+ * and threshold. Sidebar filters are kept only on the Select view,
+ * since they drive the datastreams table, not the plot.
+ *
+ * `ds` holds only plotted datastreams; the edit target and its source
+ * row are never part of it. The visibility bitmasks (`h`, `ya`) are
+ * positions into `ds`, so they share that exclusion. The zoom Y-axis
+ * index (`yz`) is the Plotly axis number instead (`0` is `y`, the edit
+ * target's axis while editing), not a position in `ds`.
  *
  * Compaction tactics used (none of them lossy):
- *   - Short query keys (`ds`, `b`, `e`, `r`, …) instead of verbose
- *     ones.
- *   - QC target is implicit (first id in `ds`); no separate `qc` key.
+ *   - Short query keys (`ds`, `ed`, `b`, `e`, `r`, …) instead of
+ *     verbose ones.
  *   - When a date preset is active (`r=0..5`), the begin/end pair is
- *     elided — recipients recompute it from "now" on load, matching
+ *     elided, recipients recompute it from "now" on load, matching
  *     the preset semantics the sender chose.
  *   - Timestamps go through `tsToBase36Seconds` (≈ 7 chars instead
  *     of 24).
  *   - Visibility state is a hex bitmask over the `ds` order rather
  *     than a list of ids.
- *   - Zoom Y axes are indexed by position in `ds`; axes at their
+ *   - Zoom Y axes are indexed by Plotly axis number; axes at their
  *     default fit are omitted entirely.
  *   - Optional bits (tab, data-points mode/threshold, zoom) are
  *     emitted only when they differ from the default.
@@ -40,13 +45,14 @@ export interface ShareState {
   workspaceId?: string | null
   /** Edit view active when true. Select view is the default. */
   editView?: boolean
+  /** The datastream being edited. Emitted only in the Edit view. */
+  editDatastreamId?: string
   /** Table tab active when true. Plot tab is the default. */
   tableTab?: boolean
-  /** Ordered list of plotted datastream ids. The first id is the QC
-   *  target. */
+  /** Ordered list of plotted datastream ids. */
   datastreamIds?: string[]
   /** History snapshots plotted as comparison lines. Kept out of `ds` so the
-   *  QC-target-is-first rule and the `h` / `ya` bitmask indices still hold. */
+   *  `h` / `ya` bitmask indices still hold. */
   snapshots?: { sessionId: string; opIndex: number }[]
   /** Date range preset id (`0..5`). When set, `begin`/`end` are
    *  omitted from the URL and the receiver resolves the window
@@ -64,11 +70,13 @@ export interface ShareState {
   observedPropertyNames?: string[]
   processingLevelNames?: string[]
   /** Eye-toggle state: `true` means visible, `false` means hidden.
-   *  Indexed by position in `datastreamIds`. */
+   *  Indexed by position in `datastreamIds` (`ds`), which holds only
+   *  plotted datastreams. The edit target and its source row are not
+   *  part of `ds`, so they're not part of this mask either. */
   traceVisibility?: boolean[]
-  /** Y-axis-toggle state for non-QC datastreams (`true` means the
-   *  axis is shown). Indexed by position in `datastreamIds` starting
-   *  at 1 (the QC stream's primary axis can't be hidden). */
+  /** Y-axis-toggle state (`true` means the axis is shown). Indexed by
+   *  position in `datastreamIds` (`ds`), same exclusions as
+   *  `traceVisibility`. */
   axisVisibility?: boolean[]
   /** Plot zoom — both x and per-axis y. */
   zoom?: ShareableZoom
@@ -128,6 +136,7 @@ export function encodeShareState(state: ShareState): Record<string, string> {
 
   if (state.workspaceId) q.ws = state.workspaceId
   if (state.editView) q.m = 'e'
+  if (state.editView && state.editDatastreamId) q.ed = state.editDatastreamId
   if (state.tableTab) q.tab = 't'
 
   if (state.datastreamIds?.length) {
@@ -185,10 +194,9 @@ export function encodeShareState(state: ShareState): Record<string, string> {
       q.z = `${tsToBase36Seconds(lo)}.${tsToBase36Seconds(hi)}`
     }
     const yEntries: string[] = []
-    const ids = state.datastreamIds ?? []
     for (const [axisName, range] of Object.entries(state.zoom.yRanges)) {
-      const idx = axisIndexFromName(axisName, ids.length)
-      if (idx < 0 || idx >= ids.length) continue
+      const idx = axisIndexFromName(axisName)
+      if (idx < 0) continue
       yEntries.push(`${idx}:${compactFloat(range[0])}~${compactFloat(range[1])}`)
     }
     if (yEntries.length) q.yz = yEntries.join(';')
@@ -218,6 +226,8 @@ export function decodeShareState(query: Record<string, unknown>): ShareState {
   if (ws) out.workspaceId = ws
 
   if (str('m') === 'e') out.editView = true
+  const ed = str('ed')
+  if (ed) out.editDatastreamId = ed
   if (str('tab') === 't') out.tableTab = true
 
   const ds = splitCsv(str('ds'))
@@ -303,13 +313,13 @@ export function decodeShareState(query: Record<string, unknown>): ShareState {
   return out
 }
 
-/** Position-in-`ds` → Plotly axis name (`0` → `y`, `1` → `y2`, …). */
+/** Axis index → Plotly axis name (`0` → `y`, `1` → `y2`, …). */
 export function axisNameFromIndex(idx: number): string {
   return idx === 0 ? 'y' : `y${idx + 1}`
 }
 
-/** Plotly axis name → position-in-`ds`. `-1` for unrecognised inputs. */
-export function axisIndexFromName(name: string, _datastreamCount: number): number {
+/** Plotly axis name → axis index. `-1` for unrecognised inputs. */
+export function axisIndexFromName(name: string): number {
   if (name === 'y') return 0
   const m = name.match(/^y(\d+)$/)
   if (!m) return -1

From 15185eb0b5ff28c7593eb1906b54cc29bae80bbc Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:00:57 -0600
Subject: [PATCH 54/93] feat: anchor editor context presets around the session
 window

Also keeps the loading overlay off the plot toolbar, drops late session
writes for a target the user already left, and clears zoom history when
the edit target changes.
---
 apps/qc-app/README.md                         |   2 +-
 apps/qc-app/docs/API_REFERENCE.md             |  41 ++++--
 apps/qc-app/docs/ARCHITECTURE.md              |   2 +-
 apps/qc-app/docs/PLOTTING.md                  |  46 +++++--
 apps/qc-app/docs/TESTING.md                   |   2 +-
 apps/qc-app/docs/USER_GUIDE.md                |  14 +-
 apps/qc-app/e2e/edit-selection.spec.ts        |  75 ++++++++++-
 apps/qc-app/e2e/support/app.ts                |  14 +-
 apps/qc-app/e2e/tooltip-threshold.spec.ts     |   7 +-
 .../components/VisualizeData/DataTable.vue    |   4 +-
 .../VisualizeData/DataVisTimeFilters.vue      |  12 +-
 .../VisualizeData/DataVisualization.vue       |  56 ++++----
 .../src/components/VisualizeData/Plot.vue     |  17 ++-
 .../VisualizeData/__tests__/DataTable.spec.ts |   4 +-
 .../__tests__/DataVisTimeFilters.spec.ts      |  14 ++
 .../__tests__/DataVisualization.spec.ts       |  18 ++-
 .../VisualizeData/__tests__/Plot.spec.ts      |  12 +-
 .../__tests__/useEditSession.spec.ts          |  88 ++++++++++++
 apps/qc-app/src/composables/useEditSession.ts |  20 ++-
 .../store/__tests__/dataVisualization.spec.ts | 125 +++++++++++++++++-
 .../src/store/__tests__/qcSession.spec.ts     |  14 ++
 apps/qc-app/src/store/dataVisualization.ts    |  64 +++++++--
 apps/qc-app/src/store/qcSession.ts            |  46 ++++---
 .../utils/__tests__/timeRangePresets.spec.ts  |  90 +++++++++++++
 apps/qc-app/src/utils/timeRangePresets.ts     |  70 ++++++++--
 25 files changed, 748 insertions(+), 109 deletions(-)

diff --git a/apps/qc-app/README.md b/apps/qc-app/README.md
index 2882285e..0207fff6 100644
--- a/apps/qc-app/README.md
+++ b/apps/qc-app/README.md
@@ -15,7 +15,7 @@ This web app facilitates QC/QA for time series observations stored in a HydroSer
 The app is the operator's view of HydroServer's QC pipeline:
 
 1. **Browse** — pick a workspace, filter datastreams by site / observed property / processing level, and plot up to five at once on a synchronized multi-axis chart.
-2. **QC one stream at a time** — the first plotted stream is the QC target. The other plotted traces are read-only context.
+2. **QC one stream at a time**: the pencil button on a datastream row picks what to edit, separately from the plot checkboxes. The editor draws that stream over its raw source, with the plotted datastreams as read-only context.
 3. **Filter / edit / add** — every operation (Value Threshold, Find Gaps, Persistence, Interpolate, Drift Correction, Fill Gaps, Add Points, etc.) commits a `HistoryItem` to a replayable edit history backed by [`@uwrl/qc-utils`](https://www.npmjs.com/package/@uwrl/qc-utils).
 4. **Save / load a QC History** — export the history as a JSON document, replay it on the same datastream a week later, or templatize across stations.
 5. **Save and commit** — save the edits to a QC session as a draft, then commit the session to push the quality-controlled observations to its managed datastream in `replace` mode.
diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 91a7e0df..1ccf16c3 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -205,16 +205,21 @@ const {
 } = useEditSession()
 ```
 
-- `beginEditing()` — resolves the QC history for the QC datastream, loads
+- `beginEditing()`: resolves the QC history for the QC datastream, loads
   its sessions, resumes the in-progress one (or sets `needsSession`); sets
   `needsHistory` when the datastream isn't QC-managed yet. Resuming rebuilds
   the shared working copy; if that rebuild is superseded it throws
   `ResumeSupersededError`, leaves `needsSession` false, and `enterEdit`
-  returns to the Select view.
-- `startSession(spec)` — creates a session and copies the source window in.
+  returns to the Select view. Entries can overlap, so it also throws
+  `ResumeSupersededError` when the edit target changes during any of its
+  fetches, and it never writes the session store (source, sessions) for a
+  target it no longer owns: sessions are fetched with `fetchSessions` and
+  applied only while still the owner.
+- `startSession(spec)`: creates a session and copies the source window in.
   When the history already has an in-progress session it resumes that one
   through the same replay instead (throwing `ResumeSupersededError` the same
-  way), so saved draft operations are never dropped.
+  way), so saved draft operations are never dropped. The same ownership rule
+  guards its session reload.
 - `saveDraft()` — persists the record's edit operations to the session
   (append-only reconcile).
 - `discardUnsavedEdits()` — drops edits made since the last save and restores
@@ -336,13 +341,13 @@ on boot.
 | `selectedData`                      | state    | `number[] \| null`                                | Index list of the active selection (lasso, box, click). |
 | `hasSelectionShape`                 | state    | `boolean`                                         | True while a box/lasso shape exists, even when it captured zero points. |
 | `loadingStates`                     | state    | `Map<string, boolean>`                            | Per-datastream in-flight observation fetches. |
-| `beginDate` / `endDate`             | state    | `Date`                                            | Active loaded window. A preset re-resolves it from the context data's (`seriesDatastreams` minus the edit target) phenomenon times on every plot rebuild; a custom range stays fixed. |
+| `beginDate` / `endDate`             | state    | `Date`                                            | Active loaded window. A preset re-resolves it on every plot rebuild: around the edit session's window while one is set (`presetAroundWindow`), otherwise back from the context data's (`seriesDatastreams` minus the edit target) end (`presetWindow`). It also re-resolves when the edit session's window loads or changes. A custom range stays fixed. |
 | `selectedDateBtnId`                 | state    | `number`                                          | Active preset id (default `1`, 1m); `-1` (`CUSTOM_PRESET_ID`) for a manual range. Presets are defined in `utils/timeRangePresets.ts`. |
 | `matchesSelectedThing`              | action   | `(ds) => boolean`                                 | Filter predicate; exposed so the table can reuse it on row updates. |
 | `matchesSelectedObservedProperty`   | action   | `(ds) => boolean`                                 | Same shape as above. |
 | `matchesSelectedProcessingLevel`    | action   | `(ds) => boolean`                                 | Same shape as above. |
 | `setDateRange`                      | action   | `({ begin?, end?, update?, custom? }) => Promise<void>` | No-ops when neither bound moves. Otherwise refreshes `seriesDatastreams`; while editing this reloads context only and preserves the zoom (`redraw(false, true)`), since the edit target's data isn't fetched here. Otherwise it clears zoom history and applies the new window. |
-| `onDateBtnClick`                    | action   | `(id: number) => Promise<void>`                   | Selects the preset and applies its window over the context data's extent. With nothing plotted, only the selection changes. |
+| `onDateBtnClick`                    | action   | `(id: number) => Promise<void>`                   | Selects the preset and applies the window the internal `resolvePresetWindow` gives it: around the edit session's window while editing one, otherwise back from the context data's end. With nothing to anchor to, only the selection changes. |
 | `refreshGraphSeriesArray`           | action   | `() => Promise<unknown[]>`                        | Reconciles `graphSeriesArray` against `seriesDatastreams` (fetch deltas + reorder + recolor), skipping the edit target (its data is owned by the edit session, not this refresh). A managed datastream with a loaded working copy (`useWorkingCopiesStore`) uses it instead of fetching. Invalidates the working copy of any managed datastream no longer in `seriesDatastreams`, except the edit target. |
 | `resetState`                        | action   | `() => void`                                      | Clears filters, the plotted set, and the edit target, and drops every working copy, on a workspace swap; preserves the preset preference. |
 | `toggleDatastream`                  | action   | `(ds: Datastream) => Promise<void>`               | Plot if absent, unplot if present. |
@@ -354,7 +359,7 @@ on boot.
 | `addSnapshotSeries`                 | action   | `(id: string, record: ObservationRecord, meta: SnapshotMeta) => Promise<void>` | Add a frozen history snapshot as an extra comparison line under the synthetic id `snap:<sessionId>:<opIndex>`. Never touches the edit target; `refreshGraphSeriesArray` skips its fetch. |
 | `removeSnapshotSeries`              | action   | `(id: string) => Promise<void>`                   | Drop one snapshot line. Leaves the edit target alone. |
 | `setPlottedDatastreams`             | action   | `(items: Datastream[]) => Promise<void>`          | Replace the plotted set wholesale (used by URL hydration). Doesn't touch the edit target. Hydrate that separately with `setEditTarget`. |
-| `setEditTarget`                     | action   | `(managedId: string) => Promise<void>`            | Begin editing `managedId`: drops snapshots, resets the `qcSession` store when the target changes (not `resumeDatastreamId`), sets `qcDatastreamId`, rebuilds the plot. Its data arrives later via `setEditRecord`. This action doesn't fetch it. |
+| `setEditTarget`                     | action   | `(managedId: string) => Promise<void>`            | Begin editing `managedId`: drops snapshots, resets the `qcSession` store (not `resumeDatastreamId`) and clears zoom history when the target changes, sets `qcDatastreamId`, rebuilds the plot. Its data arrives later via `setEditRecord`. This action doesn't fetch it. |
 | `setEditRecord`                     | action   | `(record: ObservationRecord) => Promise<void>`    | Put the edit target's record on the plot: updates its graph series in place if one exists, otherwise adds it (`buildGraphSeries`) and redraws. The only path by which the edit target's data reaches the plot. |
 | `clearEditTarget`                   | action   | `() => Promise<void>`                             | Stop editing: drops snapshots, invalidates the edit target's working copy, clears `qcDatastreamId`, removes its graph series, and rebuilds. Plotted datastreams are left exactly as they were. |
 | `rebuildPlot`                       | action   | `() => Promise<void>`                             | Serialized rebuild (refresh series, regenerate options, render). Drops zoom history in the Select view; keeps the zoom while an edit target is set. Coalesces concurrent callers. |
@@ -607,7 +612,9 @@ puts the editor in read-only mode.
 | `inProgressSession` | computed | `QualityControlSession \| null`         | The editable session, if any. |
 | `committedSessions` | computed | `QualityControlSession[]`               | Sessions with status `committed`. |
 | `viewedSession`     | computed | `QualityControlSession \| null`         | The session for `viewedSessionId`. |
-| `loadSessions`      | action   | `(historyId: string) => Promise<void>`  | Load a history's sessions; default the view to the in-progress one. |
+| `fetchSessions`     | action   | `(historyId: string) => Promise<QualityControlSession[]>` | Fetch a history's sessions with their operations without writing any state, so a caller can drop a result that went stale (see `useEditSession`). |
+| `applySessions`     | action   | `(historyId: string, sessions: QualityControlSession[]) => void` | Adopt fetched sessions; default the view to the in-progress one, else the latest committed. |
+| `loadSessions`      | action   | `(historyId: string) => Promise<void>`  | `fetchSessions` then `applySessions`, with `isLoading` set meanwhile. |
 | `viewSession`       | action   | `(sessionId: string) => void`           | View a session read-only (no-op for an unknown id). |
 | `returnToCurrent`   | action   | `() => void`                            | Return to the editable in-progress session. |
 | `reset`             | action   | `() => void`                            | Clear all state. |
@@ -645,6 +652,24 @@ columnar fetch, returns `{ datetimes: number[]; dataValues: number[] }`.
 `subtractDays`, `subtractMonths`, `subtractYears` for the time-range
 preset buttons.
 
+### `src/utils/timeRangePresets.ts`
+
+Preset definitions (`TIME_RANGE_PRESETS`, ids stable for the share URL)
+and their pure resolution:
+
+- `dataExtent(datastreams)`: earliest begin to latest end over the given
+  phenomenon times; `null` when none has observations.
+- `presetWindow(id, extent)`: the preset counted back from `extent.end`
+  (All is the whole extent, YTD starts Jan 1 of the end year). Used in the
+  Select view and while editing without a session window.
+- `presetAroundWindow(id, window, extent)`: the editor's rule while a
+  session window is known. 1w / 1m / 6m / 1y return
+  `[window.begin - span, window.end + span]`, not clipped to the data.
+  All returns `extent` widened to cover `window` (or `window` when `extent`
+  is `null`); YTD resolves the same as All. `null` for an unknown id.
+- `EDITOR_PRESETS`: the presets the editor's Context menu offers, without
+  YTD and titled for the session window.
+
 ### `src/utils/sessionWindow.ts`
 
 Pure validation for the session-window step (`SessionWindowDialog.vue`,
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 01d05646..e332307e 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -106,7 +106,7 @@ src/
 ├─ utils/
 │  ├─ plotting/                 Plotly integration (trace builders, event handlers, selection, staging).
 │  ├─ dateMath.ts               Time-range arithmetic for presets ("1w", "1m", "All", …).
-│  ├─ timeRangePresets.ts       Preset definitions, resolved against the plotted data's extent.
+│  ├─ timeRangePresets.ts       Preset definitions, resolved back from the context data's end, or around the session window while editing.
 │  ├─ observations.ts           Observation fetch helpers (paged columnar fetch).
 │  ├─ rules.ts                  Vuetify form validation rules.
 │  └─ time.ts                   Time unit conversions.
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index 6d83d58e..0a34010d 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -218,13 +218,29 @@ calls it instead of assigning `selectedSeries.value.data` directly.
 
 ## Context range
 
-The editor toolbar's **Context** menu reuses `DataVisTimeFilters` (the same
-preset chips and From / To pickers as the Select view) bound to the same
-`beginDate` / `endDate` store range. Picking a preset or a custom date calls
-`setDateRange`, which, while an edit target is set, reloads only the
-context series (`refreshGraphSeriesArray`, which never fetches the edit
-target) and redraws with `redraw(false, true)` to keep the user's current
-zoom. The edit target's working copy is never re-windowed by this control.
+The editor toolbar's **Context** menu reuses `DataVisTimeFilters` (with
+`EDITOR_PRESETS`, which leaves out YTD, and the same From / To pickers as
+the Select view) bound to the same `beginDate` / `endDate` store range.
+Picking a preset or a custom date calls `setDateRange`, which, while an
+edit target is set, reloads only the context series
+(`refreshGraphSeriesArray`, which never fetches the edit target) and
+redraws with `redraw(false, true)` to keep the user's current zoom. The edit
+target's working copy is never re-windowed by this control.
+
+`resolvePresetWindow` is the one place that picks how a preset resolves.
+While an edit target has a session window (`viewedSession`, else
+`inProgressSession`), it uses `presetAroundWindow`: 1w / 1m / 6m / 1y add
+their span before the window's start and after its end, and All (and a
+persisted YTD) is the context data extent widened to cover the window.
+Otherwise, in the Select view or before the session loads, `presetWindow`
+counts back from the context data's end. The extent comes from the context
+series only (`seriesDatastreams` minus the edit target and snapshots).
+
+The context loads on `setEditTarget`'s rebuild, before the session is known.
+The store watches the session window, and when it appears or changes it
+re-applies the active preset through `setDateRange`: one context reload,
+no edit-target fetch, zoom kept, and a no-op when the range is unchanged or
+the range is Custom.
 
 The session window itself is shaded as a layout shape (`name: 'edit-window'`
 in `options.ts`), drawn from `useQcSessionStore().viewedSession` (falling
@@ -234,15 +250,25 @@ previous target's window is never drawn over the new one.
 
 `layout.shapes` has more than one writer: `createPlotlyOption` owns
 `edit-window` and `staging.ts` owns `stage`. `Plotly.update` and
-`Plotly.relayout` replace the whole array, so every write goes through
+`Plotly.relayout` replace the whole array, so partial writes go through
 `utils/plotting/shapes.ts`: `redraw` and `cropXaxisRange` use
 `withLiveShapes` to swap in the fresh `edit-window` and keep the live
 `stage` band, and the staging flush uses `composeShapes` to do the reverse
-(stage stays first so drag events still address `shapes[0]`).
+(stage stays first so drag events still address `shapes[0]`). The
+exception is `handleNewPlot` (`events.ts`): a full rebuild calls
+`Plotly.newPlot` with the whole layout from `createPlotlyOption`, so its
+shapes are only `edit-window` and a live `stage` band is not carried over.
 
 Plot rebuilds (`rebuildPlot`, run when plotted datastreams change) keep
 the user's zoom while an edit target is set and drop it in the Select
-view.
+view. `setEditTarget` clears the zoom history when the target changes, so
+**Undo zoom** never steps back to the Select view's or a previous target's
+viewports.
+
+While observations load, `DataVisualization.vue` keeps `Plot` mounted and
+passes its loading overlay through Plot's `body-overlay` slot. The overlay
+covers the plot and table body only; the toolbar stays usable and shows its
+own spinner.
 
 ## Why `internal.ts` isn't re-exported
 
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 44ce43c1..dfef458f 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -239,7 +239,7 @@ The CI gate prints uncovered line numbers per file. Common causes:
 ```
 e2e/
 ├── support/
-│   ├── app.ts        — flow helpers (gotoHome, setupEditView, startSessionFromRow, openOp, waitForSelection)
+│   ├── app.ts        : flow helpers (gotoHome, setupEditView, startSessionFromRow, waitForEditorReady, openOp, waitForSelection)
 │   ├── fixtures.ts   — workspace / datastream / observation fixtures
 │   ├── mocks.ts      — page.route() handlers that stand in for HydroServer
 │   └── ops.ts        — op-specific preambles (selectAllPoints, expectHistoryContains)
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 104aec3b..34ae88cd 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -79,7 +79,7 @@ The select view is the default landing surface after picking a workspace. The le
 
 The filter drawer has two collapsible sections:
 
-- **Time range**: the loaded time window for the plotted datastreams. The two date pickers (`From` / `To`) show the window. The preset chips below them, **1w**, **1m**, **6m**, **1y**, **YTD**, **All**, count back from the last observation of the plotted datastreams (the newest one when several are plotted), so a preset always lands on data, even for datastreams whose observations are years old. That last observation is the one known when the workspace loaded, so data that arrives later only shows up after a reload. **1m** is the default, and the last preset you pick is remembered. Adding or removing a datastream re-anchors the preset. A `Custom` chip appears when you change a date in `From` or `To`; a custom window stays fixed while you add or remove datastreams. In the Edit view the loaded window is frozen: plotting or removing a datastream there never moves it.
+- **Time range**: the loaded time window for the plotted datastreams. The two date pickers (`From` / `To`) show the window. The preset chips below them, **1w**, **1m**, **6m**, **1y**, **YTD**, **All**, count back from the last observation of the plotted datastreams (the newest one when several are plotted), so a preset always lands on data, even for datastreams whose observations are years old. That last observation is the one known when the workspace loaded, so data that arrives later only shows up after a reload. **1m** is the default, and the last preset you pick is remembered. Adding or removing a datastream re-anchors the preset. A `Custom` chip appears when you change a date in `From` or `To`; a custom window stays fixed while you add or remove datastreams. In the Edit view the same preference drives the **Context** menu, where presets count out from the edit session's window instead.
 
 - **Datastream filters**: These filters allow you control the list of datastreams shown in the datastreams table by picking the site, observed property, and/or processing level. The list of matching datastreams updates live in the datastreams table.
 
@@ -173,9 +173,17 @@ See the [Pan and zoom across axes](#pan-and-zoom-across-axes) section below for
 Hover any toolbar icon to see its name. The left side of the toolbar flips between **Plot** and **Table** views (see below); the right side carries the **data points toggle**, a share-link button, and the `?` help menu.
 
 Between the help menu and the right edge is the **Context** menu. It holds
-the same preset chips and From / To pickers as the Select view's Time range
+preset chips and From / To pickers like the Select view's Time range
 filter, but here they control how much of the raw source and any plotted
-datastreams load around your edit, not what you're editing. Changing it
+datastreams load around your edit, not what you're editing. The presets
+count out from the edit session's window instead of back from the last
+observation: **1w**, **1m**, **6m** and **1y** load that span before the
+session's start and after its end, and **All** loads all of the context
+data (always including the session's window). **YTD** is not offered here.
+If YTD is your active preset from the Select view, the editor loads the
+context as **All** without changing that preference. When a session loads
+or you view another session, the active preset re-applies around its
+window; a `Custom` From / To range stays as you set it. Changing the range
 reloads that context data and keeps your current zoom. Your edits are
 never reloaded or re-windowed. The editor opens zoomed to the edit session's
 own window; open the Context menu (or click **All**) to see more of the
diff --git a/apps/qc-app/e2e/edit-selection.spec.ts b/apps/qc-app/e2e/edit-selection.spec.ts
index 24825230..99156204 100644
--- a/apps/qc-app/e2e/edit-selection.spec.ts
+++ b/apps/qc-app/e2e/edit-selection.spec.ts
@@ -8,6 +8,7 @@
  *   - the session window band survives staging shapes
  *   - the editor opens zoomed to the session window
  *   - changing the Context range keeps the user zoom
+ *   - Context presets count out from the session window
  */
 
 import { expect, test, type Locator, type Page } from '@playwright/test'
@@ -18,6 +19,7 @@ import {
   plotDatastreamById,
   setupEditView,
   startSessionFromRow,
+  waitForEditorReady,
 } from './support/app'
 import {
   DATASTREAM_ID,
@@ -28,7 +30,7 @@ import {
 } from './support/fixtures'
 
 type PlotRoot = HTMLElement & {
-  data?: Array<{ id?: string }>
+  data?: Array<{ id?: string; x?: Array<number | string> }>
   layout?: {
     shapes?: Array<{ name?: string }>
     xaxis?: { range?: Array<number | string> }
@@ -85,6 +87,27 @@ async function typeDateTime(field: Locator, when: Date) {
   }
 }
 
+/** First and last x of a trace in epoch ms. Plotly date strings are UTC. */
+function traceXExtent(page: Page, id: string): Promise<[number, number] | null> {
+  return page.evaluate((traceId) => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as PlotRoot
+    const xs = (gd?.data ?? []).find((t) => t.id === traceId)?.x
+    if (!xs?.length) return null
+    const toMs = (v: number | string) =>
+      typeof v === 'number' ? v : Date.parse(`${v.replace(' ', 'T')}Z`)
+    return [toMs(xs[0]!), toMs(xs[xs.length - 1]!)] as [number, number]
+  }, id)
+}
+
+/** What a `DatePickerField` shows for `when`: local MM/DD/YYYY and HH:MM. */
+function pickerText(when: Date): [string, string] {
+  const pad = (n: number) => String(n).padStart(2, '0')
+  return [
+    `${pad(when.getMonth() + 1)}/${pad(when.getDate())}/${when.getFullYear()}`,
+    `${pad(when.getHours())}:${pad(when.getMinutes())}`,
+  ]
+}
+
 async function openWindowStep(page: Page) {
   await page.getByTestId(`edit-datastream-${DATASTREAM_ID}`).click()
   await page.getByTestId(`edit-managed-${MANAGED_DATASTREAM_ID}`).click()
@@ -335,4 +358,54 @@ test.describe('edit selection', () => {
     expect(drift).toBeLessThanOrEqual(1_000)
     expect(await traceIds(page)).toContain(MANAGED_DATASTREAM_ID)
   })
+
+  test('Context presets count out from the session window', async ({ page }) => {
+    await gotoHome(page)
+    // Start halfway through the source, so the window has data before it.
+    const mid = new Date((FIXTURE_OBS_START_MS + FIXTURE_OBS_END_MS) / 2)
+    mid.setSeconds(0, 0)
+    await openWindowStep(page)
+    await typeDateTime(page.getByTestId('session-window-from'), mid)
+    await expect(page.getByTestId('session-window-start')).toBeEnabled()
+    await page.getByTestId('session-window-start').click()
+    await waitForEditorReady(page)
+
+    await page.getByTestId('context-range-btn').click()
+    const menu = page.getByTestId('context-range-menu')
+    await expect(menu.getByTestId('date-preset-YTD')).toHaveCount(0)
+    await menu.getByTestId('date-preset-1w').click()
+    await expect(menu.getByTestId('date-preset-1w')).toHaveClass(
+      /v-chip--variant-tonal/
+    )
+    await expect(page.getByTestId('data-loading-indicator')).toHaveCount(0)
+
+    // The fixture spans about 30 hours, so any 1w range loads all of it and
+    // the trace alone cannot tell the rules apart. The loaded range can:
+    // counting back from the data end would put To at the data end.
+    const expectedFrom = new Date(mid)
+    expectedFrom.setDate(expectedFrom.getDate() - 7)
+    const expectedTo = new Date(FIXTURE_OBS_END_MS)
+    expectedTo.setDate(expectedTo.getDate() + 7)
+    const fieldText = (id: string) =>
+      menu.getByTestId(id).locator('input').evaluateAll((inputs) =>
+        inputs.map((i) => (i as HTMLInputElement).value)
+      )
+    await expect.poll(() => fieldText('date-range-from')).toEqual(pickerText(expectedFrom))
+    await expect.poll(() => fieldText('date-range-to')).toEqual(pickerText(expectedTo))
+
+    // The grey source covers the whole window, from the data start (the
+    // week before the window, clamped to the fixture).
+    const tolerance = 60_000
+    await expect
+      .poll(async () => {
+        const x = await traceXExtent(page, DATASTREAM_ID)
+        return (
+          !!x &&
+          Math.abs(x[0] - FIXTURE_OBS_START_MS) <= tolerance &&
+          x[0] <= mid.getTime() &&
+          x[1] >= FIXTURE_OBS_END_MS - tolerance
+        )
+      })
+      .toBe(true)
+  })
 })
diff --git a/apps/qc-app/e2e/support/app.ts b/apps/qc-app/e2e/support/app.ts
index e9cb2253..22df5ff6 100644
--- a/apps/qc-app/e2e/support/app.ts
+++ b/apps/qc-app/e2e/support/app.ts
@@ -133,15 +133,27 @@ export async function setupEditView(page: Page): Promise<void> {
 
 /**
  * From Home: the source row's Edit button, "Start new session" on the managed
- * datastream, then Start with the default window. Waits for the editor footer.
+ * datastream, then Start with the default window. Waits for the editor to be
+ * ready.
  */
 export async function startSessionFromRow(page: Page): Promise<void> {
   await page.getByTestId(`edit-datastream-${DATASTREAM_ID}`).click()
   await page.getByTestId(`edit-managed-${MANAGED_DATASTREAM_ID}`).click()
   await page.getByTestId('session-window-start').click()
+  await waitForEditorReady(page)
+}
+
+/**
+ * Wait for the Edit view to be open with nothing loading, e.g. after a reload
+ * resumes the session. Acting earlier can race the view switch.
+ */
+export async function waitForEditorReady(page: Page): Promise<void> {
   await expect(page.getByTestId('exit-save-btn')).toBeVisible({
     timeout: 30_000,
   })
+  await expect(page.getByTestId('data-loading-indicator')).toHaveCount(0, {
+    timeout: 30_000,
+  })
 }
 
 /** Open an operation panel in the edit drawer by id (see operations.ts). */
diff --git a/apps/qc-app/e2e/tooltip-threshold.spec.ts b/apps/qc-app/e2e/tooltip-threshold.spec.ts
index a14ac575..cd1aac16 100644
--- a/apps/qc-app/e2e/tooltip-threshold.spec.ts
+++ b/apps/qc-app/e2e/tooltip-threshold.spec.ts
@@ -16,7 +16,7 @@
 
 import { expect, test, type Page } from '@playwright/test'
 import { installMocks } from './support/mocks'
-import { setupEditView } from './support/app'
+import { setupEditView, waitForEditorReady } from './support/app'
 
 /**
  * Pull the threshold number out of the auto-mode counter text. The
@@ -74,8 +74,9 @@ test.describe('data-points combobox', () => {
       localStorage.removeItem('qc.plot.tooltipsMaxDataPoints')
     )
     await page.reload()
-    // Wait for the combobox to mount before each test acts on it. In
-    // the default (auto) mode the counter cell is the visible anchor;
+    // The reload resumes the session; the counter shows before that settles.
+    await waitForEditorReady(page)
+    // In the default (auto) mode the counter cell is the visible anchor;
     // the toggle button only renders in manual mode.
     await expect(page.getByTestId('tooltips-counter')).toBeVisible()
   })
diff --git a/apps/qc-app/src/components/VisualizeData/DataTable.vue b/apps/qc-app/src/components/VisualizeData/DataTable.vue
index 7ecdc00f..11e0e4ce 100644
--- a/apps/qc-app/src/components/VisualizeData/DataTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataTable.vue
@@ -201,8 +201,8 @@ onMounted(() => {
     if (h && h !== bodyHeight.value) bodyHeight.value = h
   })
   resizeObserver.observe(bodyEl.value)
-  // Honor a zoom-to-range scroll requested while the table was unmounted
-  // (e.g. the preset was picked on the plot tab before switching here).
+  // Honor a scroll requested while the table was unmounted (e.g. the editor
+  // opened zoomed to the session window on the plot tab).
   if (tableScrollRequest.value) scrollToTime(tableScrollRequest.value.time)
 })
 
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue b/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
index 9f77dffc..3f2b23e6 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
@@ -33,7 +33,7 @@
 
     <div class="time-filters__presets">
       <v-chip
-        v-for="option in TIME_RANGE_PRESETS"
+        v-for="option in presets"
         :key="option.id"
         :data-testid="`date-preset-${option.label}`"
         :color="selectedDateBtnId === option.id ? 'primary' : undefined"
@@ -66,10 +66,18 @@ import { useDataVisStore } from '@/store/dataVisualization'
 import {
   CUSTOM_PRESET_ID,
   TIME_RANGE_PRESETS,
+  type TimeRangePreset,
 } from '@/utils/timeRangePresets'
 import { storeToRefs } from 'pinia'
 
-defineProps<{ description?: string }>()
+withDefaults(
+  defineProps<{
+    description?: string
+    /** Preset chips to offer; the editor leaves out YTD. */
+    presets?: readonly TimeRangePreset[]
+  }>(),
+  { description: undefined, presets: () => TIME_RANGE_PRESETS }
+)
 
 const { setDateRange, onDateBtnClick } = useDataVisStore()
 const { beginDate, endDate, selectedDateBtnId } = storeToRefs(useDataVisStore())
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisualization.vue b/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
index d7e6695b..4fdbf536 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
@@ -1,33 +1,34 @@
 <template>
   <!-- Stays mounted while loading so a refresh keeps the plot and its zoom. -->
-  <div
+  <Plot
     v-if="isUpdating || isDataAvailable"
-    class="fill-height position-relative"
+    class="fill-height"
+    :preview="preview"
   >
-    <Plot class="data-vis-plot fill-height" :preview="preview" />
-
-    <div
-      v-if="isUpdating"
-      class="data-vis-loading position-absolute d-flex flex-column align-center justify-center pa-6 text-center"
-      data-testid="data-loading-indicator"
-    >
-      <v-progress-circular
-        color="primary"
-        :size="56"
-        :width="4"
-        indeterminate
-        class="mb-4"
-      />
-      <div class="text-title-medium font-weight-bold mb-1">
-        Loading observations…
-      </div>
-      <div class="text-body-small text-medium-emphasis">
-        Fetching data for
-        {{ loadingCount }}
-        datastream{{ loadingCount === 1 ? '' : 's' }}
+    <template #body-overlay>
+      <div
+        v-if="isUpdating"
+        class="data-vis-loading position-absolute d-flex flex-column align-center justify-center pa-6 text-center"
+        data-testid="data-loading-indicator"
+      >
+        <v-progress-circular
+          color="primary"
+          :size="56"
+          :width="4"
+          indeterminate
+          class="mb-4"
+        />
+        <div class="text-title-medium font-weight-bold mb-1">
+          Loading observations…
+        </div>
+        <div class="text-body-small text-medium-emphasis">
+          Fetching data for
+          {{ loadingCount }}
+          datastream{{ loadingCount === 1 ? '' : 's' }}
+        </div>
       </div>
-    </div>
-  </div>
+    </template>
+  </Plot>
 
   <div
     v-else-if="seriesDatastreams.length"
@@ -152,11 +153,6 @@ const isDataAvailable = computed(() => {
 </script>
 
 <style scoped>
-/* Contains Plotly's own z-indexes so the overlay covers them. */
-.data-vis-plot {
-  isolation: isolate;
-}
-
 .data-vis-loading {
   inset: 0;
   z-index: 1;
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index 427ca528..b35539cf 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -285,7 +285,10 @@
             </v-btn>
           </template>
           <v-card width="300" class="pa-3" data-testid="context-range-menu">
-            <DataVisTimeFilters description="How much of the source and plotted datastreams to show around the session. Your edits are not reloaded." />
+            <DataVisTimeFilters
+              :presets="EDITOR_PRESETS"
+              description="How much of the source and plotted datastreams to show before and after the session window. Your edits are not reloaded."
+            />
           </v-card>
         </v-menu>
       </div>
@@ -293,8 +296,8 @@
 
     <v-divider></v-divider>
 
-    <div class="d-flex flex-row flex-grow-1">
-      <v-tabs-window v-model="tab" class="flex-grow-1">
+    <div class="plot-body position-relative d-flex flex-row flex-grow-1">
+      <v-tabs-window v-model="tab" class="plot-body__content flex-grow-1">
         <v-tabs-window-item value="plot" class="fill-height">
           <div class="fill-height position-relative d-flex flex-column">
             <div
@@ -389,6 +392,8 @@
           <DataTable v-if="tab === 'table' && !preview" class="fill-height"
         /></v-tabs-window-item>
       </v-tabs-window>
+      <!-- Covers the plot and table only, so the toolbar stays usable. -->
+      <slot name="body-overlay" />
     </div>
   </div>
 </template>
@@ -407,6 +412,7 @@ import {
 import DataTable from '@/components/VisualizeData/DataTable.vue'
 import ContextPlot from '@/components/VisualizeData/ContextPlot.vue'
 import DataVisTimeFilters from '@/components/VisualizeData/DataVisTimeFilters.vue'
+import { EDITOR_PRESETS } from '@/utils/timeRangePresets'
 import { useDataSelection } from '@/composables/useDataSelection'
 import { useBufferedNumber } from '@/composables/useBufferedNumber'
 import { usePersistedFlag } from '@/composables/useResizable'
@@ -745,6 +751,11 @@ const onTabChange = () => {
   min-height: 0;
 }
 
+/* Contains Plotly's own z-indexes so a body overlay covers them. */
+.plot-body__content {
+  isolation: isolate;
+}
+
 .plot-header {
   display: flex;
   flex-direction: column;
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
index b2ccb200..bc94fd68 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
@@ -884,11 +884,11 @@ describe('DataTable.vue onSaveChanges', () => {
   })
 })
 
-// Drives the "zoom to range" scroll: a request on the plotly store should
+// Drives `requestTableScroll`: a request on the plotly store should
 // scroll the virtual list so the first in-range row lands on top. The stub
 // exposes v-data-table-virtual's `scrollToIndex` so we can capture the index
 // the component asks to scroll to.
-describe('DataTable.vue zoom-to-range scroll', () => {
+describe('DataTable.vue scroll requests', () => {
   let scrollToIndexCalls: number[]
 
   beforeEach(() => {
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
index 14f06485..a278596e 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
@@ -30,6 +30,7 @@ vi.mock('@/components/VisualizeData/DatePickerField.vue', () => ({
 }))
 
 import DataVisTimeFilters from '@/components/VisualizeData/DataVisTimeFilters.vue'
+import { EDITOR_PRESETS } from '@/utils/timeRangePresets'
 
 function mountIt(props: Record<string, unknown> = {}) {
   return mount(DataVisTimeFilters, {
@@ -49,4 +50,17 @@ describe('DataVisTimeFilters.vue', () => {
     expect(w.text()).toContain('Context around the session.')
     expect(w.text()).not.toContain('Loaded time window')
   })
+
+  it('offers every preset, YTD included, by default', () => {
+    const w = mountIt()
+    expect(w.find('[data-testid="date-preset-YTD"]').exists()).toBe(true)
+    expect(w.find('[data-testid="date-preset-All"]').exists()).toBe(true)
+  })
+
+  it('can hide YTD with the editor presets', () => {
+    const w = mountIt({ presets: EDITOR_PRESETS })
+    expect(w.find('[data-testid="date-preset-YTD"]').exists()).toBe(false)
+    expect(w.find('[data-testid="date-preset-1w"]').exists()).toBe(true)
+    expect(w.find('[data-testid="date-preset-All"]').exists()).toBe(true)
+  })
 })
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts
index c3dde438..436a950d 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts
@@ -34,10 +34,14 @@ const lifecycle = vi.hoisted(() => ({ mounts: 0, unmounts: 0 }))
 vi.mock('@/components/VisualizeData/Plot.vue', () => ({
   default: defineComponent({
     name: 'PlotStub',
-    setup() {
+    setup(_props, { slots }) {
       onMounted(() => lifecycle.mounts++)
       onBeforeUnmount(() => lifecycle.unmounts++)
-      return () => h('div', { 'data-testid': 'plot-stub' })
+      return () =>
+        h('div', { 'data-testid': 'plot-stub' }, [
+          h('div', { 'data-testid': 'plot-stub-toolbar' }),
+          h('div', { 'data-testid': 'plot-stub-body' }, slots['body-overlay']?.()),
+        ])
     },
   }),
 }))
@@ -99,6 +103,16 @@ describe('DataVisualization.vue', () => {
     expect(lifecycle.mounts).toBe(1)
   })
 
+  it('puts the loading overlay in the plot body so the toolbar stays usable', async () => {
+    const wrapper = mountIt()
+    await setLoading(true)
+    const body = wrapper.find('[data-testid="plot-stub-body"]')
+    expect(body.find('[data-testid="data-loading-indicator"]').exists()).toBe(true)
+    expect(
+      wrapper.findAll('[data-testid="data-loading-indicator"]')
+    ).toHaveLength(1)
+  })
+
   it('counts only the datastreams being fetched, not the edit target', async () => {
     seriesDatastreams.value = [{ id: 'edit' }, { id: 'src' }, { id: 'ds-a' }]
     const wrapper = mountIt()
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
index 45139df8..ffa1fd20 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
@@ -70,9 +70,10 @@ vi.mock('plotly.js-dist', () => ({ default: { Plots: { resize: vi.fn() } } }))
 
 import Plot from '@/components/VisualizeData/Plot.vue'
 
-function mountIt(preview = true) {
+function mountIt(preview = true, slots: Record<string, string> = {}) {
   return mount(Plot, {
     props: { preview },
+    slots,
     global: {
       plugins: [createTestPinia(), createTestVuetify()],
       stubs: { ContextPlot: true, DataTable: true, DataVisTimeFilters: true },
@@ -143,4 +144,13 @@ describe('Plot.vue delayed mount', () => {
       expect(zoomXaxisTo).not.toHaveBeenCalled()
     })
   })
+
+  it('renders the body overlay under the toolbar, not over it', () => {
+    const wrapper = mountIt(false, {
+      'body-overlay': '<div data-testid="overlay-probe" />',
+    })
+    expect(wrapper.find('.plot-header [data-testid="overlay-probe"]').exists()).toBe(false)
+    expect(wrapper.find('.plot-body [data-testid="overlay-probe"]').exists()).toBe(true)
+    wrapper.unmount()
+  })
 })
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 3e7af1b8..714a91af 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -286,6 +286,70 @@ describe('useEditSession', () => {
     expect(setEditRecord).not.toHaveBeenCalled()
   })
 
+  // Another entry taking over: `setEditTarget(B)` swaps the target and resets
+  // the session store.
+  const takeOver = () => {
+    qcDatastream.value = { id: 'm-2' }
+    useQcSessionStore().reset()
+  }
+
+  it('beginEditing rejects without writing the session store when the target changes during the source fetch', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    await qc.sessions.create(h.id, WIN)
+    getItem.mockImplementationOnce(async () => {
+      takeOver()
+      return { id: 's-1', name: 'Source' }
+    })
+    const { useEditSession, ResumeSupersededError } = await import(
+      '@/composables/useEditSession'
+    )
+
+    await expect(useEditSession().beginEditing()).rejects.toBeInstanceOf(
+      ResumeSupersededError
+    )
+
+    const store = useQcSessionStore()
+    expect(store.sourceDatastream).toBeNull()
+    expect(store.historyId).toBeNull()
+    expect(store.sessions).toEqual([])
+    expect(wcRebuild).not.toHaveBeenCalled()
+  })
+
+  it('beginEditing rejects without writing the session store when the target changes while sessions load', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    await qc.sessions.create(h.id, WIN)
+    const list = qc.sessions.list.bind(qc.sessions)
+    vi.spyOn(qc.sessions, 'list').mockImplementationOnce(async (...args) => {
+      const res = await list(...args)
+      takeOver()
+      return res
+    })
+    const { useEditSession, ResumeSupersededError } = await import(
+      '@/composables/useEditSession'
+    )
+
+    await expect(useEditSession().beginEditing()).rejects.toBeInstanceOf(
+      ResumeSupersededError
+    )
+
+    const store = useQcSessionStore()
+    expect(store.historyId).toBeNull()
+    expect(store.sessions).toEqual([])
+    expect(store.currentSessionId).toBeNull()
+    expect(store.sourceDatastream).toBeNull()
+    expect(wcRebuild).not.toHaveBeenCalled()
+  })
+
   it('startSession loads the managed datastream as the working base', async () => {
     await seedHistory()
     const managedBase = makeRecord()
@@ -520,6 +584,30 @@ describe('useEditSession', () => {
     expect(store.committedSessions[0]?.description).toBe('Reviewed January spike')
   })
 
+  it('startSession rejects without writing the session store when the target changes while sessions load', async () => {
+    await seedHistory()
+    const { useEditSession, ResumeSupersededError } = await import(
+      '@/composables/useEditSession'
+    )
+    const session = useEditSession()
+    await session.beginEditing()
+    const list = qc.sessions.list.bind(qc.sessions)
+    vi.spyOn(qc.sessions, 'list').mockImplementationOnce(async (...args) => {
+      const res = await list(...args)
+      takeOver()
+      return res
+    })
+
+    await expect(session.startSession(WIN)).rejects.toBeInstanceOf(
+      ResumeSupersededError
+    )
+
+    const store = useQcSessionStore()
+    expect(store.historyId).toBeNull()
+    expect(store.sessions).toEqual([])
+    expect(fetchObservationsInRange).not.toHaveBeenCalled()
+  })
+
   it('startSession stores its base as the working copy', async () => {
     await seedHistory()
     const { useEditSession } = await import('@/composables/useEditSession')
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index f4fa8780..0c59cefd 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -151,19 +151,30 @@ export function useEditSession() {
     if (!managed) return false
     needsHistory.value = false
 
+    // Entries can overlap; never write the session store for a target
+    // another entry has since taken over.
+    const stillOwner = () => {
+      if (qcDatastream.value?.id !== managed.id) throw new ResumeSupersededError()
+    }
+
     const history = await findHistoryForDatastream(
       hs.value.qualityControlHistories,
       managed.id
     )
+    stillOwner()
     if (!history) {
       // Not a managed datastream: the caller should offer to create one
       // from it (with this datastream as the source).
       needsHistory.value = true
       return false
     }
-    sourceDatastream.value =
+    const source =
       (await hs.value.datastreams.getItem(history.sourceDatastream.id)) ?? null
-    await sessionStore.loadSessions(history.id)
+    stillOwner()
+    sourceDatastream.value = source
+    const sessions = await sessionStore.fetchSessions(history.id)
+    stillOwner()
+    sessionStore.applySessions(history.id, sessions)
 
     const inProgress = sessionStore.inProgressSession
     if (inProgress && sourceDatastream.value) {
@@ -261,7 +272,10 @@ export function useEditSession() {
       historyId,
       clampSpecToSource(spec, source)
     )
-    await sessionStore.loadSessions(historyId)
+    const sessions = await sessionStore.fetchSessions(historyId)
+    // Same ownership rule as `beginEditing`.
+    if (qcDatastream.value?.id !== managed.id) throw new ResumeSupersededError()
+    sessionStore.applySessions(historyId, sessions)
     if (resumed) {
       // It may already hold saved operations; edit their replay, not a bare base.
       await resumeWorkingCopy(managed, source, historyId, session)
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index e3037324..3a3f21c0 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -1,8 +1,8 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
 import { createPinia, setActivePinia } from 'pinia'
 import piniaPluginPersistedstate from 'pinia-plugin-persistedstate'
-import { createApp, ref, shallowRef } from 'vue'
-import { subtractMonths } from '@/utils/dateMath'
+import { createApp, nextTick, ref, shallowRef } from 'vue'
+import { subtractDays, subtractMonths } from '@/utils/dateMath'
 
 // Shared mutable stub state so each test can reset between runs.
 const mockPlotlyRef = ref<any>(null)
@@ -410,6 +410,26 @@ describe('useDataVisStore edit target', () => {
     expect(sessions.resumeDatastreamId).toBe('mgd')
   })
 
+  it('entering the editor drops the Select zoom history', async () => {
+    const { store, other } = await managedPair()
+    await store.plotDatastream(other as any)
+    mockClearZoomHistory.mockClear()
+
+    await store.setEditTarget('mgd')
+
+    expect(mockClearZoomHistory).toHaveBeenCalled()
+  })
+
+  it('setting the same edit target keeps the zoom history', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    mockClearZoomHistory.mockClear()
+
+    await store.setEditTarget('mgd')
+
+    expect(mockClearZoomHistory).not.toHaveBeenCalled()
+  })
+
   it('setting the same edit target keeps its sessions', async () => {
     const { store } = await managedPair()
     const { useQcSessionStore } = await import('@/store/qcSession')
@@ -500,6 +520,107 @@ describe('useDataVisStore.setDateRange', () => {
   })
 })
 
+describe('useDataVisStore context range around the session window', () => {
+  const WIN_START = '2025-06-01T00:00:00Z'
+  const WIN_END = '2025-06-10T00:00:00Z'
+  const loadSession = async () => {
+    const { useQcSessionStore } = await import('@/store/qcSession')
+    useQcSessionStore().applySessions('h1', [
+      {
+        id: 's1',
+        status: 'in_progress',
+        phenomenonTimeStart: WIN_START,
+        phenomenonTimeEnd: WIN_END,
+      },
+    ] as any)
+    await nextTick()
+  }
+  const settle = async () => {
+    for (let i = 0; i < 10; i++) await Promise.resolve()
+  }
+
+  it('the editor counts presets out from the session window', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    await loadSession()
+    await settle()
+
+    await store.onDateBtnClick(0)
+
+    expect(store.beginDate.getTime()).toBe(subtractDays(new Date(WIN_START), 7).getTime())
+    expect(store.endDate.getTime()).toBe(subtractDays(new Date(WIN_END), -7).getTime())
+  })
+
+  it('YTD resolves like All around the session window', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    await loadSession()
+    await settle()
+
+    await store.onDateBtnClick(3)
+
+    expect(store.beginDate.toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect(store.endDate.toISOString()).toBe('2025-12-31T00:00:00.000Z')
+  })
+
+  it('the editor counts back from the data end until a session window is known', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+
+    await store.onDateBtnClick(0)
+
+    expect(store.endDate.toISOString()).toBe('2025-12-31T00:00:00.000Z')
+    expect(store.endDate.getTime() - store.beginDate.getTime()).toBe(7 * 24 * 60 * 60 * 1000)
+  })
+
+  it('the Select view ignores a leftover session window', async () => {
+    const { store, other } = await managedPair()
+    await loadSession()
+    await store.plotDatastream(other as any)
+
+    await store.onDateBtnClick(0)
+
+    expect(store.endDate.toISOString()).toBe('2025-12-31T00:00:00.000Z')
+  })
+
+  it('re-resolves the context once the session window loads, keeping zoom', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    expect(store.endDate.toISOString()).toBe('2025-12-31T00:00:00.000Z')
+    mockFetchObservationsInRange.mockClear()
+    mockFetchGraphSeries.mockClear()
+    mockRedraw.mockClear()
+    mockClearZoomHistory.mockClear()
+
+    await loadSession()
+    await settle()
+
+    expect(store.beginDate.getTime()).toBe(subtractMonths(new Date(WIN_START), 1).getTime())
+    expect(store.endDate.getTime()).toBe(subtractMonths(new Date(WIN_END), -1).getTime())
+    expect(mockFetchObservationsInRange.mock.calls.map((c) => c[0].id)).toEqual(['src'])
+    expect(mockFetchGraphSeries).not.toHaveBeenCalled()
+    expect(mockRedraw).toHaveBeenCalledTimes(1)
+    expect(mockRedraw).toHaveBeenCalledWith(false, true)
+    expect(mockClearZoomHistory).not.toHaveBeenCalled()
+  })
+
+  it('keeps a custom context range when the session window loads', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    const begin = new Date('2025-03-01T00:00:00Z')
+    const end = new Date('2025-04-01T00:00:00Z')
+    await store.setDateRange({ begin, end })
+    mockFetchObservationsInRange.mockClear()
+
+    await loadSession()
+    await settle()
+
+    expect(store.beginDate.getTime()).toBe(begin.getTime())
+    expect(store.endDate.getTime()).toBe(end.getTime())
+    expect(mockFetchObservationsInRange).not.toHaveBeenCalled()
+  })
+})
+
 describe('useDataVisStore.resetState', () => {
   it('resets filter arrays, plotted state, and qc id', async () => {
     const { useDataVisStore } = await import('@/store/dataVisualization')
diff --git a/apps/qc-app/src/store/__tests__/qcSession.spec.ts b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
index 33ee3cf4..1ee17bc9 100644
--- a/apps/qc-app/src/store/__tests__/qcSession.spec.ts
+++ b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
@@ -55,6 +55,20 @@ describe('useQcSessionStore', () => {
     expect(store.inProgressSession?.id).toBe(inProgressId)
   })
 
+  it('fetchSessions writes nothing until the caller applies the result', async () => {
+    const { historyId, inProgressId } = await seed()
+    const store = useQcSessionStore()
+    const list = await store.fetchSessions(historyId)
+
+    expect(list).toHaveLength(2)
+    expect(store.historyId).toBeNull()
+    expect(store.sessions).toEqual([])
+
+    store.applySessions(historyId, list)
+    expect(store.historyId).toBe(historyId)
+    expect(store.viewedSessionId).toBe(inProgressId)
+  })
+
   it('viewing a committed session is read-only; returnToCurrent restores editing', async () => {
     const { historyId, committedId, inProgressId } = await seed()
     const store = useQcSessionStore()
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index 6e9dd5d2..f4521142 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -10,7 +10,9 @@ import {
   DEFAULT_PRESET_ID,
   dataExtent,
   findPreset,
+  presetAroundWindow,
   presetWindow,
+  type TimeWindow,
 } from '@/utils/timeRangePresets'
 import { isSnapshotId } from '@/utils/snapshotId'
 import { useWorkingCopiesStore } from '@/store/workingCopies'
@@ -197,8 +199,23 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   const beginDate = ref<Date>(subtractMonths(endDate.value, 1))
   const selectedDateBtnId = ref(DEFAULT_PRESET_ID)
 
-  /** The active preset's window over the plotted data; null for a custom
-   *  range or when nothing plotted has observations. */
+  /** The edit session's window, while an edit target has one. */
+  const editSessionWindow = computed<TimeWindow | null>(() => {
+    if (!qcDatastreamId.value) return null
+    const sessions = useQcSessionStore()
+    const s = sessions.viewedSession ?? sessions.inProgressSession
+    return s
+      ? {
+          begin: new Date(s.phenomenonTimeStart),
+          end: new Date(s.phenomenonTimeEnd),
+        }
+      : null
+  })
+
+  /** The active preset's window; null for a custom range or when there is
+   *  nothing to anchor it to. The single place that picks the rule: around
+   *  the session window while editing one, otherwise back from the context
+   *  data's end. */
   function resolvePresetWindow() {
     if (selectedDateBtnId.value === CUSTOM_PRESET_ID) return null
     const workingCopies = useWorkingCopiesStore()
@@ -209,6 +226,10 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
       ...context,
       ...workingCopies.extents(context.map((d) => d.id)),
     ])
+    const sessionWindow = editSessionWindow.value
+    if (sessionWindow) {
+      return presetAroundWindow(selectedDateBtnId.value, sessionWindow, extent)
+    }
     return extent ? presetWindow(selectedDateBtnId.value, extent) : null
   }
 
@@ -359,9 +380,14 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   /** Start editing `managedId`. Its data arrives later via `setEditRecord`. */
   async function setEditTarget(managedId: string) {
     dropSnapshotSeries()
-    // The session store describes the previous target until `loadSessions`
-    // lands; drop it so its window is never drawn over this one.
-    if (managedId !== qcDatastreamId.value) useQcSessionStore().reset()
+    if (managedId !== qcDatastreamId.value) {
+      // The session store describes the previous target until its sessions
+      // load; drop it so its window is never drawn over this one.
+      useQcSessionStore().reset()
+      // The rebuild below keeps zoom for an edit target, so drop viewports
+      // from the previous view or target here.
+      usePlotlyStore().clearZoomHistory()
+    }
     qcDatastreamId.value = managedId
     await rebuildPlot()
   }
@@ -574,15 +600,37 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     }
   }
 
-  const onDateBtnClick = async (selectedId: number) => {
-    if (!findPreset(selectedId)) return
-    selectedDateBtnId.value = selectedId
+  /** Load the range the active preset resolves to now. A no-op when it
+   *  already matches. */
+  const applyActivePreset = async () => {
     const range = resolvePresetWindow()
     if (range) {
       await setDateRange({ begin: range.begin, end: range.end, custom: false })
     }
   }
 
+  const onDateBtnClick = async (selectedId: number) => {
+    if (!findPreset(selectedId)) return
+    selectedDateBtnId.value = selectedId
+    await applyActivePreset()
+  }
+
+  // The editor loads its context before the session is known, so re-anchor
+  // the preset once the window arrives or changes. Like a Context change, this
+  // reloads context series only and keeps the zoom.
+  watch(
+    () =>
+      editSessionWindow.value &&
+      `${editSessionWindow.value.begin.getTime()}-${editSessionWindow.value.end.getTime()}`,
+    (key) => {
+      if (!key) return
+      applyActivePreset().catch((error) => {
+        console.error('Failed to reload the context range:', error)
+        Snackbar.error('Could not reload the context range')
+      })
+    }
+  )
+
   const updateOrFetchGraphSeries = async (
     datastream: Datastream,
     start: Date,
diff --git a/apps/qc-app/src/store/qcSession.ts b/apps/qc-app/src/store/qcSession.ts
index c16d4a93..2137fcaa 100644
--- a/apps/qc-app/src/store/qcSession.ts
+++ b/apps/qc-app/src/store/qcSession.ts
@@ -90,26 +90,36 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     ) as QualityControlSession[]
   }
 
-  /** Load a history's sessions; default the view to the in-progress session. */
+  /** A history's sessions with their operations. Writes nothing, so a caller
+   *  can drop the result if it went stale while loading. */
+  async function fetchSessions(id: string): Promise<QualityControlSession[]> {
+    const { hs } = storeToRefs(useHydroServer())
+    const list = unwrap(
+      await hs.value.qualityControlSessions.list(id, {
+        fetch_all: true,
+        expand_related: true,
+      })
+    )
+    return withOperations(hs.value, id, list)
+  }
+
+  /** Adopt a history's sessions; default the view to the in-progress session. */
+  function applySessions(id: string, list: QualityControlSession[]): void {
+    historyId.value = id
+    sessions.value = list
+    const inProgress = list.find((s) => s.status === 'in_progress') ?? null
+    currentSessionId.value = inProgress?.id ?? null
+    // Default view: the editable session, else the latest committed.
+    const latestCommitted = [...list]
+      .filter((s) => s.status === 'committed')
+      .sort((a, b) => b.phenomenonTimeStart.localeCompare(a.phenomenonTimeStart))[0]
+    viewedSessionId.value = inProgress?.id ?? latestCommitted?.id ?? null
+  }
+
   async function loadSessions(id: string): Promise<void> {
     isLoading.value = true
     try {
-      const { hs } = storeToRefs(useHydroServer())
-      const list = unwrap(
-        await hs.value.qualityControlSessions.list(id, {
-          fetch_all: true,
-          expand_related: true,
-        })
-      )
-      historyId.value = id
-      sessions.value = await withOperations(hs.value, id, list)
-      const inProgress = list.find((s) => s.status === 'in_progress') ?? null
-      currentSessionId.value = inProgress?.id ?? null
-      // Default view: the editable session, else the latest committed.
-      const latestCommitted = [...list]
-        .filter((s) => s.status === 'committed')
-        .sort((a, b) => b.phenomenonTimeStart.localeCompare(a.phenomenonTimeStart))[0]
-      viewedSessionId.value = inProgress?.id ?? latestCommitted?.id ?? null
+      applySessions(id, await fetchSessions(id))
     } finally {
       isLoading.value = false
     }
@@ -153,6 +163,8 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     inProgressSession,
     committedSessions,
     viewedSession,
+    fetchSessions,
+    applySessions,
     loadSessions,
     viewSession,
     returnToCurrent,
diff --git a/apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts b/apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts
index 22feb112..07448566 100644
--- a/apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts
@@ -2,8 +2,10 @@ import { describe, it, expect } from 'vitest'
 import {
   ALL_PRESET_ID,
   DEFAULT_PRESET_ID,
+  EDITOR_PRESETS,
   TIME_RANGE_PRESETS,
   dataExtent,
+  presetAroundWindow,
   presetWindow,
 } from '@/utils/timeRangePresets'
 
@@ -102,3 +104,91 @@ describe('presetWindow', () => {
     expect(presetWindow(-1, extent)).toBeNull()
   })
 })
+
+describe('EDITOR_PRESETS', () => {
+  it('offers every preset but YTD, with the same ids', () => {
+    expect(EDITOR_PRESETS.map((p) => [p.id, p.label])).toEqual([
+      [0, '1w'],
+      [1, '1m'],
+      [2, '6m'],
+      [4, '1y'],
+      [5, 'All'],
+    ])
+  })
+})
+
+describe('presetAroundWindow', () => {
+  const extent = { begin: new Date(2019, 0, 1), end: new Date(2022, 0, 1) }
+  const window = {
+    begin: new Date(2020, 7, 31, 6),
+    end: new Date(2020, 9, 15, 18),
+  }
+
+  it('1w adds seven days on each side of the window', () => {
+    const w = presetAroundWindow(0, window, extent)
+    expect(w?.begin.getTime()).toBe(new Date(2020, 7, 24, 6).getTime())
+    expect(w?.end.getTime()).toBe(new Date(2020, 9, 22, 18).getTime())
+  })
+
+  it('1m clamps to the end of a shorter month on each side', () => {
+    const w = presetAroundWindow(1, window, extent)
+    expect(w?.begin.getTime()).toBe(new Date(2020, 6, 31, 6).getTime())
+    expect(w?.end.getTime()).toBe(new Date(2020, 10, 15, 18).getTime())
+    const endOfJan = { begin: new Date(2021, 0, 31), end: new Date(2021, 0, 31, 12) }
+    expect(presetAroundWindow(1, endOfJan, extent)?.end.getTime()).toBe(
+      new Date(2021, 1, 28, 12).getTime()
+    )
+  })
+
+  it('6m and 1y extend the window by their span', () => {
+    const six = presetAroundWindow(2, window, extent)
+    expect(six?.begin.getTime()).toBe(new Date(2020, 1, 29, 6).getTime())
+    expect(six?.end.getTime()).toBe(new Date(2021, 3, 15, 18).getTime())
+    const year = presetAroundWindow(4, window, extent)
+    expect(year?.begin.getTime()).toBe(new Date(2019, 7, 31, 6).getTime())
+    expect(year?.end.getTime()).toBe(new Date(2021, 9, 15, 18).getTime())
+  })
+
+  it('does not clip a span to the data at the extent edges', () => {
+    const atEdges = { begin: extent.begin, end: extent.end }
+    const w = presetAroundWindow(0, atEdges, extent)
+    expect(w?.begin.getTime()).toBe(new Date(2018, 11, 25).getTime())
+    expect(w?.end.getTime()).toBe(new Date(2022, 0, 8).getTime())
+  })
+
+  it('All is the data extent', () => {
+    const w = presetAroundWindow(ALL_PRESET_ID, window, extent)
+    expect(w?.begin.getTime()).toBe(extent.begin.getTime())
+    expect(w?.end.getTime()).toBe(extent.end.getTime())
+  })
+
+  it('All still covers a window wider than the extent', () => {
+    const narrow = { begin: new Date(2020, 8, 1), end: new Date(2020, 9, 1) }
+    const w = presetAroundWindow(ALL_PRESET_ID, window, narrow)
+    expect(w?.begin.getTime()).toBe(window.begin.getTime())
+    expect(w?.end.getTime()).toBe(window.end.getTime())
+  })
+
+  it('All is the window when there is no data extent', () => {
+    const w = presetAroundWindow(ALL_PRESET_ID, window, null)
+    expect(w?.begin.getTime()).toBe(window.begin.getTime())
+    expect(w?.end.getTime()).toBe(window.end.getTime())
+  })
+
+  it('YTD resolves like All around a window', () => {
+    expect(presetAroundWindow(3, window, extent)).toEqual(
+      presetAroundWindow(ALL_PRESET_ID, window, extent)
+    )
+  })
+
+  it('never returns the caller its own dates', () => {
+    const w = presetAroundWindow(ALL_PRESET_ID, window, null)
+    expect(w?.begin).not.toBe(window.begin)
+    expect(w?.end).not.toBe(window.end)
+  })
+
+  it('is null for an unknown id', () => {
+    expect(presetAroundWindow(999, window, extent)).toBeNull()
+    expect(presetAroundWindow(-1, window, extent)).toBeNull()
+  })
+})
diff --git a/apps/qc-app/src/utils/timeRangePresets.ts b/apps/qc-app/src/utils/timeRangePresets.ts
index 65e3be31..f8c696fb 100644
--- a/apps/qc-app/src/utils/timeRangePresets.ts
+++ b/apps/qc-app/src/utils/timeRangePresets.ts
@@ -52,23 +52,77 @@ export function dataExtent(
   return { begin: new Date(begin), end: new Date(end) }
 }
 
+type SpanLabel = '1w' | '1m' | '6m' | '1y'
+
+/** `d` moved back by `times` of a preset span; a negative `times` moves forward. */
+function shiftBySpan(label: SpanLabel, d: Date, times: 1 | -1): Date {
+  switch (label) {
+    case '1w':
+      return subtractDays(d, 7 * times)
+    case '1m':
+      return subtractMonths(d, times)
+    case '6m':
+      return subtractMonths(d, 6 * times)
+    case '1y':
+      return subtractYears(d, times)
+  }
+}
+
 /** The window a preset covers, counting back from the extent's end. */
 export function presetWindow(id: number, extent: TimeWindow): TimeWindow | null {
   const preset = findPreset(id)
   if (!preset) return null
   const end = new Date(extent.end)
   switch (preset.label) {
-    case '1w':
-      return { begin: subtractDays(end, 7), end }
-    case '1m':
-      return { begin: subtractMonths(end, 1), end }
-    case '6m':
-      return { begin: subtractMonths(end, 6), end }
-    case '1y':
-      return { begin: subtractYears(end, 1), end }
     case 'YTD':
       return { begin: new Date(end.getFullYear(), 0, 1), end }
     case 'All':
       return { begin: new Date(extent.begin), end }
+    default:
+      return { begin: shiftBySpan(preset.label, end, 1), end }
+  }
+}
+
+const EDITOR_TITLES: Partial<Record<TimeRangePreset['label'], string>> = {
+  '1w': 'A week before and after the session',
+  '1m': 'A month before and after the session',
+  '6m': '6 months before and after the session',
+  '1y': 'A year before and after the session',
+}
+
+/** The editor's presets, titled for the session window. YTD is left out: it
+ *  has no meaning around a window. */
+export const EDITOR_PRESETS: readonly TimeRangePreset[] = TIME_RANGE_PRESETS
+  .filter((p) => p.label !== 'YTD')
+  .map((p) => ({ ...p, title: EDITOR_TITLES[p.label] ?? p.title }))
+
+/**
+ * The window a preset covers around an edit session's window: the span on
+ * each side. All (and YTD, not offered in the editor) is the data extent,
+ * widened to cover the window. Spans are not clipped to the data; a fetch
+ * past it just returns nothing.
+ */
+export function presetAroundWindow(
+  id: number,
+  window: TimeWindow,
+  extent: TimeWindow | null
+): TimeWindow | null {
+  const preset = findPreset(id)
+  if (!preset) return null
+  switch (preset.label) {
+    case 'YTD':
+    case 'All': {
+      const begin = window.begin.getTime()
+      const end = window.end.getTime()
+      return {
+        begin: new Date(extent ? Math.min(extent.begin.getTime(), begin) : begin),
+        end: new Date(extent ? Math.max(extent.end.getTime(), end) : end),
+      }
+    }
+    default:
+      return {
+        begin: shiftBySpan(preset.label, window.begin, 1),
+        end: shiftBySpan(preset.label, window.end, -1),
+      }
   }
 }

From 6d35837cca09c9d6d1d3443f7ee925c404831115 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:00:57 -0600
Subject: [PATCH 55/93] style: replace em-dashes with plain punctuation

---
 apps/qc-app/README.md                         | 18 +++---
 apps/qc-app/docs/API_REFERENCE.md             | 64 +++++++++----------
 apps/qc-app/docs/ARCHITECTURE.md              | 50 +++++++--------
 apps/qc-app/docs/DEPLOYMENT.md                | 34 +++++-----
 apps/qc-app/docs/ONBOARDING.md                | 26 ++++----
 apps/qc-app/docs/PERFORMANCE.md               | 16 ++---
 apps/qc-app/docs/QUALITY.md                   | 18 +++---
 apps/qc-app/docs/TESTING.md                   | 64 +++++++++----------
 apps/qc-app/e2e/all-operations.spec.ts        | 10 +--
 apps/qc-app/e2e/edit-fill-gaps.spec.ts        |  2 +-
 apps/qc-app/e2e/filter-datetime-range.spec.ts |  4 +-
 apps/qc-app/e2e/filter-gaps.spec.ts           |  6 +-
 apps/qc-app/e2e/filter-persistence.spec.ts    |  8 +--
 apps/qc-app/e2e/filter-range-toggle.spec.ts   |  6 +-
 apps/qc-app/e2e/filter-rate-of-change.spec.ts |  2 +-
 .../qc-app/e2e/filter-value-threshold.spec.ts |  4 +-
 apps/qc-app/e2e/history.spec.ts               |  4 +-
 apps/qc-app/e2e/navigation.spec.ts            |  2 +-
 .../e2e/plot-no-intended-spacing.spec.ts      |  8 +--
 apps/qc-app/e2e/qc-golden-path.spec.ts        | 22 +++----
 apps/qc-app/e2e/qc-history.spec.ts            |  4 +-
 apps/qc-app/e2e/screenshots.spec.ts           | 20 +++---
 apps/qc-app/e2e/share-url.spec.ts             |  6 +-
 apps/qc-app/e2e/support/app.ts                |  4 +-
 apps/qc-app/e2e/support/fixtures.ts           |  2 +-
 apps/qc-app/e2e/support/mocks.ts              | 10 +--
 apps/qc-app/e2e/tooltip-threshold.spec.ts     | 12 ++--
 .../VisualizeData/DataVisDatasetsTable.vue    |  2 +-
 .../VisualizeData/__tests__/DataTable.spec.ts |  4 +-
 .../__tests__/PlotSourceDialog.spec.ts        |  2 +-
 .../__tests__/PlottedDatastreams.spec.ts      |  4 +-
 .../__tests__/useDataSelection.spec.ts        |  4 +-
 .../__tests__/useResizable.spec.ts            |  4 +-
 .../src/composables/useBufferedNumber.ts      |  2 +-
 .../src/composables/useDataSelection.ts       | 12 ++--
 .../src/composables/useFilterDispatch.ts      |  8 +--
 apps/qc-app/src/composables/useQcHistory.ts   |  4 +-
 apps/qc-app/src/composables/useResizable.ts   |  8 +--
 .../composables/useWorkspacePermissions.ts    |  4 +-
 apps/qc-app/src/main.ts                       |  2 +-
 apps/qc-app/src/router/guards.ts              |  4 +-
 apps/qc-app/src/router/router.ts              |  2 +-
 .../__tests__/persistOperations.spec.ts       |  2 +-
 .../__tests__/reconstructSession.spec.ts      |  4 +-
 .../__tests__/apiReferenceCoverage.spec.ts    |  2 +-
 .../src/store/__tests__/observations.spec.ts  |  2 +-
 .../qc-app/src/store/__tests__/plotly.spec.ts |  8 +--
 apps/qc-app/src/store/dataVisualization.ts    |  6 +-
 apps/qc-app/src/store/observations.ts         |  2 +-
 apps/qc-app/src/store/plotly.ts               | 26 ++++----
 apps/qc-app/src/store/qualifiers.ts           |  4 +-
 apps/qc-app/src/store/userInterface.ts        | 10 +--
 apps/qc-app/src/store/workspaces.ts           |  8 +--
 apps/qc-app/src/types/index.ts                |  2 +-
 apps/qc-app/src/types/plotly-dist.d.ts        |  4 +-
 .../src/utils/__tests__/dateMath.spec.ts      |  4 +-
 apps/qc-app/src/utils/__tests__/share.spec.ts |  2 +-
 apps/qc-app/src/utils/csvExport.ts            |  2 +-
 apps/qc-app/src/utils/datastreamSummary.ts    |  2 +-
 apps/qc-app/src/utils/dateMath.ts             |  2 +-
 .../utils/plotting/__tests__/options.spec.ts  |  2 +-
 .../utils/plotting/__tests__/relayout.spec.ts |  2 +-
 .../utils/plotting/__tests__/selected.spec.ts |  2 +-
 .../src/utils/plotting/__tests__/zoom.spec.ts |  4 +-
 apps/qc-app/src/utils/plotting/events.ts      |  8 +--
 apps/qc-app/src/utils/plotting/interaction.ts | 18 +++---
 apps/qc-app/src/utils/plotting/operations.ts  | 14 ++--
 apps/qc-app/src/utils/plotting/options.ts     | 22 +++----
 apps/qc-app/src/utils/plotting/plotly.ts      |  2 +-
 apps/qc-app/src/utils/plotting/relayout.ts    |  4 +-
 apps/qc-app/src/utils/plotting/selected.ts    | 12 ++--
 apps/qc-app/src/utils/plotting/staging.ts     |  6 +-
 apps/qc-app/src/utils/plotting/zoom.ts        |  2 +-
 apps/qc-app/src/utils/share.ts                |  6 +-
 apps/qc-app/tsconfig.app.json                 |  4 +-
 apps/qc-app/tsconfig.test.json                |  4 +-
 apps/qc-app/vite.config.ts                    |  6 +-
 77 files changed, 351 insertions(+), 351 deletions(-)

diff --git a/apps/qc-app/README.md b/apps/qc-app/README.md
index 0207fff6..2e7bc029 100644
--- a/apps/qc-app/README.md
+++ b/apps/qc-app/README.md
@@ -14,11 +14,11 @@ This web app facilitates QC/QA for time series observations stored in a HydroSer
 
 The app is the operator's view of HydroServer's QC pipeline:
 
-1. **Browse** — pick a workspace, filter datastreams by site / observed property / processing level, and plot up to five at once on a synchronized multi-axis chart.
+1. **Browse**: pick a workspace, filter datastreams by site / observed property / processing level, and plot up to five at once on a synchronized multi-axis chart.
 2. **QC one stream at a time**: the pencil button on a datastream row picks what to edit, separately from the plot checkboxes. The editor draws that stream over its raw source, with the plotted datastreams as read-only context.
-3. **Filter / edit / add** — every operation (Value Threshold, Find Gaps, Persistence, Interpolate, Drift Correction, Fill Gaps, Add Points, etc.) commits a `HistoryItem` to a replayable edit history backed by [`@uwrl/qc-utils`](https://www.npmjs.com/package/@uwrl/qc-utils).
-4. **Save / load a QC History** — export the history as a JSON document, replay it on the same datastream a week later, or templatize across stations.
-5. **Save and commit** — save the edits to a QC session as a draft, then commit the session to push the quality-controlled observations to its managed datastream in `replace` mode.
+3. **Filter / edit / add**: every operation (Value Threshold, Find Gaps, Persistence, Interpolate, Drift Correction, Fill Gaps, Add Points, etc.) commits a `HistoryItem` to a replayable edit history backed by [`@uwrl/qc-utils`](https://www.npmjs.com/package/@uwrl/qc-utils).
+4. **Save / load a QC History**: export the history as a JSON document, replay it on the same datastream a week later, or templatize across stations.
+5. **Save and commit**: save the edits to a QC session as a draft, then commit the session to push the quality-controlled observations to its managed datastream in `replace` mode.
 
 The heavy lifting (worker-parallelized typed-array kernels, calibration, history replay, save / load wire format) lives in `@uwrl/qc-utils`. This repo is the Vue / Vuetify / Pinia / Plotly UI plus the orchestration around it.
 
@@ -39,7 +39,7 @@ During normal HydroServer development, open QC through the Data Management app a
 
 | Var                              | Required | Purpose                                                                 |
 |----------------------------------|----------|-------------------------------------------------------------------------|
-| `VITE_APP_DISABLE_COOP`          | no       | Drop the `Cross-Origin-Opener-Policy` + `Cross-Origin-Embedder-Policy` headers. Use only when the backend you're hitting doesn't serve `Cross-Origin-Resource-Policy` (older HydroServer deployments). The `qc-utils` worker layer falls back to inline kernels when SAB isn't available, so the app still works — just slower on large edits. |
+| `VITE_APP_DISABLE_COOP`          | no       | Drop the `Cross-Origin-Opener-Policy` + `Cross-Origin-Embedder-Policy` headers. Use only when the backend you're hitting doesn't serve `Cross-Origin-Resource-Policy` (older HydroServer deployments). The `qc-utils` worker layer falls back to inline kernels when SAB isn't available, so the app still works (just slower on large edits). |
 | `VITE_APP_E2E_HOOKS`             | no       | Set to `1` to expose `window.__vbwTestHooks` for Playwright. CI sets this automatically. |
 
 Cross-origin isolation is on by default (`vite.config.ts` sends `Cross-Origin-Opener-Policy: same-origin` + `Cross-Origin-Embedder-Policy: require-corp`) so `SharedArrayBuffer` is available to the qc-utils worker pool.
@@ -66,7 +66,7 @@ src/
 ├─ utils/plotting/
 │  ├─ events.ts                 plotly_click / plotly_relayout / mousemove handlers.
 │  ├─ relayout.ts               Debounced viewport recomputation, tick alignment.
-│  ├─ selected.ts               handleSelected — translates Plotly selection into a SELECTION dispatch.
+│  ├─ selected.ts               handleSelected: translates Plotly selection into a SELECTION dispatch.
 │  ├─ staging.ts                Ghost-fill markers + drag-resizable stage shape.
 │  └─ plotly.ts                 Trace builders + low-level setSelectedPoints / clearSelection.
 └─ router/                      vue-router 5 setup with workspace + auth guards.
@@ -100,7 +100,7 @@ Specs live next to source under `src/**/__tests__/`. Plotly is mocked at the mod
 
 ### End-to-end tests (Playwright)
 
-End-to-end specs cover the QC golden path: load a datastream, apply a filter, apply an edit, save and commit the session. Browser matrix: **chromium** and **firefox**. WebKit is intentionally excluded — `SharedArrayBuffer` + COOP / COEP behaviour differs in Safari and needs separate validation.
+End-to-end specs cover the QC golden path: load a datastream, apply a filter, apply an edit, save and commit the session. Browser matrix: **chromium** and **firefox**. WebKit is intentionally excluded: `SharedArrayBuffer` + COOP / COEP behaviour differs in Safari and needs separate validation.
 
 One-time setup:
 
@@ -111,14 +111,14 @@ npx playwright install chromium firefox
 Run modes:
 
 ```bash
-npm run e2e                   # headless — Chromium + Firefox (CI mode, fast for local)
+npm run e2e                   # headless: Chromium + Firefox (CI mode, fast for local)
 npm run e2e:live              # same-origin smoke via Data Management
 ```
 
 For interactive debugging, append Playwright flags, e.g. `npm run e2e -- --ui` or
 `npm run e2e -- --headed`.
 
-The Playwright config in `playwright.config.ts` auto-starts the QC Vite dev server at `http://127.0.0.1:15173` and reuses an existing one outside CI. The dev server is what serves the COOP / COEP headers `SharedArrayBuffer` needs — running e2e against a static `file://` build won't work.
+The Playwright config in `playwright.config.ts` auto-starts the QC Vite dev server at `http://127.0.0.1:15173` and reuses an existing one outside CI. The dev server is what serves the COOP / COEP headers `SharedArrayBuffer` needs; running e2e against a static `file://` build won't work.
 
 The mocked specs intercept HydroServer routes via `page.route()` and serve fixture JSON, so most runs need no backend. The live golden-path spec expects both frontends to be running and enters QC through `http://127.0.0.1:1203/qc/`.
 
diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 1ccf16c3..0de20ab1 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -1,6 +1,6 @@
 # APIs &amp; Interoperability
 
-This is a reference for the surfaces a developer integrates against —
+This is a reference for the surfaces a developer integrates against:
 both the in-app TypeScript surfaces (Pinia stores, composables, plotting
 utils) and the external HydroServer REST endpoints the app consumes.
 
@@ -75,7 +75,7 @@ hs.datastreams.createObservations(
 ```
 
 Replace mode tells HydroServer to overwrite any observation inside the
-posted window. The QC App always submits the full edited window —
+posted window. The QC App always submits the full edited window;
 incremental submission is not implemented today.
 
 ### Result qualifier codes
@@ -220,9 +220,9 @@ const {
   through the same replay instead (throwing `ResumeSupersededError` the same
   way), so saved draft operations are never dropped. The same ownership rule
   guards its session reload.
-- `saveDraft()` — persists the record's edit operations to the session
+- `saveDraft()`: persists the record's edit operations to the session
   (append-only reconcile).
-- `discardUnsavedEdits()` — drops edits made since the last save and restores
+- `discardUnsavedEdits()`: drops edits made since the last save and restores
   edited comments.
 - `commit()` — saves, verifies checksum C, pushes observations
   (`mode: 'replace'`), then locks the session.
@@ -285,7 +285,7 @@ and the plot's table-vs-chart splitter.
 
 ### `useBufferedNumber()`
 
-Debounced numeric input wrapper for filter panels — avoids dispatching
+Debounced numeric input wrapper for filter panels. Avoids dispatching
 on every keystroke when a user is typing a threshold.
 
 ## Internal: Pinia stores
@@ -302,10 +302,10 @@ Columns in the per-store tables:
   `T` inside `Ref<T>`; actions list the call signature.
 
 The "Persistence" line at the top of each store cites the
-`pinia-plugin-persistedstate` config — the storage key and the
+`pinia-plugin-persistedstate` config: the storage key and the
 specific slice picked. Stores with no Persistence line are session-only.
 
-### `useDataVisStore()` — `src/store/dataVisualization.ts`
+### `useDataVisStore()` (`src/store/dataVisualization.ts`)
 
 Catalog data (sites, datastreams, taxonomy), sidebar filters, plotted
 set + edit target, time-range window. The orchestrator for everything in
@@ -394,14 +394,14 @@ fresh.
 | `clear`      | action | `() => void`                                                                                           | Drop every cached copy and supersede every in-flight build and load; called by `useDataVisStore().resetState()` on a workspace reset. |
 | `extents`    | action | `(managedIds: string[]) => { phenomenonBeginTime: string; phenomenonEndTime: string }[]`               | The cached copies' windows for the given managed ids, in ISO form; ids with no cached copy are omitted. |
 
-### `usePlotlyStore()` — `src/store/plotly.ts`
+### `usePlotlyStore()` (`src/store/plotly.ts`)
 
 Owns the Plotly DOM ref, the per-series array driving the chart,
 viewport state (tooltips, crosshair, hover, zoom history), and the
 redraw / restyle plumbing.
 
 Persistence: `tooltipsMaxDataPoints`, `tooltipsMode`, and
-`tooltipsManualEnabled` (key `qc.plot.tooltipsMaxDataPoints`) — the
+`tooltipsManualEnabled` (key `qc.plot.tooltipsMaxDataPoints`): the
 user's data-points-mode preference. Everything else is ephemeral (DOM
 handles, live chart caches).
 
@@ -410,10 +410,10 @@ handles, live chart caches).
 | `graphSeriesArray`         | state    | `GraphSeries[]`                                   | Per-series state driving traces, colors, and axis chips. |
 | `plotlyOptions`            | state    | `PlotlyChartOptions`                              | Cached `createPlotlyOption` output; seeded empty so consumers can read without null-guards. |
 | `plotlyRef`                | state    | `AppPlotlyHTMLElement \| null`                    | Live Plotly DOM element; populated by `handleNewPlot`. |
-| `mainPlotEpoch`            | state    | `number`                                          | Monotonic counter — bumped per `handleNewPlot` so listeners can re-attach. |
+| `mainPlotEpoch`            | state    | `number`                                          | Monotonic counter, bumped per `handleNewPlot` so listeners can re-attach. |
 | `selectedSeriesIndex`      | computed | `number`                                          | Index of the edit target in `graphSeriesArray` (`-1` when none). |
 | `selectedSeries`           | computed | `GraphSeries`                                     | Convenience for `graphSeriesArray[selectedSeriesIndex]`. |
-| `editHistory`              | state    | `HistoryItem[]`                                   | Mirrors `selectedSeries.data.history` (mutated in place — never reassign). |
+| `editHistory`              | state    | `HistoryItem[]`                                   | Mirrors `selectedSeries.data.history` (mutated in place; never reassign). |
 | `suppressedEchoSelection`  | state    | `number[] \| null`                                | Sentinel armed by programmatic Plotly writes to suppress the echo SELECTION dispatch. |
 | `isUpdating`               | state    | `boolean`                                         | Surfaced in the nav rail while a redraw runs. |
 | `showLegend`               | state    | `boolean`                                         | Drives Plotly's legend visibility. |
@@ -451,7 +451,7 @@ handles, live chart caches).
 | `clearZoomHistory`         | action   | `() => void`                                      | Empty both stacks. |
 | `pushZoomState`            | action   | `(state: ZoomState) => void`                      | Called by the debounced recorder in `utils/plotting/zoom.ts`. |
 
-### `useObservationStore()` — `src/store/observations.ts`
+### `useObservationStore()` (`src/store/observations.ts`)
 
 Fetches + caches observation windows and inflates them into
 `ObservationRecord` instances.
@@ -462,7 +462,7 @@ Fetches + caches observation windows and inflates them into
 | `observationsRaw`          | state    | `Record<string, ObservationData>`                 | Typed-array cache (`Float64Array` datetimes + `Float32Array` values). |
 | `fetchObservationsInRange` | action   | `(ds: Datastream, b: Date, e: Date) => Promise<ObservationRecord>` | Extends the cached range minimally; only fetches segments outside the existing window. |
 
-### `useWorkspaceStore()` — `src/store/workspaces.ts`
+### `useWorkspaceStore()` (`src/store/workspaces.ts`)
 
 Workspace selection + role-derived edit permission.
 
@@ -480,13 +480,13 @@ restored selection on the first navigation.
 | `canEditSelected`          | computed | `boolean`                                         | True for workspace owners; for collaborators, true when their role includes an Observation create/edit permission. |
 | `loadWorkspaces`           | action   | `() => Promise<Workspace[]>`                      | Refetch + reconcile against the stored selection (drops the selection if the user lost access). |
 | `selectWorkspace`          | action   | `(id: string \| null) => Workspace \| null`       | Pick by id from `availableWorkspaces`. |
-| `applyWorkspaceById`       | action   | `(id: string) => Workspace \| null`               | Falls back to a placeholder `{ id }` when the list isn't loaded yet — used by URL hydration. |
+| `applyWorkspaceById`       | action   | `(id: string) => Workspace \| null`               | Falls back to a placeholder `{ id }` when the list isn't loaded yet. Used by URL hydration. |
 | `clearSelection`           | action   | `() => void`                                      | Drop the selection. |
 
-### `useUIStore()` — `src/store/userInterface.ts`
+### `useUIStore()` (`src/store/userInterface.ts`)
 
 Drawer / view chrome state plus the per-operation form fields read
-by every filter / edit panel. Mostly a flat bag — the panel
+by every filter / edit panel. Mostly a flat bag: the panel
 components own the validation; this store just keeps the values
 reactive between mounts.
 
@@ -496,12 +496,12 @@ defaults are reseeded from the datastream on each mount.
 
 | Name                              | Kind   | Type / signature                                  | Notes |
 |-----------------------------------|--------|---------------------------------------------------|-------|
-| `selectedDrawer`                  | state  | `DrawerType`                                      | `Edit`, `Select`, or `None` — which left drawer is active. |
+| `selectedDrawer`                  | state  | `DrawerType`                                      | `Edit`, `Select`, or `None`: which left drawer is active. |
 | `isDrawerOpen`                    | state  | `boolean`                                         | Drawer open/collapsed. |
 | `currentView`                     | state  | `'Edit' \| 'Select'`                              | Current main view (drives the nav rail's active state). |
 | `selectedOperation`               | state  | `string \| null`                                  | Open operation panel id; `null` when nothing is open. |
 | `cardHeight` / `tableHeight`      | state  | `number`                                          | Select-view top/bottom split. |
-| `operators`                       | state  | `string[]`                                        | `Object.keys(Operator)` — Change-values operator choices. |
+| `operators`                       | state  | `string[]`                                        | `Object.keys(Operator)`: Change-values operator choices. |
 | `selectedOperator`                | state  | `number`                                          | Index into `operators`. |
 | `operationValue`                  | state  | `number`                                          | Change-values numeric operand. |
 | `interpolateValues`               | state  | `boolean`                                         | Fill-gaps: interpolate vs constant. |
@@ -519,12 +519,12 @@ defaults are reseeded from the datastream on each mount.
 | `filterRangeFromTs` / `filterRangeToTs` | state | `number \| null`                            | Filter-window epoch bounds; reseed on each panel mount. |
 | `onRailItemClicked`               | action | `(title: DrawerType) => void`                     | Nav-rail click handler: toggles open/closed on repeat, switches view on first click. |
 
-### `useQualifierStore()` — `src/store/qualifiers.ts`
+### `useQualifierStore()` (`src/store/qualifiers.ts`)
 
 Workspace-scoped qualifier dictionary plus the per-observation
 applications added via the Qualifying Comments panel.
 
-Persistence: `applied` only — the dictionary is reloaded on every
+Persistence: `applied` only; the dictionary is reloaded on every
 workspace change.
 
 | Name                            | Kind     | Type / signature                                  | Notes |
@@ -535,12 +535,12 @@ workspace change.
 | `qualifierById`                 | computed | `Record<string, Qualifier>`                       | Lookup map for the chips. |
 | `loadQualifiers`                | action   | `() => Promise<void>`                             | Fetch dictionary for the active workspace; triggers a plot refresh so the qualifier band materialises. |
 | `createQualifier`               | action   | `(code: string, description: string) => Promise<Qualifier>` | Server-side create with a local-only fallback when no workspace is active. |
-| `applyQualifiers`               | action   | `(datastreamId, indices, qualifierIds, appliedBy) => void` | Idempotent merge — already-applied (qualifier, index) pairs are skipped. |
+| `applyQualifiers`               | action   | `(datastreamId, indices, qualifierIds, appliedBy) => void` | Idempotent merge: already-applied (qualifier, index) pairs are skipped. |
 | `removeQualifier`               | action   | `(datastreamId, index, qualifierId) => void`      | Drops a single (qualifier, index) application. |
 | `getApplicationsForDatastream`  | action   | `(datastreamId) => Array<{ index, qualifierId, appliedAt, appliedBy }>` | Flat list suitable for plotting. |
 | `getApplicationsAtIndex`        | action   | `(datastreamId, index) => QualifierApplication[]` | Per-point lookup. |
 
-### `useUiLayoutStore()` — `src/store/uiLayout.ts`
+### `useUiLayoutStore()` (`src/store/uiLayout.ts`)
 
 Persisted drawer / splitter geometry. A bag of values keyed by
 strings the calling composable supplies, so new resizable components
@@ -557,7 +557,7 @@ Persistence: both `sizes` and `flags` (key `qc:uiLayout:v1`).
 | `getFlag` | action | `(key: string) => boolean \| null`                | `null` lets callers distinguish "unset" from "explicit false". |
 | `setFlag` | action | `(key: string, value: boolean) => void`           | Same fresh-object pattern. |
 
-### `useOperationParamsStore()` — `src/store/operationParams.ts`
+### `useOperationParamsStore()` (`src/store/operationParams.ts`)
 
 Per-datastream remembered slots for Find Gaps / Fill Gaps
 parameters. `useUIStore` reads these to seed defaults; otherwise
@@ -571,7 +571,7 @@ Persistence: `byDatastream` (key `qc:opParams:v1`).
 | `load`         | action | `(id?: string \| null) => PersistedOpParams \| null`      | `null` when nothing's stored. |
 | `save`         | action | `(id?: string \| null, patch: PersistedOpParams) => void` | Merges; partial patches don't clobber unrelated fields. |
 
-### `useUserStore()` — `src/store/user.ts`
+### `useUserStore()` (`src/store/user.ts`)
 
 The signed-in user. Persisted in full.
 
@@ -580,10 +580,10 @@ The signed-in user. Persisted in full.
 | `user`    | state  | `User`                | Defaults to a fresh `new User()` until auth resolves. |
 | `setUser` | action | `(u: User) => void`   | Replace wholesale (called by the session resolver in `main.ts`). |
 
-### `useHydroServer()` — `src/store/hydroserver.ts`
+### `useHydroServer()` (`src/store/hydroserver.ts`)
 
 Holds the `@hydroserver/client` instance. Initialized in `main.ts`
-after settings load — every other store reaches `hs.value` through
+after settings load; every other store reaches `hs.value` through
 `storeToRefs(useHydroServer())`. Not persisted (the client carries
 ephemeral connection state).
 
@@ -591,7 +591,7 @@ ephemeral connection state).
 |------|-------|--------------------|-------|
 | `hs` | state | `Ref<HydroServer>` | Non-null after `main.ts` finishes settings load; type-asserted as non-null for ergonomic consumer code. |
 
-### `useQcSessionStore()` — `src/store/qcSession.ts`
+### `useQcSessionStore()` (`src/store/qcSession.ts`)
 
 View-mode state for QC sessions: which session is editable (the single
 in-progress one) and which is being viewed. Viewing a committed session
@@ -619,7 +619,7 @@ puts the editor in read-only mode.
 | `returnToCurrent`   | action   | `() => void`                            | Return to the editable in-progress session. |
 | `reset`             | action   | `() => void`                            | Clear all state. |
 
-### `useQcPreferencesStore()` — `src/store/qcPreferences.ts`
+### `useQcPreferencesStore()` (`src/store/qcPreferences.ts`)
 
 Persisted QC editing preferences. Persistence: key `qc:preferences:v1`,
 `pick: ['processingLevelId']`.
@@ -644,7 +644,7 @@ Re-exports:
 
 ### `src/utils/observations.ts`
 
-`fetchObservationsSync(datastream, startTime?, endTime?)` — paged
+`fetchObservationsSync(datastream, startTime?, endTime?)`: paged
 columnar fetch, returns `{ datetimes: number[]; dataValues: number[] }`.
 
 ### `src/utils/dateMath.ts`
@@ -694,7 +694,7 @@ Vuetify validation rules used across forms (required, numeric, range).
 ## Test hooks
 
 When `VITE_APP_E2E_HOOKS=1` (Playwright sets this), `src/testHooks.ts`
-attaches `window.__vbwTestHooks` with the handles E2E specs need —
+attaches `window.__vbwTestHooks` with the handles E2E specs need:
 selecting a datastream programmatically, reading the current edit
 history, asserting the plot is ready. These are e2e plumbing, not a
 public surface; treat the names as unstable.
@@ -721,8 +721,8 @@ Wire shape:
 
 ## Integrating from outside
 
-If you want to reuse the QC engine in a non-Vue context — a Jupyter
-notebook driven by Pyodide, a Node CLI, another web app — depend on
+If you want to reuse the QC engine in a non-Vue context (a Jupyter
+notebook driven by Pyodide, a Node CLI, another web app), depend on
 `@uwrl/qc-utils` directly and skip the QC App entirely. The QC App is a
 UI shell; the engine is independent.
 
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index e332307e..7b51153c 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -19,7 +19,7 @@ the end-user perspective, see [USER_GUIDE.md](./USER_GUIDE.md).
 | Build / dev      | Vite 7                                       | Fast cold start, native ESM, first-class TypeScript + Vue SFC support. The dev server is also what serves the COOP/COEP headers `SharedArrayBuffer` needs. |
 | Type system      | TypeScript 5 (strict)                        | Surfaces shape errors early; eliminates a class of regressions around `ObservationRecord` API drift. |
 | Unit tests       | Vitest + Vue Test Utils (`jsdom`)            | Shares the Vite build pipeline, runs in-process, transparent ESM. |
-| E2E tests        | Playwright (chromium + firefox)              | Cross-browser, headless-or-headed, intercepts network requests cleanly. WebKit excluded — see [QUALITY.md](./QUALITY.md). |
+| E2E tests        | Playwright (chromium + firefox)              | Cross-browser, headless-or-headed, intercepts network requests cleanly. WebKit excluded (see [QUALITY.md](./QUALITY.md)). |
 | Auth             | HydroServer session cookies                  | Authentication is owned by the data-management app; QC consumes the existing session. |
 | Package manager  | npm                                          | Stays compatible with CI cache + the published `@uwrl/qc-utils` workflow.       |
 
@@ -28,16 +28,16 @@ the end-user perspective, see [USER_GUIDE.md](./USER_GUIDE.md).
 **The QC App itself runs no database.** It is a static SPA. Persistent state
 lives in three places, in increasing order of authority:
 
-1. **In-memory typed arrays** — every loaded observation window is held in a
+1. **In-memory typed arrays**: every loaded observation window is held in a
    pair of `Float64Array` (datetimes, ms epoch) + `Float32Array` (values),
    backed by a `SharedArrayBuffer` when COOP/COEP are on so worker kernels
    can scan the same memory without copying. Lives in `ObservationRecord`
-   (qc-utils). Lost on reload — by design.
-2. **`localStorage`** — workspace selection, drawer widths, persisted
+   (qc-utils). Lost on reload, by design.
+2. **`localStorage`**: workspace selection, drawer widths, persisted
    operator inputs (`pinia-plugin-persistedstate`). Per-browser, never
    sent to the backend. Use only for ephemeral UI prefs; **never** persist
    observation data here.
-3. **HydroServer backend** — the system of record. The QC App reads
+3. **HydroServer backend**: the system of record. The QC App reads
    observations and writes edited observations back via the
    `@hydroserver/client`'s `replace`-mode bulk POST. The backend stores
    everything in HydroServer's Postgres-compatible store, and is the
@@ -80,7 +80,7 @@ always be recovered by a hard refresh.
 ```
 
 All QC computation runs in-browser. The backend never sees the intermediate
-edit state — it only sees the final observations the operator chooses to
+edit state; it only sees the final observations the operator chooses to
 submit.
 
 ## Source layout
@@ -102,7 +102,7 @@ src/
 │  ├─ useQcHistory.ts            Save / load QC Historys (calls qc-utils' serializeHistory / applyHistory).
 │  ├─ useResizable.ts           Generic drag-to-resize hook used by drawers + the plot.
 │  └─ useBufferedNumber.ts      Debounced numeric input wrapper for filter panels.
-├─ store/                       Pinia stores — see "State stores" below.
+├─ store/                       Pinia stores. See "State stores" below.
 ├─ utils/
 │  ├─ plotting/                 Plotly integration (trace builders, event handlers, selection, staging).
 │  ├─ dateMath.ts               Time-range arithmetic for presets ("1w", "1m", "All", …).
@@ -132,13 +132,13 @@ goes one direction.
 | `qualifiers.ts`       | Result qualifier codes per workspace.                                 |
 | `userInterface.ts`    | Drawer state (Select/Edit), persisted prefs, current view.            |
 | `operationParams.ts`  | Per-operation form inputs, persisted so they survive panel re-opens.  |
-| `uiLayout.ts`         | Drawer widths, table heights — persisted UI geometry.                 |
+| `uiLayout.ts`         | Drawer widths, table heights: persisted UI geometry.                  |
 | `workingCopies.ts`    | Working copy per managed datastream, keyed by its in-progress session.|
 
 The persisted stores use `pinia-plugin-persistedstate` with explicit
 `storage: localStorage` and an explicit `paths` list. **Never** persist
 ephemeral state (drawer open flags during a single session are fine; the
-current plot ref or fetched observations are not — they belong in memory).
+current plot ref or fetched observations are not; they belong in memory).
 
 ## Data flow: a QC edit
 
@@ -235,7 +235,7 @@ SDK services on the `hs` instance: `qualityControlHistories`,
 `qualityControlSessions` (with `commit`), and `qualityControlOperations`. They
 are normal SDK services built on the shared `apiMethods` layer, so they inherit
 the session auth (CSRF cookie -> `X-CSRFToken`, `credentials: 'include'`) and
-the `ApiResponse` return shape — methods never throw on HTTP errors. Bodies are
+the `ApiResponse` return shape: methods never throw on HTTP errors. Bodies are
 camelCase (`by_alias`); query parameters are snake_case (`expand_related`,
 `range_start`, `managed_datastream_id`, `ancestor_of`, ...).
 
@@ -244,7 +244,7 @@ composes those services with `@uwrl/qc-utils` and the datastream/observation
 APIs: `createManagedDatastream`, the session lifecycle (`session.ts`),
 `persistOperations`, `commitSession`, `reconstructSession`, `findHistory`, and
 the `observationsBulkBody` serializer. `unwrap` bridges `ApiResponse` to the
-thrown errors this glue surfaces. None of it is a transport — swapping the QC
+thrown errors this glue surfaces. None of it is a transport; swapping the QC
 client out is a `@hydroserver/client` change, not an app one.
 
 **Edit entry.** The row Edit button on a source (`StartEditingFlow.vue`)
@@ -261,7 +261,7 @@ owned by the session (working copy, `startSession`, `viewSession`,
 
 Two contract notes worth keeping in mind:
 
-- **The backend stores the operation DAG as metadata only — it never replays
+- **The backend stores the operation DAG as metadata only; it never replays
   operations.** The app applies ops locally (qc-utils), pushes the edited
   series to the managed datastream via `bulk-create` (replace mode), then calls
   `/commit`, which only records checksums and extends the history window.
@@ -322,7 +322,7 @@ Two contract notes worth keeping in mind:
   reproduce the final state. `reconstructCommittedSession` instead fetches the
   ancestor closure (`ancestor_of`), loads the raw source over the union of
   every window in the chain, and replays the chain in **commit order**
-  (`committedAt`, falling back to `createdAt`) — committing is what writes
+  (`committedAt`, falling back to `createdAt`): committing is what writes
   observations into the managed datastream, so it is commit order, not
   authoring order, that decides what a later session built on. The union
   window matters because operations replay against array indices: loading
@@ -339,8 +339,8 @@ Two contract notes worth keeping in mind:
   scannable. It is never sent back: the field is
   provenance, not input. Operations applied in the current session show no
   attribution until they are saved and reloaded. Comments have no author of
-  their own — `comment` is a plain nullable column that can be rewritten
-  later — so "who wrote this note" is a pending backend ask.
+  their own (`comment` is a plain nullable column that can be rewritten
+  later), so "who wrote this note" is a pending backend ask.
 - **A commit is terminal.** The API rejects updating, adding operations to,
   or re-committing a committed session, and its PATCH body carries only
   `description`. Continuing work after a commit means starting a new session;
@@ -362,7 +362,7 @@ Two contract notes worth keeping in mind:
 
 Tests stub the three services with `makeQcFake()` (a stateful in-memory double
 under `services/qualityControl/__tests__/` that returns
-`{ histories, sessions, operations }`) — the production client lives in the
+`{ histories, sessions, operations }`); the production client lives in the
 package, not the app.
 
 **Permission gating.** QC editing writes to the source datastream's workspace
@@ -404,10 +404,10 @@ Three constraints shape the implementation:
 vue-router 5, two routes (Home, Workspaces). Two guards run on
 every navigation:
 
-- **`hasAuthGuard`** — redirects unauthenticated users to the
+- **`hasAuthGuard`**: redirects unauthenticated users to the
   data-management app's `/login` route and remembers the intended QC
   destination.
-- **`hasWorkspaceGuard`** — redirects users without a selected workspace
+- **`hasWorkspaceGuard`**: redirects users without a selected workspace
   to `/workspaces`.
 
 The nav rail's "Edit" entry is enabled only while an edit target is set: it
@@ -435,10 +435,10 @@ either way (just slower on large edits).
 
 ## See also
 
-- [README](../README.md) — quick start, config, scripts
-- [ONBOARDING.md](./ONBOARDING.md) — first-day setup checklist + doc gaps
-- [DEPLOYMENT.md](./DEPLOYMENT.md) — infrastructure, operations, upgrades
-- [API_REFERENCE.md](./API_REFERENCE.md) — store / composable / util surfaces, REST integration
-- [PERFORMANCE.md](./PERFORMANCE.md) — performance characteristics and scaling
-- [QUALITY.md](./QUALITY.md) — testing posture, code quality, tech debt
-- [USER_GUIDE.md](./USER_GUIDE.md) — operator-facing feature walkthrough
+- [README](../README.md): quick start, config, scripts
+- [ONBOARDING.md](./ONBOARDING.md): first-day setup checklist + doc gaps
+- [DEPLOYMENT.md](./DEPLOYMENT.md): infrastructure, operations, upgrades
+- [API_REFERENCE.md](./API_REFERENCE.md): store / composable / util surfaces, REST integration
+- [PERFORMANCE.md](./PERFORMANCE.md): performance characteristics and scaling
+- [QUALITY.md](./QUALITY.md): testing posture, code quality, tech debt
+- [USER_GUIDE.md](./USER_GUIDE.md): operator-facing feature walkthrough
diff --git a/apps/qc-app/docs/DEPLOYMENT.md b/apps/qc-app/docs/DEPLOYMENT.md
index be58bcbd..d94ff9e3 100644
--- a/apps/qc-app/docs/DEPLOYMENT.md
+++ b/apps/qc-app/docs/DEPLOYMENT.md
@@ -27,10 +27,10 @@ A typical deployment looks like:
 
 Three required pieces:
 
-1. **Static hosting** — any object store + CDN. AWS S3 + CloudFront is
+1. **Static hosting**: any object store + CDN. AWS S3 + CloudFront is
    what the demo workflow uses (see below); GitHub Pages, Cloudflare
    Pages, Azure Blob + Front Door, or nginx in a container all work.
-2. **A HydroServer instance on the same origin** — QC expects `/api` and
+2. **A HydroServer instance on the same origin**: QC expects `/api` and
    `/login` to be served from the same origin as `/qc/`.
 3. **Response headers** at the edge:
    - `Cross-Origin-Opener-Policy: same-origin`
@@ -40,7 +40,7 @@ Three required pieces:
    them the app still works, just slower on large edits; the worker layer
    transparently falls back to inline kernels. If your HydroServer
    deployment can't yet emit `Cross-Origin-Resource-Policy`, build with
-   `VITE_APP_DISABLE_COOP=1` so the headers are dropped — otherwise the
+   `VITE_APP_DISABLE_COOP=1` so the headers are dropped; otherwise the
    browser will block the API response.
 
 ## Reference deployment: AWS S3 + CloudFront
@@ -58,7 +58,7 @@ deploys to AWS. The flow:
 3. **`verify-ci-success`** queries the GitHub API for the latest CI run
    on the chosen branch. If it didn't succeed, the deploy aborts with a
    pointer to the failing run. **You cannot deploy a branch whose CI is
-   red** — by design.
+   red**, by design.
 4. **`deploy-hydroserver-data-mgmt-app`**:
    - Assumes the IAM role via OIDC (`aws-actions/configure-aws-credentials`).
    - Checks out the chosen branch.
@@ -141,7 +141,7 @@ The QC App is browser-side software, so the observability story is
 ### What you have today
 
 - **In-app Snackbar notifications** (`Snackbar` from `qc-utils`) surface
-  successes, warnings, and failures for every user-driven action — load
+  successes, warnings, and failures for every user-driven action: load
   failures, submit results, script import reports, etc.
 - **Browser console** carries qc-utils dispatch logs and any unhandled
   promise rejections. The history panel in dev mode displays per-entry
@@ -159,7 +159,7 @@ The QC App is browser-side software, so the observability story is
   no Datadog RUM, no Google Analytics). The team has consciously kept
   the SPA telemetry-free; adding it is an opt-in deployment decision.
 - **No server-side metrics**, because there is no server-side runtime.
-- **No audit log** of QC edits — the QC History file is the closest
+- **No audit log** of QC edits. The QC History file is the closest
   equivalent (export before submitting to keep a replayable record).
 - **No alerting** beyond what your CDN / object-store provider offers.
 
@@ -179,7 +179,7 @@ shape-stable. If you ever need to break a persisted shape:
 - Bump a version key inside the affected store's persisted slice and
   drop the old value on rehydrate. The `qc-utils` calibration store
   uses this pattern (`qc-utils:calibration:v<n>`); copy it.
-- Never silently coerce — if the persisted shape might be wrong, drop
+- Never silently coerce. If the persisted shape might be wrong, drop
   it and let the user re-pick their workspace / preferences. Reset is
   cheap; data corruption is not.
 
@@ -198,7 +198,7 @@ npm run build
 ```
 
 The package is pre-1.0, so assume any minor bump may require code
-changes in the consumer — read the qc-utils commit log and re-run E2E.
+changes in the consumer. Read the qc-utils commit log and re-run E2E.
 
 For local development against unreleased qc-utils changes, run the QC app
 dev server. It aliases `@uwrl/qc-utils` to `packages/qc-utils/src`, so no
@@ -219,14 +219,14 @@ matters here is **how the QC App weathers them**:
 
 - The app talks to the backend over REST through `@hydroserver/client`.
   Schema changes that break the response shape will manifest as type
-  errors at build time (good — won't deploy) or runtime Snackbar errors
+  errors at build time (good: won't deploy) or runtime Snackbar errors
   (bad but visible).
 - The QC App consumes the local `@hydroserver/client` package from
   `packages/hydroserver-ts`. When the backend ships a schema change,
   update that client, run `npx vue-tsc --noEmit` to see the breakage, and
   patch the call sites.
 - The observation upload path uses `mode: 'replace'` on the bulk POST.
-  Replace semantics are stable across HydroServer versions — if you ever
+  Replace semantics are stable across HydroServer versions. If you ever
   need to change to append-only or upsert semantics, that's a coordinated
   release between the QC App and the backend.
 
@@ -244,9 +244,9 @@ matters here is **how the QC App weathers them**:
 
 `.github/workflows/ci.yml` runs on every push and PR to `main`:
 
-1. `npx vue-tsc --noEmit` — type-check.
-2. `npm run coverage` — Vitest with the 80% coverage threshold.
-3. `npm run build` — production build sanity check.
+1. `npx vue-tsc --noEmit`: type-check.
+2. `npm run coverage`: Vitest with the 80% coverage threshold.
+3. `npm run build`: production build sanity check.
 
 Concurrency is grouped by workflow + ref with `cancel-in-progress: true`,
 so pushing again kills the older run. The deploy workflow refuses to run
@@ -274,7 +274,7 @@ Dependabot is enabled (`.github/dependabot.yml`); a separate
 
 ## See also
 
-- [README](../README.md) — quick start, config keys
-- [ARCHITECTURE.md](./ARCHITECTURE.md) — what's running where
-- [ONBOARDING.md](./ONBOARDING.md) — first-day setup
-- [PERFORMANCE.md](./PERFORMANCE.md) — what scales and what doesn't
+- [README](../README.md): quick start, config keys
+- [ARCHITECTURE.md](./ARCHITECTURE.md): what's running where
+- [ONBOARDING.md](./ONBOARDING.md): first-day setup
+- [PERFORMANCE.md](./PERFORMANCE.md): what scales and what doesn't
diff --git a/apps/qc-app/docs/ONBOARDING.md b/apps/qc-app/docs/ONBOARDING.md
index 2588b1dc..553d7045 100644
--- a/apps/qc-app/docs/ONBOARDING.md
+++ b/apps/qc-app/docs/ONBOARDING.md
@@ -44,22 +44,22 @@ console, it's almost always one of:
 
 1. Hitting `localhost:` instead of `127.0.0.1:`.
 2. The backend doesn't serve `Cross-Origin-Resource-Policy` and COOP/COEP
-   is on — try `VITE_APP_DISABLE_COOP=1`.
+   is on. Try `VITE_APP_DISABLE_COOP=1`.
 
 ## Project tour, in reading order
 
 Read these files in order:
 
-1. `README.md` — what the app does + config.
-2. `docs/ARCHITECTURE.md` — stack, source layout, data flow.
-3. `src/App.vue` and `src/main.ts` — entry point, plugin wiring.
-4. `src/pages/Home.vue` and `src/components/VisualizeData.vue` — the
+1. `README.md`: what the app does + config.
+2. `docs/ARCHITECTURE.md`: stack, source layout, data flow.
+3. `src/App.vue` and `src/main.ts`: entry point, plugin wiring.
+4. `src/pages/Home.vue` and `src/components/VisualizeData.vue`: the
    landing experience.
-5. `src/store/dataVisualization.ts` and `src/store/plotly.ts` — the two
+5. `src/store/dataVisualization.ts` and `src/store/plotly.ts`: the two
    stores that hold "what the user sees on the plot."
-6. `src/composables/useFilterDispatch.ts` and `useEditSession.ts` —
+6. `src/composables/useFilterDispatch.ts` and `useEditSession.ts`:
    the two end-to-end flows worth tracing.
-7. `packages/qc-utils/src/utils/plotting/observation-record.ts` — the QC engine's
+7. `packages/qc-utils/src/utils/plotting/observation-record.ts`: the QC engine's
    dispatch surface. You don't need to read the kernels; the dispatcher
    is the contract.
 
@@ -84,7 +84,7 @@ cd ../../apps/qc-app
 npm run build
 ```
 
-Run `npm run coverage` before you push — that's what CI gates on.
+Run `npm run coverage` before you push; that's what CI gates on.
 
 ### Working with qc-utils changes
 
@@ -107,7 +107,7 @@ you need to verify the published package artifacts.
 - Pinia stores for cross-component state; component-local state stays in
   `ref` / `reactive` inside the component.
 - Comments only for the non-obvious _why_. Don't restate the code.
-- Commit format: `{type}({scope}): {description}` — `feat`, `fix`,
+- Commit format: `{type}({scope}): {description}`: `feat`, `fix`,
   `test`, `refactor`, `perf`, `docs`, `style`, `chore`.
 
 ## Documentation gaps
@@ -115,7 +115,7 @@ you need to verify the published package artifacts.
 1. **Result qualifiers are partial.** The `QualifyingComments` op panel
    exists and writes to the in-memory history, but the commit path
    (`services/qualityControl/observationsBody.ts`) currently serializes only
-   `phenomenonTime` and `result` — qualifier codes are deferred pending the
+   `phenomenonTime` and `result`; qualifier codes are deferred pending the
    HydroServer API adding a workable columnar response. The note in
    `observationsBody.ts` points at this.
 2. **No load-testing artifacts.** "How big a datastream can you QC in
@@ -126,11 +126,11 @@ you need to verify the published package artifacts.
 
 ## Where to ask for help
 
-- [Issue tracker](https://github.com/hydroserver2/hydroserver/issues) —
+- [Issue tracker](https://github.com/hydroserver2/hydroserver/issues):
   bugs and feature requests for the HydroServer project as a whole.
 - The qc-utils repo's issues for QC engine bugs.
 - The HydroServer documentation site at
-  <https://hydroserver2.github.io/hydroserver/> — the operator-facing
+  <https://hydroserver2.github.io/hydroserver/>: the operator-facing
   HydroServer docs cover the backend the app talks to.
 
 ## See also
diff --git a/apps/qc-app/docs/PERFORMANCE.md b/apps/qc-app/docs/PERFORMANCE.md
index 8b426a07..83d6f24c 100644
--- a/apps/qc-app/docs/PERFORMANCE.md
+++ b/apps/qc-app/docs/PERFORMANCE.md
@@ -10,13 +10,13 @@ This document answers two questions:
 It is design-review + measured-behavior, not a marketing benchmark. The
 numbers are calibrated per-device by the qc-utils calibration layer, so
 your envelope on a 2018 laptop is different from your envelope on a 2024
-desktop — that's the whole point of the calibration mechanism.
+desktop. That's the whole point of the calibration mechanism.
 
 ## Where the work happens
 
 The app is a static SPA, so "performance" almost entirely means
 **in-browser computation and memory**. The HydroServer backend handles
-metadata, auth, and observation transport — the QC App does not
+metadata, auth, and observation transport; the QC App does not
 introduce its own server-side bottleneck.
 
 | Tier                          | Bottleneck                                                                |
@@ -67,8 +67,8 @@ shouldUseWorker(EnumEditOperations.FILL_GAPS, {
 // → { useWorker: false, predictedInlineMs: 12.4, predictedWorkerMs: 53.0, ... }
 ```
 
-Spawn overhead varies hugely across devices — Windows Chrome can take
-~100 ms, macOS ~10 ms — so static thresholds baked into the library are
+Spawn overhead varies hugely across devices (Windows Chrome can take
+~100 ms, macOS ~10 ms), so static thresholds baked into the library are
 wrong for most users. The calibration layer measures three primitives
 once per device (cached in `localStorage` for 30 days):
 
@@ -92,7 +92,7 @@ serve `Cross-Origin-Opener-Policy: same-origin` +
 the demo deployment both set those headers; if your CDN doesn't, the
 worker layer falls back transparently to inline kernels.
 
-The fallback is correct but slower on large edits — expect a 2-5×
+The fallback is correct but slower on large edits: expect a 2-5×
 slowdown on multi-hundred-thousand-point operations, and a noticeable
 UI freeze on operations that take >300 ms inline.
 
@@ -155,7 +155,7 @@ deployment because:
 
 It is **less well-shaped** when:
 
-- **You need centrally orchestrated QC** — automated runs on a schedule,
+- **You need centrally orchestrated QC**: automated runs on a schedule,
   a queue of datastreams to process. The app is a single-operator GUI;
   for batch QC, drive `@uwrl/qc-utils` directly from a Node or Python
   (Pyodide) process and bypass the app.
@@ -176,7 +176,7 @@ its envelope, the things to watch:
    bundle.
 4. **The calibration widget** on representative user devices. If
    `predictedInlineMs` is much higher than reality, the calibration is
-   stale — re-benchmark.
+   stale. Re-benchmark.
 
 Today there is no automated reporter that aggregates these numbers off
 the user's machine (see [DEPLOYMENT.md "Operational observability"](./DEPLOYMENT.md#operational-observability));
@@ -191,7 +191,7 @@ windows in the hundreds-of-thousands range). The
 
 There is no published large-scale benchmark suite. If you're evaluating
 the app for a >1M-operator-monthly workload, treat that as an open
-question — the architecture has nothing structurally preventing it
+question. The architecture has nothing structurally preventing it
 (every user has their own browser), but bandwidth and HydroServer
 backend capacity are the constraints, not the QC App itself.
 
diff --git a/apps/qc-app/docs/QUALITY.md b/apps/qc-app/docs/QUALITY.md
index 81d646ed..be05bf02 100644
--- a/apps/qc-app/docs/QUALITY.md
+++ b/apps/qc-app/docs/QUALITY.md
@@ -51,7 +51,7 @@ with rationale. The high-level groups:
 | Group                                                  | Rationale                                                                |
 |--------------------------------------------------------|--------------------------------------------------------------------------|
 | `store/observations.ts`, `store/hydroserver.ts`, `store/user.ts` | Thin REST wrappers; tested via integration / E2E.                |
-| Most `components/EditData/*.vue`, all `components/FilterPoints/*.vue`, all `components/Navigation/*.vue` | One operation panel per file — heavily Vuetify-driven; mocking the v-component surface costs more than the marginal coverage. Three SFCs are unit-tested as exemplars. |
+| Most `components/EditData/*.vue`, all `components/FilterPoints/*.vue`, all `components/Navigation/*.vue` | One operation panel per file, heavily Vuetify-driven; mocking the v-component surface costs more than the marginal coverage. Three SFCs are unit-tested as exemplars. |
 | `components/VisualizeData/*.vue` (most), `pages/**`, `account/**`, `base/**` | Same Vuetify-shell rationale.                                  |
 | `utils/plotting/events.ts`, `interaction.ts`, `operations.ts`, `staging.ts` | DOM-staging / Plotly relayout seams that resist meaningful unit testing. |
 | `plugins/**`, `router/**`, `types/**`, `config/**`, `main.ts`, `*.d.ts` | Setup / declaration files with no logic.                       |
@@ -59,7 +59,7 @@ with rationale. The high-level groups:
 ### E2E
 
 Playwright specs in `e2e/`, run on **chromium and firefox only**.
-WebKit is intentionally excluded — `SharedArrayBuffer` + COOP/COEP
+WebKit is intentionally excluded: `SharedArrayBuffer` + COOP/COEP
 behavior differs in Safari and needs separate validation.
 
 Mocked specs intercept HydroServer routes via `page.route()`. The live
@@ -75,7 +75,7 @@ entrypoint so QC and Data Management share the same session.
 
 ## Areas of technical debt
 
-These are real, named, worth flagging up front. Not exhaustive — but
+These are real, named, worth flagging up front. Not exhaustive, but
 the items most likely to bite a new team in the first three months.
 
 ### 1. Result-qualifier commit path is partial
@@ -123,15 +123,15 @@ To balance the debt list:
 - **The Plotly integration is decomposed by concern.** Each file is
   short, named, and tested where testable.
 - **The QC History format is versioned**, replayable, and stable on
-  disk — this is the durability story you want.
+  disk; this is the durability story you want.
 - **CI is fast and gates the right things** (type-check, coverage,
   build).
 
 ## See also
 
-- [TESTING.md](./TESTING.md) — how to run, write, and debug tests
+- [TESTING.md](./TESTING.md): how to run, write, and debug tests
 - [ARCHITECTURE.md](./ARCHITECTURE.md)
-- [PLOTTING.md](./PLOTTING.md) — plotting-layer composition end-to-end
-- [ONBOARDING.md](./ONBOARDING.md) — documentation gaps and learning path
-- [PERFORMANCE.md](./PERFORMANCE.md) — performance characteristics
-- `vite.config.ts` — the source of truth on coverage thresholds + excludes
+- [PLOTTING.md](./PLOTTING.md): plotting-layer composition end-to-end
+- [ONBOARDING.md](./ONBOARDING.md): documentation gaps and learning path
+- [PERFORMANCE.md](./PERFORMANCE.md): performance characteristics
+- `vite.config.ts`: the source of truth on coverage thresholds + excludes
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index dfef458f..34bc60a3 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -15,7 +15,7 @@ the operator's manual; QUALITY.md is the policy.
 | Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 59 files |
 | End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 30 files |
 
-There is no separate "integration" tier — component tests live in the
+There is no separate "integration" tier; component tests live in the
 unit tier and mount real Vue components with the Vue Test Utils
 `mount()` helper, mocking only the store boundary.
 
@@ -49,11 +49,11 @@ npx vue-tsc --noEmit
 ### Running a single file or test
 
 ```bash
-# Unit — file or pattern
+# Unit: file or pattern
 npx vitest run src/utils/plotting/__tests__/options.spec.ts
 npx vitest run -t "createPlotlyOption"
 
-# E2E — file, project, repeat
+# E2E: file, project, repeat
 npx playwright test tooltip-threshold.spec.ts
 npx playwright test --project=firefox --workers=1 --headed
 npx playwright test history.spec.ts --repeat-each=5 --retries=0
@@ -69,7 +69,7 @@ For interactive debugging of a Playwright spec, prefer `--ui` over
 ### Runner config
 
 Vitest is configured in [`vite.config.ts`](../vite.config.ts) under
-the `test` key — there is no separate `vitest.config.ts`. The
+the `test` key; there is no separate `vitest.config.ts`. The
 notable knobs:
 
 - `environment: 'jsdom'` globally; `src/components/**` matches
@@ -79,7 +79,7 @@ notable knobs:
   Vitest; `setup.ts` stubs `HTMLCanvasElement.prototype.getContext`
   because jsdom doesn't implement it and Vuetify's mount cycle
   spams "Not implemented" warnings without the stub.
-- `server.deps.inline: ['vuetify']` — Vuetify ships ESM that
+- `server.deps.inline: ['vuetify']`: Vuetify ships ESM that
   Vitest's default externalization mishandles; inlining lets it
   load.
 
@@ -87,12 +87,12 @@ notable knobs:
 
 Live in `src/utils/test/`:
 
-- `pinia.ts` — `createTestPinia()` creates a fresh Pinia and calls
+- `pinia.ts`: `createTestPinia()` creates a fresh Pinia and calls
   `setActivePinia()`. No `pinia-plugin-persistedstate` is wired so
   unit tests don't read or write `localStorage`.
-- `vuetify.ts` — `createTestVuetify()` returns a default Vuetify
+- `vuetify.ts`: `createTestVuetify()` returns a default Vuetify
   instance for `mount()` plugin lists.
-- `setup.ts` — the canvas stub described above.
+- `setup.ts`: the canvas stub described above.
 
 Use them in component specs:
 
@@ -169,7 +169,7 @@ gate is:
   worth the marginal signal)
 
 The exclusion list (`coverage.exclude`) is the longest single bit
-of config in the file. Each entry is a deliberate scope decision —
+of config in the file. Each entry is a deliberate scope decision;
 read the inline comments before adding to it. The current shape:
 
 - **Untested stores** (`observations`, `hydroserver`, `user`) are
@@ -189,7 +189,7 @@ read the inline comments before adding to it. The current shape:
   `router/`, `plugins/`, `types/`, `config/`, `*.d.ts`).
 
 When you add a new file that should be tested but isn't yet,
-**do not add it to the exclude list to keep coverage green** —
+**do not add it to the exclude list to keep coverage green**;
 add a test or fail the build.
 
 ### When coverage fails
@@ -219,14 +219,14 @@ The CI gate prints uncovered line numbers per file. Common causes:
 - `fullyParallel: true` with `workers: process.env.CI ? 1 : 2`.
   The local default of 2 (instead of the Playwright default ~50%
   of cores) is **deliberate**: more workers overwhelm the shared
-  Vite dev server and starve Firefox of CPU during boot — 12 of 24
+  Vite dev server and starve Firefox of CPU during boot: 12 of 24
   specs used to time out on `waitForSelection` before this cap.
-- `retries: process.env.CI ? 2 : 1` — local runs allow one retry to
+- `retries: process.env.CI ? 2 : 1`: local runs allow one retry to
   swallow the occasional dev-server cold-start flake.
 - `projects: chromium, firefox` only. WebKit is **excluded on
   purpose**: `SharedArrayBuffer` + COOP/COEP behavior diverges in
   Safari and would need its own validation pass.
-- `baseURL: http://127.0.0.1:5173` — **never `localhost`**. The
+- `baseURL: http://127.0.0.1:5173`: **never `localhost`**. The
   backend (`playground.hydroserver.org`) CORS-allowlists
   the `127.0.0.1` origins only; using `localhost` makes API requests fail
   with `net::ERR_FAILED` and the app never mounts.
@@ -240,10 +240,10 @@ The CI gate prints uncovered line numbers per file. Common causes:
 e2e/
 ├── support/
 │   ├── app.ts        : flow helpers (gotoHome, setupEditView, startSessionFromRow, waitForEditorReady, openOp, waitForSelection)
-│   ├── fixtures.ts   — workspace / datastream / observation fixtures
-│   ├── mocks.ts      — page.route() handlers that stand in for HydroServer
-│   └── ops.ts        — op-specific preambles (selectAllPoints, expectHistoryContains)
-└── *.spec.ts         — one file per feature
+│   ├── fixtures.ts   : workspace / datastream / observation fixtures
+│   ├── mocks.ts      : page.route() handlers that stand in for HydroServer
+│   └── ops.ts        : op-specific preambles (selectAllPoints, expectHistoryContains)
+└── *.spec.ts         : one file per feature
 ```
 
 ### How a spec is structured
@@ -339,7 +339,7 @@ function observationsWithGap() {
 `import.meta.env.DEV` or `VITE_APP_E2E_HOOKS` is set. Today it
 exposes one helper:
 
-- `waitForSelectedData(minLength, timeoutMs)` — resolves when the
+- `waitForSelectedData(minLength, timeoutMs)`: resolves when the
   Pinia store's `selectedData` has at least `minLength` entries.
   Used by `support/app.ts#waitForSelection`.
 
@@ -400,7 +400,7 @@ them so the next contributor doesn't rediscover them.
 Playwright's HTML report (`playwright-report/index.html`) opens
 automatically after a local run. Per-failure trace zips
 (`test-results/<name>/trace.zip`) can be replayed with
-`npx playwright show-trace <path>` — they include screenshots, DOM
+`npx playwright show-trace <path>`. They include screenshots, DOM
 snapshots, console logs, and network traffic at every step. This
 is the single most useful debugging tool the suite has.
 
@@ -421,9 +421,9 @@ patterns above.
 `.github/workflows/ci.yml` runs on every push and on every PR
 targeting `main`. The job:
 
-1. `npx vue-tsc --noEmit` — type-check.
-2. `npm run coverage` — Vitest with the 80%/78% gates above.
-3. `npm run build` — Vite production build.
+1. `npx vue-tsc --noEmit`: type-check.
+2. `npm run coverage`: Vitest with the 80%/78% gates above.
+3. `npm run build`: Vite production build.
 
 ---
 
@@ -436,7 +436,7 @@ targeting `main`. The job:
    `setActivePinia(createPinia())` (or `createTestPinia()`) in
    `beforeEach`.
 3. Mock collaborator stores with `vi.mock(...)` and ref-backed
-   fakes — see `selected.spec.ts` for a representative shape.
+   fakes (see `selected.spec.ts` for a representative shape).
 4. Run `npx vitest run <file>` until green; run
    `npm run coverage` once to verify the gate still passes.
 
@@ -446,7 +446,7 @@ targeting `main`. The job:
 2. `mount()` the component with
    `global: { plugins: [createTestVuetify()] }`.
 3. Mock `@/store/*` and `@/composables/*` imports with refs you
-   control from the spec — never import the real stores in a
+   control from the spec; never import the real stores in a
    component test.
 4. If the component imports `@uwrl/qc-utils`, mock the enum
    constants it uses (see `EditHistory.spec.ts` line 28-77 for
@@ -471,12 +471,12 @@ targeting `main`. The job:
 ### A new test hook
 
 1. Add the implementation in `src/testHooks.ts` under the
-   `installTestHooks()` function — same registration pattern as
+   `installTestHooks()` function, same registration pattern as
    `waitForSelectedData`.
 2. Update the `Window['__vbwTestHooks']` interface in
    `e2e/support/app.ts` so spec callers get type-checking.
 3. Hooks ship only when `import.meta.env.DEV` or
-   `VITE_APP_E2E_HOOKS` is set — production builds never expose
+   `VITE_APP_E2E_HOOKS` is set; production builds never expose
    them.
 
 ---
@@ -492,7 +492,7 @@ When a Vuetify upgrade lands:
 
 When a `qc-utils` upgrade lands:
 
-- Re-run unit specs that mock `@uwrl/qc-utils` enums — the
+- Re-run unit specs that mock `@uwrl/qc-utils` enums. The
   canonical sites are `EditHistory.spec.ts` and any spec under
   `src/utils/plotting/__tests__/` that imports the enums.
 - If a new `HistoryItem` field is added, the qc-app's `EditHistory`
@@ -509,7 +509,7 @@ debounced path:
 
 When a Playwright spec starts flaking:
 
-- Check the trace.zip first — it almost always shows the cause.
+- Check the trace.zip first; it almost always shows the cause.
 - If it's `waitForSelection` timing out, the suspect is a
   `fill('...').click(button)` pattern; switch to
   `field.press('Enter')`.
@@ -528,9 +528,9 @@ When coverage drops below threshold after a code change:
 
 ## See also
 
-- [QUALITY.md](./QUALITY.md) — what is covered, what isn't, and why
-- [ONBOARDING.md](./ONBOARDING.md) — the wider learning path
-- [`vite.config.ts`](../vite.config.ts) — source of truth on
+- [QUALITY.md](./QUALITY.md): what is covered, what isn't, and why
+- [ONBOARDING.md](./ONBOARDING.md): the wider learning path
+- [`vite.config.ts`](../vite.config.ts): source of truth on
   Vitest config and coverage policy
-- [`playwright.config.ts`](../playwright.config.ts) — source of
+- [`playwright.config.ts`](../playwright.config.ts): source of
   truth on Playwright config
diff --git a/apps/qc-app/e2e/all-operations.spec.ts b/apps/qc-app/e2e/all-operations.spec.ts
index 7ca779f0..0caa5734 100644
--- a/apps/qc-app/e2e/all-operations.spec.ts
+++ b/apps/qc-app/e2e/all-operations.spec.ts
@@ -2,7 +2,7 @@
  * Single-session smoke that walks one datastream through every
  * filter / edit / add operation in order. Each step is performed
  * manually (open panel → fill controls → click commit), one after
- * another, against the same series — so any state pollution between
+ * another, against the same series, so any state pollution between
  * ops (stale selections, leftover box-select rectangles, indices
  * shifted by a prior insert/delete) surfaces as a failure on the
  * next op rather than going unnoticed.
@@ -14,7 +14,7 @@
  *     post-action highlight).
  *   - Edit ops that read indices from `history[length - 2].selected`
  *     silently no-op'd if a prior op left a non-SELECTION entry
- *     there — the panels now dispatch `[SELECTION, OP]` atomically.
+ *     there. The panels now dispatch `[SELECTION, OP]` atomically.
  */
 
 import { expect, test, type Page } from '@playwright/test'
@@ -51,7 +51,7 @@ function observationsWithGap() {
 
 /** Re-seed a contiguous selection spanning the full series. Uses
  *  Datetime Range (live-commit, no Apply button) so the selection is
- *  guaranteed to be one consecutive block — Drift Correction needs
+ *  guaranteed to be one consecutive block: Drift Correction needs
  *  groups of length > 1 and Value Threshold can yield non-contiguous
  *  hits once intermediate edits have shifted Y values around. */
 async function seedWideSelection(page: Page) {
@@ -111,7 +111,7 @@ test.describe('all operations: single-session walkthrough', () => {
     // --- Add: Fill Gaps ----------------------------------------------
     // Fill the deliberate ~4h gap with 15-minute samples. After the
     // commit, recordPostActionSelection lands an inserted-indices
-    // SELECTION in history — the next edit must NOT silently
+    // SELECTION in history, and the next edit must NOT silently
     // operate on those indices.
     await openOp(page, 'fillGaps')
     const fillPanel = page.getByTestId('operation-panel-fillGaps')
@@ -121,7 +121,7 @@ test.describe('all operations: single-session walkthrough', () => {
     await fillPanel.getByRole('button', { name: /fill gaps/i }).click()
     await expectHistoryContains(page, 'Fill Gaps')
 
-    // --- Edit: Change Values (after Fill Gaps — regression guard) ---
+    // --- Edit: Change Values (after Fill Gaps, regression guard) ---
     // Re-seed a fresh wide selection so we're operating on real data,
     // not the post-fill auto-selection. The dispatch chain in
     // ChangeValues.vue now anchors the SELECTION explicitly, so this
diff --git a/apps/qc-app/e2e/edit-fill-gaps.spec.ts b/apps/qc-app/e2e/edit-fill-gaps.spec.ts
index 400469cd..d6ebafc2 100644
--- a/apps/qc-app/e2e/edit-fill-gaps.spec.ts
+++ b/apps/qc-app/e2e/edit-fill-gaps.spec.ts
@@ -11,7 +11,7 @@ import { expectHistoryContains } from './support/ops'
 import { FIXTURE_OBS_START_MS } from './support/fixtures'
 
 function observationsWithGap() {
-  // See FIXTURE_OBS_START_MS — anchored to "now" so the series sits
+  // See FIXTURE_OBS_START_MS: anchored to "now" so the series sits
   // inside the QC app's default time range.
   const startMs = FIXTURE_OBS_START_MS
   const spacingMs = 15 * 60 * 1000
diff --git a/apps/qc-app/e2e/filter-datetime-range.spec.ts b/apps/qc-app/e2e/filter-datetime-range.spec.ts
index c6f71aaa..366ce279 100644
--- a/apps/qc-app/e2e/filter-datetime-range.spec.ts
+++ b/apps/qc-app/e2e/filter-datetime-range.spec.ts
@@ -1,7 +1,7 @@
 /**
  * Datetime-range filter: selects points whose phenomenonTime falls
  * inside [from, to] using the DatePickerField inputs. The panel is
- * live-commit — opening it seeds a selection that spans the full
+ * live-commit: opening it seeds a selection that spans the full
  * series range, without any Apply-filter button to press.
  */
 
@@ -21,7 +21,7 @@ test.describe('filter: datetime range', () => {
     await openOp(page, 'datetimeRange')
     // Live-commit: the RangeStager pushes a selection as soon as it
     // resolves its initial range, so we just wait for the selection
-    // to populate — no Apply-filter button exists.
+    // to populate (no Apply-filter button exists).
     await waitForSelection(page, 1)
 
     // Sanity: the "N points selected in range." label appears after
diff --git a/apps/qc-app/e2e/filter-gaps.spec.ts b/apps/qc-app/e2e/filter-gaps.spec.ts
index 6fe22bba..faf47cb3 100644
--- a/apps/qc-app/e2e/filter-gaps.spec.ts
+++ b/apps/qc-app/e2e/filter-gaps.spec.ts
@@ -1,10 +1,10 @@
 /**
  * Gaps filter: surfaces timestamps where Δx between adjacent points
  * exceeds a threshold. The panel is live-commit via `GapFinder`'s
- * `auto-select-endpoints` prop — no Apply button, filling the
+ * `auto-select-endpoints` prop: no Apply button, filling the
  * Amount input is enough to seed a selection once a gap is found.
  * The default fixture has a perfectly uniform 15-minute grid so a
- * small threshold still finds nothing — instead we use a custom
+ * small threshold still finds nothing. Instead we use a custom
  * fixture with one deliberate gap.
  */
 
@@ -14,7 +14,7 @@ import { openOp, setupEditView, waitForSelection } from './support/app'
 import { FIXTURE_OBS_START_MS } from './support/fixtures'
 
 function observationsWithGap() {
-  // See FIXTURE_OBS_START_MS — anchored to "now" so the series sits
+  // See FIXTURE_OBS_START_MS: anchored to "now" so the series sits
   // inside the QC app's default time range.
   const startMs = FIXTURE_OBS_START_MS
   const spacingMs = 15 * 60 * 1000
diff --git a/apps/qc-app/e2e/filter-persistence.spec.ts b/apps/qc-app/e2e/filter-persistence.spec.ts
index 4a17b71a..0e4f4917 100644
--- a/apps/qc-app/e2e/filter-persistence.spec.ts
+++ b/apps/qc-app/e2e/filter-persistence.spec.ts
@@ -1,7 +1,7 @@
 /**
  * Persistence filter: flags runs of identical consecutive Y values
  * that repeat at least N times. The default fixture is sinusoidal
- * so no run exists — we build a step-function series with a 5-point
+ * so no run exists. We build a step-function series with a 5-point
  * plateau to give the filter something to match.
  */
 
@@ -11,7 +11,7 @@ import { openOp, setupEditView, waitForSelection } from './support/app'
 import { FIXTURE_OBS_START_MS } from './support/fixtures'
 
 function observationsWithPlateau() {
-  // See FIXTURE_OBS_START_MS — anchored to "now" so the series sits
+  // See FIXTURE_OBS_START_MS: anchored to "now" so the series sits
   // inside the QC app's default time range.
   const startMs = FIXTURE_OBS_START_MS
   const spacingMs = 15 * 60 * 1000
@@ -33,13 +33,13 @@ test.describe('filter: persistence', () => {
 
   test('flags the 5-point plateau as persistent', async ({ page }) => {
     await openOp(page, 'persistence')
-    // Default threshold is 2 — accept anything >= 3 for the plateau.
+    // Default threshold is 2; accept anything >= 3 for the plateau.
     await page.locator('input[type="number"]').fill('3')
     await page.getByRole('button', { name: /apply filter/i }).click()
     await waitForSelection(page, 1)
 
     // Filter entry may be collapsed to a rolled-up "Selection" row after
-    // dispatchSelection fires — accept either as proof the filter ran.
+    // dispatchSelection fires; accept either as proof the filter ran.
     const row = page
       .locator('[data-testid^="history-item-"]')
       .filter({ hasText: /Persistence|Selection/ })
diff --git a/apps/qc-app/e2e/filter-range-toggle.spec.ts b/apps/qc-app/e2e/filter-range-toggle.spec.ts
index 4ae77182..6a14f32f 100644
--- a/apps/qc-app/e2e/filter-range-toggle.spec.ts
+++ b/apps/qc-app/e2e/filter-range-toggle.spec.ts
@@ -28,12 +28,12 @@ test.describe('filter range toggle', () => {
     await expect(page.locator(dateRangeSection)).toHaveCount(0)
     await expect(page.locator(rangePanel)).toHaveCount(0)
 
-    // Open Persistence — section appears in the empty state by default.
+    // Open Persistence: section appears in the empty state by default.
     await openOp(page, 'persistence')
     await expect(page.locator(enableBtn)).toBeVisible()
     await expect(page.locator(rangePanel)).toHaveCount(0)
 
-    // Enable — panel mounts and X icon replaces the empty state.
+    // Enable: panel mounts and X icon replaces the empty state.
     await page.locator(enableBtn).click()
     await expect(page.locator(rangePanel)).toBeVisible()
     await expect(page.locator(disableBtn)).toBeVisible()
@@ -46,7 +46,7 @@ test.describe('filter range toggle', () => {
     await page.locator(enableBtn).click()
     await expect(page.locator(rangePanel)).toBeVisible()
 
-    // Switch to datetime-range — that op brings its own picker, so the
+    // Switch to datetime-range: that op brings its own picker, so the
     // shared section and panel both go away.
     await openOp(page, 'datetimeRange')
     // Only the operation section remains, not the date-range section.
diff --git a/apps/qc-app/e2e/filter-rate-of-change.spec.ts b/apps/qc-app/e2e/filter-rate-of-change.spec.ts
index 47677f84..98665bfd 100644
--- a/apps/qc-app/e2e/filter-rate-of-change.spec.ts
+++ b/apps/qc-app/e2e/filter-rate-of-change.spec.ts
@@ -23,7 +23,7 @@ test.describe('filter: rate of change', () => {
     await waitForSelection(page, 1)
 
     // Filter entry may be collapsed to a rolled-up "Selection" row after
-    // dispatchSelection fires — accept either as proof the filter ran.
+    // dispatchSelection fires; accept either as proof the filter ran.
     const row = page
       .locator('[data-testid^="history-item-"]')
       .filter({ hasText: /Rate Of Change|Selection/ })
diff --git a/apps/qc-app/e2e/filter-value-threshold.spec.ts b/apps/qc-app/e2e/filter-value-threshold.spec.ts
index 0751dad1..35ed1db8 100644
--- a/apps/qc-app/e2e/filter-value-threshold.spec.ts
+++ b/apps/qc-app/e2e/filter-value-threshold.spec.ts
@@ -4,7 +4,7 @@
  *   - "Add filter" dispatches VALUE_THRESHOLD, populates selectedData,
  *     and adds a single replaceable filter history row
  *   - a second "Add filter" call REPLACES the filter entry (doesn't
- *     stack) — the app's dispatch collapses adjacent filter entries
+ *     stack), since the app's dispatch collapses adjacent filter entries
  */
 
 import { expect, test } from '@playwright/test'
@@ -50,7 +50,7 @@ test.describe('filter: value threshold', () => {
     await page.getByLabel('Value').fill('9')
     await page.getByRole('button', { name: /add filter/i }).click()
 
-    // Only a single filter row should remain in history — the second
+    // Only a single filter row should remain in history: the second
     // add-filter replaces the first in-place rather than stacking.
     // Accept either "Value Threshold" or a rolled-up "Selection"
     // label (both represent a single filter-class entry).
diff --git a/apps/qc-app/e2e/history.spec.ts b/apps/qc-app/e2e/history.spec.ts
index 40161659..16b9fa3a 100644
--- a/apps/qc-app/e2e/history.spec.ts
+++ b/apps/qc-app/e2e/history.spec.ts
@@ -20,7 +20,7 @@ test.describe('edit history toolbar', () => {
     await setupEditView(page)
     await selectAllPoints(page)
     await openOp(page, 'changeValues')
-    // Press Enter on the field instead of clicking Apply — same reason
+    // Press Enter on the field instead of clicking Apply, for the same reason
     // as the selectAllPoints helper: Firefox occasionally fires the
     // Apply click before Vuetify's v-text-field commits, which leaves
     // the button's `:disabled` guard tripped and silently swallows the
@@ -38,7 +38,7 @@ test.describe('edit history toolbar', () => {
     // Snapshot the post-edit row count. Edit ops emit a trailing
     // SELECTION through `recordPostActionSelection` (see
     // useFilterDispatch), so the exact count depends on whether that
-    // marker landed — we just want round-trip invariance, not a fixed
+    // marker landed. We just want round-trip invariance, not a fixed
     // integer.
     const rows = page.locator('[data-testid^="history-item-"]')
     const initial = await rows.count()
diff --git a/apps/qc-app/e2e/navigation.spec.ts b/apps/qc-app/e2e/navigation.spec.ts
index f3eefae4..d22010b6 100644
--- a/apps/qc-app/e2e/navigation.spec.ts
+++ b/apps/qc-app/e2e/navigation.spec.ts
@@ -48,7 +48,7 @@ test.describe('navigation', () => {
   })
 
   test('picking a workspace lands the user on Home', async ({ page }) => {
-    // Don't use `gotoHome` here — it pre-seeds localStorage and skips
+    // Don't use `gotoHome` here: it pre-seeds localStorage and skips
     // the picker. We want to exercise the actual pick flow.
     await page.goto('/')
     const pickButton = page
diff --git a/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts b/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts
index 8402c5d0..c7011d45 100644
--- a/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts
+++ b/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts
@@ -2,7 +2,7 @@
  * Plotting policy for datastreams without a declared `intendedTimeSpacing`.
  *
  * Two related guarantees, both covered here:
- *   1. The main trace renders as a pure scatter — `mode: 'markers'` with
+ *   1. The main trace renders as a pure scatter: `mode: 'markers'` with
  *      no companion `_gapOverlayFor` lines trace pushed alongside it.
  *   2. The "data points" toggle (manual mode + click off) does not hide
  *      the markers of such a series. Without a line fallback, honouring
@@ -57,7 +57,7 @@ function withoutIntendedSpacing(ds: DatastreamRecord): DatastreamRecord {
   // Mirror what the backend serves for an older / minimally-configured
   // datastream: both spacing fields cleared. The QC app's
   // `spacingMsFromDatastream` returns null for either missing field, so
-  // wiping the unit alone would already trip the scatter path — clearing
+  // wiping the unit alone would already trip the scatter path; clearing
   // both keeps the fixture honest about the upstream shape.
   return { ...ds, intendedTimeSpacing: null, intendedTimeSpacingUnit: null }
 }
@@ -145,7 +145,7 @@ test.describe('plot: datastream without intendedTimeSpacing', () => {
     expect(trace.mainMode).toBe('markers')
     expect(trace.overlayExists).toBe(false)
     expect(trace.anyGapOverlay).toBe(false)
-    // Initial paint must keep the scatter markers fully opaque —
+    // Initial paint must keep the scatter markers fully opaque;
     // otherwise the series has nothing visible on screen.
     expect(trace.mainMarkerOpacity).toBe(1)
   })
@@ -158,7 +158,7 @@ test.describe('plot: datastream without intendedTimeSpacing', () => {
 
     // The toggle button only renders in manual mode; default is auto,
     // so open the dropdown and pick manual first. Picking manual also
-    // fires `handleRelayout(null)` internally — that pass is the one
+    // fires `handleRelayout(null)` internally, and that pass is the one
     // that would have wiped the markers if the scatter exemption were
     // missing.
     await page.getByTestId('tooltips-mode-btn').click()
diff --git a/apps/qc-app/e2e/qc-golden-path.spec.ts b/apps/qc-app/e2e/qc-golden-path.spec.ts
index 6ac8e116..3fa0e2d7 100644
--- a/apps/qc-app/e2e/qc-golden-path.spec.ts
+++ b/apps/qc-app/e2e/qc-golden-path.spec.ts
@@ -39,7 +39,7 @@ async function loginThroughDataManagementIfNeeded(page: Page) {
 //
 // Steps:
 //   0. Enter via Data Management's same-origin /qc/ route; log in if needed.
-//   1. Navigate to app root — a fresh browser has no persisted workspace
+//   1. Navigate to app root. A fresh browser has no persisted workspace
 //      selection, so the router's workspace guard redirects to /workspaces.
 //   2. Select "Test Workspace #2" from the picker.
 //   3. Wait for datastreams table (init-complete signal on Home).
@@ -54,7 +54,7 @@ async function loginThroughDataManagementIfNeeded(page: Page) {
 // Needs a workspace source datastream with a managed datastream and QC
 // history; without one there is no session to Save or Commit.
 //
-// Live smoke — talks to the backend configured behind Data Management's
+// Live smoke: talks to the backend configured behind Data Management's
 // /api proxy. Opt in with
 // `npm run e2e:live` (E2E_LIVE=1 playwright test qc-golden-path --headed)
 // so CI runs the fast mocked suite by default; the live smoke is for
@@ -71,12 +71,12 @@ test.describe('QC golden path (live same-origin)', () => {
     // flow room to breathe on a cold dev server.
     test.setTimeout(180_000)
 
-    // Step 0 — Enter QC through the Data Management origin. If the browser
+    // Step 0: Enter QC through the Data Management origin. If the browser
     // has no valid session, Data Management handles the login page and
     // redirects back to /qc/ via the `next` query.
     await loginThroughDataManagementIfNeeded(page)
 
-    // Step 1 & 2 — Navigate to app root. Fresh browser has no stored
+    // Step 1 & 2: Navigate to app root. Fresh browser has no stored
     // workspace, so the guard redirects to /workspaces. Pick
     // "Test Workspace #2" (pinned because the datastream referenced
     // below lives there). The picker loads the workspace list
@@ -89,7 +89,7 @@ test.describe('QC golden path (live same-origin)', () => {
     await expect(pickButton).toBeVisible({ timeout: 60_000 })
     await pickButton.click()
 
-    // Step 3 — Wait for datastreams table. App.vue's onMounted fetches
+    // Step 3: Wait for datastreams table. App.vue's onMounted fetches
     // the workspace catalog; the table becomes visible once the fetch
     // settles.
     await expect(page.getByTestId('datastreams-table')).toBeVisible({
@@ -123,10 +123,10 @@ test.describe('QC golden path (live same-origin)', () => {
     await expect(saveBtn).toBeVisible({ timeout: 60_000 })
     await expect(page.getByText('Filter Data')).toBeVisible()
 
-    // Step 6 — Open "Value thresholds" operation panel, add a filter.
+    // Step 6: Open "Value thresholds" operation panel, add a filter.
     // Conservative value (0) chosen per plan 01-03 to ensure the threshold
     // selects at least some points against arbitrary live observations.
-    // The drawer row carries both the title text and a per-op testid —
+    // The drawer row carries both the title text and a per-op testid;
     // use the testid so we don't collide with the panel header that
     // renders the same title after the click.
     await page.getByTestId('op-valueThreshold').click()
@@ -135,10 +135,10 @@ test.describe('QC golden path (live same-origin)', () => {
     ).toBeVisible()
     await page.getByLabel('Value').fill('0')
     await page.getByRole('button', { name: 'Add filter' }).click()
-    // Panel dismissal isn't needed — selecting another drawer item
+    // Panel dismissal isn't needed: selecting another drawer item
     // swaps the rendered operation in the shared OperationPanel.
 
-    // Step 7 — Apply a "Change values" edit.
+    // Step 7: Apply a "Change values" edit.
     // Wait for the ValueThreshold filter to populate selectedData via the
     // app-side test hook installed from main.ts. The hook registers a Pinia
     // `watch` on useDataVisStore().selectedData and resolves as soon as
@@ -157,7 +157,7 @@ test.describe('QC golden path (live same-origin)', () => {
     await page.getByLabel('Value').fill('1')
     await page.getByRole('button', { name: 'Apply' }).click()
 
-    // Step 8 — Assert the edit landed in EditHistory. Filters now emit
+    // Step 8: Assert the edit landed in EditHistory. Filters now emit
     // their own history rows, so we can't pin to history-item-0; the
     // CHANGE_VALUES row lands after the ValueThreshold one. Find the
     // row by text instead. EditHistory formats ALL_CAPS_SNAKE as
@@ -169,7 +169,7 @@ test.describe('QC golden path (live same-origin)', () => {
 
     // Dev-only sanity check: the calibration layer attached an
     // execution-mode chip ("inline" or "worker") to the history row.
-    // This asserts the chip rendered without pinning its content —
+    // This asserts the chip rendered without pinning its content;
     // either is a legitimate calibration outcome depending on the
     // device running the test.
     const modeChip = changeRow.locator('.edit-history__mode-chip')
diff --git a/apps/qc-app/e2e/qc-history.spec.ts b/apps/qc-app/e2e/qc-history.spec.ts
index e801a057..a37310ac 100644
--- a/apps/qc-app/e2e/qc-history.spec.ts
+++ b/apps/qc-app/e2e/qc-history.spec.ts
@@ -1,5 +1,5 @@
 /**
- * QC History — round-trip e2e.
+ * QC History: round-trip e2e.
  *
  * Apply two operations (a value-threshold filter + a delete edit
  * that consumes its selection), use the EditHistory header's
@@ -37,7 +37,7 @@ test.describe('QC history: save / load round-trip', () => {
 
     // History should now have the filter + edit pair (the
     // VALUE_THRESHOLD filter, the implicit SELECTION it produced,
-    // and the DELETE_POINTS edit — selection-coupled ops keep the
+    // and the DELETE_POINTS edit; selection-coupled ops keep the
     // SELECTION between them).
     await expectHistoryContains(page, 'Delete Points')
     const beforeRows = page.locator('[data-testid^="history-item-"]')
diff --git a/apps/qc-app/e2e/screenshots.spec.ts b/apps/qc-app/e2e/screenshots.spec.ts
index 0e735df0..3fef9408 100644
--- a/apps/qc-app/e2e/screenshots.spec.ts
+++ b/apps/qc-app/e2e/screenshots.spec.ts
@@ -16,7 +16,7 @@
  * Most captures use a Locator so each PNG is tightly cropped to the
  * region the doc actually references. The viewport is intentionally
  * tall so operation panels render their full body without internal
- * scroll — the locator screenshot would otherwise clip whatever fell
+ * scroll, since the locator screenshot would otherwise clip whatever fell
  * outside the panel's visible area.
  */
 
@@ -49,7 +49,7 @@ const OUT = path.resolve(__dirname, '..', 'docs', 'images')
  * Wait for the Plotly chart in the page (if any) to finish its
  * post-mount layout. `handleNewPlot` runs inside a 200 ms setTimeout
  * in `Plot.vue`'s mount hook, and a ResizeObserver fires a frame
- * later to size the canvas to its container — both happen *after*
+ * later to size the canvas to its container. Both happen *after*
  * `setupEditView`'s "Filter Data is visible" gate. Without this
  * extra wait the screenshot captures the initial undersized layout,
  * leaving an empty stripe between the right edge of the plotting
@@ -65,11 +65,11 @@ async function waitForPlotLayoutSettled(page: Page) {
         | null
       if (!gd) return true
       // The plot div stays in the DOM behind the Table tab but its
-      // offsetWidth collapses to 0 — treat that as settled too.
+      // offsetWidth collapses to 0; treat that as settled too.
       if (gd.offsetWidth === 0) return true
       const layout = gd._fullLayout
       if (!layout?.width) return false
-      // Allow a few pixels of slop — Plotly rounds.
+      // Allow a few pixels of slop (Plotly rounds).
       return Math.abs(layout.width - gd.offsetWidth) <= 4
     },
     undefined,
@@ -106,7 +106,7 @@ const SHOULD_CAPTURE = process.env.CAPTURE_SCREENSHOTS === '1'
 test.describe('docs screenshots', () => {
   test.skip(
     !SHOULD_CAPTURE,
-    'Opt-in only — set CAPTURE_SCREENSHOTS=1 to regenerate docs/images/ PNGs.'
+    'Opt-in only: set CAPTURE_SCREENSHOTS=1 to regenerate docs/images/ PNGs.'
   )
 
   test('login page', async ({ page }) => {
@@ -246,7 +246,7 @@ test.describe('docs screenshots', () => {
   ] as const
 
   for (const id of filterOps) {
-    test(`filter panel — ${id}`, async ({ page }) => {
+    test(`filter panel: ${id}`, async ({ page }) => {
       await page.setViewportSize(TALL_VIEWPORT)
       await installMocks(page, { qcHistories: true })
       await setupEditView(page)
@@ -265,7 +265,7 @@ test.describe('docs screenshots', () => {
   ] as const
 
   for (const id of editOps) {
-    test(`edit panel — ${id}`, async ({ page }) => {
+    test(`edit panel: ${id}`, async ({ page }) => {
       await page.setViewportSize(TALL_VIEWPORT)
       await installMocks(page, { qcHistories: true })
       await setupEditView(page)
@@ -290,7 +290,7 @@ test.describe('docs screenshots', () => {
     await snapEl(panel, 'panel-date-range-mask.png')
   })
 
-  test('add panel — qualifyingComments', async ({ page }) => {
+  test('add panel: qualifyingComments', async ({ page }) => {
     await page.setViewportSize(TALL_VIEWPORT)
     await installMocks(page, { qcHistories: true })
     await setupEditView(page)
@@ -300,7 +300,7 @@ test.describe('docs screenshots', () => {
     await snapEl(panel, 'panel-qualifyingComments.png')
   })
 
-  test('add panel — addPoints', async ({ page }) => {
+  test('add panel: addPoints', async ({ page }) => {
     await page.setViewportSize(TALL_VIEWPORT)
     await installMocks(page, { qcHistories: true })
     await setupEditView(page)
@@ -309,7 +309,7 @@ test.describe('docs screenshots', () => {
     await snapEl(panel, 'panel-addPoints.png')
   })
 
-  test('add panel — fillGaps', async ({ page }) => {
+  test('add panel: fillGaps', async ({ page }) => {
     await page.setViewportSize(TALL_VIEWPORT)
     await installMocks(page, { qcHistories: true })
     await setupEditView(page)
diff --git a/apps/qc-app/e2e/share-url.spec.ts b/apps/qc-app/e2e/share-url.spec.ts
index a5b66cfa..cc0c7752 100644
--- a/apps/qc-app/e2e/share-url.spec.ts
+++ b/apps/qc-app/e2e/share-url.spec.ts
@@ -168,7 +168,7 @@ test.describe('share URL', () => {
     await setupEditView(page)
     await waitForPlotMeta(page)
 
-    // Drop a wheel event over the *left gutter* — the band between
+    // Drop a wheel event over the *left gutter*, the band between
     // the plot edge and the primary Y axis. Our custom wheel handler
     // routes that zone to the primary y axis only, so the resulting
     // relayout updates `yaxis.range` without touching X. That sweep
@@ -377,7 +377,7 @@ test.describe('share URL', () => {
     // default 30 s under worker contention.
     test.setTimeout(90_000)
     // Bug (options.ts:689): on a fresh mount with a URL `z=`, Plotly
-    // pins `_rangeInitial0/1` to the URL view — stock Reset bounces
+    // pins `_rangeInitial0/1` to the URL view, so stock Reset bounces
     // back there instead of the data extent. A cold goto to a zoomed
     // URL reproduces the same condition as the user-reported
     // zoom→reload flow. Only a plotted datastream is needed.
@@ -396,7 +396,7 @@ test.describe('share URL', () => {
     expect(hydratedSpan).toBeLessThan(fullSpan * 0.5)
 
     // Pan drag exercises Plotly's user-gesture path
-    // (`_storeDirectGUIEdit`), which re-anchors `_rangeInitial` —
+    // (`_storeDirectGUIEdit`), which re-anchors `_rangeInitial`;
     // the custom Reset button has to override that.
     await page.mouse.move(
       afterHydrate!.cursorClient.x,
diff --git a/apps/qc-app/e2e/support/app.ts b/apps/qc-app/e2e/support/app.ts
index 22df5ff6..6d2770de 100644
--- a/apps/qc-app/e2e/support/app.ts
+++ b/apps/qc-app/e2e/support/app.ts
@@ -27,7 +27,7 @@ export async function waitForHomeReady(page: Page): Promise<void> {
  * entirely. Relying on the UI to click the Select button is fragile
  * cross-browser (Firefox in particular sometimes swallows the click
  * when the v-list-item row and its nested Select button both register
- * click handlers). Seeding storage sidesteps the picker entirely —
+ * click handlers). Seeding storage sidesteps the picker entirely,
  * equivalent to a user who already chose a workspace in a previous
  * session.
  *
@@ -54,7 +54,7 @@ export async function seedWorkspaceSelection(page: Page): Promise<void> {
           JSON.stringify({ selectedWorkspace: ws })
         )
       } catch {
-        // storage disabled — fall back to UI flow
+        // storage disabled: fall back to UI flow
       }
     },
     { ws: workspace }
diff --git a/apps/qc-app/e2e/support/fixtures.ts b/apps/qc-app/e2e/support/fixtures.ts
index df748256..493032a2 100644
--- a/apps/qc-app/e2e/support/fixtures.ts
+++ b/apps/qc-app/e2e/support/fixtures.ts
@@ -2,7 +2,7 @@
  * Fixture data for the mocked HydroServer backend.
  *
  * These shapes intentionally carry only the fields the QC app reads at
- * render time — enough to light up the workspace picker, datastream
+ * render time: enough to light up the workspace picker, datastream
  * table, plot, and every filter / edit operation. Fields the server
  * would normally include but which the app ignores (audit stamps,
  * embedded documents, etc.) are omitted to keep the fixtures small.
diff --git a/apps/qc-app/e2e/support/mocks.ts b/apps/qc-app/e2e/support/mocks.ts
index aa972ecb..d906d956 100644
--- a/apps/qc-app/e2e/support/mocks.ts
+++ b/apps/qc-app/e2e/support/mocks.ts
@@ -163,7 +163,7 @@ export async function installMocks(
   ).map((ds) => ({ ...ds, ...overrides[ds.id] }))
 
   // Match only real HydroServer API calls by pathname. A bare `**/api/**`
-  // glob also catches the dev server's own source modules — the QC app
+  // glob also catches the dev server's own source modules. The QC app
   // aliases `@hydroserver/client` to `packages/hydroserver-ts/src`, whose
   // files live under `.../src/api/...` and are served from
   // `/qc/@fs/.../src/api/runtime.ts`. Those URLs contain `/api/` but their
@@ -172,7 +172,7 @@ export async function installMocks(
   // requests always have a pathname that starts with `/api/`.
   const isApiRequest = (url: URL): boolean => url.pathname.startsWith('/api/')
 
-  // Preflights for anything — the real server serves OPTIONS via
+  // Preflights for anything: the real server serves OPTIONS via
   // middleware; swallowing them here keeps the mocks happy.
   await page.route(isApiRequest, async (route) => {
     const request = route.request()
@@ -196,7 +196,7 @@ export async function installMocks(
       })
     }
     if (path.includes('/api/auth/')) {
-      // Any other auth endpoint (providers, redirects) — return OK.
+      // Any other auth endpoint (providers, redirects): return OK.
       return json(route, { status: 200, data: {}, meta: { is_authenticated: authenticated } })
     }
 
@@ -221,7 +221,7 @@ export async function installMocks(
       // cache-extension logic in `fetchObservationsInRange` (which
       // re-fetches the segment outside its cached window every time
       // the range moves) would receive the full fixture series on
-      // each call and stack duplicates into the ObservationRecord —
+      // each call and stack duplicates into the ObservationRecord,
       // visible as wrong point counts and a long phantom line
       // connecting the first and last observations.
       const tMin = parseISOorNull(params.get('phenomenon_time_min'))
@@ -296,7 +296,7 @@ export async function installMocks(
     }
 
     // --- Tags / attachments / other sub-resources the app may touch
-    //     in DatastreamInformationCard — return empty arrays so the
+    //     in DatastreamInformationCard: return empty arrays so the
     //     UI renders without errors.
     if (path.includes('/tags') || path.includes('/attachments')) {
       return json(route, { data: [] })
diff --git a/apps/qc-app/e2e/tooltip-threshold.spec.ts b/apps/qc-app/e2e/tooltip-threshold.spec.ts
index cd1aac16..3e929a38 100644
--- a/apps/qc-app/e2e/tooltip-threshold.spec.ts
+++ b/apps/qc-app/e2e/tooltip-threshold.spec.ts
@@ -5,7 +5,7 @@
  *      counter in place of the toggle button. The toggle button is
  *      hidden because the threshold drives the on/off state.
  *   2. Switching to "Manual toggle" via the caret menu hides the
- *      counter and renders the icon toggle button — clicking it
+ *      counter and renders the icon toggle button; clicking it
  *      flips data-points hover on/off.
  *   3. The mode menu carries the threshold form when auto is active:
  *      typing buffers, Apply / Enter commit, Escape (which closes
@@ -21,7 +21,7 @@ import { setupEditView, waitForEditorReady } from './support/app'
 /**
  * Pull the threshold number out of the auto-mode counter text. The
  * counter renders as "{count}/{threshold}" with locale-formatted
- * numbers — strip the comma group separators before parsing.
+ * numbers, so strip the comma group separators before parsing.
  */
 async function readDisplayedThreshold(page: Page): Promise<number> {
   const counterText = await page
@@ -37,7 +37,7 @@ async function readDisplayedThreshold(page: Page): Promise<number> {
  * underlying toolbar is interactable again. The v-menu uses
  * `close-on-content-click="false"` (so the threshold form stays open
  * during edits), which means a mode-option click does NOT auto-close
- * the menu — Escape does that reliably regardless of which mode was
+ * the menu. Escape does that reliably regardless of which mode was
  * picked.
  */
 async function pickMode(page: Page, mode: 'manual' | 'auto') {
@@ -54,7 +54,7 @@ async function openModeMenu(page: Page) {
 }
 
 test.describe('data-points combobox', () => {
-  // Each scenario takes ~18–20 s serially on Firefox — boot, plot,
+  // Each scenario takes ~18–20 s serially on Firefox: boot, plot,
   // setup, several menu open/close cycles, and a page.reload() in
   // beforeEach. With Playwright's default `workers: 2` cap, two of
   // these scenarios contending for the shared dev server can each
@@ -69,7 +69,7 @@ test.describe('data-points combobox', () => {
     // Reset the persisted preference bag so each test starts in auto
     // mode at the default threshold. The store persists `tooltipsMode`,
     // `tooltipsMaxDataPoints`, and `tooltipsManualEnabled` under a
-    // single key — clearing it resets all three.
+    // single key, so clearing it resets all three.
     await page.evaluate(() =>
       localStorage.removeItem('qc.plot.tooltipsMaxDataPoints')
     )
@@ -101,7 +101,7 @@ test.describe('data-points combobox', () => {
     await expect(toggle).toBeEnabled()
     await expect(page.getByTestId('tooltips-counter')).toHaveCount(0)
 
-    // Manual mode starts at "on" by default — flip it off and back on.
+    // Manual mode starts at "on" by default; flip it off and back on.
     await expect(toggle).toHaveAttribute('aria-pressed', 'true')
     await toggle.click()
     await expect(toggle).toHaveAttribute('aria-pressed', 'false')
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
index 0c332923..41c400ef 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
@@ -413,7 +413,7 @@ const plottedIds = computed(
 const isChecked = (item: Datastream) =>
   sourceGroupIds(item.id).some((id) => plottedIds.value.has(id))
 
-// Only managed series from this source are plotted, not the raw one — the
+// Only managed series from this source are plotted, not the raw one, so the
 // row shouldn't claim the raw line is on the plot.
 const isPartial = (item: Datastream) =>
   !plottedIds.value.has(item.id) && isChecked(item)
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
index bc94fd68..bb427b1d 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
@@ -613,7 +613,7 @@ describe('DataTable.vue onSelectChange / getRowProps', () => {
     const wrapper = createWrapperWithSlots()
     await flushPromises()
     const checkboxes = wrapper.findAllComponents({ name: 'VCheckbox' })
-    // toggle row 2 first, then row 0 — result should be sorted [0, 2]
+    // toggle row 2 first, then row 0; result should be sorted [0, 2]
     await checkboxes[2].vm.$emit('update:modelValue', true)
     await checkboxes[0].vm.$emit('update:modelValue', true)
     await flushPromises()
@@ -697,7 +697,7 @@ describe('DataTable.vue ResizeObserver integration', () => {
 
   it('uses bodyEl.clientHeight when non-zero and updates on resize', async () => {
     // The cast tells TS the class-constructor assignment widens the
-    // value back to the callable type — without it the analyzer pins
+    // value back to the callable type. Without it the analyzer pins
     // the variable to `null` after the literal initialiser.
     let capturedCallback = null as ((entries: any) => void) | null
     class CapturingRO {
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts
index 26fca03d..233bc837 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts
@@ -1,5 +1,5 @@
 /**
- * Unit tests for PlotSourceDialog.vue — the chooser shown when plotting a
+ * Unit tests for PlotSourceDialog.vue, the chooser shown when plotting a
  * source datastream that has managed (QC) datastreams derived from it.
  * Purely presentational, so no stores are mocked.
  */
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
index 0a661164..79893c15 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
@@ -1,5 +1,5 @@
 /**
- * Unit tests for PlottedDatastreams.vue — focused on the per-row load
+ * Unit tests for PlottedDatastreams.vue, focused on the per-row load
  * status (subtitle text + empty-window indicator). Mocks the two
  * stores the component reads so the rendered text reflects whatever
  * we drop into `graphSeriesArray` for each case.
@@ -107,7 +107,7 @@ function seedSeries(
   }
 }
 
-describe('PlottedDatastreams.vue — load status', () => {
+describe('PlottedDatastreams.vue: load status', () => {
   beforeEach(() => {
     plottedDatastreams.value = [datastreamA]
     qcDatastream.value = datastreamA
diff --git a/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts b/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts
index 6eb11fda..6649689f 100644
--- a/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts
@@ -196,7 +196,7 @@ describe('useDataSelection date-range helpers', () => {
 
     // formatDate is stubbed to ISO in the module mock above, so the
     // ternary `startDate.value ? formatDate(...) : ''` should emit an
-    // ISO string — the non-empty branch.
+    // ISO string (the non-empty branch).
     expect(startDateString.value).toBe('2025-01-01T00:00:00.000Z')
     expect(endDateString.value).toBe('2025-01-01T03:00:00.000Z')
   })
@@ -225,7 +225,7 @@ describe('useDataSelection date-range helpers', () => {
     const { setSelectedPoints } = await import('@/utils/plotting/plotly')
 
     const { selectDateRange } = useDataSelection()
-    // `to` earlier than `from` — startIdx > endIdx, should no-op.
+    // `to` earlier than `from`: startIdx > endIdx, should no-op.
     await selectDateRange(
       new Date('2025-01-01T03:00:00Z'),
       new Date('2025-01-01T00:30:00Z')
diff --git a/apps/qc-app/src/composables/__tests__/useResizable.spec.ts b/apps/qc-app/src/composables/__tests__/useResizable.spec.ts
index 11e7c412..496fb839 100644
--- a/apps/qc-app/src/composables/__tests__/useResizable.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useResizable.spec.ts
@@ -3,7 +3,7 @@ import { nextTick } from 'vue'
 import { createPinia, setActivePinia } from 'pinia'
 import { useResizable, usePersistedFlag } from '@/composables/useResizable'
 
-// Real `useUiLayoutStore` is fine — it's a tiny pinia store backed by a
+// Real `useUiLayoutStore` is fine: it's a tiny pinia store backed by a
 // Map and we want to exercise the persistence wiring end-to-end.
 import { useUiLayoutStore } from '@/store/uiLayout'
 
@@ -149,7 +149,7 @@ describe('useResizable', () => {
     })
     onStart(new MouseEvent('mousedown', { clientX: 0, clientY: 0 }))
     dispatchMouse('mousemove', 200, 0)
-    // Raw pixels — no percent scaling.
+    // Raw pixels, no percent scaling.
     expect(size.value).toBe(230)
     dispatchMouse('mouseup', 200, 0)
   })
diff --git a/apps/qc-app/src/composables/useBufferedNumber.ts b/apps/qc-app/src/composables/useBufferedNumber.ts
index e7b93d50..8da35aad 100644
--- a/apps/qc-app/src/composables/useBufferedNumber.ts
+++ b/apps/qc-app/src/composables/useBufferedNumber.ts
@@ -3,7 +3,7 @@ import { computed, ref, watch, type Ref } from 'vue'
 /**
  * Two-stage edit buffer over a numeric ref. Bind your input to
  * `pending` so per-keystroke writes don't bounce the underlying
- * `source` — only `apply()` (or whatever your UI wires it to) commits.
+ * `source`. Only `apply()` (or whatever your UI wires it to) commits.
  *
  * `gateOpen` re-syncs the buffer from `source` every time it
  * transitions from false → true, so opening a popover always shows
diff --git a/apps/qc-app/src/composables/useDataSelection.ts b/apps/qc-app/src/composables/useDataSelection.ts
index baa3a975..f505ae99 100644
--- a/apps/qc-app/src/composables/useDataSelection.ts
+++ b/apps/qc-app/src/composables/useDataSelection.ts
@@ -27,7 +27,7 @@ export function useDataSelection() {
    * Tell the next `plotly_relayout`-induced `handleSelected` call
    * what selection to expect from this programmatic write. The
    * relayout handler compares the expected payload against the
-   * trace's actual `selectedpoints` — a match is the echo (skip
+   * trace's actual `selectedpoints`: a match is the echo (skip
    * dispatch); a mismatch means a user gesture (box / lasso select)
    * raced through the same debounce window, so dispatch normally.
    *
@@ -45,7 +45,7 @@ export function useDataSelection() {
    * Locate the plotly trace index for the QC series we're editing.
    * Traces are rendered in the order: non-QC → QC → qualifier band, so
    * the old `data.length - 1` shortcut targeted the last qualifier-band
-   * trace whenever a qualifier band was present — selected points were
+   * trace whenever a qualifier band was present, so selected points were
    * written to an invisible trace, which matched the reported bug: the
    * "N points selected" label still tracked `selectedData`, but the QC
    * trace had no `selectedpoints` and nothing was highlighted. Match by
@@ -66,7 +66,7 @@ export function useDataSelection() {
 
   /**
    * Push `selection` into Plotly as the QC trace's `selectedpoints`
-   * and mirror it into `selectedData`. Visual-only — does NOT push
+   * and mirror it into `selectedData`. Visual-only: does NOT push
    * a SELECTION entry into the ObservationRecord history. The
    * Plotly write triggers a `plotly_relayout` that would otherwise
    * round-trip through `handleSelected` and dispatch a SELECTION
@@ -93,7 +93,7 @@ export function useDataSelection() {
    * SELECTION lands from that round-trip.
    *
    * `recordHistory` (default `true`) controls whether the clear is
-   * recorded in qc-utils history — when true, an empty SELECTION is
+   * recorded in qc-utils history. When true, an empty SELECTION is
    * dispatched, which `_selection` may use to drop the underlying
    * filter entry (the user actively cleared a filter-driven
    * selection). Programmatic callers that have already logged what
@@ -129,8 +129,8 @@ export function useDataSelection() {
 
   // `startDate` / `endDate` bracket the current selection, or the full
   // series when nothing is selected. The `|| fallback` arm that used to
-  // sit on `new Date(...)` was dead code — `new Date()` is truthy even
-  // when given `undefined` (it just produces an Invalid Date) — so the
+  // sit on `new Date(...)` was dead code: `new Date()` is truthy even
+  // when given `undefined` (it just produces an Invalid Date), so the
   // fallback was unreachable. The computeds now always return a Date,
   // and the downstream string helpers stop guarding against a value
   // that can't appear.
diff --git a/apps/qc-app/src/composables/useFilterDispatch.ts b/apps/qc-app/src/composables/useFilterDispatch.ts
index ec8911d7..43def934 100644
--- a/apps/qc-app/src/composables/useFilterDispatch.ts
+++ b/apps/qc-app/src/composables/useFilterDispatch.ts
@@ -23,8 +23,8 @@ import { useUIStore } from '@/store/userInterface'
  * + try/finally + `clearSelected`/`setPlotSelection` boilerplate that
  * drifted between panels (some called `clearSelected` first, some
  * didn't; some forgot `isUpdating`). Centralizing here keeps the
- * sequence — including the echo-suppression hand-off in
- * `setPlotSelection` — consistent.
+ * sequence (including the echo-suppression hand-off in
+ * `setPlotSelection`) consistent.
  *
  * Returns the indices the filter produced so callers can run extra
  * UI side-effects (e.g. updating local state) without re-fetching.
@@ -88,11 +88,11 @@ export function useFilterDispatch() {
   /**
    * Log a SELECTION history entry at the end of an edit / add
    * operation and visually highlight the same indices on the plot.
-   * Lets the user trace where points landed — useful for ADD_POINTS
+   * Lets the user trace where points landed, useful for ADD_POINTS
    * (newly-inserted indices), CHANGE_VALUES / INTERPOLATE /
    * DRIFT_CORRECTION (same indices, fresh marker), SHIFT_DATETIMES
    * (re-located indices), and FILL_GAPS (newly-inserted indices).
-   * No-ops on an empty array — qc-utils' empty-SELECTION rule would
+   * No-ops on an empty array: qc-utils' empty-SELECTION rule would
    * otherwise pop a preceding edit op whose `selected` was non-empty.
    */
   const recordPostActionSelection = async (indices: number[]) => {
diff --git a/apps/qc-app/src/composables/useQcHistory.ts b/apps/qc-app/src/composables/useQcHistory.ts
index 55dc8d30..de660698 100644
--- a/apps/qc-app/src/composables/useQcHistory.ts
+++ b/apps/qc-app/src/composables/useQcHistory.ts
@@ -1,5 +1,5 @@
 /**
- * QC History — Vue-side save / load wrapper.
+ * QC History: Vue-side save / load wrapper.
  *
  * Wraps qc-utils' `serializeHistory` / `parseHistory` / `applyHistory`
  * with the consumer-specific glue:
@@ -101,7 +101,7 @@ export function useQcHistory() {
    * operations. Returns the per-op report so the caller can surface
    * a Snackbar / toast summary.
    *
-   * No datastream-id matching is enforced — QC histories are reusable
+   * No datastream-id matching is enforced. QC histories are reusable
    * across datastreams (see qc-utils' QC_HISTORY.md "Stay reusable").
    */
   async function importHistory(file: File): Promise<ApplyHistoryReport> {
diff --git a/apps/qc-app/src/composables/useResizable.ts b/apps/qc-app/src/composables/useResizable.ts
index c9ba20a1..d8ce811b 100644
--- a/apps/qc-app/src/composables/useResizable.ts
+++ b/apps/qc-app/src/composables/useResizable.ts
@@ -29,7 +29,7 @@ export interface UseResizableOptions {
   /**
    * Getter for the container dimension the size is expressed as a
    * fraction of. Used when `size` is a percentage (e.g. the
-   * History / OperationPanel vertical split) — without this the
+   * History / OperationPanel vertical split). Without this the
    * raw pixel delta would be added directly to the percent value,
    * making a 30 px drag look like a 30 % swing. We capture the
    * container dimension at drag start and convert pixel delta to
@@ -42,7 +42,7 @@ export interface UseResizableReturn {
   size: Ref<number>
   /** Bind to the resize handle's `mousedown`. */
   onStart: (event: MouseEvent) => void
-  /** True while a drag is in flight — handy for styling the handle
+  /** True while a drag is in flight; handy for styling the handle
    *  (persistent hover) or pausing other work. */
   dragging: Ref<boolean>
 }
@@ -52,7 +52,7 @@ export interface UseResizableReturn {
  * a `mousedown` handler to attach to a resize-grip element.
  * Listeners are attached to `window` for the duration of the drag
  * so the gesture keeps tracking even if the cursor leaves the
- * grip — mirrors how native resize handles behave. `user-select`
+ * grip, mirroring how native resize handles behave. `user-select`
  * and `cursor` on `document.body` are toggled for the drag so text
  * doesn't get selected while the user pulls.
  */
@@ -116,7 +116,7 @@ export function useResizable(
   return { size, onStart, dragging }
 }
 
-/** Persisted boolean flag — mirrors `useResizable`'s storage story
+/** Persisted boolean flag. Mirrors `useResizable`'s storage story
  *  for the collapse toggles so they survive reloads alongside the
  *  sibling widths. */
 export function usePersistedFlag(
diff --git a/apps/qc-app/src/composables/useWorkspacePermissions.ts b/apps/qc-app/src/composables/useWorkspacePermissions.ts
index 9f081f87..4963dc81 100644
--- a/apps/qc-app/src/composables/useWorkspacePermissions.ts
+++ b/apps/qc-app/src/composables/useWorkspacePermissions.ts
@@ -4,7 +4,7 @@
  * The signed-in user's role on a workspace travels with the `Workspace`
  * object: owners have a null `collaboratorRole`, collaborators carry their
  * role's `permissions[]`, and admins (`accountType === 'admin'`) override
- * everything. No separate "am I an editor" endpoint is needed — the role is
+ * everything. No separate "am I an editor" endpoint is needed: the role is
  * embedded in `hs.workspaces.list()`. (`hs.user.can()` is the async
  * single-shot equivalent; this composable is the synchronous, reactive one
  * for gating UI.)
@@ -69,7 +69,7 @@ export function useWorkspacePermissions() {
   }
 
   /**
-   * Can run the QC edit workflow in this workspace — needs to create the
+   * Can run the QC edit workflow in this workspace. Needs to create the
    * managed datastream and/or write observations. Used to gate the editor's
    * row Edit / Save / Commit controls.
    */
diff --git a/apps/qc-app/src/main.ts b/apps/qc-app/src/main.ts
index e557ff9c..6fa08cf9 100644
--- a/apps/qc-app/src/main.ts
+++ b/apps/qc-app/src/main.ts
@@ -79,7 +79,7 @@ async function initializeApp() {
   // Run qc-utils' worker/inline calibration when the browser is idle so
   // the crossover thresholds match this device. Results are cached in
   // `localStorage`; the "Recalibrate" button in the nav rail can force
-  // a rerun. Non-critical — failures just fall back to conservative
+  // a rerun. Non-critical: failures just fall back to conservative
   // defaults baked into qc-utils.
   const kickoffCalibration = () => {
     ensureCalibration().catch((err) => {
diff --git a/apps/qc-app/src/router/guards.ts b/apps/qc-app/src/router/guards.ts
index a24bb21b..8da31581 100644
--- a/apps/qc-app/src/router/guards.ts
+++ b/apps/qc-app/src/router/guards.ts
@@ -53,7 +53,7 @@ export const guards: RouteGuard[] = [
     return null
   },
 
-  // Workspaces picker shortcut — if the user already has a selection
+  // Workspaces picker shortcut: if the user already has a selection
   // (typical reload / deep-link case), skip the picker synchronously
   // so it never flashes on the way to the intended page. The nav
   // rail's "Switch workspace" action sets `?switch=1` to opt into
@@ -66,7 +66,7 @@ export const guards: RouteGuard[] = [
     return nextLocation(to.query.next)
   },
 
-  // hasWorkspaceGuard — every data-bearing route needs an active
+  // hasWorkspaceGuard: every data-bearing route needs an active
   // HydroServer workspace context. If none is selected, bounce to the
   // picker and carry a `next` hint so we can come back here once the
   // user commits to a workspace.
diff --git a/apps/qc-app/src/router/router.ts b/apps/qc-app/src/router/router.ts
index ced30f75..853a6d53 100644
--- a/apps/qc-app/src/router/router.ts
+++ b/apps/qc-app/src/router/router.ts
@@ -8,7 +8,7 @@ const router = createRouter({
 })
 
 export function setupRouteGuards() {
-  // Return the guard's result rather than calling `next()` — the callback
+  // Return the guard's result rather than calling `next()`. The callback
   // form is deprecated in vue-router. `false` cancels, a route location
   // redirects, and null/undefined proceeds to the next guard.
   guards.forEach((fn) => {
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
index 01c17d9c..e947d68d 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
@@ -49,7 +49,7 @@ describe('sessionOperationsFromSerialized', () => {
   })
 })
 
-describe('persistSessionOperations — comments', () => {
+describe('persistSessionOperations: comments', () => {
   it('sends a comment with a newly-appended operation', async () => {
     const qc = makeQcFake()
     const { historyId, sessionId } = await sessionWith(qc)
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
index d17fb0f0..0cb5d93b 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
@@ -375,7 +375,7 @@ describe('reconstructCommittedSession', () => {
   })
 })
 
-describe('reconstructCommittedSession — chain order', () => {
+describe('reconstructCommittedSession: chain order', () => {
   // The fake allows only one in-progress session at a time, so creation
   // order always matches commit order there. Stub the services directly to
   // build the case where they disagree.
@@ -476,7 +476,7 @@ describe('reconstructCommittedSession — chain order', () => {
   })
 })
 
-describe('reconstructCommittedSession — attribution', () => {
+describe('reconstructCommittedSession: attribution', () => {
   const ok = (data: unknown) => ({ ok: true, data, status: 200, message: '' })
 
   it('carries the operation performer through to the replayed history', async () => {
diff --git a/apps/qc-app/src/store/__tests__/apiReferenceCoverage.spec.ts b/apps/qc-app/src/store/__tests__/apiReferenceCoverage.spec.ts
index 2cdbe0ff..fb4d1e08 100644
--- a/apps/qc-app/src/store/__tests__/apiReferenceCoverage.spec.ts
+++ b/apps/qc-app/src/store/__tests__/apiReferenceCoverage.spec.ts
@@ -20,7 +20,7 @@ function parseStoreExports(source: string): string[] | null {
   if (!/\bdefineStore\b/.test(source)) return null
 
   const lines = source.split(/\r?\n/)
-  // Find the LAST `  return {` line — defineStore setup functions can
+  // Find the LAST `  return {` line: defineStore setup functions can
   // have inner `return {` (e.g. inside helpers); the outer one is the
   // store's exported shape and always at indent 2.
   let start = -1
diff --git a/apps/qc-app/src/store/__tests__/observations.spec.ts b/apps/qc-app/src/store/__tests__/observations.spec.ts
index d9def570..a1282ba6 100644
--- a/apps/qc-app/src/store/__tests__/observations.spec.ts
+++ b/apps/qc-app/src/store/__tests__/observations.spec.ts
@@ -91,7 +91,7 @@ describe('useObservationStore.fetchObservationsInRange windowing', () => {
     )
     expect(second).toBe(first)
     // applyWindow is a no-op for the same window, so the data buffer is the
-    // same instance — edits/history would survive.
+    // same instance, so edits/history would survive.
     expect(second.dataX).toBe(beforeX)
   })
 
diff --git a/apps/qc-app/src/store/__tests__/plotly.spec.ts b/apps/qc-app/src/store/__tests__/plotly.spec.ts
index 097f0eed..502e18f0 100644
--- a/apps/qc-app/src/store/__tests__/plotly.spec.ts
+++ b/apps/qc-app/src/store/__tests__/plotly.spec.ts
@@ -346,7 +346,7 @@ describe('usePlotlyStore zoom history', () => {
       store.pushZoomState({ ...fakeZoom(), xRange: [i, i + 1] })
     }
     expect(store.zoomUndoStack.length).toBe(50)
-    // Oldest 10 entries should have been shifted off — first entry's xRange[0] is 10.
+    // Oldest 10 entries should have been shifted off; first entry's xRange[0] is 10.
     expect(store.zoomUndoStack[0].xRange?.[0]).toBe(10)
     expect(store.zoomUndoStack[49].xRange?.[0]).toBe(59)
   })
@@ -452,7 +452,7 @@ describe('usePlotlyStore.updateOptions + selectedSeries', () => {
   })
 })
 
-describe('usePlotlyStore — data-points (tooltips) mode', () => {
+describe('usePlotlyStore: data-points (tooltips) mode', () => {
   beforeEach(() => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
@@ -490,7 +490,7 @@ describe('usePlotlyStore — data-points (tooltips) mode', () => {
     const { usePlotlyStore } = await import('@/store/plotly')
     const store = usePlotlyStore()
     store.tooltipsMode = 'manual'
-    // Way over the cap — auto would have switched off here.
+    // Way over the cap; auto would have switched off here.
     store.tooltipsMaxDataPoints = 100
     store.visiblePoints = 100_000
 
@@ -505,7 +505,7 @@ describe('usePlotlyStore — data-points (tooltips) mode', () => {
     // The relayout pipeline reads `areTooltipsEnabled` as the single
     // source of truth. Earlier versions also re-applied the threshold
     // check on top, which silently flipped manual-on back off when
-    // the user zoomed out — the regression this guards against.
+    // the user zoomed out, the regression this guards against.
     const { usePlotlyStore } = await import('@/store/plotly')
     const store = usePlotlyStore()
     store.tooltipsMode = 'manual'
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index f4521142..1af11f2d 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -528,7 +528,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   }
 
   const filteredDatastreams = computed(() => {
-    // `?? []` — during a workspace switch `datastreams.value` is
+    // `?? []`: during a workspace switch `datastreams.value` is
     // briefly cleared in App.vue before the new catalog lands. Without
     // the fallback this computed returns `undefined`, which then
     // propagates into `tableItems.map(...)` in DataVisDatasetsTable
@@ -562,7 +562,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     // No-op when neither bound actually moved. Every sidebar path —
     // clicking the already-active preset, date-text-field blur,
     // time-text-field blur, calendar picker confirming the current
-    // day — calls this with fresh Date references whose timestamps
+    // day) calls this with fresh Date references whose timestamps
     // often match the current range. Without this guard each of those
     // clicks triggers a full data refetch + plot redraw that resets
     // the user's zoom for no reason.
@@ -737,7 +737,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   //   - setEditTarget / setEditRecord / clearEditTarget
   // Each mutation site now calls the action so side effects (time range
   // sync, graph-series rebuild, zoom history clear, Plotly re-render)
-  // happen inline and in a predictable order — no reactive cascade.
+  // happen inline and in a predictable order, with no reactive cascade.
 
   // TODO: Revisit this. Does it make sense to convert qualifierValue to a string in preprocessing
   // just to split it into an array of strings here? Maybe just save it as an array of strings instead
diff --git a/apps/qc-app/src/store/observations.ts b/apps/qc-app/src/store/observations.ts
index 29c55d94..5355cc95 100644
--- a/apps/qc-app/src/store/observations.ts
+++ b/apps/qc-app/src/store/observations.ts
@@ -55,7 +55,7 @@ export const useObservationStore = defineStore(
           observationsRaw.value[id].datetimes[0] as number
         )
 
-        // Strict `<` — skip the request entirely when the requested
+        // Strict `<`: skip the request entirely when the requested
         // begin is at or inside the cached window. `<=` used to fire a
         // 1-second range request on exact matches (e.g. re-clicking
         // the same preset), which is pure waste.
diff --git a/apps/qc-app/src/store/plotly.ts b/apps/qc-app/src/store/plotly.ts
index 18336635..68420685 100644
--- a/apps/qc-app/src/store/plotly.ts
+++ b/apps/qc-app/src/store/plotly.ts
@@ -37,7 +37,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
   const showLegend = ref(true)
   const showTooltip = ref(false)
   const isUpdating = ref(false)
-  // Persisted as a user preference — large plots are cheap on fast machines
+  // Persisted as a user preference. Large plots are cheap on fast machines
   // and expensive on slow ones, so let the user pick. Bounded in the UI
   // but not hard-clamped here so power users can override via storage.
   // Persistence is wired through pinia-plugin-persistedstate at the
@@ -45,9 +45,9 @@ export const usePlotlyStore = defineStore('Plotly', () => {
   const tooltipsMaxDataPoints = ref<number>(10 * 1000)
   const visiblePoints: Ref<number> = ref(0)
   // Two-mode toggle for individual data-point rendering / hover.
-  //   - 'manual' — user controls on/off via `tooltipsManualEnabled`.
+  //   - 'manual': user controls on/off via `tooltipsManualEnabled`.
   //                Threshold ignored.
-  //   - 'auto'   — threshold-driven: on while visiblePoints <=
+  //   - 'auto':   threshold-driven: on while visiblePoints <=
   //                threshold, off otherwise. Default keeps backward
   //                behavior for existing users.
   const tooltipsMode = ref<'manual' | 'auto'>('auto')
@@ -55,7 +55,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
   // `auto`. Defaults to `true` so the first manual click feels like
   // an explicit toggle off.
   const tooltipsManualEnabled = ref(true)
-  // Derived "is hover currently rendering?" — read by the relayout
+  // Derived "is hover currently rendering?", read by the relayout
   // pipeline and the toolbar UI. Auto mode reads the live threshold;
   // manual mode reads the user's explicit on/off.
   const areTooltipsEnabled = computed(() => {
@@ -75,7 +75,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
    * `processMouseMove` on every frame. Lives outside Plotly's
    * `showspikes` because the built-in spikes are gated on
    * `hoverinfo !== 'skip'` and so disappear when tooltips auto-
-   * disable at high point counts — users want the crosshair to stay
+   * disable at high point counts, and users want the crosshair to stay
    * regardless of tooltip state. The CSS driver also avoids the
    * noticeable lag behind the cursor that Plotly's spike layer has
    * on scattergl.
@@ -93,7 +93,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
   /**
    * Datastream IDs whose non-QC right-side y-axis is currently hidden.
    * `createPlotlyOption` reads this while building overlay axes and
-   * sets `visible: false` on matches — the trace itself keeps
+   * sets `visible: false` on matches. The trace itself keeps
    * rendering, only the axis chrome (line, ticks, labels, title) goes
    * away, and autoshift reclaims the column's horizontal space. Keyed
    * by datastream id so trace reorders and QC promotions don't
@@ -138,7 +138,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
   /**
    * Horizontal title chips rendered above each non-QC right-side axis
    * (see `.plot-axis-chip` in Plot.vue). Replaces Plotly's rotated
-   * vertical axis titles — horizontal text is much easier to scan
+   * vertical axis titles, since horizontal text is much easier to scan
    * when several right-side axes stack up. Populated by
    * `updateAxisChips` after each plot/relayout; each entry carries
    * the datastream id, the axis line's pixel position (for
@@ -169,7 +169,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
    * Sentinel armed by programmatic Plotly writes (`setPlotSelection`,
    * `clearSelected`). When the next `plotly_relayout`-induced
    * `handleSelected` call fires, we compare the current selection
-   * against this expected payload — if they match it's the echo of
+   * against this expected payload: if they match it's the echo of
    * our own write (skip the SELECTION dispatch); if they differ a
    * user gesture (box/lasso select) raced through the same debounce
    * window, so we let the dispatch proceed. `handleClick` (the
@@ -201,7 +201,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
   /**
    * Monotonic counter bumped once per `handleNewPlot` run. `Plotly.newPlot`
    * reuses the same DOM element (so `plotlyRef.value`'s identity does not
-   * change) but purges every externally-attached event listener — code
+   * change) but purges every externally-attached event listener, so code
    * outside `handleNewPlot` that subscribes to `plotly_relayout` /
    * `plotly_restyle` (e.g. `ContextPlot.vue`) needs a positive signal to
    * re-attach. Watch this ref instead of trying to detect element
@@ -231,7 +231,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
   // }
 
   /**
-   * Zoom history — separate from `editHistory` (which tracks QC data
+   * Zoom history, separate from `editHistory` (which tracks QC data
    * edits). Each entry captures the plot's visible ranges at a point in
    * time so the user can step back/forward through viewport changes.
    *
@@ -250,7 +250,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
 
   /**
    * Live ranges currently shown on the plot. Top of the undo stack
-   * after every relayout (zoom, pan, axis drag) — `null` when nothing
+   * after every relayout (zoom, pan, axis drag), or `null` when nothing
    * has been recorded yet (initial mount before the first layout
    * settles). The share URL watcher subscribes to this so the link
    * reflects the latest viewport without poking into the stack
@@ -307,7 +307,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
    * @param preserveZoom When true (default), the current live x/y ranges
    *   are copied onto the fresh layout so QC edits don't reset the user's
    *   zoom. Pass `false` when the caller *wants* the new layout's default
-   *   range to apply — notably when the user changed the date filter
+   *   range to apply, notably when the user changed the date filter
    *   (`useDataVisStore#setDateRange`), where preserving the old range
    *   would defeat the action.
    */
@@ -456,7 +456,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
    *
    * Race-safe because the walk is synchronous and reads the array
    * exactly once. Stable across reloads because the walk order is
-   * the user-facing legend order, not fetch-completion order — so
+   * the user-facing legend order, not fetch-completion order, so
    * the same `plottedDatastreams` configuration yields the same
    * colour assignment every time.
    *
diff --git a/apps/qc-app/src/store/qualifiers.ts b/apps/qc-app/src/store/qualifiers.ts
index 77a3eb0f..692d8845 100644
--- a/apps/qc-app/src/store/qualifiers.ts
+++ b/apps/qc-app/src/store/qualifiers.ts
@@ -96,7 +96,7 @@ export const useQualifierStore = defineStore(
 /**
      * Create a ResultQualifier on the server for the active workspace.
      * Falls back to a purely local record (no `workspaceId`) if no
-     * workspace is selected — that's the only path on which an entry
+     * workspace is selected. That's the only path on which an entry
      * can exist without a server id; callers that need persistence
      * should wait until a workspace is active.
      */
@@ -230,7 +230,7 @@ export const useQualifierStore = defineStore(
     }
   },
   {
-    // Persist only the per-observation application map — the
+    // Persist only the per-observation application map. The
     // dictionary is reloaded from the server on every workspace
     // change, and persisting it would compete with that fresh data.
     persist: {
diff --git a/apps/qc-app/src/store/userInterface.ts b/apps/qc-app/src/store/userInterface.ts
index ef023fe5..4dd08840 100644
--- a/apps/qc-app/src/store/userInterface.ts
+++ b/apps/qc-app/src/store/userInterface.ts
@@ -52,7 +52,7 @@ export const useUIStore = defineStore('userInterface', () => {
   // View
   const currentView = ref<View>(DrawerType.Select)
 
-  // Operation panel — which operation's details are shown in the right
+  // Operation panel: which operation's details are shown in the right
   // sidebar below the edit history. `null` means no panel is open.
   const selectedOperation = ref<string | null>(null)
 
@@ -95,11 +95,11 @@ export const useUIStore = defineStore('userInterface', () => {
   // Seed gap / fill defaults when the QC datastream changes. Preference
   // order:
   //   1. Per-datastream persisted values (user's last commit for this
-  //      series) — so reopening a panel feels continuous.
+  //      series), so reopening a panel feels continuous.
   //   2. Datastream's declared `intendedTimeSpacing` /
-  //      `intendedTimeSpacingUnit` / `noDataValue` — a reasonable
+  //      `intendedTimeSpacingUnit` / `noDataValue`, a reasonable
   //      starting point for first-time use.
-  //   3. Current ref values — keep whatever the user had if the
+  //   3. Current ref values, keeping whatever the user had if the
   //      datastream lacks metadata.
   // A null change (unset) is skipped so closing and reopening the QC
   // drawer without a datastream loaded doesn't clobber form state.
@@ -214,7 +214,7 @@ export const useUIStore = defineStore('userInterface', () => {
 }, {
   // Persist only the user-toggleable preference. The from/to of the
   // active filter window are panel state and reseed from data bounds
-  // on each mount — restoring stale ms across sessions would point at
+  // on each mount; restoring stale ms across sessions would point at
   // a window that no longer overlaps the current datastream's data.
   // `:v1` suffix matches the `qc-utils:calibration:v1` template so we
   // can invalidate the persisted value by bumping the version.
diff --git a/apps/qc-app/src/store/workspaces.ts b/apps/qc-app/src/store/workspaces.ts
index 44746bf1..6db130d8 100644
--- a/apps/qc-app/src/store/workspaces.ts
+++ b/apps/qc-app/src/store/workspaces.ts
@@ -7,8 +7,8 @@ import { useHydroServer } from '@/store/hydroserver'
  * Central state for HydroServer workspace context.
  *
  * The QC app inherits HydroServer's role-based access control at the
- * workspace level. Every CRUD-ish interaction — things, datastreams,
- * observations, qualifiers — is scoped to a workspace the signed-in
+ * workspace level. Every CRUD-ish interaction (things, datastreams,
+ * observations, qualifiers) is scoped to a workspace the signed-in
  * user has been granted a role on. `hs.workspaces.list()` already
  * filters server-side to workspaces the user can see; we trust that
  * listing and let the user pick one before any data is fetched.
@@ -42,7 +42,7 @@ export const useWorkspaceStore = defineStore('workspaces', () => {
   const canEditSelected = computed(() => {
     const ws = selectedWorkspace.value
     if (!ws) return false
-    // Owner: `collaboratorRole` is null on owned workspaces — the role
+    // Owner: `collaboratorRole` is null on owned workspaces; the role
     // machinery only applies to invited collaborators.
     if (!ws.collaboratorRole) return true
     const perms = ws.collaboratorRole.permissions ?? []
@@ -102,7 +102,7 @@ export const useWorkspaceStore = defineStore('workspaces', () => {
    * Apply a workspace selection by id, falling back to a placeholder
    * `{ id }` object when the full Workspace isn't in
    * `availableWorkspaces` yet. Used by shared-link hydration in
-   * `main.ts` and the matching router guard — those can fire before
+   * `main.ts` and the matching router guard. Those can fire before
    * `loadWorkspaces` has populated the list (e.g. cross-origin dev
    * setups where the session endpoint 401s silently, or a fresh boot
    * where the list fetch is still in flight). The downstream catalog
diff --git a/apps/qc-app/src/types/index.ts b/apps/qc-app/src/types/index.ts
index 4218d6a0..aa46d90c 100644
--- a/apps/qc-app/src/types/index.ts
+++ b/apps/qc-app/src/types/index.ts
@@ -46,7 +46,7 @@ export interface GraphSeries {
    * the plot breaks the line wherever consecutive observations sit
    * farther apart than this value, so true data gaps render as
    * disconnected segments. Null/undefined when the datastream has no
-   * declared cadence — those series draw lines through every gap as
+   * declared cadence; those series draw lines through every gap as
    * before.
    */
   intendedSpacingMs?: number | null
diff --git a/apps/qc-app/src/types/plotly-dist.d.ts b/apps/qc-app/src/types/plotly-dist.d.ts
index 3c828e3b..3b7b7018 100644
--- a/apps/qc-app/src/types/plotly-dist.d.ts
+++ b/apps/qc-app/src/types/plotly-dist.d.ts
@@ -1,6 +1,6 @@
 // Module shim: route the bare specifier `plotly.js-dist` (the runtime bundle
 // shipped without its own .d.ts) to the typed `plotly.js` namespace provided
-// by @types/plotly.js. See CONV-013 — Plotly types are owned by this package
+// by @types/plotly.js. See CONV-013: Plotly types are owned by this package
 // and `src/utils/plotting/plotly.ts`.
 declare module 'plotly.js-dist' {
   export * from 'plotly.js'
@@ -13,7 +13,7 @@ declare module 'plotly.js-dist' {
 // used by `handleMouseMove` to convert mouse pixel positions into data
 // coordinates. They are NOT covered by `@types/plotly.js`.
 //
-// Intentionally NOT a global augmentation of `Plotly.PlotlyHTMLElement` —
+// Intentionally NOT a global augmentation of `Plotly.PlotlyHTMLElement`,
 // keeping the private surface scoped to a separate exported interface so
 // only the seam (which performs an explicit `as PrivatePlotlyHTMLElement`
 // cast at the call site) can see it. Callers consuming the published
diff --git a/apps/qc-app/src/utils/__tests__/dateMath.spec.ts b/apps/qc-app/src/utils/__tests__/dateMath.spec.ts
index 1a3a6365..30a39585 100644
--- a/apps/qc-app/src/utils/__tests__/dateMath.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/dateMath.spec.ts
@@ -5,7 +5,7 @@ import {
   subtractYears,
 } from '@/utils/dateMath'
 
-// The helpers clone their input — these tests double as immutability
+// The helpers clone their input, so these tests double as immutability
 // assertions so callers don't need to defensively copy.
 
 describe('dateMath.subtractDays', () => {
@@ -51,7 +51,7 @@ describe('dateMath.subtractMonths', () => {
   })
 
   it('clamps day-of-month when target month is shorter (Aug 31 - 6M → Feb 28)', () => {
-    // The reason this helper exists — JS's native setMonth overflows
+    // The reason this helper exists: JS's native setMonth overflows
     // Feb 31 into March 3, which silently misrouted the 6-month preset.
     const result = subtractMonths(new Date(2025, 7, 31), 6) // Aug 31 - 6M
     expect(result.getFullYear()).toBe(2025)
diff --git a/apps/qc-app/src/utils/__tests__/share.spec.ts b/apps/qc-app/src/utils/__tests__/share.spec.ts
index b3aac2ce..653c4a20 100644
--- a/apps/qc-app/src/utils/__tests__/share.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/share.spec.ts
@@ -61,7 +61,7 @@ describe('share encoding primitives', () => {
   })
 })
 
-describe('encodeShareState — omits defaults', () => {
+describe('encodeShareState: omits defaults', () => {
   it('emits an empty query for the empty state', () => {
     expect(encodeShareState({})).toEqual({})
   })
diff --git a/apps/qc-app/src/utils/csvExport.ts b/apps/qc-app/src/utils/csvExport.ts
index c6db7904..126f0607 100644
--- a/apps/qc-app/src/utils/csvExport.ts
+++ b/apps/qc-app/src/utils/csvExport.ts
@@ -5,7 +5,7 @@ import { downloadBlob } from '@/utils/download'
 /**
  * App-side CSV export delivery.
  *
- * The client exposes a single data-only primitive — `fetchCsvBlob` — that
+ * The client exposes a single data-only primitive, `fetchCsvBlob`, that
  * follows the Result model and never touches the DOM. Everything
  * environment-specific lives here: batching, ZIP bundling, and the browser
  * anchor-click download. These helpers throw on failure so existing try/catch
diff --git a/apps/qc-app/src/utils/datastreamSummary.ts b/apps/qc-app/src/utils/datastreamSummary.ts
index cbdedc0c..5b693034 100644
--- a/apps/qc-app/src/utils/datastreamSummary.ts
+++ b/apps/qc-app/src/utils/datastreamSummary.ts
@@ -9,7 +9,7 @@ const NUMBER = new Intl.NumberFormat()
 /**
  * One-line recap of a datastream, so several derived from the same source
  * are distinguishable in a chooser: processing level, observation count,
- * and — when sessions are supplied — how many QC sessions it carries.
+ * and (when sessions are supplied) how many QC sessions it carries.
  */
 export function datastreamSummary(
   datastream: Datastream & Partial<DatastreamExtended>,
diff --git a/apps/qc-app/src/utils/dateMath.ts b/apps/qc-app/src/utils/dateMath.ts
index 37b1c9c2..2c8a983d 100644
--- a/apps/qc-app/src/utils/dateMath.ts
+++ b/apps/qc-app/src/utils/dateMath.ts
@@ -4,7 +4,7 @@
  * `Mar 3`), which made `subtractMonths(Aug 31, 6)` land three days
  * into March instead of on the Feb/Mar boundary. Clone the input,
  * subtract, and clamp the day-of-month back to the target month's
- * last day — matching the arithmetic Plotly's `rangeselector`
+ * last day, matching the arithmetic Plotly's `rangeselector`
  * performs internally.
  */
 
diff --git a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
index f8e02968..bd290fd2 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
@@ -249,7 +249,7 @@ describe('createPlotlyOption', () => {
 
   it('emits a lines-only gap overlay when intendedSpacingMs is set', () => {
     // Spacing 1.5 → the [1,2,3,4,5] x grid (Δ=1) never exceeds, so the
-    // overlay's x/y match the main trace verbatim — but the trace
+    // overlay's x/y match the main trace verbatim, but the trace
     // pair is still emitted so the line-drawing path is exercised
     // regardless of whether real gaps are present.
     qcDatastream.value = { id: 'qc' }
diff --git a/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
index 469d9eb7..6d4cfe5b 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
@@ -471,7 +471,7 @@ describe('handleRelayout', () => {
 
   it('keeps scatter-only series markers visible past DENSITY_HIDE_MARKERS in auto mode', async () => {
     // Auto-mode + visible >> threshold normally trips the density fade.
-    // The scatter-only exemption has to win here too — otherwise a busy
+    // The scatter-only exemption has to win here too, otherwise a busy
     // view of a no-cadence series wipes itself out as soon as the user
     // pans into a dense zoom level.
     qcDatastream.value = { id: 'qc-target' }
diff --git a/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts
index cbc5ec15..548ec78f 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts
@@ -126,7 +126,7 @@ describe('handleSelected', () => {
     selectedSeries.value = { data: { dispatchFilter } }
     plotlyRef.value = makePlot('qc', [4, 5])
     // Programmatic write expected [9, 10], user gesture landed [4, 5]
-    // — mismatch should fall through to dispatch.
+    // so the mismatch should fall through to dispatch.
     suppressedEchoSelection.value = [9, 10]
     await handleSelected({} as any, { fromRelayout: true })
     expect(dispatchFilter).toHaveBeenCalledWith('SELECTION', [4, 5])
diff --git a/apps/qc-app/src/utils/plotting/__tests__/zoom.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/zoom.spec.ts
index 8fa95ffa..f6185a26 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/zoom.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/zoom.spec.ts
@@ -13,7 +13,7 @@ vi.mock('plotly.js-dist', () => ({
 // plotlyRef is accessed via `storeToRefs` (returns ref) in captureCurrentZoomState,
 // and via raw property access (`store.plotlyRef`) in applyZoomState. Other fields
 // like zoomUndoStack/zoomRedoStack/suppressZoomHistory are mutated directly on the
-// store object (not via `.value`), so they must be plain properties — not refs.
+// store object (not via `.value`), so they must be plain properties, not refs.
 const plotlyRefRef = ref<any>(null)
 
 type ZoomSnap = { xRange: [number, number] | null; yRanges: Record<string, [number, number]>; source: 'user' | 'init' }
@@ -294,7 +294,7 @@ describe('applyZoomState', () => {
     plotlyRefRef.value = { layout: {} }
     await applyZoomState({
       xRange: [0, 10],
-      // `y` (trace-axis ref) — applyZoomState converts to `yaxis`
+      // `y` (trace-axis ref): applyZoomState converts to `yaxis`
       // before emitting the Plotly relayout payload.
       yRanges: { y: [0, 1] },
       source: 'user',
diff --git a/apps/qc-app/src/utils/plotting/events.ts b/apps/qc-app/src/utils/plotting/events.ts
index bd1ce6a2..d56d1d1e 100644
--- a/apps/qc-app/src/utils/plotting/events.ts
+++ b/apps/qc-app/src/utils/plotting/events.ts
@@ -157,7 +157,7 @@ export const handleNewPlot = async (
   // real data.
   //
   // Note: this does NOT need to preserve Plotly's `_rangeInitial0/1`
-  // for Reset Axes — the custom Reset button in `options.ts`
+  // for Reset Axes: the custom Reset button in `options.ts`
   // computes the data extent directly from `trace.x` rather than
   // relying on Plotly's internal anchors.
   if (pendingShareZoom.value && plotlyOptions.value.traces.length) {
@@ -184,7 +184,7 @@ export const handleNewPlot = async (
   // Plotly.newPlot reuses the same DOM node, so `plotlyRef.value`'s
   // identity is unchanged and Vue's ref watchers don't refire. It does
   // wipe externally-attached listeners (the ContextPlot's brush sync,
-  // etc.) — bump an epoch so those subscribers know to re-attach.
+  // etc.), so bump an epoch so those subscribers know to re-attach.
   mainPlotEpoch.value++
 
   // Debounce long enough that a rapid scroll-wheel burst collapses
@@ -196,7 +196,7 @@ export const handleNewPlot = async (
   handleRelayout(null)
 
   // Only listen to `plotly_relayout`. We used to also wire
-  // `plotly_redraw`, which fires on every Plotly re-paint — so each
+  // `plotly_redraw`, which fires on every Plotly re-paint, so each
   // scroll tick routed through BOTH debouncers (one per event
   // type) and handleRelayout ran twice per gesture, each heavy pass
   // competing with the user's in-progress zoom. The relayout event
@@ -206,7 +206,7 @@ export const handleNewPlot = async (
     'plotly_relayout',
     debounce(handleRelayout, debounceDelay)
   )
-  // Zoom-history recorder — runs on its own 350 ms debouncer so a single
+  // Zoom-history recorder: runs on its own 350 ms debouncer so a single
   // drag/scroll gesture collapses to one entry. Kept independent of the
   // relayout handler above, which does tooltip/visible-point work.
   installZoomTracking(plotlyRef.value)
diff --git a/apps/qc-app/src/utils/plotting/interaction.ts b/apps/qc-app/src/utils/plotting/interaction.ts
index cfd87732..a3e3c137 100644
--- a/apps/qc-app/src/utils/plotting/interaction.ts
+++ b/apps/qc-app/src/utils/plotting/interaction.ts
@@ -39,7 +39,7 @@ const processMouseMove = (event: MouseEvent) => {
 
   // PRIVATE-API: `_fullLayout` (and the `xaxis.p2c` / `yaxis.p2c` pixel-to-data
   // converters it exposes) are undocumented Plotly internals used here for
-  // mouse-move coordinate conversion. They have no public type — see
+  // mouse-move coordinate conversion. They have no public type; see
   // `src/types/plotly-dist.d.ts` for the local `PrivatePlotlyHTMLElement`
   // augmentation. If these break in a future Plotly version, switch to a
   // published gestures API or recompute coordinates from `layout.range`.
@@ -88,7 +88,7 @@ const processMouseMove = (event: MouseEvent) => {
   if (!insidePlot) {
     // Leaving the plot area while still over the root element (e.g.
     // cursor dropped onto the qualifier band or a tick gutter) should
-    // hide the readout and crosshair — otherwise they freeze at the
+    // hide the readout and crosshair, otherwise they freeze at the
     // last in-plot position.
     showCoordinates.value = false
     if (crosshair.value.visible) crosshair.value.visible = false
@@ -189,7 +189,7 @@ const collectRightAxes = (
     // user sees the blue hover tint on top of the chart (from the
     // `.drag.cursor-*:hover` styling in Plot.vue) in a region that
     // isn't a visible axis column. The wheel-zoom picker has the
-    // same concern — routing scroll to an axis with no visible
+    // same concern: routing scroll to an axis with no visible
     // chrome is surprising.
     if (ax.visible === false) continue
     if (typeof ax._mainLinePosition !== 'number') continue
@@ -236,7 +236,7 @@ export const widenYAxisDragRects = (gd: HTMLElement): void => {
   type Span = { subplotId: string; zoneLeft: number; zoneRight: number }
   const spans: Span[] = []
 
-  // Primary y-axis (side: 'left'). No `_shift` — autoshift only
+  // Primary y-axis (side: 'left'). No `_shift`: autoshift only
   // applies to overlays with `anchor: 'free'`. Zone is from the
   // graph's left edge out to the axis line.
   const primaryY = fl.yaxis as
@@ -304,7 +304,7 @@ export const widenYAxisDragRects = (gd: HTMLElement): void => {
  * the counter-axis origin (plot-left), so the hidden axis's drag
  * column lands right next to the primary QC axis. With the hover
  * tint from `.drag.cursor-*:hover` in Plot.vue, that renders as a
- * stray blue rectangle hugging the QC axis — even though the axis
+ * stray blue rectangle hugging the QC axis, even though the axis
  * chrome itself is gone. Reproduces with as few as two plotted
  * datastreams (one QC + one hidden overlay).
  *
@@ -315,7 +315,7 @@ export const widenYAxisDragRects = (gd: HTMLElement): void => {
  * selectors in Plot.vue can't match. `pointer-events: none` is a
  * belt-and-braces in case any sibling CSS still widens them.
  *
- * Must re-run after every `plotly_afterplot` — Plotly rebuilds drag
+ * Must re-run after every `plotly_afterplot`: Plotly rebuilds drag
  * rects on each relayout and our DOM edits don't survive a rebuild.
  */
 export const suppressHiddenAxisDragRects = (gd: HTMLElement): void => {
@@ -332,7 +332,7 @@ export const suppressHiddenAxisDragRects = (gd: HTMLElement): void => {
   ]
 
   for (const key of Object.keys(fl)) {
-    // Guard the primary yaxis — it's always visible and owns the QC
+    // Guard the primary yaxis: it's always visible and owns the QC
     // axis's drag column; zeroing it would break rescale-drag on QC.
     if (!Y_AXIS_KEY_RE.test(key) || key === 'yaxis') continue
     const ax = fl[key] as
@@ -391,7 +391,7 @@ export const updateAxisChips = (gd: PlotlyHTMLElement | null): void => {
   const graphWidth = (gd as HTMLElement).clientWidth
   let leftIdx = 0
   let rightIdx = 0
-  // Iterate `graphSeriesArray` — it's the authoritative sidebar
+  // Iterate `graphSeriesArray`: it's the authoritative sidebar
   // order; `gd.data` gets reversed so traces paint top-over-bottom
   // (see `createPlotlyOption`), which would flip the stacked chips.
   for (const series of graphSeriesArray.value) {
@@ -563,7 +563,7 @@ export const handleWheel = (event: WheelEvent) => {
   // `ax.range`), so on the second-and-later wheel tick in a burst
   // the pivot would land tens of milliseconds off the cursor's
   // actual data position. That offset accumulates and reads as a
-  // pan on top of the zoom — exactly what the user reported.
+  // pan on top of the zoom, exactly what the user reported.
   // Computing the pivot manually from the up-to-date range
   // sidesteps the stale closure entirely.
   const live = (gd as { layout?: Record<string, unknown> } | null)?.layout
diff --git a/apps/qc-app/src/utils/plotting/operations.ts b/apps/qc-app/src/utils/plotting/operations.ts
index a8bf252c..6b101f7d 100644
--- a/apps/qc-app/src/utils/plotting/operations.ts
+++ b/apps/qc-app/src/utils/plotting/operations.ts
@@ -17,7 +17,7 @@ import type { AppPlotlyTrace } from './options'
 /**
  * Zoom the x-axis to an explicit [start, end] window without touching
  * the data. Used by the editor's preset buttons, which should be a
- * visual-only zoom — unlike the Select-view sidebar presets, which
+ * visual-only zoom, unlike the Select-view sidebar presets, which
  * drive a fresh fetch + redraw via `useDataVisStore#onDateBtnClick`.
  */
 export const zoomXaxisTo = async (
@@ -54,8 +54,8 @@ export const toggleTraceVisibility = async (
  *
  * A dot-path relayout (`{'yaxisN.visible': false}`) hides the chrome
  * but doesn't rerun the autoshift pass, so the column stays reserved.
- * We rebuild via `handleNewPlot(preserveZoom)` — the same replot path
- * used for QC swaps and trace reorders — which regenerates the layout
+ * We rebuild via `handleNewPlot(preserveZoom)` (the same replot path
+ * used for QC swaps and trace reorders), which regenerates the layout
  * from `createPlotlyOption` with the updated `hiddenAxisIds` and
  * preserves the user's viewport.
  *
@@ -199,7 +199,7 @@ export const cropXaxisRange = async () => {
  * points. Mirror of `fitYaxisToVisible`: for each trace we consider the
  * points already within the live x-range, keep those whose y also falls
  * inside the trace's live y-range, and shrink the x-axis to span their
- * min/max x (with 10% padding — same amount as the Y variant).
+ * min/max x (with 10% padding, same amount as the Y variant).
  * @param _eventData unused; preserved for the modebar click signature.
  */
 export const fitXaxisToVisible = async (_eventData?: unknown) => {
@@ -280,7 +280,7 @@ export const fitXaxisToVisible = async (_eventData?: unknown) => {
 /**
  * Crops the QC trace's y axis to the extent of its currently visible
  * points. Only the primary `yaxis` (where QC always lives, per
- * `createPlotlyOption`) is rescaled — every non-QC overlay keeps the
+ * `createPlotlyOption`) is rescaled; every non-QC overlay keeps the
  * range the user set on it, so clicking this button never reshuffles
  * the companion axes. The data considered is exclusively the QC
  * trace's y-values within the current live x-range.
@@ -343,10 +343,10 @@ export const fitYaxisToVisible = async (_eventData?: unknown) => {
 
     if (yMax === yMin || !Number.isFinite(yMin) || !Number.isFinite(yMax)) return
 
-    // Fit Y means fit; no padding. Mirrors `fitXaxisToVisible` — any
+    // Fit Y means fit; no padding. Mirrors `fitXaxisToVisible`: any
     // padding reads as a margin Plotly conjured up.
     // Reuse the Plotly.update layout-object form (same shape the old
-    // loop-over-all-axes seam used) — a previous attempt with
+    // loop-over-all-axes seam used). A previous attempt with
     // Plotly.relayout + dot-path keys silently no-op'd in some cases
     // and left the y-axis showing the whole extent.
     const layoutUpdates: Partial<Layout> & Record<string, Partial<LayoutAxis>> = {
diff --git a/apps/qc-app/src/utils/plotting/options.ts b/apps/qc-app/src/utils/plotting/options.ts
index 5a25e09d..0841d66f 100644
--- a/apps/qc-app/src/utils/plotting/options.ts
+++ b/apps/qc-app/src/utils/plotting/options.ts
@@ -89,7 +89,7 @@ export type AxisChip = {
 // from the lighter non-QC companions). Slots 1..8 are the light half
 // of d3's category20 with red and pink removed.
 export const COLORS = [
-  '#3f3f3f', // QC — medium grey
+  '#3f3f3f', // QC: medium grey
   '#aec7e8', // light blue
   '#ffbb78', // light orange
   '#98df8a', // light green
@@ -102,7 +102,7 @@ export const COLORS = [
 
 // Darkened companion palette used for tick labels and axis titles.
 export const LABEL_COLORS = [
-  '#3f3f3f', // QC — dark grey (unchanged)
+  '#3f3f3f', // QC: dark grey (unchanged)
   '#1f77b4', // blue
   '#c06a00', // orange
   '#208020', // green
@@ -143,7 +143,7 @@ const primaryAxis = (
 })
 
 /**
- * Indices `i` (i >= 1) where `x[i] - x[i-1] > thresholdMs` — every spot
+ * Indices `i` (i >= 1) where `x[i] - x[i-1] > thresholdMs`: every spot
  * where a synthetic break should be inserted to disconnect the rendered
  * line. Callers pass `intendedSpacingMs`; a strict `>` matches "wider
  * than the intended cadence" without flagging cadence-tight points.
@@ -193,7 +193,7 @@ export const insertGapBreaks = (
 }
 
 // Colour palette for qualifier-flag markers along the bottom of the plot.
-// Reds and pinks are deliberately excluded — red is reserved for point
+// Reds and pinks are deliberately excluded; red is reserved for point
 // selection.
 export const QUALIFIER_COLORS = [
   '#ff7f0e', // orange
@@ -211,7 +211,7 @@ const iconRescaleY = {
   height: 600,
   path: 'M182.6 9.4c-12.5-12.5-32.8-12.5-45.3 0l-96 96c-12.5 12.5-12.5 32.8 0 45.3s32.8 12.5 45.3 0L128 109.3l0 293.5L86.6 361.4c-12.5-12.5-32.8-12.5-45.3 0s-12.5 32.8 0 45.3l96 96c12.5 12.5 32.8 12.5 45.3 0l96-96c12.5-12.5 12.5-32.8 0-45.3s-32.8-12.5-45.3 0L192 402.7l0-293.5 41.4 41.4c12.5 12.5 32.8 12.5 45.3 0s12.5-32.8 0-45.3l-96-96z',
 }
-// Horizontal double-arrow — FontAwesome `arrows-alt-h`.
+// Horizontal double-arrow (FontAwesome `arrows-alt-h`).
 const iconRescaleX = {
   width: 512,
   height: 512,
@@ -362,7 +362,7 @@ function buildQualifierBand(
       const ts = new Date(e.appliedAt)
       const when = isNaN(ts.getTime()) ? e.appliedAt : ts.toLocaleString()
       texts.push(
-        `<b>${code}</b>${e.description ? ' — ' + e.description : ''}` +
+        `<b>${code}</b>${e.description ? ': ' + e.description : ''}` +
         `<br>Applied: ${when}` +
         `<br>By: ${e.appliedBy}`
       )
@@ -483,7 +483,7 @@ export const createPlotlyOption = (
     // Series with a declared cadence get a sibling lines overlay; series
     // without one render as a pure scatter. Hoisted here so the marker-
     // opacity seed below can skip the density fade for scatter-only
-    // series — they have no line fallback to read once markers vanish.
+    // series, since they have no line fallback to read once markers vanish.
     const spacingMs = s.intendedSpacingMs ?? null
     const hasLineFallback = !!(spacingMs && spacingMs > 0)
 
@@ -503,7 +503,7 @@ export const createPlotlyOption = (
     // intended cadence, a sibling overlay trace draws the connecting line
     // with NaN-y breaks at gaps; otherwise the series renders as a pure
     // scatter plot. Either way the main trace owns selection and point
-    // indices, so its `line` attribute would be dead config — omitted.
+    // indices, so its `line` attribute would be dead config, so it is omitted.
     const trace: AppPlotlyTrace = {
       id: s.id,
       x: xData,
@@ -523,7 +523,7 @@ export const createPlotlyOption = (
       trace.selected = { marker: { color: 'red', opacity: 1 } }
 
       // Data is pre-windowed in the record, so plot indices already align
-      // with it — no offset needed.
+      // with it; no offset needed.
       trace._windowStartIdx = 0
 
       ;(yaxis as Record<string, Partial<LayoutAxis>>)[axisKey] = primaryAxis(
@@ -601,7 +601,7 @@ export const createPlotlyOption = (
     // intended cadence, a sibling overlay trace draws the connecting
     // line with NaN-y break points at every gap wider than the cadence,
     // so the rendered polyline visually disconnects across real gaps.
-    // Series without a declared cadence stay as a pure scatter — the
+    // Series without a declared cadence stay as a pure scatter. The
     // main trace already renders markers-only, so simply not pushing
     // an overlay is enough. The overlay carries no `id`, so
     // selection/lookup logic that finds traces by datastream id keeps
@@ -752,7 +752,7 @@ export const createPlotlyOption = (
   // Custom Reset Axes button. Replaces Plotly's stock `resetScale2d`,
   // which restores from `_rangeInitial0/1`. Those markers get pinned
   // to whatever range was active when the user first interacted with
-  // the plot — which on a fresh reload with a URL zoom is the URL
+  // the plot, which on a fresh reload with a URL zoom is the URL
   // view, not the data extent. Reset then bounces back to the URL
   // view instead of the data extent, which the user perceived as
   // "Reset doesn't work."
diff --git a/apps/qc-app/src/utils/plotting/plotly.ts b/apps/qc-app/src/utils/plotting/plotly.ts
index 67ca8027..683bd334 100644
--- a/apps/qc-app/src/utils/plotting/plotly.ts
+++ b/apps/qc-app/src/utils/plotting/plotly.ts
@@ -1,5 +1,5 @@
 // Barrel for the plotting module. Each sub-module owns a concern.
-// internal.ts is intentionally NOT re-exported — its helpers are private.
+// internal.ts is intentionally NOT re-exported; its helpers are private.
 export * from './options'
 export * from './zoom'
 export * from './relayout'
diff --git a/apps/qc-app/src/utils/plotting/relayout.ts b/apps/qc-app/src/utils/plotting/relayout.ts
index 2308b530..1a130636 100644
--- a/apps/qc-app/src/utils/plotting/relayout.ts
+++ b/apps/qc-app/src/utils/plotting/relayout.ts
@@ -221,7 +221,7 @@ export const handleRelayout = async (
       // markers fade past the density threshold so the lines stay
       // readable. `areTooltipsEnabled` already encodes the mode
       // policy (manual override vs. threshold-driven auto), so the
-      // threshold isn't checked again here — doing so would make a
+      // threshold isn't checked again here; doing so would make a
       // user's manual "on" silently flip off past the cap.
       //
       // Scatter-only series (datastreams without a declared
@@ -260,7 +260,7 @@ export const handleRelayout = async (
         // density-driven `marker.opacity` for non-QC traces so they
         // stay opted out of Plotly's global selection-fade no matter
         // what density bucket they land in. Pass `null` for the QC
-        // trace to leave its `selected.marker` config untouched —
+        // trace to leave its `selected.marker` config untouched.
         // Plotly's restyle skips entries whose array slot is null.
         const perTraceUnselectedOpacity = perTraceOpacity.map((o, i) => {
           const isQc = qcId != null && traces[i]?.id === qcId
diff --git a/apps/qc-app/src/utils/plotting/selected.ts b/apps/qc-app/src/utils/plotting/selected.ts
index 0c215f76..51dd1394 100644
--- a/apps/qc-app/src/utils/plotting/selected.ts
+++ b/apps/qc-app/src/utils/plotting/selected.ts
@@ -22,11 +22,11 @@ import type { AppPlotlyTrace } from './options'
  * `suppressedEchoSelection` with the indices their write will land
  * on. When the next `plotly_relayout`-induced `handleSelected` call
  * fires (`fromRelayout: true`) we compare the trace's actual
- * `selectedpoints` against that expected payload — equal means the
+ * `selectedpoints` against that expected payload: equal means the
  * echo (skip dispatch); different means a user gesture (box / lasso
  * select) raced through the same debounce window, so dispatch
  * normally. The click-induced path (`fromRelayout` falsy, default)
- * ignores the sentinel entirely — user clicks always dispatch.
+ * ignores the sentinel entirely; user clicks always dispatch.
  */
 export const handleSelected = async (
   eventData?: PlotMouseEvent | PlotRelayoutEvent | PlotSelectionEvent | null,
@@ -63,16 +63,16 @@ export const handleSelected = async (
   // Only dispatch the SELECTION filter on user gestures. Two
   // suppression signals layer here:
   //
-  //  1. `isUpdating` — set by undo/redo and dispatch-helper composables
+  //  1. `isUpdating`: set by undo/redo and dispatch-helper composables
   //     so a programmatic re-render doesn't echo back as a SELECTION
   //     and clobber the redo stack.
-  //  2. `suppressedEchoSelection` — payload-keyed sentinel armed by
+  //  2. `suppressedEchoSelection`: payload-keyed sentinel armed by
   //     `setPlotSelection` / `clearSelected`. We always clear it (one-
   //     shot), but only suppress when the current selection MATCHES
   //     what the echo should carry. A mismatch means a real user
   //     gesture overlapped the programmatic write's debounce window,
   //     and dropping it would be the bug we're guarding against.
-  //     Click and direct-call paths bypass — they're real gestures.
+  //     Click and direct-call paths bypass; they're real gestures.
   if (opts.fromRelayout && suppressedEchoSelection.value != null) {
     const expected = suppressedEchoSelection.value
     suppressedEchoSelection.value = null
@@ -89,7 +89,7 @@ export const handleSelected = async (
 
 /** Order-preserving equality for selection payloads. Selection
  *  arrays are produced sorted by both Plotly and our setters, so a
- *  positional compare is enough — no need to allocate a Set. */
+ *  positional compare is enough, no need to allocate a Set. */
 const sameSelection = (a: number[], b: number[]): boolean => {
   if (a.length !== b.length) return false
   for (let i = 0; i < a.length; i++) {
diff --git a/apps/qc-app/src/utils/plotting/staging.ts b/apps/qc-app/src/utils/plotting/staging.ts
index ace49967..01f66421 100644
--- a/apps/qc-app/src/utils/plotting/staging.ts
+++ b/apps/qc-app/src/utils/plotting/staging.ts
@@ -77,7 +77,7 @@ function currentDragmode(): string {
 /**
  * Force the plot back into pan mode. Called by the staging-based
  * operation panels (Find Gaps / Fill Gaps) when they open so the
- * range overlay is immediately interactive — otherwise a user who
+ * range overlay is immediately interactive. Otherwise a user who
  * was last in zoom/select/lasso mode would open the panel to a
  * hidden band (we drop it outside pan mode to keep those tools
  * unobstructed) and have to switch tools themselves to resize it.
@@ -93,7 +93,7 @@ export async function enterPanMode(): Promise<void> {
  * Add (or replace) the single editable range shape that represents
  * the operation's staged date window. The shape spans the full
  * y-axis so the user can grab it anywhere vertically. It only
- * actually renders in pan mode (see `flushShapes`) — zoom /
+ * actually renders in pan mode (see `flushShapes`); zoom /
  * select / lasso modes drop it entirely to keep their box-drag
  * gestures unobstructed.
  */
@@ -245,7 +245,7 @@ export function onStageDrag(
     // Rebuild the shape with the horizontal update applied and y
     // pinned to the full paper span. Pushing the whole shape (not
     // a dotted-path y0/y1 relayout) overrides Plotly's in-progress
-    // drag state in a single write — the earlier split approach
+    // drag state in a single write. The earlier split approach
     // (x via the parent's watcher, y via a separate relayout) was
     // racing, leaving vertical edits visible and horizontal edits
     // dropped.
diff --git a/apps/qc-app/src/utils/plotting/zoom.ts b/apps/qc-app/src/utils/plotting/zoom.ts
index 04cb3d02..b5462582 100644
--- a/apps/qc-app/src/utils/plotting/zoom.ts
+++ b/apps/qc-app/src/utils/plotting/zoom.ts
@@ -101,7 +101,7 @@ export const recordZoomIfSettled = (
   store.pushZoomState(snap)
 }
 
-/** 350 ms debounce — slightly longer than the relayout handler so a single
+/** 350 ms debounce, slightly longer than the relayout handler so a single
  * drag gesture collapses to one entry even when the user pauses briefly
  * mid-gesture. Module-private: wire via `installZoomTracking` instead. */
 const recordZoomDebounced = debounce(
diff --git a/apps/qc-app/src/utils/share.ts b/apps/qc-app/src/utils/share.ts
index eb44d4e4..d448584e 100644
--- a/apps/qc-app/src/utils/share.ts
+++ b/apps/qc-app/src/utils/share.ts
@@ -64,7 +64,7 @@ export interface ShareState {
   /** Custom-window end (epoch ms). Only emitted when no preset is
    *  active. */
   endMs?: number | null
-  /** Sidebar filters — only emitted when the recipient lands on the
+  /** Sidebar filters. Only emitted when the recipient lands on the
    *  Select view (they don't affect the plot). */
   thingIds?: string[]
   observedPropertyNames?: string[]
@@ -78,7 +78,7 @@ export interface ShareState {
    *  position in `datastreamIds` (`ds`), same exclusions as
    *  `traceVisibility`. */
   axisVisibility?: boolean[]
-  /** Plot zoom — both x and per-axis y. */
+  /** Plot zoom: both x and per-axis y. */
   zoom?: ShareableZoom
   /** Data-points marker mode. Default is `auto`. */
   dataPointsMode?: 'auto' | 'manualOn' | 'manualOff'
@@ -175,7 +175,7 @@ export function encodeShareState(state: ShareState): Record<string, string> {
     q.pl = state.processingLevelNames.join(',')
   }
 
-  // Visibility — only serialise when at least one trace is hidden
+  // Visibility: only serialise when at least one trace is hidden
   // (false). Empty/all-visible → omit.
   if (state.traceVisibility?.some((v) => v === false)) {
     const hidden = state.traceVisibility.map((v) => v === false)
diff --git a/apps/qc-app/tsconfig.app.json b/apps/qc-app/tsconfig.app.json
index 99de6af4..fa0cf90e 100644
--- a/apps/qc-app/tsconfig.app.json
+++ b/apps/qc-app/tsconfig.app.json
@@ -1,7 +1,7 @@
 {
   "extends": "@vue/tsconfig/tsconfig.dom.json",
   "compilerOptions": {
-    // `paths` without `baseUrl` — TS 5+ resolves each pattern relative
+    // `paths` without `baseUrl`: TS 5+ resolves each pattern relative
     // to this tsconfig's directory, so `@/*` still maps to `./src/*`.
     // `baseUrl` itself is deprecated in TS 6 and removed in TS 7.
     "paths": {
@@ -39,7 +39,7 @@
     "src/**/*.vue",
     "vitest.config.ts"
   ],
-  // Tests live under `tsconfig.test.json` (more lenient — see comments
+  // Tests live under `tsconfig.test.json` (more lenient; see comments
   // there); excluding them from the app project keeps production code
   // strict while the suite still type-checks under its own config.
   "exclude": [
diff --git a/apps/qc-app/tsconfig.test.json b/apps/qc-app/tsconfig.test.json
index ac0c941c..7ff9310c 100644
--- a/apps/qc-app/tsconfig.test.json
+++ b/apps/qc-app/tsconfig.test.json
@@ -7,7 +7,7 @@
     // mocked-store arrays positionally on every line. Under strict
     // `noUncheckedIndexedAccess` each access widens to `T | undefined`
     // and the tests would need a non-null assertion on every line for
-    // no real safety gain — the indices are deterministic in the test
+    // no real safety gain, since the indices are deterministic in the test
     // fixtures. Disabling the check here keeps production code strict
     // while letting specs read naturally.
     "noUncheckedIndexedAccess": false
@@ -19,7 +19,7 @@
     // Source must transitively compile when a test imports it, but the
     // ambient module shims (`plotly.js-dist`, vite/client, vuetify
     // styles) live in `src/types/` and need to be visible to this
-    // project too — declarations are project-scoped, not loaded
+    // project too: declarations are project-scoped, not loaded
     // through the file graph.
     "src/**/*.ts",
     "src/**/*.vue",
diff --git a/apps/qc-app/vite.config.ts b/apps/qc-app/vite.config.ts
index 3719a762..d7f59b7e 100644
--- a/apps/qc-app/vite.config.ts
+++ b/apps/qc-app/vite.config.ts
@@ -53,7 +53,7 @@ export default defineConfig(({ command, mode }) => {
       },
       // COOP/COEP headers enable SharedArrayBuffer-backed workers (fast
       // shared-memory data ops) but also block cross-origin responses
-      // that don't carry CORP — including the `playground.hydroserver.org`
+      // that don't carry CORP, including the `playground.hydroserver.org`
       // API. On by default; set `VITE_APP_DISABLE_COOP=1` to drop them
       // when you need to talk to a backend that doesn't serve CORP headers.
       // The worker layer gracefully falls back to inline execution when
@@ -142,11 +142,11 @@ export default defineConfig(({ command, mode }) => {
           '**/src/utils/plotting/events.ts',
           '**/src/utils/plotting/interaction.ts',
           '**/src/utils/plotting/operations.ts',
-          // Plotly DOM-staging seam — same shape as `events.ts` /
+          // Plotly DOM-staging seam, same shape as `events.ts` /
           // `interaction.ts`. Mostly Plotly relayout calls + drag-
           // gesture wiring that resists meaningful unit testing.
           '**/src/utils/plotting/staging.ts',
-          // Barrel re-export — no logic to cover, but appears in the
+          // Barrel re-export: no logic to cover, but appears in the
           // coverage report at 0% because nothing imports it directly
           // from a test (everything goes through the source modules).
           '**/src/utils/plotting/plotly.ts',

From de2655fd1b4f69336ebd38e57b135869b07545ff Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:00:58 -0600
Subject: [PATCH 56/93] fix: show the multiplication sign in gap interval
 labels

---
 apps/qc-app/src/components/EditData/FillGaps.vue                | 2 +-
 apps/qc-app/src/components/EditData/ShiftDatetimes.vue          | 2 +-
 apps/qc-app/src/components/FilterPoints/GapFinder.vue           | 2 +-
 .../qc-app/src/components/Navigation/PerformanceCalibration.vue | 2 +-
 4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/apps/qc-app/src/components/EditData/FillGaps.vue b/apps/qc-app/src/components/EditData/FillGaps.vue
index c7672c81..4865c2a6 100644
--- a/apps/qc-app/src/components/EditData/FillGaps.vue
+++ b/apps/qc-app/src/components/EditData/FillGaps.vue
@@ -358,7 +358,7 @@ const fillSnapChips = computed<SnapChip[]>(() => {
   return [0.5, 1, 2].map((m) => {
     const amount = i.amount * m
     return {
-      label: `${m}× intended (${amount} ${i.unit.toLowerCase()})`,
+      label: `${m}× intended (${amount} ${i.unit.toLowerCase()})`,
       amount,
       unit: i.unit,
       active:
diff --git a/apps/qc-app/src/components/EditData/ShiftDatetimes.vue b/apps/qc-app/src/components/EditData/ShiftDatetimes.vue
index cbfafff6..9ad3a719 100644
--- a/apps/qc-app/src/components/EditData/ShiftDatetimes.vue
+++ b/apps/qc-app/src/components/EditData/ShiftDatetimes.vue
@@ -104,7 +104,7 @@ const snapChips = computed<SnapChip[]>(() => {
   return [0.5, 1, 2].map((m) => {
     const amount = n * m
     return {
-      label: `${m}× intended (${amount} ${unitKey.toLowerCase()})`,
+      label: `${m}× intended (${amount} ${unitKey.toLowerCase()})`,
       amount,
       unit: unitKey,
       active:
diff --git a/apps/qc-app/src/components/FilterPoints/GapFinder.vue b/apps/qc-app/src/components/FilterPoints/GapFinder.vue
index 6ff539be..ba35bea7 100644
--- a/apps/qc-app/src/components/FilterPoints/GapFinder.vue
+++ b/apps/qc-app/src/components/FilterPoints/GapFinder.vue
@@ -256,7 +256,7 @@ const snapChips = computed<SnapChip[]>(() => {
   return multipliers.map((m) => {
     const amount = n * m
     return {
-      label: `${m}× intended (${amount} ${unitKey.toLowerCase()})`,
+      label: `${m}× intended (${amount} ${unitKey.toLowerCase()})`,
       amount,
       unit: unitKey,
       active:
diff --git a/apps/qc-app/src/components/Navigation/PerformanceCalibration.vue b/apps/qc-app/src/components/Navigation/PerformanceCalibration.vue
index 17d6e90e..7ed7cca2 100644
--- a/apps/qc-app/src/components/Navigation/PerformanceCalibration.vue
+++ b/apps/qc-app/src/components/Navigation/PerformanceCalibration.vue
@@ -158,7 +158,7 @@
                   the algorithm, not the machine, so it's shipped with qc-utils,
                   not measured at runtime. Recalibration only re-measures the
                   three device primitives above; weights stay fixed. The
-                  dispatch formula is <code>weight × N / throughput</code>, so
+                  dispatch formula is <code>weight × N / throughput</code>, so
                   one universal weight per op plus your per-device throughputs
                   covers the full operation catalog.
                 </p>

From 3dce0ac1ee91db8e64f58a93ac29ee17b935cc2c Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:00:58 -0600
Subject: [PATCH 57/93] fix: serialize plot loads and draw once the plot
 element exists

Drops context responses for a stale range, guards the post-commit
session refresh, highlights All for a saved YTD in the editor and adds
a truthful editor-ready signal for the e2e suite.
---
 apps/qc-app/docs/API_REFERENCE.md             |  27 +-
 apps/qc-app/docs/ARCHITECTURE.md              |   4 +-
 apps/qc-app/docs/DEPLOYMENT.md                |   4 +-
 apps/qc-app/docs/PERFORMANCE.md               |  10 +
 apps/qc-app/docs/PLOTTING.md                  |  40 ++-
 apps/qc-app/docs/TESTING.md                   |  14 +-
 apps/qc-app/docs/USER_GUIDE.md                |   3 +-
 apps/qc-app/e2e/support/app.ts                |  18 +-
 apps/qc-app/e2e/support/ops.ts                |   4 +-
 .../EditData/__tests__/SessionList.spec.ts    |   6 +-
 apps/qc-app/src/components/VisualizeData.vue  |   5 +-
 .../VisualizeData/DataVisTimeFilters.vue      |  12 +-
 .../src/components/VisualizeData/Plot.vue     | 103 +++---
 .../VisualizeData/PlottedDatastreams.vue      |   3 +-
 .../__tests__/DataVisTimeFilters.spec.ts      |  15 +
 .../VisualizeData/__tests__/Plot.spec.ts      |  66 ++++
 .../__tests__/PlottedDatastreams.spec.ts      |  19 ++
 .../__tests__/useEditSession.spec.ts          |  28 ++
 apps/qc-app/src/composables/useEditSession.ts |   9 +-
 .../store/__tests__/dataVisualization.spec.ts | 294 +++++++++++++++++-
 .../src/store/__tests__/observations.spec.ts  |  52 ++++
 .../src/store/__tests__/qcSession.spec.ts     |  14 +-
 apps/qc-app/src/store/dataVisualization.ts    | 267 ++++++++++------
 apps/qc-app/src/store/observations.ts         |  35 ++-
 apps/qc-app/src/store/qcSession.ts            |  13 -
 .../utils/__tests__/timeRangePresets.spec.ts  |  17 +
 apps/qc-app/src/utils/timeRangePresets.ts     |  11 +
 apps/qc-app/vite.config.ts                    |   4 +-
 28 files changed, 890 insertions(+), 207 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 0de20ab1..0eff6684 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -224,9 +224,12 @@ const {
   (append-only reconcile).
 - `discardUnsavedEdits()`: drops edits made since the last save and restores
   edited comments.
-- `commit()` — saves, verifies checksum C, pushes observations
-  (`mode: 'replace'`), then locks the session.
-- `hasUnsavedChanges` / `unsavedEditCount` — the working copy compared with
+- `commit()`: saves, verifies checksum C, pushes observations
+  (`mode: 'replace'`), then locks the session and reloads the sessions. If
+  another target took over the editor while they reloaded, it leaves the
+  session store and the saved-edits baseline alone and still resolves, since
+  the commit itself went through.
+- `hasUnsavedChanges` / `unsavedEditCount`: the working copy compared with
   the saved-edits baseline (`qcSession.savedEdits` / `savedComments`). The
   baseline lives in the store, so every caller agrees: the editor footer and
   the nav rail's exit guard both read it.
@@ -340,13 +343,15 @@ on boot.
 | `selectedQualifier`                 | state    | `string`                                          | Active qualifier in the picker. |
 | `selectedData`                      | state    | `number[] \| null`                                | Index list of the active selection (lasso, box, click). |
 | `hasSelectionShape`                 | state    | `boolean`                                         | True while a box/lasso shape exists, even when it captured zero points. |
-| `loadingStates`                     | state    | `Map<string, boolean>`                            | Per-datastream in-flight observation fetches. |
+| `loadingStates`                     | state    | `Map<string, boolean>`                            | Per-datastream in-flight observation fetches. Only the latest request for a datastream clears its flag. |
+| `isEditorReady`                     | computed | `boolean`                                         | True while editing once the session window is known, the edit record is on the plot, no session is opening (`isSwitchingSession`), and no plot work is pending: no queued or running plot load (including the context re-anchor around the session window) and no tracked draw. Bound to `data-editor-ready` on the edit view's `edit-plot-column`; e2e `waitForEditorReady` waits on it. |
+| `trackPlotWork`                     | action   | `(work: () => Promise<void>) => Promise<void>`    | Run `work` counted as pending plot work for `isEditorReady`. Plot loads and `setEditRecord` use it, and `Plot.vue` counts its first draw from the moment the plot element appears. |
 | `beginDate` / `endDate`             | state    | `Date`                                            | Active loaded window. A preset re-resolves it on every plot rebuild: around the edit session's window while one is set (`presetAroundWindow`), otherwise back from the context data's (`seriesDatastreams` minus the edit target) end (`presetWindow`). It also re-resolves when the edit session's window loads or changes. A custom range stays fixed. |
 | `selectedDateBtnId`                 | state    | `number`                                          | Active preset id (default `1`, 1m); `-1` (`CUSTOM_PRESET_ID`) for a manual range. Presets are defined in `utils/timeRangePresets.ts`. |
 | `matchesSelectedThing`              | action   | `(ds) => boolean`                                 | Filter predicate; exposed so the table can reuse it on row updates. |
 | `matchesSelectedObservedProperty`   | action   | `(ds) => boolean`                                 | Same shape as above. |
 | `matchesSelectedProcessingLevel`    | action   | `(ds) => boolean`                                 | Same shape as above. |
-| `setDateRange`                      | action   | `({ begin?, end?, update?, custom? }) => Promise<void>` | No-ops when neither bound moves. Otherwise refreshes `seriesDatastreams`; while editing this reloads context only and preserves the zoom (`redraw(false, true)`), since the edit target's data isn't fetched here. Otherwise it clears zoom history and applies the new window. |
+| `setDateRange`                      | action   | `({ begin?, end?, update?, custom? }) => Promise<void>` | No-ops when neither bound moves. Otherwise sets the bounds at once and queues a range reload as a plot load (see `rebuildPlot`), which joins a queued rebuild instead of loading on its own and is skipped when an earlier load already caught up with the range. While editing it reloads context only and preserves the zoom (`redraw(false, true)`), since the edit target's data isn't fetched here. Otherwise it clears zoom history and applies the new window. |
 | `onDateBtnClick`                    | action   | `(id: number) => Promise<void>`                   | Selects the preset and applies the window the internal `resolvePresetWindow` gives it: around the edit session's window while editing one, otherwise back from the context data's end. With nothing to anchor to, only the selection changes. |
 | `refreshGraphSeriesArray`           | action   | `() => Promise<unknown[]>`                        | Reconciles `graphSeriesArray` against `seriesDatastreams` (fetch deltas + reorder + recolor), skipping the edit target (its data is owned by the edit session, not this refresh). A managed datastream with a loaded working copy (`useWorkingCopiesStore`) uses it instead of fetching. Invalidates the working copy of any managed datastream no longer in `seriesDatastreams`, except the edit target. |
 | `resetState`                        | action   | `() => void`                                      | Clears filters, the plotted set, and the edit target, and drops every working copy, on a workspace swap; preserves the preset preference. |
@@ -360,9 +365,9 @@ on boot.
 | `removeSnapshotSeries`              | action   | `(id: string) => Promise<void>`                   | Drop one snapshot line. Leaves the edit target alone. |
 | `setPlottedDatastreams`             | action   | `(items: Datastream[]) => Promise<void>`          | Replace the plotted set wholesale (used by URL hydration). Doesn't touch the edit target. Hydrate that separately with `setEditTarget`. |
 | `setEditTarget`                     | action   | `(managedId: string) => Promise<void>`            | Begin editing `managedId`: drops snapshots, resets the `qcSession` store (not `resumeDatastreamId`) and clears zoom history when the target changes, sets `qcDatastreamId`, rebuilds the plot. Its data arrives later via `setEditRecord`. This action doesn't fetch it. |
-| `setEditRecord`                     | action   | `(record: ObservationRecord) => Promise<void>`    | Put the edit target's record on the plot: updates its graph series in place if one exists, otherwise adds it (`buildGraphSeries`) and redraws. The only path by which the edit target's data reaches the plot. |
+| `setEditRecord`                     | action   | `(record: ObservationRecord) => Promise<void>`    | Put the edit target's record on the plot: updates its graph series in place if one exists, otherwise adds it (`buildGraphSeries`) and redraws. The only path by which the edit target's data reaches the plot. Counted as plot work until drawn. |
 | `clearEditTarget`                   | action   | `() => Promise<void>`                             | Stop editing: drops snapshots, invalidates the edit target's working copy, clears `qcDatastreamId`, removes its graph series, and rebuilds. Plotted datastreams are left exactly as they were. |
-| `rebuildPlot`                       | action   | `() => Promise<void>`                             | Serialized rebuild (refresh series, regenerate options, render). Drops zoom history in the Select view; keeps the zoom while an edit target is set. Coalesces concurrent callers. |
+| `rebuildPlot`                       | action   | `() => Promise<void>`                             | Rebuild (refresh series, regenerate options, render) as a plot load. Plot loads (rebuilds and `setDateRange` range reloads) run one at a time; requests made while one runs share one queued follow-up, which is a rebuild if any of them was, and every caller resolves only after a load that started after its change. A load whose range moves while it loads drops those responses and loads again before drawing. Drops zoom history in the Select view; keeps the zoom while an edit target is set. |
 
 ### `useWorkingCopiesStore()` (`src/store/workingCopies.ts`)
 
@@ -460,7 +465,7 @@ Fetches + caches observation windows and inflates them into
 |----------------------------|----------|---------------------------------------------------|-------|
 | `observations`             | state    | `Record<string, ObservationRecord>`               | Per-datastream record; reused across rebuilds. |
 | `observationsRaw`          | state    | `Record<string, ObservationData>`                 | Typed-array cache (`Float64Array` datetimes + `Float32Array` values). |
-| `fetchObservationsInRange` | action   | `(ds: Datastream, b: Date, e: Date) => Promise<ObservationRecord>` | Extends the cached range minimally; only fetches segments outside the existing window. |
+| `fetchObservationsInRange` | action   | `(ds: Datastream, b: Date, e: Date) => Promise<ObservationRecord>` | Extends the cached range minimally; only fetches segments outside the existing window. Requests for one datastream run in order, so the record ends on the latest requested window and the cache never merges the same segment twice; a request matching the last queued range shares its promise. |
 
 ### `useWorkspaceStore()` (`src/store/workspaces.ts`)
 
@@ -604,7 +609,6 @@ puts the editor in read-only mode.
 | `sessions`          | state    | `QualityControlSession[]`               | Committed + in-progress sessions for the history. |
 | `currentSessionId`  | state    | `string \| null`                        | The single in-progress (editable) session. |
 | `viewedSessionId`   | state    | `string \| null`                        | The session currently being viewed. |
-| `isLoading`         | state    | `boolean`                               | True while `loadSessions` is in flight. |
 | `isSwitchingSession`| state    | `boolean`                               | True while another session's data and operations load. The operations panel renders a loading state instead of the outgoing session's entries, which would otherwise linger and read as the incoming session's. |
 | `savedEdits`        | state    | `HistoryItem[]`                         | Edit history entries (by reference) at the last load or save. `useEditSession` compares the working copy against it for `hasUnsavedChanges`; kept in the store so the editor footer and the nav rail's exit guard agree. |
 | `savedComments`     | state    | `string[]`                              | Comment text of `savedEdits`, since comments are edited in place. |
@@ -614,7 +618,6 @@ puts the editor in read-only mode.
 | `viewedSession`     | computed | `QualityControlSession \| null`         | The session for `viewedSessionId`. |
 | `fetchSessions`     | action   | `(historyId: string) => Promise<QualityControlSession[]>` | Fetch a history's sessions with their operations without writing any state, so a caller can drop a result that went stale (see `useEditSession`). |
 | `applySessions`     | action   | `(historyId: string, sessions: QualityControlSession[]) => void` | Adopt fetched sessions; default the view to the in-progress one, else the latest committed. |
-| `loadSessions`      | action   | `(historyId: string) => Promise<void>`  | `fetchSessions` then `applySessions`, with `isLoading` set meanwhile. |
 | `viewSession`       | action   | `(sessionId: string) => void`           | View a session read-only (no-op for an unknown id). |
 | `returnToCurrent`   | action   | `() => void`                            | Return to the editable in-progress session. |
 | `reset`             | action   | `() => void`                            | Clear all state. |
@@ -669,6 +672,10 @@ and their pure resolution:
   is `null`); YTD resolves the same as All. `null` for an unknown id.
 - `EDITOR_PRESETS`: the presets the editor's Context menu offers, without
   YTD and titled for the session window.
+- `shownPresetId(selectedId, presets)`: the chip that shows the selection
+  among `presets`. YTD shows as All where it is not offered; `null` for
+  Custom or an unknown id. Display only: `DataVisTimeFilters` highlights it
+  and `selectedDateBtnId` is unchanged.
 
 ### `src/utils/sessionWindow.ts`
 
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 7b51153c..63fcb152 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -427,8 +427,8 @@ COOP/COEP are on by default. `vite.config.ts` sets
 `Cross-Origin-Opener-Policy: same-origin` and
 `Cross-Origin-Embedder-Policy: require-corp` so `SharedArrayBuffer` is
 available for the qc-utils worker pool. The trade-off is that any
-cross-origin response without `Cross-Origin-Resource-Policy` gets blocked
-— so older HydroServer deployments that don't yet serve CORP headers need
+cross-origin response without `Cross-Origin-Resource-Policy` gets blocked,
+so older HydroServer deployments that don't yet serve CORP headers need
 `VITE_APP_DISABLE_COOP=1` to drop the headers. When SAB is unavailable
 the worker layer falls back to inline kernels, so the app keeps working
 either way (just slower on large edits).
diff --git a/apps/qc-app/docs/DEPLOYMENT.md b/apps/qc-app/docs/DEPLOYMENT.md
index d94ff9e3..6a8a8063 100644
--- a/apps/qc-app/docs/DEPLOYMENT.md
+++ b/apps/qc-app/docs/DEPLOYMENT.md
@@ -6,8 +6,8 @@ don't) get out of the box, and how upgrades and migrations work.
 
 ## Shape of a deployment
 
-The QC App builds to a fully static SPA — HTML, JS, CSS, fonts, icons,
-and inline worker bundles. Anything that can serve static files over
+The QC App builds to a fully static SPA (HTML, JS, CSS, fonts, icons,
+and inline worker bundles). Anything that can serve static files over
 HTTPS and add a few response headers can host it. There is **no server
 component, no database, and no background job** to operate alongside the
 app itself; all the moving parts live in the user's browser.
diff --git a/apps/qc-app/docs/PERFORMANCE.md b/apps/qc-app/docs/PERFORMANCE.md
index 83d6f24c..f6999375 100644
--- a/apps/qc-app/docs/PERFORMANCE.md
+++ b/apps/qc-app/docs/PERFORMANCE.md
@@ -112,6 +112,16 @@ Key fetch optimizations already in place:
   in `rawData` and slices it to the selected `[begin, end]` range via
   `applyWindow`, so the plot, table and counts only ever touch the current
   window.
+- **One plot load at a time.** Rebuilds and context range reloads are
+  serialized in `useDataVisStore`. A range reload that lands while a rebuild
+  is queued joins it, and one that finds its range already loaded skips, so
+  overlapping triggers (a session window arriving during a rebuild) do not
+  fetch the same range twice.
+- **Ordered requests per datastream.** Overlapping requests for one
+  datastream from different callers (a context load and a session base
+  load, for example) run in order, so each fills only what the previous left
+  missing, and a request for the range already queued shares that request
+  instead of fetching again.
 - **Loading state per datastream.** The UI shows a per-stream spinner so
   one slow fetch doesn't block the others.
 
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index 0a34010d..b52a7581 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -191,7 +191,7 @@ so members absent from it are unplotted in the same pass.
 Doing it in one pass matters even with nothing to promote: looping
 `plotDatastream` / `unplotDatastream` would trigger a `rebuildPlot` per
 change, where the batched action means exactly one, against the final set.
-The coalescing lock never has to absorb a burst.
+The plot load queue never has to absorb a burst.
 
 Additions are appended in `ids` order, which the dialog builds in display
 order (raw first, then managed). Plotting never singles one out as an edit
@@ -219,7 +219,8 @@ calls it instead of assigning `selectedSeries.value.data` directly.
 ## Context range
 
 The editor toolbar's **Context** menu reuses `DataVisTimeFilters` (with
-`EDITOR_PRESETS`, which leaves out YTD, and the same From / To pickers as
+`EDITOR_PRESETS`, which leaves out YTD and highlights All for a persisted
+YTD through `shownPresetId`, and the same From / To pickers as
 the Select view) bound to the same `beginDate` / `endDate` store range.
 Picking a preset or a custom date calls `setDateRange`, which, while an
 edit target is set, reloads only the context series
@@ -240,7 +241,23 @@ The context loads on `setEditTarget`'s rebuild, before the session is known.
 The store watches the session window, and when it appears or changes it
 re-applies the active preset through `setDateRange`: one context reload,
 no edit-target fetch, zoom kept, and a no-op when the range is unchanged or
-the range is Custom.
+the range is Custom. It is a watch rather than a call from each session
+action because the window follows every session store write (sessions
+applied, a session viewed, returning to the current one, a failed view
+reverted), and a missed call would silently leave the context on the wrong
+range.
+
+Plot loads run one at a time. A rebuild (`rebuildPlot`) and a range reload
+(`setDateRange`) are both plot loads: a request made while one runs waits
+in a single queued follow-up, which becomes a rebuild if any request it
+absorbed was one, since a rebuild resolves the preset itself. So a re-anchor
+that lands while a rebuild is queued joins it instead of loading on its own,
+and a rebuild requested during a range reload waits for it. `setDateRange`
+still moves `beginDate` / `endDate` at once, so a load already running can
+find its range moved: `updateOrFetchGraphSeries` drops every response whose
+range no longer matches, and the load loads the new range before drawing.
+A range reload queued behind it then finds the range already loaded and
+skips.
 
 The session window itself is shaded as a layout shape (`name: 'edit-window'`
 in `options.ts`), drawn from `useQcSessionStore().viewedSession` (falling
@@ -270,6 +287,23 @@ passes its loading overlay through Plot's `body-overlay` slot. The overlay
 covers the plot and table body only; the toolbar stays usable and shows its
 own spinner.
 
+`Plot` draws its first frame when its plot element renders, not on mount.
+The Plot tab's window item renders lazily, so a view that opens on the
+Table tab (a share link with the table tab) has no element until the Plot
+tab first opens; the draw waits for it. Until then `plotlyRef` is null, and
+every caller that redraws an existing plot (`handleNewPlot(undefined)`)
+skips, since that first draw renders the latest options anyway.
+
+## Editor readiness
+
+`useDataVisStore().isEditorReady` says the editor is settled: the session
+window is known, the edit record is on the plot, no session is opening, and
+no plot work is pending. Plot work is counted by `trackPlotWork`: every plot
+load (queued or running), `setEditRecord`'s draw, and `Plot`'s first draw
+from the moment its element appears through the mount delay. The edit view
+binds it to `data-editor-ready` on `edit-plot-column`, which the e2e helper
+`waitForEditorReady` waits on.
+
 ## Why `internal.ts` isn't re-exported
 
 `plotly.ts` is a barrel for everything the rest of the app needs.
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 34bc60a3..1b74b467 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -74,8 +74,8 @@ notable knobs:
 
 - `environment: 'jsdom'` globally; `src/components/**` matches
   `jsdom` again via `environmentMatchGlobs` for symmetry.
-- `setupFiles: ['@vitest/web-worker', './src/utils/test/setup.ts']`
-  — the worker plugin lets `?worker&inline` imports resolve in
+- `setupFiles: ['@vitest/web-worker', './src/utils/test/setup.ts']`:
+  the worker plugin lets `?worker&inline` imports resolve in
   Vitest; `setup.ts` stubs `HTMLCanvasElement.prototype.getContext`
   because jsdom doesn't implement it and Vuetify's mount cycle
   spams "Not implemented" warnings without the stub.
@@ -395,6 +395,16 @@ them so the next contributor doesn't rediscover them.
    foreground OS window. If a previously-green spec started
    failing only after you alt-tabbed, that's why.
 
+5. **Wait for the editor to be ready, not just its buttons.** Entering
+   the editor or reloading into it loads the sessions, places the edit
+   record, then reloads the context range around the session window, and
+   the plot draws its first frame after a mount delay. The footer buttons
+   and a hidden loading overlay can both show before that settles.
+   `waitForEditorReady` waits for `edit-plot-column` to carry
+   `data-editor-ready="true"` (`useDataVisStore().isEditorReady`: the
+   session window is known, the edit record is on the plot, no session is
+   opening, and no plot load or tracked draw is pending).
+
 ### Debugging a failure
 
 Playwright's HTML report (`playwright-report/index.html`) opens
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 34ae88cd..d4b4995f 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -181,7 +181,8 @@ observation: **1w**, **1m**, **6m** and **1y** load that span before the
 session's start and after its end, and **All** loads all of the context
 data (always including the session's window). **YTD** is not offered here.
 If YTD is your active preset from the Select view, the editor loads the
-context as **All** without changing that preference. When a session loads
+context as **All** and highlights the **All** chip, without changing that
+preference. When a session loads
 or you view another session, the active preset re-applies around its
 window; a `Custom` From / To range stays as you set it. Changing the range
 reloads that context data and keeps your current zoom. Your edits are
diff --git a/apps/qc-app/e2e/support/app.ts b/apps/qc-app/e2e/support/app.ts
index 6d2770de..657ba590 100644
--- a/apps/qc-app/e2e/support/app.ts
+++ b/apps/qc-app/e2e/support/app.ts
@@ -144,16 +144,18 @@ export async function startSessionFromRow(page: Page): Promise<void> {
 }
 
 /**
- * Wait for the Edit view to be open with nothing loading, e.g. after a reload
- * resumes the session. Acting earlier can race the view switch.
+ * Wait for the Edit view to be ready: its session and edit record loaded, and
+ * every plot load and draw finished, including the context reload around the
+ * session window that follows the session load (see `isEditorReady` in the
+ * data visualization store). Acting earlier can race the view switch or that
+ * reload, e.g. after a reload resumes the session.
  */
 export async function waitForEditorReady(page: Page): Promise<void> {
-  await expect(page.getByTestId('exit-save-btn')).toBeVisible({
-    timeout: 30_000,
-  })
-  await expect(page.getByTestId('data-loading-indicator')).toHaveCount(0, {
-    timeout: 30_000,
-  })
+  await expect(page.getByTestId('edit-plot-column')).toHaveAttribute(
+    'data-editor-ready',
+    'true',
+    { timeout: 30_000 }
+  )
 }
 
 /** Open an operation panel in the edit drawer by id (see operations.ts). */
diff --git a/apps/qc-app/e2e/support/ops.ts b/apps/qc-app/e2e/support/ops.ts
index 7163b5c3..4a71c808 100644
--- a/apps/qc-app/e2e/support/ops.ts
+++ b/apps/qc-app/e2e/support/ops.ts
@@ -17,8 +17,8 @@ import { openOp, waitForSelection } from './app'
  * "Add filter" button. The button path occasionally flaked on Firefox
  * because Playwright fired the click before Vuetify's v-text-field
  * committed the typed value via `update:modelValue`, leaving the
- * button's `:disabled` guard "still true" and swallowing the click
- * — the panel saw no add-filter event, `selectedData` stayed empty,
+ * button's `:disabled` guard "still true" and swallowing the click.
+ * The panel saw no add-filter event, `selectedData` stayed empty,
  * and `waitForSelection` timed out. Pressing Enter on the focused
  * input commits and dispatches in one event, so there's no window
  * for the value commit to race the click.
diff --git a/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts b/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
index 29b8d83b..f170d3e8 100644
--- a/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
@@ -54,7 +54,7 @@ async function seedAndLoad() {
     await qc.sessions.create(h.id, win('2025-02-01T00:00:00Z', '2025-03-01T00:00:00Z'))
   )
   const store = useQcSessionStore()
-  await store.loadSessions(h.id)
+  store.applySessions(h.id, await store.fetchSessions(h.id))
   return { store, committedId: committed.id, inProgressId: inProgress.id }
 }
 
@@ -108,7 +108,7 @@ describe('SessionList', () => {
     const first = unwrap(await qc.sessions.create(h.id, sameWindow))
     await qc.sessions.commit(h.id, first.id)
     const second = unwrap(await qc.sessions.create(h.id, sameWindow))
-    await useQcSessionStore().loadSessions(h.id)
+    useQcSessionStore().applySessions(h.id, await useQcSessionStore().fetchSessions(h.id))
 
     const wrapper = mountList()
     await flushPromises()
@@ -188,7 +188,7 @@ describe('SessionList', () => {
     ])
     await qc.sessions.commit(h.id, committed.id)
     await qc.sessions.create(h.id, range)
-    await useQcSessionStore().loadSessions(h.id)
+    useQcSessionStore().applySessions(h.id, await useQcSessionStore().fetchSessions(h.id))
 
     const wrapper = mountList()
     await flushPromises()
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index 3395bab8..40214b7f 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -98,6 +98,8 @@
     />
 
     <div
+      data-testid="edit-plot-column"
+      :data-editor-ready="isEditorReady"
       class="edit-view__col edit-view__col--plot d-flex flex-column flex-fill pa-3 overflow-hidden"
     >
       <v-card class="fill-height d-flex flex-column" elevation="1">
@@ -491,6 +493,7 @@ const { toggleSnapshot } = useHistorySnapshots()
 const {
   plottedDatastreams,
   qcDatastreamId,
+  isEditorReady,
   datastreams,
   things,
   beginDate,
@@ -857,7 +860,7 @@ const hydrateFromUrl = () => {
   // Snapshots replay against the session store, which the resume hook loads
   // asynchronously and independently of the plot. Wait for the editor to
   // actually be open on the `ed` target (an in-progress or viewed session
-  // means `loadSessions` has landed) rather than chaining off
+  // means its sessions were applied) rather than chaining off
   // `setPlottedDatastreams`, which only settles the plot.
   const snapshots = state.snapshots ?? []
   const editTargetId = state.editView ? state.editDatastreamId : undefined
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue b/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
index 3f2b23e6..45cc3f2d 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
@@ -36,8 +36,8 @@
         v-for="option in presets"
         :key="option.id"
         :data-testid="`date-preset-${option.label}`"
-        :color="selectedDateBtnId === option.id ? 'primary' : undefined"
-        :variant="selectedDateBtnId === option.id ? 'tonal' : 'outlined'"
+        :color="shownId === option.id ? 'primary' : undefined"
+        :variant="shownId === option.id ? 'tonal' : 'outlined'"
         size="small"
         :title="option.title"
         class="time-filters__preset-chip justify-center"
@@ -66,11 +66,13 @@ import { useDataVisStore } from '@/store/dataVisualization'
 import {
   CUSTOM_PRESET_ID,
   TIME_RANGE_PRESETS,
+  shownPresetId,
   type TimeRangePreset,
 } from '@/utils/timeRangePresets'
 import { storeToRefs } from 'pinia'
+import { computed } from 'vue'
 
-withDefaults(
+const props = withDefaults(
   defineProps<{
     description?: string
     /** Preset chips to offer; the editor leaves out YTD. */
@@ -81,6 +83,10 @@ withDefaults(
 
 const { setDateRange, onDateBtnClick } = useDataVisStore()
 const { beginDate, endDate, selectedDateBtnId } = storeToRefs(useDataVisStore())
+
+const shownId = computed(() =>
+  shownPresetId(selectedDateBtnId.value, props.presets)
+)
 </script>
 
 <style scoped>
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index b35539cf..90716f9e 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -446,6 +446,7 @@ const {
 } = storeToRefs(usePlotlyStore())
 const { selectedData, hasSelectionShape, qcDatastream } =
   storeToRefs(useDataVisStore())
+const { trackPlotWork } = useDataVisStore()
 const { viewedSession, inProgressSession } = storeToRefs(useQcSessionStore())
 
 const tooltipsAutoDisabled = computed(
@@ -667,65 +668,91 @@ const keyboardShortcuts = [
 
 let plotResizeObserver: ResizeObserver | null = null
 let pendingResizeFrame: number | null = null
-let mountDrawTimer: ReturnType<typeof setTimeout> | null = null
+let cancelFirstDraw: (() => void) | null = null
 // Deferred work checks this so it never touches a detached plot.
 let isUnmounted = false
 
-onMounted(async () => {
+onMounted(() => {
   // Flip before handleNewPlot so createPlotlyOption emits the
   // preview layout (no qualifier band, no title, tight margins).
   previewMode.value = !!props.preview
   updateOptions()
+})
+
+// The plot tab renders its element lazily, so a view that opens on the
+// Table tab has no element at mount. Draw when the element appears.
+watch(
+  plot,
+  (target) => {
+    if (target) scheduleFirstDraw(target)
+  },
+  { flush: 'post' }
+)
 
-  // Wait for the view-switch animation to expand the container.
-  mountDrawTimer = setTimeout(async () => {
-    mountDrawTimer = null
+// Plot work from the moment the element appears, so the editor is not
+// reported ready before the plot the user sees is drawn.
+function scheduleFirstDraw(target: HTMLDivElement) {
+  cancelFirstDraw?.()
+  const drawn = trackPlotWork(async () => {
+    // Wait for the view-switch animation to expand the container.
+    let cancel!: () => void
+    const due = await new Promise<boolean>((resolve) => {
+      const timer = setTimeout(() => resolve(true), 200)
+      cancel = () => {
+        clearTimeout(timer)
+        resolve(false)
+      }
+      cancelFirstDraw = cancel
+    })
+    // A newer schedule owns the flag by now, so only clear our own.
+    if (cancelFirstDraw === cancel) cancelFirstDraw = null
+    if (!due || isUnmounted) return
     updateOptions()
     // A share-URL zoom, if any, is applied and cleared inside
     // handleNewPlot itself, so capture it before that happens: it
     // should win over the session-window zoom below.
     const hadPendingShareZoom = !!pendingShareZoom.value
-    await handleNewPlot(plot.value)
+    await handleNewPlot(target)
     if (isUnmounted) return
     if (!props.preview && !hadPendingShareZoom) zoomToEditWindow()
+    observePlotSize(target)
+  })
+  drawn.catch((e) => console.error('First plot draw failed', e))
+}
 
-    const target = plot.value
-    if (target && typeof ResizeObserver !== 'undefined') {
-      // The plot can be laid out before the container reaches its final
-      // size, so resize whenever the two differ, including on the first
-      // notification.
-      plotResizeObserver = new ResizeObserver(() => {
-        if (pendingResizeFrame != null) return
-        pendingResizeFrame = requestAnimationFrame(() => {
-          pendingResizeFrame = null
-          const gd = plot.value as
-            | (HTMLElement & { _fullLayout?: { width: number; height: number } })
-            | null
-          const layout = gd?._fullLayout
-          // Plotly.Plots.resize rejects a hidden div (Table tab).
-          if (!gd || !layout || !gd.offsetWidth || !gd.offsetHeight) return
-          if (
-            Math.abs(layout.width - gd.offsetWidth) < 1 &&
-            Math.abs(layout.height - gd.offsetHeight) < 1
-          ) {
-            return
-          }
-          void Plotly.Plots.resize(gd as unknown as Plotly.Root)
-        })
-      })
-      plotResizeObserver.observe(target)
-    }
-  }, 200)
-})
+function observePlotSize(target: HTMLDivElement) {
+  if (typeof ResizeObserver === 'undefined') return
+  plotResizeObserver?.disconnect()
+  // The plot can be laid out before the container reaches its final
+  // size, so resize whenever the two differ, including on the first
+  // notification.
+  plotResizeObserver = new ResizeObserver(() => {
+    if (pendingResizeFrame != null) return
+    pendingResizeFrame = requestAnimationFrame(() => {
+      pendingResizeFrame = null
+      const gd = plot.value as
+        | (HTMLElement & { _fullLayout?: { width: number; height: number } })
+        | null
+      const layout = gd?._fullLayout
+      // Plotly.Plots.resize rejects a hidden div (Table tab).
+      if (!gd || !layout || !gd.offsetWidth || !gd.offsetHeight) return
+      if (
+        Math.abs(layout.width - gd.offsetWidth) < 1 &&
+        Math.abs(layout.height - gd.offsetHeight) < 1
+      ) {
+        return
+      }
+      void Plotly.Plots.resize(gd as unknown as Plotly.Root)
+    })
+  })
+  plotResizeObserver.observe(target)
+}
 
 onBeforeUnmount(() => {
   isUnmounted = true
   // Reset so the next Plot mount in Edit view doesn't inherit preview.
   if (previewMode.value) previewMode.value = false
-  if (mountDrawTimer != null) {
-    clearTimeout(mountDrawTimer)
-    mountDrawTimer = null
-  }
+  cancelFirstDraw?.()
   if (pendingResizeFrame != null) {
     cancelAnimationFrame(pendingResizeFrame)
     pendingResizeFrame = null
diff --git a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
index fb8a51dd..8f2ac3a7 100644
--- a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
+++ b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
@@ -361,7 +361,8 @@ async function onDrop(index: number) {
   if (from === null || from === index || isPinnedAt(index)) return
   if (!reorder(from, index)) return
   updateOptions()
-  await handleNewPlot(undefined, { preserveZoom: true })
+  // With no plot yet, its first draw picks up the new order.
+  if (plotlyRef.value) await handleNewPlot(undefined, { preserveZoom: true })
 }
 
 function onDragEnd() {
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
index a278596e..d3c9dd49 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
@@ -63,4 +63,19 @@ describe('DataVisTimeFilters.vue', () => {
     expect(w.find('[data-testid="date-preset-1w"]').exists()).toBe(true)
     expect(w.find('[data-testid="date-preset-All"]').exists()).toBe(true)
   })
+
+  it('highlights All for a saved YTD preference with the editor presets, keeping the preference', () => {
+    selectedDateBtnId.value = 3
+    const w = mountIt({ presets: EDITOR_PRESETS })
+    expect(w.find('[data-testid="date-preset-All"]').classes()).toContain('v-chip--variant-tonal')
+    expect(w.find('[data-testid="date-preset-1w"]').classes()).toContain('v-chip--variant-outlined')
+    expect(selectedDateBtnId.value).toBe(3)
+  })
+
+  it('highlights YTD itself where it is offered', () => {
+    selectedDateBtnId.value = 3
+    const w = mountIt()
+    expect(w.find('[data-testid="date-preset-YTD"]').classes()).toContain('v-chip--variant-tonal')
+    expect(w.find('[data-testid="date-preset-All"]').classes()).toContain('v-chip--variant-outlined')
+  })
 })
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
index ffa1fd20..c6c28c9d 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
@@ -28,12 +28,23 @@ vi.mock('@/store/plotly', () => ({
     reactive({ ...plotly, updateOptions, requestTableScroll }),
 }))
 
+const pendingPlotWork = ref(0)
+async function trackPlotWork(work: () => Promise<void>) {
+  pendingPlotWork.value++
+  try {
+    await work()
+  } finally {
+    pendingPlotWork.value--
+  }
+}
+
 vi.mock('@/store/dataVisualization', () => ({
   useDataVisStore: () =>
     reactive({
       selectedData: ref(null),
       hasSelectionShape: ref(false),
       qcDatastream: ref(null),
+      trackPlotWork,
     }),
 }))
 
@@ -88,6 +99,7 @@ describe('Plot.vue delayed mount', () => {
     zoomXaxisTo.mockClear()
     plotly.plotlyRef.value = null
     viewedSession.value = null
+    pendingPlotWork.value = 0
   })
 
   afterEach(() => {
@@ -104,6 +116,60 @@ describe('Plot.vue delayed mount', () => {
     wrapper.unmount()
   })
 
+  describe('opening on the Table tab', () => {
+    beforeEach(() => {
+      plotly.activeTab.value = 'table'
+    })
+    afterEach(() => {
+      plotly.activeTab.value = 'plot'
+    })
+
+    it('waits for the plot element instead of drawing into nothing', async () => {
+      const wrapper = mountIt()
+      await vi.advanceTimersByTimeAsync(200)
+      expect(wrapper.find('[data-testid="main-plot"]').exists()).toBe(false)
+      expect(handleNewPlot).not.toHaveBeenCalled()
+      wrapper.unmount()
+    })
+
+    it('draws once the Plot tab renders the element', async () => {
+      const wrapper = mountIt()
+      await vi.advanceTimersByTimeAsync(200)
+      plotly.activeTab.value = 'plot'
+      await flushPromises()
+      await vi.advanceTimersByTimeAsync(200)
+      expect(handleNewPlot).toHaveBeenCalledTimes(1)
+      expect(handleNewPlot).toHaveBeenCalledWith(
+        wrapper.find('[data-testid="main-plot"]').element
+      )
+      wrapper.unmount()
+    })
+  })
+
+  it('counts the mount draw as plot work from mount until it is drawn', async () => {
+    let finishDraw!: () => void
+    handleNewPlot.mockImplementationOnce(
+      () => new Promise<void>((r) => (finishDraw = r))
+    )
+    const wrapper = mountIt()
+    await flushPromises()
+    expect(pendingPlotWork.value).toBe(1)
+    await vi.advanceTimersByTimeAsync(200)
+    expect(pendingPlotWork.value).toBe(1)
+    finishDraw()
+    await flushPromises()
+    expect(pendingPlotWork.value).toBe(0)
+    wrapper.unmount()
+  })
+
+  it('releases the plot work when unmounted before the delay fires', async () => {
+    const wrapper = mountIt()
+    await flushPromises()
+    wrapper.unmount()
+    await flushPromises()
+    expect(pendingPlotWork.value).toBe(0)
+  })
+
   it('does not draw when unmounted before the delay fires', async () => {
     const wrapper = mountIt()
     wrapper.unmount()
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
index 79893c15..45646c9f 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
@@ -75,6 +75,7 @@ vi.mock('@/utils/plotting/plotly', () => ({
 }))
 
 import PlottedDatastreams from '@/components/VisualizeData/PlottedDatastreams.vue'
+import { handleNewPlot } from '@/utils/plotting/plotly'
 
 function mountIt() {
   return mount(PlottedDatastreams, {
@@ -425,6 +426,24 @@ describe('PlottedDatastreams row kinds', () => {
     expect(updateOptions).not.toHaveBeenCalled()
   })
 
+  it('redraws a reorder only once a plot exists', async () => {
+    vi.mocked(handleNewPlot).mockClear()
+    plotlyRef.value = null
+    const wrapper = mountIt()
+    await row(wrapper, 'ctx2').trigger('dragstart')
+    await row(wrapper, 'ctx').trigger('drop')
+    await flushPromises()
+    expect(plottedIds()).toEqual(['ctx2', 'ctx'])
+    expect(handleNewPlot).not.toHaveBeenCalled()
+
+    plotlyRef.value = {}
+    await row(wrapper, 'ctx').trigger('dragstart')
+    await row(wrapper, 'ctx2').trigger('drop')
+    await flushPromises()
+    expect(handleNewPlot).toHaveBeenCalledTimes(1)
+    plotlyRef.value = null
+  })
+
   it('reorders plotted datastreams in Select', async () => {
     const wrapper = mountIt()
     await row(wrapper, 'ctx2').trigger('dragstart')
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 714a91af..0f1c4c06 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -608,6 +608,34 @@ describe('useEditSession', () => {
     expect(fetchObservationsInRange).not.toHaveBeenCalled()
   })
 
+  it('commit completes without writing the session store when the target changes while sessions reload', async () => {
+    await seedHistory()
+    selectedSeries.value = { data: makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }]) }
+    const refreshed = { id: 'm-1', phenomenonEndTime: '2025-02-01T00:00:00Z' }
+    getItem.mockImplementation(async (id: string) =>
+      id === 'm-1' ? refreshed : { id: 's-1', name: 'Source' }
+    )
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    const list = qc.sessions.list.bind(qc.sessions)
+    vi.spyOn(qc.sessions, 'list').mockImplementationOnce(async (...args) => {
+      const res = await list(...args)
+      takeOver()
+      return res
+    })
+
+    await expect(session.commit()).resolves.toBeUndefined()
+
+    expect(createObservations).toHaveBeenCalled()
+    const store = useQcSessionStore()
+    expect(store.historyId).toBeNull()
+    expect(store.sessions).toEqual([])
+    expect(store.savedEdits).toEqual([])
+    expect(replaceDatastream).toHaveBeenCalledWith(refreshed)
+  })
+
   it('startSession stores its base as the working copy', async () => {
     await seedHistory()
     const { useEditSession } = await import('@/composables/useEditSession')
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index 0c59cefd..a23fce9e 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -381,8 +381,13 @@ export function useEditSession() {
       },
     })
     workingCopies.invalidate(managed.id)
-    await sessionStore.loadSessions(historyId)
-    snapshotSavedEdits()
+    const sessions = await sessionStore.fetchSessions(historyId)
+    // The commit went through either way. If another target took over the
+    // editor meanwhile, the session store is its now: leave it alone.
+    if (qcDatastream.value?.id === managed.id) {
+      sessionStore.applySessions(historyId, sessions)
+      snapshotSavedEdits()
+    }
     // The push moved the managed datastream's phenomenon times. Refresh last so
     // a network failure does not abort the commit (session is already locked).
     try {
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index 3a3f21c0..07a225b4 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -520,6 +520,206 @@ describe('useDataVisStore.setDateRange', () => {
   })
 })
 
+describe('useDataVisStore overlapping plot loads', () => {
+  const deferred = <T>() => {
+    let resolve!: (v: T) => void
+    const promise = new Promise<T>((r) => (resolve = r))
+    return { promise, resolve }
+  }
+  const flush = () => new Promise((r) => setTimeout(r, 0))
+  const FEB = new Date('2025-02-01T00:00:00Z')
+  const MAR = new Date('2025-03-01T00:00:00Z')
+  const APR = new Date('2025-04-01T00:00:00Z')
+  const MAY = new Date('2025-05-01T00:00:00Z')
+
+  it('drops a context response whose range was superseded', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    mockRedraw.mockClear()
+    const first = deferred<any>()
+    const second = deferred<any>()
+    mockFetchObservationsInRange
+      .mockImplementationOnce(() => first.promise)
+      .mockImplementationOnce(() => second.promise)
+
+    const older = store.setDateRange({ begin: FEB, end: MAR })
+    const newer = store.setDateRange({ begin: APR, end: MAY })
+    first.resolve({ id: 'feb' })
+    await flush()
+    second.resolve({ id: 'apr' })
+    await Promise.all([older, newer])
+
+    const src = mockGraphSeriesArray.value.find((s) => s.id === 'src')
+    expect(src.data).toEqual({ id: 'apr' })
+    expect(store.loadingStates.get('src')).toBe(false)
+    expect(mockRedraw).toHaveBeenCalledTimes(1)
+  })
+
+  it('a rebuild whose range moves while loading draws the new range once', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    mockPlotlyRef.value = {}
+    const { handleNewPlot } = await import('@/utils/plotting/plotly')
+    const drawn: unknown[] = []
+    vi.mocked(handleNewPlot).mockImplementationOnce(async () => {
+      drawn.push(mockGraphSeriesArray.value.find((s) => s.id === 'src')?.data)
+    })
+    mockFetchObservationsInRange.mockClear()
+    mockRedraw.mockClear()
+    const first = deferred<any>()
+    const second = deferred<any>()
+    mockFetchObservationsInRange
+      .mockImplementationOnce(() => first.promise)
+      .mockImplementationOnce(() => second.promise)
+
+    const rebuild = store.rebuildPlot()
+    await flush()
+    const reload = store.setDateRange({ begin: APR, end: MAY })
+    first.resolve({ id: 'old' })
+    await flush()
+    second.resolve({ id: 'new' })
+    await Promise.all([rebuild, reload])
+
+    expect(drawn).toEqual([{ id: 'new' }])
+    expect(mockFetchObservationsInRange.mock.calls.map((c) => c[1])).toEqual([
+      expect.any(Date),
+      APR,
+    ])
+    expect(mockRedraw).not.toHaveBeenCalled()
+  })
+
+  it('a rebuild requested while a context reload loads waits for it', async () => {
+    const { store, other } = await managedPair()
+    await store.setEditTarget('mgd')
+    mockFetchObservationsInRange.mockClear()
+    mockFetchGraphSeries.mockClear()
+    const pending = deferred<any>()
+    mockFetchObservationsInRange.mockImplementationOnce(() => pending.promise)
+
+    const reload = store.setDateRange({ begin: APR, end: MAY })
+    const rebuild = store.plotDatastream(other as any)
+    await flush()
+
+    expect(mockFetchObservationsInRange).toHaveBeenCalledTimes(1)
+    expect(mockFetchGraphSeries).not.toHaveBeenCalled()
+
+    pending.resolve({ id: 'apr' })
+    await Promise.all([reload, rebuild])
+    expect(mockFetchGraphSeries.mock.calls.map((c: any[]) => [c[0].id, c[1]])).toEqual([
+      ['other', APR],
+    ])
+  })
+
+  it('a context reload requested while a rebuild is queued joins it', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    mockPlotlyRef.value = {}
+    mockRedraw.mockClear()
+    const pending = deferred<any>()
+    mockFetchObservationsInRange.mockClear()
+    mockFetchObservationsInRange.mockImplementationOnce(() => pending.promise)
+
+    const inFlight = store.rebuildPlot()
+    await flush()
+    const queued = store.rebuildPlot()
+    const reload = store.setDateRange({ begin: APR, end: MAY })
+    pending.resolve({ id: 'old' })
+    await Promise.all([inFlight, queued, reload])
+
+    const starts = mockFetchObservationsInRange.mock.calls.map((c) => c[1])
+    expect(starts.filter((d) => d === APR)).toHaveLength(2)
+    expect(starts).toHaveLength(3)
+    expect(mockRedraw).not.toHaveBeenCalled()
+  })
+
+  // A load requested in the microtasks between one finishing and its queued
+  // follow-up starting must join the follow-up, not start a load beside it.
+  // The exact hop count is an implementation detail, so sweep the window.
+  it('does not start a second load when one is requested as a load settles', async () => {
+    const runScenario = async (hops: number) => {
+      const { store } = await managedPair()
+      await store.setEditTarget('mgd')
+      mockPlotlyRef.value = {}
+      let active = 0
+      let maxActive = 0
+      let firstFetch: Promise<unknown> | null = null
+      mockFetchObservationsInRange.mockClear()
+      mockFetchObservationsInRange.mockImplementation(() => {
+        const fetched = (async () => {
+          active++
+          maxActive = Math.max(maxActive, active)
+          await flush()
+          active--
+          return { id: 'x' }
+        })()
+        firstFetch ??= fetched
+        return fetched
+      })
+
+      const inFlight = store.rebuildPlot()
+      await flush()
+      const queued = store.rebuildPlot()
+      // Count hops from the first load's own fetch, so the request lands in
+      // the microtasks where that load is settling.
+      const late = firstFetch!.then(async () => {
+        for (let i = 0; i < hops; i++) await Promise.resolve()
+        return store.rebuildPlot()
+      })
+      await Promise.all([inFlight, queued, late])
+      mockFetchObservationsInRange.mockResolvedValue({ id: 'stub', data: {} })
+      return maxActive
+    }
+
+    for (let hops = 0; hops <= 24; hops++) {
+      expect([hops, await runScenario(hops)]).toEqual([hops, 1])
+    }
+  })
+
+  it('does not add a series from a superseded range', async () => {
+    const { store, other } = await managedPair()
+    const first = deferred<any>()
+    const second = deferred<any>()
+    mockFetchGraphSeries
+      .mockImplementationOnce(() => first.promise)
+      .mockImplementationOnce(() => second.promise)
+
+    const rebuild = store.plotDatastream(other as any)
+    await flush()
+    const reload = store.setDateRange({ begin: APR, end: MAY })
+    first.resolve({ id: 'other', data: 'old range' })
+    await flush()
+    second.resolve({ id: 'other', data: 'new range' })
+    await Promise.all([rebuild, reload])
+
+    expect(mockGraphSeriesArray.value.map((s) => s.data)).toEqual(['new range'])
+  })
+
+  it('a rebuild requested while another runs resolves only after the follow-up rebuild', async () => {
+    const { store, other } = await managedPair()
+    const extra = makeDs({ id: 'extra', phenomenonBeginTime: '2025-01-01T00:00:00Z', phenomenonEndTime: '2025-12-31T00:00:00Z' })
+    store.datastreams = [...store.datastreams, extra] as any
+    const first = deferred<any>()
+    const second = deferred<any>()
+    mockFetchGraphSeries
+      .mockImplementationOnce(() => first.promise)
+      .mockImplementationOnce(() => second.promise)
+
+    const inFlight = store.plotDatastream(other as any)
+    await flush()
+    let queuedDone = false
+    const queued = store.plotDatastream(extra as any).then(() => {
+      queuedDone = true
+    })
+    first.resolve({ id: 'other', data: {} })
+    await flush()
+
+    expect(queuedDone).toBe(false)
+    second.resolve({ id: 'extra', data: {} })
+    await Promise.all([inFlight, queued])
+    expect(mockGraphSeriesArray.value.map((s) => s.id)).toEqual(['other', 'extra'])
+  })
+})
+
 describe('useDataVisStore context range around the session window', () => {
   const WIN_START = '2025-06-01T00:00:00Z'
   const WIN_END = '2025-06-10T00:00:00Z'
@@ -535,9 +735,7 @@ describe('useDataVisStore context range around the session window', () => {
     ] as any)
     await nextTick()
   }
-  const settle = async () => {
-    for (let i = 0; i < 10; i++) await Promise.resolve()
-  }
+  const settle = () => new Promise((r) => setTimeout(r, 0))
 
   it('the editor counts presets out from the session window', async () => {
     const { store } = await managedPair()
@@ -604,6 +802,96 @@ describe('useDataVisStore context range around the session window', () => {
     expect(mockClearZoomHistory).not.toHaveBeenCalled()
   })
 
+  it('reports the editor ready only once the range around the session window has loaded', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    await store.setEditRecord({ dataX: [1], history: [] } as any)
+    expect(store.isEditorReady).toBe(false)
+    let resolveFetch!: (v: unknown) => void
+    mockFetchObservationsInRange.mockImplementationOnce(
+      () => new Promise((r) => (resolveFetch = r))
+    )
+
+    await loadSession()
+    await settle()
+    expect(store.beginDate.getTime()).toBe(subtractMonths(new Date(WIN_START), 1).getTime())
+    expect(store.isEditorReady).toBe(false)
+
+    resolveFetch({ id: 'stub', data: {} })
+    await settle()
+    expect(store.isEditorReady).toBe(true)
+  })
+
+  it('reports the editor ready when the session window leaves the range unchanged', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    await store.setEditRecord({ dataX: [1], history: [] } as any)
+    await store.setDateRange({ begin: new Date('2025-03-01T00:00:00Z'), end: new Date('2025-04-01T00:00:00Z') })
+
+    await loadSession()
+    await settle()
+
+    expect(store.isEditorReady).toBe(true)
+  })
+
+  it('does not report the editor ready while a plot load is still drawing', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    await store.setEditRecord({ dataX: [1], history: [] } as any)
+    await loadSession()
+    await settle()
+    expect(store.isEditorReady).toBe(true)
+    mockPlotlyRef.value = {}
+    const { handleNewPlot } = await import('@/utils/plotting/plotly')
+    let finishDraw!: () => void
+    vi.mocked(handleNewPlot).mockImplementationOnce(
+      () => new Promise<void>((r) => (finishDraw = r))
+    )
+
+    const rebuild = store.rebuildPlot()
+    await settle()
+    expect(store.isEditorReady).toBe(false)
+
+    finishDraw()
+    await rebuild
+    expect(store.isEditorReady).toBe(true)
+  })
+
+  it('does not report the editor ready until the edit record is drawn', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    await loadSession()
+    await settle()
+    expect(store.isEditorReady).toBe(false)
+    mockPlotlyRef.value = {}
+    const { handleNewPlot } = await import('@/utils/plotting/plotly')
+    let finishDraw!: () => void
+    vi.mocked(handleNewPlot).mockImplementationOnce(
+      () => new Promise<void>((r) => (finishDraw = r))
+    )
+
+    const placing = store.setEditRecord({ dataX: [1], history: [] } as any)
+    await settle()
+    expect(store.isEditorReady).toBe(false)
+
+    finishDraw()
+    await placing
+    expect(store.isEditorReady).toBe(true)
+  })
+
+  it('does not report the editor ready while another session is opening', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    await store.setEditRecord({ dataX: [1], history: [] } as any)
+    await loadSession()
+    await settle()
+    const { useQcSessionStore } = await import('@/store/qcSession')
+
+    useQcSessionStore().isSwitchingSession = true
+
+    expect(store.isEditorReady).toBe(false)
+  })
+
   it('keeps a custom context range when the session window loads', async () => {
     const { store } = await managedPair()
     await store.setEditTarget('mgd')
diff --git a/apps/qc-app/src/store/__tests__/observations.spec.ts b/apps/qc-app/src/store/__tests__/observations.spec.ts
index a1282ba6..4c3a591f 100644
--- a/apps/qc-app/src/store/__tests__/observations.spec.ts
+++ b/apps/qc-app/src/store/__tests__/observations.spec.ts
@@ -106,3 +106,55 @@ describe('useObservationStore.fetchObservationsInRange windowing', () => {
     expect(rec.dataX.length).toBe(0)
   })
 })
+
+describe('useObservationStore.fetchObservationsInRange overlapping requests', () => {
+  const deferred = <T>() => {
+    let resolve!: (v: T) => void
+    const promise = new Promise<T>((r) => (resolve = r))
+    return { promise, resolve }
+  }
+  const rangeOf = (begin: number, end: number) => {
+    const datetimes = ALL_TIMES.filter((t) => t >= begin && t <= end)
+    return { datetimes, dataValues: datetimes.map((t) => t * 10) }
+  }
+
+  it('applies the latest requested window and keeps the cache free of duplicates when responses land out of order', async () => {
+    const slow = deferred<ReturnType<typeof rangeOf>>()
+    ;(fetchObservationsSync as any)
+      .mockImplementationOnce(() => slow.promise)
+      .mockImplementationOnce(async () => rangeOf(5, 10))
+    const store = useObservationStore()
+
+    const first = store.fetchObservationsInRange(datastream, new Date(0), new Date(10))
+    const second = store.fetchObservationsInRange(datastream, new Date(5), new Date(10))
+    await Promise.resolve()
+    slow.resolve(rangeOf(0, 10))
+    const [, rec] = await Promise.all([first, second])
+
+    expect(rec.dataX.length).toBe(6)
+    expect(Array.from(store.observationsRaw['ds-1'].datetimes)).toEqual(ALL_TIMES)
+  })
+
+  it('shares one request for an identical range already in flight', async () => {
+    const store = useObservationStore()
+
+    const [a, b] = await Promise.all([
+      store.fetchObservationsInRange(datastream, new Date(2), new Date(8)),
+      store.fetchObservationsInRange(datastream, new Date(2), new Date(8)),
+    ])
+
+    expect(a).toBe(b)
+    expect(fetchObservationsSync).toHaveBeenCalledTimes(1)
+  })
+
+  it('still serves later requests after a failed one', async () => {
+    ;(fetchObservationsSync as any).mockRejectedValueOnce(new Error('offline'))
+    const store = useObservationStore()
+
+    const failed = store.fetchObservationsInRange(datastream, new Date(0), new Date(10))
+    const next = store.fetchObservationsInRange(datastream, new Date(0), new Date(4))
+
+    await expect(failed).rejects.toThrow('offline')
+    expect((await next).dataX.length).toBe(5)
+  })
+})
diff --git a/apps/qc-app/src/store/__tests__/qcSession.spec.ts b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
index 1ee17bc9..078096cf 100644
--- a/apps/qc-app/src/store/__tests__/qcSession.spec.ts
+++ b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
@@ -44,7 +44,7 @@ describe('useQcSessionStore', () => {
   it('loads sessions and defaults the view to the in-progress session', async () => {
     const { historyId, inProgressId } = await seed()
     const store = useQcSessionStore()
-    await store.loadSessions(historyId)
+    store.applySessions(historyId, await store.fetchSessions(historyId))
 
     expect(store.historyId).toBe(historyId)
     expect(store.sessions).toHaveLength(2)
@@ -72,7 +72,7 @@ describe('useQcSessionStore', () => {
   it('viewing a committed session is read-only; returnToCurrent restores editing', async () => {
     const { historyId, committedId, inProgressId } = await seed()
     const store = useQcSessionStore()
-    await store.loadSessions(historyId)
+    store.applySessions(historyId, await store.fetchSessions(historyId))
 
     store.viewSession(committedId)
     expect(store.viewedSessionId).toBe(committedId)
@@ -87,7 +87,7 @@ describe('useQcSessionStore', () => {
   it('ignores viewSession for an unknown session id', async () => {
     const { historyId, inProgressId } = await seed()
     const store = useQcSessionStore()
-    await store.loadSessions(historyId)
+    store.applySessions(historyId, await store.fetchSessions(historyId))
     store.viewSession('does-not-exist')
     expect(store.viewedSessionId).toBe(inProgressId)
   })
@@ -109,7 +109,7 @@ describe('useQcSessionStore', () => {
     await qc.sessions.commit(h.id, b.id)
 
     const store = useQcSessionStore()
-    await store.loadSessions(h.id)
+    store.applySessions(h.id, await store.fetchSessions(h.id))
 
     expect(store.currentSessionId).toBeNull()
     expect(store.viewedSessionId).toBe(b.id)
@@ -126,7 +126,7 @@ describe('useQcSessionStore', () => {
     await qc.sessions.create(h.id, win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'))
 
     const store = useQcSessionStore()
-    await store.loadSessions(h.id)
+    store.applySessions(h.id, await store.fetchSessions(h.id))
     store.resumeDatastreamId = 'm-1'
 
     // `reset` clears the live session context (leaving a datastream), but the
@@ -167,7 +167,7 @@ describe('useQcSessionStore', () => {
     } as any
 
     const store = useQcSessionStore()
-    await store.loadSessions(h.id)
+    store.applySessions(h.id, await store.fetchSessions(h.id))
 
     const loaded = store.sessions.find((x) => x.id === s.id) as any
     expect(loaded.operations.map((o: any) => o.operationType)).toEqual([
@@ -197,7 +197,7 @@ describe('useQcSessionStore', () => {
       qualityControlOperations: { ...qc.operations, list: opsList },
     } as any
 
-    await useQcSessionStore().loadSessions(h.id)
+    useQcSessionStore().applySessions(h.id, await useQcSessionStore().fetchSessions(h.id))
     expect(opsList).not.toHaveBeenCalled()
   })
 
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index 1af11f2d..ff13921e 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -189,6 +189,19 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   /** True when a box/lasso selection shape is drawn on the plot, even if it captured 0 points. */
   const hasSelectionShape = ref(false)
 
+  /** Plot loads and draws started and not yet finished. */
+  const pendingPlotWork = ref(0)
+
+  /** Run `work` counted as pending plot work, see `isEditorReady`. */
+  async function trackPlotWork(work: () => Promise<void>): Promise<void> {
+    pendingPlotWork.value++
+    try {
+      await work()
+    } finally {
+      pendingPlotWork.value--
+    }
+  }
+
   /** Track the loading status of each datastream to be plotted.
    * Set to true when we get a response from the API. Keyed by datastream id. */
   const loadingStates = ref(new Map<string, boolean>())
@@ -393,20 +406,22 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   }
 
   /** Put the edit target's record on the plot, adding its series if needed. */
-  async function setEditRecord(record: ObservationRecord) {
-    const edit = qcDatastream.value
-    if (!edit) return
-    const existing = graphSeriesArray.value.find((s) => s.id === edit.id)
-    if (existing) {
-      existing.data = record
-      await usePlotlyStore().redraw()
-      return
-    }
-    graphSeriesArray.value.push(buildGraphSeries(edit, record))
-    orderAndColorSeries()
-    updateOptions()
-    const { plotlyRef } = storeToRefs(usePlotlyStore())
-    if (plotlyRef.value) await handleNewPlot(undefined, { preserveZoom: true })
+  function setEditRecord(record: ObservationRecord): Promise<void> {
+    return trackPlotWork(async () => {
+      const edit = qcDatastream.value
+      if (!edit) return
+      const existing = graphSeriesArray.value.find((s) => s.id === edit.id)
+      if (existing) {
+        existing.data = record
+        await usePlotlyStore().redraw()
+        return
+      }
+      graphSeriesArray.value.push(buildGraphSeries(edit, record))
+      orderAndColorSeries()
+      updateOptions()
+      const { plotlyRef } = storeToRefs(usePlotlyStore())
+      if (plotlyRef.value) await handleNewPlot(undefined, { preserveZoom: true })
+    })
   }
 
   /** Stop editing. Plotted datastreams stay as they were. */
@@ -421,67 +436,89 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     await rebuildPlot()
   }
 
-  // Coalescing lock for `rebuildPlot`. Rapid checkbox toggles in the
-  // Select view call `plotDatastream` in quick succession; each call
-  // pushes into `plottedDatastreams` and then awaits `rebuildPlot`.
-  // Without a lock, the rebuilds interleave: two concurrent
-  // `refreshGraphSeriesArray` passes both see the same
-  // `graphSeriesArray` before either has pushed, both enter the
-  // "fetch new series" branch for the same datastream, and each
-  // pushes its own copy — so the downstream `createPlotlyOption`
-  // emits duplicate right-side y-axes. The `handleNewPlot` calls
-  // also race, which can leave stale axis chrome from the earlier
-  // render on top of the latest plot.
-  //
-  // Strategy: serialize rebuilds and coalesce queued ones. While a
-  // rebuild is in flight, extra callers just flip `rebuildQueued`
-  // and await the outcome. When the in-flight rebuild finishes and
-  // there's at least one queued request, we run exactly one more
-  // rebuild with the latest `plottedDatastreams` snapshot. N rapid
-  // clicks therefore settle into at most two rebuilds (in-flight +
-  // final), with no concurrent fetches or concurrent `newPlot`s.
-  let rebuildInFlight: Promise<void> | null = null
-  let rebuildQueued = false
-
-  async function rebuildPlot(): Promise<void> {
-    if (rebuildInFlight) {
-      rebuildQueued = true
-      try {
-        await rebuildInFlight
-      } catch {
-        /* swallow — original error already surfaced to its caller */
-      }
-      // Another queued caller may have already claimed the follow-up
-      // rebuild (and cleared the flag). Only the caller that still
-      // sees the flag kicks off the coalesced final rebuild.
-      if (!rebuildQueued) return
-      rebuildQueued = false
-      return rebuildPlot()
+  // Plot loads (a rebuild, or a reload of the context range) run one at a
+  // time. Concurrent `refreshGraphSeriesArray` passes would both push a series
+  // for the same new datastream (duplicate right-side axes) and race their
+  // draws. A request made while one runs joins the single queued follow-up,
+  // which is a rebuild if any joined request was one, so N rapid clicks settle
+  // into at most two loads. Every caller resolves only once a load that
+  // started after its change has finished.
+  type PlotLoadKind = 'rebuild' | 'range'
+  /** Guard against a range that never settles. */
+  const MAX_RANGE_PASSES = 5
+  let loadInFlight: Promise<void> | null = null
+  let queuedLoad: { kind: PlotLoadKind; promise: Promise<void> } | null = null
+  /** The range the series were last loaded for, as `begin-end` ms. */
+  let loadedRangeKey: string | null = null
+
+  const rangeKey = (begin: Date, end: Date) =>
+    `${begin.getTime()}-${end.getTime()}`
+
+  function queuePlotLoad(kind: PlotLoadKind): Promise<void> {
+    return trackPlotWork(() => nextPlotLoad(kind))
+  }
+
+  function nextPlotLoad(kind: PlotLoadKind): Promise<void> {
+    // Check the queue first: the in-flight load clears `loadInFlight` a couple
+    // of microtasks before its follow-up starts, and a caller landing in that
+    // gap would otherwise start a second concurrent load.
+    if (queuedLoad) {
+      if (kind === 'rebuild') queuedLoad.kind = 'rebuild'
+      return queuedLoad.promise
     }
-    rebuildInFlight = doRebuildPlot()
-    try {
-      await rebuildInFlight
-    } finally {
-      rebuildInFlight = null
-    }
-    // A rebuild queued while we were running needs exactly one more
-    // pass to reflect the latest state.
-    if (rebuildQueued) {
-      rebuildQueued = false
-      return rebuildPlot()
+    if (!loadInFlight) return startPlotLoad(kind)
+    const queued = { kind, promise: Promise.resolve() }
+    queued.promise = loadInFlight
+      .catch(() => undefined)
+      .then(() => {
+        queuedLoad = null
+        return startPlotLoad(queued.kind)
+      })
+    queuedLoad = queued
+    return queued.promise
+  }
+
+  function startPlotLoad(kind: PlotLoadKind): Promise<void> {
+    const run: Promise<void> = (
+      kind === 'rebuild' ? doRebuildPlot() : doReloadRange()
+    ).finally(() => {
+      if (loadInFlight === run) loadInFlight = null
+    })
+    loadInFlight = run
+    return run
+  }
+
+  function rebuildPlot(): Promise<void> {
+    return queuePlotLoad('rebuild')
+  }
+
+  /** Load every series for the current range. Responses for a range that
+   *  moved meanwhile are dropped, so load again until it holds still. */
+  async function loadCurrentRange(): Promise<void> {
+    for (let pass = 0; pass < MAX_RANGE_PASSES; pass++) {
+      const begin = beginDate.value
+      const end = endDate.value
+      await refreshGraphSeriesArray()
+      if (isCurrentRange(begin, end)) {
+        loadedRangeKey = rangeKey(begin, end)
+        return
+      }
     }
+    console.warn(
+      `Plot range still moving after ${MAX_RANGE_PASSES} loads, drawing anyway.`
+    )
   }
 
   /** Rebuild the plot from scratch: rebuild the graph-series array from
    *  `seriesDatastreams`, regenerate Plotly options, and re-render. Select
    *  drops the zoom; the editor keeps it, since context changes never move
-   *  the user's view of the session. Must run serialized, see the lock in
-   *  `rebuildPlot` above. */
-  async function doRebuildPlot() {
+   *  the user's view of the session. Runs as a plot load, see above. */
+  async function doRebuildPlot(): Promise<void> {
     hasSelectionShape.value = false
     if (!seriesDatastreams.value.length) {
       invalidateUnplottedWorkingCopies()
       clearChartState()
+      loadedRangeKey = null
       return
     }
     const keepZoom = !!qcDatastreamId.value
@@ -492,7 +529,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
       beginDate.value = presetRange.begin
       endDate.value = presetRange.end
     }
-    await refreshGraphSeriesArray()
+    await loadCurrentRange()
     updateOptions()
     const { plotlyRef } = storeToRefs(usePlotlyStore())
     if (plotlyRef.value) {
@@ -500,6 +537,24 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     }
   }
 
+  /** Reload the series for a new range and redraw. Skipped when an earlier
+   *  load already caught up with the range. */
+  async function doReloadRange(): Promise<void> {
+    if (!seriesDatastreams.value.length) return
+    if (loadedRangeKey === rangeKey(beginDate.value, endDate.value)) return
+    await loadCurrentRange()
+    const { redraw, clearZoomHistory } = usePlotlyStore()
+    if (qcDatastreamId.value) {
+      // Context reload in the editor: the user's view of the session stays put.
+      await redraw(false, true)
+    } else {
+      // The zoom stack refers to the old range, so drop it and let the new
+      // range apply instead of copying the live range over it.
+      clearZoomHistory()
+      await redraw(false, false)
+    }
+  }
+
   // Note: datastreams are loaded with `expand_related: true`, so these carry
   // nested `thing`/`observedProperty`/`processingLevel` objects rather than
   // the flat `*Id` fields on the bare Datastream type.
@@ -559,8 +614,8 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     update = true,
     custom = true,
   }: SetDateRangeParams) => {
-    // No-op when neither bound actually moved. Every sidebar path —
-    // clicking the already-active preset, date-text-field blur,
+    // No-op when neither bound actually moved. Every sidebar path
+    // (clicking the already-active preset, date-text-field blur,
     // time-text-field blur, calendar picker confirming the current
     // day) calls this with fresh Date references whose timestamps
     // often match the current range. Without this guard each of those
@@ -575,28 +630,8 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     if (end) endDate.value = end
     if (custom) selectedDateBtnId.value = CUSTOM_PRESET_ID
 
-    if (
-      update &&
-      beginDate.value &&
-      endDate.value &&
-      seriesDatastreams.value.length
-    ) {
-      const { redraw, clearZoomHistory } = usePlotlyStore()
-      await refreshGraphSeriesArray()
-      if (qcDatastreamId.value) {
-        // Context reload in the editor: the user's view of the session stays put.
-        await redraw(false, true)
-      } else {
-        // A date-filter change refetches data and drops the zoom window:
-        // the recorded zoom stack refers to the OLD time range and would
-        // be meaningless after redraw, so clear it.
-        clearZoomHistory()
-        // The user explicitly changed the date filter, so they expect the
-        // new window to actually apply, so opt out of the zoom-preserving
-        // path in `redraw` (which otherwise copies the live range over
-        // the fresh layout and the plot would stay on the old window).
-        redraw(false, false)
-      }
+    if (update && seriesDatastreams.value.length) {
+      await queuePlotLoad('range')
     }
   }
 
@@ -617,11 +652,14 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
 
   // The editor loads its context before the session is known, so re-anchor
   // the preset once the window arrives or changes. Like a Context change, this
-  // reloads context series only and keeps the zoom.
+  // reloads context series only and keeps the zoom. A watch rather than a call
+  // at each session store write: the window follows sessions being applied, a
+  // session viewed, a return to the current one and a failed view reverted.
+  // It queues behind any rebuild, and joins a queued one.
   watch(
     () =>
       editSessionWindow.value &&
-      `${editSessionWindow.value.begin.getTime()}-${editSessionWindow.value.end.getTime()}`,
+      rangeKey(editSessionWindow.value.begin, editSessionWindow.value.end),
     (key) => {
       if (!key) return
       applyActivePreset().catch((error) => {
@@ -631,11 +669,31 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     }
   )
 
+  /** True while editing once the session window is known, the edit record is
+   *  on the plot, no session is opening, and every plot load and draw started
+   *  so far (including the re-anchor around the window) has finished. */
+  const isEditorReady = computed(
+    (): boolean =>
+      pendingPlotWork.value === 0 &&
+      !useQcSessionStore().isSwitchingSession &&
+      !!editSessionWindow.value &&
+      graphSeriesArray.value.some((s) => s.id === qcDatastreamId.value)
+  )
+
+  const isCurrentRange = (start: Date, end: Date) =>
+    start.getTime() === beginDate.value.getTime() &&
+    end.getTime() === endDate.value.getTime()
+
+  /** The latest load per datastream; only it may clear the loading flag. */
+  const latestLoads = new Map<string, object>()
+
   const updateOrFetchGraphSeries = async (
     datastream: Datastream,
     start: Date,
     end: Date
   ) => {
+    const load = {}
+    latestLoads.set(datastream.id, load)
     try {
       // A managed datastream with a session in progress plots its working
       // copy: it spans the session window and is never re-windowed, since a
@@ -663,19 +721,17 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
           console.error('Failed to fetch observations:', error)
           return null
         })
-        if (obsRecord && graphSeriesArray.value[seriesIndex]) {
-          graphSeriesArray.value[seriesIndex].data = obsRecord
-        }
+        // The range moved while fetching: the request for the new range writes.
+        if (!isCurrentRange(start, end)) return
+        // Re-find: the array may have been filtered while fetching.
+        const series = graphSeriesArray.value.find((s) => s.id === datastream.id)
+        if (obsRecord && series) series.data = obsRecord
       } else {
-        // Add new graph series. The await spans a network fetch, so
-        // another caller may have already pushed a series for this
-        // datastream by the time we resume (the `rebuildPlot` lock
-        // prevents the main rapid-click path, but other callers of
-        // `refreshGraphSeriesArray` — edit history, navigation rail —
-        // aren't serialized with it). Re-check before pushing so we
-        // don't stack duplicate series, which would each spawn their
-        // own right-side y-axis in `createPlotlyOption`.
         const newSeries = await fetchGraphSeries(datastream, start, end)
+        if (!isCurrentRange(start, end)) return
+        // Callers of `refreshGraphSeriesArray` outside the plot load queue
+        // (the edit history reload) may have pushed this series meanwhile. A
+        // duplicate would spawn its own right-side y-axis.
         const alreadyPresent = graphSeriesArray.value.some(
           (s) => s.id === datastream.id
         )
@@ -687,7 +743,10 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
         error
       )
     } finally {
-      loadingStates.value.set(datastream.id, false)
+      if (latestLoads.get(datastream.id) === load) {
+        latestLoads.delete(datastream.id)
+        loadingStates.value.set(datastream.id, false)
+      }
     }
   }
 
@@ -794,6 +853,8 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     beginDate,
     endDate,
     loadingStates,
+    isEditorReady,
+    trackPlotWork,
     selectedDateBtnId,
     qcDatastream,
     qcDatastreamId,
diff --git a/apps/qc-app/src/store/observations.ts b/apps/qc-app/src/store/observations.ts
index 5355cc95..2f39f82e 100644
--- a/apps/qc-app/src/store/observations.ts
+++ b/apps/qc-app/src/store/observations.ts
@@ -29,17 +29,50 @@ export const useObservationStore = defineStore(
     //   }
     // }
 
+    /** The last request queued per datastream. */
+    const queued = new Map<
+      string,
+      { begin: number; end: number; promise: Promise<ObservationRecord> }
+    >()
+
     /**
      * Fetches requested observations that aren't currently in the pinia store,
      * updates the store, then returns the corresponding `ObservationRecord`.
+     *
+     * Requests for one datastream run in order: each fills the cache from what
+     * the previous one left, and the record ends on the latest requested
+     * window. A request matching the last queued range shares it.
      */
-    const fetchObservationsInRange = async (
+    const fetchObservationsInRange = (
       datastream: Datastream,
       beginTime: Date,
       endTime: Date
     ): Promise<ObservationRecord> => {
       const id = datastream.id
+      const begin = beginTime.getTime()
+      const end = endTime.getTime()
+      const last = queued.get(id)
+      if (last && last.begin === begin && last.end === end) return last.promise
+
+      const previous = last?.promise.catch(() => undefined) ?? Promise.resolve()
+      const promise = previous.then(() =>
+        loadRange(datastream, beginTime, endTime)
+      )
+      const entry = { begin, end, promise }
+      queued.set(id, entry)
+      const release = () => {
+        if (queued.get(id) === entry) queued.delete(id)
+      }
+      promise.then(release, release)
+      return promise
+    }
 
+    const loadRange = async (
+      datastream: Datastream,
+      beginTime: Date,
+      endTime: Date
+    ): Promise<ObservationRecord> => {
+      const id = datastream.id
 
       let beginDataPromise: Promise<{
         datetimes: number[]
diff --git a/apps/qc-app/src/store/qcSession.ts b/apps/qc-app/src/store/qcSession.ts
index 2137fcaa..f2085fdf 100644
--- a/apps/qc-app/src/store/qcSession.ts
+++ b/apps/qc-app/src/store/qcSession.ts
@@ -26,7 +26,6 @@ export const useQcSessionStore = defineStore('qcSession', () => {
   const currentSessionId = ref<string | null>(null)
   /** The session currently being viewed. */
   const viewedSessionId = ref<string | null>(null)
-  const isLoading = ref(false)
   const isSwitchingSession = ref(false)
   /** Edit history entries (by reference) at the last load or save, the
    *  baseline `useEditSession` compares against for unsaved edits. Kept here
@@ -116,15 +115,6 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     viewedSessionId.value = inProgress?.id ?? latestCommitted?.id ?? null
   }
 
-  async function loadSessions(id: string): Promise<void> {
-    isLoading.value = true
-    try {
-      applySessions(id, await fetchSessions(id))
-    } finally {
-      isLoading.value = false
-    }
-  }
-
   /** View a session read-only (no-op for an unknown id). */
   function viewSession(sessionId: string): void {
     if (sessions.value.some((s) => s.id === sessionId)) {
@@ -143,7 +133,6 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     sessions.value = []
     currentSessionId.value = null
     viewedSessionId.value = null
-    isLoading.value = false
     savedEdits.value = []
     savedComments.value = []
   }
@@ -155,7 +144,6 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     sessions,
     currentSessionId,
     viewedSessionId,
-    isLoading,
     isSwitchingSession,
     savedEdits,
     savedComments,
@@ -165,7 +153,6 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     viewedSession,
     fetchSessions,
     applySessions,
-    loadSessions,
     viewSession,
     returnToCurrent,
     reset,
diff --git a/apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts b/apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts
index 07448566..f78c0bb5 100644
--- a/apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/timeRangePresets.spec.ts
@@ -7,6 +7,7 @@ import {
   dataExtent,
   presetAroundWindow,
   presetWindow,
+  shownPresetId,
 } from '@/utils/timeRangePresets'
 
 const DAY = 24 * 60 * 60 * 1000
@@ -192,3 +193,19 @@ describe('presetAroundWindow', () => {
     expect(presetAroundWindow(-1, window, extent)).toBeNull()
   })
 })
+
+describe('shownPresetId', () => {
+  it('shows an offered preset as itself', () => {
+    expect(shownPresetId(3, TIME_RANGE_PRESETS)).toBe(3)
+    expect(shownPresetId(0, EDITOR_PRESETS)).toBe(0)
+  })
+
+  it('shows YTD as All where YTD is not offered', () => {
+    expect(shownPresetId(3, EDITOR_PRESETS)).toBe(ALL_PRESET_ID)
+  })
+
+  it('shows nothing for Custom or an unknown id', () => {
+    expect(shownPresetId(-1, EDITOR_PRESETS)).toBeNull()
+    expect(shownPresetId(99, EDITOR_PRESETS)).toBeNull()
+  })
+})
diff --git a/apps/qc-app/src/utils/timeRangePresets.ts b/apps/qc-app/src/utils/timeRangePresets.ts
index f8c696fb..6b270063 100644
--- a/apps/qc-app/src/utils/timeRangePresets.ts
+++ b/apps/qc-app/src/utils/timeRangePresets.ts
@@ -96,6 +96,17 @@ export const EDITOR_PRESETS: readonly TimeRangePreset[] = TIME_RANGE_PRESETS
   .filter((p) => p.label !== 'YTD')
   .map((p) => ({ ...p, title: EDITOR_TITLES[p.label] ?? p.title }))
 
+/** The chip that shows `selectedId` among `presets`, or null for none. YTD
+ *  shows as All where it is not offered, since it loads like All there. The
+ *  selection itself is unchanged. */
+export function shownPresetId(
+  selectedId: number,
+  presets: readonly TimeRangePreset[]
+): number | null {
+  if (presets.some((p) => p.id === selectedId)) return selectedId
+  return findPreset(selectedId)?.label === 'YTD' ? ALL_PRESET_ID : null
+}
+
 /**
  * The window a preset covers around an edit session's window: the span on
  * each side. All (and YTD, not offered in the editor) is the data extent,
diff --git a/apps/qc-app/vite.config.ts b/apps/qc-app/vite.config.ts
index d7f59b7e..c25075c5 100644
--- a/apps/qc-app/vite.config.ts
+++ b/apps/qc-app/vite.config.ts
@@ -160,8 +160,8 @@ export default defineConfig(({ command, mode }) => {
           functions: 80,
           // Branches sits a couple points below the others because the
           // last few uncovered branches live in the qualifier-band path
-          // of `options.ts` and the relayout-echo path of `selected.ts`
-          // — both require heavy Plotly-DOM fixture setup for marginal
+          // of `options.ts` and the relayout-echo path of `selected.ts`.
+          // Both require heavy Plotly-DOM fixture setup for marginal
           // signal. Raise back to 80 once those are covered.
           branches: 78,
         },

From 91cded61fb821f99c952669486dec328cdc4f014 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:00:59 -0600
Subject: [PATCH 58/93] feat: default new sessions to the full source range

---
 apps/qc-app/docs/API_REFERENCE.md             |  8 ++---
 apps/qc-app/docs/USER_GUIDE.md                |  6 ++--
 .../EditData/SessionWindowDialog.vue          |  7 +++--
 .../__tests__/SessionWindowDialog.spec.ts     | 21 +++++++++++--
 .../src/utils/__tests__/sessionWindow.spec.ts | 30 ++++++++++++-------
 apps/qc-app/src/utils/sessionWindow.ts        | 15 +++-------
 6 files changed, 52 insertions(+), 35 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 0eff6684..fb1c814d 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -682,10 +682,10 @@ and their pure resolution:
 Pure validation for the session-window step (`SessionWindowDialog.vue`,
 opened from the row Edit chooser or the editor footer's **New session**):
 
-- `defaultSessionWindow(source, sessions)`. The step's default window:
-  starts where the committed history ends, or at the source's own start if
-  nothing is committed yet; runs to the end of the source data. `null` when
-  the source has no observations.
+- `defaultSessionWindow(source)`. The step's default window: the source's
+  full extent, begin to end. `null` when the source has no observations.
+  The rules below accept it whenever the committed history lies inside that
+  extent; the dialog reports the rare case where it does not.
 - `validateSessionWindow(window, source, sessions)`: `null` when `window`
   is valid, otherwise an error string. The window must lie inside the
   source's extent, and it can't leave a gap before or after the committed
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index d4b4995f..1a92c15b 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -122,9 +122,9 @@ session window step.
 
 The session window step shows the source's data extent and, if the managed
 datastream already has committed history, that history's extent too. The
-default window starts where the committed history ends (or at the start of
-the source data, if nothing is committed yet) and runs to the end of the
-source data.
+default window is the source's own begin and end datetimes, so a new session
+covers the whole record and overlaps whatever is already committed. Narrow it
+whenever you only want to work on part of the record.
 
 You can adjust the From / To pickers, but two rules apply:
 
diff --git a/apps/qc-app/src/components/EditData/SessionWindowDialog.vue b/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
index f464204b..db138d83 100644
--- a/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
+++ b/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
@@ -5,8 +5,9 @@
         New session on "{{ managedName }}"
       </div>
       <div class="text-body-small text-medium-emphasis mt-1">
-        Pick the period to edit. It can overlap committed sessions but cannot
-        leave a gap before or after them.
+        The window starts out covering the whole source record. Narrow it to
+        the period you want to edit: it can overlap committed sessions but
+        cannot leave a gap before or after them.
       </div>
     </div>
 
@@ -108,7 +109,7 @@ const emit = defineEmits<{
   (e: 'cancel'): void
 }>()
 
-const initial = defaultSessionWindow(props.source, props.sessions)
+const initial = defaultSessionWindow(props.source)
 const begin = ref<Date | null>(initial?.begin ?? null)
 const end = ref<Date | null>(initial?.end ?? null)
 
diff --git a/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
index 03e0b7ab..ce1f878c 100644
--- a/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
@@ -64,11 +64,12 @@ const pickers = (w: ReturnType<typeof mount>) =>
   w.findAllComponents(DatePickerStub)
 
 describe('SessionWindowDialog', () => {
-  it('prefills the window from the end of the committed history', () => {
+  it('prefills the window with the whole source extent', () => {
     const w = mountDialog()
     const [from, to] = pickers(w)
-    expect((from!.props('modelValue') as Date).toISOString()).toBe('2025-05-01T00:00:00.000Z')
+    expect((from!.props('modelValue') as Date).toISOString()).toBe('2025-01-01T00:00:00.000Z')
     expect((to!.props('modelValue') as Date).toISOString()).toBe('2025-12-31T00:00:00.000Z')
+    expect(w.find('[data-testid="session-window-error"]').exists()).toBe(false)
   })
 
   it('shows the committed history range', () => {
@@ -85,10 +86,24 @@ describe('SessionWindowDialog', () => {
     const w = mountDialog()
     await w.find('[data-testid="session-window-start"]').trigger('click')
     const [window] = w.emitted('confirm')![0] as [{ begin: Date; end: Date }]
-    expect(window.begin.toISOString()).toBe('2025-05-01T00:00:00.000Z')
+    expect(window.begin.toISOString()).toBe('2025-01-01T00:00:00.000Z')
     expect(window.end.toISOString()).toBe('2025-12-31T00:00:00.000Z')
   })
 
+  it('blocks Start when the committed history sits outside the source', () => {
+    const w = mountDialog({
+      sessions: [
+        {
+          status: 'committed',
+          phenomenonTimeStart: '2026-03-01T00:00:00Z',
+          phenomenonTimeEnd: '2026-04-01T00:00:00Z',
+        },
+      ],
+    })
+    expect(w.find('[data-testid="session-window-error"]').text()).toMatch(/gap before/)
+    expect(w.find('[data-testid="session-window-start"]').attributes('disabled')).toBeDefined()
+  })
+
   it('blocks Start and explains a gap after the history', async () => {
     const w = mountDialog()
     pickers(w)[0]!.vm.$emit('update:modelValue', new Date('2025-07-01T00:00:00Z'))
diff --git a/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts b/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
index 2c960d63..94b7fbd0 100644
--- a/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
@@ -36,27 +36,35 @@ describe('committedExtent', () => {
 
 describe('defaultSessionWindow', () => {
   it('covers the whole source when nothing is committed', () => {
-    expect(defaultSessionWindow(source, [])).toEqual(
+    expect(defaultSessionWindow(source)).toEqual(
       win('2025-01-01T00:00:00Z', '2025-12-31T00:00:00Z')
     )
   })
 
-  it('starts at the end of the committed history', () => {
+  it('is null when the source has no observations', () => {
+    expect(defaultSessionWindow({})).toBeNull()
+  })
+})
+
+describe('the default window against the rules', () => {
+  it('is valid while committed history sits inside the source', () => {
     const sessions = [committed('2025-01-01T00:00:00Z', '2025-05-01T00:00:00Z')]
-    expect(defaultSessionWindow(source, sessions)).toEqual(
-      win('2025-05-01T00:00:00Z', '2025-12-31T00:00:00Z')
-    )
+    const window = defaultSessionWindow(source)!
+    expect(validateSessionWindow(window, source, sessions)).toBeNull()
   })
 
-  it('covers the whole source when history already reaches its end', () => {
+  it('is valid when the history already covers the whole source', () => {
     const sessions = [committed('2025-01-01T00:00:00Z', '2025-12-31T00:00:00Z')]
-    expect(defaultSessionWindow(source, sessions)).toEqual(
-      win('2025-01-01T00:00:00Z', '2025-12-31T00:00:00Z')
-    )
+    const window = defaultSessionWindow(source)!
+    expect(validateSessionWindow(window, source, sessions)).toBeNull()
   })
 
-  it('is null when the source has no observations', () => {
-    expect(defaultSessionWindow({}, [])).toBeNull()
+  // The source extent can shrink away from history committed earlier, and
+  // then even the full extent leaves a gap. The dialog reports it.
+  it('is rejected when the committed history sits outside the source', () => {
+    const sessions = [committed('2026-03-01T00:00:00Z', '2026-04-01T00:00:00Z')]
+    const window = defaultSessionWindow(source)!
+    expect(validateSessionWindow(window, source, sessions)).toMatch(/gap before/)
   })
 })
 
diff --git a/apps/qc-app/src/utils/sessionWindow.ts b/apps/qc-app/src/utils/sessionWindow.ts
index 859558c2..2da7a0b1 100644
--- a/apps/qc-app/src/utils/sessionWindow.ts
+++ b/apps/qc-app/src/utils/sessionWindow.ts
@@ -33,17 +33,10 @@ export function committedExtent(
   return { begin: new Date(begin), end: new Date(end) }
 }
 
-export function defaultSessionWindow(
-  source: SourceExtent,
-  sessions: readonly SessionRange[]
-): TimeWindow | null {
-  const extent = dataExtent([source])
-  if (!extent) return null
-  const history = committedExtent(sessions)
-  if (history && history.end.getTime() < extent.end.getTime()) {
-    return { begin: new Date(history.end), end: extent.end }
-  }
-  return extent
+/** The source's own extent, which the rules above always accept while the
+ *  committed history lies inside it. */
+export function defaultSessionWindow(source: SourceExtent): TimeWindow | null {
+  return dataExtent([source])
 }
 
 export function validateSessionWindow(

From b47501e613b4555bcde0e9a6ffdc861731372177 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:00:59 -0600
Subject: [PATCH 59/93] feat: set description, status and method when creating
 a managed datastream

---
 apps/qc-app/docs/API_REFERENCE.md             |  28 ++++-
 apps/qc-app/docs/TESTING.md                   |   1 +
 apps/qc-app/docs/USER_GUIDE.md                |  19 +++
 .../e2e/create-managed-datastream.spec.ts     |  82 ++++++++++++
 apps/qc-app/e2e/support/fixtures.ts           |  10 ++
 apps/qc-app/e2e/support/mocks.ts              |  52 +++++++-
 .../EditData/CreateDatastreamForm.vue         |  93 ++++++++++++++
 .../components/EditData/StartEditingFlow.vue  |  13 +-
 .../__tests__/CreateDatastreamForm.spec.ts    | 117 +++++++++++++-----
 .../__tests__/StartEditingFlow.spec.ts        |   7 +-
 .../useCreateManagedDatastream.spec.ts        |  21 ++++
 .../__tests__/useDatastreamMetadata.spec.ts   |  78 ++++++++++++
 .../composables/useCreateManagedDatastream.ts |  13 +-
 .../src/composables/useDatastreamMetadata.ts  |  53 ++++++++
 .../__tests__/createManagedDatastream.spec.ts |  16 +++
 15 files changed, 563 insertions(+), 40 deletions(-)
 create mode 100644 apps/qc-app/e2e/create-managed-datastream.spec.ts
 create mode 100644 apps/qc-app/src/composables/__tests__/useDatastreamMetadata.spec.ts
 create mode 100644 apps/qc-app/src/composables/useDatastreamMetadata.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index fb1c814d..7fa9c275 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -238,9 +238,21 @@ const {
 
 ```ts
 const { create } = useCreateManagedDatastream()
-const { managedDatastream, history } = await create({ source, processingLevelId, name })
+const { managedDatastream, history } = await create({
+  source,
+  processingLevelId,
+  name,
+  description,
+  status, // omitted to create the datastream without one
+  sensorId, // the datastream's method
+})
 ```
 
+`description`, `status` and `sensorId` are writable on a datastream create
+(see the client's `writableKeys`); there is no separate method field, since a
+datastream's method is its sensor. They are passed as overrides, so they win
+over the values copied from the source.
+
 Delegates to the tested `createManagedDatastream` orchestration with the
 live client (`hs.datastreams` + `hs.qualityControlHistories`). The datastream
 is created with `expand_related: true`, so `managedDatastream` comes back in
@@ -262,6 +274,20 @@ histories and fetches each one's sessions. Feeds the row Edit button's
 chooser (`StartEditingFlow.vue`), which lists managed datastreams with their
 in-progress/committed sessions.
 
+### `useDatastreamMetadata()`
+
+```ts
+const { sensors, statuses, load } = useDatastreamMetadata()
+await load()
+```
+
+Reference data for the create-datastream form: the active workspace's sensors
+(`hs.sensors.list`, including the system-level ones) and the datastream status
+vocabulary (`hs.datastreams.getStatuses`). Loaded on demand when the form
+opens rather than with the workspace catalog in `App.vue`, since nothing else
+needs it. Neither request throws: a list that fails to load stays empty and
+the form falls back to the source datastream's own value.
+
 ### `useWorkspacePermissions()`
 
 Synchronous, reactive role/permission checks for gating UI. The role
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 1b74b467..bf71ce01 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -288,6 +288,7 @@ Per-spec overrides go through `options`:
 await installMocks(page, {
   observations: { phenomenonTime: [...], result: [...] },
   submissions: collectedSubmissions,   // accumulates bulk POSTs for assertion
+  datastreamCreates: collectedBodies,   // accumulates datastream create bodies
   authenticated: false,                 // simulate signed-out
   qcHistories: true,                    // give DATASTREAM_ID a managed datastream
   qcSessionState: sessions,             // live QC sessions, for assertions
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 1a92c15b..1f212a73 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -118,6 +118,25 @@ straight to the create-datastream form. Pick a managed datastream to
 continue its in-progress session, or **Start new session** to open the
 session window step.
 
+#### Creating a managed datastream
+
+The create-datastream form builds the quality-controlled datastream from the
+source's metadata. Everything you can change is prefilled from the source:
+
+- **New processing level**: required, and it has to differ from the source's.
+  Use **Add processing level** if the workspace has no suitable one yet.
+- **Name**: the source's name with "(QC)" appended.
+- **Description**: required, since the datastream cannot be saved without one.
+- **Status**: the HydroServer status vocabulary. It can be left empty.
+- **Select method**: the workspace's methods, which are its sensors. A
+  datastream's method is the sensor that produced it.
+
+If the method or status list cannot be loaded, the form keeps the source's own
+value so creation still goes through. Everything else (site, observed
+property, unit, medium, aggregation) is copied from the source and cannot be
+changed here; edit the datastream in the Data Management app afterwards if you
+need to.
+
 #### The session window
 
 The session window step shows the source's data extent and, if the managed
diff --git a/apps/qc-app/e2e/create-managed-datastream.spec.ts b/apps/qc-app/e2e/create-managed-datastream.spec.ts
new file mode 100644
index 00000000..92ca32e4
--- /dev/null
+++ b/apps/qc-app/e2e/create-managed-datastream.spec.ts
@@ -0,0 +1,82 @@
+/**
+ * Creating a managed datastream from a source that has none: the form
+ * defaults every field from the source, and what the user edits (name,
+ * description, status, method) is what gets posted.
+ */
+
+import { expect, test } from '@playwright/test'
+import { installMocks } from './support/mocks'
+import { gotoHome } from './support/app'
+import { DATASTREAM_ID, QC_PROC_LEVEL_ID, SENSOR_ID_B } from './support/fixtures'
+
+/** Open a Vuetify select and pick one of its options. */
+async function pick(page: import('@playwright/test').Page, testId: string, option: string) {
+  await page.getByTestId(testId).click()
+  await page.getByRole('option', { name: option, exact: true }).click()
+}
+
+test.describe('create managed datastream', () => {
+  test.beforeEach(() => test.slow())
+
+  test('posts the edited fields and moves on to the session window', async ({
+    page,
+  }) => {
+    const datastreamCreates: Array<Record<string, any>> = []
+    await installMocks(page, { datastreamCreates })
+    await gotoHome(page)
+
+    // No managed datastream yet, so Edit opens the create form directly.
+    await page.getByTestId(`edit-datastream-${DATASTREAM_ID}`).click()
+    await expect(page.getByTestId('create-confirm')).toBeVisible()
+
+    // Defaults come from the source datastream.
+    await expect(page.getByTestId('create-name').locator('input')).toHaveValue(
+      'Streamflow Datastream (QC)'
+    )
+    await expect(
+      page.getByTestId('create-description').getByRole('textbox')
+    ).toHaveValue('Synthetic sine-wave dataset for QC tests')
+    await expect(page.getByTestId('create-status')).toContainText('ongoing')
+    await expect(page.getByTestId('create-sensor')).toContainText(
+      'ADVelocity Sensor'
+    )
+
+    await pick(page, 'create-processing-level', 'Quality controlled')
+    await page
+      .getByTestId('create-description')
+      .getByRole('textbox')
+      .fill('Quality-controlled streamflow')
+    await pick(page, 'create-status', 'complete')
+    await pick(page, 'create-sensor', 'Thermistor Sensor')
+
+    await page.getByTestId('create-confirm').click()
+
+    await expect.poll(() => datastreamCreates.length).toBe(1)
+    expect(datastreamCreates[0]).toMatchObject({
+      name: 'Streamflow Datastream (QC)',
+      description: 'Quality-controlled streamflow',
+      status: 'complete',
+      sensorId: SENSOR_ID_B,
+      processingLevelId: QC_PROC_LEVEL_ID,
+      valueCount: 0,
+    })
+
+    // The flow continues into the session window for the new datastream.
+    await expect(page.getByTestId('session-window-start')).toBeVisible()
+  })
+
+  test('blocks create without a description', async ({ page }) => {
+    await installMocks(page)
+    await gotoHome(page)
+
+    await page.getByTestId(`edit-datastream-${DATASTREAM_ID}`).click()
+    await pick(page, 'create-processing-level', 'Quality controlled')
+    await expect(page.getByTestId('create-confirm')).toBeEnabled()
+
+    await page.getByTestId('create-description').getByRole('textbox').fill('')
+    await expect(page.getByTestId('create-confirm')).toBeDisabled()
+    await expect(page.getByTestId('create-description')).toContainText(
+      'A description is required'
+    )
+  })
+})
diff --git a/apps/qc-app/e2e/support/fixtures.ts b/apps/qc-app/e2e/support/fixtures.ts
index 493032a2..ae3fb98d 100644
--- a/apps/qc-app/e2e/support/fixtures.ts
+++ b/apps/qc-app/e2e/support/fixtures.ts
@@ -79,8 +79,18 @@ export const processingLevels = [
     definition: 'Raw data',
     explanation: 'Unprocessed readings',
   },
+  {
+    id: QC_PROC_LEVEL_ID,
+    workspaceId: WORKSPACE_ID,
+    code: 'QC',
+    definition: 'Quality controlled',
+    explanation: 'Reviewed readings',
+  },
 ]
 
+/** The datastream status vocabulary, as `/datastreams/statuses` serves it. */
+export const datastreamStatuses = ['ongoing', 'complete', 'planned']
+
 export const observedProperties = [
   {
     id: OBSERVED_PROP_ID,
diff --git a/apps/qc-app/e2e/support/mocks.ts b/apps/qc-app/e2e/support/mocks.ts
index d906d956..83e88603 100644
--- a/apps/qc-app/e2e/support/mocks.ts
+++ b/apps/qc-app/e2e/support/mocks.ts
@@ -28,6 +28,7 @@ import {
   processingLevels,
   QC_SESSION_AUTHOR,
   QC_SOURCE_CHECKSUM,
+  datastreamStatuses,
   qcHistories,
   qcSessions,
   resultQualifiers,
@@ -67,6 +68,11 @@ export interface MockOptions {
    * payload contents without installing a second route handler.
    */
   submissions?: Array<{ mode: string | null; body: any }>
+  /**
+   * Accumulates every datastream create body the app posts, so a spec can
+   * assert on what the create-datastream form sent.
+   */
+  datastreamCreates?: Array<Record<string, any>>
   /**
    * Serve the QC history fixture, so `DATASTREAM_ID` has a managed
    * datastream derived from it. Off by default: most specs want a catalog
@@ -151,6 +157,7 @@ export async function installMocks(
   const observations = options.observations ?? buildObservations()
   const observationsById = options.observationsById ?? {}
   const submissions = options.submissions ?? []
+  const datastreamCreates = options.datastreamCreates ?? []
   const withQcHistories = options.qcHistories ?? false
   const sessionState = options.qcSessionState ?? []
   if (withQcHistories) {
@@ -260,10 +267,15 @@ export async function installMocks(
     if (qcSessionRoute) {
       return handleQcSessions(route, sessionState, qcSessionRoute)
     }
-    if (
-      path.endsWith('/api/data/quality-control/histories') &&
-      method === 'GET'
-    ) {
+    if (path.endsWith('/api/data/quality-control/histories')) {
+      if (method === 'POST') {
+        const body = await safeJson(request)
+        return json(
+          route,
+          { data: { id: 'qch-e2e-new', ...body } },
+          201
+        )
+      }
       return json(route, { data: withQcHistories ? qcHistories : [] })
     }
 
@@ -271,9 +283,39 @@ export async function installMocks(
     if (path.endsWith('/api/data/things') && method === 'GET') {
       return json(route, { data: things })
     }
-    if (path.endsWith('/api/data/datastreams') && method === 'GET') {
+    if (path.endsWith('/api/data/datastreams')) {
+      if (method === 'POST') {
+        const body = await safeJson(request)
+        datastreamCreates.push(body)
+        // Echo the body back the way the server does, with an assigned id
+        // and the nested relations `expand_related: true` asks for.
+        const created = { ...body, id: 'ds-qc-e2e-created' }
+        return json(
+          route,
+          {
+            data: {
+              ...created,
+              thing: things.find((t) => t.id === created.thingId),
+              unit: units.find((u) => u.id === created.unitId),
+              sensor: sensors.find((s) => s.id === created.sensorId),
+              observedProperty: observedProperties.find(
+                (o) => o.id === created.observedPropertyId
+              ),
+              processingLevel: processingLevels.find(
+                (p) => p.id === created.processingLevelId
+              ),
+            },
+          },
+          201
+        )
+      }
       return json(route, { data: catalog })
     }
+    // Ahead of the single-datastream route, which would otherwise treat
+    // "statuses" as a datastream id.
+    if (path.endsWith('/api/data/datastreams/statuses') && method === 'GET') {
+      return json(route, { data: datastreamStatuses })
+    }
     if (path.endsWith('/api/data/processing-levels') && method === 'GET') {
       return json(route, { data: processingLevels })
     }
diff --git a/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue b/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
index 7cf1dd6d..5622ef77 100644
--- a/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
+++ b/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
@@ -116,6 +116,43 @@
       label="Name"
       density="compact"
       hide-details="auto"
+      class="mb-2"
+    />
+
+    <v-textarea
+      v-model="description"
+      data-testid="create-description"
+      label="Description"
+      rows="2"
+      auto-grow
+      density="compact"
+      :error-messages="descriptionError"
+      hide-details="auto"
+      class="mb-2"
+    />
+
+    <v-select
+      v-model="status"
+      data-testid="create-status"
+      :items="statusItems"
+      label="Status"
+      density="compact"
+      clearable
+      hide-details="auto"
+      class="mb-2"
+    />
+
+    <v-select
+      v-model="sensorId"
+      data-testid="create-sensor"
+      :items="sensorItems"
+      item-title="title"
+      item-value="value"
+      label="Select method"
+      no-data-text="No available methods"
+      density="compact"
+      :error-messages="sensorError"
+      hide-details="auto"
       class="mb-3"
     />
 
@@ -146,15 +183,30 @@ interface ProcessingLevelOption {
   code?: string
 }
 
+interface SensorOption {
+  id: string
+  name: string
+}
+
 export interface CreateDatastreamSpec {
   source: Datastream
   processingLevelId: string
   name?: string
+  description: string
+  /** Left out when the user clears it: the field is nullable. */
+  status?: string
+  /** A datastream's method is its sensor. */
+  sensorId: string
 }
 
 const props = defineProps<{
   source: Datastream
   processingLevels: ProcessingLevelOption[]
+  /** The workspace's methods. Empty when the list could not be loaded, and
+   *  the source's own method is then the only option. */
+  sensors?: SensorOption[]
+  /** The datastream status vocabulary. Empty when it could not be loaded. */
+  statuses?: string[]
   defaultProcessingLevelId?: string | null
   /** When set, the user can't create datastreams here: shown as a warning
    *  and the confirm button is disabled. */
@@ -191,6 +243,42 @@ const processingLevelId = ref<string | null>(
     : null
 )
 const name = ref(`${props.source.name} (QC)`)
+const description = ref(props.source.description ?? '')
+const status = ref<string | null>(props.source.status ?? null)
+
+// Datastreams load expand_related, so the sensor arrives nested.
+const sourceSensor = computed(() => {
+  const s = props.source as Datastream & { sensor?: SensorOption }
+  return { id: props.source.sensorId ?? s.sensor?.id ?? '', name: s.sensor?.name }
+})
+const sensorId = ref(sourceSensor.value.id)
+
+// The source's own value stays selectable when the list is missing it, so a
+// failed load never silently changes what gets created.
+const statusItems = computed(() => {
+  const items = [...(props.statuses ?? [])]
+  const current = props.source.status
+  if (current && !items.includes(current)) items.unshift(current)
+  return items
+})
+
+const sensorItems = computed(() => {
+  const items = (props.sensors ?? []).map((s) => ({
+    title: s.name,
+    value: s.id,
+  }))
+  const { id, name: sensorName } = sourceSensor.value
+  if (id && !items.some((i) => i.value === id)) {
+    items.unshift({ title: sensorName || "The source's method", value: id })
+  }
+  return items
+})
+
+const descriptionError = computed(() =>
+  description.value.trim() ? '' : 'A description is required'
+)
+
+const sensorError = computed(() => (sensorId.value ? '' : 'A method is required'))
 
 const processingLevelItems = computed(() =>
   props.processingLevels.map((p) => ({
@@ -218,6 +306,8 @@ const isValid = computed(
     !!processingLevelId.value &&
     knownLevelIds.value.has(processingLevelId.value) &&
     !processingLevelError.value &&
+    !descriptionError.value &&
+    !!sensorId.value &&
     !props.permissionError
 )
 
@@ -227,6 +317,9 @@ function onConfirm(): void {
     source: props.source,
     processingLevelId: processingLevelId.value,
     name: name.value.trim() || undefined,
+    description: description.value.trim(),
+    status: status.value || undefined,
+    sensorId: sensorId.value,
   })
 }
 
diff --git a/apps/qc-app/src/components/EditData/StartEditingFlow.vue b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
index 516238ee..526abaa3 100644
--- a/apps/qc-app/src/components/EditData/StartEditingFlow.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
@@ -16,8 +16,11 @@
   <v-dialog v-model="showCreateDatastream" max-width="560" persistent>
     <v-card v-if="chooserSource" rounded="lg">
       <CreateDatastreamForm
+        :key="chooserSource.id"
         :source="chooserSource"
         :processing-levels="processingLevels"
+        :sensors="sensors"
+        :statuses="statuses"
         :default-processing-level-id="qcPreferences.processingLevelId"
         :on-create-processing-level="onCreateProcessingLevel"
         :permission-error="createPermissionError"
@@ -48,7 +51,7 @@
  * then enter the editor.
  */
 
-import { computed, ref } from 'vue'
+import { computed, ref, watch } from 'vue'
 import { storeToRefs } from 'pinia'
 import { Snackbar } from '@uwrl/qc-utils'
 import type { Datastream, QualityControlSession } from '@hydroserver/client'
@@ -68,6 +71,7 @@ import {
   useManagedDatastreams,
   type ManagedDatastreamOption,
 } from '@/composables/useManagedDatastreams'
+import { useDatastreamMetadata } from '@/composables/useDatastreamMetadata'
 import { useProcessingLevels } from '@/composables/useProcessingLevels'
 import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
 import { collectDeletionChain } from '@/utils/sessionGraph'
@@ -98,6 +102,8 @@ const { loadForSource, deleteManaged, deleteSessionChain } =
   useManagedDatastreams()
 const { create: createManaged } = useCreateManagedDatastream()
 const { createProcessingLevel } = useProcessingLevels()
+const { sensors, statuses, load: loadDatastreamMetadata } =
+  useDatastreamMetadata()
 const { canCreateDatastream, roleName } = useWorkspacePermissions()
 const { enterEdit, startSessionOver, leaveEdit } = useEditEntry()
 
@@ -113,6 +119,11 @@ const chooserOptions = ref<ManagedDatastreamOption[]>([])
 const chooserSource = ref<Datastream | null>(null)
 const showCreateDatastream = ref(false)
 
+// The form's method and status lists are only needed once it opens.
+watch(showCreateDatastream, (open) => {
+  if (open) void loadDatastreamMetadata()
+})
+
 const windowTarget = ref<WindowTarget | null>(null)
 let windowTargetCount = 0
 function openWindow(target: Omit<WindowTarget, 'key'>) {
diff --git a/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts b/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
index b7a1c72f..2ea5fcbb 100644
--- a/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
@@ -1,4 +1,4 @@
-import { mount, flushPromises } from '@vue/test-utils'
+import { mount, flushPromises, type VueWrapper } from '@vue/test-utils'
 import { describe, it, expect, vi } from 'vitest'
 import { createTestVuetify } from '@/utils/test/vuetify'
 import CreateDatastreamForm from '@/components/EditData/CreateDatastreamForm.vue'
@@ -8,21 +8,39 @@ import CreateDatastreamForm from '@/components/EditData/CreateDatastreamForm.vue
   disconnect() {}
 }
 
-const source = { id: 's-1', name: 'Raw Temp', processingLevelId: 'pl-raw' } as any
+const source = {
+  id: 's-1',
+  name: 'Raw Temp',
+  description: 'Raw temperature readings',
+  status: 'ongoing',
+  sensorId: 'sn-1',
+  processingLevelId: 'pl-raw',
+} as any
 
 const processingLevels = [
   { id: 'pl-raw', definition: 'Raw' },
   { id: 'pl-qc', definition: 'Quality Controlled' },
 ]
 
-const mountForm = () =>
+const sensors = [
+  { id: 'sn-1', name: 'Thermistor' },
+  { id: 'sn-2', name: 'Weather station' },
+]
+
+const statuses = ['ongoing', 'complete']
+
+const mountForm = (props: Record<string, unknown> = {}) =>
   mount(CreateDatastreamForm, {
-    props: { source, processingLevels },
+    props: { source, processingLevels, sensors, statuses, ...props },
     global: { plugins: [createTestVuetify()] },
   })
 
+// Every field below is a Vuetify component carrying the test id.
+const field = (w: ReturnType<typeof mountForm>, testId: string) =>
+  w.findComponent(`[data-testid="${testId}"]`) as VueWrapper
+
 const levelSelect = (w: ReturnType<typeof mountForm>) =>
-  w.findComponent({ name: 'VSelect' })
+  field(w, 'create-processing-level')
 
 describe('CreateDatastreamForm', () => {
   it('disables create until a different processing level is chosen', async () => {
@@ -37,7 +55,7 @@ describe('CreateDatastreamForm', () => {
     expect(confirm().attributes('disabled')).toBeUndefined()
   })
 
-  it('emits the create spec on confirm, defaulting the name from the source', async () => {
+  it('emits the create spec on confirm, defaulting the fields from the source', async () => {
     const w = mountForm()
     await levelSelect(w).vm.$emit('update:modelValue', 'pl-qc')
     await w.find('[data-testid="create-confirm"]').trigger('click')
@@ -46,17 +64,69 @@ describe('CreateDatastreamForm', () => {
       source: { id: string }
       processingLevelId: string
       name?: string
+      description: string
+      status?: string
+      sensorId: string
     }
     expect(spec.source.id).toBe('s-1')
     expect(spec.processingLevelId).toBe('pl-qc')
     expect(spec.name).toBe('Raw Temp (QC)')
+    expect(spec.description).toBe('Raw temperature readings')
+    expect(spec.status).toBe('ongoing')
+    expect(spec.sensorId).toBe('sn-1')
   })
 
-  it('uses the provided default processing level', async () => {
-    const w = mount(CreateDatastreamForm, {
-      props: { source, processingLevels, defaultProcessingLevelId: 'pl-qc' },
-      global: { plugins: [createTestVuetify()] },
+  it('emits the edited description, status and method', async () => {
+    const w = mountForm({ defaultProcessingLevelId: 'pl-qc' })
+    await field(w, 'create-description').vm.$emit('update:modelValue', 'QC pass')
+    await field(w, 'create-status').vm.$emit('update:modelValue', 'complete')
+    await field(w, 'create-sensor').vm.$emit('update:modelValue', 'sn-2')
+    await w.find('[data-testid="create-confirm"]').trigger('click')
+
+    const spec = w.emitted('confirm')![0][0] as {
+      description: string
+      status?: string
+      sensorId: string
+    }
+    expect(spec.description).toBe('QC pass')
+    expect(spec.status).toBe('complete')
+    expect(spec.sensorId).toBe('sn-2')
+  })
+
+  it('requires a description, and allows an empty status', async () => {
+    const w = mountForm({ defaultProcessingLevelId: 'pl-qc' })
+    const confirm = () => w.find('[data-testid="create-confirm"]')
+
+    await field(w, 'create-status').vm.$emit('update:modelValue', null)
+    expect(confirm().attributes('disabled')).toBeUndefined()
+
+    await field(w, 'create-description').vm.$emit('update:modelValue', '  ')
+    expect(confirm().attributes('disabled')).toBeDefined()
+
+    await field(w, 'create-description').vm.$emit('update:modelValue', 'QC pass')
+    expect(confirm().attributes('disabled')).toBeUndefined()
+    await confirm().trigger('click')
+    const spec = w.emitted('confirm')![0][0] as { status?: string }
+    expect(spec.status).toBeUndefined()
+  })
+
+  it('keeps the source values when the lists could not be loaded', async () => {
+    const w = mountForm({
+      defaultProcessingLevelId: 'pl-qc',
+      sensors: [],
+      statuses: [],
     })
+    await w.find('[data-testid="create-confirm"]').trigger('click')
+    const spec = w.emitted('confirm')![0][0] as {
+      status?: string
+      sensorId: string
+    }
+    expect(spec.status).toBe('ongoing')
+    expect(spec.sensorId).toBe('sn-1')
+  })
+
+  it('uses the provided default processing level', async () => {
+    const w = mountForm({ defaultProcessingLevelId: 'pl-qc' })
     // Valid immediately since the default differs from the source's level.
     expect(w.find('[data-testid="create-confirm"]').attributes('disabled')).toBeUndefined()
     await w.find('[data-testid="create-confirm"]').trigger('click')
@@ -71,15 +141,8 @@ describe('CreateDatastreamForm', () => {
   })
 
   it('ignores a remembered default not in this workspace, and stays invalid until a real level is picked', async () => {
-    const w = mount(CreateDatastreamForm, {
-      props: {
-        source,
-        processingLevels,
-        // A level id persisted from another workspace/backend.
-        defaultProcessingLevelId: 'pl-from-elsewhere',
-      },
-      global: { plugins: [createTestVuetify()] },
-    })
+    // A level id persisted from another workspace/backend.
+    const w = mountForm({ defaultProcessingLevelId: 'pl-from-elsewhere' })
     // Stale default dropped -> confirm disabled (not submitted as-is).
     expect(
       w.find('[data-testid="create-confirm"]').attributes('disabled')
@@ -93,10 +156,7 @@ describe('CreateDatastreamForm', () => {
   it('adds a processing level inline and selects the new one', async () => {
     const created = { id: 'pl-new', code: 'Quality Controlled' }
     const onCreateProcessingLevel = vi.fn().mockResolvedValue(created)
-    const w = mount(CreateDatastreamForm, {
-      props: { source, processingLevels, onCreateProcessingLevel },
-      global: { plugins: [createTestVuetify()] },
-    })
+    const w = mountForm({ onCreateProcessingLevel })
 
     // Open the inline add panel and submit a new level.
     await w.find('[data-testid="add-level-toggle"]').trigger('click')
@@ -118,14 +178,9 @@ describe('CreateDatastreamForm', () => {
   })
 
   it('blocks create and shows a warning when permissionError is set', async () => {
-    const w = mount(CreateDatastreamForm, {
-      props: {
-        source,
-        processingLevels,
-        defaultProcessingLevelId: 'pl-qc', // otherwise valid
-        permissionError: 'You cannot create datastreams here.',
-      },
-      global: { plugins: [createTestVuetify()] },
+    const w = mountForm({
+      defaultProcessingLevelId: 'pl-qc', // otherwise valid
+      permissionError: 'You cannot create datastreams here.',
     })
     expect(w.find('[data-testid="create-permission-error"]').exists()).toBe(true)
     expect(w.text()).toContain('You cannot create datastreams here.')
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
index 84a8b57e..63b2b2fb 100644
--- a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
@@ -426,7 +426,12 @@ describe('StartEditingFlow', () => {
     await flushPromises()
     wrapper
       .findComponent(CreateStub)
-      .vm.$emit('confirm', { source, processingLevelId: 'pl' })
+      .vm.$emit('confirm', {
+        source,
+        processingLevelId: 'pl',
+        description: 'Cleaned series',
+        sensorId: 'sn-1',
+      })
     await flushPromises()
 
     expect(addQcHistory).toHaveBeenCalledWith({ id: 'h-new' })
diff --git a/apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts b/apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts
index 33a8a0cd..e6f0f40c 100644
--- a/apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts
@@ -54,11 +54,17 @@ describe('useCreateManagedDatastream', () => {
       source: makeSource(),
       processingLevelId: 'pl-qc',
       name: 'My QC',
+      description: 'Cleaned series',
+      status: 'complete',
+      sensorId: 'sn-2',
     })
 
     const body = hsCreate.mock.calls[0][0]
     expect(body.processingLevelId).toBe('pl-qc')
     expect(body.name).toBe('My QC')
+    expect(body.description).toBe('Cleaned series')
+    expect(body.status).toBe('complete')
+    expect(body.sensorId).toBe('sn-2')
     expect(body.valueCount).toBe(0)
 
     expect(result.history.managedDatastream.id).toBe('managed-1')
@@ -69,4 +75,19 @@ describe('useCreateManagedDatastream', () => {
       expect.objectContaining({ value: 'managed-1' })
     )
   })
+
+  it('sends no status when the spec leaves it out', async () => {
+    const { useCreateManagedDatastream } = await import(
+      '@/composables/useCreateManagedDatastream'
+    )
+    const { create } = useCreateManagedDatastream()
+    await create({
+      source: makeSource({ status: 'ongoing' }),
+      processingLevelId: 'pl-qc',
+      description: 'Cleaned series',
+      sensorId: 'sn-1',
+    })
+
+    expect(hsCreate.mock.calls[0][0].status).toBeUndefined()
+  })
 })
diff --git a/apps/qc-app/src/composables/__tests__/useDatastreamMetadata.spec.ts b/apps/qc-app/src/composables/__tests__/useDatastreamMetadata.spec.ts
new file mode 100644
index 00000000..6cc777f3
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useDatastreamMetadata.spec.ts
@@ -0,0 +1,78 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { setActivePinia, createPinia } from 'pinia'
+import { useDatastreamMetadata } from '../useDatastreamMetadata'
+import { useHydroServer } from '@/store/hydroserver'
+import { useWorkspaceStore } from '@/store/workspaces'
+
+const sensorList = [{ id: 'sn-1', name: 'Thermistor' }]
+
+function setHs(listAllItems: any, getStatuses: any) {
+  useHydroServer().hs = {
+    sensors: { listAllItems },
+    datastreams: { getStatuses },
+  } as any
+}
+
+describe('useDatastreamMetadata', () => {
+  beforeEach(() => {
+    setActivePinia(createPinia())
+    useWorkspaceStore().selectedWorkspace = { id: 'ws-1' } as any
+    vi.spyOn(console, 'warn').mockImplementation(() => {})
+  })
+
+  it('loads the workspace sensors and the status vocabulary', async () => {
+    const listAllItems = vi.fn().mockResolvedValue(sensorList)
+    const getStatuses = vi
+      .fn()
+      .mockResolvedValue({ ok: true, data: ['ongoing', 'complete'] })
+    setHs(listAllItems, getStatuses)
+
+    const { sensors, statuses, load } = useDatastreamMetadata()
+    await load()
+
+    // System-level sensors (workspace null) are selectable too, and every
+    // page is fetched so a long list is not truncated.
+    expect(listAllItems.mock.calls[0][0]).toMatchObject({
+      workspace_id: ['ws-1', 'null'],
+    })
+    expect(sensors.value).toEqual(sensorList)
+    expect(statuses.value).toEqual(['ongoing', 'complete'])
+  })
+
+  it('leaves a list empty when its request fails, without throwing', async () => {
+    setHs(
+      vi.fn().mockRejectedValue(new Error('offline')),
+      vi.fn().mockResolvedValue({ ok: false, data: null })
+    )
+
+    const { sensors, statuses, load } = useDatastreamMetadata()
+    await expect(load()).resolves.toBeUndefined()
+    expect(sensors.value).toEqual([])
+    expect(statuses.value).toEqual([])
+    expect(console.warn).toHaveBeenCalledTimes(2)
+  })
+
+  it('keeps the list that loaded when the other one fails', async () => {
+    setHs(
+      vi.fn().mockRejectedValue(new Error('offline')),
+      vi.fn().mockResolvedValue({ ok: true, data: ['ongoing'] })
+    )
+
+    const { sensors, statuses, load } = useDatastreamMetadata()
+    await load()
+    expect(sensors.value).toEqual([])
+    expect(statuses.value).toEqual(['ongoing'])
+  })
+
+  it('loads nothing when no workspace is selected', async () => {
+    const listAllItems = vi.fn()
+    const getStatuses = vi.fn()
+    setHs(listAllItems, getStatuses)
+    useWorkspaceStore().selectedWorkspace = undefined as any
+
+    const { load } = useDatastreamMetadata()
+    await load()
+    expect(listAllItems).not.toHaveBeenCalled()
+    expect(getStatuses).not.toHaveBeenCalled()
+  })
+})
diff --git a/apps/qc-app/src/composables/useCreateManagedDatastream.ts b/apps/qc-app/src/composables/useCreateManagedDatastream.ts
index 2d170429..2f576a28 100644
--- a/apps/qc-app/src/composables/useCreateManagedDatastream.ts
+++ b/apps/qc-app/src/composables/useCreateManagedDatastream.ts
@@ -16,6 +16,11 @@ export interface CreateManagedDatastreamSpec {
   source: Datastream
   processingLevelId: string
   name?: string
+  description: string
+  /** Left out to create the datastream without a status. */
+  status?: string
+  /** A datastream's method is its sensor. */
+  sensorId: string
 }
 
 export function useCreateManagedDatastream() {
@@ -27,7 +32,13 @@ export function useCreateManagedDatastream() {
     return createManagedDatastream(hs.value, hs.value.qualityControlHistories, {
       source: spec.source,
       processingLevelId: spec.processingLevelId,
-      overrides: spec.name ? { name: spec.name } : undefined,
+      overrides: {
+        ...(spec.name ? { name: spec.name } : {}),
+        description: spec.description,
+        // Explicit, so clearing the status drops the source's one.
+        status: spec.status,
+        sensorId: spec.sensorId,
+      },
     })
   }
 
diff --git a/apps/qc-app/src/composables/useDatastreamMetadata.ts b/apps/qc-app/src/composables/useDatastreamMetadata.ts
new file mode 100644
index 00000000..b8d23d28
--- /dev/null
+++ b/apps/qc-app/src/composables/useDatastreamMetadata.ts
@@ -0,0 +1,53 @@
+/**
+ * Reference data the create-datastream form offers: the workspace's sensors
+ * (a datastream's "method" in HydroServer) and the datastream status
+ * vocabulary. Loaded on demand rather than with the workspace catalog, since
+ * only that one form needs them. A list that fails to load stays empty and
+ * the form falls back to the source datastream's own value.
+ */
+
+import { ref } from 'vue'
+import { storeToRefs } from 'pinia'
+import type { Sensor } from '@hydroserver/client'
+import { useHydroServer } from '@/store/hydroserver'
+import { useWorkspaceStore } from '@/store/workspaces'
+
+export function useDatastreamMetadata() {
+  const { hs } = storeToRefs(useHydroServer())
+  const { selectedWorkspaceId } = storeToRefs(useWorkspaceStore())
+
+  const sensors = ref<Sensor[]>([])
+  const statuses = ref<string[]>([])
+
+  async function load(): Promise<void> {
+    const workspaceId = selectedWorkspaceId.value
+    if (!hs.value || !workspaceId) return
+
+    const [sensorList, statusList] = await Promise.allSettled([
+      // 'null' keeps the system-level sensors, which are selectable too.
+      hs.value.sensors.listAllItems({
+        workspace_id: [workspaceId, 'null'] as (string | 'null')[],
+        order_by: ['name'] as const,
+      }),
+      hs.value.datastreams.getStatuses(),
+    ])
+
+    if (sensorList.status === 'rejected') {
+      console.warn('Could not load the method list', sensorList.reason)
+    }
+    sensors.value =
+      sensorList.status === 'fulfilled' ? (sensorList.value as Sensor[]) : []
+
+    if (statusList.status === 'rejected') {
+      console.warn('Could not load the status list', statusList.reason)
+      statuses.value = []
+    } else if (!statusList.value.ok) {
+      console.warn('Could not load the status list', statusList.value)
+      statuses.value = []
+    } else {
+      statuses.value = statusList.value.data
+    }
+  }
+
+  return { sensors, statuses, load }
+}
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
index 2b42ab5b..1de1ab7e 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
@@ -60,8 +60,24 @@ describe('buildManagedDatastreamBody', () => {
   it('applies overrides last', () => {
     const body = buildManagedDatastreamBody(makeSource(), 'pl-qc', {
       name: 'QC Temperature',
+      description: 'Cleaned temperature',
+      status: 'complete',
+      sensorId: 'sensor-2',
     })
     expect(body.name).toBe('QC Temperature')
+    expect(body.description).toBe('Cleaned temperature')
+    expect(body.status).toBe('complete')
+    expect(body.sensorId).toBe('sensor-2')
+  })
+
+  // The API takes no status rather than an empty one.
+  it('drops the status when the override clears it', () => {
+    const body = buildManagedDatastreamBody(
+      makeSource({ status: 'ongoing' } as Partial<Datastream>),
+      'pl-qc',
+      { status: undefined }
+    )
+    expect(body.status).toBeUndefined()
   })
 
   it('extracts flat ids from an expand_related (nested) source', () => {

From 653e1ebf3ee24b9ab34e281dc9d9534b92ab7a76 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:01:00 -0600
Subject: [PATCH 60/93] feat: keep the edit session open in the Select view

---
 apps/qc-app/docs/API_REFERENCE.md             |  41 +++-
 apps/qc-app/docs/ARCHITECTURE.md              |  37 ++-
 apps/qc-app/docs/PLOTTING.md                  |  25 +-
 apps/qc-app/docs/TESTING.md                   |   4 +-
 apps/qc-app/docs/USER_GUIDE.md                |  41 +++-
 apps/qc-app/e2e/navigation.spec.ts            |  37 ++-
 apps/qc-app/e2e/select-while-editing.spec.ts  | 227 ++++++++++++++++++
 .../components/EditData/EditTargetPanel.vue   |  98 ++++++++
 .../components/EditData/StartEditingFlow.vue  |  14 +-
 .../__tests__/EditTargetPanel.spec.ts         |  92 +++++++
 .../__tests__/StartEditingFlow.spec.ts        |   8 +-
 .../components/Navigation/NavigationRail.vue  |  13 +-
 .../__tests__/NavigationRail.spec.ts          |  74 +++++-
 apps/qc-app/src/components/VisualizeData.vue  |  92 +++++--
 .../VisualizeData/DataVisualization.vue       |  10 +-
 .../src/components/VisualizeData/Plot.vue     |  51 ++--
 .../VisualizeData/__tests__/Plot.spec.ts      |   9 +-
 .../__tests__/useEditEntry.spec.ts            |  71 ++++++
 .../__tests__/useFilterDispatch.spec.ts       |   2 +-
 .../__tests__/useResumeEditSession.spec.ts    |  16 ++
 apps/qc-app/src/composables/useEditEntry.ts   |  40 ++-
 .../qc-app/src/composables/useLeaveSession.ts |  17 ++
 .../src/composables/useResumeEditSession.ts   |   7 +-
 .../src/store/__tests__/userInterface.spec.ts |  53 +++-
 apps/qc-app/src/store/plotly.ts               |   9 -
 apps/qc-app/src/store/userInterface.ts        |  45 +++-
 apps/qc-app/src/utils/__tests__/share.spec.ts |  13 +-
 .../utils/plotting/__tests__/events.spec.ts   | 100 +++++++-
 .../utils/plotting/__tests__/options.spec.ts  |  13 +-
 apps/qc-app/src/utils/plotting/events.ts      |  10 +-
 apps/qc-app/src/utils/plotting/options.ts     |   6 +-
 apps/qc-app/src/utils/share.ts                |   9 +-
 32 files changed, 1122 insertions(+), 162 deletions(-)
 create mode 100644 apps/qc-app/e2e/select-while-editing.spec.ts
 create mode 100644 apps/qc-app/src/components/EditData/EditTargetPanel.vue
 create mode 100644 apps/qc-app/src/components/EditData/__tests__/EditTargetPanel.spec.ts
 create mode 100644 apps/qc-app/src/composables/useLeaveSession.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 7fa9c275..0b61f626 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -115,36 +115,57 @@ await clearSelected({ recordHistory: false })  // skip history append on cleanup
 ### `useEditEntry()`
 
 ```ts
-const { enterEdit, startSessionOver, leaveEdit } = useEditEntry()
+const { enterEdit, startSessionOver, openEditor, leaveEdit } = useEditEntry()
 
 const result = await enterEdit(managedId, window)
-// result: 'editing' | 'needs-window' | 'not-managed' | 'superseded'
+// result: 'editing' | 'needs-window' | 'not-managed' | 'superseded' | 'kept'
 ```
 
 Sets the edit target and switches to the Edit view. Shared by the row Edit
 flow (`StartEditingFlow.vue`), reload resume (`useResumeEditSession()`
 below), and share-link hydration (the `ed` query param).
 
-- `enterEdit(managedId, window?)`: sets the edit target
+- `enterEdit(managedId, window?, view?)`: sets the edit target
   (`setEditTarget`), resolves its QC history via `beginEditing()`, and
   resumes an in-progress session. With no session and no `window`, returns
   `'needs-window'` so the caller opens `SessionWindowDialog.vue` (the
   session-window step) and calls back in with the chosen window, or calls
   `startSessionOver` instead; with a `window`, starts the session
-  immediately.
+  immediately. `view` is the layout to land on, `Edit` by default; a share
+  link made from the Select view passes `Select`, so the session reopens
+  behind it. Called on the target already open, with no `window`, it only
+  shows that view: nothing about the session is re-entered. Taking over from
+  another open session first asks `useLeaveSession().canLeaveSession()`, and
+  returns `'kept'` when it says no.
 - `startSessionOver(window)`: starts a new session on the current edit
   target over `window` (a `utils/timeRangePresets.ts` `TimeWindow`); backs
   **Start new session** and the editor footer's **New session**.
+- `openEditor()`: shows the Edit layout for the current target. Pure
+  navigation, used by the nav rail's Edit button and the Select view's
+  **Back to editor**.
 - `leaveEdit()`: returns to the Select view, clears the resume pointer and
   the edit target (`clearEditTarget`). Every exit uses it, including the nav
   rail's Home and Workspaces buttons, so a reload never reopens an editor the
-  user left.
+  user left. Switching views is not an exit and does not use it.
 
 Entries can overlap (a reload resume and a click land close together);
 whichever call sets the edit target last owns the view: a call that finds
 the target changed after an `await` stands down without touching view
 state, the target, or the resume pointer.
 
+### `useLeaveSession()`
+
+```ts
+const { canLeaveSession } = useLeaveSession()
+if (!(await canLeaveSession())) return
+```
+
+The one decision point for abandoning an open edit session: taking over with
+another edit target, and every other exit that ends it. Switching between the
+Select and Edit views is not an exit, so it never asks. Leaving is always
+allowed today; the prompts that offer to save, discard or keep the session
+belong here.
+
 ### `useResumeEditSession()`
 
 ```ts
@@ -159,8 +180,11 @@ Reopens the editor after a page reload, using the persisted
 datastream id (normally wired to `StartEditingFlow`'s exposed `resume`, which
 enters through `useEditEntry()` and opens the session-window step when there
 is no session to continue). Resumes at
-most once. A pointer to a datastream missing from the catalog (deleted, or
-another workspace) is dropped rather than retried.
+most once, and the chance closes as soon as the catalog lands: a pointer set
+after that belongs to an entry already navigating on its own, and a later
+catalog refresh (a commit rewrites the managed datastream) must not re-enter
+behind the user. A pointer to a datastream missing from the catalog (deleted,
+or another workspace) is dropped rather than retried.
 
 Note the watcher must not use Vue's `once` together with `immediate`: the
 immediate call fires on the initial empty catalog and stops the watcher, so
@@ -463,7 +487,6 @@ handles, live chart caches).
 | `tableScrollRequest`       | state    | `{ time: number; seq: number } \| null`           | Set when the plot zooms to the session window; `DataTable` scrolls to the first row at/after `time`. `seq` re-triggers on repeats. |
 | `requestTableScroll`       | action   | `(time: number) => void`                          | Publish a `tableScrollRequest` for the given epoch-ms range start (bumps `seq`). |
 | `axisChips`                | state    | `AxisChip[]`                                      | Horizontal axis title chips (replaces Plotly's rotated titles). |
-| `previewMode`              | state    | `boolean`                                         | Strips select/lasso/etc when the chart is rendered in the Select view's preview slot. |
 | `zoomUndoStack`            | state    | `ZoomState[]`                                     | Captured viewports for the modebar's Undo zoom button. |
 | `zoomRedoStack`            | state    | `ZoomState[]`                                     | Cleared on every new user-initiated zoom. |
 | `suppressZoomHistory`      | state    | `boolean`                                         | Flipped on during programmatic restores so the recorder doesn't double-capture. |
@@ -530,6 +553,7 @@ defaults are reseeded from the datastream on each mount.
 | `selectedDrawer`                  | state  | `DrawerType`                                      | `Edit`, `Select`, or `None`: which left drawer is active. |
 | `isDrawerOpen`                    | state  | `boolean`                                         | Drawer open/collapsed. |
 | `currentView`                     | state  | `'Edit' \| 'Select'`                              | Current main view (drives the nav rail's active state). |
+| `isPlotPreview`                   | computed | `boolean`                                       | True only in the Select view with nothing being edited; an open edit target keeps the full plot chrome in both views. |
 | `selectedOperation`               | state  | `string \| null`                                  | Open operation panel id; `null` when nothing is open. |
 | `cardHeight` / `tableHeight`      | state  | `number`                                          | Select-view top/bottom split. |
 | `operators`                       | state  | `string[]`                                        | `Object.keys(Operator)`: Change-values operator choices. |
@@ -549,6 +573,7 @@ defaults are reseeded from the datastream on each mount.
 | `filterRangeActive`               | state  | `boolean`                                         | Toggles the shared filter-window UX; the only persisted field. |
 | `filterRangeFromTs` / `filterRangeToTs` | state | `number \| null`                            | Filter-window epoch bounds; reseed on each panel mount. |
 | `onRailItemClicked`               | action | `(title: DrawerType) => void`                     | Nav-rail click handler: toggles open/closed on repeat, switches view on first click. |
+| `showView`                        | action | `(view: View) => void`                            | Switch layouts (view + drawer). Layout only: an open edit session is untouched. |
 
 ### `useQualifierStore()` (`src/store/qualifiers.ts`)
 
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 63fcb152..d5316a68 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -130,7 +130,7 @@ goes one direction.
 | `dataVisualization.ts`| Selected datastream, plotted streams, QC datastream, selectedData.    |
 | `plotly.ts`           | Plot ref, edit history, redraw, `suppressedEchoSelection` sentinel.    |
 | `qualifiers.ts`       | Result qualifier codes per workspace.                                 |
-| `userInterface.ts`    | Drawer state (Select/Edit), persisted prefs, current view.            |
+| `userInterface.ts`    | Drawer state (Select/Edit), persisted prefs, current view, `isPlotPreview`. |
 | `operationParams.ts`  | Per-operation form inputs, persisted so they survive panel re-opens.  |
 | `uiLayout.ts`         | Drawer widths, table heights: persisted UI geometry.                  |
 | `workingCopies.ts`    | Working copy per managed datastream, keyed by its in-progress session.|
@@ -187,6 +187,33 @@ Invariants:
   `selectedSeries.data`, the edit target's series; without an edit target
   `selectedSeries` is undefined.
 
+## Views and the shared plot
+
+`VisualizeData.vue` renders two layouts, Select and Edit, and `userInterface`
+holds which one is showing. A view is chrome only: `dataVisualization` owns the
+edit target (`qcDatastreamId`), so switching views never touches the session,
+the working copy or unsaved edits. Only `useEditEntry.leaveEdit` ends a
+session, and `useLeaveSession` is the one place that decides whether an open
+session may be left behind.
+
+Consequences of that split:
+
+- The Edit layout stays mounted whenever an edit target is set, hidden with
+  `v-show` while the Select layout shows. The open operation panel, its staged
+  range and every panel's own state are component state, and a round trip
+  through Select must not reset them.
+- Both layouts share **one** `DataVisualization` instance, teleported into the
+  active layout's plot host (`#qc-plot-host-select` / `#qc-plot-host-edit`).
+  Moving the Plotly graph div keeps its zoom, its live `layout.shapes` and its
+  WebGL traces; rebuilding it would drop all three. The teleport renders after
+  this component mounts, since the target is resolved with
+  `document.querySelector` and the hosts are still detached before then.
+- `isPlotPreview` (Select view, nothing being edited) drives the plot's
+  preview chrome. An edit target keeps the full plot in both views, so the
+  Select view shows the same series, the session band and the Context control.
+  The flag only flips together with a rebuild (`setEditTarget` /
+  `clearEditTarget`), so no extra redraw is needed for it.
+
 ## Plotly integration
 
 `src/utils/plotting/` is split by concern so each file has isolated mocks
@@ -305,10 +332,10 @@ Two contract notes worth keeping in mind:
   the editor closes through `exitToSelect` (Close, Save and close, or close
   without saving), on commit, when its session or managed datastream is
   deleted, and when a managed datastream other than the edit target stops being
-  plotted; `resetState` clears every copy on a workspace reset. A nav rail
-  switch to Select keeps the copy, which then equals the saved state because
-  the exit guard saves or discards first. The next preview or resume rebuilds
-  it from what was actually saved.
+  plotted; `resetState` clears every copy on a workspace reset. Switching to
+  the Select view keeps the copy untouched, unsaved edits and all: it does not
+  end the session. The next preview or resume rebuilds it from what was
+  actually saved.
 - **Editing never starts over saved draft operations without replaying
   them.** The save reconciles operations by position, so a base without the
   replayed draft would delete the server's operations on the next save.
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index b52a7581..6ef1e04d 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -221,7 +221,9 @@ calls it instead of assigning `selectedSeries.value.data` directly.
 The editor toolbar's **Context** menu reuses `DataVisTimeFilters` (with
 `EDITOR_PRESETS`, which leaves out YTD and highlights All for a persisted
 YTD through `shownPresetId`, and the same From / To pickers as
-the Select view) bound to the same `beginDate` / `endDate` store range.
+the Select view) bound to the same `beginDate` / `endDate` store range. The
+Select view's own Time range drives that same state, so while an edit target
+is set the two are one control.
 Picking a preset or a custom date calls `setDateRange`, which, while an
 edit target is set, reloads only the context series
 (`refreshGraphSeriesArray`, which never fetches the edit target) and
@@ -271,10 +273,10 @@ previous target's window is never drawn over the new one.
 `utils/plotting/shapes.ts`: `redraw` and `cropXaxisRange` use
 `withLiveShapes` to swap in the fresh `edit-window` and keep the live
 `stage` band, and the staging flush uses `composeShapes` to do the reverse
-(stage stays first so drag events still address `shapes[0]`). The
-exception is `handleNewPlot` (`events.ts`): a full rebuild calls
-`Plotly.newPlot` with the whole layout from `createPlotlyOption`, so its
-shapes are only `edit-window` and a live `stage` band is not carried over.
+(stage stays first so drag events still address `shapes[0]`).
+`handleNewPlot` (`events.ts`) does the same on a re-plot of the live element,
+so an operation's staged band survives a rebuild; a first mount has no live
+layout to carry anything from.
 
 Plot rebuilds (`rebuildPlot`, run when plotted datastreams change) keep
 the user's zoom while an edit target is set and drop it in the Select
@@ -282,6 +284,19 @@ view. `setEditTarget` clears the zoom history when the target changes, so
 **Undo zoom** never steps back to the Select view's or a previous target's
 viewports.
 
+One `Plot` serves both views. `VisualizeData.vue` teleports a single
+`DataVisualization` into whichever layout is showing, so a Select / Edit round
+trip moves the Plotly graph div instead of rebuilding it: the zoom, the live
+`layout.shapes` and the WebGL traces all stay. `userInterface.isPlotPreview`
+(Select view, nothing being edited) drives the preview chrome; with an edit
+target the full plot shows in both views, and the flag only flips alongside a
+rebuild (`setEditTarget` / `clearEditTarget`).
+
+A share link's zoom (`pendingShareZoom`) is an explicit viewport, so it beats
+the editor's default "open on the session window" once: the first session
+window that arrives afterwards does not re-zoom, later ones (viewing another
+session) do.
+
 While observations load, `DataVisualization.vue` keeps `Plot` mounted and
 passes its loading overlay through Plot's `body-overlay` slot. The overlay
 covers the plot and table body only; the toolbar stays usable and shows its
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index bf71ce01..a6a23bff 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -12,8 +12,8 @@ the operator's manual; QUALITY.md is the policy.
 
 | Layer      | Runner                          | Where                        | Count    |
 | ---------- | ------------------------------- | ---------------------------- | -------- |
-| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 59 files |
-| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 30 files |
+| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 72 files |
+| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 33 files |
 
 There is no separate "integration" tier; component tests live in the
 unit tier and mount real Vue components with the Vue Test Utils
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 1f212a73..b0753594 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -63,13 +63,15 @@ A thin, always-visible column of icons.
 | Icon | Action |
 |------|--------|
 | HydroServer logo | Top left. Go home. Resets the current view. Prompts before discarding unsaved edits. |
-| Cursor (Select) | Top left. Show the datastream Select drawer + plot. |
+| Cursor (Select) | Top left. Show the datastream Select drawer + plot. While you are editing, this keeps the session open behind it. |
 | Pencil (Edit) | Top left. Return to the Edit view. **Disabled** until you're editing: pick a datastream to edit with its row's pencil button first. |
 | Stopwatch (Performance) | Bottom left. Open the Performance Calibration dialog. See "Performance" below. |
 | Briefcase (Workspaces) | Bottom left. Switch workspace, or Continue in the current one. |
 | Logout | Bottom left. Sign out. |
 
-If you click any of these while the Edit view has edits not yet saved to the session, the app shows an "Unsaved edits" dialog with **Save & continue** / **Discard** / **Cancel**. **Save & continue** saves a draft to the session before leaving; it is unavailable when no session is open. **Discard** drops the edits made since the last save, and they cannot be recovered.
+Switching between **Select** and **Edit** never ends an edit session, so neither asks anything: the datastream you are editing, its session, and any unsaved edits stay exactly as they were.
+
+Home, Workspaces and Logout do leave the session. If you click one of them while there are edits not yet saved to the session, the app shows an "Unsaved edits" dialog with **Save & continue** / **Discard** / **Cancel**. **Save & continue** saves a draft to the session before leaving; it is unavailable when no session is open. **Discard** drops the edits made since the last save, and they cannot be recovered.
 
 ### Select view
 
@@ -85,13 +87,40 @@ The filter drawer has two collapsible sections:
 
 The main area to the right of the window is split top/bottom:
 
-- **Top card** carries the preview plot and a header showing how many datastreams are plotted ("N datastreams plotted" or "No datastream plotted"). The right pane of the card lists currently plotted datastreams.
+- **Top card** carries the preview plot and a header showing how many datastreams are plotted ("N datastreams plotted" or "No datastream plotted"). The right pane of the card lists currently plotted datastreams, and, while you are editing, an **Editing** panel above it.
 - **Bottom card** is the **Datastreams selection table**: listing every datastream the filters match. Each row has a **Plot** toggle (check box) and an **Edit** column (pencil button). See [Datastreams with quality-controlled versions](#datastreams-with-quality-controlled-versions) below for what the Edit button does.
 
 ![Select view with one datastream plotted](./images/home-plotted.png)
 
 Clicking a datastream row shows the full metadata for a datastream. Clicking the plot toggle (check box) in the row plots the datastream as read-only context: plotting never picks what you edit, and the first plotted datastream has no special role.
 
+### The Select view while you are editing
+
+Clicking **Select** in the rail during an edit session does not leave it. The
+session, its working copy and every unsaved edit stay open; the Select view is
+just a different way to look at the same thing, so you can change what is
+plotted *around* the data you are editing.
+
+What changes:
+
+- An **Editing** panel appears in the top card's right pane: the managed
+  datastream's name, the session's time window, whether anything is unsaved,
+  and a **Back to editor** button. Editing operations stay in the editor.
+- The preview plot draws the same series the editor does, the edit target and
+  its raw source included, with the session window shaded.
+- The row check boxes add and remove **context** datastreams. The edit target
+  and its source stay on the plot and cannot be removed here.
+- The filter drawer's **Time range** is the editor's Context range: the same
+  window the Context menu sets, with presets counting out from the session
+  window. Your edits are never reloaded by it.
+- The rail's **Edit** button and the panel's **Back to editor** button both
+  return to the editor with the session, your zoom and any staged range intact.
+- The row **Edit** button on the datastream you are already editing returns to
+  the editor too. On a different datastream it starts the usual entry flow.
+
+A link copied from here carries the edit target, so opening it reopens the
+session behind the Select view.
+
 ### Datastreams with quality-controlled versions
 
 A row with a number badge next to its check box has that many **managed
@@ -258,7 +287,7 @@ The URL encodes everything needed to reproduce what the sender is looking at - q
 - **View** (Select vs Edit; `m=e` for Edit)
 - **Active center-column tab** (`tab=t` for Table)
 - **Plotted datastreams** (`ds`), in order.
-- **Edit target** (`ed`), the datastream you're editing, present only in the Edit view.
+- **Edit target** (`ed`), the datastream you're editing. It travels from either view, since the Select view keeps the session open.
 - **Time window**: either a preset id (`r=0..5`) which the recipient resolves against the plotted data, or an explicit `from` / `to` pair as base36 second-epochs when the sender used a custom range.
 - **Per-trace eye-toggle visibility** (`h`) and **per-axis visibility** (`ya`) as hex bitmasks over the `ds` order.
 - **Plot zoom**: X zoom (`z`) plus optional per-Y-axis zoom (`yz`) for axes that aren't at their default fit.
@@ -551,7 +580,8 @@ Things worth knowing:
   snapshots of any other session replay what was saved to the server.
 - Snapshots travel in the share link, so a link reproduces the comparison.
   Each one replays on load, so a link carrying several is slower to open.
-- Leaving the editor for the Select view drops every snapshot.
+- Closing the session drops every snapshot. Switching to the Select view does
+  not, since the session stays open.
 
 ## Deleting a session
 
@@ -675,6 +705,7 @@ Click **Workspaces** in the nav rail and **Select** another, or **Continue** to
 | Blank page on load | Wrong API URL or `localhost` vs `127.0.0.1` mismatch. | See [DEPLOYMENT.md](./DEPLOYMENT.md). |
 | Plot stays empty after picking a datastream | A `Custom` time range that doesn't overlap the datastream's observations, or the datastream has no observations. The plotted row shows a database-off icon and the subtitle reads `0 pts loaded`. | Click a preset such as **1m** or **All** in Time range. |
 | The Edit rail item is greyed out | You are not editing. | Click a row's pencil button. |
+| The Select view shows an "Editing" panel | A session is still open. | That is expected: click **Back to editor** to return to it, or **Close** in the editor to end it. |
 | Big edits freeze the page | `SharedArrayBuffer` not available; running inline. | Have your admin re-enable COOP/COEP headers, or accept the slower fallback. |
 | Save fails with a backend error | Permissions / workspace issue / network. | The Snackbar shows the backend message verbatim. Share that with your admin. |
 | The history shows a red failed entry after Load | The QC history referenced something missing in this datastream. | The rest of the QC history still ran. Click the chevron on the row to see its arguments. |
diff --git a/apps/qc-app/e2e/navigation.spec.ts b/apps/qc-app/e2e/navigation.spec.ts
index d22010b6..45efdb29 100644
--- a/apps/qc-app/e2e/navigation.spec.ts
+++ b/apps/qc-app/e2e/navigation.spec.ts
@@ -3,8 +3,9 @@
  *   - fresh browser redirects to /workspaces
  *   - picking a workspace navigates to Home
  *   - the nav-rail workspace-switch button offers to revisit the picker
- *   - the edit rail item is enabled only while editing
- *   - leaving Edit with edits not saved to the session asks first
+ *   - the edit rail item is enabled only while an edit target is set
+ *   - leaving the workspace with edits not saved to the session asks first
+ *   - switching to the Select view keeps the session and asks nothing
  */
 
 import { expect, test, type Page } from '@playwright/test'
@@ -31,6 +32,12 @@ function exitDialog(page: Page) {
   return page.getByRole('dialog').filter({ hasText: 'Unsaved edits' })
 }
 
+function expectWorkspacePicker(page: Page) {
+  return expect(page.getByTestId(`workspace-pick-${WORKSPACE_ID}`)).toBeVisible({
+    timeout: 30_000,
+  })
+}
+
 test.describe('navigation', () => {
   test.beforeEach(async ({ page }) => {
     await installMocks(page, { qcHistories: true })
@@ -117,14 +124,12 @@ test.describe('navigation: leaving a QC session', () => {
   }) => {
     await applyChangeValues(page)
 
-    await page.getByTestId('nav-rail-item-select').click()
+    await page.getByTestId('nav-rail-workspaces').click()
     const dialog = exitDialog(page)
     await expect(dialog).toBeVisible()
     await dialog.getByRole('button', { name: /save & continue/i }).click()
 
-    await expect(page.getByTestId('datastreams-table')).toBeVisible({
-      timeout: 30_000,
-    })
+    await expectWorkspacePicker(page)
     const session = sessions.find((s) => s.status === 'in_progress')
     expect(session!.operations.map((o) => o.operationType)).toContain(
       'CHANGE_VALUES'
@@ -135,14 +140,12 @@ test.describe('navigation: leaving a QC session', () => {
   test('Discard continues without saving', async ({ page }) => {
     await applyChangeValues(page)
 
-    await page.getByTestId('nav-rail-item-select').click()
+    await page.getByTestId('nav-rail-workspaces').click()
     const dialog = exitDialog(page)
     await expect(dialog).toBeVisible()
     await dialog.getByRole('button', { name: /discard/i }).click()
 
-    await expect(page.getByTestId('datastreams-table')).toBeVisible({
-      timeout: 30_000,
-    })
+    await expectWorkspacePicker(page)
     const session = sessions.find((s) => s.status === 'in_progress')
     expect(session!.operations).toHaveLength(0)
     expect(submissions).toHaveLength(0)
@@ -153,10 +156,24 @@ test.describe('navigation: leaving a QC session', () => {
     await page.getByTestId('exit-save-btn').click()
     await expect(page.getByText('Draft saved.')).toBeVisible()
 
+    await page.getByTestId('nav-rail-workspaces').click()
+    await expectWorkspacePicker(page)
+    await expect(exitDialog(page)).toHaveCount(0)
+  })
+
+  // Switching views is not an exit, so it never asks.
+  test('the Select view keeps the session with unsaved edits', async ({
+    page,
+  }) => {
+    await applyChangeValues(page)
+
     await page.getByTestId('nav-rail-item-select').click()
     await expect(page.getByTestId('datastreams-table')).toBeVisible({
       timeout: 30_000,
     })
     await expect(exitDialog(page)).toHaveCount(0)
+    await expect(page.getByTestId('edit-target-panel')).toBeVisible()
+    const session = sessions.find((s) => s.status === 'in_progress')
+    expect(session!.operations).toHaveLength(0)
   })
 })
diff --git a/apps/qc-app/e2e/select-while-editing.spec.ts b/apps/qc-app/e2e/select-while-editing.spec.ts
new file mode 100644
index 00000000..c3613a02
--- /dev/null
+++ b/apps/qc-app/e2e/select-while-editing.spec.ts
@@ -0,0 +1,227 @@
+/**
+ * The Select view while an edit session is open:
+ *   - the Select rail item keeps the session, the working copy and unsaved
+ *     edits, and asks nothing
+ *   - the edit panel says what is being edited and leads back
+ *   - the round trip keeps the plot's zoom and the editor's staged range
+ *   - checkboxes add context around the edit target, which stays plotted
+ *   - the share URL keeps `ed` without `m`, and a reload restores Select
+ *   - the row Edit button on the datastream being edited returns to the editor
+ */
+
+import { expect, test, type Page } from '@playwright/test'
+import { installMocks } from './support/mocks'
+import {
+  gotoHome,
+  openOp,
+  setupEditView,
+  startSessionFromRow,
+  waitForEditorReady,
+} from './support/app'
+import { expectHistoryContains, selectAllPoints } from './support/ops'
+import {
+  DATASTREAM_ID,
+  DATASTREAM_ID_B,
+  MANAGED_DATASTREAM_ID,
+  buildObservations,
+  buildTemperatureObservations,
+} from './support/fixtures'
+
+const editPanel = (page: Page) => page.getByTestId('edit-target-panel')
+/** The Select view's right column. The editor stays mounted behind it, so its
+ *  own plotted list carries the same test ids. */
+const sidePanel = (page: Page) => page.getByTestId('select-side-panel')
+
+async function goToSelect(page: Page) {
+  await page.getByTestId('nav-rail-item-select').click()
+  await expect(page.getByTestId('datastreams-table')).toBeVisible({
+    timeout: 30_000,
+  })
+}
+
+async function goToEditor(page: Page) {
+  await page.getByTestId('nav-rail-item-edit').click()
+  await waitForEditorReady(page)
+}
+
+/** The live X range of the main plot, as `[loMs, hiMs]`. */
+async function plotXRange(page: Page): Promise<[number, number]> {
+  return page.evaluate(async () => {
+    const wait = (ms: number) => new Promise((r) => setTimeout(r, ms))
+    const start = Date.now()
+    while (Date.now() - start < 10_000) {
+      const gd = document.querySelector('[data-testid="main-plot"]') as
+        | (HTMLElement & {
+            _fullLayout?: {
+              xaxis?: { range?: [number | string, number | string] }
+            }
+          })
+        | null
+      const range = gd?._fullLayout?.xaxis?.range
+      if (range) {
+        const toMs = (v: number | string) =>
+          typeof v === 'string' ? Date.parse(v) : v
+        return [toMs(range[0]), toMs(range[1])] as [number, number]
+      }
+      await wait(100)
+    }
+    throw new Error('plot never reported an x range')
+  })
+}
+
+/** The staged filter band's x span on the live plot, or null when absent. */
+async function stageBandSpan(page: Page): Promise<string | null> {
+  return page.evaluate(() => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as
+      | (HTMLElement & {
+          layout?: { shapes?: { name?: string; x0?: unknown; x1?: unknown }[] }
+        })
+      | null
+    const shape = (gd?.layout?.shapes ?? []).find((s) => s.name === 'stage')
+    return shape ? `${String(shape.x0)}..${String(shape.x1)}` : null
+  })
+}
+
+/** Ids of the traces currently drawn on the main plot. */
+async function plottedTraceIds(page: Page): Promise<string[]> {
+  return page.evaluate(() => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as
+      | (HTMLElement & { data?: { id?: string }[] })
+      | null
+    return (gd?.data ?? [])
+      .map((t) => t.id)
+      .filter((id): id is string => !!id)
+  })
+}
+
+test.describe('Select while editing', () => {
+  test.beforeEach(async ({ page }) => {
+    test.slow()
+    await installMocks(page, {
+      qcHistories: true,
+      observationsById: {
+        [DATASTREAM_ID]: buildObservations(),
+        [DATASTREAM_ID_B]: buildTemperatureObservations(),
+      },
+    })
+  })
+
+  test('keeps unsaved edits and leads back to the editor', async ({ page }) => {
+    await setupEditView(page)
+    await selectAllPoints(page)
+    await openOp(page, 'changeValues')
+    await page.getByLabel('Value').fill('1')
+    await page.getByRole('button', { name: 'Apply' }).click()
+    await expectHistoryContains(page, 'Change Values')
+
+    await goToSelect(page)
+
+    // No prompt: nothing is being left behind.
+    await expect(
+      page.getByRole('dialog').filter({ hasText: 'Unsaved edits' })
+    ).toHaveCount(0)
+    await expect(editPanel(page)).toBeVisible()
+    await expect(editPanel(page)).toContainText('Streamflow Datastream (QC)')
+    await expect(page.getByTestId('edit-target-unsaved')).toContainText(
+      'unsaved edit'
+    )
+
+    // The edit target is still drawn, so the user sees what they changed.
+    expect(await plottedTraceIds(page)).toContain(MANAGED_DATASTREAM_ID)
+
+    await page.getByTestId('back-to-editor-btn').click()
+    await waitForEditorReady(page)
+    await expectHistoryContains(page, 'Change Values')
+  })
+
+  test('keeps the plot zoom and a staged range across the round trip', async ({
+    page,
+  }) => {
+    await setupEditView(page)
+    // Staging a filter window draws an editable band over the plot.
+    await openOp(page, 'persistence')
+    await page.getByTestId('filter-range-enable-btn').click()
+    await expect(page.getByTestId('filter-range-panel')).toBeVisible()
+    const fromField = page
+      .getByTestId('filter-range-panel')
+      .getByPlaceholder('MM/DD/YYYY')
+      .first()
+    const stagedFrom = await fromField.inputValue()
+    const stagedBand = await stageBandSpan(page)
+    expect(stagedBand).not.toBeNull()
+
+    const before = await plotXRange(page)
+
+    await goToSelect(page)
+    await goToEditor(page)
+
+    const after = await plotXRange(page)
+    expect(after[0]).toBeCloseTo(before[0], -4)
+    expect(after[1]).toBeCloseTo(before[1], -4)
+
+    expect(await stageBandSpan(page)).toBe(stagedBand)
+    await expect(page.getByTestId('filter-range-panel')).toBeVisible()
+    await expect(fromField).toHaveValue(stagedFrom)
+  })
+
+  test('plots context around the edit target from the table', async ({
+    page,
+  }) => {
+    await setupEditView(page)
+    await goToSelect(page)
+
+    await page.getByTestId(`plot-checkbox-${DATASTREAM_ID_B}`).click()
+    await page
+      .getByTestId('data-loading-indicator')
+      .waitFor({ state: 'hidden', timeout: 30_000 })
+
+    const ids = await plottedTraceIds(page)
+    expect(ids).toContain(DATASTREAM_ID_B)
+    expect(ids).toContain(MANAGED_DATASTREAM_ID)
+
+    // The edit target and its source are pinned in the plotted list.
+    await expect(
+      sidePanel(page).getByTestId(`plotted-item-${MANAGED_DATASTREAM_ID}`)
+    ).toBeVisible()
+    await expect(
+      sidePanel(page).getByTestId(`plotted-item-${DATASTREAM_ID}`)
+    ).toBeVisible()
+  })
+
+  test('keeps ed in the share URL and restores Select on reload', async ({
+    page,
+  }) => {
+    await setupEditView(page)
+    await goToSelect(page)
+
+    const params = new URLSearchParams(
+      await page.evaluate(() => window.location.search)
+    )
+    expect(params.get('ed')).toBe(MANAGED_DATASTREAM_ID)
+    expect(params.has('m')).toBe(false)
+
+    await page.reload()
+    await expect(page.getByTestId('datastreams-table')).toBeVisible({
+      timeout: 30_000,
+    })
+    await expect(editPanel(page)).toBeVisible({ timeout: 30_000 })
+    await expect(page.getByTestId('nav-rail-item-edit')).toHaveAttribute(
+      'aria-disabled',
+      'false'
+    )
+  })
+
+  test('the row Edit button returns to the session being edited', async ({
+    page,
+  }) => {
+    await gotoHome(page)
+    await startSessionFromRow(page)
+    await goToSelect(page)
+
+    await page.getByTestId(`edit-datastream-${DATASTREAM_ID}`).click()
+    // The session id is generated, so match the Continue button by prefix.
+    await page.locator('[data-testid^="continue-session-"]').click()
+    await waitForEditorReady(page)
+    await expect(page.getByText('Filter Data')).toBeVisible()
+  })
+})
diff --git a/apps/qc-app/src/components/EditData/EditTargetPanel.vue b/apps/qc-app/src/components/EditData/EditTargetPanel.vue
new file mode 100644
index 00000000..610caa9a
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/EditTargetPanel.vue
@@ -0,0 +1,98 @@
+<template>
+  <div
+    v-if="qcDatastream"
+    data-testid="edit-target-panel"
+    class="edit-target-panel"
+  >
+    <div class="edit-target-panel__header d-flex align-center ga-1 px-3 py-1">
+      <v-icon icon="mdi-pencil" color="primary" size="16" />
+      <span class="text-body-small font-weight-medium">Editing</span>
+    </div>
+
+    <v-divider />
+
+    <div class="d-flex flex-column ga-2 pa-3">
+      <div style="min-width: 0">
+        <div
+          class="text-body-medium font-weight-medium"
+          :title="qcDatastream.name"
+        >
+          {{ qcDatastream.name }}
+        </div>
+        <div class="text-body-small text-medium-emphasis">
+          Session {{ sessionWindow }}
+        </div>
+      </div>
+
+      <div class="d-flex align-center ga-2">
+        <v-icon
+          :icon="unsavedEditCount ? 'mdi-content-save-alert-outline' : 'mdi-check-circle-outline'"
+          :color="unsavedEditCount ? 'warning' : 'success'"
+          size="16"
+        />
+        <span class="text-body-small" data-testid="edit-target-unsaved">
+          <template v-if="unsavedEditCount">
+            {{ unsavedEditCount }} unsaved
+            edit{{ unsavedEditCount === 1 ? '' : 's' }}
+          </template>
+          <template v-else>All edits saved</template>
+        </span>
+      </div>
+
+      <div class="text-body-small text-medium-emphasis">
+        Plot other datastreams around this session, then go back to edit it.
+      </div>
+
+      <v-btn
+        data-testid="back-to-editor-btn"
+        size="small"
+        variant="flat"
+        color="primary"
+        prepend-icon="mdi-arrow-left"
+        block
+        @click="openEditor"
+      >
+        Back to editor
+      </v-btn>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+/**
+ * What the Select view says about an open edit session: the managed
+ * datastream, its session window, whether anything is unsaved, and the way
+ * back. Editing itself stays in the editor.
+ */
+
+import { computed } from 'vue'
+import { storeToRefs } from 'pinia'
+import { useDataVisStore } from '@/store/dataVisualization'
+import { useQcSessionStore } from '@/store/qcSession'
+import { useEditSession } from '@/composables/useEditSession'
+import { useEditEntry } from '@/composables/useEditEntry'
+import { formatDateRange } from '@/utils/time'
+
+const { qcDatastream } = storeToRefs(useDataVisStore())
+const { inProgressSession, viewedSession } = storeToRefs(useQcSessionStore())
+const { unsavedEditCount } = useEditSession()
+const { openEditor } = useEditEntry()
+
+// Viewing history overrides the live session, matching the plot's band.
+const sessionWindow = computed(() => {
+  const session = viewedSession.value ?? inProgressSession.value
+  return session
+    ? formatDateRange(
+        session.phenomenonTimeStart,
+        session.phenomenonTimeEnd
+      )
+    : 'not started'
+})
+</script>
+
+<style scoped>
+.edit-target-panel__header {
+  background-color: rgba(var(--v-theme-primary), 0.04);
+  min-height: 28px;
+}
+</style>
diff --git a/apps/qc-app/src/components/EditData/StartEditingFlow.vue b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
index 526abaa3..97c00bec 100644
--- a/apps/qc-app/src/components/EditData/StartEditingFlow.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
@@ -76,6 +76,7 @@ import { useProcessingLevels } from '@/composables/useProcessingLevels'
 import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
 import { collectDeletionChain } from '@/utils/sessionGraph'
 import type { TimeWindow } from '@/utils/timeRangePresets'
+import type { View } from '@/store/userInterface'
 
 // 'entering': the editor is not open yet. 'resume': it opened with no session.
 // 'footer': it is open on committed history.
@@ -194,13 +195,18 @@ function openNewSession() {
   openSessionWindow('footer')
 }
 
-async function resume(managedId: string) {
-  await runEnter(managedId)
+/** Reload / share-link entry. `view` is the layout the link was made from. */
+async function resume(managedId: string, view?: View) {
+  await runEnter(managedId, undefined, view)
 }
 
-async function runEnter(managedId: string, window?: TimeWindow) {
+async function runEnter(
+  managedId: string,
+  window?: TimeWindow,
+  view?: View
+) {
   try {
-    const result = await enterEdit(managedId, window)
+    const result = await enterEdit(managedId, window, view)
     if (result === 'needs-window' && !openSessionWindow('resume')) {
       // No source to window, so the open editor has nothing to edit.
       await leaveEdit()
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditTargetPanel.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditTargetPanel.spec.ts
new file mode 100644
index 00000000..8512d8f1
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/__tests__/EditTargetPanel.spec.ts
@@ -0,0 +1,92 @@
+import { mount } from '@vue/test-utils'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+import { createTestVuetify } from '@/utils/test/vuetify'
+
+const { qcDatastream, inProgressSession, viewedSession, unsavedEditCount, openEditor } =
+  vi.hoisted(() => {
+    const { ref: r } = require('vue') as typeof import('vue')
+    return {
+      qcDatastream: r<{ id: string; name: string } | null>(null),
+      inProgressSession: r<Record<string, string> | null>(null),
+      viewedSession: r<Record<string, string> | null>(null),
+      unsavedEditCount: r(0),
+      openEditor: vi.fn(),
+    }
+  })
+
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () => ({ qcDatastream }),
+}))
+
+vi.mock('@/store/qcSession', () => ({
+  useQcSessionStore: () => ({ inProgressSession, viewedSession }),
+}))
+
+vi.mock('@/composables/useEditSession', () => ({
+  useEditSession: () => ({ unsavedEditCount }),
+}))
+
+vi.mock('@/composables/useEditEntry', () => ({
+  useEditEntry: () => ({ openEditor }),
+}))
+
+import EditTargetPanel from '@/components/EditData/EditTargetPanel.vue'
+import { formatDateRange } from '@/utils/time'
+
+const mountPanel = () =>
+  mount(EditTargetPanel, {
+    global: { plugins: [createTestPinia(), createTestVuetify()] },
+  })
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  qcDatastream.value = { id: 'mgd-1', name: 'Managed Temperature' }
+  inProgressSession.value = {
+    phenomenonTimeStart: '2025-01-05T00:00:00Z',
+    phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+  }
+  viewedSession.value = null
+  unsavedEditCount.value = 0
+})
+
+describe('EditTargetPanel', () => {
+  it('names what is being edited and the session window', () => {
+    const text = mountPanel().text()
+    expect(text).toContain('Managed Temperature')
+    expect(text).toContain(
+      formatDateRange('2025-01-05T00:00:00Z', '2025-02-01T00:00:00Z')
+    )
+  })
+
+  it('reports unsaved edits, and says so when there are none', async () => {
+    expect(mountPanel().text()).toContain('All edits saved')
+    unsavedEditCount.value = 3
+    expect(mountPanel().text()).toContain('3 unsaved edits')
+    unsavedEditCount.value = 1
+    expect(mountPanel().text()).toContain('1 unsaved edit')
+  })
+
+  it('shows the window of the session being viewed over the live one', () => {
+    viewedSession.value = {
+      phenomenonTimeStart: '2024-03-01T00:00:00Z',
+      phenomenonTimeEnd: '2024-04-01T00:00:00Z',
+    }
+    expect(mountPanel().text()).toContain(
+      formatDateRange('2024-03-01T00:00:00Z', '2024-04-01T00:00:00Z')
+    )
+  })
+
+  it('goes back to the editor without touching the session', async () => {
+    const wrapper = mountPanel()
+    await wrapper.find('[data-testid="back-to-editor-btn"]').trigger('click')
+    expect(openEditor).toHaveBeenCalledTimes(1)
+  })
+
+  it('renders nothing without an edit target', () => {
+    qcDatastream.value = null
+    expect(mountPanel().find('[data-testid="edit-target-panel"]').exists()).toBe(
+      false
+    )
+  })
+})
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
index 63b2b2fb..90734044 100644
--- a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
@@ -244,7 +244,7 @@ describe('StartEditingFlow', () => {
     wrapper.findComponent(ChooserStub).vm.$emit('edit', option)
     await flushPromises()
     expect(enterEdit).toHaveBeenCalledTimes(1)
-    expect(enterEdit.mock.calls[0]).toEqual(['mgd', undefined])
+    expect(enterEdit.mock.calls[0]).toEqual(['mgd', undefined, undefined])
     expect(present('window-stub')).toBe(false)
   })
 
@@ -261,7 +261,7 @@ describe('StartEditingFlow', () => {
 
     wrapper.findComponent(WindowStub).vm.$emit('confirm', window)
     await flushPromises()
-    expect(enterEdit).toHaveBeenCalledWith('mgd', window)
+    expect(enterEdit).toHaveBeenCalledWith('mgd', window, undefined)
     expect(present('window-stub')).toBe(false)
   })
 
@@ -272,7 +272,7 @@ describe('StartEditingFlow', () => {
     const { flow } = mountFlow()
     await flow.resume('mgd')
     await flushPromises()
-    expect(enterEdit.mock.calls[0]).toEqual(['mgd', undefined])
+    expect(enterEdit.mock.calls[0]).toEqual(['mgd', undefined, undefined])
     expect(present('window-stub')).toBe(true)
   })
 
@@ -443,6 +443,6 @@ describe('StartEditingFlow', () => {
 
     wrapper.findComponent(WindowStub).vm.$emit('confirm', window)
     await flushPromises()
-    expect(enterEdit).toHaveBeenCalledWith('mgd-new', window)
+    expect(enterEdit).toHaveBeenCalledWith('mgd-new', window, undefined)
   })
 })
diff --git a/apps/qc-app/src/components/Navigation/NavigationRail.vue b/apps/qc-app/src/components/Navigation/NavigationRail.vue
index 30ec1947..881e4b5f 100644
--- a/apps/qc-app/src/components/Navigation/NavigationRail.vue
+++ b/apps/qc-app/src/components/Navigation/NavigationRail.vue
@@ -70,6 +70,7 @@
         <span v-if="item.title === 'Edit' && !qcDatastream">
           Edit a datastream from its row first.
         </span>
+        <span v-else-if="item.title === 'Edit'">Back to the editor</span>
         <span v-else>{{ item.title }}</span>
       </v-tooltip>
     </div>
@@ -205,8 +206,9 @@ const { hasUnsavedChanges, unsavedEditCount, saveDraft, discardUnsavedEdits } =
   useEditSession()
 const { leaveEdit } = useEditEntry()
 
+// Home, the workspace switch and log out are the exits that end the session.
 const needsExitConfirm = computed(
-  () => currentView.value === DrawerType.Edit && hasUnsavedChanges.value
+  () => !!qcDatastream.value && hasUnsavedChanges.value
 )
 // With no in-progress session there is nothing to save to.
 const canSave = computed(() => !!inProgressSession.value)
@@ -286,13 +288,12 @@ const items = ref([
   { title: 'Edit', icon: 'mdi-pencil' },
 ])
 
+// Switching views never ends the session, so neither item prompts: Select
+// keeps the editor's target, session and unsaved edits alive behind it, and
+// Edit brings them back.
 function onMainRailItemClicked(item: DrawerType) {
   if (item === DrawerType.Edit && !qcDatastream.value) return
-  if (item === DrawerType.Select && currentView.value === DrawerType.Edit) {
-    guardExit(() => leaveEdit())
-    return
-  }
-  guardExit(() => onRailItemClicked(item))
+  onRailItemClicked(item)
 }
 
 async function onLogout() {
diff --git a/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts b/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
index efee9def..0a9e8261 100644
--- a/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
+++ b/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
@@ -22,9 +22,13 @@ const {
   hasUnsavedChanges,
   push,
   assign,
+  showView,
 } = vi.hoisted(() => {
   const { ref: r, computed: c } = require('vue') as typeof import('vue')
   const qcDatastreamId = r<string | null>(null)
+  const currentView = r('Edit')
+  const selectedDrawer = r('Edit')
+  const isDrawerOpen = r(true)
   return {
     qcDatastreamId,
     qcDatastream: c(() =>
@@ -36,13 +40,18 @@ const {
     clearEditTarget: vi.fn(async () => {
       qcDatastreamId.value = null
     }),
-    currentView: r('Edit'),
-    selectedDrawer: r('Edit'),
-    isDrawerOpen: r(true),
+    currentView,
+    selectedDrawer,
+    isDrawerOpen,
     resumeDatastreamId: r<string | null>(null),
     hasUnsavedChanges: r(false),
     push: vi.fn(async () => {}),
     assign: vi.fn(),
+    showView: vi.fn((view: string) => {
+      currentView.value = view
+      selectedDrawer.value = view
+      isDrawerOpen.value = true
+    }),
   }
 })
 
@@ -67,7 +76,12 @@ vi.mock('@/store/userInterface', async () => {
       currentView,
       selectedDrawer,
       isDrawerOpen,
-      onRailItemClicked: vi.fn(),
+      showView,
+      onRailItemClicked: vi.fn((title: string) => {
+        if (selectedDrawer.value === title) {
+          isDrawerOpen.value = !isDrawerOpen.value
+        } else showView(title)
+      }),
     })),
   }
 })
@@ -155,6 +169,58 @@ function mountRail() {
   )
 }
 
+describe('NavigationRail view switching', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    qcDatastreamId.value = 'm-1'
+    resumeDatastreamId.value = 'm-1'
+    currentView.value = 'Edit'
+    selectedDrawer.value = 'Edit'
+    hasUnsavedChanges.value = true
+  })
+
+  it('Select keeps the session, even with unsaved edits', async () => {
+    const wrapper = mountRail()
+    await wrapper.find('[data-testid="nav-rail-item-select"]').trigger('click')
+    await flushPromises()
+
+    expect(currentView.value).toBe('Select')
+    expect(qcDatastreamId.value).toBe('m-1')
+    expect(resumeDatastreamId.value).toBe('m-1')
+    expect(clearEditTarget).not.toHaveBeenCalled()
+    expect(wrapper.find('.v-dialog').exists()).toBe(false)
+    wrapper.unmount()
+  })
+
+  it('Edit returns to the editor from the Select view', async () => {
+    currentView.value = 'Select'
+    selectedDrawer.value = 'Select'
+    const wrapper = mountRail()
+    const editRail = wrapper.find('[data-testid="nav-rail-item-edit"]')
+    expect(editRail.attributes('aria-disabled')).toBe('false')
+    await editRail.trigger('click')
+    await flushPromises()
+
+    expect(currentView.value).toBe('Edit')
+    expect(qcDatastreamId.value).toBe('m-1')
+    wrapper.unmount()
+  })
+
+  it('Edit stays disabled with no edit target', async () => {
+    qcDatastreamId.value = null
+    currentView.value = 'Select'
+    selectedDrawer.value = 'Select'
+    const wrapper = mountRail()
+    const editRail = wrapper.find('[data-testid="nav-rail-item-edit"]')
+    expect(editRail.attributes('aria-disabled')).toBe('true')
+    await editRail.trigger('click')
+    await flushPromises()
+
+    expect(currentView.value).toBe('Select')
+    wrapper.unmount()
+  })
+})
+
 describe('NavigationRail leaving the editor', () => {
   beforeEach(() => {
     vi.clearAllMocks()
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index 40214b7f..31e93eeb 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -1,6 +1,6 @@
 <template>
   <div
-    v-if="currentView === DrawerType.Select"
+    v-if="!isEditLayout"
     class="select-view fill-height d-flex flex-column pa-4"
   >
     <v-card class="select-view__card d-flex flex-column mb-3">
@@ -18,7 +18,10 @@
             <template v-else>No datastream plotted</template>
           </span>
           <span class="text-body-small text-medium-emphasis">
-            <template v-if="plottedDatastreams.length">
+            <template v-if="qcDatastream">
+              Choose what to plot around the session you are editing
+            </template>
+            <template v-else-if="plottedDatastreams.length">
               Use the pencil button on a row to edit a datastream
             </template>
             <template v-else>
@@ -31,15 +34,24 @@
       <v-divider />
 
       <div class="select-view__card-body d-flex flex-grow-1">
-        <div class="select-view__plot-body flex-grow-1 pa-2">
-          <DataVisualization preview />
-        </div>
+        <div
+          id="qc-plot-host-select"
+          class="select-view__plot-body flex-grow-1 pa-2"
+        ></div>
 
-        <template v-if="plottedDatastreams.length">
+        <template v-if="plottedDatastreams.length || qcDatastream">
           <v-divider vertical class="select-view__divider-vertical" />
           <v-divider class="select-view__divider-horizontal" />
-          <div class="select-view__plotted d-flex flex-column flex-grow-0 flex-shrink-0 overflow-hidden">
-            <div class="select-view__plotted-body flex-grow-1 overflow-y-auto">
+          <div
+            data-testid="select-side-panel"
+            class="select-view__side d-flex flex-column flex-grow-0 flex-shrink-0 overflow-hidden"
+          >
+            <EditTargetPanel v-if="qcDatastream" />
+            <v-divider v-if="qcDatastream && plottedDatastreams.length" />
+            <div
+              v-if="plottedDatastreams.length"
+              class="select-view__plotted-body flex-grow-1 overflow-y-auto"
+            >
               <PlottedDatastreams />
             </div>
           </div>
@@ -55,8 +67,12 @@
     </v-card>
   </div>
 
+  <!-- Kept mounted while an edit target is set, even in the Select view: the
+       open operation, its staged range and the panels' own state survive the
+       round trip because nothing here is torn down. -->
   <div
-    v-else-if="currentView === DrawerType.Edit"
+    v-if="qcDatastream"
+    v-show="isEditLayout"
     class="edit-view d-flex bg-background"
   >
     <aside
@@ -103,9 +119,11 @@
       class="edit-view__col edit-view__col--plot d-flex flex-column flex-fill pa-3 overflow-hidden"
     >
       <v-card class="fill-height d-flex flex-column" elevation="1">
-        <div class="flex-grow-1 pa-2" style="min-height: 0">
-          <DataVisualization />
-        </div>
+        <div
+          id="qc-plot-host-edit"
+          class="flex-grow-1 pa-2"
+          style="min-height: 0"
+        ></div>
       </v-card>
     </div>
 
@@ -445,6 +463,14 @@
     </v-dialog>
 </div>
 
+  <!-- One plot for both layouts. Moving it instead of rebuilding it is what
+       keeps the zoom, the session band and a staged range across a view
+       switch. The hosts only exist once this component is in the document,
+       hence the mounted gate. -->
+  <Teleport v-if="isMounted" :to="plotHost">
+    <DataVisualization />
+  </Teleport>
+
   <StartEditingFlow ref="startEditing" />
 </template>
 
@@ -455,6 +481,7 @@ import EditHistory from '@/components/EditData/EditHistory.vue'
 import OperationPanel from '@/components/EditData/OperationPanel.vue'
 import EditDrawer from '@/components/Navigation/EditDrawer.vue'
 import StartEditingFlow from '@/components/EditData/StartEditingFlow.vue'
+import EditTargetPanel from '@/components/EditData/EditTargetPanel.vue'
 
 import { useDataVisStore } from '@/store/dataVisualization'
 import { storeToRefs } from 'pinia'
@@ -492,6 +519,7 @@ const { resetState } = useDataVisStore()
 const { toggleSnapshot } = useHistorySnapshots()
 const {
   plottedDatastreams,
+  qcDatastream,
   qcDatastreamId,
   isEditorReady,
   datastreams,
@@ -565,6 +593,22 @@ const { leaveEdit } = useEditEntry()
 const canEditWorkspace = computed(() => canEdit())
 const workspaceRole = computed(() => roleName())
 
+// The Edit layout needs a resolved target to render; without one the Select
+// layout shows, so exactly one plot host is in the DOM at any time.
+const isEditLayout = computed(
+  () => currentView.value === DrawerType.Edit && !!qcDatastream.value
+)
+
+// Teleport resolves its target with `document.querySelector`, and this
+// component's own DOM is still detached while it first mounts.
+const isMounted = ref(false)
+onMounted(() => {
+  isMounted.value = true
+})
+const plotHost = computed(() =>
+  isEditLayout.value ? '#qc-plot-host-edit' : '#qc-plot-host-select'
+)
+
 const startEditing =
   useTemplateRef<InstanceType<typeof StartEditingFlow>>('startEditing')
 
@@ -577,14 +621,19 @@ const router = useRouter()
 // A share link's `ed` becomes the resume pointer. Set it before registering
 // the resume hook, which checks the pointer only once.
 const initialShareState = decodeShareState(route.query as Record<string, unknown>)
-if (initialShareState.editView && initialShareState.editDatastreamId) {
+if (initialShareState.editDatastreamId) {
   resumeDatastreamId.value = initialShareState.editDatastreamId
 }
+// `m` says which layout the link was made from. Without it the session is
+// reopened behind the Select view, exactly as the sender left it.
+const initialView = initialShareState.editView
+  ? DrawerType.Edit
+  : DrawerType.Select
 
-// Reopens the editor after a page reload; waits for the catalog to land.
+// Reopens the session after a page reload; waits for the catalog to land.
 useResumeEditSession(async (id) => {
   await flowMounted
-  await startEditing.value?.resume(id)
+  await startEditing.value?.resume(id, initialView)
 })
 
 const editCount = computed(() => editHistory.value?.length ?? 0)
@@ -863,7 +912,7 @@ const hydrateFromUrl = () => {
   // means its sessions were applied) rather than chaining off
   // `setPlottedDatastreams`, which only settles the plot.
   const snapshots = state.snapshots ?? []
-  const editTargetId = state.editView ? state.editDatastreamId : undefined
+  const editTargetId = state.editDatastreamId
   // No `immediate`: resuming the editor always takes at least one await, so
   // the ready condition cannot already hold when this watch is created.
   if (snapshots.length && editTargetId) {
@@ -942,7 +991,8 @@ watch(
     const state: ShareState = {
       workspaceId: selectedWorkspaceId.value || null,
       editView: isEdit,
-      editDatastreamId: isEdit ? (qcDatastreamId.value ?? undefined) : undefined,
+      // The Select view keeps the session open, so the target travels too.
+      editDatastreamId: qcDatastreamId.value ?? undefined,
       tableTab: activeTab.value === 'table',
       datastreamIds: ids,
       snapshots,
@@ -1004,13 +1054,13 @@ onUnmounted(() => {
 
 .select-view__card-body,
 .select-view__plot-body,
-.select-view__plotted,
+.select-view__side,
 .select-view__table {
   min-height: 0;
 }
 
 .select-view__card-body {
-  /* Inner row: plot + vertical divider + plotted list. */
+  /* Inner row: plot + vertical divider + side panels. */
   overflow: hidden;
 }
 
@@ -1022,7 +1072,7 @@ onUnmounted(() => {
   display: none;
 }
 
-.select-view__plotted {
+.select-view__side {
   width: 280px;
 }
 
@@ -1041,7 +1091,7 @@ onUnmounted(() => {
   .select-view__card-body {
     flex-direction: column;
   }
-  .select-view__plotted {
+  .select-view__side {
     width: 100%;
     max-height: 240px;
   }
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisualization.vue b/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
index 4fdbf536..ff3c445d 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
@@ -1,10 +1,6 @@
 <template>
   <!-- Stays mounted while loading so a refresh keeps the plot and its zoom. -->
-  <Plot
-    v-if="isUpdating || isDataAvailable"
-    class="fill-height"
-    :preview="preview"
-  >
+  <Plot v-if="isUpdating || isDataAvailable" class="fill-height">
     <template #body-overlay>
       <div
         v-if="isUpdating"
@@ -132,10 +128,6 @@ import { storeToRefs } from 'pinia'
 import { usePlotlyStore } from '@/store/plotly'
 import Plot from '@/components/VisualizeData/Plot.vue'
 
-defineProps<{
-  preview?: boolean
-}>()
-
 const { plotlyOptions } = storeToRefs(usePlotlyStore())
 
 const { loadingStates, seriesDatastreams } = storeToRefs(useDataVisStore())
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index 90716f9e..a4cb97c0 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -3,7 +3,7 @@
     <div class="plot-header">
       <div class="plot-toolbar d-flex align-center flex-wrap ga-1 px-3 py-1">
         <v-btn-toggle
-          v-if="!preview"
+          v-if="!isPlotPreview"
           v-model="tab"
           density="compact"
           color="primary"
@@ -31,7 +31,7 @@
         />
 
         <v-chip
-          v-if="(selectedData?.length || hasSelectionShape) && !preview"
+          v-if="(selectedData?.length || hasSelectionShape) && !isPlotPreview"
           class="plot-toolbar__selection flex-grow-0 flex-shrink-0"
           size="small"
           color="red"
@@ -48,7 +48,7 @@
         <v-spacer />
 
         <div
-          v-if="preview || tab === 'plot'"
+          v-if="isPlotPreview || tab === 'plot'"
           class="plot-toolbar__points-combo d-inline-flex align-stretch rounded-lg"
           :class="{
             'plot-toolbar__points-combo--on': areTooltipsEnabled,
@@ -184,7 +184,7 @@
         </div>
 
         <v-btn
-          v-if="tab === 'plot' && !preview"
+          v-if="tab === 'plot' && !isPlotPreview"
           size="small"
           variant="text"
           icon="mdi-share-variant-outline"
@@ -195,7 +195,7 @@
 
         <v-menu
           v-model="showHelp"
-          v-if="tab === 'plot' && !preview"
+          v-if="tab === 'plot' && !isPlotPreview"
           :close-on-content-click="false"
           location="bottom end"
           offset="6"
@@ -266,7 +266,7 @@
         </v-menu>
 
         <v-menu
-          v-if="!preview"
+          v-if="!isPlotPreview"
           v-model="contextRangeOpen"
           :close-on-content-click="false"
           location="bottom end"
@@ -306,7 +306,7 @@
               class="flex-fill"
               style="min-height: 0"
             ></div>
-            <template v-if="!preview">
+            <template v-if="!isPlotPreview">
               <div
                 class="plot-context-strip d-flex align-center justify-center cursor-pointer user-select-none"
                 :title="
@@ -389,7 +389,7 @@
 
         <v-tabs-window-item value="table" class="fill-height">
           <!-- Don't keep DataTable mounted when not on the table tab. -->
-          <DataTable v-if="tab === 'table' && !preview" class="fill-height"
+          <DataTable v-if="tab === 'table' && !isPlotPreview" class="fill-height"
         /></v-tabs-window-item>
       </v-tabs-window>
       <!-- Covers the plot and table only, so the toolbar stays usable. -->
@@ -419,11 +419,7 @@ import { usePersistedFlag } from '@/composables/useResizable'
 import { formatDate, Snackbar } from '@uwrl/qc-utils'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { useQcSessionStore } from '@/store/qcSession'
-
-// Preview strips the in-plot chrome for the Select view.
-const props = defineProps<{
-  preview?: boolean
-}>()
+import { useUIStore } from '@/store/userInterface'
 
 const { setPlotSelection, clearSelected } = useDataSelection()
 const { updateOptions, requestTableScroll } = usePlotlyStore()
@@ -439,7 +435,6 @@ const {
   showCoordinates,
   crosshair,
   axisChips,
-  previewMode,
   plotlyRef,
   activeTab,
   pendingShareZoom,
@@ -448,6 +443,9 @@ const { selectedData, hasSelectionShape, qcDatastream } =
   storeToRefs(useDataVisStore())
 const { trackPlotWork } = useDataVisStore()
 const { viewedSession, inProgressSession } = storeToRefs(useQcSessionStore())
+// The Select view previews the plot only when nothing is being edited; an
+// open session keeps the full chrome in both views.
+const { isPlotPreview } = storeToRefs(useUIStore())
 
 const tooltipsAutoDisabled = computed(
   () =>
@@ -588,10 +586,26 @@ function zoomToEditWindow() {
   requestTableScroll(w.begin)
 }
 
+// A share link's zoom is an explicit viewport, so it wins over that default,
+// once: later windows (viewing another session) zoom as usual.
+let shareZoomWins = false
+watch(
+  pendingShareZoom,
+  (zoom) => {
+    if (zoom) shareZoomWins = true
+  },
+  { immediate: true }
+)
+
 watch(
   () => editWindow.value && `${editWindow.value.begin}-${editWindow.value.end}`,
   () => {
-    if (!props.preview) zoomToEditWindow()
+    if (isPlotPreview.value) return
+    if (shareZoomWins) {
+      shareZoomWins = false
+      return
+    }
+    zoomToEditWindow()
   }
 )
 
@@ -673,9 +687,6 @@ let cancelFirstDraw: (() => void) | null = null
 let isUnmounted = false
 
 onMounted(() => {
-  // Flip before handleNewPlot so createPlotlyOption emits the
-  // preview layout (no qualifier band, no title, tight margins).
-  previewMode.value = !!props.preview
   updateOptions()
 })
 
@@ -714,7 +725,7 @@ function scheduleFirstDraw(target: HTMLDivElement) {
     const hadPendingShareZoom = !!pendingShareZoom.value
     await handleNewPlot(target)
     if (isUnmounted) return
-    if (!props.preview && !hadPendingShareZoom) zoomToEditWindow()
+    if (!isPlotPreview.value && !hadPendingShareZoom) zoomToEditWindow()
     observePlotSize(target)
   })
   drawn.catch((e) => console.error('First plot draw failed', e))
@@ -750,8 +761,6 @@ function observePlotSize(target: HTMLDivElement) {
 
 onBeforeUnmount(() => {
   isUnmounted = true
-  // Reset so the next Plot mount in Edit view doesn't inherit preview.
-  if (previewMode.value) previewMode.value = false
   cancelFirstDraw?.()
   if (pendingResizeFrame != null) {
     cancelAnimationFrame(pendingResizeFrame)
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
index c6c28c9d..2b7bf7ac 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
@@ -15,7 +15,6 @@ const plotly = {
   showCoordinates: ref(false),
   crosshair: ref({ visible: false }),
   axisChips: ref<any[]>([]),
-  previewMode: ref(false),
   plotlyRef: ref<any>(null),
   activeTab: ref('plot'),
   pendingShareZoom: ref<any>(null),
@@ -62,6 +61,12 @@ vi.mock('@/composables/useDataSelection', () => ({
   }),
 }))
 
+const isPlotPreview = ref(true)
+
+vi.mock('@/store/userInterface', () => ({
+  useUIStore: () => reactive({ isPlotPreview }),
+}))
+
 vi.mock('@/composables/useResizable', () => ({
   usePersistedFlag: (_key: string, initial: boolean) => ref(initial),
 }))
@@ -82,8 +87,8 @@ vi.mock('plotly.js-dist', () => ({ default: { Plots: { resize: vi.fn() } } }))
 import Plot from '@/components/VisualizeData/Plot.vue'
 
 function mountIt(preview = true, slots: Record<string, string> = {}) {
+  isPlotPreview.value = preview
   return mount(Plot, {
-    props: { preview },
     slots,
     global: {
       plugins: [createTestPinia(), createTestVuetify()],
diff --git a/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts b/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
index 15be4fd1..5acec515 100644
--- a/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
@@ -17,6 +17,7 @@ const {
   resumeDatastreamId,
   success,
   error,
+  canLeaveSession,
   ResumeSupersededError,
 } = vi.hoisted(() => {
   const { ref: r } = require('vue') as typeof import('vue')
@@ -40,6 +41,7 @@ const {
     resumeDatastreamId: r<string | null>(null),
     success: vi.fn(),
     error: vi.fn(),
+    canLeaveSession: vi.fn(),
     ResumeSupersededError,
   }
 })
@@ -64,10 +66,19 @@ vi.mock('@/store/userInterface', async () => {
       currentView,
       selectedDrawer,
       isDrawerOpen,
+      showView: (view: string) => {
+        currentView.value = view
+        selectedDrawer.value = view
+        isDrawerOpen.value = true
+      },
     })),
   }
 })
 
+vi.mock('@/composables/useLeaveSession', () => ({
+  useLeaveSession: () => ({ canLeaveSession }),
+}))
+
 vi.mock('@/store/qcSession', async () => {
   const { defineStore } = await import('pinia')
   return {
@@ -88,6 +99,7 @@ vi.mock('@/composables/useEditSession', () => ({
 vi.mock('@uwrl/qc-utils', () => ({ Snackbar: { success, error } }))
 
 import { useEditEntry } from '../useEditEntry'
+import { DrawerType } from '@/store/userInterface'
 
 const window = {
   begin: new Date('2025-01-01T00:00:00Z'),
@@ -112,6 +124,7 @@ beforeEach(() => {
   })
   beginEditing.mockResolvedValue(true)
   startSession.mockResolvedValue(undefined)
+  canLeaveSession.mockResolvedValue(true)
 })
 
 describe('useEditEntry', () => {
@@ -262,6 +275,64 @@ describe('useEditEntry', () => {
     expect(await startSessionOver(window)).toBe(false)
   })
 
+  it('lands on Select when the caller asks for it', async () => {
+    const result = await useEditEntry().enterEdit(
+      'mgd',
+      undefined,
+      DrawerType.Select
+    )
+    expect(result).toBe('editing')
+    expect(currentView.value).toBe('Select')
+    expect(selectedDrawer.value).toBe('Select')
+    expect(resumeDatastreamId.value).toBe('mgd')
+  })
+
+  it('shows the editor again without re-entering its own target', async () => {
+    qcDatastream.value = { id: 'mgd' }
+    currentView.value = 'Select'
+    expect(await useEditEntry().enterEdit('mgd')).toBe('editing')
+    expect(currentView.value).toBe('Edit')
+    expect(setEditTarget).not.toHaveBeenCalled()
+    expect(beginEditing).not.toHaveBeenCalled()
+  })
+
+  it('still starts a session over a window on its own target', async () => {
+    qcDatastream.value = { id: 'mgd' }
+    beginEditing.mockImplementation(async () => {
+      needsSession.value = true
+      return false
+    })
+    expect(await useEditEntry().enterEdit('mgd', window)).toBe('editing')
+    expect(startSession).toHaveBeenCalled()
+  })
+
+  it('asks before taking over from another open session', async () => {
+    qcDatastream.value = { id: 'other' }
+    canLeaveSession.mockResolvedValueOnce(false)
+    expect(await useEditEntry().enterEdit('mgd')).toBe('kept')
+    expect(setEditTarget).not.toHaveBeenCalled()
+    expect(qcDatastream.value).toEqual({ id: 'other' })
+    expect(currentView.value).toBe('Select')
+  })
+
+  it('takes over when leaving the open session is allowed', async () => {
+    qcDatastream.value = { id: 'other' }
+    expect(await useEditEntry().enterEdit('mgd')).toBe('editing')
+    expect(canLeaveSession).toHaveBeenCalled()
+    expect(setEditTarget).toHaveBeenCalledWith('mgd')
+  })
+
+  it('openEditor shows the editor without touching the target', () => {
+    qcDatastream.value = { id: 'mgd' }
+    resumeDatastreamId.value = 'mgd'
+    useEditEntry().openEditor()
+    expect(currentView.value).toBe('Edit')
+    expect(selectedDrawer.value).toBe('Edit')
+    expect(isDrawerOpen.value).toBe(true)
+    expect(clearEditTarget).not.toHaveBeenCalled()
+    expect(resumeDatastreamId.value).toBe('mgd')
+  })
+
   it('leaveEdit returns to Select and forgets the resume target', async () => {
     resumeDatastreamId.value = 'mgd'
     currentView.value = 'Edit'
diff --git a/apps/qc-app/src/composables/__tests__/useFilterDispatch.spec.ts b/apps/qc-app/src/composables/__tests__/useFilterDispatch.spec.ts
index 0cb695b4..a2bd4424 100644
--- a/apps/qc-app/src/composables/__tests__/useFilterDispatch.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useFilterDispatch.spec.ts
@@ -71,7 +71,7 @@ vi.mock('@/store/observations', () => ({
 }))
 
 vi.mock('@/store/operationParams', () => ({
-  useOperationParamsStore: () => ({}),
+  useOperationParamsStore: () => ({ load: () => null }),
 }))
 
 describe('useFilterDispatch.getActiveFilterRange', () => {
diff --git a/apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts
index 8eae6ec3..14249e2d 100644
--- a/apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useResumeEditSession.spec.ts
@@ -122,6 +122,22 @@ describe('useResumeEditSession', () => {
     expect(resume).toHaveBeenCalledTimes(1)
   })
 
+  // Entering the editor sets the pointer itself, and a later catalog
+  // refresh (a commit rewrites the managed datastream) must not re-enter
+  // behind the user's back.
+  it('ignores a pointer set after the catalog landed', async () => {
+    datastreams.value = [{ id: 'mgd-1' }]
+    mountHost()
+    await flushPromises()
+
+    resumeDatastreamId.value = 'mgd-1'
+    datastreams.value = [{ id: 'mgd-1' }, { id: 'mgd-2' }]
+    await flushPromises()
+
+    expect(resume).not.toHaveBeenCalled()
+    expect(resumeDatastreamId.value).toBe('mgd-1')
+  })
+
   it('surfaces a failure instead of failing silently', async () => {
     resumeDatastreamId.value = 'mgd-1'
     resume.mockRejectedValueOnce(new Error('boom'))
diff --git a/apps/qc-app/src/composables/useEditEntry.ts b/apps/qc-app/src/composables/useEditEntry.ts
index a3714c05..2ba1879c 100644
--- a/apps/qc-app/src/composables/useEditEntry.ts
+++ b/apps/qc-app/src/composables/useEditEntry.ts
@@ -6,17 +6,22 @@
  * Entries can overlap (a reload resume and a click). Whichever set the target
  * last owns the view, so an entry that finds the target changed after an await
  * stands down without touching view state, the target or the resume pointer.
+ *
+ * Only `leaveEdit` ends a session. Switching between the Select and Edit views
+ * keeps the target, so `openEditor` and an entry on the open target are pure
+ * navigation.
  */
 
 import { storeToRefs } from 'pinia'
 import { Snackbar } from '@uwrl/qc-utils'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { useQcSessionStore } from '@/store/qcSession'
-import { DrawerType, useUIStore } from '@/store/userInterface'
+import { DrawerType, useUIStore, type View } from '@/store/userInterface'
 import {
   ResumeSupersededError,
   useEditSession,
 } from '@/composables/useEditSession'
+import { useLeaveSession } from '@/composables/useLeaveSession'
 import type { TimeWindow } from '@/utils/timeRangePresets'
 
 export type EnterEditResult =
@@ -24,6 +29,8 @@ export type EnterEditResult =
   | 'needs-window'
   | 'not-managed'
   | 'superseded'
+  /** The open session was kept, so the requested target was not entered. */
+  | 'kept'
 
 type StartOutcome = 'started' | 'superseded' | 'failed'
 
@@ -31,20 +38,25 @@ export function useEditEntry() {
   const dataVis = useDataVisStore()
   const { qcDatastream } = storeToRefs(dataVis)
   const { setEditTarget, clearEditTarget } = dataVis
-  const { currentView, selectedDrawer, isDrawerOpen } = storeToRefs(useUIStore())
+  const { showView } = useUIStore()
   const { resumeDatastreamId } = storeToRefs(useQcSessionStore())
   const { beginEditing, startSession, needsSession, needsHistory } =
     useEditSession()
+  const { canLeaveSession } = useLeaveSession()
 
   const owns = (id: string | undefined) => qcDatastream.value?.id === id
   // A cleared target is not a takeover: leaving again is harmless.
   const takenOver = (id: string | undefined) =>
     !!qcDatastream.value && !owns(id)
 
+  /** Show the editor on the current target. Nothing about the session, the
+   *  working copy or the staged edits changes. */
+  function openEditor() {
+    showView(DrawerType.Edit)
+  }
+
   async function leaveEdit() {
-    currentView.value = DrawerType.Select
-    selectedDrawer.value = DrawerType.Select
-    isDrawerOpen.value = true
+    showView(DrawerType.Select)
     resumeDatastreamId.value = null
     await clearEditTarget()
   }
@@ -81,8 +93,18 @@ export function useEditEntry() {
 
   async function enterEdit(
     managedId: string,
-    window?: TimeWindow
+    window?: TimeWindow,
+    view: View = DrawerType.Edit
   ): Promise<EnterEditResult> {
+    // Already this target: show the view again and leave the session, the
+    // working copy and any unsaved edits alone. A window means "start a
+    // session over it", which still has work to do.
+    if (owns(managedId) && !window) {
+      showView(view)
+      resumeDatastreamId.value = managedId
+      return 'editing'
+    }
+    if (takenOver(managedId) && !(await canLeaveSession())) return 'kept'
     try {
       await setEditTarget(managedId)
       await beginEditing()
@@ -99,9 +121,7 @@ export function useEditEntry() {
       Snackbar.error('This datastream is not set up for QC editing.')
       return 'not-managed'
     }
-    currentView.value = DrawerType.Edit
-    selectedDrawer.value = DrawerType.Edit
-    isDrawerOpen.value = true
+    showView(view)
     resumeDatastreamId.value = managedId
     if (!needsSession.value) return 'editing'
     if (!window) return 'needs-window'
@@ -110,5 +130,5 @@ export function useEditEntry() {
     return outcome === 'failed' ? 'needs-window' : 'superseded'
   }
 
-  return { enterEdit, startSessionOver, leaveEdit }
+  return { enterEdit, startSessionOver, openEditor, leaveEdit }
 }
diff --git a/apps/qc-app/src/composables/useLeaveSession.ts b/apps/qc-app/src/composables/useLeaveSession.ts
new file mode 100644
index 00000000..84aac7bd
--- /dev/null
+++ b/apps/qc-app/src/composables/useLeaveSession.ts
@@ -0,0 +1,17 @@
+/**
+ * The one decision point for abandoning the open edit session: taking over
+ * with another edit target, and every other exit that ends it.
+ *
+ * Switching between the Select and Edit views does not end the session, so it
+ * never asks. Today leaving is always allowed; the prompts that offer to save,
+ * discard or keep the session belong here.
+ */
+
+export function useLeaveSession() {
+  /** True when the open session may be left behind. */
+  async function canLeaveSession(): Promise<boolean> {
+    return true
+  }
+
+  return { canLeaveSession }
+}
diff --git a/apps/qc-app/src/composables/useResumeEditSession.ts b/apps/qc-app/src/composables/useResumeEditSession.ts
index 791263b0..04a3ef58 100644
--- a/apps/qc-app/src/composables/useResumeEditSession.ts
+++ b/apps/qc-app/src/composables/useResumeEditSession.ts
@@ -39,11 +39,16 @@ export function useResumeEditSession(
     }
   }
 
+  // The chance to resume closes as soon as the catalog lands: a pointer set
+  // after that belongs to an entry that is already navigating on its own, and
+  // a later catalog refresh (a commit rewrites the managed datastream) must
+  // not re-enter behind the user.
   watch(
     datastreams,
     (list) => {
-      if (attempted || !list.length || !resumeDatastreamId.value) return
+      if (attempted || !list.length) return
       attempted = true
+      if (!resumeDatastreamId.value) return
       run().catch((e) => {
         Snackbar.error(
           e instanceof Error ? e.message : 'Could not reopen the edit session.'
diff --git a/apps/qc-app/src/store/__tests__/userInterface.spec.ts b/apps/qc-app/src/store/__tests__/userInterface.spec.ts
index d2d0acd2..e94aafe8 100644
--- a/apps/qc-app/src/store/__tests__/userInterface.spec.ts
+++ b/apps/qc-app/src/store/__tests__/userInterface.spec.ts
@@ -6,9 +6,18 @@ const { qcDatastreamRef } = vi.hoisted(() => {
   return { qcDatastreamRef: ref<{ id: string } | null>(null) }
 })
 
-vi.mock('@/store/dataVisualization', () => ({
-  useDataVisStore: () => ({ qcDatastream: qcDatastreamRef }),
-}))
+// A real setup store, so `storeToRefs` and ref unwrapping behave as they do
+// in production.
+vi.mock('@/store/dataVisualization', async () => {
+  const { defineStore } = await import('pinia')
+  const { computed } = await import('vue')
+  return {
+    useDataVisStore: defineStore('dataVisualization', () => ({
+      qcDatastream: qcDatastreamRef,
+      qcDatastreamId: computed(() => qcDatastreamRef.value?.id ?? null),
+    })),
+  }
+})
 vi.mock('@/store/operationParams', () => ({
   useOperationParamsStore: () => ({ load: () => null, save: () => {} }),
 }))
@@ -30,7 +39,10 @@ describe('timeSpacingUnitToTimeUnitKey', () => {
 })
 
 describe('useUIStore drawer', () => {
-  beforeEach(() => { setActivePinia(createPinia()); qcDatastreamRef.value = null })
+  beforeEach(() => {
+    setActivePinia(createPinia())
+    qcDatastreamRef.value = null
+  })
 
   it('defaults to Select drawer open', () => {
     const store = useUIStore()
@@ -53,6 +65,39 @@ describe('useUIStore drawer', () => {
     store.onRailItemClicked(DrawerType.Select)
     expect(store.isDrawerOpen).toBe(true)
   })
+  it('showView moves both the view and the drawer', () => {
+    const store = useUIStore()
+    store.showView(DrawerType.Edit)
+    expect(store.currentView).toBe(DrawerType.Edit)
+    expect(store.selectedDrawer).toBe(DrawerType.Edit)
+    expect(store.isDrawerOpen).toBe(true)
+    store.showView(DrawerType.Select)
+    expect(store.currentView).toBe(DrawerType.Select)
+    expect(store.selectedDrawer).toBe(DrawerType.Select)
+  })
+})
+
+describe('useUIStore isPlotPreview', () => {
+  beforeEach(() => {
+    setActivePinia(createPinia())
+    qcDatastreamRef.value = null
+  })
+
+  it('previews in the Select view with nothing being edited', () => {
+    expect(useUIStore().isPlotPreview).toBe(true)
+  })
+
+  it('keeps the full plot in the Select view while editing', () => {
+    const store = useUIStore()
+    qcDatastreamRef.value = { id: 'mgd' }
+    expect(store.isPlotPreview).toBe(false)
+  })
+
+  it('never previews in the Edit view', () => {
+    const store = useUIStore()
+    store.showView(DrawerType.Edit)
+    expect(store.isPlotPreview).toBe(false)
+  })
 })
 
 describe('useUIStore defaults', () => {
diff --git a/apps/qc-app/src/store/plotly.ts b/apps/qc-app/src/store/plotly.ts
index 68420685..693688e8 100644
--- a/apps/qc-app/src/store/plotly.ts
+++ b/apps/qc-app/src/store/plotly.ts
@@ -146,14 +146,6 @@ export const usePlotlyStore = defineStore('Plotly', () => {
    */
   const axisChips = ref<AxisChip[]>([])
 
-  /**
-   * Toggles a lightweight preview layout in `createPlotlyOption`: the
-   * qualifier flag band, the plot title, select/lasso modebar buttons,
-   * and the custom Y-autoscale button are all suppressed. Plot.vue
-   * flips this based on its `preview` prop so the Select-view chart
-   * stays uncluttered.
-   */
-  const previewMode = ref(false)
   /** The index of the series that represents the datastream selected for quality control */
   const selectedSeriesIndex = computed(() => {
     const { qcDatastream } = storeToRefs(useDataVisStore())
@@ -561,7 +553,6 @@ export const usePlotlyStore = defineStore('Plotly', () => {
     tableScrollRequest,
     requestTableScroll,
     axisChips,
-    previewMode,
     // Zoom history
     zoomUndoStack,
     zoomRedoStack,
diff --git a/apps/qc-app/src/store/userInterface.ts b/apps/qc-app/src/store/userInterface.ts
index 4dd08840..03623dff 100644
--- a/apps/qc-app/src/store/userInterface.ts
+++ b/apps/qc-app/src/store/userInterface.ts
@@ -1,6 +1,6 @@
 import { LogicalOperation, Operator, TimeUnit } from '@uwrl/qc-utils'
-import { defineStore, storeToRefs } from 'pinia'
-import { ref, watch } from 'vue'
+import { defineStore } from 'pinia'
+import { computed, ref, watch } from 'vue'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { useOperationParamsStore } from '@/store/operationParams'
 
@@ -42,7 +42,7 @@ export enum DrawerType {
   None = '',
 }
 
-type View = DrawerType.Edit | DrawerType.Select
+export type View = DrawerType.Edit | DrawerType.Select
 
 export const useUIStore = defineStore('userInterface', () => {
   // Navigation Drawer
@@ -60,17 +60,34 @@ export const useUIStore = defineStore('userInterface', () => {
   const cardHeight = ref(40)
   const tableHeight = ref(35)
 
+  /** Switch layouts. Neither view owns the edit session, so this only moves
+   *  the chrome: the target, its session and its working copy stay put. */
+  const showView = (view: View) => {
+    currentView.value = view
+    selectedDrawer.value = view
+    isDrawerOpen.value = true
+  }
+
   const onRailItemClicked = (title: DrawerType) => {
-    if (selectedDrawer.value === title) {
-      isDrawerOpen.value = !isDrawerOpen.value
-    } else {
-      selectedDrawer.value = title
-      if (title === DrawerType.Edit) currentView.value = DrawerType.Edit
-      if (title === DrawerType.Select) currentView.value = DrawerType.Select
-      isDrawerOpen.value = true
-    }
+    if (selectedDrawer.value === title) isDrawerOpen.value = !isDrawerOpen.value
+    else if (title !== DrawerType.None) showView(title)
   }
 
+  /** True while the plot is the Select view's read-only preview. An edit
+   *  target keeps the full plot in both views, so the user always sees the
+   *  session band, the overview strip and the Context range control.
+   *
+   *  The data-vis store is read lazily: building the plot's first options
+   *  instantiates this store from inside that one's own setup, so
+   *  `storeToRefs` would hand back an empty object there. Nothing is being
+   *  edited that early anyway.
+   */
+  const isPlotPreview = computed(
+    () =>
+      currentView.value === DrawerType.Select &&
+      !useDataVisStore().qcDatastreamId
+  )
+
   // Change Values
   const operators = [...Object.keys(Operator)]
   const selectedOperator = ref(0)
@@ -103,9 +120,9 @@ export const useUIStore = defineStore('userInterface', () => {
   //      datastream lacks metadata.
   // A null change (unset) is skipped so closing and reopening the QC
   // drawer without a datastream loaded doesn't clobber form state.
-  const { qcDatastream } = storeToRefs(useDataVisStore())
+  // Read through a getter for the same reason as `isPlotPreview` above.
   watch(
-    qcDatastream,
+    () => useDataVisStore().qcDatastream,
     (ds) => {
       if (!ds) return
       const persisted = useOperationParamsStore().load(ds.id)
@@ -181,10 +198,12 @@ export const useUIStore = defineStore('userInterface', () => {
     selectedDrawer,
     isDrawerOpen,
     currentView,
+    isPlotPreview,
     selectedOperation,
     cardHeight,
     tableHeight,
     onRailItemClicked,
+    showView,
     shiftUnits,
     selectedShiftUnit,
     shiftAmount,
diff --git a/apps/qc-app/src/utils/__tests__/share.spec.ts b/apps/qc-app/src/utils/__tests__/share.spec.ts
index 653c4a20..cedc8daf 100644
--- a/apps/qc-app/src/utils/__tests__/share.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/share.spec.ts
@@ -168,7 +168,7 @@ describe('encodeShareState: omits defaults', () => {
     expect(q.yz).toBe('1:1~2')
   })
 
-  it('writes ed only in the Edit view', () => {
+  it('writes ed whenever an edit target is set, in either view', () => {
     expect(
       encodeShareState({
         editView: true,
@@ -176,9 +176,14 @@ describe('encodeShareState: omits defaults', () => {
         datastreamIds: ['a'],
       }).ed
     ).toBe('mgd')
-    expect(
-      encodeShareState({ editView: false, editDatastreamId: 'mgd' }).ed
-    ).toBeUndefined()
+    // Select view with the session still open: the target travels, `m` does not.
+    const select = encodeShareState({
+      editView: false,
+      editDatastreamId: 'mgd',
+    })
+    expect(select.ed).toBe('mgd')
+    expect(select.m).toBeUndefined()
+    expect(encodeShareState({ editView: true }).ed).toBeUndefined()
   })
 
   it('omits data points keys when the mode is auto and threshold is default', () => {
diff --git a/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
index a379671d..c4b2939f 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
@@ -1,22 +1,118 @@
 import { describe, it, expect, beforeEach, vi } from 'vitest'
 import { createPinia, setActivePinia } from 'pinia'
+import { reactive, ref } from 'vue'
+
+const newPlot = vi.fn()
 
 vi.mock('plotly.js-dist', () => ({
   default: {
-    newPlot: vi.fn(),
+    newPlot,
     update: vi.fn(),
     restyle: vi.fn(),
     relayout: vi.fn(),
   },
 }))
 
-describe('plotting/events exports', () => {
+const { plotlyOptions, plotlyRef, mainPlotEpoch, pendingShareZoom } = vi.hoisted(
+  () => {
+    const { ref: r } = require('vue') as typeof import('vue')
+    return {
+      plotlyOptions: r<any>({ traces: [], layout: {}, config: {} }),
+      plotlyRef: r<any>(null),
+      mainPlotEpoch: r(0),
+      pendingShareZoom: r<any>(null),
+    }
+  }
+)
+
+vi.mock('@/store/plotly', () => ({
+  usePlotlyStore: () =>
+    reactive({
+      plotlyOptions,
+      plotlyRef,
+      mainPlotEpoch,
+      pendingShareZoom,
+      zoomUndoStack: ref([]),
+      pushZoomState: vi.fn(),
+    }),
+}))
+
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () => reactive({ hasSelectionShape: ref(false) }),
+}))
+
+vi.mock('../relayout', () => ({ handleRelayout: vi.fn() }))
+vi.mock('../selected', () => ({ handleSelected: vi.fn() }))
+vi.mock('../zoom', () => ({
+  captureCurrentZoomState: vi.fn(() => null),
+  installZoomTracking: vi.fn(),
+}))
+vi.mock('../interaction', () => ({
+  handleMouseMove: vi.fn(),
+  handleMouseOut: vi.fn(),
+  handleWheel: vi.fn(),
+  widenYAxisDragRects: vi.fn(),
+  suppressHiddenAxisDragRects: vi.fn(),
+  updateAxisChips: vi.fn(),
+}))
+
+/** Enough of a Plotly graph div for `handleNewPlot` to re-plot onto. */
+function fakeGraphDiv(layout: Record<string, unknown>) {
+  const el = document.createElement('div') as unknown as Record<string, unknown>
+  el.data = []
+  el.layout = layout
+  el.on = vi.fn()
+  el.removeListener = vi.fn()
+  return el
+}
+
+describe('handleNewPlot', () => {
   beforeEach(() => {
     setActivePinia(createPinia())
+    newPlot.mockReset().mockImplementation(async (el: unknown) => el)
+    plotlyOptions.value = { traces: [], layout: {}, config: {} }
+    plotlyRef.value = null
+    pendingShareZoom.value = null
   })
 
   it('exports handleNewPlot as a function', async () => {
     const { handleNewPlot } = await import('@/utils/plotting/events')
     expect(typeof handleNewPlot).toBe('function')
   })
+
+  it('carries other writers shapes through a re-plot', async () => {
+    const { handleNewPlot } = await import('@/utils/plotting/events')
+    plotlyRef.value = fakeGraphDiv({
+      shapes: [
+        { name: 'stage', type: 'rect' },
+        { name: 'edit-window', type: 'rect', x0: 1 },
+      ],
+    })
+    plotlyOptions.value = {
+      traces: [],
+      layout: { shapes: [{ name: 'edit-window', type: 'rect', x0: 2 }] },
+      config: {},
+    }
+
+    await handleNewPlot()
+
+    const layout = newPlot.mock.calls[0]?.[2] as { shapes: any[] }
+    // The stage band survives; the window band is replaced by the fresh one.
+    expect(layout.shapes.map((s) => s.name)).toEqual(['stage', 'edit-window'])
+    expect(layout.shapes.find((s) => s.name === 'edit-window').x0).toBe(2)
+  })
+
+  it('draws a first mount with the layout it was given', async () => {
+    const { handleNewPlot } = await import('@/utils/plotting/events')
+    plotlyOptions.value = {
+      traces: [],
+      layout: { shapes: [{ name: 'edit-window', type: 'rect' }] },
+      config: {},
+    }
+
+    await handleNewPlot(fakeGraphDiv({}) as unknown as HTMLElement)
+
+    const layout = newPlot.mock.calls[0]?.[2] as { shapes: any[] }
+    expect(layout.shapes.map((s) => s.name)).toEqual(['edit-window'])
+  })
 })
diff --git a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
index bd290fd2..3917f11d 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
@@ -49,20 +49,23 @@ vi.mock('@/store/qcSession', () => ({
   })),
 }))
 
-const previewMode = ref(false)
+const isPlotPreview = ref(false)
 const hiddenAxisIds = ref<Set<string>>(new Set())
 const plotlyRef = ref<unknown>(null)
 const editHistory = ref<unknown[]>([])
 
 vi.mock('@/store/plotly', () => ({
   usePlotlyStore: () => ({
-    previewMode,
     hiddenAxisIds,
     plotlyRef,
     editHistory,
   }),
 }))
 
+vi.mock('@/store/userInterface', () => ({
+  useUIStore: () => ({ isPlotPreview }),
+}))
+
 const applications: Array<{ qualifierId: string; index: number; appliedAt: string; appliedBy: string }> = []
 const qualifierById: Record<string, { code: string; description: string }> = {}
 
@@ -193,7 +196,7 @@ describe('createPlotlyOption', () => {
     inProgressSession.value = null
     beginDate.value = null
     endDate.value = null
-    previewMode.value = false
+    isPlotPreview.value = false
     hiddenAxisIds.value = new Set()
     plotlyRef.value = null
     applications.length = 0
@@ -228,8 +231,8 @@ describe('createPlotlyOption', () => {
     expect((opts.traces[1] as any).id).toBe('a')
   })
 
-  it('omits select/lasso modebar buttons when previewMode is true', () => {
-    previewMode.value = true
+  it('omits select/lasso modebar buttons in the Select preview', () => {
+    isPlotPreview.value = true
     const opts = createPlotlyOption([])
     const cfg = opts.config as any
     const flat = cfg.modeBarButtons.flat()
diff --git a/apps/qc-app/src/utils/plotting/events.ts b/apps/qc-app/src/utils/plotting/events.ts
index d56d1d1e..8bf620cb 100644
--- a/apps/qc-app/src/utils/plotting/events.ts
+++ b/apps/qc-app/src/utils/plotting/events.ts
@@ -22,6 +22,7 @@ import {
   updateAxisChips,
 } from './interaction'
 import type { AppPlotlyTrace } from './options'
+import { withLiveShapes } from './shapes'
 
 const handleClick = async (eventData: PlotMouseEvent) => {
   const { plotlyRef } = storeToRefs(usePlotlyStore())
@@ -139,12 +140,19 @@ export const handleNewPlot = async (
     }
   }
 
+  // A re-plot keeps the shapes other writers own (the staged range band);
+  // `createPlotlyOption` only rebuilds the session window band. A first
+  // mount has no live plot to read them from.
+  const layout = element
+    ? plotlyOptions.value.layout
+    : withLiveShapes(plotlyOptions.value.layout, plotlyRef.value?.layout)
+
   // `Plotly.newPlot` returns `Promise<PlotlyHTMLElement>`. The store's
   // `plotlyRef` is now typed as `PlotlyHTMLElement | null`
   const newElement = await Plotly.newPlot(
     element || plotlyRef.value as Plotly.Root,
     plotlyOptions.value.traces,
-    plotlyOptions.value.layout,
+    layout,
     plotlyOptions.value.config
   )
   plotlyRef.value = newElement as unknown as typeof plotlyRef.value
diff --git a/apps/qc-app/src/utils/plotting/options.ts b/apps/qc-app/src/utils/plotting/options.ts
index 0841d66f..c88b6de6 100644
--- a/apps/qc-app/src/utils/plotting/options.ts
+++ b/apps/qc-app/src/utils/plotting/options.ts
@@ -14,6 +14,7 @@ import { storeToRefs } from 'pinia'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { useQcSessionStore } from '@/store/qcSession'
 import { useQualifierStore } from '@/store/qualifiers'
+import { useUIStore } from '@/store/userInterface'
 import { findFirstGreaterOrEqual } from '@uwrl/qc-utils'
 import { DENSITY_HIDE_MARKERS, Y_AXIS_KEY_RE } from './internal'
 import { undoZoom, redoZoom } from './zoom'
@@ -410,11 +411,12 @@ export const createPlotlyOption = (
 ): PlotlyChartOptions => {
   const { qcDatastream, sourceContextDatastream, beginDate, endDate } =
     storeToRefs(useDataVisStore())
-  const { previewMode, hiddenAxisIds, hiddenTraceIds, plotlyRef } = storeToRefs(
+  const { hiddenAxisIds, hiddenTraceIds, plotlyRef } = storeToRefs(
     usePlotlyStore()
   )
   const { viewedSession, inProgressSession } = storeToRefs(useQcSessionStore())
-  const isPreview = previewMode?.value ?? false
+  const { isPlotPreview } = storeToRefs(useUIStore())
+  const isPreview = isPlotPreview?.value ?? false
   const hiddenAxes = hiddenAxisIds?.value ?? new Set<string>()
   const hiddenTraces = hiddenTraceIds?.value ?? new Set<string>()
 
diff --git a/apps/qc-app/src/utils/share.ts b/apps/qc-app/src/utils/share.ts
index d448584e..c5f8b57c 100644
--- a/apps/qc-app/src/utils/share.ts
+++ b/apps/qc-app/src/utils/share.ts
@@ -3,8 +3,8 @@
  *
  * The URL keeps every plot-relevant piece of state the sender sees so
  * a recipient lands on an identical view: plotted datastreams (`ds`),
- * the datastream being edited (`ed`, Edit view only), time range,
- * current view (Select / Edit), Plot/Table tab, per-trace visibility,
+ * the datastream being edited (`ed`), time range,
+ * current view (`m`, Select / Edit), Plot/Table tab, per-trace visibility,
  * per-Y-axis visibility, X/Y zoom, and the data-points (markers) mode
  * and threshold. Sidebar filters are kept only on the Select view,
  * since they drive the datastreams table, not the plot.
@@ -45,7 +45,8 @@ export interface ShareState {
   workspaceId?: string | null
   /** Edit view active when true. Select view is the default. */
   editView?: boolean
-  /** The datastream being edited. Emitted only in the Edit view. */
+  /** The datastream being edited. Travels in both views: the Select view
+   *  keeps the session open, so a link from it reopens the same session. */
   editDatastreamId?: string
   /** Table tab active when true. Plot tab is the default. */
   tableTab?: boolean
@@ -136,7 +137,7 @@ export function encodeShareState(state: ShareState): Record<string, string> {
 
   if (state.workspaceId) q.ws = state.workspaceId
   if (state.editView) q.m = 'e'
-  if (state.editView && state.editDatastreamId) q.ed = state.editDatastreamId
+  if (state.editDatastreamId) q.ed = state.editDatastreamId
   if (state.tableTab) q.tab = 't'
 
   if (state.datastreamIds?.length) {

From 0364eeafdf70194533c7c9fa859a7e7cfeda8aaa Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:01:00 -0600
Subject: [PATCH 61/93] feat: ask before leaving an edit session

---
 apps/qc-app/docs/API_REFERENCE.md             |  62 ++-
 apps/qc-app/docs/ARCHITECTURE.md              |  41 +-
 apps/qc-app/docs/TESTING.md                   |   4 +-
 apps/qc-app/docs/USER_GUIDE.md                |  39 +-
 apps/qc-app/e2e/edit-selection.spec.ts        |   2 +
 apps/qc-app/e2e/leave-session.spec.ts         | 218 +++++++++++
 apps/qc-app/e2e/navigation.spec.ts            |  45 +--
 apps/qc-app/src/App.vue                       |   3 +
 .../EditData/LeaveSessionDialog.vue           | 155 ++++++++
 .../__tests__/LeaveSessionDialog.spec.ts      | 151 ++++++++
 .../components/Navigation/NavigationRail.vue  | 155 +-------
 .../__tests__/NavigationRail.spec.ts          |  57 ++-
 apps/qc-app/src/components/VisualizeData.vue  |  70 +---
 .../__tests__/useEditEntry.spec.ts            |  45 ++-
 .../__tests__/useLeaveSession.spec.ts         | 353 ++++++++++++++++++
 apps/qc-app/src/composables/useEditEntry.ts   |  24 +-
 apps/qc-app/src/composables/useEditSession.ts |   2 +-
 .../qc-app/src/composables/useLeaveSession.ts | 206 +++++++++-
 .../src/router/__tests__/guards.spec.ts       |  73 ++++
 apps/qc-app/src/router/guards.ts              |  20 +
 .../src/store/__tests__/qcSession.spec.ts     |  54 +++
 apps/qc-app/src/store/qcSession.ts            |  18 +-
 22 files changed, 1494 insertions(+), 303 deletions(-)
 create mode 100644 apps/qc-app/e2e/leave-session.spec.ts
 create mode 100644 apps/qc-app/src/components/EditData/LeaveSessionDialog.vue
 create mode 100644 apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts
 create mode 100644 apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
 create mode 100644 apps/qc-app/src/router/__tests__/guards.spec.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 0b61f626..8c2cf3fb 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -115,7 +115,8 @@ await clearSelected({ recordHistory: false })  // skip history append on cleanup
 ### `useEditEntry()`
 
 ```ts
-const { enterEdit, startSessionOver, openEditor, leaveEdit } = useEditEntry()
+const { enterEdit, startSessionOver, openEditor, leaveEdit, closeEditor } =
+  useEditEntry()
 
 const result = await enterEdit(managedId, window)
 // result: 'editing' | 'needs-window' | 'not-managed' | 'superseded' | 'kept'
@@ -135,8 +136,8 @@ below), and share-link hydration (the `ed` query param).
   link made from the Select view passes `Select`, so the session reopens
   behind it. Called on the target already open, with no `window`, it only
   shows that view: nothing about the session is re-entered. Taking over from
-  another open session first asks `useLeaveSession().canLeaveSession()`, and
-  returns `'kept'` when it says no.
+  another open session first asks `useLeaveSession().requestLeave()`, and
+  returns `'kept'` when the user keeps the open one.
 - `startSessionOver(window)`: starts a new session on the current edit
   target over `window` (a `utils/timeRangePresets.ts` `TimeWindow`); backs
   **Start new session** and the editor footer's **New session**.
@@ -144,9 +145,14 @@ below), and share-link hydration (the `ed` query param).
   navigation, used by the nav rail's Edit button and the Select view's
   **Back to editor**.
 - `leaveEdit()`: returns to the Select view, clears the resume pointer and
-  the edit target (`clearEditTarget`). Every exit uses it, including the nav
-  rail's Home and Workspaces buttons, so a reload never reopens an editor the
-  user left. Switching views is not an exit and does not use it.
+  the edit target (`clearEditTarget`). The user has already been asked by
+  then, or there was nothing to ask about. Switching views is not an exit and
+  does not use it.
+- `closeEditor()`: what a user's exit calls. Runs the leave flow
+  (`useLeaveSession().requestLeave()`) and, once it resolves true, ends the
+  session with `leaveEdit()`. Returns false when the user chose to stay, so
+  the caller abandons its own exit. Backs the editor footer's **Close** and
+  the nav rail's Home and Log out.
 
 Entries can overlap (a reload resume and a click land close together);
 whichever call sets the edit target last owns the view: a call that finds
@@ -156,15 +162,40 @@ state, the target, or the resume pointer.
 ### `useLeaveSession()`
 
 ```ts
-const { canLeaveSession } = useLeaveSession()
-if (!(await canLeaveSession())) return
+const { requestLeave } = useLeaveSession()
+if (!(await requestLeave())) return // the user stayed; nothing changed
 ```
 
-The one decision point for abandoning an open edit session: taking over with
-another edit target, and every other exit that ends it. Switching between the
-Select and Edit views is not an exit, so it never asks. Leaving is always
-allowed today; the prompts that offer to save, discard or keep the session
-belong here.
+The one decision point for leaving an open edit session, used by every exit:
+the editor footer's **Close**, the nav rail's Home, workspace switch and log
+out, the row Edit button on a different datastream, and in-app navigation
+(`leaveSessionGuard` in `router/guards.ts`). Switching between the Select and
+Edit views is not an exit, so it never asks. The prompt state is module-wide
+and `LeaveSessionDialog.vue`, mounted once in `App.vue`, shows it.
+
+- `leaveCase()`: which of the four situations applies right now.
+  `'none'` (nothing being edited, no session, or committed history being
+  viewed) leaves silently; `'unsaved'` has edits that never reached the
+  session; `'empty'` is a session holding no operations at all; `'saved'` is
+  a session whose work is all saved.
+- `requestLeave()`: decides the case, shows the matching prompt and carries
+  the answer out. Resolves true when the caller may go on, false when the
+  user stays. A second request supersedes the first, whose caller stays put.
+- `saveAndLeave()` / `discardEditsAndLeave()`: the `'unsaved'` answers.
+  Saving is refused with no session open. Discarding that empties the session
+  falls through to the `'empty'` prompt rather than leaving silently.
+- `keepSession()` / `discardSessionAndLeave()`: the `'empty'` answers.
+  Discarding deletes the session through
+  `useManagedDatastreams().deleteSessionChain`, and refuses if another
+  session was somehow built on it. A failed delete keeps the user in the
+  session.
+- `closeSession()`: the `'saved'` answer, keeping the session as it is.
+- `cancelLeave()`: stay, with the zoom, staged band and unsaved edits intact.
+- `forgetSession()`: drop the resume pointer only. For exits that unmount the
+  editor, where clearing the edit target would have the editor's URL writer
+  replace the route mid-navigation.
+- `leavePrompt` / `leaveWork`: what the dialog renders, and which answer is
+  currently running.
 
 ### `useResumeEditSession()`
 
@@ -256,7 +287,7 @@ const {
 - `hasUnsavedChanges` / `unsavedEditCount`: the working copy compared with
   the saved-edits baseline (`qcSession.savedEdits` / `savedComments`). The
   baseline lives in the store, so every caller agrees: the editor footer and
-  the nav rail's exit guard both read it.
+  the leave flow both read it.
 
 ### `useCreateManagedDatastream()`
 
@@ -661,12 +692,13 @@ puts the editor in read-only mode.
 | `currentSessionId`  | state    | `string \| null`                        | The single in-progress (editable) session. |
 | `viewedSessionId`   | state    | `string \| null`                        | The session currently being viewed. |
 | `isSwitchingSession`| state    | `boolean`                               | True while another session's data and operations load. The operations panel renders a loading state instead of the outgoing session's entries, which would otherwise linger and read as the incoming session's. |
-| `savedEdits`        | state    | `HistoryItem[]`                         | Edit history entries (by reference) at the last load or save. `useEditSession` compares the working copy against it for `hasUnsavedChanges`; kept in the store so the editor footer and the nav rail's exit guard agree. |
+| `savedEdits`        | state    | `HistoryItem[]`                         | Edit history entries (by reference) at the last load or save. `useEditSession` compares the working copy against it for `hasUnsavedChanges`; kept in the store so the editor footer and the leave flow agree. |
 | `savedComments`     | state    | `string[]`                              | Comment text of `savedEdits`, since comments are edited in place. |
 | `isReadOnly`        | computed | `boolean`                               | True when sessions exist and the viewed one isn't the in-progress session. Guarded on `sessions.length` so plain editing outside the session workflow isn't treated as read-only. |
 | `inProgressSession` | computed | `QualityControlSession \| null`         | The editable session, if any. |
 | `committedSessions` | computed | `QualityControlSession[]`               | Sessions with status `committed`. |
 | `viewedSession`     | computed | `QualityControlSession \| null`         | The session for `viewedSessionId`. |
+| `hasSessionOperations` | computed | `boolean`                            | True when the in-progress session holds any work: the operations the server returned with it, plus anything saved since (a save leaves them in `savedEdits` before the sessions are re-fetched). The leave flow tells an untouched session from one worth keeping with it. |
 | `fetchSessions`     | action   | `(historyId: string) => Promise<QualityControlSession[]>` | Fetch a history's sessions with their operations without writing any state, so a caller can drop a result that went stale (see `useEditSession`). |
 | `applySessions`     | action   | `(historyId: string, sessions: QualityControlSession[]) => void` | Adopt fetched sessions; default the view to the in-progress one, else the latest committed. |
 | `viewSession`       | action   | `(sessionId: string) => void`           | View a session read-only (no-op for an unknown id). |
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index d5316a68..0de275e3 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -194,7 +194,7 @@ holds which one is showing. A view is chrome only: `dataVisualization` owns the
 edit target (`qcDatastreamId`), so switching views never touches the session,
 the working copy or unsaved edits. Only `useEditEntry.leaveEdit` ends a
 session, and `useLeaveSession` is the one place that decides whether an open
-session may be left behind.
+session may be left behind (see "Leaving a session").
 
 Consequences of that split:
 
@@ -428,9 +428,11 @@ Three constraints shape the implementation:
 
 ## Routing and auth
 
-vue-router 5, two routes (Home, Workspaces). Two guards run on
+vue-router 5, two routes (Home, Workspaces). Three guards run on
 every navigation:
 
+- **`leaveSessionGuard`**: a change of page ends an open edit session, so it
+  runs the leave flow first and cancels the navigation if the user stays.
 - **`hasAuthGuard`**: redirects unauthenticated users to the
   data-management app's `/login` route and remembers the intended QC
   destination.
@@ -438,12 +440,35 @@ every navigation:
   to `/workspaces`.
 
 The nav rail's "Edit" entry is enabled only while an edit target is set: it
-returns to the open editor and never picks one itself. Leaving
-the Edit view with edits not yet saved to the session runs the "Unsaved
-edits" dialog: Save & continue saves a draft to the in-progress session and
-Discard returns to the last save. The unsaved state comes from
-`useEditSession`, whose saved-edits baseline lives in the `qcSession` store so
-the rail and the editor footer agree (see `NavigationRail.vue`).
+returns to the open editor and never picks one itself.
+
+## Leaving a session
+
+A user never leaves an edit session without choosing to, and the choice says
+what happens to the work. `useLeaveSession.requestLeave()` decides which of
+four situations applies and shows the matching prompt; `LeaveSessionDialog`,
+mounted once in `App.vue`, renders it, and the caller proceeds only when the
+promise resolves true.
+
+| Situation | Prompt | What it does |
+|---|---|---|
+| Unsaved edits | Save and close / Discard changes and close / Cancel | Save writes a draft to the in-progress session, and is disabled with no session open. Discarding that empties the session falls through to the next row. |
+| The session holds nothing at all | Keep session / Discard session / Cancel | Discard deletes it with `useManagedDatastreams().deleteSessionChain`. A failed delete keeps the user in the session. |
+| Everything saved | Close / Cancel | Nothing. The session stays in progress. |
+| Nothing being edited, or committed history being viewed | none | Leaves silently. |
+
+The unsaved signal comes from `useEditSession`, whose saved-edits baseline
+lives in the `qcSession` store; whether the session holds any operations at
+all comes from `qcSession.hasSessionOperations`, which reads the server's
+operations plus anything saved since. Neither is a guess about the UI.
+
+Every exit routes through it: the editor footer's **Close** and the nav rail's
+Home and Log out via `useEditEntry.closeEditor()`, the row Edit button on
+another datastream via `enterEdit`, and in-app navigation (including the
+workspace switch) via `leaveSessionGuard`. Cancelling leaves the user exactly
+where they were, zoom, staged band and unsaved edits intact. The native
+`beforeunload` prompt still covers a reload or a closed tab, with the
+browser's own wording.
 
 Auth itself is delegated to HydroServer's Django AllAuth setup; the app
 keeps no credentials of its own. The browser holds a session cookie.
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index a6a23bff..38517281 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -12,8 +12,8 @@ the operator's manual; QUALITY.md is the policy.
 
 | Layer      | Runner                          | Where                        | Count    |
 | ---------- | ------------------------------- | ---------------------------- | -------- |
-| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 72 files |
-| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 33 files |
+| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 75 files |
+| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 34 files |
 
 There is no separate "integration" tier; component tests live in the
 unit tier and mount real Vue components with the Vue Test Utils
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index b0753594..41e66b02 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -62,7 +62,7 @@ A thin, always-visible column of icons.
 
 | Icon | Action |
 |------|--------|
-| HydroServer logo | Top left. Go home. Resets the current view. Prompts before discarding unsaved edits. |
+| HydroServer logo | Top left. Go home. Resets the current view. Asks what should happen to an open edit session first. |
 | Cursor (Select) | Top left. Show the datastream Select drawer + plot. While you are editing, this keeps the session open behind it. |
 | Pencil (Edit) | Top left. Return to the Edit view. **Disabled** until you're editing: pick a datastream to edit with its row's pencil button first. |
 | Stopwatch (Performance) | Bottom left. Open the Performance Calibration dialog. See "Performance" below. |
@@ -71,7 +71,7 @@ A thin, always-visible column of icons.
 
 Switching between **Select** and **Edit** never ends an edit session, so neither asks anything: the datastream you are editing, its session, and any unsaved edits stay exactly as they were.
 
-Home, Workspaces and Logout do leave the session. If you click one of them while there are edits not yet saved to the session, the app shows an "Unsaved edits" dialog with **Save & continue** / **Discard** / **Cancel**. **Save & continue** saves a draft to the session before leaving; it is unavailable when no session is open. **Discard** drops the edits made since the last save, and they cannot be recovered.
+Home, Workspaces and Logout do leave the session, so they go through the same question as the editor's **Close** button. See "Leaving a session" below.
 
 ### Select view
 
@@ -647,7 +647,7 @@ When you're satisfied with the edits, hit one of the action buttons at the botto
 - **Commit**: materializes the session into the managed datastream and locks it into the history.
 - **Discard**: drops every edit made since the last save, returning the session to its last saved state. Edits already saved to the session stay. Disabled when there is nothing unsaved, and it asks for confirmation first.
 - **New session**: replaces Save and Commit once the session is committed. Opens the session window step, starting from the state the last commit left behind.
-- **Close**: leaves the editor. If you have unsaved edits, a "Save before closing?" dialog lets you save first or close without saving.
+- **Close**: leaves the editor, after asking what should happen to the session. See "Leaving a session" below.
 
 Clicking Commit opens a confirmation dialog so a misclick won't push data to the server.
 
@@ -659,6 +659,36 @@ The dialog lets you add an optional session description. Once you confirm:
 2. The session is committed and becomes read-only. The Snackbar shows "Session committed." and the footer swaps Save and Commit for **New session**.
 3. On failure, the Snackbar shows the backend's error message verbatim. Show that to your administrator if you need help.
 
+## Leaving a session
+
+You never leave an edit session by accident. Anything that ends it, the
+editor's **Close** button, the HydroServer logo, the workspace switch, Log
+out, the browser's Back button, or editing a different datastream, asks the
+same question first and tells you what happens to your work.
+
+Which question depends on where the session stands:
+
+- **You have edits that are not saved to the session yet.** Choose **Save and
+  close** to write them to the session first, **Discard changes and close** to
+  drop everything since your last save, or **Cancel** to stay. Either way the
+  session stays in progress and you can pick it up later. Save is unavailable
+  when no session is open, because there is nowhere to save to. If discarding
+  leaves the session with nothing in it, the next question is the one below.
+- **The session has no edits at all**, nothing saved and nothing unsaved (you
+  started it and then changed your mind). Choose **Keep session** to leave it
+  in progress, or **Discard session** to delete it from the server. Keeping it
+  is the safe choice: it costs nothing and you can resume it.
+- **Everything is saved.** The dialog only confirms that the session stays in
+  progress; **Close** leaves, **Cancel** stays.
+
+To come back to a session you left in progress, click the pencil button on
+that datastream's row again. It reopens where you left off.
+
+**Cancel** always puts you back exactly where you were: same zoom, same staged
+range, same unsaved edits. Switching between the **Select** and **Edit** views
+is not leaving, so it never asks. Reloading the page or closing the tab still
+shows the browser's own "leave site?" prompt, which the app cannot reword.
+
 ## Performance and big datastreams
 
 For datastreams with hundreds of thousands of points:
@@ -696,7 +726,7 @@ See [PERFORMANCE.md](./PERFORMANCE.md) for the envelope details.
 
 ### "I picked the wrong workspace."
 
-Click **Workspaces** in the nav rail and **Select** another, or **Continue** to stay in the current one. If you have unsaved edits, the app asks first.
+Click **Workspaces** in the nav rail and **Select** another, or **Continue** to stay in the current one. If an edit session is open, the app asks what should happen to it first.
 
 ## Troubleshooting
 
@@ -706,6 +736,7 @@ Click **Workspaces** in the nav rail and **Select** another, or **Continue** to
 | Plot stays empty after picking a datastream | A `Custom` time range that doesn't overlap the datastream's observations, or the datastream has no observations. The plotted row shows a database-off icon and the subtitle reads `0 pts loaded`. | Click a preset such as **1m** or **All** in Time range. |
 | The Edit rail item is greyed out | You are not editing. | Click a row's pencil button. |
 | The Select view shows an "Editing" panel | A session is still open. | That is expected: click **Back to editor** to return to it, or **Close** in the editor to end it. |
+| I closed the editor and cannot find my session | It is still in progress, just not open. | Click the pencil button on that datastream's row to resume it. Sessions you discarded on the way out are gone for good. |
 | Big edits freeze the page | `SharedArrayBuffer` not available; running inline. | Have your admin re-enable COOP/COEP headers, or accept the slower fallback. |
 | Save fails with a backend error | Permissions / workspace issue / network. | The Snackbar shows the backend message verbatim. Share that with your admin. |
 | The history shows a red failed entry after Load | The QC history referenced something missing in this datastream. | The rest of the QC history still ran. Click the chevron on the row to see its arguments. |
diff --git a/apps/qc-app/e2e/edit-selection.spec.ts b/apps/qc-app/e2e/edit-selection.spec.ts
index 99156204..713e6b04 100644
--- a/apps/qc-app/e2e/edit-selection.spec.ts
+++ b/apps/qc-app/e2e/edit-selection.spec.ts
@@ -182,6 +182,8 @@ test.describe('edit selection', () => {
     await plotDatastreamById(page, DATASTREAM_ID_B)
     await startSessionFromRow(page)
     await page.getByTestId('exit-close-btn').click()
+    // The session was never touched, so closing asks what to do with it.
+    await page.getByTestId('leave-keep-btn').click()
     await expect(page.getByTestId('datastreams-table')).toBeVisible({
       timeout: 30_000,
     })
diff --git a/apps/qc-app/e2e/leave-session.spec.ts b/apps/qc-app/e2e/leave-session.spec.ts
new file mode 100644
index 00000000..46c21366
--- /dev/null
+++ b/apps/qc-app/e2e/leave-session.spec.ts
@@ -0,0 +1,218 @@
+/**
+ * Leaving an edit session. One dialog, one decision function, three cases:
+ *   - unsaved edits: save, discard, or stay
+ *   - a session with nothing in it: keep it, or delete it from the server
+ *   - everything saved: a notice that the session stays in progress
+ * plus the exits that all run it: the footer Close, the Home logo, the
+ * workspace switch (through the router guard), Log out, and the row Edit
+ * button on another datastream. Viewing committed history leaves silently.
+ */
+
+import { expect, test, type Page } from '@playwright/test'
+import { installMocks, type MockQcSession } from './support/mocks'
+import {
+  gotoHome,
+  openOp,
+  setupEditView,
+  waitForEditorReady,
+} from './support/app'
+import { expectHistoryContains, selectAllPoints } from './support/ops'
+import { DATASTREAM_ID_B } from './support/fixtures'
+
+const leaveDialog = (page: Page) => page.getByTestId('leave-session-dialog')
+const editPanel = (page: Page) => page.getByTestId('edit-target-panel')
+
+const inProgress = (sessions: MockQcSession[]) =>
+  sessions.find((s) => s.status === 'in_progress')
+
+async function applyChangeValues(page: Page) {
+  await selectAllPoints(page)
+  await openOp(page, 'changeValues')
+  await page.getByLabel('Value').fill('1')
+  await page.getByRole('button', { name: 'Apply' }).click()
+  await expectHistoryContains(page, 'Change Values')
+}
+
+async function goToSelect(page: Page) {
+  await page.getByTestId('nav-rail-item-select').click()
+  await expect(page.getByTestId('datastreams-table')).toBeVisible({
+    timeout: 30_000,
+  })
+}
+
+/** The editor is closed: the Select view is showing with nothing being edited. */
+async function expectClosed(page: Page) {
+  await expect(page.getByTestId('datastreams-table')).toBeVisible({
+    timeout: 30_000,
+  })
+  await expect(editPanel(page)).toHaveCount(0)
+  await expect(page.getByTestId('nav-rail-item-edit')).toHaveAttribute(
+    'aria-disabled',
+    'true'
+  )
+}
+
+test.describe('leaving a session', () => {
+  let sessions: MockQcSession[]
+
+  test.beforeEach(async ({ page }) => {
+    test.slow()
+    sessions = []
+    await installMocks(page, {
+      qcHistories: true,
+      qcSessionState: sessions,
+    })
+    await setupEditView(page)
+  })
+
+  test('an untouched session offers keep or discard, and Cancel stays', async ({
+    page,
+  }) => {
+    await page.getByTestId('exit-close-btn').click()
+    await expect(leaveDialog(page)).toContainText('This session has no edits')
+
+    await page.getByTestId('leave-cancel-btn').click()
+    await expect(leaveDialog(page)).toHaveCount(0)
+    await waitForEditorReady(page)
+    expect(inProgress(sessions)).toBeDefined()
+
+    await page.getByTestId('exit-close-btn').click()
+    await page.getByTestId('leave-keep-btn').click()
+
+    await expectClosed(page)
+    expect(inProgress(sessions)).toBeDefined()
+  })
+
+  test('discarding an untouched session deletes it from the server', async ({
+    page,
+  }) => {
+    const sessionId = inProgress(sessions)!.id
+
+    await page.getByTestId('exit-close-btn').click()
+    await page.getByTestId('leave-discard-session-btn').click()
+
+    await expect(page.getByText('Session discarded.')).toBeVisible()
+    await expectClosed(page)
+    expect(sessions.some((s) => s.id === sessionId)).toBe(false)
+  })
+
+  test('unsaved edits can be saved on the way out', async ({ page }) => {
+    await applyChangeValues(page)
+
+    await page.getByTestId('exit-close-btn').click()
+    await expect(leaveDialog(page)).toContainText('Unsaved edits')
+    await page.getByTestId('leave-save-btn').click()
+
+    await expectClosed(page)
+    expect(inProgress(sessions)!.operations.map((o) => o.operationType)).toContain(
+      'CHANGE_VALUES'
+    )
+  })
+
+  test('discarding the edits asks about the now empty session', async ({
+    page,
+  }) => {
+    await applyChangeValues(page)
+
+    await page.getByTestId('exit-close-btn').click()
+    await page.getByTestId('leave-discard-edits-btn').click()
+
+    // Nothing was ever saved, so the session is empty now.
+    await expect(leaveDialog(page)).toContainText('This session has no edits')
+    await page.getByTestId('leave-keep-btn').click()
+
+    await expectClosed(page)
+    expect(inProgress(sessions)!.operations).toHaveLength(0)
+  })
+
+  test('a fully saved session only confirms', async ({ page }) => {
+    await applyChangeValues(page)
+    await page.getByTestId('exit-save-btn').click()
+    await expect(page.getByText('Draft saved.')).toBeVisible()
+
+    await page.getByTestId('exit-close-btn').click()
+    await expect(leaveDialog(page)).toContainText('Close this session?')
+    await expect(leaveDialog(page)).toContainText('stays in progress')
+    await page.getByTestId('leave-close-btn').click()
+
+    await expectClosed(page)
+    expect(inProgress(sessions)!.operations.length).toBeGreaterThan(0)
+  })
+
+  test('viewing committed history closes without a question', async ({
+    page,
+  }) => {
+    await page.getByTestId('session-header-qcs-e2e-1').click()
+    await expect(page.getByTestId('session-return-current')).toBeVisible()
+
+    await page.getByTestId('exit-close-btn').click()
+
+    await expect(leaveDialog(page)).toHaveCount(0)
+    await expectClosed(page)
+  })
+
+  test('the Home logo asks first', async ({ page }) => {
+    await page.getByRole('button', { name: 'Home' }).click()
+    await expect(leaveDialog(page)).toBeVisible()
+
+    await page.getByTestId('leave-cancel-btn').click()
+    await waitForEditorReady(page)
+
+    await page.getByRole('button', { name: 'Home' }).click()
+    await page.getByTestId('leave-keep-btn').click()
+
+    // Home reloads the app, and the session is no longer the one to resume.
+    await expectClosed(page)
+    expect(inProgress(sessions)).toBeDefined()
+  })
+
+  test('the workspace switch asks through the router guard', async ({
+    page,
+  }) => {
+    await page.getByTestId('nav-rail-workspaces').click()
+    await expect(leaveDialog(page)).toBeVisible()
+
+    // Cancelling cancels the navigation too.
+    await page.getByTestId('leave-cancel-btn').click()
+    await waitForEditorReady(page)
+    await expect(page.getByTestId('datastreams-table')).toHaveCount(0)
+
+    await page.getByTestId('nav-rail-workspaces').click()
+    await page.getByTestId('leave-keep-btn').click()
+
+    await expect(page.getByTestId('workspace-current-hint')).toBeVisible({
+      timeout: 30_000,
+    })
+  })
+
+  test('logging out asks first', async ({ page }) => {
+    await page.getByTestId('nav-rail-logout').click()
+    await expect(leaveDialog(page)).toBeVisible()
+
+    await page.getByTestId('leave-cancel-btn').click()
+    await waitForEditorReady(page)
+    expect(inProgress(sessions)).toBeDefined()
+  })
+
+  test('editing another datastream asks about the open session', async ({
+    page,
+  }) => {
+    await goToSelect(page)
+
+    // Datastream B has no managed datastream yet, so Edit goes through the
+    // create step before it can take the editor over.
+    await page.getByTestId(`edit-datastream-${DATASTREAM_ID_B}`).click()
+    await page.getByTestId('create-processing-level').click()
+    await page
+      .getByRole('option', { name: 'Quality controlled', exact: true })
+      .click()
+    await page.getByTestId('create-confirm').click()
+    await page.getByTestId('session-window-start').click()
+
+    await expect(leaveDialog(page)).toBeVisible()
+    await page.getByTestId('leave-cancel-btn').click()
+
+    // The original session is still the one being edited.
+    await expect(editPanel(page)).toContainText('Streamflow Datastream (QC)')
+  })
+})
diff --git a/apps/qc-app/e2e/navigation.spec.ts b/apps/qc-app/e2e/navigation.spec.ts
index 45efdb29..f73a7256 100644
--- a/apps/qc-app/e2e/navigation.spec.ts
+++ b/apps/qc-app/e2e/navigation.spec.ts
@@ -4,7 +4,7 @@
  *   - picking a workspace navigates to Home
  *   - the nav-rail workspace-switch button offers to revisit the picker
  *   - the edit rail item is enabled only while an edit target is set
- *   - leaving the workspace with edits not saved to the session asks first
+ *   - leaving the workspace runs the leave flow (see leave-session.spec.ts)
  *   - switching to the Select view keeps the session and asks nothing
  */
 
@@ -28,8 +28,8 @@ async function applyChangeValues(page: Page) {
   await expectHistoryContains(page, 'Change Values')
 }
 
-function exitDialog(page: Page) {
-  return page.getByRole('dialog').filter({ hasText: 'Unsaved edits' })
+function leaveDialog(page: Page) {
+  return page.getByTestId('leave-session-dialog')
 }
 
 function expectWorkspacePicker(page: Page) {
@@ -43,7 +43,10 @@ test.describe('navigation', () => {
     await installMocks(page, { qcHistories: true })
   })
 
+  // The two picker tests boot the app without a seeded workspace, so they
+  // pay the dev server's cold start when this file runs first.
   test('fresh browser redirects to the workspace picker', async ({ page }) => {
+    test.slow()
     await page.goto('/')
     // Either still /workspaces, or the picker's select button is visible.
     await expect(
@@ -55,6 +58,7 @@ test.describe('navigation', () => {
   })
 
   test('picking a workspace lands the user on Home', async ({ page }) => {
+    test.slow()
     // Don't use `gotoHome` here: it pre-seeds localStorage and skips
     // the picker. We want to exercise the actual pick flow.
     await page.goto('/')
@@ -119,15 +123,15 @@ test.describe('navigation: leaving a QC session', () => {
     await setupEditView(page)
   })
 
-  test('Save & continue saves the draft without posting observations', async ({
+  test('the workspace switch saves the draft on the way out', async ({
     page,
   }) => {
     await applyChangeValues(page)
 
     await page.getByTestId('nav-rail-workspaces').click()
-    const dialog = exitDialog(page)
-    await expect(dialog).toBeVisible()
-    await dialog.getByRole('button', { name: /save & continue/i }).click()
+    const dialog = leaveDialog(page)
+    await expect(dialog).toContainText('Unsaved edits')
+    await page.getByTestId('leave-save-btn').click()
 
     await expectWorkspacePicker(page)
     const session = sessions.find((s) => s.status === 'in_progress')
@@ -137,30 +141,21 @@ test.describe('navigation: leaving a QC session', () => {
     expect(submissions).toHaveLength(0)
   })
 
-  test('Discard continues without saving', async ({ page }) => {
+  test('cancelling the workspace switch cancels the navigation', async ({
+    page,
+  }) => {
     await applyChangeValues(page)
 
     await page.getByTestId('nav-rail-workspaces').click()
-    const dialog = exitDialog(page)
-    await expect(dialog).toBeVisible()
-    await dialog.getByRole('button', { name: /discard/i }).click()
+    await expect(leaveDialog(page)).toBeVisible()
+    await page.getByTestId('leave-cancel-btn').click()
 
-    await expectWorkspacePicker(page)
-    const session = sessions.find((s) => s.status === 'in_progress')
-    expect(session!.operations).toHaveLength(0)
+    await expect(leaveDialog(page)).toHaveCount(0)
+    await expect(page.getByTestId('edit-plot-column')).toBeVisible()
+    await expectHistoryContains(page, 'Change Values')
     expect(submissions).toHaveLength(0)
   })
 
-  test('saved edits leave without asking', async ({ page }) => {
-    await applyChangeValues(page)
-    await page.getByTestId('exit-save-btn').click()
-    await expect(page.getByText('Draft saved.')).toBeVisible()
-
-    await page.getByTestId('nav-rail-workspaces').click()
-    await expectWorkspacePicker(page)
-    await expect(exitDialog(page)).toHaveCount(0)
-  })
-
   // Switching views is not an exit, so it never asks.
   test('the Select view keeps the session with unsaved edits', async ({
     page,
@@ -171,7 +166,7 @@ test.describe('navigation: leaving a QC session', () => {
     await expect(page.getByTestId('datastreams-table')).toBeVisible({
       timeout: 30_000,
     })
-    await expect(exitDialog(page)).toHaveCount(0)
+    await expect(leaveDialog(page)).toHaveCount(0)
     await expect(page.getByTestId('edit-target-panel')).toBeVisible()
     const session = sessions.find((s) => s.status === 'in_progress')
     expect(session!.operations).toHaveLength(0)
diff --git a/apps/qc-app/src/App.vue b/apps/qc-app/src/App.vue
index 37df6228..ea40d608 100644
--- a/apps/qc-app/src/App.vue
+++ b/apps/qc-app/src/App.vue
@@ -6,11 +6,14 @@
     </v-main>
 
     <Notifications />
+    <!-- One dialog for every exit from an edit session, wherever it starts. -->
+    <LeaveSessionDialog />
   </v-app>
 </template>
 
 <script setup lang="ts">
 import Notifications from '@/components/base/Notifications.vue'
+import LeaveSessionDialog from '@/components/EditData/LeaveSessionDialog.vue'
 import FullScreenLoader from '@/components/base/FullScreenLoader.vue'
 
 import { setupRouteGuards } from '@/router/router'
diff --git a/apps/qc-app/src/components/EditData/LeaveSessionDialog.vue b/apps/qc-app/src/components/EditData/LeaveSessionDialog.vue
new file mode 100644
index 00000000..23b83442
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/LeaveSessionDialog.vue
@@ -0,0 +1,155 @@
+<template>
+  <v-dialog :model-value="!!leavePrompt" max-width="520" persistent>
+    <v-card v-if="leavePrompt" rounded="lg" data-testid="leave-session-dialog">
+      <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
+        <v-avatar :color="copy.color" variant="tonal" size="40">
+          <v-icon :icon="copy.icon" size="22" />
+        </v-avatar>
+        <div class="d-flex flex-column">
+          <div class="text-title-large font-weight-bold">{{ copy.title }}</div>
+          <div class="text-body-small text-medium-emphasis">
+            {{ copy.subtitle }}
+          </div>
+        </div>
+      </div>
+
+      <v-card-text class="text-body-medium pt-2 pb-4 px-6">
+        {{ copy.body }}
+      </v-card-text>
+
+      <v-divider />
+
+      <v-card-actions class="d-flex align-center ga-2 px-4 py-3">
+        <v-btn
+          variant="text"
+          data-testid="leave-cancel-btn"
+          :disabled="!!leaveWork"
+          @click="cancelLeave"
+        >
+          Cancel
+        </v-btn>
+        <v-spacer />
+
+        <template v-if="leavePrompt.kind === 'unsaved'">
+          <v-btn
+            color="error"
+            variant="tonal"
+            prepend-icon="mdi-backup-restore"
+            data-testid="leave-discard-edits-btn"
+            :disabled="!!leaveWork"
+            :loading="leaveWork === 'discard-edits'"
+            @click="discardEditsAndLeave"
+          >
+            Discard changes and close
+          </v-btn>
+          <v-btn
+            color="primary"
+            variant="flat"
+            prepend-icon="mdi-content-save-outline"
+            data-testid="leave-save-btn"
+            :disabled="!!leaveWork || !leavePrompt.canSave"
+            :loading="leaveWork === 'save'"
+            @click="saveAndLeave"
+          >
+            Save and close
+          </v-btn>
+        </template>
+
+        <template v-else-if="leavePrompt.kind === 'empty'">
+          <v-btn
+            color="error"
+            variant="tonal"
+            prepend-icon="mdi-delete-outline"
+            data-testid="leave-discard-session-btn"
+            :disabled="!!leaveWork"
+            :loading="leaveWork === 'discard-session'"
+            @click="discardSessionAndLeave"
+          >
+            Discard session
+          </v-btn>
+          <v-btn
+            color="primary"
+            variant="flat"
+            prepend-icon="mdi-pause-circle-outline"
+            data-testid="leave-keep-btn"
+            :disabled="!!leaveWork"
+            @click="keepSession"
+          >
+            Keep session
+          </v-btn>
+        </template>
+
+        <template v-else>
+          <v-btn
+            color="primary"
+            variant="flat"
+            prepend-icon="mdi-close"
+            data-testid="leave-close-btn"
+            :disabled="!!leaveWork"
+            @click="closeSession"
+          >
+            Close
+          </v-btn>
+        </template>
+      </v-card-actions>
+    </v-card>
+  </v-dialog>
+</template>
+
+<script setup lang="ts">
+/**
+ * The one dialog for leaving an edit session. Every exit routes through
+ * `useLeaveSession`, which decides which of the three cases applies and
+ * carries out the answer; this component only shows it.
+ */
+
+import { computed } from 'vue'
+import { useLeaveSession } from '@/composables/useLeaveSession'
+
+const {
+  leavePrompt,
+  leaveWork,
+  cancelLeave,
+  keepSession,
+  closeSession,
+  saveAndLeave,
+  discardEditsAndLeave,
+  discardSessionAndLeave,
+} = useLeaveSession()
+
+const RESUME = "Resume it any time from the datastream's Edit button."
+
+const copy = computed(() => {
+  const prompt = leavePrompt.value
+  if (prompt?.kind === 'unsaved') {
+    const count = prompt.unsavedCount
+    return {
+      color: 'warning',
+      icon: 'mdi-content-save-alert-outline',
+      title: 'Unsaved edits',
+      subtitle: count
+        ? `${count} edit${count === 1 ? '' : 's'} not yet saved to the session`
+        : 'You have changes that are not in the session',
+      body: prompt.canSave
+        ? `Save them to the session, or discard them for good. Either way the session stays in progress. ${RESUME}`
+        : `No session is open, so these edits cannot be saved. Discarding them closes the editor.`,
+    }
+  }
+  if (prompt?.kind === 'empty') {
+    return {
+      color: 'error',
+      icon: 'mdi-help-circle-outline',
+      title: 'This session has no edits',
+      subtitle: 'Nothing saved, nothing unsaved',
+      body: `Keep it and it stays in progress. ${RESUME} Discard it and it is deleted from the server.`,
+    }
+  }
+  return {
+    color: 'primary',
+    icon: 'mdi-pause-circle-outline',
+    title: 'Close this session?',
+    subtitle: 'Every edit is saved to the session',
+    body: `Your work stays where it is. The session stays in progress. ${RESUME}`,
+  }
+})
+</script>
diff --git a/apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts
new file mode 100644
index 00000000..4764e454
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts
@@ -0,0 +1,151 @@
+import { mount } from '@vue/test-utils'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+import { createTestVuetify } from '@/utils/test/vuetify'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+// jsdom has no visualViewport; Vuetify's overlay positioning reads it.
+;(globalThis as any).visualViewport ||= {
+  addEventListener() {},
+  removeEventListener() {},
+  offsetLeft: 0,
+  offsetTop: 0,
+  width: 1024,
+  height: 768,
+  scale: 1,
+}
+
+const {
+  leavePrompt,
+  leaveWork,
+  cancelLeave,
+  keepSession,
+  closeSession,
+  saveAndLeave,
+  discardEditsAndLeave,
+  discardSessionAndLeave,
+} = vi.hoisted(() => {
+  const { ref: r } = require('vue') as typeof import('vue')
+  return {
+    leavePrompt: r<Record<string, unknown> | null>(null),
+    leaveWork: r<string | null>(null),
+    cancelLeave: vi.fn(),
+    keepSession: vi.fn(),
+    closeSession: vi.fn(),
+    saveAndLeave: vi.fn(),
+    discardEditsAndLeave: vi.fn(),
+    discardSessionAndLeave: vi.fn(),
+  }
+})
+
+vi.mock('@/composables/useLeaveSession', () => ({
+  useLeaveSession: () => ({
+    leavePrompt,
+    leaveWork,
+    cancelLeave,
+    keepSession,
+    closeSession,
+    saveAndLeave,
+    discardEditsAndLeave,
+    discardSessionAndLeave,
+  }),
+}))
+
+import LeaveSessionDialog from '@/components/EditData/LeaveSessionDialog.vue'
+
+const mountDialog = () =>
+  mount(LeaveSessionDialog, {
+    attachTo: document.body,
+    global: { plugins: [createTestPinia(), createTestVuetify()] },
+  })
+
+const dialogText = () =>
+  document.querySelector('[data-testid="leave-session-dialog"]')?.textContent ??
+  ''
+
+const button = (testId: string) =>
+  document.querySelector<HTMLElement>(`[data-testid="${testId}"]`)
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  document.body.innerHTML = ''
+  leavePrompt.value = null
+  leaveWork.value = null
+})
+
+describe('LeaveSessionDialog', () => {
+  it('shows nothing while no exit is pending', () => {
+    mountDialog()
+    expect(button('leave-session-dialog')).toBeNull()
+  })
+
+  it('offers save, discard and cancel for unsaved edits', async () => {
+    leavePrompt.value = { kind: 'unsaved', unsavedCount: 2, canSave: true }
+    const wrapper = mountDialog()
+    await wrapper.vm.$nextTick()
+
+    const text = dialogText()
+    expect(text).toContain('2 edits')
+    expect(text).toContain('Resume it')
+    expect(button('leave-save-btn')).not.toBeNull()
+    expect(button('leave-save-btn')?.getAttribute('disabled')).toBeNull()
+    expect(button('leave-discard-edits-btn')).not.toBeNull()
+
+    button('leave-discard-edits-btn')?.click()
+    expect(discardEditsAndLeave).toHaveBeenCalled()
+    button('leave-save-btn')?.click()
+    expect(saveAndLeave).toHaveBeenCalled()
+    button('leave-cancel-btn')?.click()
+    expect(cancelLeave).toHaveBeenCalled()
+  })
+
+  it('disables saving when no session is open', async () => {
+    leavePrompt.value = { kind: 'unsaved', unsavedCount: 1, canSave: false }
+    const wrapper = mountDialog()
+    await wrapper.vm.$nextTick()
+
+    expect(button('leave-save-btn')?.getAttribute('disabled')).not.toBeNull()
+    expect(dialogText()).toContain('No session is open')
+  })
+
+  it('offers keep or discard for a session with no edits', async () => {
+    leavePrompt.value = { kind: 'empty', unsavedCount: 0, canSave: true }
+    const wrapper = mountDialog()
+    await wrapper.vm.$nextTick()
+
+    expect(dialogText()).toContain('no edits')
+    expect(button('leave-save-btn')).toBeNull()
+
+    button('leave-keep-btn')?.click()
+    expect(keepSession).toHaveBeenCalled()
+    button('leave-discard-session-btn')?.click()
+    expect(discardSessionAndLeave).toHaveBeenCalled()
+  })
+
+  it('says the session stays in progress when everything is saved', async () => {
+    leavePrompt.value = { kind: 'saved', unsavedCount: 0, canSave: true }
+    const wrapper = mountDialog()
+    await wrapper.vm.$nextTick()
+
+    expect(dialogText()).toContain('stays in progress')
+    expect(button('leave-discard-session-btn')).toBeNull()
+
+    button('leave-close-btn')?.click()
+    expect(closeSession).toHaveBeenCalled()
+  })
+
+  it('locks the choices while one is running', async () => {
+    leavePrompt.value = { kind: 'unsaved', unsavedCount: 1, canSave: true }
+    leaveWork.value = 'save'
+    const wrapper = mountDialog()
+    await wrapper.vm.$nextTick()
+
+    expect(button('leave-cancel-btn')?.getAttribute('disabled')).not.toBeNull()
+    expect(
+      button('leave-discard-edits-btn')?.getAttribute('disabled')
+    ).not.toBeNull()
+  })
+})
diff --git a/apps/qc-app/src/components/Navigation/NavigationRail.vue b/apps/qc-app/src/components/Navigation/NavigationRail.vue
index 881e4b5f..bfddc95f 100644
--- a/apps/qc-app/src/components/Navigation/NavigationRail.vue
+++ b/apps/qc-app/src/components/Navigation/NavigationRail.vue
@@ -4,7 +4,7 @@
       <button
         class="home-icon-btn"
         aria-label="Home"
-        @click="guardExit(goHome)"
+        @click="goHome"
       >
         <v-img
           :src="HydroServerIcon"
@@ -83,7 +83,7 @@
             v-bind="tipProps"
             class="rail-btn rail-btn-secondary"
             data-testid="nav-rail-workspaces"
-            @click.prevent="guardExit(onSwitchWorkspace)"
+            @click.prevent="onSwitchWorkspace"
           >
             <span class="rail-pill rail-pill-secondary">
               <v-icon icon="mdi-briefcase-outline" size="22" />
@@ -103,7 +103,8 @@
           <button
             v-bind="tipProps"
             class="rail-btn rail-btn-secondary"
-            @click.prevent="guardExit(onLogout)"
+            data-testid="nav-rail-logout"
+            @click.prevent="onLogout"
           >
             <span class="rail-pill rail-pill-secondary">
               <v-icon icon="mdi-logout" size="22" />
@@ -117,67 +118,10 @@
   </v-navigation-drawer>
 
   <SelectDrawer v-if="isDrawerOpen && selectedDrawer === DrawerType.Select" />
-
-  <v-dialog v-model="showExitConfirm" max-width="520" persistent>
-    <v-card rounded="lg">
-      <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
-        <v-avatar color="warning" variant="tonal" size="40">
-          <v-icon icon="mdi-alert-outline" size="22" />
-        </v-avatar>
-        <div class="d-flex flex-column">
-          <div class="text-title-large font-weight-bold">Unsaved edits</div>
-          <div class="text-body-small text-medium-emphasis">
-            <template v-if="unsavedEditCount > 0">
-              {{ unsavedEditCount }} edit{{ unsavedEditCount === 1 ? '' : 's' }}
-              not yet saved to the session
-            </template>
-            <template v-else>You have unsaved changes</template>
-          </div>
-        </div>
-      </div>
-      <v-card-text class="text-body-medium pt-2 pb-4 px-6">
-        <template v-if="canSave">
-          Save your edits to the session before leaving, or discard the edits
-          made since the last save. Discarded edits cannot be recovered.
-        </template>
-        <template v-else>
-          No session is open, so these edits cannot be saved. Discard them to
-          continue. Discarded edits cannot be recovered.
-        </template>
-      </v-card-text>
-      <v-divider />
-      <v-card-actions class="d-flex align-center ga-2 px-4 py-3">
-        <v-btn variant="text" :disabled="isBusy" @click="cancelExit">
-          Cancel
-        </v-btn>
-        <v-spacer />
-        <v-btn
-          color="error"
-          variant="tonal"
-          prepend-icon="mdi-delete-outline"
-          :disabled="isBusy"
-          :loading="isBusy && exitAction === 'discard'"
-          @click="discardAndContinue"
-        >
-          Discard
-        </v-btn>
-        <v-btn
-          color="primary"
-          variant="flat"
-          prepend-icon="mdi-content-save-outline"
-          :disabled="isBusy || !canSave"
-          :loading="isBusy && exitAction === 'save'"
-          @click="saveAndContinue"
-        >
-          Save &amp; continue
-        </v-btn>
-      </v-card-actions>
-    </v-card>
-  </v-dialog>
 </template>
 
 <script setup lang="ts">
-import { computed, ref } from 'vue'
+import { ref } from 'vue'
 import HydroServerIcon from '@/assets/icon-color-thick.svg'
 import SelectDrawer from '@/components/Navigation/SelectDrawer.vue'
 import PerformanceCalibration from '@/components/Navigation/PerformanceCalibration.vue'
@@ -188,9 +132,6 @@ import { useDataVisStore } from '@/store/dataVisualization'
 import router from '@/router/router'
 import { useHydroServer } from '@/store/hydroserver'
 import { useWorkspaceStore } from '@/store/workspaces'
-import { usePlotlyStore } from '@/store/plotly'
-import { useQcSessionStore } from '@/store/qcSession'
-import { useEditSession } from '@/composables/useEditSession'
 import { useEditEntry } from '@/composables/useEditEntry'
 
 const { onRailItemClicked } = useUIStore()
@@ -200,86 +141,12 @@ const { qcDatastream } = storeToRefs(useDataVisStore())
 const { hs } = storeToRefs(useHydroServer())
 const workspaceStore = useWorkspaceStore()
 const { selectedWorkspace } = storeToRefs(workspaceStore)
-const { redraw } = usePlotlyStore()
-const { inProgressSession } = storeToRefs(useQcSessionStore())
-const { hasUnsavedChanges, unsavedEditCount, saveDraft, discardUnsavedEdits } =
-  useEditSession()
-const { leaveEdit } = useEditEntry()
-
-// Home, the workspace switch and log out are the exits that end the session.
-const needsExitConfirm = computed(
-  () => !!qcDatastream.value && hasUnsavedChanges.value
-)
-// With no in-progress session there is nothing to save to.
-const canSave = computed(() => !!inProgressSession.value)
-
-const showExitConfirm = ref(false)
-const exitAction = ref<'save' | 'discard' | null>(null)
-const isBusy = ref(false)
-let pendingAction: (() => void | Promise<void>) | null = null
-
-function guardExit(action: () => void | Promise<void>) {
-  if (needsExitConfirm.value) {
-    pendingAction = action
-    showExitConfirm.value = true
-  } else {
-    action()
-  }
-}
-
-function cancelExit() {
-  if (isBusy.value) return
-  pendingAction = null
-  showExitConfirm.value = false
-}
-
-async function continueExit() {
-  const next = pendingAction
-  pendingAction = null
-  showExitConfirm.value = false
-  await next?.()
-}
-
-async function saveAndContinue() {
-  if (isBusy.value || !canSave.value) return
-  isBusy.value = true
-  exitAction.value = 'save'
-  try {
-    await saveDraft()
-    Snackbar.success('Draft saved.')
-  } catch (e) {
-    Snackbar.error(e instanceof Error ? e.message : 'Could not save the draft.')
-    return
-  } finally {
-    isBusy.value = false
-    exitAction.value = null
-  }
-  await continueExit()
-}
-
-async function discardAndContinue() {
-  if (isBusy.value) return
-  isBusy.value = true
-  exitAction.value = 'discard'
-  try {
-    await discardUnsavedEdits()
-    await redraw()
-  } catch (e) {
-    Snackbar.error(
-      e instanceof Error ? e.message : 'Could not discard the edits.'
-    )
-    return
-  } finally {
-    isBusy.value = false
-    exitAction.value = null
-  }
-  await continueExit()
-}
+const { closeEditor } = useEditEntry()
 
+// Home and log out reload the page, so they end the session themselves.
 async function goHome() {
+  if (!(await closeEditor())) return
   resetState()
-  // Clears the resume pointer, or the reload would reopen the editor.
-  await leaveEdit()
   window.location.assign('/')
 }
 
@@ -297,6 +164,7 @@ function onMainRailItemClicked(item: DrawerType) {
 }
 
 async function onLogout() {
+  if (!(await closeEditor())) return
   await hs.value.session.logout()
   workspaceStore.clearSelection()
   Snackbar.info('You have logged out')
@@ -304,11 +172,10 @@ async function onLogout() {
 }
 
 async function onSwitchWorkspace() {
-  // Navigate BEFORE mutating refs: VisualizeData's deep watcher syncs
-  // filters to router.replace, racing our push and stranding the user.
+  // In-app navigation goes through the router's leave guard, which asks and
+  // tears the session down once the navigation is on its way.
   // `switch=1` prevents the Workspaces picker from auto-redirecting back.
   await router.push({ name: 'Workspaces', query: { switch: '1' } })
-  await leaveEdit()
 }
 </script>
 
diff --git a/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts b/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
index 0a9e8261..5209c2aa 100644
--- a/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
+++ b/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
@@ -20,6 +20,8 @@ const {
   isDrawerOpen,
   resumeDatastreamId,
   hasUnsavedChanges,
+  inProgressSession,
+  hasSessionOperations,
   push,
   assign,
   showView,
@@ -45,6 +47,8 @@ const {
     isDrawerOpen,
     resumeDatastreamId: r<string | null>(null),
     hasUnsavedChanges: r(false),
+    inProgressSession: r<{ id: string } | null>(null),
+    hasSessionOperations: r(false),
     push: vi.fn(async () => {}),
     assign: vi.fn(),
     showView: vi.fn((view: string) => {
@@ -92,7 +96,11 @@ vi.mock('@/store/qcSession', async () => {
   return {
     useQcSessionStore: defineStore('qcSession', () => ({
       resumeDatastreamId,
-      inProgressSession: ref(null),
+      historyId: ref('h-1'),
+      sessions: ref([]),
+      inProgressSession,
+      isReadOnly: ref(false),
+      hasSessionOperations,
     })),
   }
 })
@@ -101,7 +109,9 @@ vi.mock('@/store/hydroserver', async () => {
   const { defineStore } = await import('pinia')
   const { ref } = await import('vue')
   return {
-    useHydroServer: defineStore('hydroserver', () => ({ hs: ref({}) })),
+    useHydroServer: defineStore('hydroserver', () => ({
+      hs: ref({ session: { logout: vi.fn() } }),
+    })),
   }
 })
 
@@ -151,6 +161,7 @@ vi.mock('@/components/Navigation/PerformanceCalibration.vue', () => ({
 }))
 
 import NavigationRail from '@/components/Navigation/NavigationRail.vue'
+import { useLeaveSession } from '@/composables/useLeaveSession'
 
 function mountRail() {
   return mount(
@@ -229,6 +240,8 @@ describe('NavigationRail leaving the editor', () => {
     currentView.value = 'Edit'
     selectedDrawer.value = 'Edit'
     hasUnsavedChanges.value = false
+    inProgressSession.value = null
+    hasSessionOperations.value = false
     vi.stubGlobal('location', { ...window.location, assign })
   })
 
@@ -244,11 +257,20 @@ describe('NavigationRail leaving the editor', () => {
     wrapper.unmount()
   })
 
-  it('switching workspace navigates first, then clears the resume pointer', async () => {
-    let pointerAtPush: string | null = null
-    push.mockImplementationOnce(async () => {
-      pointerAtPush = resumeDatastreamId.value
-    })
+  it('logging out ends the session first', async () => {
+    const wrapper = mountRail()
+    await wrapper.find('[data-testid="nav-rail-logout"]').trigger('click')
+    await flushPromises()
+
+    expect(resumeDatastreamId.value).toBeNull()
+    expect(qcDatastreamId.value).toBeNull()
+    expect(assign).toHaveBeenCalledWith('/login')
+    wrapper.unmount()
+  })
+
+  // The router guard runs the leave flow for in-app navigation, so the rail
+  // only navigates.
+  it('switching workspace navigates and leaves the session to the guard', async () => {
     const wrapper = mountRail()
     await wrapper.find('[data-testid="nav-rail-workspaces"]').trigger('click')
     await flushPromises()
@@ -257,20 +279,27 @@ describe('NavigationRail leaving the editor', () => {
       name: 'Workspaces',
       query: { switch: '1' },
     })
-    expect(pointerAtPush).toBe('m-1')
-    expect(resumeDatastreamId.value).toBeNull()
-    expect(clearEditTarget).toHaveBeenCalled()
-    expect(currentView.value).toBe('Select')
+    expect(clearEditTarget).not.toHaveBeenCalled()
+    expect(resumeDatastreamId.value).toBe('m-1')
     wrapper.unmount()
   })
 
-  it('asks before leaving with unsaved edits', async () => {
+  it('asks about unsaved edits and stays put when the user cancels', async () => {
     hasUnsavedChanges.value = true
+    inProgressSession.value = { id: 'qcs-1' }
     const wrapper = mountRail()
-    await wrapper.find('[data-testid="nav-rail-workspaces"]').trigger('click')
+    await wrapper.find('button[aria-label="Home"]').trigger('click')
     await flushPromises()
 
-    expect(push).not.toHaveBeenCalled()
+    const { leavePrompt, cancelLeave } = useLeaveSession()
+    expect(leavePrompt.value?.kind).toBe('unsaved')
+    expect(assign).not.toHaveBeenCalled()
+
+    cancelLeave()
+    await flushPromises()
+
+    expect(assign).not.toHaveBeenCalled()
+    expect(qcDatastreamId.value).toBe('m-1')
     expect(resumeDatastreamId.value).toBe('m-1')
     wrapper.unmount()
   })
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index 31e93eeb..ecff27ba 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -422,45 +422,6 @@
         </v-card-actions>
       </v-card>
     </v-dialog>
-
-    <v-dialog v-model="showCloseConfirm" max-width="520">
-      <v-card rounded="lg">
-        <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
-          <v-avatar color="primary" variant="tonal" size="40">
-            <v-icon icon="mdi-content-save-outline" size="22" />
-          </v-avatar>
-          <div class="d-flex flex-column">
-            <div class="text-title-large font-weight-bold">Save before closing?</div>
-            <div class="text-body-small text-medium-emphasis">
-              <template v-if="unsavedEditCount > 0">
-                {{ unsavedEditCount }} edit{{ unsavedEditCount === 1 ? '' : 's' }}
-                not yet saved to the session
-              </template>
-              <template v-else> You have unsaved changes </template>
-            </div>
-          </div>
-        </div>
-        <v-card-text class="text-body-medium pt-2 pb-4 px-6">
-          Save your edits to the in-progress session before closing, or close
-          without saving (unsaved changes are dropped; previously-saved draft
-          operations stay in the session).
-        </v-card-text>
-        <v-divider />
-        <v-card-actions class="d-flex align-center ga-2 px-4 py-3">
-          <v-btn variant="text" @click="showCloseConfirm = false">Cancel</v-btn>
-          <v-spacer />
-          <v-btn variant="text" @click="closeWithoutSaving">Close without saving</v-btn>
-          <v-btn
-            color="primary"
-            variant="flat"
-            prepend-icon="mdi-content-save-outline"
-            @click="saveDraftAndClose"
-          >
-            Save &amp; close
-          </v-btn>
-        </v-card-actions>
-      </v-card>
-    </v-dialog>
 </div>
 
   <!-- One plot for both layouts. Moving it instead of rebuilding it is what
@@ -586,7 +547,7 @@ async function onViewSession(sessionId: string) {
 const { isReadOnly, inProgressSession, viewedSession, resumeDatastreamId } =
   storeToRefs(useQcSessionStore())
 const { canEdit, roleName } = useWorkspacePermissions()
-const { leaveEdit } = useEditEntry()
+const { closeEditor } = useEditEntry()
 
 // Gate the editor footer so a read-only collaborator sees a disabled state
 // instead of a 403 mid-flow.
@@ -638,7 +599,6 @@ useResumeEditSession(async (id) => {
 
 const editCount = computed(() => editHistory.value?.length ?? 0)
 const showCommitConfirm = ref(false)
-const showCloseConfirm = ref(false)
 const commitDescription = ref('')
 const isSavingDraft = ref(false)
 const isCommitting = ref(false)
@@ -742,10 +702,6 @@ const historyPaneStyle = computed(() => {
   return { flex: '1 1 auto' }
 })
 
-function exitToSelect() {
-  void leaveEdit()
-}
-
 async function onDiscardUnsaved() {
   showDiscardConfirm.value = false
   isDiscarding.value = true
@@ -776,10 +732,6 @@ async function onSaveDraft(): Promise<boolean> {
   }
 }
 
-async function onSaveAndClose() {
-  if (await onSaveDraft()) exitToSelect()
-}
-
 // Prefill the description with the session's current one so committing
 // preserves/edits it rather than blanking it.
 function openCommit() {
@@ -801,24 +753,10 @@ async function onCommit() {
   }
 }
 
+// Closing goes through the one leave flow, which decides what happens to the
+// session and says so before anything is lost.
 function requestClose() {
-  // Only prompt to save when there are edits not yet persisted to the
-  // session; otherwise close straight back to the select view.
-  if (hasUnsavedChanges.value) {
-    showCloseConfirm.value = true
-  } else {
-    exitToSelect()
-  }
-}
-
-async function saveDraftAndClose() {
-  showCloseConfirm.value = false
-  await onSaveAndClose()
-}
-
-function closeWithoutSaving() {
-  showCloseConfirm.value = false
-  exitToSelect()
+  void closeEditor()
 }
 
 // Hydrate state from the URL once datastream metadata is available.
diff --git a/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts b/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
index 5acec515..0ef62f30 100644
--- a/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
@@ -17,7 +17,8 @@ const {
   resumeDatastreamId,
   success,
   error,
-  canLeaveSession,
+  requestLeave,
+  forgetSession,
   ResumeSupersededError,
 } = vi.hoisted(() => {
   const { ref: r } = require('vue') as typeof import('vue')
@@ -27,6 +28,7 @@ const {
       this.name = 'ResumeSupersededError'
     }
   }
+  const resumeDatastreamId = r<string | null>(null)
   return {
     qcDatastream: r<{ id: string } | null>(null),
     setEditTarget: vi.fn(),
@@ -38,10 +40,13 @@ const {
     currentView: r('Select'),
     selectedDrawer: r('Select'),
     isDrawerOpen: r(false),
-    resumeDatastreamId: r<string | null>(null),
+    resumeDatastreamId,
     success: vi.fn(),
     error: vi.fn(),
-    canLeaveSession: vi.fn(),
+    requestLeave: vi.fn(),
+    forgetSession: vi.fn(() => {
+      resumeDatastreamId.value = null
+    }),
     ResumeSupersededError,
   }
 })
@@ -76,7 +81,7 @@ vi.mock('@/store/userInterface', async () => {
 })
 
 vi.mock('@/composables/useLeaveSession', () => ({
-  useLeaveSession: () => ({ canLeaveSession }),
+  useLeaveSession: () => ({ requestLeave, forgetSession }),
 }))
 
 vi.mock('@/store/qcSession', async () => {
@@ -124,7 +129,7 @@ beforeEach(() => {
   })
   beginEditing.mockResolvedValue(true)
   startSession.mockResolvedValue(undefined)
-  canLeaveSession.mockResolvedValue(true)
+  requestLeave.mockResolvedValue(true)
 })
 
 describe('useEditEntry', () => {
@@ -308,7 +313,7 @@ describe('useEditEntry', () => {
 
   it('asks before taking over from another open session', async () => {
     qcDatastream.value = { id: 'other' }
-    canLeaveSession.mockResolvedValueOnce(false)
+    requestLeave.mockResolvedValueOnce(false)
     expect(await useEditEntry().enterEdit('mgd')).toBe('kept')
     expect(setEditTarget).not.toHaveBeenCalled()
     expect(qcDatastream.value).toEqual({ id: 'other' })
@@ -318,7 +323,7 @@ describe('useEditEntry', () => {
   it('takes over when leaving the open session is allowed', async () => {
     qcDatastream.value = { id: 'other' }
     expect(await useEditEntry().enterEdit('mgd')).toBe('editing')
-    expect(canLeaveSession).toHaveBeenCalled()
+    expect(requestLeave).toHaveBeenCalled()
     expect(setEditTarget).toHaveBeenCalledWith('mgd')
   })
 
@@ -341,4 +346,30 @@ describe('useEditEntry', () => {
     expect(resumeDatastreamId.value).toBeNull()
     expect(clearEditTarget).toHaveBeenCalled()
   })
+
+  it('closeEditor ends the session once the user agrees to leave', async () => {
+    qcDatastream.value = { id: 'mgd' }
+    resumeDatastreamId.value = 'mgd'
+    currentView.value = 'Edit'
+
+    expect(await useEditEntry().closeEditor()).toBe(true)
+
+    expect(requestLeave).toHaveBeenCalled()
+    expect(clearEditTarget).toHaveBeenCalled()
+    expect(currentView.value).toBe('Select')
+    expect(resumeDatastreamId.value).toBeNull()
+  })
+
+  it('closeEditor leaves everything in place when the user stays', async () => {
+    qcDatastream.value = { id: 'mgd' }
+    resumeDatastreamId.value = 'mgd'
+    currentView.value = 'Edit'
+    requestLeave.mockResolvedValueOnce(false)
+
+    expect(await useEditEntry().closeEditor()).toBe(false)
+
+    expect(clearEditTarget).not.toHaveBeenCalled()
+    expect(currentView.value).toBe('Edit')
+    expect(resumeDatastreamId.value).toBe('mgd')
+  })
 })
diff --git a/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts b/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
new file mode 100644
index 00000000..62a4bd36
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
@@ -0,0 +1,353 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+
+const {
+  qcDatastream,
+  resumeDatastreamId,
+  historyId,
+  sessions,
+  inProgressSession,
+  isReadOnly,
+  hasSessionOperations,
+  hasUnsavedChanges,
+  unsavedEditCount,
+  saveDraft,
+  discardUnsavedEdits,
+  deleteSessionChain,
+  applySessions,
+  redraw,
+  invalidate,
+  success,
+  error,
+} = vi.hoisted(() => {
+  const { ref: r } = require('vue') as typeof import('vue')
+  return {
+    qcDatastream: r<{ id: string } | null>(null),
+    resumeDatastreamId: r<string | null>(null),
+    historyId: r<string | null>('h-1'),
+    sessions: r<Array<{ id: string; dependencyIds?: string[] }>>([]),
+    inProgressSession: r<{ id: string } | null>(null),
+    isReadOnly: r(false),
+    hasSessionOperations: r(false),
+    hasUnsavedChanges: r(false),
+    unsavedEditCount: r(0),
+    saveDraft: vi.fn(),
+    discardUnsavedEdits: vi.fn(),
+    deleteSessionChain: vi.fn(),
+    applySessions: vi.fn(),
+    redraw: vi.fn(),
+    invalidate: vi.fn(),
+    success: vi.fn(),
+    error: vi.fn(),
+  }
+})
+
+vi.mock('@/store/dataVisualization', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useDataVisStore: defineStore('dataVisualization', () => ({ qcDatastream })),
+  }
+})
+
+vi.mock('@/store/qcSession', async () => {
+  const { defineStore } = await import('pinia')
+  return {
+    useQcSessionStore: defineStore('qcSession', () => ({
+      resumeDatastreamId,
+      historyId,
+      sessions,
+      inProgressSession,
+      isReadOnly,
+      hasSessionOperations,
+      applySessions,
+    })),
+  }
+})
+
+vi.mock('@/store/plotly', () => ({ usePlotlyStore: () => ({ redraw }) }))
+
+vi.mock('@/store/workingCopies', () => ({
+  useWorkingCopiesStore: () => ({ invalidate }),
+}))
+
+vi.mock('@/composables/useEditSession', () => ({
+  useEditSession: () => ({
+    hasUnsavedChanges,
+    unsavedEditCount,
+    saveDraft,
+    discardUnsavedEdits,
+  }),
+}))
+
+vi.mock('@/composables/useManagedDatastreams', () => ({
+  useManagedDatastreams: () => ({ deleteSessionChain }),
+}))
+
+vi.mock('@uwrl/qc-utils', () => ({ Snackbar: { success, error } }))
+
+import { useLeaveSession } from '../useLeaveSession'
+
+/** Nothing saved, nothing unsaved: an untouched in-progress session. */
+function emptySession() {
+  qcDatastream.value = { id: 'mgd-1' }
+  inProgressSession.value = { id: 'qcs-1' }
+  sessions.value = [{ id: 'qcs-1' }]
+  hasSessionOperations.value = false
+  hasUnsavedChanges.value = false
+}
+
+beforeEach(() => {
+  createTestPinia()
+  vi.clearAllMocks()
+  qcDatastream.value = { id: 'mgd-1' }
+  resumeDatastreamId.value = 'mgd-1'
+  historyId.value = 'h-1'
+  sessions.value = []
+  inProgressSession.value = null
+  isReadOnly.value = false
+  hasSessionOperations.value = false
+  hasUnsavedChanges.value = false
+  unsavedEditCount.value = 0
+  saveDraft.mockResolvedValue(undefined)
+  discardUnsavedEdits.mockResolvedValue(undefined)
+  deleteSessionChain.mockResolvedValue(['qcs-1'])
+})
+
+describe('leaveCase', () => {
+  it('is none with nothing being edited', () => {
+    qcDatastream.value = null
+    expect(useLeaveSession().leaveCase()).toBe('none')
+  })
+
+  it('is none while viewing committed history', () => {
+    emptySession()
+    isReadOnly.value = true
+    expect(useLeaveSession().leaveCase()).toBe('none')
+  })
+
+  it('is none when the editor is open with no session', () => {
+    expect(useLeaveSession().leaveCase()).toBe('none')
+  })
+
+  it('is unsaved when edits have not reached the session', () => {
+    emptySession()
+    hasUnsavedChanges.value = true
+    expect(useLeaveSession().leaveCase()).toBe('unsaved')
+  })
+
+  it('is empty when the session holds nothing', () => {
+    emptySession()
+    expect(useLeaveSession().leaveCase()).toBe('empty')
+  })
+
+  it('is saved when the session holds operations', () => {
+    emptySession()
+    hasSessionOperations.value = true
+    expect(useLeaveSession().leaveCase()).toBe('saved')
+  })
+})
+
+describe('requestLeave', () => {
+  it('leaves without asking when there is nothing to decide', async () => {
+    const { requestLeave, leavePrompt } = useLeaveSession()
+    expect(await requestLeave()).toBe(true)
+    expect(leavePrompt.value).toBeNull()
+  })
+
+  it('asks about unsaved edits and reports whether saving is possible', async () => {
+    emptySession()
+    hasUnsavedChanges.value = true
+    unsavedEditCount.value = 3
+    const { requestLeave, leavePrompt, cancelLeave } = useLeaveSession()
+    const pending = requestLeave()
+
+    expect(leavePrompt.value).toEqual({
+      kind: 'unsaved',
+      unsavedCount: 3,
+      canSave: true,
+    })
+    cancelLeave()
+    expect(await pending).toBe(false)
+    expect(leavePrompt.value).toBeNull()
+  })
+
+  it('cannot save with no session open', async () => {
+    qcDatastream.value = { id: 'mgd-1' }
+    hasUnsavedChanges.value = true
+    const { requestLeave, leavePrompt, saveAndLeave, cancelLeave } =
+      useLeaveSession()
+    const pending = requestLeave()
+
+    expect(leavePrompt.value?.canSave).toBe(false)
+    await saveAndLeave()
+    expect(saveDraft).not.toHaveBeenCalled()
+    expect(leavePrompt.value).not.toBeNull()
+
+    cancelLeave()
+    await pending
+  })
+
+  it('saves the draft and leaves', async () => {
+    emptySession()
+    hasUnsavedChanges.value = true
+    const { requestLeave, saveAndLeave, leavePrompt } = useLeaveSession()
+    const pending = requestLeave()
+
+    await saveAndLeave()
+
+    expect(saveDraft).toHaveBeenCalled()
+    expect(await pending).toBe(true)
+    expect(leavePrompt.value).toBeNull()
+  })
+
+  it('keeps the user in the session when the save fails', async () => {
+    emptySession()
+    hasUnsavedChanges.value = true
+    saveDraft.mockRejectedValueOnce(new Error('offline'))
+    const { requestLeave, saveAndLeave, leavePrompt, cancelLeave } =
+      useLeaveSession()
+    const pending = requestLeave()
+
+    await saveAndLeave()
+
+    expect(error).toHaveBeenCalledWith('offline')
+    expect(leavePrompt.value?.kind).toBe('unsaved')
+
+    cancelLeave()
+    expect(await pending).toBe(false)
+  })
+
+  it('discards the edits and leaves when the session keeps operations', async () => {
+    emptySession()
+    hasUnsavedChanges.value = true
+    hasSessionOperations.value = true
+    const { requestLeave, discardEditsAndLeave } = useLeaveSession()
+    const pending = requestLeave()
+
+    await discardEditsAndLeave()
+
+    expect(discardUnsavedEdits).toHaveBeenCalled()
+    expect(redraw).toHaveBeenCalled()
+    expect(await pending).toBe(true)
+  })
+
+  it('asks about the empty session when discarding leaves nothing', async () => {
+    emptySession()
+    hasUnsavedChanges.value = true
+    const { requestLeave, discardEditsAndLeave, leavePrompt, keepSession } =
+      useLeaveSession()
+    const pending = requestLeave()
+
+    await discardEditsAndLeave()
+
+    expect(leavePrompt.value?.kind).toBe('empty')
+    keepSession()
+    expect(await pending).toBe(true)
+  })
+
+  it('keeps the user in the session when discarding the edits fails', async () => {
+    emptySession()
+    hasUnsavedChanges.value = true
+    discardUnsavedEdits.mockRejectedValueOnce(new Error('no record'))
+    const { requestLeave, discardEditsAndLeave, leavePrompt, cancelLeave } =
+      useLeaveSession()
+    const pending = requestLeave()
+
+    await discardEditsAndLeave()
+
+    expect(error).toHaveBeenCalledWith('no record')
+    expect(leavePrompt.value?.kind).toBe('unsaved')
+    cancelLeave()
+    expect(await pending).toBe(false)
+  })
+
+  it('keeps an empty session on the server when asked to', async () => {
+    emptySession()
+    const { requestLeave, keepSession } = useLeaveSession()
+    const pending = requestLeave()
+
+    keepSession()
+
+    expect(await pending).toBe(true)
+    expect(deleteSessionChain).not.toHaveBeenCalled()
+  })
+
+  it('deletes only the empty session itself', async () => {
+    emptySession()
+    const { requestLeave, discardSessionAndLeave } = useLeaveSession()
+    const pending = requestLeave()
+
+    await discardSessionAndLeave()
+
+    expect(deleteSessionChain).toHaveBeenCalledWith('h-1', ['qcs-1'])
+    expect(invalidate).toHaveBeenCalledWith('mgd-1')
+    // The store no longer lists a session the server does not have.
+    expect(applySessions).toHaveBeenCalledWith('h-1', [])
+    expect(await pending).toBe(true)
+  })
+
+  it('refuses to discard a session another one was built on', async () => {
+    emptySession()
+    sessions.value = [
+      { id: 'qcs-1' },
+      { id: 'qcs-2', dependencyIds: ['qcs-1'] },
+    ]
+    const { requestLeave, discardSessionAndLeave, leavePrompt, cancelLeave } =
+      useLeaveSession()
+    const pending = requestLeave()
+
+    await discardSessionAndLeave()
+
+    expect(deleteSessionChain).not.toHaveBeenCalled()
+    expect(error).toHaveBeenCalled()
+    expect(leavePrompt.value?.kind).toBe('empty')
+    cancelLeave()
+    expect(await pending).toBe(false)
+  })
+
+  it('keeps the user in the session when the delete fails', async () => {
+    emptySession()
+    deleteSessionChain.mockRejectedValueOnce(new Error('server said no'))
+    const { requestLeave, discardSessionAndLeave, leavePrompt, cancelLeave } =
+      useLeaveSession()
+    const pending = requestLeave()
+
+    await discardSessionAndLeave()
+
+    expect(error).toHaveBeenCalledWith('server said no')
+    expect(leavePrompt.value?.kind).toBe('empty')
+    cancelLeave()
+    expect(await pending).toBe(false)
+  })
+
+  it('asks before leaving a session whose edits are all saved', async () => {
+    emptySession()
+    hasSessionOperations.value = true
+    const { requestLeave, leavePrompt, closeSession } = useLeaveSession()
+    const pending = requestLeave()
+
+    expect(leavePrompt.value?.kind).toBe('saved')
+    closeSession()
+
+    expect(await pending).toBe(true)
+    expect(deleteSessionChain).not.toHaveBeenCalled()
+  })
+
+  it('a second request supersedes the first, which stays put', async () => {
+    emptySession()
+    const { requestLeave, cancelLeave } = useLeaveSession()
+    const first = requestLeave()
+    const second = requestLeave()
+
+    expect(await first).toBe(false)
+    cancelLeave()
+    expect(await second).toBe(false)
+  })
+})
+
+describe('forgetSession', () => {
+  it('drops the pointer that reopens the editor', () => {
+    useLeaveSession().forgetSession()
+    expect(resumeDatastreamId.value).toBeNull()
+  })
+})
diff --git a/apps/qc-app/src/composables/useEditEntry.ts b/apps/qc-app/src/composables/useEditEntry.ts
index 2ba1879c..2aa6587d 100644
--- a/apps/qc-app/src/composables/useEditEntry.ts
+++ b/apps/qc-app/src/composables/useEditEntry.ts
@@ -7,9 +7,9 @@
  * last owns the view, so an entry that finds the target changed after an await
  * stands down without touching view state, the target or the resume pointer.
  *
- * Only `leaveEdit` ends a session. Switching between the Select and Edit views
- * keeps the target, so `openEditor` and an entry on the open target are pure
- * navigation.
+ * Only `closeEditor` and `leaveEdit` end a session. Switching between the
+ * Select and Edit views keeps the target, so `openEditor` and an entry on the
+ * open target are pure navigation.
  */
 
 import { storeToRefs } from 'pinia'
@@ -42,7 +42,7 @@ export function useEditEntry() {
   const { resumeDatastreamId } = storeToRefs(useQcSessionStore())
   const { beginEditing, startSession, needsSession, needsHistory } =
     useEditSession()
-  const { canLeaveSession } = useLeaveSession()
+  const { requestLeave, forgetSession } = useLeaveSession()
 
   const owns = (id: string | undefined) => qcDatastream.value?.id === id
   // A cleared target is not a takeover: leaving again is harmless.
@@ -55,12 +55,22 @@ export function useEditEntry() {
     showView(DrawerType.Edit)
   }
 
+  /** End the session and show the Select view. The caller has already asked
+   *  the user, or there was nothing to ask about. */
   async function leaveEdit() {
     showView(DrawerType.Select)
-    resumeDatastreamId.value = null
+    forgetSession()
     await clearEditTarget()
   }
 
+  /** Leave the editor the way a user asks to: decide what happens to the
+   *  session, then end it. False means they chose to stay. */
+  async function closeEditor(): Promise<boolean> {
+    if (!(await requestLeave())) return false
+    await leaveEdit()
+    return true
+  }
+
   // A superseded start has already left the editor (unless a newer entry
   // owns it); a failed one leaves it open with no session.
   async function tryStartSession(window: TimeWindow): Promise<StartOutcome> {
@@ -104,7 +114,7 @@ export function useEditEntry() {
       resumeDatastreamId.value = managedId
       return 'editing'
     }
-    if (takenOver(managedId) && !(await canLeaveSession())) return 'kept'
+    if (takenOver(managedId) && !(await requestLeave())) return 'kept'
     try {
       await setEditTarget(managedId)
       await beginEditing()
@@ -130,5 +140,5 @@ export function useEditEntry() {
     return outcome === 'failed' ? 'needs-window' : 'superseded'
   }
 
-  return { enterEdit, startSessionOver, openEditor, leaveEdit }
+  return { enterEdit, startSessionOver, openEditor, leaveEdit, closeEditor }
 }
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index a23fce9e..31a90675 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -86,7 +86,7 @@ export function useEditSession() {
   const workingCopies = useWorkingCopiesStore()
 
   // The saved-edits snapshot lives in the store so every caller (the editor
-  // footer and the nav rail's exit guard) sees the same unsaved state.
+  // footer and the leave flow) sees the same unsaved state.
   const { sourceDatastream, savedEdits, savedComments } =
     storeToRefs(sessionStore)
   /** True when the managed datastream has no in-progress session to resume. */
diff --git a/apps/qc-app/src/composables/useLeaveSession.ts b/apps/qc-app/src/composables/useLeaveSession.ts
index 84aac7bd..65fc7319 100644
--- a/apps/qc-app/src/composables/useLeaveSession.ts
+++ b/apps/qc-app/src/composables/useLeaveSession.ts
@@ -1,17 +1,205 @@
 /**
- * The one decision point for abandoning the open edit session: taking over
- * with another edit target, and every other exit that ends it.
+ * Leaving an edit session: the one decision every exit goes through, and the
+ * work the user's choice implies.
+ *
+ * Every way out of the editor (the footer's Close, the nav rail's Home,
+ * workspace switch and log out, editing a different datastream, and in-app
+ * navigation) calls `requestLeave` and only proceeds when it resolves true.
+ * The prompt state is module-wide because one dialog serves all of them, and
+ * the stores are resolved on use, since the dialog and the router guard hold
+ * this composable long before anything is being edited.
  *
  * Switching between the Select and Edit views does not end the session, so it
- * never asks. Today leaving is always allowed; the prompts that offer to save,
- * discard or keep the session belong here.
+ * never asks. The native `beforeunload` prompt covers a reload or a closed
+ * tab, where the browser allows no dialog of ours.
  */
 
-export function useLeaveSession() {
-  /** True when the open session may be left behind. */
-  async function canLeaveSession(): Promise<boolean> {
-    return true
+import { ref } from 'vue'
+import { Snackbar } from '@uwrl/qc-utils'
+import { useDataVisStore } from '@/store/dataVisualization'
+import { usePlotlyStore } from '@/store/plotly'
+import { useQcSessionStore } from '@/store/qcSession'
+import { useWorkingCopiesStore } from '@/store/workingCopies'
+import { useEditSession } from '@/composables/useEditSession'
+import { useManagedDatastreams } from '@/composables/useManagedDatastreams'
+import { hasDependents } from '@/utils/sessionGraph'
+
+/** What the user stands to lose by leaving right now. */
+export type LeaveCase =
+  /** Nothing to decide: no edit target, no session, or committed history. */
+  | 'none'
+  /** Edits that never reached the session. */
+  | 'unsaved'
+  /** A session holding no operations at all. */
+  | 'empty'
+  /** Everything saved; the session stays in progress. */
+  | 'saved'
+
+export interface LeavePrompt {
+  kind: Exclude<LeaveCase, 'none'>
+  /** Edits made since the last save. */
+  unsavedCount: number
+  /** False when no session is open, so there is nowhere to save to. */
+  canSave: boolean
+}
+
+/** The choice being carried out, while it runs. */
+export type LeaveWork = 'save' | 'discard-edits' | 'discard-session' | null
+
+const leavePrompt = ref<LeavePrompt | null>(null)
+const leaveWork = ref<LeaveWork>(null)
+let answer: ((leave: boolean) => void) | null = null
+
+const messageOf = (e: unknown, fallback: string) =>
+  e instanceof Error ? e.message : fallback
+
+/** What leaving costs, decided the moment the exit is requested. */
+function leaveCase(): LeaveCase {
+  const { qcDatastream } = useDataVisStore()
+  const session = useQcSessionStore()
+  if (!qcDatastream) return 'none'
+  // Viewing committed history edits nothing, so there is nothing to lose.
+  if (session.isReadOnly) return 'none'
+  if (useEditSession().hasUnsavedChanges.value) return 'unsaved'
+  if (!session.inProgressSession) return 'none'
+  return session.hasSessionOperations ? 'saved' : 'empty'
+}
+
+function show(kind: Exclude<LeaveCase, 'none'>): void {
+  leavePrompt.value = {
+    kind,
+    unsavedCount: useEditSession().unsavedEditCount.value,
+    canSave: !!useQcSessionStore().inProgressSession,
+  }
+}
+
+function settle(leave: boolean): void {
+  const resolve = answer
+  answer = null
+  leavePrompt.value = null
+  leaveWork.value = null
+  resolve?.(leave)
+}
+
+/**
+ * Ask what should happen to the open session and carry the answer out.
+ * Resolves true when the caller may go on with its exit, false when the user
+ * chose to stay, in which case nothing has changed.
+ */
+async function requestLeave(): Promise<boolean> {
+  const kind = leaveCase()
+  if (kind === 'none') return true
+  // A second exit request supersedes the first, whose caller stays put.
+  answer?.(false)
+  show(kind)
+  return new Promise<boolean>((resolve) => {
+    answer = resolve
+  })
+}
+
+/** Stay in the session, exactly where the user was. */
+function cancelLeave(): void {
+  if (!leaveWork.value) settle(false)
+}
+
+/** Leave with the session untouched, still in progress. */
+function keepSession(): void {
+  if (!leaveWork.value) settle(true)
+}
+
+/** Keeping it, under the word the user sees when nothing is at stake. */
+function closeSession(): void {
+  keepSession()
+}
+
+async function saveAndLeave(): Promise<void> {
+  if (leaveWork.value || !leavePrompt.value?.canSave) return
+  leaveWork.value = 'save'
+  try {
+    await useEditSession().saveDraft()
+    Snackbar.success('Draft saved.')
+  } catch (e) {
+    Snackbar.error(messageOf(e, 'Could not save the draft.'))
+    return
+  } finally {
+    leaveWork.value = null
   }
+  settle(true)
+}
 
-  return { canLeaveSession }
+async function discardEditsAndLeave(): Promise<void> {
+  if (leaveWork.value) return
+  leaveWork.value = 'discard-edits'
+  try {
+    await useEditSession().discardUnsavedEdits()
+    await usePlotlyStore().redraw()
+  } catch (e) {
+    Snackbar.error(messageOf(e, 'Could not discard the edits.'))
+    return
+  } finally {
+    leaveWork.value = null
+  }
+  // Dropping them can leave the session with nothing in it at all, which is
+  // its own decision rather than a silent exit.
+  const session = useQcSessionStore()
+  if (session.inProgressSession && !session.hasSessionOperations) {
+    show('empty')
+    return
+  }
+  settle(true)
+}
+
+async function discardSessionAndLeave(): Promise<void> {
+  const store = useQcSessionStore()
+  const session = store.inProgressSession
+  const historyId = store.historyId
+  if (leaveWork.value || !session || !historyId) return
+  leaveWork.value = 'discard-session'
+  try {
+    // The in-progress session is the newest one, so nothing can be built on
+    // it. Refuse rather than cascade the delete into committed work.
+    if (hasDependents(store.sessions, session.id)) {
+      throw new Error(
+        'Another session was built on this one, so it cannot be discarded.'
+      )
+    }
+    await useManagedDatastreams().deleteSessionChain(historyId, [session.id])
+  } catch (e) {
+    Snackbar.error(messageOf(e, 'Could not discard the session.'))
+    return
+  } finally {
+    leaveWork.value = null
+  }
+  // The session is gone from the server, so drop it here too rather than
+  // wait for the next load to contradict the store.
+  store.applySessions(
+    historyId,
+    store.sessions.filter((s) => s.id !== session.id)
+  )
+  const { qcDatastream } = useDataVisStore()
+  if (qcDatastream) useWorkingCopiesStore().invalidate(qcDatastream.id)
+  Snackbar.success('Session discarded.')
+  settle(true)
+}
+
+/** Forget which datastream the editor was on, so the next load starts in the
+ *  Select view. Exits that unmount the editor need nothing else. */
+function forgetSession(): void {
+  useQcSessionStore().resumeDatastreamId = null
+}
+
+export function useLeaveSession() {
+  return {
+    leaveCase,
+    requestLeave,
+    cancelLeave,
+    keepSession,
+    closeSession,
+    saveAndLeave,
+    discardEditsAndLeave,
+    discardSessionAndLeave,
+    forgetSession,
+    leavePrompt,
+    leaveWork,
+  }
 }
diff --git a/apps/qc-app/src/router/__tests__/guards.spec.ts b/apps/qc-app/src/router/__tests__/guards.spec.ts
new file mode 100644
index 00000000..b732495e
--- /dev/null
+++ b/apps/qc-app/src/router/__tests__/guards.spec.ts
@@ -0,0 +1,73 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+import type { RouteLocationNormalized } from 'vue-router'
+
+const { requestLeave, forgetSession } = vi.hoisted(() => ({
+  requestLeave: vi.fn(),
+  forgetSession: vi.fn(),
+}))
+
+vi.mock('@/composables/useLeaveSession', () => ({
+  useLeaveSession: () => ({ requestLeave, forgetSession }),
+}))
+
+vi.mock('@hydroserver/client', () => ({ default: { session: {} } }))
+
+vi.mock('@/store/workspaces', () => ({
+  useWorkspaceStore: () => ({
+    selectedWorkspaceId: 'ws-1',
+    hasSelection: true,
+    applyWorkspaceById: vi.fn(),
+  }),
+}))
+
+import { leaveSessionGuard } from '@/router/guards'
+
+const route = (name: string | undefined, query: Record<string, string> = {}) =>
+  ({ name, query, meta: {}, fullPath: '/' }) as unknown as RouteLocationNormalized
+
+beforeEach(() => {
+  createTestPinia()
+  vi.clearAllMocks()
+  requestLeave.mockResolvedValue(true)
+})
+
+describe('leaveSessionGuard', () => {
+  it('asks before navigating away from the editor', async () => {
+    expect(await leaveSessionGuard(route('Workspaces'), route('Home'))).toBe(
+      null
+    )
+    expect(requestLeave).toHaveBeenCalled()
+    expect(forgetSession).toHaveBeenCalled()
+  })
+
+  it('cancels the navigation when the user stays', async () => {
+    requestLeave.mockResolvedValueOnce(false)
+    expect(await leaveSessionGuard(route('Workspaces'), route('Home'))).toBe(
+      false
+    )
+    expect(forgetSession).not.toHaveBeenCalled()
+  })
+
+  // The editor rewrites its own URL constantly; that is not an exit.
+  it('ignores a new URL for the page it is already on', async () => {
+    expect(
+      await leaveSessionGuard(route('Home', { ds: 'a' }), route('Home'))
+    ).toBe(null)
+    expect(requestLeave).not.toHaveBeenCalled()
+  })
+
+  // The flow answers "nothing to decide" itself, so the guard needs no
+  // knowledge of which page holds the editor.
+  it('consults the flow on any change of page', async () => {
+    expect(await leaveSessionGuard(route('Home'), route('Workspaces'))).toBe(
+      null
+    )
+    expect(requestLeave).toHaveBeenCalled()
+  })
+
+  it('ignores the first navigation of the session', async () => {
+    expect(await leaveSessionGuard(route('Home'), route(undefined))).toBe(null)
+    expect(requestLeave).not.toHaveBeenCalled()
+  })
+})
diff --git a/apps/qc-app/src/router/guards.ts b/apps/qc-app/src/router/guards.ts
index 8da31581..c66885fd 100644
--- a/apps/qc-app/src/router/guards.ts
+++ b/apps/qc-app/src/router/guards.ts
@@ -1,4 +1,5 @@
 import { useWorkspaceStore } from '@/store/workspaces'
+import { useLeaveSession } from '@/composables/useLeaveSession'
 import hs from '@hydroserver/client'
 import {
   RouteLocationNormalized,
@@ -31,8 +32,27 @@ const redirectToDataManagementLogin = (to: RouteLocationNormalized) => {
   return false as const
 }
 
+/**
+ * Navigating to another page ends an open edit session, so it goes through
+ * the leave flow first. Staying on the page (the editor rewrites its own URL
+ * as the user works) is not an exit.
+ *
+ * Only the resume pointer is cleared here. The page being left unmounts the
+ * editor, which resets the rest; clearing the edit target now would have the
+ * editor's URL writer replace the route in the middle of this navigation.
+ */
+export const leaveSessionGuard: RouteGuard = async (to, from) => {
+  if (!from.name || to.name === from.name) return null
+  const { requestLeave, forgetSession } = useLeaveSession()
+  if (!(await requestLeave())) return false
+  forgetSession()
+  return null
+}
+
 /** Guards are executed in the order they appear in this array */
 export const guards: RouteGuard[] = [
+  leaveSessionGuard,
+
   (to) => {
     if (!to.meta?.hasAuthGuard) return null
     if (hs.session?.isAuthenticated) return null
diff --git a/apps/qc-app/src/store/__tests__/qcSession.spec.ts b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
index 078096cf..971a2488 100644
--- a/apps/qc-app/src/store/__tests__/qcSession.spec.ts
+++ b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
@@ -211,3 +211,57 @@ describe('useQcSessionStore', () => {
     expect(store.sourceDatastream).toBeNull()
   })
 })
+
+describe('hasSessionOperations', () => {
+  it('is false for a session the server returned empty', async () => {
+    const { historyId } = await seed()
+    const store = useQcSessionStore()
+    store.applySessions(historyId, await store.fetchSessions(historyId))
+
+    expect(store.inProgressSession).not.toBeNull()
+    expect(store.hasSessionOperations).toBe(false)
+  })
+
+  it('reads the in-progress session operations the server returned', async () => {
+    const { historyId, inProgressId } = await seed()
+    await qc.operations.create(historyId, inProgressId, [
+      { operationType: 'DELETE_POINTS' as any, order: 0 },
+    ])
+    const store = useQcSessionStore()
+    store.applySessions(historyId, await store.fetchSessions(historyId))
+
+    expect(store.hasSessionOperations).toBe(true)
+  })
+
+  it('ignores operations that belong to a committed session', async () => {
+    const { historyId, committedId } = await seed()
+    const store = useQcSessionStore()
+    const sessions = await store.fetchSessions(historyId)
+    store.applySessions(
+      historyId,
+      sessions.map((s) =>
+        s.id === committedId
+          ? ({ ...s, operations: [{ id: 'op-1' }] } as any)
+          : s
+      )
+    )
+
+    expect(store.hasSessionOperations).toBe(false)
+  })
+
+  // A save writes operations the store's copy of the session doesn't have
+  // yet; the saved-edits snapshot is what it left behind.
+  it('counts edits saved since the sessions were loaded', async () => {
+    const { historyId } = await seed()
+    const store = useQcSessionStore()
+    store.applySessions(historyId, await store.fetchSessions(historyId))
+
+    store.savedEdits = [{ type: 'DELETE_POINTS' } as any]
+
+    expect(store.hasSessionOperations).toBe(true)
+  })
+
+  it('is false with no session at all', () => {
+    expect(useQcSessionStore().hasSessionOperations).toBe(false)
+  })
+})
diff --git a/apps/qc-app/src/store/qcSession.ts b/apps/qc-app/src/store/qcSession.ts
index f2085fdf..afce0870 100644
--- a/apps/qc-app/src/store/qcSession.ts
+++ b/apps/qc-app/src/store/qcSession.ts
@@ -29,7 +29,7 @@ export const useQcSessionStore = defineStore('qcSession', () => {
   const isSwitchingSession = ref(false)
   /** Edit history entries (by reference) at the last load or save, the
    *  baseline `useEditSession` compares against for unsaved edits. Kept here
-   *  so the editor and the nav rail's exit guard agree. */
+   *  so the editor and the leave flow agree. */
   const savedEdits = ref<HistoryItem[]>([])
   /** Comment text of `savedEdits`, since comments are edited in place. */
   const savedComments = ref<string[]>([])
@@ -60,6 +60,21 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     () => sessions.value.find((s) => s.id === viewedSessionId.value) ?? null
   )
 
+  /**
+   * True when the in-progress session holds any work at all: the operations
+   * the server returned with it, plus anything saved since (a save writes
+   * operations this copy of the session doesn't have yet, and leaves them in
+   * `savedEdits`). The leave flow uses it to tell an untouched session from
+   * one worth keeping.
+   */
+  const hasSessionOperations = computed(() => {
+    const session = inProgressSession.value as
+      | { operations?: unknown[] }
+      | null
+    if (!session) return false
+    return (session.operations?.length ?? 0) > 0 || savedEdits.value.length > 0
+  })
+
   /**
    * Guarantee every session carries its `operations`, which the previews
    * read. `expand_related` should embed them, but a backend that ignores it
@@ -151,6 +166,7 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     inProgressSession,
     committedSessions,
     viewedSession,
+    hasSessionOperations,
     fetchSessions,
     applySessions,
     viewSession,

From b0ebb86c3f9fd015e01b62bf38ceb573110e0a08 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:01:01 -0600
Subject: [PATCH 62/93] test: warm the dev server before the e2e suite

---
 apps/qc-app/docs/TESTING.md             |  8 +++++
 apps/qc-app/e2e/navigation.spec.ts      |  4 ---
 apps/qc-app/e2e/support/global-setup.ts | 48 +++++++++++++++++++++++++
 apps/qc-app/playwright.config.ts        |  2 ++
 4 files changed, 58 insertions(+), 4 deletions(-)
 create mode 100644 apps/qc-app/e2e/support/global-setup.ts

diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 38517281..bfc68834 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -233,6 +233,13 @@ The CI gate prints uncovered line numbers per file. Common causes:
 - `webServer.command: npm run dev` with
   `env: { VITE_APP_E2E_HOOKS: '1' }`. The env var arms test hooks
   (see [Test hooks](#test-hooks)).
+- `globalSetup: './e2e/support/global-setup.ts'`. Playwright starts
+  `webServer` before global setup, so the setup boots the app once
+  (same mocks, same seeded workspace) and waits for the datastreams
+  table. Vite transforms the module graph on demand, and paying for
+  it inside a test made the first `page.goto('/')` of a file exceed
+  the 30 s test timeout; warming it once moves that cost out of the
+  tests and every worker reuses the cached transform.
 
 ### Support layout
 
@@ -241,6 +248,7 @@ e2e/
 ├── support/
 │   ├── app.ts        : flow helpers (gotoHome, setupEditView, startSessionFromRow, waitForEditorReady, openOp, waitForSelection)
 │   ├── fixtures.ts   : workspace / datastream / observation fixtures
+│   ├── global-setup.ts : warms the dev server before the first test
 │   ├── mocks.ts      : page.route() handlers that stand in for HydroServer
 │   └── ops.ts        : op-specific preambles (selectAllPoints, expectHistoryContains)
 └── *.spec.ts         : one file per feature
diff --git a/apps/qc-app/e2e/navigation.spec.ts b/apps/qc-app/e2e/navigation.spec.ts
index f73a7256..f22cc848 100644
--- a/apps/qc-app/e2e/navigation.spec.ts
+++ b/apps/qc-app/e2e/navigation.spec.ts
@@ -43,10 +43,7 @@ test.describe('navigation', () => {
     await installMocks(page, { qcHistories: true })
   })
 
-  // The two picker tests boot the app without a seeded workspace, so they
-  // pay the dev server's cold start when this file runs first.
   test('fresh browser redirects to the workspace picker', async ({ page }) => {
-    test.slow()
     await page.goto('/')
     // Either still /workspaces, or the picker's select button is visible.
     await expect(
@@ -58,7 +55,6 @@ test.describe('navigation', () => {
   })
 
   test('picking a workspace lands the user on Home', async ({ page }) => {
-    test.slow()
     // Don't use `gotoHome` here: it pre-seeds localStorage and skips
     // the picker. We want to exercise the actual pick flow.
     await page.goto('/')
diff --git a/apps/qc-app/e2e/support/global-setup.ts b/apps/qc-app/e2e/support/global-setup.ts
new file mode 100644
index 00000000..114510b6
--- /dev/null
+++ b/apps/qc-app/e2e/support/global-setup.ts
@@ -0,0 +1,48 @@
+/**
+ * Warm the dev server before the first test runs.
+ *
+ * Vite transforms the module graph on demand, so the first `page.goto('/')`
+ * of a run used to pay for the whole app (Plotly and Vuetify included) inside
+ * a 30 s test timeout, and the first test of a file lost that race. Playwright
+ * starts `webServer` before global setup, so loading the app once here moves
+ * that cost out of the tests; every worker afterwards gets the cached
+ * transform.
+ *
+ * The app only pulls the rest of the graph once it mounts, so this boots it
+ * the way a spec does: the same route mocks and the same seeded workspace.
+ */
+
+import { chromium, type FullConfig } from '@playwright/test'
+import { seedWorkspaceSelection } from './app'
+import { installMocks } from './mocks'
+
+const WARMUP_TIMEOUT = 180_000
+
+export default async function warmDevServer(config: FullConfig): Promise<void> {
+  const baseURL = config.projects.map((p) => p.use.baseURL).find(Boolean)
+  if (!baseURL) {
+    throw new Error('No baseURL is configured, so the dev server cannot be warmed.')
+  }
+
+  const browser = await chromium.launch()
+  try {
+    const page = await browser.newPage({ baseURL })
+    await installMocks(page, { qcHistories: true })
+    await seedWorkspaceSelection(page)
+
+    const response = await page.goto(baseURL, {
+      waitUntil: 'load',
+      timeout: WARMUP_TIMEOUT,
+    })
+    if (!response?.ok()) {
+      throw new Error(
+        `The dev server answered ${response?.status() ?? 'nothing'} for ${baseURL}.`
+      )
+    }
+    await page.waitForSelector('[data-testid="datastreams-table"]', {
+      timeout: WARMUP_TIMEOUT,
+    })
+  } finally {
+    await browser.close()
+  }
+}
diff --git a/apps/qc-app/playwright.config.ts b/apps/qc-app/playwright.config.ts
index 0f56d063..f8e2adb6 100644
--- a/apps/qc-app/playwright.config.ts
+++ b/apps/qc-app/playwright.config.ts
@@ -18,6 +18,8 @@ const reporter = process.env.CI
 
 export default defineConfig({
   testDir: './e2e',
+  // Loads the app once so the first test does not pay Vite's cold transform.
+  globalSetup: './e2e/support/global-setup.ts',
   fullyParallel: true,
   forbidOnly: !!process.env.CI,
   retries: process.env.CI ? 2 : 1,

From cd12318bae515e9182e50172a09c500d0de5c216 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:01:02 -0600
Subject: [PATCH 63/93] fix: ask before creating a datastream and scope share
 link zoom to its edit target

Also refuses a second leave request while one is in flight.
---
 apps/qc-app/docs/API_REFERENCE.md             | 10 ++-
 apps/qc-app/docs/ARCHITECTURE.md              | 22 ++++--
 apps/qc-app/docs/PLOTTING.md                  |  8 +-
 apps/qc-app/docs/TESTING.md                   |  7 ++
 apps/qc-app/docs/USER_GUIDE.md                | 14 +++-
 apps/qc-app/e2e/leave-session.spec.ts         | 26 ++++---
 apps/qc-app/e2e/share-url.spec.ts             | 75 ++++++++++++++++++-
 .../components/EditData/StartEditingFlow.vue  | 21 +++++-
 .../__tests__/StartEditingFlow.spec.ts        | 62 ++++++++++++++-
 apps/qc-app/src/components/VisualizeData.vue  |  5 ++
 .../src/components/VisualizeData/Plot.vue     | 22 ++----
 .../VisualizeData/__tests__/Plot.spec.ts      | 63 +++++++++++++++-
 .../__tests__/useLeaveSession.spec.ts         | 26 +++++++
 .../qc-app/src/composables/useLeaveSession.ts |  4 +
 apps/qc-app/src/store/plotly.ts               |  9 +++
 15 files changed, 326 insertions(+), 48 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 8c2cf3fb..de6e970f 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -137,7 +137,10 @@ below), and share-link hydration (the `ed` query param).
   behind it. Called on the target already open, with no `window`, it only
   shows that view: nothing about the session is re-entered. Taking over from
   another open session first asks `useLeaveSession().requestLeave()`, and
-  returns `'kept'` when the user keeps the open one.
+  returns `'kept'` when the user keeps the open one. The create step in
+  `StartEditingFlow.vue` asks earlier, through `closeEditor()`: a created
+  managed datastream is always a new target, so the takeover is certain and
+  the question must come before anything reaches the server.
 - `startSessionOver(window)`: starts a new session on the current edit
   target over `window` (a `utils/timeRangePresets.ts` `TimeWindow`); backs
   **Start new session** and the editor footer's **New session**.
@@ -180,7 +183,9 @@ and `LeaveSessionDialog.vue`, mounted once in `App.vue`, shows it.
   a session whose work is all saved.
 - `requestLeave()`: decides the case, shows the matching prompt and carries
   the answer out. Resolves true when the caller may go on, false when the
-  user stays. A second request supersedes the first, whose caller stays put.
+  user stays. A second request supersedes the first, whose caller stays put;
+  once an answer is being carried out (`leaveWork`), a second request is
+  refused outright rather than inheriting that answer.
 - `saveAndLeave()` / `discardEditsAndLeave()`: the `'unsaved'` answers.
   Saving is refused with no session open. Discarding that empties the session
   falls through to the `'empty'` prompt rather than leaving silently.
@@ -522,6 +527,7 @@ handles, live chart caches).
 | `zoomRedoStack`            | state    | `ZoomState[]`                                     | Cleared on every new user-initiated zoom. |
 | `suppressZoomHistory`      | state    | `boolean`                                         | Flipped on during programmatic restores so the recorder doesn't double-capture. |
 | `pendingShareZoom`         | state    | `ZoomState \| null`                               | URL-hydrated zoom; applied once on mount then cleared. |
+| `shareZoomEditTarget`      | state    | `string \| null`                                  | The `ed` target of that link, when it carried one: the one session window the share zoom outranks. |
 | `canUndoZoom`              | computed | `boolean`                                         | `zoomUndoStack.length > 1`. |
 | `canRedoZoom`              | computed | `boolean`                                         | `zoomRedoStack.length > 0`. |
 | `currentZoom`              | computed | `ZoomState \| null`                               | Top of the undo stack; what the share URL writer subscribes to. |
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 0de275e3..dd0a1513 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -329,10 +329,11 @@ Two contract notes worth keeping in mind:
   built on a copy of the base (`reconstructSession`/`loadLatestBase`'s
   `cloneRecord`), never the observation store's cached record, so the raw
   datastream's cached observations are never edited. It is invalidated when
-  the editor closes through `exitToSelect` (Close, Save and close, or close
-  without saving), on commit, when its session or managed datastream is
-  deleted, and when a managed datastream other than the edit target stops being
-  plotted; `resetState` clears every copy on a workspace reset. Switching to
+  the edit target is cleared (`dataVisualization.clearEditTarget`, which every
+  exit reaches through `useEditEntry.leaveEdit`), on commit, when its session
+  or managed datastream is deleted, and when a managed datastream other than
+  the edit target stops being plotted; `resetState` clears every copy when the
+  page unmounts or the workspace is reset. Switching to
   the Select view keeps the copy untouched, unsaved edits and all: it does not
   end the session. The next preview or resume rebuilds it from what was
   actually saved.
@@ -465,10 +466,15 @@ operations plus anything saved since. Neither is a guess about the UI.
 Every exit routes through it: the editor footer's **Close** and the nav rail's
 Home and Log out via `useEditEntry.closeEditor()`, the row Edit button on
 another datastream via `enterEdit`, and in-app navigation (including the
-workspace switch) via `leaveSessionGuard`. Cancelling leaves the user exactly
-where they were, zoom, staged band and unsaved edits intact. The native
-`beforeunload` prompt still covers a reload or a closed tab, with the
-browser's own wording.
+workspace switch) via `leaveSessionGuard`. When that row Edit button leads to
+a create step, `StartEditingFlow` asks through `closeEditor()` before the
+create rather than waiting for `enterEdit`: a new managed datastream is always
+a new target, so cancelling later would have left an orphan datastream and its
+QC history on the server. While an answer is being carried out, a second exit
+request is refused rather than resolved by work the user did not answer for.
+Cancelling leaves the user exactly where they were, zoom, staged band and
+unsaved edits intact. The native `beforeunload` prompt still covers a reload
+or a closed tab, with the browser's own wording.
 
 Auth itself is delegated to HydroServer's Django AllAuth setup; the app
 keeps no credentials of its own. The browser holds a session cookie.
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index 6ef1e04d..a6f7c13a 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -293,9 +293,11 @@ target the full plot shows in both views, and the flag only flips alongside a
 rebuild (`setEditTarget` / `clearEditTarget`).
 
 A share link's zoom (`pendingShareZoom`) is an explicit viewport, so it beats
-the editor's default "open on the session window" once: the first session
-window that arrives afterwards does not re-zoom, later ones (viewing another
-session) do.
+the editor's default "open on the session window" for the session that link
+itself opens. `plotly.shareZoomEditTarget` holds the link's `ed` target for
+exactly that, and `Plot.vue` drops it at the first session window it sees: a
+link without an edit target, or an editor opened later in the page's life,
+zooms to its session window as usual.
 
 While observations load, `DataVisualization.vue` keeps `Plot` mounted and
 passes its loading overlay through Plot's `body-overlay` slot. The overlay
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index bfc68834..aa17ba81 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -414,6 +414,13 @@ them so the next contributor doesn't rediscover them.
    session window is known, the edit record is on the plot, no session is
    opening, and no plot load or tracked draw is pending).
 
+6. **With an edit target set, the editor stays mounted behind the Select
+   view.** It is hidden with `v-show`, not unmounted, so its test ids exist
+   twice in the DOM: `plotted-item-*`, `history-item-*` and the rest resolve
+   to two elements and a bare `getByTestId` is strict-mode ambiguous. Scope
+   Select-view assertions through the panel, as `select-while-editing.spec.ts`
+   does with `page.getByTestId('select-side-panel')`.
+
 ### Debugging a failure
 
 Playwright's HTML report (`playwright-report/index.html`) opens
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 41e66b02..e22f05cd 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -664,7 +664,9 @@ The dialog lets you add an optional session description. Once you confirm:
 You never leave an edit session by accident. Anything that ends it, the
 editor's **Close** button, the HydroServer logo, the workspace switch, Log
 out, the browser's Back button, or editing a different datastream, asks the
-same question first and tells you what happens to your work.
+same question first and tells you what happens to your work. If that other
+datastream has no QC datastream yet, the question comes before the create
+form, so cancelling leaves nothing new behind.
 
 Which question depends on where the session stands:
 
@@ -673,7 +675,8 @@ Which question depends on where the session stands:
   drop everything since your last save, or **Cancel** to stay. Either way the
   session stays in progress and you can pick it up later. Save is unavailable
   when no session is open, because there is nowhere to save to. If discarding
-  leaves the session with nothing in it, the next question is the one below.
+  leaves the session with nothing in it, the next question is the one below;
+  the edits are gone by then, so that question is only about the session.
 - **The session has no edits at all**, nothing saved and nothing unsaved (you
   started it and then changed your mind). Choose **Keep session** to leave it
   in progress, or **Discard session** to delete it from the server. Keeping it
@@ -684,8 +687,11 @@ Which question depends on where the session stands:
 To come back to a session you left in progress, click the pencil button on
 that datastream's row again. It reopens where you left off.
 
-**Cancel** always puts you back exactly where you were: same zoom, same staged
-range, same unsaved edits. Switching between the **Select** and **Edit** views
+**Cancel** puts you back where you were: same zoom, same staged range, same
+unsaved edits. The one exception is the question that follows **Discard
+changes and close**, where the edits were already dropped by the choice you
+made: cancelling there keeps you in the session, not in your edits. Switching
+between the **Select** and **Edit** views
 is not leaving, so it never asks. Reloading the page or closing the tab still
 shows the browser's own "leave site?" prompt, which the app cannot reword.
 
diff --git a/apps/qc-app/e2e/leave-session.spec.ts b/apps/qc-app/e2e/leave-session.spec.ts
index 46c21366..254ae6f0 100644
--- a/apps/qc-app/e2e/leave-session.spec.ts
+++ b/apps/qc-app/e2e/leave-session.spec.ts
@@ -194,25 +194,31 @@ test.describe('leaving a session', () => {
     expect(inProgress(sessions)).toBeDefined()
   })
 
-  test('editing another datastream asks about the open session', async ({
+  test('editing another datastream asks before anything is created', async ({
     page,
   }) => {
     await goToSelect(page)
 
-    // Datastream B has no managed datastream yet, so Edit goes through the
-    // create step before it can take the editor over.
+    // Datastream B has no managed datastream yet, so Edit would create one.
+    // Cancelling must leave nothing behind on the server.
     await page.getByTestId(`edit-datastream-${DATASTREAM_ID_B}`).click()
-    await page.getByTestId('create-processing-level').click()
-    await page
-      .getByRole('option', { name: 'Quality controlled', exact: true })
-      .click()
-    await page.getByTestId('create-confirm').click()
-    await page.getByTestId('session-window-start').click()
-
     await expect(leaveDialog(page)).toBeVisible()
     await page.getByTestId('leave-cancel-btn').click()
 
+    await expect(page.getByTestId('create-confirm')).toHaveCount(0)
     // The original session is still the one being edited.
     await expect(editPanel(page)).toContainText('Streamflow Datastream (QC)')
   })
+
+  test('answering the question opens the create step', async ({ page }) => {
+    await goToSelect(page)
+
+    await page.getByTestId(`edit-datastream-${DATASTREAM_ID_B}`).click()
+    await page.getByTestId('leave-keep-btn').click()
+
+    await expect(page.getByTestId('create-confirm')).toBeVisible()
+    // The session was left the moment the user said so, not at the end.
+    await expect(editPanel(page)).toHaveCount(0)
+    expect(inProgress(sessions)).toBeDefined()
+  })
 })
diff --git a/apps/qc-app/e2e/share-url.spec.ts b/apps/qc-app/e2e/share-url.spec.ts
index cc0c7752..d2ea5a52 100644
--- a/apps/qc-app/e2e/share-url.spec.ts
+++ b/apps/qc-app/e2e/share-url.spec.ts
@@ -12,7 +12,7 @@
 
 import { expect, test, type Page } from '@playwright/test'
 import { installMocks } from './support/mocks'
-import { setupEditView } from './support/app'
+import { setupEditView, startSessionFromRow } from './support/app'
 import {
   DATASTREAM_ID,
   DATASTREAM_ID_B,
@@ -100,6 +100,48 @@ async function clickResetAxes(page: Page) {
   }, selector)
 }
 
+/** The live X range of the main plot as `[loMs, hiMs]`, or null. */
+async function plotXRange(page: Page): Promise<[number, number] | null> {
+  return page.evaluate(() => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as
+      | (HTMLElement & {
+          _fullLayout?: { xaxis?: { range?: [number | string, number | string] } }
+        })
+      | null
+    const range = gd?._fullLayout?.xaxis?.range
+    if (!range) return null
+    const toMs = (v: number | string) =>
+      typeof v === 'number' ? v : Date.parse(v)
+    return [toMs(range[0]), toMs(range[1])] as [number, number]
+  })
+}
+
+/**
+ * Whether the plot is showing the session-window band the editor draws, which
+ * is what "the editor opened on its session window" looks like from outside.
+ */
+async function zoomedToSessionWindow(page: Page): Promise<string> {
+  return page.evaluate(() => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as
+      | (HTMLElement & {
+          _fullLayout?: { xaxis?: { range?: [number | string, number | string] } }
+          layout?: { shapes?: { name?: string; x0?: unknown; x1?: unknown }[] }
+        })
+      | null
+    const range = gd?._fullLayout?.xaxis?.range
+    const band = (gd?.layout?.shapes ?? []).find((s) => s.name === 'edit-window')
+    if (!range) return 'no plot'
+    if (!band) return 'no session band'
+    const toMs = (v: unknown) =>
+      typeof v === 'number' ? v : Date.parse(String(v))
+    const near = (a: number, b: number) => Math.abs(a - b) <= 2000
+    return near(toMs(range[0]), toMs(band.x0)) &&
+      near(toMs(range[1]), toMs(band.x1))
+      ? 'session window'
+      : 'another range'
+  })
+}
+
 /** Wait until the live plot draws a trace for every id in `ids`. */
 async function waitForTraces(page: Page, ids: string[]) {
   await page.waitForFunction(
@@ -313,6 +355,37 @@ test.describe('share URL', () => {
     )
   })
 
+  test('a Select-view zoom leaves a later editor on its session window', async ({
+    page,
+  }) => {
+    test.setTimeout(90_000)
+    // A link with `z=` but no `ed=` carries the sender's Select viewport. It
+    // must not outrank the session window of an editor opened afterwards,
+    // which is the only zoom telling the user what they are editing.
+    const fullSpan = FIXTURE_OBS_END_MS - FIXTURE_OBS_START_MS
+    const xHi = FIXTURE_OBS_END_MS - fullSpan * 0.1
+    const xLo = xHi - fullSpan * 0.2
+    const toS36 = (ms: number) => Math.floor(ms / 1000).toString(36)
+    await page.goto(
+      `/?ws=${WORKSPACE_ID}&ds=${DATASTREAM_ID}&z=${toS36(xLo)}.${toS36(xHi)}`
+    )
+    await waitForTraces(page, [DATASTREAM_ID])
+
+    // The link's own zoom lands first, so the assertion below is about the
+    // editor overriding it rather than the zoom never arriving.
+    await expect
+      .poll(async () => (await plotXRange(page))?.[0] ?? null, {
+        timeout: 30_000,
+      })
+      .toBeCloseTo(xLo, -4)
+
+    await startSessionFromRow(page)
+
+    await expect
+      .poll(() => zoomedToSessionWindow(page), { timeout: 30_000 })
+      .toBe('session window')
+  })
+
   test('hydrator hides traces flagged by the URL `h=` bitmask on a cold load', async ({
     page,
   }) => {
diff --git a/apps/qc-app/src/components/EditData/StartEditingFlow.vue b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
index 97c00bec..7ea50ded 100644
--- a/apps/qc-app/src/components/EditData/StartEditingFlow.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
@@ -106,7 +106,7 @@ const { createProcessingLevel } = useProcessingLevels()
 const { sensors, statuses, load: loadDatastreamMetadata } =
   useDatastreamMetadata()
 const { canCreateDatastream, roleName } = useWorkspacePermissions()
-const { enterEdit, startSessionOver, leaveEdit } = useEditEntry()
+const { enterEdit, startSessionOver, leaveEdit, closeEditor } = useEditEntry()
 
 const createPermissionError = computed(() =>
   canCreateDatastream()
@@ -139,6 +139,17 @@ const showWindow = computed({
   },
 })
 
+/**
+ * A created managed datastream is always a new edit target, so an open
+ * session is certainly abandoned. Ask and end it before anything reaches the
+ * server, rather than after a create the user might not want. False means the
+ * session was kept and the create step must not open.
+ */
+async function leaveOpenSession(): Promise<boolean> {
+  if (!qcDatastream.value) return true
+  return closeEditor()
+}
+
 /** Row Edit button: choose a managed datastream, or create the first one. */
 async function openFor(source: Datastream) {
   chooserSource.value = source
@@ -155,7 +166,7 @@ async function openFor(source: Datastream) {
     chooserLoading.value = false
   }
   if (chooserOptions.value.length) showChooser.value = true
-  else showCreateDatastream.value = true
+  else if (await leaveOpenSession()) showCreateDatastream.value = true
 }
 
 async function onChooserEdit(option: ManagedDatastreamOption) {
@@ -249,9 +260,11 @@ async function onWindowCancel() {
   }
 }
 
-function onChooserCreate() {
+async function onChooserCreate() {
   showChooser.value = false
-  showCreateDatastream.value = true
+  // Keeping the session returns to the chooser, where Continue is still there.
+  if (await leaveOpenSession()) showCreateDatastream.value = true
+  else showChooser.value = true
 }
 
 // Cancelling create returns to the chooser it was opened from, if any.
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
index 90734044..6cce8abf 100644
--- a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
@@ -27,6 +27,7 @@ const {
   enterEdit,
   startSessionOver,
   leaveEdit,
+  closeEditor,
   createManaged,
   addQcHistory,
   removeManagedDatastream,
@@ -47,6 +48,7 @@ const {
     enterEdit: vi.fn(),
     startSessionOver: vi.fn(),
     leaveEdit: vi.fn(),
+    closeEditor: vi.fn(),
     createManaged: vi.fn(),
     addQcHistory: vi.fn(),
     removeManagedDatastream: vi.fn(),
@@ -101,7 +103,7 @@ vi.mock('@/composables/useManagedDatastreams', () => ({
 }))
 
 vi.mock('@/composables/useEditEntry', () => ({
-  useEditEntry: () => ({ enterEdit, startSessionOver, leaveEdit }),
+  useEditEntry: () => ({ enterEdit, startSessionOver, leaveEdit, closeEditor }),
 }))
 
 vi.mock('@/composables/useCreateManagedDatastream', () => ({
@@ -213,6 +215,7 @@ beforeEach(() => {
   enterEdit.mockResolvedValue('editing')
   startSessionOver.mockResolvedValue(true)
   leaveEdit.mockResolvedValue(undefined)
+  closeEditor.mockResolvedValue(true)
 })
 
 describe('StartEditingFlow', () => {
@@ -414,6 +417,63 @@ describe('StartEditingFlow', () => {
     expect(present('window-stub')).toBe(false)
   })
 
+  it('leaves the open session before the create form, not after the create', async () => {
+    loadForSource.mockResolvedValue([])
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    const { flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    expect(closeEditor).toHaveBeenCalled()
+    expect(present('create-stub')).toBe(true)
+  })
+
+  it('creates nothing when the open session is kept', async () => {
+    loadForSource.mockResolvedValue([])
+    closeEditor.mockResolvedValue(false)
+    qcDatastream.value = { id: 'mgd', name: 'Temp (QC)' }
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    expect(dialogOpen(wrapper, 1)).toBe(false)
+    expect(createManaged).not.toHaveBeenCalled()
+  })
+
+  it('asks before the create form opened from the chooser', async () => {
+    const option = optionWith([inProgress])
+    loadForSource.mockResolvedValue([option])
+    qcDatastream.value = { id: 'other', name: 'Other (QC)' }
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    wrapper.findComponent(ChooserStub).vm.$emit('create')
+    await flushPromises()
+    expect(closeEditor).toHaveBeenCalled()
+    expect(present('create-stub')).toBe(true)
+  })
+
+  it('returns to the chooser when the open session is kept', async () => {
+    const option = optionWith([inProgress])
+    loadForSource.mockResolvedValue([option])
+    closeEditor.mockResolvedValue(false)
+    qcDatastream.value = { id: 'other', name: 'Other (QC)' }
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    wrapper.findComponent(ChooserStub).vm.$emit('create')
+    await flushPromises()
+    expect(dialogOpen(wrapper, 1)).toBe(false)
+    expect(dialogOpen(wrapper, 0)).toBe(true)
+  })
+
+  it('does not ask when nothing is being edited', async () => {
+    loadForSource.mockResolvedValue([])
+    const { flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    expect(closeEditor).not.toHaveBeenCalled()
+    expect(present('create-stub')).toBe(true)
+  })
+
   it('asks for a window for a newly created managed datastream', async () => {
     loadForSource.mockResolvedValue([])
     const managed = { id: 'mgd-new', name: 'Temp (QC new)' }
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index ecff27ba..ac6ff0a9 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -503,6 +503,7 @@ const {
   hiddenAxisIds,
   currentZoom,
   pendingShareZoom,
+  shareZoomEditTarget,
   tooltipsMode,
   tooltipsManualEnabled,
   tooltipsMaxDataPoints,
@@ -839,6 +840,10 @@ const hydrateFromUrl = () => {
         source: 'user',
       }
     : null
+  // It only outranks the session window when the link opens a session at all.
+  shareZoomEditTarget.value = state.zoom
+    ? (state.editDatastreamId ?? null)
+    : null
 
   // `resumeDatastreamId` is already set from `ed` by the early block above,
   // before the resume hook is registered. Nothing to do here for it.
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index a4cb97c0..d600ce2e 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -438,6 +438,7 @@ const {
   plotlyRef,
   activeTab,
   pendingShareZoom,
+  shareZoomEditTarget,
 } = storeToRefs(usePlotlyStore())
 const { selectedData, hasSelectionShape, qcDatastream } =
   storeToRefs(useDataVisStore())
@@ -586,24 +587,17 @@ function zoomToEditWindow() {
   requestTableScroll(w.begin)
 }
 
-// A share link's zoom is an explicit viewport, so it wins over that default,
-// once: later windows (viewing another session) zoom as usual.
-let shareZoomWins = false
-watch(
-  pendingShareZoom,
-  (zoom) => {
-    if (zoom) shareZoomWins = true
-  },
-  { immediate: true }
-)
-
+// A share link's zoom is an explicit viewport, so it wins over that default
+// for the session the link itself opened. Any other window zooms as usual and
+// drops the pointer, so the link cannot reach a later editing session.
 watch(
   () => editWindow.value && `${editWindow.value.begin}-${editWindow.value.end}`,
   () => {
     if (isPlotPreview.value) return
-    if (shareZoomWins) {
-      shareZoomWins = false
-      return
+    const shareTarget = shareZoomEditTarget.value
+    if (shareTarget) {
+      shareZoomEditTarget.value = null
+      if (shareTarget === qcDatastream.value?.id) return
     }
     zoomToEditWindow()
   }
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
index 2b7bf7ac..4b33f51e 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
@@ -18,6 +18,7 @@ const plotly = {
   plotlyRef: ref<any>(null),
   activeTab: ref('plot'),
   pendingShareZoom: ref<any>(null),
+  shareZoomEditTarget: ref<string | null>(null),
 }
 const updateOptions = vi.fn()
 const requestTableScroll = vi.fn()
@@ -37,12 +38,14 @@ async function trackPlotWork(work: () => Promise<void>) {
   }
 }
 
+const qcDatastream = ref<{ id: string } | null>(null)
+
 vi.mock('@/store/dataVisualization', () => ({
   useDataVisStore: () =>
     reactive({
       selectedData: ref(null),
       hasSelectionShape: ref(false),
-      qcDatastream: ref(null),
+      qcDatastream,
       trackPlotWork,
     }),
 }))
@@ -216,6 +219,64 @@ describe('Plot.vue delayed mount', () => {
     })
   })
 
+  describe('a share link zoom against the session window', () => {
+    const windowA = {
+      phenomenonTimeStart: '2020-01-01T00:00:00Z',
+      phenomenonTimeEnd: '2020-01-02T00:00:00Z',
+    }
+    const windowB = {
+      phenomenonTimeStart: '2020-02-01T00:00:00Z',
+      phenomenonTimeEnd: '2020-02-02T00:00:00Z',
+    }
+
+    beforeEach(() => {
+      plotly.plotlyRef.value = {}
+      plotly.pendingShareZoom.value = { xRange: [1, 2], source: 'user' }
+      handleNewPlot.mockResolvedValue(undefined)
+    })
+
+    afterEach(() => {
+      plotly.pendingShareZoom.value = null
+      plotly.shareZoomEditTarget.value = null
+      qcDatastream.value = null
+    })
+
+    it('leaves a later editor to open on its own session window', async () => {
+      // A Select-view link: a zoom, and no edit target for it to outrank.
+      const wrapper = mountIt(true)
+      await vi.advanceTimersByTimeAsync(200)
+      await flushPromises()
+      expect(zoomXaxisTo).not.toHaveBeenCalled()
+
+      // The user opens the editor later in the same page life.
+      isPlotPreview.value = false
+      qcDatastream.value = { id: 'mgd-1' }
+      viewedSession.value = windowA
+      await flushPromises()
+
+      expect(zoomXaxisTo).toHaveBeenCalledTimes(1)
+      wrapper.unmount()
+    })
+
+    it('outranks the session window of the editor the link opened', async () => {
+      plotly.shareZoomEditTarget.value = 'mgd-1'
+      qcDatastream.value = { id: 'mgd-1' }
+      const wrapper = mountIt(false)
+      await vi.advanceTimersByTimeAsync(200)
+      await flushPromises()
+
+      viewedSession.value = windowA
+      await flushPromises()
+      expect(zoomXaxisTo).not.toHaveBeenCalled()
+
+      // Only the window the link landed on; viewing another one zooms.
+      viewedSession.value = windowB
+      await flushPromises()
+      expect(zoomXaxisTo).toHaveBeenCalledTimes(1)
+      wrapper.unmount()
+    })
+  })
+
   it('renders the body overlay under the toolbar, not over it', () => {
     const wrapper = mountIt(false, {
       'body-overlay': '<div data-testid="overlay-probe" />',
diff --git a/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts b/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
index 62a4bd36..e6228c7a 100644
--- a/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
@@ -87,6 +87,9 @@ vi.mock('@uwrl/qc-utils', () => ({ Snackbar: { success, error } }))
 
 import { useLeaveSession } from '../useLeaveSession'
 
+/** Let every already-settled promise deliver before asserting. */
+const tick = () => new Promise((resolve) => setTimeout(resolve, 0))
+
 /** Nothing saved, nothing unsaved: an untouched in-progress session. */
 function emptySession() {
   qcDatastream.value = { id: 'mgd-1' }
@@ -343,6 +346,29 @@ describe('requestLeave', () => {
     cancelLeave()
     expect(await second).toBe(false)
   })
+
+  it('refuses a second request while the first answer is being carried out', async () => {
+    emptySession()
+    hasUnsavedChanges.value = true
+    let finishSave!: () => void
+    saveDraft.mockImplementationOnce(
+      () => new Promise<void>((resolve) => (finishSave = resolve))
+    )
+    const { requestLeave, saveAndLeave } = useLeaveSession()
+    const first = requestLeave()
+    void saveAndLeave()
+    await tick()
+
+    // The save is still running, so the second exit is refused outright
+    // rather than inheriting the answer the user gave the first one.
+    const second = requestLeave()
+    expect(await Promise.race([second, tick().then(() => 'unanswered')])).toBe(
+      false
+    )
+
+    finishSave()
+    expect(await first).toBe(true)
+  })
 })
 
 describe('forgetSession', () => {
diff --git a/apps/qc-app/src/composables/useLeaveSession.ts b/apps/qc-app/src/composables/useLeaveSession.ts
index 65fc7319..4b3bf919 100644
--- a/apps/qc-app/src/composables/useLeaveSession.ts
+++ b/apps/qc-app/src/composables/useLeaveSession.ts
@@ -87,6 +87,10 @@ function settle(leave: boolean): void {
  * chose to stay, in which case nothing has changed.
  */
 async function requestLeave(): Promise<boolean> {
+  // The user's answer to the open prompt is already being carried out, and
+  // it belongs to the exit that asked for it. Refuse rather than let this
+  // request inherit it or leave it stranded behind disabled buttons.
+  if (leaveWork.value) return false
   const kind = leaveCase()
   if (kind === 'none') return true
   // A second exit request supersedes the first, whose caller stays put.
diff --git a/apps/qc-app/src/store/plotly.ts b/apps/qc-app/src/store/plotly.ts
index 693688e8..e0b817a6 100644
--- a/apps/qc-app/src/store/plotly.ts
+++ b/apps/qc-app/src/store/plotly.ts
@@ -262,6 +262,14 @@ export const usePlotlyStore = defineStore('Plotly', () => {
    */
   const pendingShareZoom = ref<ZoomState | null>(null)
 
+  /**
+   * The edit target whose session window the share zoom outranks, set with
+   * `pendingShareZoom` only when the link also carried one. `Plot.vue` drops
+   * it at the first session window it sees, so a link without an edit target
+   * never suppresses the zoom of an editor opened later in the page's life.
+   */
+  const shareZoomEditTarget = ref<string | null>(null)
+
   function clearZoomHistory() {
     zoomUndoStack.value = []
     zoomRedoStack.value = []
@@ -561,6 +569,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
     canRedoZoom,
     currentZoom,
     pendingShareZoom,
+    shareZoomEditTarget,
     clearZoomHistory,
     pushZoomState,
   }

From c38adb6647fcb1806079d965a2b3e9fc94654418 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:01:02 -0600
Subject: [PATCH 64/93] feat: only allow deleting the newest session and drop
 the in-progress chip

---
 apps/qc-app/docs/API_REFERENCE.md             |  10 +-
 apps/qc-app/docs/ARCHITECTURE.md              |  17 +-
 apps/qc-app/docs/USER_GUIDE.md                |  26 +--
 .../EditData/StartEditingDialog.vue           | 156 +++---------------
 .../components/EditData/StartEditingFlow.vue  |  24 +--
 .../__tests__/StartEditingDialog.spec.ts      | 130 +++++++--------
 .../__tests__/StartEditingFlow.spec.ts        |   6 +-
 .../__tests__/useLeaveSession.spec.ts         |  18 +-
 .../__tests__/useManagedDatastreams.spec.ts   |  17 +-
 .../qc-app/src/composables/useLeaveSession.ts |   2 +-
 .../src/composables/useManagedDatastreams.ts  |  41 ++---
 .../src/utils/__tests__/sessionGraph.spec.ts  |  50 ------
 apps/qc-app/src/utils/sessionGraph.ts         |  28 ----
 13 files changed, 162 insertions(+), 363 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index de6e970f..dec6db26 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -191,7 +191,7 @@ and `LeaveSessionDialog.vue`, mounted once in `App.vue`, shows it.
   falls through to the `'empty'` prompt rather than leaving silently.
 - `keepSession()` / `discardSessionAndLeave()`: the `'empty'` answers.
   Discarding deletes the session through
-  `useManagedDatastreams().deleteSessionChain`, and refuses if another
+  `useManagedDatastreams().deleteSession`, and refuses if another
   session was somehow built on it. A failed delete keeps the user in the
   session.
 - `closeSession()`: the `'saved'` answer, keeping the session as it is.
@@ -334,6 +334,12 @@ histories and fetches each one's sessions. Feeds the row Edit button's
 chooser (`StartEditingFlow.vue`), which lists managed datastreams with their
 in-progress/committed sessions.
 
+- `deleteManaged(historyId, managedId)`: removes the QC history, then the
+  managed datastream.
+- `deleteSession(historyId, sessionId)`: removes one session and throws the
+  server's message when it refuses (the session has dependents). Callers only
+  offer the newest session, or an empty in-progress one.
+
 ### `useDatastreamMetadata()`
 
 ```ts
@@ -342,7 +348,7 @@ await load()
 ```
 
 Reference data for the create-datastream form: the active workspace's sensors
-(`hs.sensors.list`, including the system-level ones) and the datastream status
+(`hs.sensors.listAllItems`, every page, including the system-level ones) and the datastream status
 vocabulary (`hs.datastreams.getStatuses`). Loaded on demand when the form
 opens rather than with the workspace catalog in `App.vue`, since nothing else
 needs it. Neither request throws: a list that fails to load stays empty and
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index dd0a1513..2f1ca77f 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -374,15 +374,14 @@ Two contract notes worth keeping in mind:
   `description`. Continuing work after a commit means starting a new session;
   the backend links it to every committed session its window overlaps, which
   is how the DAG gets built.
-- **Deleting a session takes its descendants with it.** The API allows
+- **Only a session with no dependents can be deleted.** The API allows
   deleting any session with no dependents, committed or not, and rejects one
-  that still has them. Sessions carry `dependencyIds` (their ancestors) on the
-  detail response only, so `utils/sessionGraph.ts` inverts that over a
-  `expand_related: true` listing to find descendants, and
-  `deleteSessionChain()` removes them deepest-first so each delete meets the
-  server's rule. There is no transaction: a failure part-way leaves the
-  earlier deletes in place, which is why the error names how many are already
-  gone and the chooser reloads from the server rather than guessing. Reopening the most recent commit
+  that still has them. `StartEditingDialog.vue` therefore only offers the
+  trash icon on the newest session in the timeline, which by construction has
+  none, and `useManagedDatastreams().deleteSession()` removes that one session.
+  Sessions carry `dependencyIds` (their ancestors) on an `expand_related: true`
+  listing; `utils/sessionGraph.ts` inverts that so the leave flow can refuse to
+  discard a session something was built on. Reopening the most recent commit
   is a pending backend ask (see the TODO in `store/qcSession.ts`) and needs
   more than lifting the status guard, since a commit also writes observations
   to the managed datastream and rolls the history's checksum and extent
@@ -454,7 +453,7 @@ promise resolves true.
 | Situation | Prompt | What it does |
 |---|---|---|
 | Unsaved edits | Save and close / Discard changes and close / Cancel | Save writes a draft to the in-progress session, and is disabled with no session open. Discarding that empties the session falls through to the next row. |
-| The session holds nothing at all | Keep session / Discard session / Cancel | Discard deletes it with `useManagedDatastreams().deleteSessionChain`. A failed delete keeps the user in the session. |
+| The session holds nothing at all | Keep session / Discard session / Cancel | Discard deletes it with `useManagedDatastreams().deleteSession`. A failed delete keeps the user in the session. |
 | Everything saved | Close / Cancel | Nothing. The session stays in progress. |
 | Nothing being edited, or committed history being viewed | none | Leaves silently. |
 
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index e22f05cd..8f2e5ab8 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -586,24 +586,26 @@ Things worth knowing:
 ## Deleting a session
 
 The **Edit** chooser shows every session on a managed datastream as a
-timeline, oldest first, each with a trash icon. Both in-progress and committed
-sessions can be deleted. When no session is open, the timeline is headed by a
-**Start new session** node.
+timeline, oldest first. Only the last row, the most recent session, carries a
+trash icon. When no session is open, the timeline is headed by a **Start new
+session** node.
 
 Sessions build on each other: a session started after a commit records that it
-began from that commit's result. Deleting a session therefore also deletes
-every session built on top of it, since those describe edits to data that would
-no longer exist. Hovering the trash icon says how far the delete reaches.
+began from that commit's result. Deleting an older session would therefore have
+to delete every session built on top of it, so the chooser does not offer it.
+The most recent session cannot have anything built on it, which makes it the
+only one safe to remove. It can be in progress or committed. To unwind further,
+delete the newest session, then the next, one at a time.
 
-The confirmation dialog names every session that will go, marks the one you
-picked, and orders them newest first, which is the order they are removed in.
-When the delete reaches beyond the session you picked, you also have to tick a
-box acknowledging the count. **None of this can be undone.**
+The confirmation dialog names the session and says its operations go with it.
+Earlier sessions are untouched. **This cannot be undone.**
 
-If the server rejects a delete part-way through a chain, the sessions already
-removed stay removed. The error says how many, and the chooser reloads so the
+If the server rejects the delete, the chooser reloads from the server so the
 list reflects what actually survived.
 
+To remove a managed datastream and all of its sessions at once, use the trash
+icon on the datastream's own header row instead.
+
 ## Save / load a QC history
 
 The QC history is the canonical save format. It's a JSON file you can keep, re-apply, share, or version-control.
diff --git a/apps/qc-app/src/components/EditData/StartEditingDialog.vue b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
index ada96b6d..7f6646da 100644
--- a/apps/qc-app/src/components/EditData/StartEditingDialog.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
@@ -130,7 +130,7 @@
             </v-timeline-item>
 
             <v-timeline-item
-              v-for="s in orderedSessions(opt.sessions)"
+              v-for="(s, i) in orderedSessions(opt.sessions)"
               :key="s.id"
               fill-dot
               size="x-small"
@@ -156,16 +156,22 @@
                   >
                     {{ sessionPeriod(s) }}
                   </div>
+                  <!-- The Continue button carries this state visually, so the
+                       row only needs it spelled out for screen readers. -->
+                  <span v-if="s.status === 'in_progress'" class="d-sr-only">
+                    In progress
+                  </span>
                 </div>
 
                 <v-chip
+                  v-if="s.status !== 'in_progress'"
                   size="x-small"
-                  :color="s.status === 'in_progress' ? 'warning' : 'grey'"
+                  color="grey"
                   variant="tonal"
                   label
                   class="flex-shrink-0"
                 >
-                  {{ s.status === 'in_progress' ? 'In progress' : 'Committed' }}
+                  Committed
                 </v-chip>
 
                 <v-btn
@@ -179,20 +185,22 @@
                 >
                   Continue
                 </v-btn>
+                <!-- Only the newest session can be deleted: anything older may
+                     have sessions built on it. -->
                 <v-btn
+                  v-if="i === opt.sessions.length - 1"
                   icon="mdi-trash-can-outline"
                   size="x-small"
                   variant="text"
                   color="error"
                   class="flex-shrink-0"
                   :data-testid="`delete-session-${s.id}`"
-                  :title="deleteHint(opt, s)"
+                  title="Delete this session"
                   @click="openConfirm(opt, s)"
                 />
               </div>
             </v-timeline-item>
           </v-timeline>
-
         </v-card>
 
         <v-btn
@@ -218,7 +226,7 @@
     </v-card-actions>
   </v-card>
 
-  <v-dialog v-model="confirmOpen" max-width="560" persistent>
+  <v-dialog v-model="confirmOpen" max-width="480" persistent>
     <v-card rounded="lg" data-testid="delete-session-dialog">
       <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
         <v-avatar color="error" variant="tonal" size="40">
@@ -226,11 +234,7 @@
         </v-avatar>
         <div class="d-flex flex-column">
           <div class="text-title-large font-weight-bold">
-            {{
-              chainCount > 1
-                ? `Delete ${chainCount} sessions?`
-                : 'Delete this session?'
-            }}
+            Delete this session?
           </div>
           <div class="text-body-small text-medium-emphasis">
             This permanently removes quality control work
@@ -238,91 +242,15 @@
         </div>
       </div>
 
-      <v-card-text class="pt-2 pb-4 px-6">
+      <v-card-text v-if="confirmingSession" class="pt-2 pb-4 px-6">
         <p class="text-body-medium mb-3">
-          <template v-if="chainCount > 1">
-            Later sessions were built on this one, so they go too. Deleting
-            only the one you picked would leave them describing edits that no
-            longer exist.
-          </template>
-          <template v-else>
-            Its operations and the record of them are removed.
-          </template>
+          "{{ sessionLabel(confirmingSession) }}" and the record of its
+          operations are removed. Earlier sessions are untouched.
         </p>
 
-        <div class="text-body-small font-weight-medium mb-1">
-          {{ chainCount > 1 ? 'Will be deleted, newest first' : 'Will be deleted' }}
-        </div>
-        <v-list
-          density="compact"
-          class="py-0 mb-3 rounded border"
-          :lines="false"
-          data-testid="delete-session-chain"
-        >
-          <v-list-item
-            v-for="s in chainSessions"
-            :key="s.id"
-            :data-testid="`delete-chain-item-${s.id}`"
-          >
-            <template #prepend>
-              <v-icon
-                :icon="
-                  s.id === confirmingSessionId
-                    ? 'mdi-target'
-                    : 'mdi-subdirectory-arrow-right'
-                "
-                :color="s.id === confirmingSessionId ? 'error' : 'warning'"
-                size="16"
-                class="mr-2"
-              />
-            </template>
-            <v-list-item-title class="text-body-small">
-              {{ sessionLabel(s) }}
-              <span v-if="s.id === confirmingSessionId" class="text-medium-emphasis">
-                (the one you picked)
-              </span>
-            </v-list-item-title>
-            <template #append>
-              <v-chip
-                size="x-small"
-                :color="s.status === 'in_progress' ? 'warning' : 'grey'"
-                variant="tonal"
-                label
-              >
-                {{ s.status === 'in_progress' ? 'In progress' : 'Committed' }}
-              </v-chip>
-            </template>
-          </v-list-item>
-        </v-list>
-
-        <div
-          class="text-body-small text-medium-emphasis mb-3"
-          data-testid="delete-session-dependents-note"
-        >
-          Any session built on this one is deleted with it.
-        </div>
-
         <v-alert type="error" variant="tonal" density="compact">
           <span class="text-body-small">This cannot be undone.</span>
         </v-alert>
-
-        <!-- Extra affirmation only when the cascade reaches beyond the
-             session the user actually pointed at. -->
-        <v-checkbox
-          v-if="chainCount > 1"
-          v-model="cascadeAcknowledged"
-          density="compact"
-          hide-details
-          color="error"
-          class="mt-2"
-          data-testid="delete-session-acknowledge"
-        >
-          <template #label>
-            <span class="text-body-small">
-              I understand {{ chainCount }} sessions will be deleted
-            </span>
-          </template>
-        </v-checkbox>
       </v-card-text>
 
       <v-divider />
@@ -335,11 +263,10 @@
           color="error"
           variant="flat"
           prepend-icon="mdi-delete-outline"
-          :disabled="confirmDisabled"
-          :data-testid="`confirm-delete-session-${confirmingSessionId}`"
+          :data-testid="`confirm-delete-session-${confirmingSession?.id}`"
           @click="onDeleteSession"
         >
-          {{ chainCount > 1 ? `Delete ${chainCount} sessions` : 'Delete session' }}
+          Delete session
         </v-btn>
       </v-card-actions>
     </v-card>
@@ -351,7 +278,6 @@ import { computed, ref } from 'vue'
 import type { Datastream, QualityControlSession } from '@hydroserver/client'
 import type { ManagedDatastreamOption } from '@/composables/useManagedDatastreams'
 import { formatDateRange } from '@/utils/time'
-import { collectDeletionChain } from '@/utils/sessionGraph'
 import { datastreamSummary } from '@/utils/datastreamSummary'
 
 defineProps<{
@@ -370,54 +296,26 @@ const emit = defineEmits<{
 
 // historyId of the managed datastream whose delete is awaiting confirmation.
 const confirmingDeleteId = ref<string | null>(null)
-// id of the session whose delete is awaiting confirmation, and the option
-// it belongs to, so the chain can be resolved against that option's sessions.
-const confirmingSessionId = ref<string | null>(null)
+// The session whose delete is awaiting confirmation, and the option it
+// belongs to, so the emit can name both.
+const confirmingSession = ref<QualityControlSession | null>(null)
 const confirmingOption = ref<ManagedDatastreamOption | null>(null)
-const cascadeAcknowledged = ref(false)
 
 const confirmOpen = computed({
-  get: () => !!confirmingSessionId.value,
+  get: () => !!confirmingSession.value,
   set: (open: boolean) => {
     if (!open) closeConfirm()
   },
 })
 
-/** The sessions this delete would remove, in the order they go. */
-const chainSessions = computed<QualityControlSession[]>(() => {
-  const opt = confirmingOption.value
-  const targetId = confirmingSessionId.value
-  if (!opt || !targetId) return []
-  const byId = new Map(opt.sessions.map((s) => [s.id, s]))
-  return collectDeletionChain(opt.sessions, targetId)
-    .map((id) => byId.get(id))
-    .filter((s): s is QualityControlSession => !!s)
-})
-
-const chainCount = computed(() => chainSessions.value.length)
-
-// A cascade needs the checkbox; a single session just needs the button.
-const confirmDisabled = computed(
-  () => chainCount.value > 1 && !cascadeAcknowledged.value
-)
-
-const deleteHint = (opt: ManagedDatastreamOption, s: QualityControlSession) => {
-  const count = collectDeletionChain(opt.sessions, s.id).length
-  return count > 1
-    ? `Delete this session and the ${count - 1} built on it`
-    : 'Delete this session'
-}
-
 function openConfirm(opt: ManagedDatastreamOption, s: QualityControlSession) {
   confirmingOption.value = opt
-  confirmingSessionId.value = s.id
-  cascadeAcknowledged.value = false
+  confirmingSession.value = s
 }
 
 function closeConfirm() {
-  confirmingSessionId.value = null
+  confirmingSession.value = null
   confirmingOption.value = null
-  cascadeAcknowledged.value = false
 }
 
 function onDelete(opt: ManagedDatastreamOption) {
@@ -427,7 +325,7 @@ function onDelete(opt: ManagedDatastreamOption) {
 
 function onDeleteSession() {
   const opt = confirmingOption.value
-  const sessionId = confirmingSessionId.value
+  const sessionId = confirmingSession.value?.id
   closeConfirm()
   if (opt && sessionId) emit('deleteSession', opt, sessionId)
 }
diff --git a/apps/qc-app/src/components/EditData/StartEditingFlow.vue b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
index 7ea50ded..ebd9fbf7 100644
--- a/apps/qc-app/src/components/EditData/StartEditingFlow.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
@@ -74,7 +74,6 @@ import {
 import { useDatastreamMetadata } from '@/composables/useDatastreamMetadata'
 import { useProcessingLevels } from '@/composables/useProcessingLevels'
 import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
-import { collectDeletionChain } from '@/utils/sessionGraph'
 import type { TimeWindow } from '@/utils/timeRangePresets'
 import type { View } from '@/store/userInterface'
 
@@ -99,7 +98,7 @@ const { sourceDatastream, sessions, inProgressSession } =
   storeToRefs(useQcSessionStore())
 const qcPreferences = useQcPreferencesStore()
 const workingCopies = useWorkingCopiesStore()
-const { loadForSource, deleteManaged, deleteSessionChain } =
+const { loadForSource, deleteManaged, deleteSession } =
   useManagedDatastreams()
 const { create: createManaged } = useCreateManagedDatastream()
 const { createProcessingLevel } = useProcessingLevels()
@@ -332,32 +331,23 @@ async function onChooserDelete(option: ManagedDatastreamOption) {
   }
 }
 
-// Only the chosen session and those built on it go; the managed datastream
-// and its other commits remain.
+// The chooser only offers the newest session, which nothing builds on, so
+// the managed datastream and its earlier sessions remain.
 async function onChooserDeleteSession(
   option: ManagedDatastreamOption,
   sessionId: string
 ) {
   try {
-    // Same input the confirmation previewed, so the two cannot disagree.
-    const deleted = await deleteSessionChain(
-      option.historyId,
-      collectDeletionChain(option.sessions, sessionId)
-    )
+    await deleteSession(option.historyId, sessionId)
     workingCopies.invalidate(option.managed.id)
-    const gone = new Set(deleted)
     chooserOptions.value = chooserOptions.value.map((o) =>
       o.historyId === option.historyId
-        ? { ...o, sessions: o.sessions.filter((s) => !gone.has(s.id)) }
+        ? { ...o, sessions: o.sessions.filter((s) => s.id !== sessionId) }
         : o
     )
-    Snackbar.success(
-      deleted.length === 1
-        ? 'Session deleted.'
-        : `${deleted.length} sessions deleted.`
-    )
+    Snackbar.success('Session deleted.')
   } catch (e) {
-    // Refresh regardless: a partial cascade leaves the chooser's copy wrong.
+    // The server may have changed under the chooser, so reload its copy.
     if (chooserSource.value) {
       chooserOptions.value = await loadForSource(chooserSource.value.id)
     }
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
index 56fd9656..0667680b 100644
--- a/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
@@ -126,14 +126,26 @@ describe('StartEditingDialog', () => {
     expect(item('s-0').find('.mdi-check').exists()).toBe(true)
   })
 
-  it('offers delete on every session, and Continue only in progress', () => {
+  it('offers delete only on the most recent session', () => {
     const w = mountDialog()
+    // s-1 is the newest, so nothing can be built on it.
     expect(w.find('[data-testid="delete-session-s-1"]').exists()).toBe(true)
-    // Committed sessions are deletable now that the API allows it.
-    expect(w.find('[data-testid="delete-session-s-0"]').exists()).toBe(true)
+    expect(w.find('[data-testid="delete-session-s-0"]').exists()).toBe(false)
     expect(w.find('[data-testid="continue-session-s-0"]').exists()).toBe(false)
   })
 
+  it('drops the In progress chip but keeps the state readable', () => {
+    const w = mountDialog()
+    const active = w.find('[data-testid="chooser-session-s-1"]')
+    // The Continue button already says the row is in progress.
+    expect(active.find('.v-chip').exists()).toBe(false)
+    expect(active.find('.d-sr-only').text()).toBe('In progress')
+    // A committed row has no button, so its chip is the only cue.
+    const done = w.find('[data-testid="chooser-session-s-0"]')
+    expect(done.find('.v-chip').text()).toBe('Committed')
+    expect(done.find('.d-sr-only').exists()).toBe(false)
+  })
+
   it('asks for confirmation, then emits deleteSession for that session', async () => {
     const w = mountDialog()
     await w.find('[data-testid="delete-session-s-1"]').trigger('click')
@@ -151,20 +163,16 @@ describe('StartEditingDialog', () => {
     expect(w.emitted('deleteSession')).toBeUndefined()
   })
 
-  it('warns that a lone delete cannot be undone', async () => {
+  it('warns that the delete cannot be undone', async () => {
     const w = mountDialog()
     await w.find('[data-testid="delete-session-s-1"]').trigger('click')
     const dialog = inDialog('delete-session-dialog')!
+    expect(dialog.textContent).toContain('Delete this session?')
     expect(dialog.textContent).toContain('cannot be undone')
-    // One session, so it lists only that one and needs no extra tick-box.
-    expect(
-      document.querySelectorAll('[data-testid^="delete-chain-item-"]')
-    ).toHaveLength(1)
+    // Only the newest session is deletable, so nothing cascades.
+    expect(inDialog('delete-session-chain')).toBeNull()
+    expect(inDialog('delete-session-dependents-note')).toBeNull()
     expect(inDialog('delete-session-acknowledge')).toBeNull()
-    // The listing can be incomplete, so the warning stands on its own.
-    expect(inDialog('delete-session-dependents-note')!.textContent).toContain(
-      'deleted with it'
-    )
   })
 
   it('formats session date ranges readably', () => {
@@ -219,7 +227,7 @@ describe('StartEditingDialog', () => {
   })
 })
 
-describe('StartEditingDialog cascade delete', () => {
+describe('StartEditingDialog session deletion', () => {
   // s-0 committed, s-1 built on it, s-2 built on s-1.
   const chained = [
     {
@@ -270,74 +278,54 @@ describe('StartEditingDialog cascade delete', () => {
     return w
   }
 
-  it('lists the whole chain, target last, when deleting the root', async () => {
-    await openDeleteFor('s-0')
-    const ids = [
-      ...document.querySelectorAll('[data-testid^="delete-chain-item-"]'),
-    ].map((el) => el.getAttribute('data-testid'))
-    expect(ids).toEqual([
-      'delete-chain-item-s-2',
-      'delete-chain-item-s-1',
-      'delete-chain-item-s-0',
-    ])
+  it('offers delete on the newest session only, whatever came before', () => {
+    const w = mountChained()
+    expect(w.find('[data-testid="delete-session-s-2"]').exists()).toBe(true)
+    expect(w.find('[data-testid="delete-session-s-1"]').exists()).toBe(false)
+    expect(w.find('[data-testid="delete-session-s-0"]').exists()).toBe(false)
   })
 
-  it('says how many sessions go and names them', async () => {
-    await openDeleteFor('s-0')
+  it('names the session and deletes it without an acknowledgement', async () => {
+    const w = await openDeleteFor('s-2')
     const dialog = inDialog('delete-session-dialog')!
-    expect(dialog.textContent).toContain('Delete 3 sessions?')
-    expect(dialog.textContent).toContain('First pass')
-    expect(dialog.textContent).toContain('Second pass')
+    expect(dialog.textContent).toContain('Delete this session?')
     expect(dialog.textContent).toContain('Third pass')
-    expect(dialog.textContent).toContain('cannot be undone')
-  })
-
-  it('marks which session the user actually picked', async () => {
-    await openDeleteFor('s-0')
-    const target = inDialog('delete-chain-item-s-0')!
-    expect(target.textContent).toContain('the one you picked')
-    expect(inDialog('delete-chain-item-s-1')!.textContent).not.toContain(
-      'the one you picked'
-    )
-  })
-
-  it('holds the delete until the cascade is acknowledged', async () => {
-    const w = await openDeleteFor('s-0')
-    const confirm = inDialog('confirm-delete-session-s-0') as HTMLButtonElement
-    expect(confirm.disabled).toBe(true)
-
-    confirm.click()
-    await flushPromises()
-    expect(w.emitted('deleteSession')).toBeUndefined()
-
-    // v-checkbox puts the test id on its wrapper; the input carries the model.
-    const box = document.querySelector(
-      '[data-testid="delete-session-acknowledge"] input'
-    ) as HTMLInputElement
-    box.click()
-    await flushPromises()
-    expect(
-      (inDialog('confirm-delete-session-s-0') as HTMLButtonElement).disabled
-    ).toBe(false)
-    await clickInDialog('confirm-delete-session-s-0')
-    expect(w.emitted('deleteSession')![0][1]).toBe('s-0')
-  })
-
-  it('needs no acknowledgement for a leaf session', async () => {
-    await openDeleteFor('s-2')
     expect(inDialog('delete-session-acknowledge')).toBeNull()
-    expect(
-      (inDialog('confirm-delete-session-s-2') as HTMLButtonElement).disabled
-    ).toBe(false)
+
+    const confirm = inDialog('confirm-delete-session-s-2') as HTMLButtonElement
+    expect(confirm.disabled).toBe(false)
+    await clickInDialog('confirm-delete-session-s-2')
+    expect(w.emitted('deleteSession')![0][1]).toBe('s-2')
   })
 
-  it('tells the user on the row how far the delete reaches', async () => {
+  it('labels the trash button for the one session it removes', () => {
     const w = mountChained()
-    expect(
-      w.find('[data-testid="delete-session-s-0"]').attributes('title')
-    ).toBe('Delete this session and the 2 built on it')
     expect(
       w.find('[data-testid="delete-session-s-2"]').attributes('title')
     ).toBe('Delete this session')
   })
+
+  it('offers delete on the only session of a managed datastream', () => {
+    const lone = [
+      {
+        historyId: 'h-9',
+        managed: { id: 'mgd-9', name: 'Temp (QC solo)' },
+        sessions: [
+          {
+            id: 's-solo',
+            status: 'committed',
+            createdAt: '2025-03-01T09:00:00Z',
+            description: 'Only pass',
+            phenomenonTimeStart: '2025-03-01T00:00:00Z',
+            phenomenonTimeEnd: '2025-03-05T00:00:00Z',
+            dependencyIds: [],
+          },
+        ],
+      },
+    ] as any
+    const w = mountDialog({ options: lone })
+    expect(w.find('[data-testid="delete-session-s-solo"]').exists()).toBe(true)
+    // The managed datastream keeps its own delete, separate from the session.
+    expect(w.find('[data-testid="delete-managed-mgd-9"]').exists()).toBe(true)
+  })
 })
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
index 6cce8abf..d5cab844 100644
--- a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
@@ -23,7 +23,7 @@ import { createTestVuetify } from '@/utils/test/vuetify'
 const {
   loadForSource,
   deleteManaged,
-  deleteSessionChain,
+  deleteSession,
   enterEdit,
   startSessionOver,
   leaveEdit,
@@ -44,7 +44,7 @@ const {
   return {
     loadForSource: vi.fn(),
     deleteManaged: vi.fn(),
-    deleteSessionChain: vi.fn(),
+    deleteSession: vi.fn(),
     enterEdit: vi.fn(),
     startSessionOver: vi.fn(),
     leaveEdit: vi.fn(),
@@ -98,7 +98,7 @@ vi.mock('@/composables/useManagedDatastreams', () => ({
   useManagedDatastreams: () => ({
     loadForSource,
     deleteManaged,
-    deleteSessionChain,
+    deleteSession,
   }),
 }))
 
diff --git a/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts b/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
index e6228c7a..4835843a 100644
--- a/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
@@ -13,7 +13,7 @@ const {
   unsavedEditCount,
   saveDraft,
   discardUnsavedEdits,
-  deleteSessionChain,
+  deleteSession,
   applySessions,
   redraw,
   invalidate,
@@ -33,7 +33,7 @@ const {
     unsavedEditCount: r(0),
     saveDraft: vi.fn(),
     discardUnsavedEdits: vi.fn(),
-    deleteSessionChain: vi.fn(),
+    deleteSession: vi.fn(),
     applySessions: vi.fn(),
     redraw: vi.fn(),
     invalidate: vi.fn(),
@@ -80,7 +80,7 @@ vi.mock('@/composables/useEditSession', () => ({
 }))
 
 vi.mock('@/composables/useManagedDatastreams', () => ({
-  useManagedDatastreams: () => ({ deleteSessionChain }),
+  useManagedDatastreams: () => ({ deleteSession }),
 }))
 
 vi.mock('@uwrl/qc-utils', () => ({ Snackbar: { success, error } }))
@@ -113,7 +113,7 @@ beforeEach(() => {
   unsavedEditCount.value = 0
   saveDraft.mockResolvedValue(undefined)
   discardUnsavedEdits.mockResolvedValue(undefined)
-  deleteSessionChain.mockResolvedValue(['qcs-1'])
+  deleteSession.mockResolvedValue(undefined)
 })
 
 describe('leaveCase', () => {
@@ -272,7 +272,7 @@ describe('requestLeave', () => {
     keepSession()
 
     expect(await pending).toBe(true)
-    expect(deleteSessionChain).not.toHaveBeenCalled()
+    expect(deleteSession).not.toHaveBeenCalled()
   })
 
   it('deletes only the empty session itself', async () => {
@@ -282,7 +282,7 @@ describe('requestLeave', () => {
 
     await discardSessionAndLeave()
 
-    expect(deleteSessionChain).toHaveBeenCalledWith('h-1', ['qcs-1'])
+    expect(deleteSession).toHaveBeenCalledWith('h-1', 'qcs-1')
     expect(invalidate).toHaveBeenCalledWith('mgd-1')
     // The store no longer lists a session the server does not have.
     expect(applySessions).toHaveBeenCalledWith('h-1', [])
@@ -301,7 +301,7 @@ describe('requestLeave', () => {
 
     await discardSessionAndLeave()
 
-    expect(deleteSessionChain).not.toHaveBeenCalled()
+    expect(deleteSession).not.toHaveBeenCalled()
     expect(error).toHaveBeenCalled()
     expect(leavePrompt.value?.kind).toBe('empty')
     cancelLeave()
@@ -310,7 +310,7 @@ describe('requestLeave', () => {
 
   it('keeps the user in the session when the delete fails', async () => {
     emptySession()
-    deleteSessionChain.mockRejectedValueOnce(new Error('server said no'))
+    deleteSession.mockRejectedValueOnce(new Error('server said no'))
     const { requestLeave, discardSessionAndLeave, leavePrompt, cancelLeave } =
       useLeaveSession()
     const pending = requestLeave()
@@ -333,7 +333,7 @@ describe('requestLeave', () => {
     closeSession()
 
     expect(await pending).toBe(true)
-    expect(deleteSessionChain).not.toHaveBeenCalled()
+    expect(deleteSession).not.toHaveBeenCalled()
   })
 
   it('a second request supersedes the first, which stays put', async () => {
diff --git a/apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts b/apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts
index 65b93958..2d2c80a2 100644
--- a/apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts
@@ -8,6 +8,7 @@ const historiesBySource = new Map<string, any[]>()
 const listAllItems = vi.fn()
 const deleteHistory = vi.fn()
 const deleteDatastream = vi.fn()
+const deleteSessionCall = vi.fn()
 
 vi.mock('@/store/dataVisualization', () => ({
   useDataVisStore: () => ({ datastreams, historiesBySource }),
@@ -15,7 +16,7 @@ vi.mock('@/store/dataVisualization', () => ({
 vi.mock('@/store/hydroserver', () => ({
   useHydroServer: () => ({
     hs: {
-      qualityControlSessions: { listAllItems },
+      qualityControlSessions: { listAllItems, delete: deleteSessionCall },
       qualityControlHistories: { delete: deleteHistory },
       datastreams: { delete: deleteDatastream },
     },
@@ -90,4 +91,18 @@ describe('useManagedDatastreams.loadForSource', () => {
     await expect(deleteManaged('h-1', 'mgd-1')).rejects.toThrow('history busy')
     expect(deleteDatastream).not.toHaveBeenCalled()
   })
+
+  it('deleteSession removes one session', async () => {
+    deleteSessionCall.mockResolvedValue({ ok: true })
+    const { deleteSession } = useManagedDatastreams()
+    await deleteSession('h-1', 'qcs-2')
+    expect(deleteSessionCall).toHaveBeenCalledTimes(1)
+    expect(deleteSessionCall).toHaveBeenCalledWith('h-1', 'qcs-2')
+  })
+
+  it('deleteSession throws the server message when the delete is refused', async () => {
+    deleteSessionCall.mockResolvedValue({ ok: false, message: 'has dependents' })
+    const { deleteSession } = useManagedDatastreams()
+    await expect(deleteSession('h-1', 'qcs-1')).rejects.toThrow('has dependents')
+  })
 })
diff --git a/apps/qc-app/src/composables/useLeaveSession.ts b/apps/qc-app/src/composables/useLeaveSession.ts
index 4b3bf919..de12439a 100644
--- a/apps/qc-app/src/composables/useLeaveSession.ts
+++ b/apps/qc-app/src/composables/useLeaveSession.ts
@@ -167,7 +167,7 @@ async function discardSessionAndLeave(): Promise<void> {
         'Another session was built on this one, so it cannot be discarded.'
       )
     }
-    await useManagedDatastreams().deleteSessionChain(historyId, [session.id])
+    await useManagedDatastreams().deleteSession(historyId, session.id)
   } catch (e) {
     Snackbar.error(messageOf(e, 'Could not discard the session.'))
     return
diff --git a/apps/qc-app/src/composables/useManagedDatastreams.ts b/apps/qc-app/src/composables/useManagedDatastreams.ts
index 95ecb246..17c6087c 100644
--- a/apps/qc-app/src/composables/useManagedDatastreams.ts
+++ b/apps/qc-app/src/composables/useManagedDatastreams.ts
@@ -40,8 +40,7 @@ export function useManagedDatastreams() {
             id: managedId,
             name: h.managedDatastream?.name ?? managedId,
           } as Datastream)
-        // expand_related so each session carries `dependencyIds`, which the
-        // deletion chain is derived from.
+        // expand_related so each session carries `dependencyIds`.
         const sessions = await hs.qualityControlSessions.listAllItems(
           historyId,
           { expand_related: true }
@@ -74,39 +73,19 @@ export function useManagedDatastreams() {
   }
 
   /**
-   * Delete `chain` in order, deepest dependent first so each delete meets
-   * the server's "no dependents" rule. Resolves with the ids removed. On
-   * failure it throws, and the message names how many were already gone,
-   * because the deletes that landed cannot be rolled back.
-   *
-   * Takes the chain rather than deriving one: it must be exactly what the
-   * confirmation showed. If the server has since gained a dependent the
-   * delete is refused there, which is the safe direction to be wrong in.
+   * Delete one session. The server refuses a session that others were built
+   * on, so callers only offer sessions without dependents.
    */
-  async function deleteSessionChain(
+  async function deleteSession(
     historyId: string,
-    chain: string[]
-  ): Promise<string[]> {
+    sessionId: string
+  ): Promise<void> {
     const { hs } = useHydroServer()
-    if (!chain.length) {
-      throw new Error('That session no longer exists.')
-    }
-
-    const deleted: string[] = []
-    for (const id of chain) {
-      const res = await hs.qualityControlSessions.delete(historyId, id)
-      if (!res.ok) {
-        const reason = res.message || 'Could not delete the session.'
-        throw new Error(
-          deleted.length
-            ? `${reason} ${deleted.length} of ${chain.length} sessions were already deleted and cannot be restored.`
-            : reason
-        )
-      }
-      deleted.push(id)
+    const res = await hs.qualityControlSessions.delete(historyId, sessionId)
+    if (!res.ok) {
+      throw new Error(res.message || 'Could not delete the session.')
     }
-    return deleted
   }
 
-  return { loadForSource, deleteManaged, deleteSessionChain }
+  return { loadForSource, deleteManaged, deleteSession }
 }
diff --git a/apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts b/apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts
index 7246ce8e..4fafb8ab 100644
--- a/apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts
@@ -1,7 +1,6 @@
 import { describe, it, expect } from 'vitest'
 import {
   buildDependentsMap,
-  collectDeletionChain,
   hasDependents,
 } from '@/utils/sessionGraph'
 import type { SessionNode } from '@/utils/sessionGraph'
@@ -37,52 +36,3 @@ describe('hasDependents', () => {
     expect(hasDependents(chain, 'c')).toBe(false)
   })
 })
-
-describe('collectDeletionChain', () => {
-  it('returns just the target when nothing depends on it', () => {
-    expect(collectDeletionChain(chain, 'c')).toEqual(['c'])
-  })
-
-  it('orders descendants before the target', () => {
-    expect(collectDeletionChain(chain, 'a')).toEqual(['c', 'b', 'a'])
-  })
-
-  it('deletes a shared descendant before either of its parents', () => {
-    // t -> a, t -> b, and both a and b -> d.
-    const diamond = [
-      node('t'),
-      node('a', ['t']),
-      node('b', ['t']),
-      node('d', ['a', 'b']),
-    ]
-    const order = collectDeletionChain(diamond, 't')
-
-    expect(order).toHaveLength(4)
-    expect(order.at(-1)).toBe('t')
-    // Every session precedes the ones it was built on.
-    expect(order.indexOf('d')).toBeLessThan(order.indexOf('a'))
-    expect(order.indexOf('d')).toBeLessThan(order.indexOf('b'))
-    expect(order.indexOf('a')).toBeLessThan(order.indexOf('t'))
-    expect(order.indexOf('b')).toBeLessThan(order.indexOf('t'))
-  })
-
-  it('lists each session once when reachable by several paths', () => {
-    const diamond = [
-      node('t'),
-      node('a', ['t']),
-      node('b', ['t']),
-      node('d', ['a', 'b']),
-    ]
-    const order = collectDeletionChain(diamond, 't')
-    expect(new Set(order).size).toBe(order.length)
-  })
-
-  it('ignores sessions on an unrelated branch', () => {
-    const forked = [...chain, node('x'), node('y', ['x'])]
-    expect(collectDeletionChain(forked, 'b')).toEqual(['c', 'b'])
-  })
-
-  it('returns nothing for an unknown target', () => {
-    expect(collectDeletionChain(chain, 'nope')).toEqual([])
-  })
-})
diff --git a/apps/qc-app/src/utils/sessionGraph.ts b/apps/qc-app/src/utils/sessionGraph.ts
index 1c4673d5..660d4c7a 100644
--- a/apps/qc-app/src/utils/sessionGraph.ts
+++ b/apps/qc-app/src/utils/sessionGraph.ts
@@ -38,31 +38,3 @@ export function hasDependents(
 ): boolean {
   return (buildDependentsMap(sessions).get(sessionId)?.length ?? 0) > 0
 }
-
-/**
- * The sessions that must be removed to delete `targetId`, ordered so each
- * one is free of dependents by the time its turn comes: descendants first,
- * target last. Empty when the target is not in `sessions`.
- */
-export function collectDeletionChain(
-  sessions: SessionNode[],
-  targetId: string
-): string[] {
-  if (!sessions.some((s) => s.id === targetId)) return []
-
-  const dependents = buildDependentsMap(sessions)
-  const order: string[] = []
-  const visited = new Set<string>()
-
-  // Post-order over the dependents edges: a node is appended only after
-  // everything reachable from it, which is exactly delete order.
-  const visit = (id: string) => {
-    if (visited.has(id)) return
-    visited.add(id)
-    for (const childId of dependents.get(id) ?? []) visit(childId)
-    order.push(id)
-  }
-  visit(targetId)
-
-  return order
-}

From 6a55c0a9704523cb8a72206b171c4731ad14c58e Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:01:03 -0600
Subject: [PATCH 65/93] feat: reserve a plot slot for the edit target and label
 the row Edit button

---
 apps/qc-app/docs/API_REFERENCE.md             |  2 +-
 apps/qc-app/docs/PERFORMANCE.md               |  7 +++---
 apps/qc-app/docs/TESTING.md                   |  4 +--
 apps/qc-app/docs/USER_GUIDE.md                | 24 +++++++++---------
 .../VisualizeData/DataVisDatasetsTable.vue    | 25 +++++++++----------
 .../VisualizeData/PlotSourceDialog.vue        |  4 +--
 .../__tests__/DataVisDatasetsTable.spec.ts    | 16 ++++++++++++
 7 files changed, 49 insertions(+), 33 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index dec6db26..95e556d2 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -426,7 +426,7 @@ on boot.
 | `selectedObservedPropertyNames`     | state    | `string[]`                                        | Observed-property filter selection. |
 | `selectedProcessingLevelNames`      | state    | `string[]`                                        | Processing-level filter selection. |
 | `filteredDatastreams`               | computed | `(Datastream & DatastreamExtended)[]`             | `datastreams` narrowed by the three filter selections, with managed (QC) datastreams excluded. |
-| `plottedDatastreams`                | state    | `Datastream[]`                                    | Up to 5 streams the user chose to plot. Editing never adds to or removes from it (snapshots are the exception, dropped on leave); the 5-stream cap doesn't count the edit target or its source. |
+| `plottedDatastreams`                | state    | `Datastream[]`                                    | Up to 4 streams the user chose to plot. Editing never adds to or removes from it (snapshots are the exception, dropped on leave); the 4-stream cap doesn't count the edit target or its source, so the plot holds at most 6 series while editing. |
 | `qcDatastreamId`                    | state    | `string \| null`                                  | The edit target's id. Set only by the edit flow (`setEditTarget` / `clearEditTarget`); null in the Select view. |
 | `qcDatastream`                      | computed | `Datastream \| null`                              | Live catalog lookup of `qcDatastreamId` in `datastreams`, not `plottedDatastreams`, since the edit target isn't a plotted entry. |
 | `sourceContextDatastream`           | computed | `Datastream \| null`                              | The edit target's catalog source, resolved through `qcHistories`; null without an edit target. Drawn behind the edit target as context. |
diff --git a/apps/qc-app/docs/PERFORMANCE.md b/apps/qc-app/docs/PERFORMANCE.md
index f6999375..8d2b71dc 100644
--- a/apps/qc-app/docs/PERFORMANCE.md
+++ b/apps/qc-app/docs/PERFORMANCE.md
@@ -47,8 +47,9 @@ Order-of-magnitude envelope:
 | 5 million obs     | ~60 MB            | Still feasible on a modern desktop. Initial fetch is the long pole.    |
 | 50 million obs    | ~600 MB           | Browser tab approaching its memory budget. Avoid plotting more than 1 stream. Performance becomes dominated by Plotly redraw, not qc-utils. |
 
-The architecture caps the plot at **5 concurrent streams** in the UI to
-keep the multi-axis chart readable and the memory envelope predictable.
+The architecture caps the plot at **4 checked streams** in the UI, plus the
+datastream being edited and its source, to keep the multi-axis chart readable
+and the memory envelope predictable.
 
 ## Worker pool and calibration
 
@@ -139,7 +140,7 @@ synchronized plotting at high point density is the dominant interactive
 cost. Practical guidance:
 
 - **One edit target, its source, and one or two context streams** is the sweet spot.
-  Five plotted streams is the hard cap; in practice three is comfortable
+  Four checked streams is the hard cap; in practice two is comfortable
   at multi-hundred-thousand points.
 - **Tick alignment and viewport recompute are debounced.** See
   `src/utils/plotting/relayout.ts`.
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index aa17ba81..50627e31 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -280,7 +280,7 @@ frontends to be running, and enters QC through the Data Management
 same-origin entrypoint. It edits through a QC session, so the workspace
 needs a source datastream with a managed datastream and QC history: the
 spec plots the first source showing the managed-count badge, clicks its
-row's pencil (Edit) button, and ends by saving and committing the session.
+row's Edit button, and ends by saving and committing the session.
 
 ### Mocks
 
@@ -315,7 +315,7 @@ a spec can pass a `qcSessionState` array and assert on what was persisted
 (see `submit.spec.ts`).
 
 Save and Commit only appear once a session is open. `setupEditView` enters
-through a row's pencil (Edit) button rather than the nav rail, so it needs
+through a row's Edit button rather than the nav rail, so it needs
 `installMocks(page, { qcHistories: true })` to give the source a managed
 datastream for the chooser to pick. Use `startSessionFromRow` (also with
 `qcHistories: true`) when a spec needs to plot or pick a range first and
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 8f2e5ab8..c4f07e9a 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -13,8 +13,8 @@ If you are looking for developer / deployment docs for the QC App, start with [A
 The QC App is the operator's view of HydroServer's quality control pipeline. With it you can:
 
 1. **Browse** monitoring sites and datastreams in a HydroServer workspace.
-2. **Plot up to five datastreams** on a synchronized multi-axis chart for visual context.
-3. **Pick one datastream to edit with its row's pencil button.** Plotted datastreams are read-only context.
+2. **Plot up to four datastreams** on a synchronized multi-axis chart for visual context, keeping the fifth plot slot for the datastream you edit.
+3. **Pick one datastream to edit with its row's Edit button.** Plotted datastreams are read-only context.
 4. **Filter** suspicious points using value thresholds, time windows, change detection, rate-of-change limits, gap detection, or persistence runs.
 5. **Edit** the selected points: change values, interpolate, drift-correct, shift datetimes, delete, fill gaps, add points, attach qualifier flags.
 6. **Save your edits as a QC history** (a JSON file you can replay on the same datastream later).
@@ -64,7 +64,7 @@ A thin, always-visible column of icons.
 |------|--------|
 | HydroServer logo | Top left. Go home. Resets the current view. Asks what should happen to an open edit session first. |
 | Cursor (Select) | Top left. Show the datastream Select drawer + plot. While you are editing, this keeps the session open behind it. |
-| Pencil (Edit) | Top left. Return to the Edit view. **Disabled** until you're editing: pick a datastream to edit with its row's pencil button first. |
+| Pencil (Edit) | Top left. Return to the Edit view. **Disabled** until you're editing: pick a datastream to edit with its row's Edit button first. |
 | Stopwatch (Performance) | Bottom left. Open the Performance Calibration dialog. See "Performance" below. |
 | Briefcase (Workspaces) | Bottom left. Switch workspace, or Continue in the current one. |
 | Logout | Bottom left. Sign out. |
@@ -88,7 +88,7 @@ The filter drawer has two collapsible sections:
 The main area to the right of the window is split top/bottom:
 
 - **Top card** carries the preview plot and a header showing how many datastreams are plotted ("N datastreams plotted" or "No datastream plotted"). The right pane of the card lists currently plotted datastreams, and, while you are editing, an **Editing** panel above it.
-- **Bottom card** is the **Datastreams selection table**: listing every datastream the filters match. Each row has a **Plot** toggle (check box) and an **Edit** column (pencil button). See [Datastreams with quality-controlled versions](#datastreams-with-quality-controlled-versions) below for what the Edit button does.
+- **Bottom card** is the **Datastreams selection table**: listing every datastream the filters match. Each row has a **Plot** toggle (check box) and an **Edit** column (Edit button). See [Datastreams with quality-controlled versions](#datastreams-with-quality-controlled-versions) below for what the Edit button does.
 
 ![Select view with one datastream plotted](./images/home-plotted.png)
 
@@ -135,13 +135,13 @@ a raw series against the quality-controlled version derived from it.
 
 Clicking the check box again reopens the chooser with the current selection, so
 that is also where you change or clear it. Unticking everything and applying
-unplots every series from that source. The five-series plot cap still applies:
-once it is reached, unticked rows are disabled.
+unplots every series from that source. The four-datastream plot cap still
+applies: once it is reached, unticked rows are disabled.
 
 The row's check box reflects the whole group. It is filled when the raw
 datastream is plotted, and shows a partial mark when only managed versions are.
 
-The row's pencil (**Edit**) button is separate from plotting: it opens a
+The row's **Edit** button is separate from plotting: it opens a
 chooser of that source's managed datastreams, or, the first time, skips
 straight to the create-datastream form. Pick a managed datastream to
 continue its in-progress session, or **Start new session** to open the
@@ -314,7 +314,7 @@ While editing, the edit target's row shows the session's working data: committed
 
 ### Plotting multiple datastreams
 
-You can plot up to **5 datastreams at a time**. The plotted count and cap are surfaced in the Datastreams table toolbar as a chip ("`N/5 plotted`"). Once you hit the cap, the unchecked rows disable their plot toggles and a tooltip explains why. Unplot a row from either the table or the list to free up a slot. While editing, the edit target and its raw source are shown in addition to the 5 plotted datastreams. They don't count against the cap.
+You can check up to **4 datastreams at a time**. The fifth plot slot is kept for the datastream you edit, so there is always room for it. The plotted count and cap are surfaced in the Datastreams table toolbar as a chip ("`N/4 plotted`"). Once you hit the cap, the unchecked rows disable their plot toggles and a tooltip explains why. Unplot a row from either the table or the list to free up a slot. While editing, the edit target and its raw source are shown in addition to the 4 plotted datastreams. They don't count against the cap.
 
 ![Two datastreams on independent y-axes](./images/home-multi-datastreams.png)
 
@@ -686,7 +686,7 @@ Which question depends on where the session stands:
 - **Everything is saved.** The dialog only confirms that the session stays in
   progress; **Close** leaves, **Cancel** stays.
 
-To come back to a session you left in progress, click the pencil button on
+To come back to a session you left in progress, click the Edit button on
 that datastream's row again. It reopens where you left off.
 
 **Cancel** puts you back where you were: same zoom, same staged range, same
@@ -712,7 +712,7 @@ See [PERFORMANCE.md](./PERFORMANCE.md) for the envelope details.
 
 ### "I just want to drop everything above 1000 and re-submit."
 
-1. Pick a workspace, then click the pencil button on the row of the datastream you want to QC.
+1. Pick a workspace, then click the Edit button on the row of the datastream you want to QC.
 2. Expand **Value thresholds**, set `Greater than: 1000`, press Enter.
 3. Expand **Delete points**, click Delete.
 4. Click **Save** at the bottom of the Edit history panel, then **Commit** and confirm in the dialog.
@@ -742,9 +742,9 @@ Click **Workspaces** in the nav rail and **Select** another, or **Continue** to
 |---------|--------------|-----|
 | Blank page on load | Wrong API URL or `localhost` vs `127.0.0.1` mismatch. | See [DEPLOYMENT.md](./DEPLOYMENT.md). |
 | Plot stays empty after picking a datastream | A `Custom` time range that doesn't overlap the datastream's observations, or the datastream has no observations. The plotted row shows a database-off icon and the subtitle reads `0 pts loaded`. | Click a preset such as **1m** or **All** in Time range. |
-| The Edit rail item is greyed out | You are not editing. | Click a row's pencil button. |
+| The Edit rail item is greyed out | You are not editing. | Click a row's Edit button. |
 | The Select view shows an "Editing" panel | A session is still open. | That is expected: click **Back to editor** to return to it, or **Close** in the editor to end it. |
-| I closed the editor and cannot find my session | It is still in progress, just not open. | Click the pencil button on that datastream's row to resume it. Sessions you discarded on the way out are gone for good. |
+| I closed the editor and cannot find my session | It is still in progress, just not open. | Click the Edit button on that datastream's row to resume it. Sessions you discarded on the way out are gone for good. |
 | Big edits freeze the page | `SharedArrayBuffer` not available; running inline. | Have your admin re-enable COOP/COEP headers, or accept the slower fallback. |
 | Save fails with a backend error | Permissions / workspace issue / network. | The Snackbar shows the backend message verbatim. Share that with your admin. |
 | The history shows a red failed entry after Load | The QC history referenced something missing in this datastream. | The rest of the QC history still ran. Click the chevron on the row to see its arguments. |
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
index 41c400ef..29b5fd1c 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
@@ -126,19 +126,14 @@
         </template>
 
         <template #header.edit>
-          <v-icon
-            icon="mdi-pencil"
-            size="18"
-            title="Edit"
-            aria-label="Edit"
-          />
+          <span class="text-body-small">Edit</span>
         </template>
 
         <template v-slot:item.plot="{ item }">
           <v-tooltip
             :disabled="!isAtCap(item)"
             location="top"
-            :text="`Maximum of ${PLOT_CAP} datastreams plotted; remove one to add another`"
+            :text="`Maximum of ${PLOT_CAP} datastreams plotted, one slot is kept for the datastream you edit; remove one to add another`"
           >
             <template #activator="{ props: tooltipProps }">
               <div class="d-flex align-center" v-bind="tooltipProps">
@@ -195,15 +190,18 @@
               <!-- A disabled button passes clicks to this wrapper; keep them off the row. -->
               <span v-bind="tooltipProps" @click.stop>
                 <v-btn
-                  icon="mdi-pencil"
+                  prepend-icon="mdi-pencil"
                   size="small"
-                  variant="text"
+                  variant="tonal"
+                  color="primary"
                   density="comfortable"
                   :data-testid="`edit-datastream-${item.id}`"
                   :aria-label="`Edit ${item.name}`"
                   :disabled="!canEditWorkspace"
                   @click.stop="emit('edit', item)"
-                />
+                >
+                  Edit
+                </v-btn>
               </span>
             </template>
           </v-tooltip>
@@ -297,8 +295,9 @@ import {
 import { Snackbar } from '@uwrl/qc-utils'
 import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
 
-/** Maximum series the plot holds at once. */
-const PLOT_CAP = 5
+/** Datastreams the user can check. The fifth plot slot is kept for the
+ *  datastream being edited. */
+const PLOT_CAP = 4
 
 const emit = defineEmits<{
   (e: 'edit', datastream: Datastream & DatastreamExtended): void
@@ -491,7 +490,7 @@ const getRowProps = ({ item }: { item: Datastream }) => ({
 const search = ref()
 const headers = reactive([
   { title: 'Plot', key: 'plot', visible: true, width: 96, sortable: false },
-  { title: 'Edit', key: 'edit', visible: true, width: 56, sortable: false },
+  { title: 'Edit', key: 'edit', visible: true, width: 96, sortable: false },
   {
     title: 'Name',
     key: 'name',
diff --git a/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue b/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
index 8dfe6bd9..23df6e7e 100644
--- a/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
+++ b/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
@@ -60,7 +60,7 @@
           class="text-body-small text-medium-emphasis"
           data-testid="plot-source-cap"
         >
-          Maximum of 5 datastreams plotted; remove one to add another.
+          Maximum of 4 datastreams plotted; remove one to add another.
         </div>
       </template>
     </div>
@@ -95,7 +95,7 @@ const props = defineProps<{
   options: ManagedDatastreamOption[]
   plottedIds: string[]
   loading?: boolean
-  /** Free slots under the 5-series plot cap, ignoring this source's group. */
+  /** Free slots under the 4-datastream plot cap, ignoring this source's group. */
   slotsLeft: number
 }>()
 
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
index ded898c3..55cfb97e 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
@@ -143,6 +143,22 @@ describe('DataVisDatasetsTable plot checkbox', () => {
     expect(plotSourceSelection).toHaveBeenCalledWith('src', ['mgd'])
   })
 
+  // The fifth plot slot belongs to the datastream being edited.
+  it('stops at four checked datastreams', async () => {
+    plottedDatastreams.value = [
+      { id: 'a' },
+      { id: 'b' },
+      { id: 'c' },
+      { id: 'd' },
+    ]
+    const wrapper = await mountTable()
+    expect(wrapper.text()).toContain('4/4 plotted')
+
+    await checkbox(wrapper, 'solo').trigger('click')
+    await flushPromises()
+    expect(toggleDatastream).not.toHaveBeenCalled()
+  })
+
   it('shows a checked box when the raw datastream is plotted', async () => {
     plottedDatastreams.value = [raw]
     const wrapper = await mountTable()

From 093e474c49742165c564f78186d226cea915a8e9 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 17 Sep 2026 22:01:03 -0600
Subject: [PATCH 66/93] feat: add presets and one-click fixes to the session
 window dialog

---
 apps/qc-app/docs/API_REFERENCE.md             |  23 +-
 apps/qc-app/docs/ARCHITECTURE.md              |   4 +-
 apps/qc-app/e2e/edit-selection.spec.ts        |  40 ++++
 .../EditData/SessionWindowDialog.vue          |  92 +++++++-
 .../__tests__/SessionWindowDialog.spec.ts     |  94 ++++++++
 .../src/utils/__tests__/sessionWindow.spec.ts | 195 +++++++++++++++--
 apps/qc-app/src/utils/__tests__/time.spec.ts  |  23 +-
 apps/qc-app/src/utils/sessionWindow.ts        | 203 +++++++++++++++++-
 apps/qc-app/src/utils/time.ts                 |   7 +
 9 files changed, 633 insertions(+), 48 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 95e556d2..343c4840 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -781,11 +781,24 @@ opened from the row Edit chooser or the editor footer's **New session**):
   full extent, begin to end. `null` when the source has no observations.
   The rules below accept it whenever the committed history lies inside that
   extent; the dialog reports the rare case where it does not.
-- `validateSessionWindow(window, source, sessions)`: `null` when `window`
-  is valid, otherwise an error string. The window must lie inside the
-  source's extent, and it can't leave a gap before or after the committed
-  history (history spec 7.2.3 / 7.2.4). Touching an edge or overlapping is
-  fine.
+- `sessionWindowIssue(window, source, sessions)`: `null` when `window` is
+  valid, otherwise `{ kind, message, fix }` for the first broken rule. The
+  window must lie inside the source's extent, and it can't leave a gap
+  before or after the committed history (history spec 7.2.3 / 7.2.4).
+  Touching an edge or overlapping is fine. `fix` carries only the
+  endpoints at fault, set to the nearest valid value (source start or end,
+  committed end for a gap after, committed start for a gap before), plus
+  button copy naming the date. It is `null` when the start is not before
+  the end, or when the correction would itself break a rule. The dialog
+  shows it as a button on the warning.
+- `NO_SOURCE_DATA_ISSUE`: the issue for a source without observations.
+- `sessionWindowPresets(source, sessions)`: the dialog's preset chips.
+  `All` (the whole record, the default), then `1y`, `6m`, `1m` counting back
+  from the source's last observation through `presetWindow`, and, when
+  something is committed, `Since commit` (committed end to source end).
+  Each carries its `window` and a short `disabledReason` when that window
+  breaks a rule; the dialog disables it and shows the reason as its
+  tooltip. Empty when the source has no observations.
 - `committedExtent(sessions)`: the earliest committed start to the latest
   committed end, or `null` with nothing committed.
 
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 2f1ca77f..e07d66b3 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -280,7 +280,9 @@ as the edit target and switches to the Edit view, resuming an in-progress
 session, or opening the session-window step when one is needed. Session
 window validity (inside the source's extent, no gap before or after
 committed history) is a pure util, `utils/sessionWindow.ts`, used by the
-session-window dialog. `startSession` only clamps the chosen window to the
+session-window dialog. The same util builds the dialog's preset chips
+(disabled with a reason when they break a rule) and the one-click fix on
+its warning, which moves only the endpoint at fault. `startSession` only clamps the chosen window to the
 source's extent (`clampSpecToSource`) as a backstop. Once editing, context refresh
 (`refreshGraphSeriesArray`) never fetches the edit target: its data is
 owned by the session (working copy, `startSession`, `viewSession`,
diff --git a/apps/qc-app/e2e/edit-selection.spec.ts b/apps/qc-app/e2e/edit-selection.spec.ts
index 713e6b04..caf55df1 100644
--- a/apps/qc-app/e2e/edit-selection.spec.ts
+++ b/apps/qc-app/e2e/edit-selection.spec.ts
@@ -3,6 +3,7 @@
  * editor draws around the session.
  *   - Edit on a row does not plot it
  *   - the window step prefills a valid window and blocks an invalid one
+ *   - the window step offers presets and a one-click fix
  *   - the editor draws the edit target, its source, and plotted datastreams
  *   - closing the editor keeps the plotted datastreams
  *   - the session window band survives staging shapes
@@ -152,6 +153,45 @@ test.describe('edit selection', () => {
     await expect(page.getByTestId('session-window-start')).toBeDisabled()
   })
 
+  test('the window step offers presets and a one-click fix', async ({ page }) => {
+    await gotoHome(page)
+    await openWindowStep(page)
+
+    // The fixture record spans about a day, fully committed: only All fits.
+    await expect(page.getByTestId('session-window-preset-all')).toHaveAttribute(
+      'aria-pressed',
+      'true'
+    )
+    await expect(page.getByTestId('session-window-preset-1m')).toHaveClass(
+      /v-chip--disabled/
+    )
+    await expect(
+      page.getByTestId('session-window-preset-slot-1m')
+    ).toHaveAttribute('title', 'Longer than the record')
+
+    const start = new Date(FIXTURE_OBS_START_MS)
+    const end = new Date(FIXTURE_OBS_END_MS)
+    await typeDateTime(
+      page.getByTestId('session-window-from'),
+      new Date(FIXTURE_OBS_START_MS - 2 * 24 * 60 * 60 * 1000)
+    )
+    await expect(page.getByTestId('session-window-start')).toBeDisabled()
+    const fix = page.getByTestId('session-window-fix')
+    await expect(fix).toContainText('Start at')
+    await fix.click()
+
+    await expect(page.getByTestId('session-window-error')).toBeHidden()
+    await expect(page.getByTestId('session-window-start')).toBeEnabled()
+    const [fromDate, fromTime] = pickerText(start)
+    const [toDate, toTime] = pickerText(end)
+    const from = page.getByTestId('session-window-from').locator('input')
+    const to = page.getByTestId('session-window-to').locator('input')
+    await expect(from.nth(0)).toHaveValue(fromDate)
+    await expect(from.nth(1)).toHaveValue(fromTime)
+    await expect(to.nth(0)).toHaveValue(toDate)
+    await expect(to.nth(1)).toHaveValue(toTime)
+  })
+
   test('the editor draws the source and plotted datastreams around the session', async ({
     page,
   }) => {
diff --git a/apps/qc-app/src/components/EditData/SessionWindowDialog.vue b/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
index db138d83..3defe949 100644
--- a/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
+++ b/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
@@ -29,6 +29,29 @@
       </div>
 
       <template v-if="begin && end">
+        <div v-if="presets.length" class="session-window__presets">
+          <span
+            v-for="preset in presets"
+            :key="preset.id"
+            :data-testid="`session-window-preset-slot-${preset.id}`"
+            :title="preset.disabledReason ?? preset.title"
+          >
+            <v-chip
+              :data-testid="`session-window-preset-${preset.id}`"
+              :color="activePresetId === preset.id ? 'primary' : undefined"
+              :variant="activePresetId === preset.id ? 'tonal' : 'outlined'"
+              size="small"
+              role="button"
+              :aria-pressed="String(activePresetId === preset.id)"
+              :disabled="!!preset.disabledReason"
+              class="session-window__preset-chip"
+              @click="applyPreset(preset)"
+            >
+              {{ preset.label }}
+            </v-chip>
+          </span>
+        </div>
+
         <div>
           <div class="text-body-small text-medium-emphasis mb-1">From</div>
           <DatePickerField
@@ -48,13 +71,23 @@
       </template>
 
       <v-alert
-        v-if="error"
+        v-if="issue"
         type="warning"
         variant="tonal"
         density="compact"
         data-testid="session-window-error"
       >
-        <span class="text-body-small">{{ error }}</span>
+        <span class="text-body-small">{{ issue.message }}</span>
+        <div v-if="issue.fix" class="mt-2">
+          <v-btn
+            size="small"
+            variant="tonal"
+            data-testid="session-window-fix"
+            @click="applyFix(issue.fix)"
+          >
+            {{ issue.fix.label }}
+          </v-btn>
+        </div>
       </v-alert>
     </div>
 
@@ -74,7 +107,7 @@
         color="primary"
         variant="flat"
         data-testid="session-window-start"
-        :disabled="!!error"
+        :disabled="!!issue"
         :loading="loading"
         @click="onStart"
       >
@@ -90,10 +123,15 @@ import DatePickerField from '@/components/VisualizeData/DatePickerField.vue'
 import { formatDateRange } from '@/utils/time'
 import type { TimeWindow } from '@/utils/timeRangePresets'
 import {
+  NO_SOURCE_DATA_ISSUE,
   committedExtent,
   defaultSessionWindow,
-  validateSessionWindow,
+  sessionWindowIssue,
+  sessionWindowPresets,
   type SessionRange,
+  type SessionWindowFix,
+  type SessionWindowIssue,
+  type SessionWindowPreset,
   type SourceExtent,
 } from '@/utils/sessionWindow'
 
@@ -114,20 +152,54 @@ const begin = ref<Date | null>(initial?.begin ?? null)
 const end = ref<Date | null>(initial?.end ?? null)
 
 const history = computed(() => committedExtent(props.sessions))
+const presets = computed(() => sessionWindowPresets(props.source, props.sessions))
 
-const error = computed(() => {
-  if (!begin.value || !end.value) {
-    return 'The source datastream has no observations to edit.'
-  }
-  return validateSessionWindow(
+const issue = computed<SessionWindowIssue | null>(() => {
+  if (!begin.value || !end.value) return NO_SOURCE_DATA_ISSUE
+  return sessionWindowIssue(
     { begin: begin.value, end: end.value },
     props.source,
     props.sessions
   )
 })
 
+const activePresetId = computed(() => {
+  const from = begin.value?.getTime()
+  const to = end.value?.getTime()
+  if (from === undefined || to === undefined) return null
+  return (
+    presets.value.find(
+      (p) => p.window.begin.getTime() === from && p.window.end.getTime() === to
+    )?.id ?? null
+  )
+})
+
+function applyPreset(preset: SessionWindowPreset) {
+  begin.value = new Date(preset.window.begin)
+  end.value = new Date(preset.window.end)
+}
+
+// Only the endpoints the fix names move; a valid one is left as the user set it.
+function applyFix(fix: SessionWindowFix) {
+  if (fix.begin) begin.value = new Date(fix.begin)
+  if (fix.end) end.value = new Date(fix.end)
+}
+
 function onStart() {
-  if (error.value || !begin.value || !end.value) return
+  if (issue.value || !begin.value || !end.value) return
   emit('confirm', { begin: begin.value, end: end.value })
 }
 </script>
+
+<style scoped>
+.session-window__presets {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 4px;
+}
+
+.session-window__preset-chip {
+  font-size: 0.75rem !important;
+  height: 26px !important;
+}
+</style>
diff --git a/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
index ce1f878c..cf891103 100644
--- a/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
@@ -118,3 +118,97 @@ describe('SessionWindowDialog', () => {
     expect(w.emitted('cancel')).toHaveLength(1)
   })
 })
+
+describe('SessionWindowDialog presets', () => {
+  const preset = (w: ReturnType<typeof mount>, id: string) =>
+    w.find(`[data-testid="session-window-preset-${id}"]`)
+
+  it('offers the record spans, plus one starting at the committed end', () => {
+    const w = mountDialog()
+    for (const id of ['all', '1y', '6m', '1m', 'since']) {
+      expect(preset(w, id).exists()).toBe(true)
+    }
+  })
+
+  it('leaves out the committed preset when nothing is committed', () => {
+    const w = mountDialog({ sessions: [] })
+    expect(preset(w, 'since').exists()).toBe(false)
+    expect(preset(w, 'all').exists()).toBe(true)
+  })
+
+  it('sets both fields from a preset', async () => {
+    const w = mountDialog()
+    await preset(w, 'since').trigger('click')
+    await flushPromises()
+    const [from, to] = pickers(w)
+    expect((from!.props('modelValue') as Date).toISOString()).toBe('2025-05-01T00:00:00.000Z')
+    expect((to!.props('modelValue') as Date).toISOString()).toBe('2025-12-31T00:00:00.000Z')
+    expect(w.find('[data-testid="session-window-error"]').exists()).toBe(false)
+  })
+
+  it('disables a preset that would break a rule and says why', () => {
+    const w = mountDialog()
+    expect(preset(w, '1m').attributes('disabled')).toBeDefined()
+    expect(w.find('[data-testid="session-window-preset-slot-1m"]').attributes('title')).toBe(
+      'Leaves a gap after the committed work'
+    )
+    expect(preset(w, 'all').attributes('disabled')).toBeUndefined()
+  })
+
+  it('marks the preset matching the current window', async () => {
+    const w = mountDialog()
+    expect(preset(w, 'all').attributes('aria-pressed')).toBe('true')
+    await preset(w, 'since').trigger('click')
+    await flushPromises()
+    expect(preset(w, 'since').attributes('aria-pressed')).toBe('true')
+    expect(preset(w, 'all').attributes('aria-pressed')).toBe('false')
+  })
+})
+
+describe('SessionWindowDialog one-click fix', () => {
+  const fix = (w: ReturnType<typeof mount>) =>
+    w.find('[data-testid="session-window-fix"]')
+
+  it('offers the nearest valid start and leaves the end alone', async () => {
+    const w = mountDialog()
+    pickers(w)[0]!.vm.$emit('update:modelValue', new Date('2025-07-01T00:00:00Z'))
+    await flushPromises()
+    expect(fix(w).exists()).toBe(true)
+
+    await fix(w).trigger('click')
+    await flushPromises()
+    const [from, to] = pickers(w)
+    expect((from!.props('modelValue') as Date).toISOString()).toBe('2025-05-01T00:00:00.000Z')
+    expect((to!.props('modelValue') as Date).toISOString()).toBe('2025-12-31T00:00:00.000Z')
+    expect(w.find('[data-testid="session-window-error"]').exists()).toBe(false)
+    expect(w.find('[data-testid="session-window-start"]').attributes('disabled')).toBeUndefined()
+  })
+
+  it('offers the nearest valid end and leaves the start alone', async () => {
+    const w = mountDialog()
+    pickers(w)[1]!.vm.$emit('update:modelValue', new Date('2026-06-01T00:00:00Z'))
+    await flushPromises()
+    expect(fix(w).text()).toMatch(/^End at/)
+
+    await fix(w).trigger('click')
+    await flushPromises()
+    const [from, to] = pickers(w)
+    expect((from!.props('modelValue') as Date).toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect((to!.props('modelValue') as Date).toISOString()).toBe('2025-12-31T00:00:00.000Z')
+    expect(w.find('[data-testid="session-window-error"]').exists()).toBe(false)
+  })
+
+  it('offers no fix when no single correction would be valid', () => {
+    const w = mountDialog({
+      sessions: [
+        {
+          status: 'committed',
+          phenomenonTimeStart: '2026-03-01T00:00:00Z',
+          phenomenonTimeEnd: '2026-04-01T00:00:00Z',
+        },
+      ],
+    })
+    expect(w.find('[data-testid="session-window-error"]').exists()).toBe(true)
+    expect(fix(w).exists()).toBe(false)
+  })
+})
diff --git a/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts b/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
index 94b7fbd0..09ac78ec 100644
--- a/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
@@ -2,8 +2,11 @@ import { describe, it, expect } from 'vitest'
 import {
   committedExtent,
   defaultSessionWindow,
-  validateSessionWindow,
+  sessionWindowIssue,
+  sessionWindowPresets,
 } from '@/utils/sessionWindow'
+import { subtractMonths } from '@/utils/dateMath'
+import { formatStamp } from '@/utils/time'
 
 const source = {
   phenomenonBeginTime: '2025-01-01T00:00:00Z',
@@ -18,6 +21,11 @@ const win = (begin: string, end: string) => ({
   begin: new Date(begin),
   end: new Date(end),
 })
+const stamp = (iso: string) => formatStamp(new Date(iso))
+const message = (
+  window: { begin: Date; end: Date },
+  sessions: ReturnType<typeof committed>[] = []
+) => sessionWindowIssue(window, source, sessions)?.message ?? null
 
 describe('committedExtent', () => {
   it('spans the earliest committed start to the latest committed end', () => {
@@ -50,13 +58,13 @@ describe('the default window against the rules', () => {
   it('is valid while committed history sits inside the source', () => {
     const sessions = [committed('2025-01-01T00:00:00Z', '2025-05-01T00:00:00Z')]
     const window = defaultSessionWindow(source)!
-    expect(validateSessionWindow(window, source, sessions)).toBeNull()
+    expect(sessionWindowIssue(window, source, sessions)).toBeNull()
   })
 
   it('is valid when the history already covers the whole source', () => {
     const sessions = [committed('2025-01-01T00:00:00Z', '2025-12-31T00:00:00Z')]
     const window = defaultSessionWindow(source)!
-    expect(validateSessionWindow(window, source, sessions)).toBeNull()
+    expect(sessionWindowIssue(window, source, sessions)).toBeNull()
   })
 
   // The source extent can shrink away from history committed earlier, and
@@ -64,58 +72,203 @@ describe('the default window against the rules', () => {
   it('is rejected when the committed history sits outside the source', () => {
     const sessions = [committed('2026-03-01T00:00:00Z', '2026-04-01T00:00:00Z')]
     const window = defaultSessionWindow(source)!
-    expect(validateSessionWindow(window, source, sessions)).toMatch(/gap before/)
+    expect(message(window, sessions)).toMatch(/gap before/)
   })
 })
 
-describe('validateSessionWindow', () => {
+describe('sessionWindowIssue', () => {
   const history = [committed('2025-03-01T00:00:00Z', '2025-06-01T00:00:00Z')]
 
   it('accepts a window that starts exactly at the history end', () => {
     expect(
-      validateSessionWindow(win('2025-06-01T00:00:00Z', '2025-08-01T00:00:00Z'), source, history)
+      sessionWindowIssue(win('2025-06-01T00:00:00Z', '2025-08-01T00:00:00Z'), source, history)
     ).toBeNull()
   })
 
   it('accepts a window overlapping committed sessions', () => {
     expect(
-      validateSessionWindow(win('2025-05-01T00:00:00Z', '2025-08-01T00:00:00Z'), source, history)
+      sessionWindowIssue(win('2025-05-01T00:00:00Z', '2025-08-01T00:00:00Z'), source, history)
     ).toBeNull()
   })
 
   it('accepts a window that ends exactly at the history start', () => {
     expect(
-      validateSessionWindow(win('2025-01-01T00:00:00Z', '2025-03-01T00:00:00Z'), source, history)
+      sessionWindowIssue(win('2025-01-01T00:00:00Z', '2025-03-01T00:00:00Z'), source, history)
     ).toBeNull()
   })
 
   it('rejects a gap after the history', () => {
-    expect(
-      validateSessionWindow(win('2025-07-01T00:00:00Z', '2025-08-01T00:00:00Z'), source, history)
-    ).toMatch(/gap after/)
+    expect(message(win('2025-07-01T00:00:00Z', '2025-08-01T00:00:00Z'), history)).toMatch(
+      /gap after/
+    )
   })
 
   it('rejects a gap before the history', () => {
-    expect(
-      validateSessionWindow(win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'), source, history)
-    ).toMatch(/gap before/)
+    expect(message(win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'), history)).toMatch(
+      /gap before/
+    )
   })
 
   it('rejects a window past the source end', () => {
-    expect(
-      validateSessionWindow(win('2025-06-01T00:00:00Z', '2026-02-01T00:00:00Z'), source, history)
-    ).toMatch(/source data/)
+    expect(message(win('2025-06-01T00:00:00Z', '2026-02-01T00:00:00Z'), history)).toMatch(
+      /source data/
+    )
   })
 
   it('rejects a start that is not before the end', () => {
-    expect(
-      validateSessionWindow(win('2025-08-01T00:00:00Z', '2025-08-01T00:00:00Z'), source, [])
-    ).toMatch(/before the end/)
+    expect(message(win('2025-08-01T00:00:00Z', '2025-08-01T00:00:00Z'))).toMatch(
+      /before the end/
+    )
   })
 
   it('accepts any in-source window when nothing is committed', () => {
     expect(
-      validateSessionWindow(win('2025-09-01T00:00:00Z', '2025-10-01T00:00:00Z'), source, [])
+      sessionWindowIssue(win('2025-09-01T00:00:00Z', '2025-10-01T00:00:00Z'), source, [])
     ).toBeNull()
   })
+
+  it('reports no observations when the source has none', () => {
+    const issue = sessionWindowIssue(win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'), {}, [])
+    expect(issue?.kind).toBe('no-data')
+    expect(issue?.fix).toBeNull()
+  })
+})
+
+describe('the one-click fix', () => {
+  const history = [committed('2025-03-01T00:00:00Z', '2025-06-01T00:00:00Z')]
+
+  it('moves only the start when it falls before the source', () => {
+    const issue = sessionWindowIssue(
+      win('2024-06-01T00:00:00Z', '2025-08-01T00:00:00Z'),
+      source,
+      history
+    )
+    expect(issue?.kind).toBe('starts-before-source')
+    expect(issue?.fix?.begin?.toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect(issue?.fix?.end).toBeUndefined()
+    expect(issue?.fix?.label).toBe(`Start at ${stamp(source.phenomenonBeginTime)}`)
+  })
+
+  it('moves only the end when it falls past the source', () => {
+    const issue = sessionWindowIssue(
+      win('2025-06-01T00:00:00Z', '2026-02-01T00:00:00Z'),
+      source,
+      history
+    )
+    expect(issue?.kind).toBe('ends-after-source')
+    expect(issue?.fix?.end?.toISOString()).toBe('2025-12-31T00:00:00.000Z')
+    expect(issue?.fix?.begin).toBeUndefined()
+    expect(issue?.fix?.label).toBe(`End at ${stamp(source.phenomenonEndTime)}`)
+  })
+
+  it('moves both ends when both fall outside the source', () => {
+    const issue = sessionWindowIssue(
+      win('2024-06-01T00:00:00Z', '2026-02-01T00:00:00Z'),
+      source,
+      history
+    )
+    expect(issue?.kind).toBe('outside-source')
+    expect(issue?.fix?.begin?.toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect(issue?.fix?.end?.toISOString()).toBe('2025-12-31T00:00:00.000Z')
+    expect(issue?.fix?.label).toBe(
+      `Use ${stamp(source.phenomenonBeginTime)} to ${stamp(source.phenomenonEndTime)}`
+    )
+  })
+
+  it('pulls the start back to the history end on a gap after', () => {
+    const issue = sessionWindowIssue(
+      win('2025-07-01T00:00:00Z', '2025-08-01T00:00:00Z'),
+      source,
+      history
+    )
+    expect(issue?.kind).toBe('gap-after-history')
+    expect(issue?.fix?.begin?.toISOString()).toBe('2025-06-01T00:00:00.000Z')
+    expect(issue?.fix?.end).toBeUndefined()
+  })
+
+  it('pushes the end out to the history start on a gap before', () => {
+    const issue = sessionWindowIssue(
+      win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'),
+      source,
+      history
+    )
+    expect(issue?.kind).toBe('gap-before-history')
+    expect(issue?.fix?.end?.toISOString()).toBe('2025-03-01T00:00:00.000Z')
+    expect(issue?.fix?.begin).toBeUndefined()
+  })
+
+  it('offers nothing when the start is not before the end', () => {
+    expect(
+      sessionWindowIssue(win('2025-08-01T00:00:00Z', '2025-08-01T00:00:00Z'), source, [])?.fix
+    ).toBeNull()
+  })
+
+  // The history sits outside the source, so closing the gap would push the
+  // end past the source and swap one error for another.
+  it('offers nothing when the correction is itself invalid', () => {
+    const issue = sessionWindowIssue(defaultSessionWindow(source)!, source, [
+      committed('2026-03-01T00:00:00Z', '2026-04-01T00:00:00Z'),
+    ])
+    expect(issue?.kind).toBe('gap-before-history')
+    expect(issue?.fix).toBeNull()
+  })
+
+  it('always yields a valid window when it is offered', () => {
+    const window = win('2024-06-01T00:00:00Z', '2026-02-01T00:00:00Z')
+    const fix = sessionWindowIssue(window, source, history)!.fix!
+    const fixed = { begin: fix.begin ?? window.begin, end: fix.end ?? window.end }
+    expect(sessionWindowIssue(fixed, source, history)).toBeNull()
+  })
+})
+
+describe('sessionWindowPresets', () => {
+  const ids = (sessions: ReturnType<typeof committed>[]) =>
+    sessionWindowPresets(source, sessions).map((p) => p.id)
+
+  it('offers the whole record and shorter spans anchored at its end', () => {
+    expect(ids([])).toEqual(['all', '1y', '6m', '1m'])
+    const presets = sessionWindowPresets(source, [])
+    const all = presets.find((p) => p.id === 'all')!
+    expect(all.window).toEqual(defaultSessionWindow(source))
+    const month = presets.find((p) => p.id === '1m')!
+    const recordEnd = new Date(source.phenomenonEndTime)
+    expect(month.window.end.getTime()).toBe(recordEnd.getTime())
+    expect(month.window.begin.getTime()).toBe(
+      subtractMonths(recordEnd, 1).getTime()
+    )
+  })
+
+  it('adds a preset starting where the committed work ends', () => {
+    const sessions = [committed('2025-01-01T00:00:00Z', '2025-05-01T00:00:00Z')]
+    expect(ids(sessions)).toEqual(['all', '1y', '6m', '1m', 'since'])
+    const since = sessionWindowPresets(source, sessions).find((p) => p.id === 'since')!
+    expect(since.window.begin.toISOString()).toBe('2025-05-01T00:00:00.000Z')
+    expect(since.window.end.toISOString()).toBe('2025-12-31T00:00:00.000Z')
+    expect(since.disabledReason).toBeNull()
+  })
+
+  it('disables a span that would leave a gap after the committed work', () => {
+    const sessions = [committed('2025-01-01T00:00:00Z', '2025-05-01T00:00:00Z')]
+    const presets = sessionWindowPresets(source, sessions)
+    expect(presets.find((p) => p.id === '1m')!.disabledReason).toBe(
+      'Leaves a gap after the committed work'
+    )
+    expect(presets.find((p) => p.id === 'all')!.disabledReason).toBeNull()
+  })
+
+  it('disables a span longer than the source record', () => {
+    const short = {
+      phenomenonBeginTime: '2025-11-01T00:00:00Z',
+      phenomenonEndTime: '2025-12-01T00:00:00Z',
+    }
+    const presets = sessionWindowPresets(short, [])
+    expect(presets.find((p) => p.id === '1y')!.disabledReason).toBe(
+      'Longer than the record'
+    )
+    expect(presets.find((p) => p.id === 'all')!.disabledReason).toBeNull()
+  })
+
+  it('offers nothing when the source has no observations', () => {
+    expect(sessionWindowPresets({}, [])).toEqual([])
+  })
 })
diff --git a/apps/qc-app/src/utils/__tests__/time.spec.ts b/apps/qc-app/src/utils/__tests__/time.spec.ts
index 70a2c177..b1ae345d 100644
--- a/apps/qc-app/src/utils/__tests__/time.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/time.spec.ts
@@ -1,5 +1,10 @@
 import { describe, it, expect } from 'vitest'
-import { formatDateInput, formatDateRange, formatDayStamp } from '../time'
+import {
+  formatDateInput,
+  formatDateRange,
+  formatDayStamp,
+  formatStamp,
+} from '../time'
 
 // Datetime strings without a timezone offset are parsed as local time, so
 // these assertions are stable regardless of the test runner's zone.
@@ -70,3 +75,19 @@ describe('formatDayStamp', () => {
     expect(formatDayStamp('nonsense')).toBe('nonsense')
   })
 })
+
+describe('formatStamp', () => {
+  it('drops the clock on a whole-day boundary', () => {
+    expect(formatStamp(new Date(2026, 2, 14))).toBe('Mar 14, 2026')
+  })
+
+  it('keeps the clock at any other time', () => {
+    expect(formatStamp(new Date(2026, 2, 14, 14, 30))).toBe(
+      'Mar 14, 2026, 2:30 PM'
+    )
+  })
+
+  it('falls back for an unparseable date', () => {
+    expect(formatStamp(new Date(NaN))).toBe('–')
+  })
+})
diff --git a/apps/qc-app/src/utils/sessionWindow.ts b/apps/qc-app/src/utils/sessionWindow.ts
index 2da7a0b1..7b776c50 100644
--- a/apps/qc-app/src/utils/sessionWindow.ts
+++ b/apps/qc-app/src/utils/sessionWindow.ts
@@ -4,7 +4,14 @@
  * after the committed history. Overlapping committed sessions is allowed.
  */
 
-import { dataExtent, type TimeWindow } from '@/utils/timeRangePresets'
+import { formatStamp } from '@/utils/time'
+import {
+  ALL_PRESET_ID,
+  TIME_RANGE_PRESETS,
+  dataExtent,
+  presetWindow,
+  type TimeWindow,
+} from '@/utils/timeRangePresets'
 
 export interface SourceExtent {
   phenomenonBeginTime?: string | null
@@ -17,7 +24,30 @@ export interface SessionRange {
   phenomenonTimeEnd: string
 }
 
-const formatMs = (ms: number) => new Date(ms).toLocaleString()
+export type SessionWindowIssueKind =
+  | 'no-data'
+  | 'order'
+  | 'starts-before-source'
+  | 'ends-after-source'
+  | 'outside-source'
+  | 'gap-after-history'
+  | 'gap-before-history'
+
+/** A one-click correction. It carries only the endpoints at fault, so the
+ *  one the user already got right is never moved. */
+export interface SessionWindowFix {
+  begin?: Date
+  end?: Date
+  /** Button copy, naming the date it sets. */
+  label: string
+}
+
+export interface SessionWindowIssue {
+  kind: SessionWindowIssueKind
+  message: string
+  /** Null when no correction of the offending endpoints would be valid. */
+  fix: SessionWindowFix | null
+}
 
 export function committedExtent(
   sessions: readonly SessionRange[]
@@ -39,27 +69,180 @@ export function defaultSessionWindow(source: SourceExtent): TimeWindow | null {
   return dataExtent([source])
 }
 
-export function validateSessionWindow(
+export const NO_SOURCE_DATA_ISSUE: SessionWindowIssue = {
+  kind: 'no-data',
+  message: 'The source datastream has no observations to edit.',
+  fix: null,
+}
+
+/** The first broken rule, with the correction it suggests. The correction is
+ *  not yet checked against the other rules; `sessionWindowIssue` does that. */
+function firstProblem(
   window: TimeWindow,
   source: SourceExtent,
   sessions: readonly SessionRange[]
-): string | null {
+): SessionWindowIssue | null {
   const extent = dataExtent([source])
-  if (!extent) return 'The source datastream has no observations to edit.'
+  if (!extent) return NO_SOURCE_DATA_ISSUE
+
   const begin = window.begin.getTime()
   const end = window.end.getTime()
-  if (!(begin < end)) return 'The start must be before the end.'
-  if (begin < extent.begin.getTime() || end > extent.end.getTime()) {
-    return `The window must be within the source data (${formatMs(extent.begin.getTime())} to ${formatMs(extent.end.getTime())}).`
+  if (!(begin < end)) {
+    return { kind: 'order', message: 'The start must be before the end.', fix: null }
+  }
+
+  const early = begin < extent.begin.getTime()
+  const late = end > extent.end.getTime()
+  if (early && late) {
+    return {
+      kind: 'outside-source',
+      message: `The window must be within the source data (${formatStamp(extent.begin)} to ${formatStamp(extent.end)}).`,
+      fix: {
+        begin: new Date(extent.begin),
+        end: new Date(extent.end),
+        label: `Use ${formatStamp(extent.begin)} to ${formatStamp(extent.end)}`,
+      },
+    }
+  }
+  if (early) {
+    return {
+      kind: 'starts-before-source',
+      message: `The start is before the source data, which begins ${formatStamp(extent.begin)}.`,
+      fix: {
+        begin: new Date(extent.begin),
+        label: `Start at ${formatStamp(extent.begin)}`,
+      },
+    }
   }
+  if (late) {
+    return {
+      kind: 'ends-after-source',
+      message: `The end is after the source data, which ends ${formatStamp(extent.end)}.`,
+      fix: { end: new Date(extent.end), label: `End at ${formatStamp(extent.end)}` },
+    }
+  }
+
   const history = committedExtent(sessions)
   if (history) {
     if (begin > history.end.getTime()) {
-      return `This leaves a gap after the committed history, which ends ${formatMs(history.end.getTime())}. Start on or before that.`
+      return {
+        kind: 'gap-after-history',
+        message: `This leaves a gap after the committed history, which ends ${formatStamp(history.end)}. Start on or before that.`,
+        fix: {
+          begin: new Date(history.end),
+          label: `Start at ${formatStamp(history.end)}`,
+        },
+      }
     }
     if (end < history.begin.getTime()) {
-      return `This leaves a gap before the committed history, which starts ${formatMs(history.begin.getTime())}. End on or after that.`
+      return {
+        kind: 'gap-before-history',
+        message: `This leaves a gap before the committed history, which starts ${formatStamp(history.begin)}. End on or after that.`,
+        fix: {
+          end: new Date(history.begin),
+          label: `End at ${formatStamp(history.begin)}`,
+        },
+      }
     }
   }
   return null
 }
+
+/** `null` when `window` is valid, otherwise the broken rule and the fix for
+ *  it. A fix is only offered when applying it leaves a valid window. */
+export function sessionWindowIssue(
+  window: TimeWindow,
+  source: SourceExtent,
+  sessions: readonly SessionRange[]
+): SessionWindowIssue | null {
+  const issue = firstProblem(window, source, sessions)
+  if (!issue?.fix) return issue
+  const fixed = {
+    begin: issue.fix.begin ?? window.begin,
+    end: issue.fix.end ?? window.end,
+  }
+  return firstProblem(fixed, source, sessions) ? { ...issue, fix: null } : issue
+}
+
+export interface SessionWindowPreset {
+  id: string
+  label: string
+  /** Tooltip, or the reason when the preset is disabled. */
+  title: string
+  window: TimeWindow
+  /** Short reason the preset breaks a rule, or `null` when it is usable. */
+  disabledReason: string | null
+}
+
+const PRESET_REASONS: Record<SessionWindowIssueKind, string> = {
+  'no-data': 'No observations',
+  order: 'Empty window',
+  'starts-before-source': 'Longer than the record',
+  'ends-after-source': 'Past the end of the record',
+  'outside-source': 'Longer than the record',
+  'gap-after-history': 'Leaves a gap after the committed work',
+  'gap-before-history': 'Leaves a gap before the committed work',
+}
+
+/** Whole record first, then shorter spans counting back from its end. */
+const SPAN_PRESETS: readonly { id: string; label: string }[] = [
+  { id: 'all', label: 'All' },
+  { id: '1y', label: '1y' },
+  { id: '6m', label: '6m' },
+  { id: '1m', label: '1m' },
+]
+
+function toPreset(
+  id: string,
+  label: string,
+  title: string,
+  window: TimeWindow,
+  source: SourceExtent,
+  sessions: readonly SessionRange[]
+): SessionWindowPreset {
+  const issue = firstProblem(window, source, sessions)
+  return {
+    id,
+    label,
+    title,
+    window,
+    disabledReason: issue ? PRESET_REASONS[issue.kind] : null,
+  }
+}
+
+/**
+ * One-click windows for the session-window step, each already checked
+ * against the rules. Spans anchor to the source's last observation, and a
+ * committed history adds the "continue where the commit ended" case.
+ */
+export function sessionWindowPresets(
+  source: SourceExtent,
+  sessions: readonly SessionRange[]
+): SessionWindowPreset[] {
+  const extent = dataExtent([source])
+  if (!extent) return []
+
+  const presets: SessionWindowPreset[] = []
+  for (const span of SPAN_PRESETS) {
+    const base = TIME_RANGE_PRESETS.find((p) => p.label === span.label)
+    const window = base ? presetWindow(base.id, extent) : null
+    if (!base || !window) continue
+    const title = base.id === ALL_PRESET_ID ? 'The whole source record' : base.title
+    presets.push(toPreset(span.id, span.label, title, window, source, sessions))
+  }
+
+  const history = committedExtent(sessions)
+  if (history) {
+    presets.push(
+      toPreset(
+        'since',
+        'Since commit',
+        'From the end of the committed work to the end of the record',
+        { begin: new Date(history.end), end: new Date(extent.end) },
+        source,
+        sessions
+      )
+    )
+  }
+  return presets
+}
diff --git a/apps/qc-app/src/utils/time.ts b/apps/qc-app/src/utils/time.ts
index 28dab676..41ec8113 100644
--- a/apps/qc-app/src/utils/time.ts
+++ b/apps/qc-app/src/utils/time.ts
@@ -66,6 +66,13 @@ export function formatDayStamp(iso?: string | null): string {
 const isMidnight = (d: Date) =>
   d.getHours() === 0 && d.getMinutes() === 0 && d.getSeconds() === 0
 
+/** One boundary: `Mar 14, 2026`, or `Mar 14, 2026, 2:30 PM` off midnight. */
+export function formatStamp(when: Date): string {
+  if (Number.isNaN(when.getTime())) return '–'
+  const day = MONTH_DAY_YEAR.format(when)
+  return isMidnight(when) ? day : `${day}, ${CLOCK.format(when)}`
+}
+
 /**
  * A session's phenomenon-time window, readable at a glance:
  *   `Jan 5 – Feb 1, 2025`                    whole days in one year

From 69813a31861d2d93ad317a43abc24cc9f5c55402 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Mon, 21 Sep 2026 16:47:16 -0600
Subject: [PATCH 67/93] feat: draw source context around the edit window,
 preview before editing and preview history steps

---
 apps/qc-app/docs/API_REFERENCE.md             |  24 +-
 apps/qc-app/docs/ARCHITECTURE.md              |  21 +-
 apps/qc-app/docs/PERFORMANCE.md               |  15 +-
 apps/qc-app/docs/PLOTTING.md                  |  67 +++--
 apps/qc-app/docs/USER_GUIDE.md                |  97 ++++---
 apps/qc-app/e2e/edit-selection.spec.ts        | 162 ++++++++---
 apps/qc-app/e2e/leave-session.spec.ts         |   6 +-
 .../e2e/plot-no-intended-spacing.spec.ts      |  10 +-
 apps/qc-app/e2e/select-while-editing.spec.ts  |  66 ++++-
 apps/qc-app/e2e/share-url.spec.ts             |  68 +++--
 apps/qc-app/e2e/support/app.ts                |   9 +-
 .../src/components/EditData/EditHistory.vue   |  92 +++---
 .../components/EditData/EditTargetPanel.vue   |  11 +-
 .../EditData/LeaveSessionDialog.vue           |   7 +-
 .../components/EditData/OperationPanel.vue    |  16 +-
 .../components/EditData/StartEditingFlow.vue  |  29 +-
 .../EditData/__tests__/EditHistory.spec.ts    | 196 ++++++++-----
 .../__tests__/EditTargetPanel.spec.ts         |   2 +-
 .../__tests__/LeaveSessionDialog.spec.ts      |  37 ++-
 .../__tests__/StartEditingFlow.spec.ts        |  68 ++++-
 .../components/Navigation/SelectDrawer.vue    |   7 +
 apps/qc-app/src/components/VisualizeData.vue  |  23 +-
 .../components/VisualizeData/ContextPlot.vue  |  52 +++-
 .../components/VisualizeData/DataTable.vue    |   9 +-
 .../VisualizeData/DataVisDatasetsTable.vue    |  55 +++-
 .../VisualizeData/DataVisTimeFilters.vue      |   6 +-
 .../src/components/VisualizeData/Plot.vue     | 146 ++++++++--
 .../VisualizeData/PlotSourceDialog.vue        |  19 +-
 .../VisualizeData/PlottedDatastreams.vue      |  54 ++--
 .../VisualizeData/__tests__/DataTable.spec.ts |   1 +
 .../__tests__/DataVisDatasetsTable.spec.ts    |  46 +++
 .../__tests__/DataVisTimeFilters.spec.ts      |  10 +-
 .../VisualizeData/__tests__/Plot.spec.ts      |   3 +
 .../__tests__/PlottedDatastreams.spec.ts      |  54 +++-
 .../__tests__/useDataSelection.spec.ts        |   1 +
 .../__tests__/useEditSession.spec.ts          |  28 +-
 .../__tests__/useLeaveSession.spec.ts         |   9 +-
 .../src/composables/useDataSelection.ts       |   5 +-
 apps/qc-app/src/composables/useEditSession.ts |  10 +-
 .../src/composables/useHistorySnapshots.ts    |  27 +-
 .../qc-app/src/composables/useLeaveSession.ts |   3 +
 .../store/__tests__/dataVisualization.spec.ts | 201 ++++++++++---
 .../src/store/__tests__/observations.spec.ts  |  67 +++++
 .../qc-app/src/store/__tests__/plotly.spec.ts |  33 +--
 .../src/store/__tests__/workingCopies.spec.ts |  10 +-
 apps/qc-app/src/store/dataVisualization.ts    | 132 +++++++--
 apps/qc-app/src/store/observations.ts         | 274 ++++++++++--------
 apps/qc-app/src/store/plotly.ts               |  30 +-
 apps/qc-app/src/store/workingCopies.ts        |   4 +-
 .../src/utils/__tests__/timeIntervals.spec.ts |  39 +++
 apps/qc-app/src/utils/contextSeriesId.ts      |  15 +
 .../utils/plotting/__tests__/events.spec.ts   |  27 +-
 .../utils/plotting/__tests__/options.spec.ts  | 177 ++++++++---
 .../utils/plotting/__tests__/relayout.spec.ts |   4 +-
 .../utils/plotting/__tests__/selected.spec.ts |  17 ++
 .../utils/plotting/__tests__/shapes.spec.ts   |  38 +--
 .../utils/plotting/__tests__/staging.spec.ts  |  24 +-
 apps/qc-app/src/utils/plotting/events.ts      |  13 +-
 apps/qc-app/src/utils/plotting/interaction.ts |   4 +
 apps/qc-app/src/utils/plotting/operations.ts  |   6 +-
 apps/qc-app/src/utils/plotting/options.ts     | 222 ++++++++++----
 apps/qc-app/src/utils/plotting/relayout.ts    |   8 +-
 apps/qc-app/src/utils/plotting/selected.ts    |  12 +-
 apps/qc-app/src/utils/plotting/shapes.ts      |  30 +-
 apps/qc-app/src/utils/plotting/staging.ts     |  29 +-
 apps/qc-app/src/utils/timeIntervals.ts        |  36 +++
 packages/qc-utils/docs/API_REFERENCE.md       |   7 +-
 packages/qc-utils/docs/ARCHITECTURE.md        |   5 +-
 packages/qc-utils/docs/QUALITY.md             |   6 +-
 .../__tests__/observation-record.spec.ts      | 142 ++++++++-
 .../src/utils/plotting/observation-record.ts  | 124 ++++++--
 71 files changed, 2316 insertions(+), 991 deletions(-)
 create mode 100644 apps/qc-app/src/utils/__tests__/timeIntervals.spec.ts
 create mode 100644 apps/qc-app/src/utils/contextSeriesId.ts
 create mode 100644 apps/qc-app/src/utils/timeIntervals.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 343c4840..5997073e 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -146,7 +146,8 @@ below), and share-link hydration (the `ed` query param).
   **Start new session** and the editor footer's **New session**.
 - `openEditor()`: shows the Edit layout for the current target. Pure
   navigation, used by the nav rail's Edit button and the Select view's
-  **Back to editor**.
+  **Open editor**. A datastream picked from a row enters with
+  `view = DrawerType.Select`, so it lands on that preview first.
 - `leaveEdit()`: returns to the Select view, clears the resume pointer and
   the edit target (`clearEditTarget`). The user has already been asked by
   then, or there was nothing to ask about. Switching views is not an exit and
@@ -199,8 +200,9 @@ and `LeaveSessionDialog.vue`, mounted once in `App.vue`, shows it.
 - `forgetSession()`: drop the resume pointer only. For exits that unmount the
   editor, where clearing the edit target would have the editor's URL writer
   replace the route mid-navigation.
-- `leavePrompt` / `leaveWork`: what the dialog renders, and which answer is
-  currently running.
+- `leavePrompt` / `leaveWork`: what the dialog renders (the case, the name of
+  the datastream being left, the unsaved count and whether saving is
+  possible), and which answer is currently running.
 
 ### `useResumeEditSession()`
 
@@ -429,7 +431,7 @@ on boot.
 | `plottedDatastreams`                | state    | `Datastream[]`                                    | Up to 4 streams the user chose to plot. Editing never adds to or removes from it (snapshots are the exception, dropped on leave); the 4-stream cap doesn't count the edit target or its source, so the plot holds at most 6 series while editing. |
 | `qcDatastreamId`                    | state    | `string \| null`                                  | The edit target's id. Set only by the edit flow (`setEditTarget` / `clearEditTarget`); null in the Select view. |
 | `qcDatastream`                      | computed | `Datastream \| null`                              | Live catalog lookup of `qcDatastreamId` in `datastreams`, not `plottedDatastreams`, since the edit target isn't a plotted entry. |
-| `sourceContextDatastream`           | computed | `Datastream \| null`                              | The edit target's catalog source, resolved through `qcHistories`; null without an edit target. Drawn behind the edit target as context. |
+| `sourceContextDatastream`           | computed | `Datastream \| null`                              | The source drawn around the edit target as context (light grey, cut around the session window). Null without an edit target, with context switched off, or when the user plotted the source, which then draws as an ordinary series. |
 | `seriesDatastreams`                 | computed | `Datastream[]`                                    | What the plot actually draws, in order: `[edit target, its source, ...plotted minus those]` while editing, otherwise `plottedDatastreams` unchanged. Refresh, colour assignment, series ordering, working-copy invalidation, `PlottedDatastreams`, the share watcher, and snapshots all iterate this instead of `plottedDatastreams`. |
 | `qualifierSet`                      | state    | `Set<string>`                                     | Qualifier codes seen on the edit target's loaded points. |
 | `selectedQualifier`                 | state    | `string`                                          | Active qualifier in the picker. |
@@ -439,7 +441,13 @@ on boot.
 | `isEditorReady`                     | computed | `boolean`                                         | True while editing once the session window is known, the edit record is on the plot, no session is opening (`isSwitchingSession`), and no plot work is pending: no queued or running plot load (including the context re-anchor around the session window) and no tracked draw. Bound to `data-editor-ready` on the edit view's `edit-plot-column`; e2e `waitForEditorReady` waits on it. |
 | `trackPlotWork`                     | action   | `(work: () => Promise<void>) => Promise<void>`    | Run `work` counted as pending plot work for `isEditorReady`. Plot loads and `setEditRecord` use it, and `Plot.vue` counts its first draw from the moment the plot element appears. |
 | `beginDate` / `endDate`             | state    | `Date`                                            | Active loaded window. A preset re-resolves it on every plot rebuild: around the edit session's window while one is set (`presetAroundWindow`), otherwise back from the context data's (`seriesDatastreams` minus the edit target) end (`presetWindow`). It also re-resolves when the edit session's window loads or changes. A custom range stays fixed. |
-| `selectedDateBtnId`                 | state    | `number`                                          | Active preset id (default `1`, 1m); `-1` (`CUSTOM_PRESET_ID`) for a manual range. Presets are defined in `utils/timeRangePresets.ts`. |
+| `selectedDateBtnId`                 | state    | `number`                                          | The Select view's Time range preset id (default `1`, 1m); `-1` (`CUSTOM_PRESET_ID`) for a manual range. Presets are defined in `utils/timeRangePresets.ts`. |
+| `contextPresetId`                   | state    | `number`                                          | The editor's Context range preset id, remembered apart from `selectedDateBtnId` so neither moves the other. Persisted. |
+| `activePresetId`                    | computed | `number` (writable)                               | The preset the loaded range follows: `contextPresetId` while an edit target is set, else `selectedDateBtnId`. `onDateBtnClick` and a custom `setDateRange` write it. |
+| `editSessionWindow`                 | computed | `TimeWindow \| null`                             | The viewed session's window, else the in-progress one's; null without an edit target. |
+| `editSourceDatastream`              | computed | `Datastream \| null`                             | The edit target's source datastream, however it is drawn. |
+| `showSourceContext`                 | state    | `boolean`                                         | Whether the edit target's source is drawn around it as context (default `true`). Persisted with `selectedDateBtnId`. |
+| `setShowSourceContext`              | action   | `(show: boolean) => Promise<void>`                | Turn the source context on or off from the Context menu; rebuilds the plot while editing. |
 | `matchesSelectedThing`              | action   | `(ds) => boolean`                                 | Filter predicate; exposed so the table can reuse it on row updates. |
 | `matchesSelectedObservedProperty`   | action   | `(ds) => boolean`                                 | Same shape as above. |
 | `matchesSelectedProcessingLevel`    | action   | `(ds) => boolean`                                 | Same shape as above. |
@@ -511,6 +519,7 @@ handles, live chart caches).
 | `selectedSeriesIndex`      | computed | `number`                                          | Index of the edit target in `graphSeriesArray` (`-1` when none). |
 | `selectedSeries`           | computed | `GraphSeries`                                     | Convenience for `graphSeriesArray[selectedSeriesIndex]`. |
 | `editHistory`              | state    | `HistoryItem[]`                                   | Mirrors `selectedSeries.data.history` (mutated in place; never reassign). |
+| `previewIndex`             | computed | `number \| null`                                  | The earlier history step the edit target shows (`-1` for the starting state), or null for the whole history. While set, edits wait: operations, plot selections and table saves are held. |
 | `suppressedEchoSelection`  | state    | `number[] \| null`                                | Sentinel armed by programmatic Plotly writes to suppress the echo SELECTION dispatch. |
 | `isUpdating`               | state    | `boolean`                                         | Surfaced in the nav rail while a redraw runs. |
 | `showLegend`               | state    | `boolean`                                         | Drives Plotly's legend visibility. |
@@ -538,7 +547,7 @@ handles, live chart caches).
 | `canRedoZoom`              | computed | `boolean`                                         | `zoomRedoStack.length > 0`. |
 | `currentZoom`              | computed | `ZoomState \| null`                               | Top of the undo stack; what the share URL writer subscribes to. |
 | `updateOptions`            | action   | `() => void`                                      | Rebuild `plotlyOptions` from `graphSeriesArray`. |
-| `redraw`                   | action   | `(recomputeXaxisRange?: boolean, preserveZoom?: boolean) => Promise<void>` | Push typed-array updates + restyle; preserves the user's zoom by default. Keeps shapes other writers own (the staging band) and replaces `edit-window`. |
+| `redraw`                   | action   | `(recomputeXaxisRange?: boolean, preserveZoom?: boolean) => Promise<void>` | Push typed-array updates + restyle; preserves the user's zoom by default, except on a y axis that had no points drawn. Leaves `layout.shapes` (the staging band) alone. |
 | `clearChartState`          | action   | `() => void`                                      | Drop all series + zoom history (used on workspace swap). |
 | `fetchGraphSeries`         | action   | `(ds, start: Date, end: Date) => Promise<GraphSeries>` | Fetch observations for `ds` over `[start, end]` and build a `GraphSeries` via `buildGraphSeries`. |
 | `buildGraphSeries`         | action   | `(ds: Datastream, data: ObservationRecord) => GraphSeries` | Build a `GraphSeries` from an already-loaded record, no fetch. Used directly for a managed datastream's working copy. |
@@ -557,7 +566,8 @@ Fetches + caches observation windows and inflates them into
 |----------------------------|----------|---------------------------------------------------|-------|
 | `observations`             | state    | `Record<string, ObservationRecord>`               | Per-datastream record; reused across rebuilds. |
 | `observationsRaw`          | state    | `Record<string, ObservationData>`                 | Typed-array cache (`Float64Array` datetimes + `Float32Array` values). |
-| `fetchObservationsInRange` | action   | `(ds: Datastream, b: Date, e: Date) => Promise<ObservationRecord>` | Extends the cached range minimally; only fetches segments outside the existing window. Requests for one datastream run in order, so the record ends on the latest requested window and the cache never merges the same segment twice; a request matching the last queued range shares its promise. |
+| `fetchObservationsInRange` | action   | `(ds: Datastream, b: Date, e: Date, exclude?: { begin: Date; end: Date }) => Promise<ObservationRecord>` | Fetches only the parts of `[b, e]` never asked for before, skipping `exclude`, and returns the shared record windowed to `[b, e]`. Requests for one datastream run in order, so the record ends on the latest requested window; a request matching the last queued one shares its promise. |
+| `fetchDetachedRecord`      | action   | `(ds: Datastream, b: Date, e: Date) => Promise<ObservationRecord>` | Fills the same cache through the same per-datastream queue, but returns a record of its own windowed to `[b, e]`. For working copies and snapshots: never re-windows the shared record the plot draws. |
 
 ### `useWorkspaceStore()` (`src/store/workspaces.ts`)
 
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index e07d66b3..6ca38ae8 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -416,10 +416,11 @@ Three constraints shape the implementation:
   base window misaligns the replay. A snapshot is therefore frozen at
   creation and never refetched.
 - **Record isolation.** `useObservationStore.fetchObservationsInRange` hands
-  back one shared `ObservationRecord` per datastream, and the replay mutates
-  whatever it is given. Snapshots inject a *detached* fetcher that warms the
-  raw cache and then constructs its own `ObservationRecord`, so a build never
-  disturbs the plot's series or a previous snapshot.
+  back one shared `ObservationRecord` per datastream, the one the plot draws,
+  and the replay mutates whatever it is given. Snapshots and working copies
+  build on `fetchDetachedRecord`, which fills the same cache through the same
+  queue but constructs a record of its own, so a build never disturbs the
+  plot's series or a previous snapshot.
 - **Identity.** Snapshots ride in `plottedDatastreams` under the synthetic id
   `snap:<sessionId>:<opIndex>` so legend rendering, colour assignment,
   visibility and reorder work unchanged. `isSnapshotId()` guards the paths
@@ -467,11 +468,13 @@ operations plus anything saved since. Neither is a guess about the UI.
 Every exit routes through it: the editor footer's **Close** and the nav rail's
 Home and Log out via `useEditEntry.closeEditor()`, the row Edit button on
 another datastream via `enterEdit`, and in-app navigation (including the
-workspace switch) via `leaveSessionGuard`. When that row Edit button leads to
-a create step, `StartEditingFlow` asks through `closeEditor()` before the
-create rather than waiting for `enterEdit`: a new managed datastream is always
-a new target, so cancelling later would have left an orphan datastream and its
-QC history on the server. While an answer is being carried out, a second exit
+workspace switch) via `leaveSessionGuard`. When the row Edit flow picks a
+different target, `StartEditingFlow` asks through `closeEditor()` right at the
+pick, before the window step or a create step opens, rather than waiting for
+`enterEdit`. Asking later reads as a question about the new session, and a
+create cancelled late would leave an orphan datastream and its QC history on
+the server. Keeping the session returns to the chooser. The prompt names the
+datastream being left (`LeavePrompt.datastreamName`). While an answer is being carried out, a second exit
 request is refused rather than resolved by work the user did not answer for.
 Cancelling leaves the user exactly where they were, zoom, staged band and
 unsaved edits intact. The native `beforeunload` prompt still covers a reload
diff --git a/apps/qc-app/docs/PERFORMANCE.md b/apps/qc-app/docs/PERFORMANCE.md
index 8d2b71dc..42ea5625 100644
--- a/apps/qc-app/docs/PERFORMANCE.md
+++ b/apps/qc-app/docs/PERFORMANCE.md
@@ -107,12 +107,15 @@ because the row format times out on ~35k-point ranges (see comments in
 Key fetch optimizations already in place:
 
 - **Cache-aware ranging.** `useObservationStore.fetchObservationsInRange`
-  only requests segments outside the existing cached window (strict
-  `<` / `>` comparison, see comments around `observations.ts:62`). The full
-  fetched history stays in `observationsRaw`; the `ObservationRecord` keeps it
-  in `rawData` and slices it to the selected `[begin, end]` range via
-  `applyWindow`, so the plot, table and counts only ever touch the current
-  window.
+  tracks the ranges already asked of the server per datastream (found or
+  not) and requests only the missing parts (`utils/timeIntervals.ts`). An
+  optional `exclude` stretch is never requested: the source's context skips
+  the session window this way. Fetched chunks are merged into
+  `observationsRaw` in one linear pass. The `ObservationRecord` keeps the
+  cache in `rawData` and slices it to the selected `[begin, end]` range via
+  `applyWindow(begin, end, rawData)`, which re-slices when either the window
+  or the cache changed, so the plot, table and counts only ever touch the
+  current window.
 - **One plot load at a time.** Rebuilds and context range reloads are
   serialized in `useDataVisStore`. A range reload that lands while a rebuild
   is queued joins it, and one that finds its range already loaded skips, so
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index a6f7c13a..c69a1f25 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -22,7 +22,7 @@ contributor can read the modules in the order the code runs.
 | `interaction.ts`  | DOM-level handlers: throttled mousemove crosshair, wheel-zoom, axis-chip placement, y-axis drag-rect widening.         |
 | `operations.ts`   | Imperative helpers callable from components: `zoomXaxisTo`, `toggleTraceVisibility`, `setSelectedPoints`, etc.         |
 | `staging.ts`      | Visual-only overlays while a Find-Gaps / Fill-Gaps operation is staged. Ghost-fill trace + drag-resizable shape.       |
-| `shapes.ts`       | `composeShapes` / `withLiveShapes`: each `layout.shapes` writer replaces only the shapes it names.                     |
+| `shapes.ts`       | `withLiveShapes`: carries the live stage band onto a fresh layout when the plot is re-created.                         |
 
 The split is not by file size, it's by the Plotly API each module
 talks to. `options.ts` owns trace + layout construction. `events.ts`
@@ -222,8 +222,9 @@ The editor toolbar's **Context** menu reuses `DataVisTimeFilters` (with
 `EDITOR_PRESETS`, which leaves out YTD and highlights All for a persisted
 YTD through `shownPresetId`, and the same From / To pickers as
 the Select view) bound to the same `beginDate` / `endDate` store range. The
-Select view's own Time range drives that same state, so while an edit target
-is set the two are one control.
+Select view's own Time range is hidden while an edit target is set, and each
+remembers its own preset: `activePresetId` is `contextPresetId` while editing
+and `selectedDateBtnId` otherwise, and every range action goes through it.
 Picking a preset or a custom date calls `setDateRange`, which, while an
 edit target is set, reloads only the context series
 (`refreshGraphSeriesArray`, which never fetches the edit target) and
@@ -261,21 +262,53 @@ range no longer matches, and the load loads the new range before drawing.
 A range reload queued behind it then finds the range already loaded and
 skips.
 
-The session window itself is shaded as a layout shape (`name: 'edit-window'`
-in `options.ts`), drawn from `useQcSessionStore().viewedSession` (falling
-back to `inProgressSession`), only while an edit target is set.
+The session window is not drawn as a shape: a shape would sit over the plot,
+and with `edits.shapePosition` on (needed to drag the stage band) Plotly
+makes every shape grab the mouse, which blocks box select. The data shows it
+instead. The edit target is one trace in QC grey (`COLORS[0]`), and its
+working copy spans only the session window. The source is the context around
+it, drawn light grey (`SOURCE_CONTEXT_COLOR`) and only outside
+`editSessionWindow` (the viewed session's window, else the in-progress one).
+`sourceContextDatastream` is that drawn source: null when the Context menu's
+switch is off (`showSourceContext`), or when the user plotted the source
+themselves, which then draws whole as an ordinary series and counts toward
+the 4. `editSourceDatastream` is the edit target's source either way.
 `setEditTarget` resets the session store when the target changes, so the
-previous target's window is never drawn over the new one.
-
-`layout.shapes` has more than one writer: `createPlotlyOption` owns
-`edit-window` and `staging.ts` owns `stage`. `Plotly.update` and
-`Plotly.relayout` replace the whole array, so partial writes go through
-`utils/plotting/shapes.ts`: `redraw` and `cropXaxisRange` use
-`withLiveShapes` to swap in the fresh `edit-window` and keep the live
-`stage` band, and the staging flush uses `composeShapes` to do the reverse
-(stage stays first so drag events still address `shapes[0]`).
-`handleNewPlot` (`events.ts`) does the same on a re-plot of the live element,
-so an operation's staged band survives a rebuild; a first mount has no live
+previous target's window is never applied to the new one.
+
+- **Pieces**: `splitAroundWindow` cuts the source record into the points
+  before and after the window, as `subarray` views of the same buffers, so
+  nothing is copied. The main trace (which keeps the series `id`) draws the
+  before points; the after points ride on a companion trace tagged
+  `_partOf`, as do the gap overlays, one per piece so no line crosses the
+  window. `_partOf` routes visibility toggles and replot carries to every
+  trace of the series.
+- **Bridge**: `bridgeAcrossWindow` adds one more `_partOf` line joining the
+  source's last point before the window to the edit target's first, and the
+  target's last to the source's first after it. A step wider than the
+  cadence stays open, as it would inside a line. Every source trace always
+  exists (pieces are empty without a window), so the trace count never
+  changes between `redraw`s. `ContextPlot` draws the source with the same
+  gap.
+- **Loading**: the source waits for the window, and its context load passes
+  the window as `exclude` to `fetchObservationsInRange`, so the window's
+  observations are never requested for it. Working copies and snapshots build
+  on `fetchDetachedRecord`, so they never re-window the shared record the
+  plot draws; building a base over the window used to cut the plotted
+  source down to the window, leaving no context.
+- **Axes**: a re-plot (`handleNewPlot`) or `redraw` keeps the live y range
+  only for an axis that had points drawn; an empty axis sits on Plotly's
+  default range, and carrying it would put the new data off the plot. Series
+  sharing an axis (the edit target and its source) share one axis chip. The window watch runs a rebuild,
+  since the source's traces first appear then and only a rebuild adds
+  traces. The window is part of the `loadKey` a range reload checks.
+
+`layout.shapes` belongs to `staging.ts` alone (the `stage` band, always
+`shapes[0]`). `createPlotlyOption` builds layouts without shapes, so
+`Plotly.update` from `redraw` and `cropXaxisRange` leaves the band alone.
+`handleNewPlot` (`events.ts`) re-creates the plot, so on a re-plot of the
+live element it carries the live shapes over with `withLiveShapes`, and an
+operation's staged band survives a rebuild; a first mount has no live
 layout to carry anything from.
 
 Plot rebuilds (`rebuildPlot`, run when plotted datastreams change) keep
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index c4f07e9a..8ee3e715 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -31,7 +31,7 @@ Everything runs in your web browser. The backend never sees your edit history un
 | **Datastream** | A single time-series at a site: one variable, one sensor, one processing level. |
 | **Observation** | A single (timestamp, value) measurement. |
 | **Edit target** | The datastream you are editing, chosen with a row's Edit button. |
-| **Context traces** | The raw source and any plotted datastreams shown around it. |
+| **Context traces** | The raw source, drawn around the session window, and any plotted datastreams shown around it. |
 | **History** | The ordered list of filters + selections + edits you've applied in the current edit session. Undo / redo / save / load all operate on this list. |
 | **Selection** | The set of point indices a filter (or your click / lasso) produced. Edits operate on the current selection. |
 | **QC history** | A JSON file holding your editing history for a datastream. This is the canonical save format. |
@@ -81,7 +81,7 @@ The select view is the default landing surface after picking a workspace. The le
 
 The filter drawer has two collapsible sections:
 
-- **Time range**: the loaded time window for the plotted datastreams. The two date pickers (`From` / `To`) show the window. The preset chips below them, **1w**, **1m**, **6m**, **1y**, **YTD**, **All**, count back from the last observation of the plotted datastreams (the newest one when several are plotted), so a preset always lands on data, even for datastreams whose observations are years old. That last observation is the one known when the workspace loaded, so data that arrives later only shows up after a reload. **1m** is the default, and the last preset you pick is remembered. Adding or removing a datastream re-anchors the preset. A `Custom` chip appears when you change a date in `From` or `To`; a custom window stays fixed while you add or remove datastreams. In the Edit view the same preference drives the **Context** menu, where presets count out from the edit session's window instead.
+- **Time range**: the loaded time window for the plotted datastreams. The two date pickers (`From` / `To`) show the window. The preset chips below them, **1w**, **1m**, **6m**, **1y**, **YTD**, **All**, count back from the last observation of the plotted datastreams (the newest one when several are plotted), so a preset always lands on data, even for datastreams whose observations are years old. That last observation is the one known when the workspace loaded, so data that arrives later only shows up after a reload. **1m** is the default, and the last preset you pick is remembered. Adding or removing a datastream re-anchors the preset. A `Custom` chip appears when you change a date in `From` or `To`; a custom window stays fixed while you add or remove datastreams. While you are editing, this section is hidden: the **Context** menu above the plot sets the range instead, with presets that count out from the edit session's window. The two remember their own preset, so neither changes the other.
 
 - **Datastream filters**: These filters allow you control the list of datastreams shown in the datastreams table by picking the site, observed property, and/or processing level. The list of matching datastreams updates live in the datastreams table.
 
@@ -105,18 +105,31 @@ What changes:
 
 - An **Editing** panel appears in the top card's right pane: the managed
   datastream's name, the session's time window, whether anything is unsaved,
-  and a **Back to editor** button. Editing operations stay in the editor.
-- The preview plot draws the same series the editor does, the edit target and
-  its raw source included, with the session window shaded.
-- The row check boxes add and remove **context** datastreams. The edit target
-  and its source stay on the plot and cannot be removed here.
-- The filter drawer's **Time range** is the editor's Context range: the same
-  window the Context menu sets, with presets counting out from the session
-  window. Your edits are never reloaded by it.
-- The rail's **Edit** button and the panel's **Back to editor** button both
-  return to the editor with the session, your zoom and any staged range intact.
-- The row **Edit** button on the datastream you are already editing returns to
-  the editor too. On a different datastream it starts the usual entry flow.
+  and an **Open editor** button. Editing operations stay in the editor.
+- Picking a datastream with a row's **Edit** button lands here first, in this
+  preview, so you can set up its context before editing. **Open editor** takes
+  you in.
+- The preview plot draws the same series the editor does: the edit target,
+  and its raw source around the session window.
+- **Clear plot**, above the plotted list in the right pane, empties the
+  plot. While a datastream is being edited it closes it first, with the
+  usual question about its session; choosing to stay keeps everything as it
+  was.
+- The row you are editing shows an **Editing** chip and a **Close** button in
+  place of **Edit**. **Close** ends the session the same way the editor's
+  **Close** does.
+- The row check boxes add and remove **context** datastreams, up to 4. That
+  includes the edit target's own source and its other quality-controlled
+  versions: a source you plot this way draws in full, in its own colour, like
+  any plotted datastream. The edit target itself is marked **Editing** in the
+  chooser and cannot be picked, since it is already on the plot.
+- The filter drawer's **Time range** is hidden. The **Context** menu above the
+  plot sets the range around the session, with presets counting out from the
+  session window. Your edits are never reloaded by it.
+- The rail's **Edit** button and the panel's **Open editor** button both
+  open the editor with the session, your zoom and any staged range intact.
+- The row **Edit** button on a different datastream starts the usual entry
+  flow.
 
 A link copied from here carries the edit target, so opening it reopens the
 session behind the Select view.
@@ -187,17 +200,20 @@ window is invalid.
 
 ### Edit view
 
-The Edit View opens once you finish the row Edit button's chooser and session
-window step (see [Datastreams with quality-controlled versions](#datastreams-with-quality-controlled-versions)
+The Edit View opens from the Select view's **Editing** panel (**Open editor**)
+once you finish the row Edit button's chooser and session window step (see
+[Datastreams with quality-controlled versions](#datastreams-with-quality-controlled-versions)
 above), or when you click the pencil icon in the navigation rail to return to
 a session already open.
 
 ![Edit view (full layout)](./images/edit-view.png)
 
-On the plot, the datastream you're editing draws in black on the primary
-axis; its raw source draws underneath in grey as read-only context. Any
+On the plot, the datastream you're editing draws in dark grey on the primary
+axis. Its raw source draws in light grey as read-only context, only before
+and after the session window, and joins the edited data at the window's
+edges. Nothing covers the plot, so box and lasso select work anywhere. Any
 other plotted datastreams render on their own axes to the right, same as the
-Select view. The edit session's time window is shaded.
+Select view.
 
 The Edit View consists of three columns, each independently resizable / collapsible:
 
@@ -220,17 +236,21 @@ See the [Pan and zoom across axes](#pan-and-zoom-across-axes) section below for
 
 Hover any toolbar icon to see its name. The left side of the toolbar flips between **Plot** and **Table** views (see below); the right side carries the **data points toggle**, a share-link button, and the `?` help menu.
 
-Between the help menu and the right edge is the **Context** menu. It holds
+Between the help menu and the right edge is the **Context** button. It reads
+**Context · 1m** (the active range) while the source context is shown, and
+**Context off** when it is not. Its menu starts with a **Show source
+context** switch: off, the raw source leaves the plot entirely. Below it are
 preset chips and From / To pickers like the Select view's Time range
 filter, but here they control how much of the raw source and any plotted
-datastreams load around your edit, not what you're editing. The presets
+datastreams load around your edit, not what you're editing. The raw source
+is never loaded inside the session window. The presets
 count out from the edit session's window instead of back from the last
 observation: **1w**, **1m**, **6m** and **1y** load that span before the
 session's start and after its end, and **All** loads all of the context
 data (always including the session's window). **YTD** is not offered here.
-If YTD is your active preset from the Select view, the editor loads the
-context as **All** and highlights the **All** chip, without changing that
-preference. When a session loads
+The Context menu remembers its own preset, apart from the Select view's Time
+range. A remembered **YTD** loads the context as **All** and highlights the
+**All** chip, without changing that preference. When a session loads
 or you view another session, the active preset re-applies around its
 window; a `Custom` From / To range stays as you set it. Changing the range
 reloads that context data and keeps your current zoom. Your edits are
@@ -300,10 +320,10 @@ URL only on the Select view, because they drive the datastreams table rather tha
 
 The right-hand list (visible on both Select and Edit views) is the roster of everything currently on the plot. Each row carries:
 
-- A drag handle to reorder the list (the line colors track the order). While editing, the **edit target** and its **raw source** are pinned at the top and can't be reordered or unplotted; only the other plotted datastreams can be dragged.
+- A drag handle to reorder the list (the line colors track the order). While editing, the **edit target** is pinned at the top and can't be reordered or unplotted; only the plotted datastreams can be dragged. The raw source context is not listed: it is switched from the **Context** menu.
 - An **eye** toggle that hides the trace from the plot without unplotting it. Hidden rows render with a strikethrough.
 - A **Y-axis** toggle (non-primary rows only) that collapses that datastream's secondary axis to provide more horizontal space for the plot. The edit target and its source share the primary axis, so neither row has this toggle.
-- The datastream name and a subtitle showing the number of points loaded **in the current time window**, e.g. `1,248 pts loaded`. While the fetch is still in flight, the subtitle reads `loading…`. The source row carries a `raw source` chip.
+- The datastream name and a subtitle showing the number of points loaded **in the current time window**, e.g. `1,248 pts loaded`. While the fetch is still in flight, the subtitle reads `loading…`.
 - An `×` button to unplot the row. The edit target and its source can't be unplotted this way. Leave the editor to drop them.
 
 If a plotted datastream has no observations in the current window (either because the dataset is empty there or because the chosen time range doesn't cover its data), the row title shows a small warning-tinted database icon. Hover it for the tooltip "No observations in the current time window". Widening the time range (or clicking **All** in the Time range / Context section) usually clears it.
@@ -314,13 +334,13 @@ While editing, the edit target's row shows the session's working data: committed
 
 ### Plotting multiple datastreams
 
-You can check up to **4 datastreams at a time**. The fifth plot slot is kept for the datastream you edit, so there is always room for it. The plotted count and cap are surfaced in the Datastreams table toolbar as a chip ("`N/4 plotted`"). Once you hit the cap, the unchecked rows disable their plot toggles and a tooltip explains why. Unplot a row from either the table or the list to free up a slot. While editing, the edit target and its raw source are shown in addition to the 4 plotted datastreams. They don't count against the cap.
+You can check up to **4 datastreams at a time**. The fifth plot slot is kept for the datastream you edit, so there is always room for it. The plotted count and cap are surfaced in the Datastreams table toolbar as a chip ("`N/4 plotted`"). Once you hit the cap, the unchecked rows disable their plot toggles and a tooltip explains why. Unplot a row from either the table or the list to free up a slot. While editing, the edit target and its raw source context are shown in addition to the 4 plotted datastreams. They don't count against the cap; a source you plot yourself does.
 
 ![Two datastreams on independent y-axes](./images/home-multi-datastreams.png)
 
 When more than one datastream is on the plot:
 
-- In the Select view, the **first** datastream you plot uses the primary (left) Y axis; in the Edit view, the edit target and its raw source share it instead. In the Select view that line keeps its own series colour; in the Edit view the edit target draws in black with a point marker on every observation, and the raw source draws underneath it in grey.
+- In the Select view, the **first** datastream you plot uses the primary (left) Y axis; in the Edit view, the edit target and its raw source share it instead. In the Select view that line keeps its own series colour; in the Edit view the edit target draws in dark grey with a point marker on every observation, and the raw source draws in light grey around the session window.
 - Every other datastream gets its **own Y axis** on the right side of the plot. The chip at the top of each axis carries the datastream's display name plus its unit (e.g. `Water Temperature (°C)`). Up to four secondary axes stack side by side.
 - Axis chips are colored to match their line so you can tell at a glance which trace goes with which axis.
 
@@ -532,17 +552,18 @@ The body shows:
   - A duration badge.
   - In dev mode, a small chip showing whether the op ran inline or on a worker.
   - A **plot-this-step** button that adds that point in history to the plot as a comparison line.
-  - Clicking the row itself replays history up to that step. Steps after the one on screen are dimmed, since they were not replayed and so are recorded but not reflected in the plot. Replaying re-measures each step's duration but keeps its comment and attribution.
-  - An **undo** button on the trailing entry only (older entries are undone via Reload-from-this-step).
+  - Clicking the row previews the data as it was at that step; a selection step also selects its points on the plot. Every step stays in the history: steps after the one on screen are dimmed, since they are recorded but not reflected in the plot. A banner reads **Previewing step N of M** with a **Back to latest** button (clicking the last step works too). While previewing, editing waits: operation panels, plot selections and table saves hold until you are back on the latest step. Replaying re-measures each step's duration but keeps its comment and attribution.
+  - An **undo** button on the trailing entry only. Undo and redo are the only ways to change the history; from a preview, undo acts on the whole history and ends the preview.
 - A chevron toggles an inline "Arguments" drawer that shows the raw qc-utils call arguments.
 
-### Reload from this step vs. reload from server
+### Preview a step vs. reload from server
 
 These two look like siblings on the **Data loaded** row and mostly produce the
 same picture, but they are not the same operation:
 
-- **Reload from this step** replays the raw observations already held in the
-  browser. No network, and it can only ever restore what you loaded.
+- **Previewing a step** (or the **Data loaded** row, the starting state)
+  replays the raw observations already held in the browser and keeps the
+  history. No network, and it can only ever restore what you loaded.
 - **Discard edits and reload from server** (cloud icon) re-fetches the
   observations and throws the history away.
 
@@ -666,9 +687,11 @@ The dialog lets you add an optional session description. Once you confirm:
 You never leave an edit session by accident. Anything that ends it, the
 editor's **Close** button, the HydroServer logo, the workspace switch, Log
 out, the browser's Back button, or editing a different datastream, asks the
-same question first and tells you what happens to your work. If that other
-datastream has no QC datastream yet, the question comes before the create
-form, so cancelling leaves nothing new behind.
+same question first and tells you what happens to your work. The question
+names the datastream you are leaving. When you edit a different datastream, it
+comes as soon as you pick that datastream, before the session window or the
+create form, so cancelling leaves nothing new behind and puts you back in the
+chooser.
 
 Which question depends on where the session stands:
 
@@ -743,7 +766,7 @@ Click **Workspaces** in the nav rail and **Select** another, or **Continue** to
 | Blank page on load | Wrong API URL or `localhost` vs `127.0.0.1` mismatch. | See [DEPLOYMENT.md](./DEPLOYMENT.md). |
 | Plot stays empty after picking a datastream | A `Custom` time range that doesn't overlap the datastream's observations, or the datastream has no observations. The plotted row shows a database-off icon and the subtitle reads `0 pts loaded`. | Click a preset such as **1m** or **All** in Time range. |
 | The Edit rail item is greyed out | You are not editing. | Click a row's Edit button. |
-| The Select view shows an "Editing" panel | A session is still open. | That is expected: click **Back to editor** to return to it, or **Close** in the editor to end it. |
+| The Select view shows an "Editing" panel | A session is open, or you just picked a datastream to edit. | That is expected: click **Open editor** to edit it, or **Close** on its row (or in the editor) to end it. |
 | I closed the editor and cannot find my session | It is still in progress, just not open. | Click the Edit button on that datastream's row to resume it. Sessions you discarded on the way out are gone for good. |
 | Big edits freeze the page | `SharedArrayBuffer` not available; running inline. | Have your admin re-enable COOP/COEP headers, or accept the slower fallback. |
 | Save fails with a backend error | Permissions / workspace issue / network. | The Snackbar shows the backend message verbatim. Share that with your admin. |
diff --git a/apps/qc-app/e2e/edit-selection.spec.ts b/apps/qc-app/e2e/edit-selection.spec.ts
index caf55df1..ea14d1d9 100644
--- a/apps/qc-app/e2e/edit-selection.spec.ts
+++ b/apps/qc-app/e2e/edit-selection.spec.ts
@@ -6,7 +6,9 @@
  *   - the window step offers presets and a one-click fix
  *   - the editor draws the edit target, its source, and plotted datastreams
  *   - closing the editor keeps the plotted datastreams
- *   - the session window band survives staging shapes
+ *   - the source is drawn only around the session window, with no shape
+ *   - box select works over the session window, and Clear drops it
+ *   - the stage band survives a redraw
  *   - the editor opens zoomed to the session window
  *   - changing the Context range keeps the user zoom
  *   - Context presets count out from the session window
@@ -16,6 +18,7 @@ import { expect, test, type Locator, type Page } from '@playwright/test'
 import { installMocks } from './support/mocks'
 import {
   gotoHome,
+  openEditor,
   openOp,
   plotDatastreamById,
   setupEditView,
@@ -45,6 +48,41 @@ function shapeNames(page: Page): Promise<string[]> {
   })
 }
 
+// `COLORS[0]` and `SOURCE_CONTEXT_COLOR` in `src/utils/plotting/options.ts`.
+const QC_COLOR = '#3f3f3f'
+const SOURCE_COLOR = '#cfcfcf'
+
+type DrawnTrace = {
+  id?: string
+  _partOf?: string
+  x?: ArrayLike<number | string>
+  line?: { color?: string }
+  marker?: { color?: string }
+  _isGapOverlay?: boolean
+}
+
+/** Every trace drawn for datastream `id`: its colour and x extent. */
+function tracesOf(page: Page, id: string) {
+  return page.evaluate((id) => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as
+      | (HTMLElement & { data?: DrawnTrace[] })
+      | null
+    const ms = (v: number | string) =>
+      typeof v === 'number' ? v : Date.parse(v)
+    return (gd?.data ?? [])
+      .filter((t) => t.id === id || t._partOf === id)
+      .map((t) => {
+        const xs = Array.from(t.x ?? [], ms).filter(Number.isFinite)
+        return {
+          line: !!t._isGapOverlay,
+          color: t._isGapOverlay ? t.line?.color : t.marker?.color,
+          count: xs.length,
+          max: xs.length ? Math.max(...xs) : null,
+        }
+      })
+  }, id)
+}
+
 type ReloadProbe = { __contextStart: number; __afterplots: number[] }
 
 function traceIds(page: Page): Promise<string[]> {
@@ -204,7 +242,7 @@ test.describe('edit selection', () => {
       .toEqual(
         expect.arrayContaining([
           MANAGED_DATASTREAM_ID,
-          DATASTREAM_ID,
+          `ctx:${DATASTREAM_ID}`,
           DATASTREAM_ID_B,
         ])
       )
@@ -235,14 +273,65 @@ test.describe('edit selection', () => {
     ).toHaveAttribute('aria-pressed', 'false')
   })
 
-  test('the session window band survives staging shapes', async ({ page }) => {
+  test('the source is drawn only around the session window, with no shape', async ({
+    page,
+  }) => {
+    await gotoHome(page)
+    // Start the session halfway through the source, so the source has
+    // context before it and nothing after.
+    const mid = new Date((FIXTURE_OBS_START_MS + FIXTURE_OBS_END_MS) / 2)
+    mid.setSeconds(0, 0)
+    await openWindowStep(page)
+    await typeDateTime(page.getByTestId('session-window-from'), mid)
+    await page.getByTestId('session-window-start').click()
+    await openEditor(page)
+
+    await expect
+      .poll(async () => (await tracesOf(page, `ctx:${DATASTREAM_ID}`)).length)
+      .toBeGreaterThan(0)
+    const source = await tracesOf(page, `ctx:${DATASTREAM_ID}`)
+    expect(source.every((t) => t.color === SOURCE_COLOR)).toBe(true)
+    expect(source.some((t) => t.count > 0)).toBe(true)
+    // Points only: the bridge line reaches the edit target's first point.
+    expect(
+      source
+        .filter((t) => !t.line)
+        .every((t) => t.max === null || t.max < mid.getTime())
+    ).toBe(true)
+
+    const edit = await tracesOf(page, MANAGED_DATASTREAM_ID)
+    expect(edit.every((t) => t.color === QC_COLOR)).toBe(true)
+    expect(await shapeNames(page)).toEqual([])
+  })
+
+  test('box select works over the session window, and Clear drops it', async ({
+    page,
+  }) => {
+    await setupEditView(page)
+    await page.locator('.modebar-btn[data-title="Box Select"]').first().click()
+    const box = (await page
+      .locator('[data-testid="main-plot"] .nsewdrag')
+      .first()
+      .boundingBox())!
+    await page.mouse.move(box.x + box.width * 0.4, box.y + box.height * 0.05)
+    await page.mouse.down()
+    await page.mouse.move(box.x + box.width * 0.6, box.y + box.height * 0.95, {
+      steps: 10,
+    })
+    await page.mouse.up()
+
+    const clear = page.getByTestId('clear-selection-btn')
+    await expect(clear).toBeVisible()
+    await clear.click()
+    await expect(clear).toHaveCount(0)
+  })
+
+  test('the stage band survives a redraw', async ({ page }) => {
     await setupEditView(page)
-    await expect.poll(() => shapeNames(page)).toContain('edit-window')
 
     await openOp(page, 'gaps')
     await page.getByTestId('filter-range-enable-btn').click()
     await expect.poll(() => shapeNames(page)).toContain('stage')
-    expect(await shapeNames(page)).toContain('edit-window')
 
     // A context reload redraws the plot; the stage band must survive it.
     await page.evaluate(() => {
@@ -262,13 +351,10 @@ test.describe('edit selection', () => {
       () => (window as unknown as { __updates: number }).__updates > 0
     )
     await page.keyboard.press('Escape')
-    expect(await shapeNames(page)).toEqual(
-      expect.arrayContaining(['stage', 'edit-window'])
-    )
+    expect(await shapeNames(page)).toEqual(['stage'])
 
     await page.getByTestId('filter-range-disable-btn').click()
-    await expect.poll(() => shapeNames(page)).not.toContain('stage')
-    expect(await shapeNames(page)).toContain('edit-window')
+    await expect.poll(() => shapeNames(page)).toEqual([])
   })
 
   test('the editor opens zoomed to the session window', async ({ page }) => {
@@ -284,9 +370,7 @@ test.describe('edit selection', () => {
     await typeDateTime(page.getByTestId('session-window-from'), mid)
     await expect(page.getByTestId('session-window-start')).toBeEnabled()
     await page.getByTestId('session-window-start').click()
-    await expect(page.getByTestId('exit-save-btn')).toBeVisible({
-      timeout: 30_000,
-    })
+    await openEditor(page)
 
     const tolerance = 60_000
     await expect
@@ -336,16 +420,6 @@ test.describe('edit selection', () => {
       w.__afterplots = []
       gd.on('plotly_afterplot', () => w.__afterplots.push(performance.now()))
     })
-    // The reload refetches the source and plotted datastreams, never the edit target.
-    const reloaded = Promise.all(
-      [DATASTREAM_ID, DATASTREAM_ID_B].map((id) =>
-        page.waitForResponse(
-          (r) =>
-            r.url().includes(`/api/data/datastreams/${id}/observations`) &&
-            r.request().method() === 'GET'
-        )
-      )
-    )
     // 1w, not All: All is already the context, so picking it changes nothing.
     await menu.getByTestId('date-preset-1w').click()
     await expect(menu.getByTestId('date-preset-1w')).toHaveClass(
@@ -354,26 +428,23 @@ test.describe('edit selection', () => {
     await expect(menu.getByTestId('date-preset-All')).not.toHaveClass(
       /v-chip--variant-tonal/
     )
-    await reloaded
-
-    // Redrawn after both refetches landed: a plotly_afterplot later than the
-    // last observations response in the page's resource timeline.
-    await page.waitForFunction((ids) => {
+    // The reload redraws the context. All was loaded already, so the cache
+    // answers it without a request, and the edit target is never refetched.
+    await page.waitForFunction(
+      () => (window as unknown as ReloadProbe).__afterplots.length > 0
+    )
+    await expect(page.getByTestId('data-loading-indicator')).toHaveCount(0)
+    const editFetched = await page.evaluate((id) => {
       const w = window as unknown as ReloadProbe
-      const fetched = performance
+      return performance
         .getEntriesByType('resource')
-        .filter((e) => e.startTime >= w.__contextStart)
-      const ends = ids.map((id) =>
-        Math.max(
-          ...fetched
-            .filter((e) => e.name.includes(`/datastreams/${id}/observations`))
-            .map((e) => (e as PerformanceResourceTiming).responseEnd)
+        .some(
+          (e) =>
+            e.startTime >= w.__contextStart &&
+            e.name.includes(`/datastreams/${id}/observations`)
         )
-      )
-      if (ends.some((t) => !Number.isFinite(t))) return false
-      const landed = Math.max(...ends)
-      return w.__afterplots.some((t) => t > landed)
-    }, [DATASTREAM_ID, DATASTREAM_ID_B])
+    }, MANAGED_DATASTREAM_ID)
+    expect(editFetched).toBe(false)
     expect(await traceIds(page)).toContain(MANAGED_DATASTREAM_ID)
 
     // Sample the range over several frames so a late redraw would show up.
@@ -410,7 +481,7 @@ test.describe('edit selection', () => {
     await typeDateTime(page.getByTestId('session-window-from'), mid)
     await expect(page.getByTestId('session-window-start')).toBeEnabled()
     await page.getByTestId('session-window-start').click()
-    await waitForEditorReady(page)
+    await openEditor(page)
 
     await page.getByTestId('context-range-btn').click()
     const menu = page.getByTestId('context-range-menu')
@@ -435,17 +506,16 @@ test.describe('edit selection', () => {
     await expect.poll(() => fieldText('date-range-from')).toEqual(pickerText(expectedFrom))
     await expect.poll(() => fieldText('date-range-to')).toEqual(pickerText(expectedTo))
 
-    // The grey source covers the whole window, from the data start (the
-    // week before the window, clamped to the fixture).
+    // The grey source runs from the data start (the week before the window,
+    // clamped to the fixture) and stops at the window.
     const tolerance = 60_000
     await expect
       .poll(async () => {
-        const x = await traceXExtent(page, DATASTREAM_ID)
+        const x = await traceXExtent(page, `ctx:${DATASTREAM_ID}`)
         return (
           !!x &&
           Math.abs(x[0] - FIXTURE_OBS_START_MS) <= tolerance &&
-          x[0] <= mid.getTime() &&
-          x[1] >= FIXTURE_OBS_END_MS - tolerance
+          x[1] < mid.getTime()
         )
       })
       .toBe(true)
diff --git a/apps/qc-app/e2e/leave-session.spec.ts b/apps/qc-app/e2e/leave-session.spec.ts
index 254ae6f0..21f7dfcb 100644
--- a/apps/qc-app/e2e/leave-session.spec.ts
+++ b/apps/qc-app/e2e/leave-session.spec.ts
@@ -69,7 +69,7 @@ test.describe('leaving a session', () => {
     page,
   }) => {
     await page.getByTestId('exit-close-btn').click()
-    await expect(leaveDialog(page)).toContainText('This session has no edits')
+    await expect(leaveDialog(page)).toContainText('has no edits')
 
     await page.getByTestId('leave-cancel-btn').click()
     await expect(leaveDialog(page)).toHaveCount(0)
@@ -118,7 +118,7 @@ test.describe('leaving a session', () => {
     await page.getByTestId('leave-discard-edits-btn').click()
 
     // Nothing was ever saved, so the session is empty now.
-    await expect(leaveDialog(page)).toContainText('This session has no edits')
+    await expect(leaveDialog(page)).toContainText('has no edits')
     await page.getByTestId('leave-keep-btn').click()
 
     await expectClosed(page)
@@ -131,7 +131,7 @@ test.describe('leaving a session', () => {
     await expect(page.getByText('Draft saved.')).toBeVisible()
 
     await page.getByTestId('exit-close-btn').click()
-    await expect(leaveDialog(page)).toContainText('Close this session?')
+    await expect(leaveDialog(page)).toContainText('Close your session on')
     await expect(leaveDialog(page)).toContainText('stays in progress')
     await page.getByTestId('leave-close-btn').click()
 
diff --git a/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts b/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts
index c7011d45..ad998a15 100644
--- a/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts
+++ b/apps/qc-app/e2e/plot-no-intended-spacing.spec.ts
@@ -3,7 +3,7 @@
  *
  * Two related guarantees, both covered here:
  *   1. The main trace renders as a pure scatter: `mode: 'markers'` with
- *      no companion `_gapOverlayFor` lines trace pushed alongside it.
+ *      no companion `_partOf` lines trace pushed alongside it.
  *   2. The "data points" toggle (manual mode + click off) does not hide
  *      the markers of such a series. Without a line fallback, honouring
  *      the toggle would leave the series invisible, so the relayout
@@ -31,7 +31,7 @@ type RoutedTrace = {
   mode?: string
   marker?: { opacity?: number }
   _isGapOverlay?: boolean
-  _gapOverlayFor?: string
+  _partOf?: string
 }
 
 /**
@@ -129,16 +129,16 @@ test.describe('plot: datastream without intendedTimeSpacing', () => {
         | null
       const traces = gd?.data ?? []
       const main = traces.find((t) => t.id === id) ?? null
-      const overlay = traces.find((t) => t._gapOverlayFor === id) ?? null
+      const overlay = traces.find((t) => t._partOf === id) ?? null
       return {
         mainMode: main?.mode ?? null,
         mainMarkerOpacity: main?.marker?.opacity ?? null,
         overlayExists: !!overlay,
         // Sanity: there should also be no `mode: 'lines'` trace
         // pointing at this series via the gap-overlay channel. The
-        // app reserves `_gapOverlayFor` exclusively for that role,
+        // app reserves `_partOf` exclusively for that role,
         // so its absence is the precise signal.
-        anyGapOverlay: traces.some((t) => t._gapOverlayFor === id),
+        anyGapOverlay: traces.some((t) => t._partOf === id),
       }
     }, MANAGED_DATASTREAM_ID)
 
diff --git a/apps/qc-app/e2e/select-while-editing.spec.ts b/apps/qc-app/e2e/select-while-editing.spec.ts
index c3613a02..1b7ec5d2 100644
--- a/apps/qc-app/e2e/select-while-editing.spec.ts
+++ b/apps/qc-app/e2e/select-while-editing.spec.ts
@@ -129,7 +129,7 @@ test.describe('Select while editing', () => {
     // The edit target is still drawn, so the user sees what they changed.
     expect(await plottedTraceIds(page)).toContain(MANAGED_DATASTREAM_ID)
 
-    await page.getByTestId('back-to-editor-btn').click()
+    await page.getByTestId('open-editor-btn').click()
     await waitForEditorReady(page)
     await expectHistoryContains(page, 'Change Values')
   })
@@ -179,13 +179,14 @@ test.describe('Select while editing', () => {
     expect(ids).toContain(DATASTREAM_ID_B)
     expect(ids).toContain(MANAGED_DATASTREAM_ID)
 
-    // The edit target and its source are pinned in the plotted list.
+    // The edit target is pinned in the plotted list. Its source is drawn as
+    // context, which the Context menu switches, so it gets no row.
     await expect(
       sidePanel(page).getByTestId(`plotted-item-${MANAGED_DATASTREAM_ID}`)
     ).toBeVisible()
     await expect(
       sidePanel(page).getByTestId(`plotted-item-${DATASTREAM_ID}`)
-    ).toBeVisible()
+    ).toHaveCount(0)
   })
 
   test('keeps ed in the share URL and restores Select on reload', async ({
@@ -211,17 +212,64 @@ test.describe('Select while editing', () => {
     )
   })
 
-  test('the row Edit button returns to the session being edited', async ({
+  test('the edited row offers Close, which ends the session', async ({
     page,
   }) => {
     await gotoHome(page)
     await startSessionFromRow(page)
     await goToSelect(page)
 
-    await page.getByTestId(`edit-datastream-${DATASTREAM_ID}`).click()
-    // The session id is generated, so match the Continue button by prefix.
-    await page.locator('[data-testid^="continue-session-"]').click()
-    await waitForEditorReady(page)
-    await expect(page.getByText('Filter Data')).toBeVisible()
+    await expect(page.getByTestId(`edit-datastream-${DATASTREAM_ID}`)).toHaveCount(0)
+    await expect(page.getByTestId('editing-chip')).toBeVisible()
+    await page.getByTestId(`close-datastream-${DATASTREAM_ID}`).click()
+    // The fresh session holds no edits, so leaving asks what to do with it.
+    await page.getByTestId('leave-keep-btn').click()
+    await expect(page.getByTestId('editing-chip')).toHaveCount(0)
+    await expect(page.getByTestId(`edit-datastream-${DATASTREAM_ID}`)).toBeVisible()
+  })
+
+  test('plotting the raw source keeps the grey context beside it', async ({
+    page,
+  }) => {
+    await gotoHome(page)
+    await startSessionFromRow(page)
+    await goToSelect(page)
+
+    await page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`).click()
+    await expect(page.getByTestId('plot-source-dialog')).toBeVisible()
+    // The edit target is already on the plot, so it is not offered.
+    await expect(
+      page.getByTestId(`plot-option-editing-${MANAGED_DATASTREAM_ID}`)
+    ).toBeVisible()
+    await page.getByTestId(`plot-option-${DATASTREAM_ID}`).locator('input').check()
+    await page.getByTestId('plot-source-apply').click()
+
+    await expect
+      .poll(() => plottedTraceIds(page))
+      .toEqual(expect.arrayContaining([DATASTREAM_ID, `ctx:${DATASTREAM_ID}`]))
+    // The plotted source is an ordinary row; the grey context gets none.
+    await expect(
+      sidePanel(page).getByTestId(`plotted-item-${DATASTREAM_ID}`)
+    ).toBeVisible()
+  })
+
+  test('Clear plot closes the edited datastream and unplots everything', async ({
+    page,
+  }) => {
+    await gotoHome(page)
+    await startSessionFromRow(page)
+    await goToSelect(page)
+    await page.getByTestId(`plot-checkbox-${DATASTREAM_ID_B}`).click()
+    await expect
+      .poll(() => plottedTraceIds(page))
+      .toEqual(expect.arrayContaining([DATASTREAM_ID_B]))
+
+    await page.getByTestId('clear-plot-btn').click()
+    // The fresh session holds no edits, so closing asks what to do with it.
+    await page.getByTestId('leave-keep-btn').click()
+
+    await expect(page.getByTestId('editing-chip')).toHaveCount(0)
+    await expect(page.getByTestId('clear-plot-btn')).toHaveCount(0)
+    await expect(page.getByText('No datastream plotted')).toBeVisible()
   })
 })
diff --git a/apps/qc-app/e2e/share-url.spec.ts b/apps/qc-app/e2e/share-url.spec.ts
index d2ea5a52..63f4bf79 100644
--- a/apps/qc-app/e2e/share-url.spec.ts
+++ b/apps/qc-app/e2e/share-url.spec.ts
@@ -116,30 +116,40 @@ async function plotXRange(page: Page): Promise<[number, number] | null> {
   })
 }
 
-/**
- * Whether the plot is showing the session-window band the editor draws, which
- * is what "the editor opened on its session window" looks like from outside.
- */
-async function zoomedToSessionWindow(page: Page): Promise<string> {
-  return page.evaluate(() => {
-    const gd = document.querySelector('[data-testid="main-plot"]') as
-      | (HTMLElement & {
-          _fullLayout?: { xaxis?: { range?: [number | string, number | string] } }
-          layout?: { shapes?: { name?: string; x0?: unknown; x1?: unknown }[] }
-        })
-      | null
-    const range = gd?._fullLayout?.xaxis?.range
-    const band = (gd?.layout?.shapes ?? []).find((s) => s.name === 'edit-window')
-    if (!range) return 'no plot'
-    if (!band) return 'no session band'
-    const toMs = (v: unknown) =>
-      typeof v === 'number' ? v : Date.parse(String(v))
-    const near = (a: number, b: number) => Math.abs(a - b) <= 2000
-    return near(toMs(range[0]), toMs(band.x0)) &&
-      near(toMs(range[1]), toMs(band.x1))
-      ? 'session window'
-      : 'another range'
-  })
+type SessionWindow = { begin: number; end: number }
+
+/** Resolves with the window of the next session the app creates. */
+function nextSessionWindow(page: Page): Promise<SessionWindow> {
+  return page
+    .waitForResponse(
+      (r) =>
+        r.request().method() === 'POST' &&
+        /\/quality-control\/histories\/[^/]+\/sessions$/.test(
+          new URL(r.url()).pathname
+        )
+    )
+    .then(async (r) => {
+      const { data: s } = (await r.json()) as {
+        data: { phenomenonTimeStart: string; phenomenonTimeEnd: string }
+      }
+      return {
+        begin: Date.parse(s.phenomenonTimeStart),
+        end: Date.parse(s.phenomenonTimeEnd),
+      }
+    })
+}
+
+/** Whether the plot's x range is the session window. */
+async function zoomedToSessionWindow(
+  page: Page,
+  w: SessionWindow
+): Promise<string> {
+  const range = await plotXRange(page)
+  if (!range) return 'no plot'
+  const near = (a: number, b: number) => Math.abs(a - b) <= 2000
+  return near(range[0], w.begin) && near(range[1], w.end)
+    ? 'session window'
+    : 'another range'
 }
 
 /** Wait until the live plot draws a trace for every id in `ids`. */
@@ -379,10 +389,14 @@ test.describe('share URL', () => {
       })
       .toBeCloseTo(xLo, -4)
 
+    const created = nextSessionWindow(page)
     await startSessionFromRow(page)
+    const sessionWindow = await created
 
     await expect
-      .poll(() => zoomedToSessionWindow(page), { timeout: 30_000 })
+      .poll(() => zoomedToSessionWindow(page, sessionWindow), {
+        timeout: 30_000,
+      })
       .toBe('session window')
   })
 
@@ -410,7 +424,7 @@ test.describe('share URL', () => {
               data?: Array<{
                 id?: string
                 visible?: boolean | 'legendonly' | undefined
-                _gapOverlayFor?: string
+                _partOf?: string
               }>
             })
           | null
@@ -422,7 +436,7 @@ test.describe('share URL', () => {
         const reportFor = (id: string): State | null => {
           const main = traces.find((t) => t.id === id)
           if (!main) return null
-          const overlay = traces.find((t) => t._gapOverlayFor === id)
+          const overlay = traces.find((t) => t._partOf === id)
           return {
             visible: main.visible,
             overlayHidden: overlay ? overlay.visible === false : true,
diff --git a/apps/qc-app/e2e/support/app.ts b/apps/qc-app/e2e/support/app.ts
index 657ba590..09f6b1bd 100644
--- a/apps/qc-app/e2e/support/app.ts
+++ b/apps/qc-app/e2e/support/app.ts
@@ -133,13 +133,20 @@ export async function setupEditView(page: Page): Promise<void> {
 
 /**
  * From Home: the source row's Edit button, "Start new session" on the managed
- * datastream, then Start with the default window. Waits for the editor to be
+ * datastream, then Start with the default window. That lands on the Select
+ * view's preview; `openEditor` then opens the editor. Waits for it to be
  * ready.
  */
 export async function startSessionFromRow(page: Page): Promise<void> {
   await page.getByTestId(`edit-datastream-${DATASTREAM_ID}`).click()
   await page.getByTestId(`edit-managed-${MANAGED_DATASTREAM_ID}`).click()
   await page.getByTestId('session-window-start').click()
+  await openEditor(page)
+}
+
+/** From the Select view's preview of an edit target, open the editor. */
+export async function openEditor(page: Page): Promise<void> {
+  await page.getByTestId('open-editor-btn').click({ timeout: 30_000 })
   await waitForEditorReady(page)
 }
 
diff --git a/apps/qc-app/src/components/EditData/EditHistory.vue b/apps/qc-app/src/components/EditData/EditHistory.vue
index 41347467..69221951 100644
--- a/apps/qc-app/src/components/EditData/EditHistory.vue
+++ b/apps/qc-app/src/components/EditData/EditHistory.vue
@@ -131,7 +131,7 @@
         class="edit-history__row edit-history__row--baseline px-3 py-2 d-flex align-center"
         :class="{ 'edit-history__row--clickable': canStepTo }"
         data-testid="history-reload-step-baseline"
-        :title="canStepTo ? `Reload the session's starting state` : undefined"
+        :title="canStepTo ? `Preview the session's starting state` : undefined"
         @click="onRowReload(SNAPSHOT_BASELINE_INDEX)"
       >
         <v-icon
@@ -149,7 +149,7 @@
           class="edit-history__step text-body-small font-weight-medium flex-grow-1 text-truncate"
           data-testid="history-step-btn-baseline"
           :disabled="!canStepTo"
-          :title="canStepTo ? `Reload the session's starting state` : undefined"
+          :title="canStepTo ? `Preview the session's starting state` : undefined"
           @click.stop="onRowReload(SNAPSHOT_BASELINE_INDEX)"
         >
           {{ selectedSeries?.data.isLoading ? 'Loading data…' : 'Data loaded' }}
@@ -234,6 +234,32 @@
 
       <v-divider />
 
+      <div
+        v-if="previewIndex !== null && !isSwitchingSession"
+        class="edit-history__preview d-flex align-center ga-2 px-3 py-2"
+        data-testid="history-preview-banner"
+      >
+        <v-icon icon="mdi-eye-outline" size="16" color="primary" />
+        <span class="text-body-small flex-grow-1">
+          Previewing
+          {{
+            previewIndex < 0
+              ? 'the starting state'
+              : `step ${previewIndex + 1} of ${editCount}`
+          }}. Editing waits until you are back on the latest step.
+        </span>
+        <v-btn
+          data-testid="history-back-to-latest-btn"
+          size="small"
+          variant="flat"
+          color="primary"
+          :disabled="isUpdating"
+          @click.stop="onBackToLatest"
+        >
+          Back to latest
+        </v-btn>
+      </div>
+
       <!-- Or the outgoing session's operations linger as if they were these. -->
       <div
         v-if="isSwitchingSession"
@@ -578,7 +604,7 @@ const onHeaderClick = (e: MouseEvent) => {
   toggleCollapsed()
 }
 
-const { editHistory, selectedSeries, isUpdating } =
+const { editHistory, selectedSeries, isUpdating, previewIndex } =
   storeToRefs(usePlotlyStore())
 const { selectedOperation } = storeToRefs(useUIStore())
 const { redraw } = usePlotlyStore()
@@ -588,15 +614,11 @@ const fileInputRef = ref<HTMLInputElement | null>(null)
 
 const openIndex = ref<number | null>(null)
 
-/** Invalidated by any mutation of the history. */
-const loadedStepIndex = ref<number | null>(null)
-
-/** The step the plot reflects: the explicit choice, else the last entry. */
+/** The step the plot reflects: the one previewed, else the last entry. */
 const shownStepIndex = computed<number | null>(() => {
   const last = editHistory.value.length - 1
   if (last < 0) return null
-  const chosen = loadedStepIndex.value
-  return chosen !== null && chosen <= last ? chosen : last
+  return previewIndex.value ?? last
 })
 
 // Committed sessions are immutable server-side, so their comments are shown
@@ -634,7 +656,7 @@ const canRedo = computed(
 /** Stepping needs something to replay and a settled dispatch. */
 const canStepTo = computed(() => !isUpdating.value && editCount.value > 0)
 
-/** Row click / Enter / Space: replay to that step. */
+/** Row click / Enter / Space: preview that step. */
 const onRowReload = (index: number) => {
   if (!canStepTo.value) return
   if (index >= 0 && editHistory.value[index]?.execution?.inFlight) return
@@ -652,8 +674,8 @@ const isApplied = (index: number) =>
 
 const stepTitle = (index: number) =>
   isApplied(index)
-    ? 'Reload from this step'
-    : 'Not applied in the step currently shown. Click to reload from here.'
+    ? 'Preview this step'
+    : 'Not applied in the step currently shown. Click to preview it.'
 
 function formatMethod(method: string) {
   if (!method) return ''
@@ -685,7 +707,6 @@ function formatArg(arg: unknown): string {
 }
 
 const onReload = async () => {
-  loadedStepIndex.value = null
   if (isReadOnly.value || isUpdating.value) return
   isUpdating.value = true
   closeStaleStagingPanel()
@@ -709,29 +730,26 @@ const onReload = async () => {
   })
 }
 
+// Previewing only shows a step: every step stays in the history, and only
+// undo and redo change it. The last step is the whole history again.
 const onReloadHistory = async (index: number) => {
-  if (index < editHistory.value.length) {
-    isUpdating.value = true
-    closeStaleStagingPanel()
-    // `reloadHistory` truncates to `0..index`; a committed session's
-    // operations must survive stepping through them.
-    const record = selectedSeries.value?.data
-    const preserved = isReadOnly.value ? [...(record?.history ?? [])] : null
-    loadedStepIndex.value = index
-    setTimeout(async () => {
-      const newSelection = await record?.reloadHistory(index)
-      if (preserved && record) {
-        // Append only what the replay dropped. Restoring `preserved`
-        // wholesale would put the pre-replay timings back.
-        const tail = preserved.slice(index + 1)
-        record.history.push(...tail)
-        loadedStepIndex.value = record.history.length - tail.length - 1
-      }
-
+  if (index >= editHistory.value.length) return
+  const record = selectedSeries.value?.data
+  if (!record) return
+  isUpdating.value = true
+  closeStaleStagingPanel()
+  setTimeout(async () => {
+    try {
+      await applyReplayedSelection(await record.previewHistory(index))
+    } finally {
       isUpdating.value = false
-      await applyReplayedSelection(newSelection)
-    })
-  }
+    }
+  })
+}
+
+const onBackToLatest = () => {
+  const last = editHistory.value.length - 1
+  if (last >= 0) void onReloadHistory(last)
 }
 
 const onSaveHistory = async () => {
@@ -799,7 +817,6 @@ const closeStaleStagingPanel = () => {
 }
 
 const onUndo = async () => {
-  loadedStepIndex.value = null
   // Also guards the Ctrl+Z shortcut, which bypasses the disabled button.
   if (isReadOnly.value || !canUndo.value || isUpdating.value) return
   isUpdating.value = true
@@ -815,7 +832,6 @@ const onUndo = async () => {
 }
 
 const onRedo = async () => {
-  loadedStepIndex.value = null
   if (isReadOnly.value || !canRedo.value || isUpdating.value) return
   isUpdating.value = true
   closeStaleStagingPanel()
@@ -923,6 +939,10 @@ onBeforeUnmount(() => window.removeEventListener('keydown', onKeydown))
 
 /* Steps past the one on screen were not replayed, so they are dimmed to
    separate what the plot reflects from what is merely recorded. */
+.edit-history__preview {
+  background-color: rgba(var(--v-theme-primary), 0.06);
+}
+
 .edit-history__row--unapplied {
   opacity: 0.45;
 }
diff --git a/apps/qc-app/src/components/EditData/EditTargetPanel.vue b/apps/qc-app/src/components/EditData/EditTargetPanel.vue
index 610caa9a..50ce5c32 100644
--- a/apps/qc-app/src/components/EditData/EditTargetPanel.vue
+++ b/apps/qc-app/src/components/EditData/EditTargetPanel.vue
@@ -40,19 +40,19 @@
       </div>
 
       <div class="text-body-small text-medium-emphasis">
-        Plot other datastreams around this session, then go back to edit it.
+        Plot other datastreams around this session, then open the editor.
       </div>
 
       <v-btn
-        data-testid="back-to-editor-btn"
+        data-testid="open-editor-btn"
         size="small"
         variant="flat"
         color="primary"
-        prepend-icon="mdi-arrow-left"
+        prepend-icon="mdi-pencil"
         block
         @click="openEditor"
       >
-        Back to editor
+        Open editor
       </v-btn>
     </div>
   </div>
@@ -62,7 +62,8 @@
 /**
  * What the Select view says about an open edit session: the managed
  * datastream, its session window, whether anything is unsaved, and the way
- * back. Editing itself stays in the editor.
+ * into the editor. Picking a datastream to edit lands here first, so its
+ * context can be set up before editing.
  */
 
 import { computed } from 'vue'
diff --git a/apps/qc-app/src/components/EditData/LeaveSessionDialog.vue b/apps/qc-app/src/components/EditData/LeaveSessionDialog.vue
index 23b83442..214f9115 100644
--- a/apps/qc-app/src/components/EditData/LeaveSessionDialog.vue
+++ b/apps/qc-app/src/components/EditData/LeaveSessionDialog.vue
@@ -121,12 +121,13 @@ const RESUME = "Resume it any time from the datastream's Edit button."
 
 const copy = computed(() => {
   const prompt = leavePrompt.value
+  const on = prompt?.datastreamName ? ` on ${prompt.datastreamName}` : ''
   if (prompt?.kind === 'unsaved') {
     const count = prompt.unsavedCount
     return {
       color: 'warning',
       icon: 'mdi-content-save-alert-outline',
-      title: 'Unsaved edits',
+      title: `Unsaved edits${on}`,
       subtitle: count
         ? `${count} edit${count === 1 ? '' : 's'} not yet saved to the session`
         : 'You have changes that are not in the session',
@@ -139,7 +140,7 @@ const copy = computed(() => {
     return {
       color: 'error',
       icon: 'mdi-help-circle-outline',
-      title: 'This session has no edits',
+      title: `Your session${on} has no edits`,
       subtitle: 'Nothing saved, nothing unsaved',
       body: `Keep it and it stays in progress. ${RESUME} Discard it and it is deleted from the server.`,
     }
@@ -147,7 +148,7 @@ const copy = computed(() => {
   return {
     color: 'primary',
     icon: 'mdi-pause-circle-outline',
-    title: 'Close this session?',
+    title: `Close your session${on}?`,
     subtitle: 'Every edit is saved to the session',
     body: `Your work stays where it is. The session stays in progress. ${RESUME}`,
   }
diff --git a/apps/qc-app/src/components/EditData/OperationPanel.vue b/apps/qc-app/src/components/EditData/OperationPanel.vue
index fafa87f9..9009a8e8 100644
--- a/apps/qc-app/src/components/EditData/OperationPanel.vue
+++ b/apps/qc-app/src/components/EditData/OperationPanel.vue
@@ -37,7 +37,19 @@
     <v-divider />
 
     <div
-      v-if="op.requiresSelection && !selectedData?.length"
+      v-if="previewIndex !== null"
+      class="px-3 py-4 d-flex flex-column align-center text-center"
+      data-testid="operation-preview-blocked"
+    >
+      <v-icon icon="mdi-eye-outline" size="28" color="primary" class="mb-2" />
+      <div class="text-body-small text-medium-emphasis">
+        An earlier history step is being previewed. Go back to the latest
+        step in the edit history to continue editing.
+      </div>
+    </div>
+
+    <div
+      v-else-if="op.requiresSelection && !selectedData?.length"
       class="px-3 py-4 d-flex flex-column align-center text-center"
     >
       <v-icon
@@ -106,6 +118,7 @@
 </template>
 
 <script setup lang="ts">
+import { usePlotlyStore } from '@/store/plotly'
 import { computed } from 'vue'
 import { storeToRefs } from 'pinia'
 import { useUIStore } from '@/store/userInterface'
@@ -115,6 +128,7 @@ import FilterRangePanel from '@/components/FilterPoints/FilterRangePanel.vue'
 
 const { selectedOperation, filterRangeActive } = storeToRefs(useUIStore())
 const { selectedData } = storeToRefs(useDataVisStore())
+const { previewIndex } = storeToRefs(usePlotlyStore())
 
 const op = computed(() =>
   selectedOperation.value ? operationsById[selectedOperation.value] : null
diff --git a/apps/qc-app/src/components/EditData/StartEditingFlow.vue b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
index ebd9fbf7..40c1f2c0 100644
--- a/apps/qc-app/src/components/EditData/StartEditingFlow.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
@@ -75,10 +75,11 @@ import { useDatastreamMetadata } from '@/composables/useDatastreamMetadata'
 import { useProcessingLevels } from '@/composables/useProcessingLevels'
 import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
 import type { TimeWindow } from '@/utils/timeRangePresets'
-import type { View } from '@/store/userInterface'
+import { DrawerType, type View } from '@/store/userInterface'
 
-// 'entering': the editor is not open yet. 'resume': it opened with no session.
-// 'footer': it is open on committed history.
+// 'entering': picked from a row, so it opens in the Select view's preview.
+// 'resume': the editor opened with no session. 'footer': it is open on
+// committed history.
 type WindowOrigin = 'entering' | 'resume' | 'footer'
 
 interface WindowTarget {
@@ -139,13 +140,12 @@ const showWindow = computed({
 })
 
 /**
- * A created managed datastream is always a new edit target, so an open
- * session is certainly abandoned. Ask and end it before anything reaches the
- * server, rather than after a create the user might not want. False means the
- * session was kept and the create step must not open.
+ * Picking another target abandons the open session. Ask and end it right
+ * away, before the window step or a create reaches the server. False means
+ * the session was kept and the next step must not open.
  */
-async function leaveOpenSession(): Promise<boolean> {
-  if (!qcDatastream.value) return true
+async function leaveOpenSession(nextId?: string): Promise<boolean> {
+  if (!qcDatastream.value || qcDatastream.value.id === nextId) return true
   return closeEditor()
 }
 
@@ -172,8 +172,15 @@ async function onChooserEdit(option: ManagedDatastreamOption) {
   showChooser.value = false
   const source = chooserSource.value
   if (!source) return
+  // Keeping the session returns to the chooser.
+  if (!(await leaveOpenSession(option.managed.id))) {
+    showChooser.value = true
+    return
+  }
+  // Picked from a row: preview it in the Select view to set up its context
+  // first; the side panel opens the editor.
   if (option.sessions.some((s) => s.status === 'in_progress')) {
-    await runEnter(option.managed.id)
+    await runEnter(option.managed.id, undefined, DrawerType.Select)
     return
   }
   openWindow({
@@ -236,7 +243,7 @@ async function onWindowConfirm(window: TimeWindow) {
   let pickAgain = false
   try {
     if (target.origin === 'entering') {
-      await runEnter(target.managedId, window)
+      await runEnter(target.managedId, window, DrawerType.Select)
     } else {
       const started = await startSessionOver(window)
       // A plain failure keeps the editor on this target; a superseded one left.
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
index 29ed5ced..cd816a29 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
@@ -1,5 +1,5 @@
 import { mount, flushPromises, enableAutoUnmount } from '@vue/test-utils'
-import { ref } from 'vue'
+import { computed, reactive, ref } from 'vue'
 import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
 import { createTestPinia } from '@/utils/test/pinia'
 import { createTestVuetify } from '@/utils/test/vuetify'
@@ -20,10 +20,31 @@ const isUpdating = ref(false)
 const redraw = vi.fn().mockResolvedValue(undefined)
 const refreshGraphSeriesArray = vi.fn().mockResolvedValue(undefined)
 
+const previewIndex = computed(() => selectedSeries.value?.data?.previewIndex ?? null)
+
+// Reactive like a real store, so `storeToRefs` unwraps the computed.
 vi.mock('@/store/plotly', () => ({
-  usePlotlyStore: () => ({ editHistory, selectedSeries, isUpdating, redraw }),
+  usePlotlyStore: () =>
+    reactive({
+      editHistory,
+      selectedSeries,
+      isUpdating,
+      redraw,
+      previewIndex,
+    }),
 }))
 
+/**
+ * Stands in for `ObservationRecord.previewHistory`: shows step `index`
+ * without dropping the later ones, and previewing the last step returns to
+ * the whole history. Called on the record, so `this` is the series data.
+ */
+const previewing = (selection: number[] = []) =>
+  vi.fn(async function (this: any, index: number) {
+    this.previewIndex = index >= this.history.length - 1 ? null : index
+    return selection
+  })
+
 const qcDatastream = ref<any>(null)
 vi.mock('@/store/dataVisualization', () => ({
   useDataVisStore: () => ({ refreshGraphSeriesArray, qcDatastream }),
@@ -110,7 +131,7 @@ function makeSeries(overrides: Partial<any> = {}) {
       undo: vi.fn().mockResolvedValue([1, 2]),
       redo: vi.fn().mockResolvedValue([]),
       reload: vi.fn().mockResolvedValue(undefined),
-      reloadHistory: vi.fn().mockResolvedValue([3]),
+      previewHistory: previewing([3]),
       ...overrides,
     },
   }
@@ -330,10 +351,10 @@ describe('EditHistory.vue actions', () => {
     vi.useRealTimers()
   })
 
-  it('clicking a step row calls reloadHistory with its index', async () => {
+  it('clicking a step row previews it', async () => {
     vi.useFakeTimers()
     editHistory.value = [makeEntry('ADD_POINTS')]
-    selectedSeries.value.data.reloadHistory = vi.fn().mockResolvedValue([9])
+    selectedSeries.value.data.previewHistory = vi.fn().mockResolvedValue([9])
     const wrapper = createWrapper()
     await flushPromises()
     const entry = wrapper.find('[data-testid="history-item-0"]')
@@ -341,7 +362,7 @@ describe('EditHistory.vue actions', () => {
     expect(reloadBtn).toBeTruthy()
     await reloadBtn!.trigger('click')
     await vi.runAllTimersAsync()
-    expect(selectedSeries.value.data.reloadHistory).toHaveBeenCalledWith(0)
+    expect(selectedSeries.value.data.previewHistory).toHaveBeenCalledWith(0)
     expect(setPlotSelection).toHaveBeenCalledWith([9])
     vi.useRealTimers()
   })
@@ -446,25 +467,25 @@ describe('EditHistory.vue actions', () => {
     }
 
     /**
-     * Stands in for the real `reloadHistory`: truncate to `0..index` and
-     * replace the survivors with freshly dispatched entries carrying the
-     * timings the replay just measured. Splices through the ref's proxy;
-     * mutating the raw array wouldn't trigger reactivity.
+     * Stands in for the real `previewHistory`: replace the steps up to
+     * `index` with freshly dispatched entries carrying the timings the replay
+     * just measured, and keep the later ones. Splices through the ref's
+     * proxy; mutating the raw array wouldn't trigger reactivity.
      */
-    const replayingReloadHistory = () =>
-      vi.fn(async (index: number) => {
+    const replayingPreview = () =>
+      vi.fn(async function (this: any, index: number) {
         const fresh = editHistory.value.slice(0, index + 1).map((h, i) => ({
           method: h.method,
           args: h.args,
           execution: { inFlight: false, status: 'success', durationMs: 900 + i },
         }))
-        editHistory.value.splice(0, editHistory.value.length, ...fresh)
+        editHistory.value.splice(0, index + 1, ...fresh)
+        this.previewIndex = index
         return []
       })
 
-    // The committed-session path restores the full operation list after the
-    // replay so the user can keep stepping through it. That restore must not
-    // drag the pre-replay timings back with it.
+    // A preview keeps the later steps listed; they must not show the timings
+    // of a run that no longer holds in this view.
     it('shows the timings the replay produced, not the originals', async () => {
       const history = [
         makeEntry('SELECTION', [], { durationMs: 10 }),
@@ -476,7 +497,7 @@ describe('EditHistory.vue actions', () => {
         data: {
           history: editHistory.value,
           redoStack: [],
-          reloadHistory: replayingReloadHistory(),
+          previewHistory: replayingPreview(),
         },
       }
       const w = createWrapper()
@@ -486,8 +507,7 @@ describe('EditHistory.vue actions', () => {
       await w
         .find('[data-testid="history-item-1"] .edit-history__row')
         .trigger('click')
-      // `loadedStepIndex` moves synchronously, so waiting on the marker
-      // would race the replay. Wait for the dispatch itself to settle.
+      // Wait for the preview itself to settle.
       await vi.waitFor(() => expect(isUpdating.value).toBe(false))
       await flushPromises()
 
@@ -540,35 +560,56 @@ describe('EditHistory.vue actions', () => {
       expect(btn.attributes('disabled')).toBeUndefined()
     })
 
-    it('keeps the entries below when reloading from a step', async () => {
-      const history = [
-        { method: 'SELECTION', args: [], execution: {} },
-        { method: 'DELETE_POINTS', args: [], execution: {} },
-        { method: 'INTERPOLATE', args: [], execution: {} },
-      ]
-      editHistory.value = history
-      // Stand in for the engine: truncate to `0..index`, as qc-utils does.
-      const reloadHistory = vi.fn(async (index: number) => {
-        history.splice(index + 1)
-        return []
-      })
-      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+  })
 
+  describe('previewing a step', () => {
+    const threeSteps = () => [
+      { method: 'SELECTION', args: [], execution: {} },
+      { method: 'DELETE_POINTS', args: [], execution: {} },
+      { method: 'INTERPOLATE', args: [], execution: {} },
+    ]
+
+    it('keeps every step listed and offers the way back', async () => {
+      const history = threeSteps()
+      editHistory.value = history
+      const previewHistory = previewing()
+      selectedSeries.value = { data: { history, redoStack: [], previewHistory } }
       const w = createWrapper()
-      await readOnly()
       await flushPromises()
+      expect(w.find('[data-testid="history-preview-banner"]').exists()).toBe(false)
 
       await w.find('[data-testid="history-item-0"] .edit-history__row').trigger('click')
-      await vi.waitFor(() => expect(reloadHistory).toHaveBeenCalledWith(0))
+      await vi.waitFor(() =>
+        expect(w.find('[data-testid="history-preview-banner"]').exists()).toBe(true)
+      )
+      expect(previewHistory).toHaveBeenCalledWith(0)
+      expect(w.findAll('[data-testid^="history-item-"]').length).toBe(3)
+      expect(w.find('[data-testid="history-preview-banner"]').text()).toContain(
+        'step 1 of 3'
+      )
+
+      await vi.waitFor(() => expect(isUpdating.value).toBe(false))
+      await w.find('[data-testid="history-back-to-latest-btn"]').trigger('click')
+      await vi.waitFor(() => expect(previewHistory).toHaveBeenLastCalledWith(2))
+      await vi.waitFor(() =>
+        expect(w.find('[data-testid="history-preview-banner"]').exists()).toBe(false)
+      )
+    })
 
-      // Data stepped back, but the record of what the session did survives.
-      // Polled: the restore runs after an await inside the handler's timeout.
+    it('names the starting state when previewing it', async () => {
+      const history = threeSteps()
+      editHistory.value = history
+      selectedSeries.value = {
+        data: { history, redoStack: [], previewHistory: previewing() },
+      }
+      const w = createWrapper()
+      await flushPromises()
+
+      await w.find('[data-testid="history-reload-step-baseline"]').trigger('click')
       await vi.waitFor(() =>
-        expect(history.map((h) => h.method)).toEqual([
-          'SELECTION',
-          'DELETE_POINTS',
-          'INTERPOLATE',
-        ])
+        expect(w.find('[data-testid="history-preview-banner"]').text()).toContain(
+          'the starting state'
+        )
       )
     })
   })
@@ -580,9 +621,12 @@ describe('EditHistory.vue actions', () => {
         { method: 'DELETE_POINTS', args: [], execution: {} },
       ]
       editHistory.value = history
-      const reloadHistory = vi.fn(async () => [])
-      const undo = vi.fn(async () => [])
-      selectedSeries.value = { data: { history, redoStack: [], reloadHistory, undo } }
+      const previewHistory = previewing()
+      const undo = vi.fn(async function (this: any) {
+        this.previewIndex = null
+        return []
+      })
+      selectedSeries.value = { data: { history, redoStack: [], previewHistory, undo } }
 
       const w = createWrapper()
       await flushPromises()
@@ -606,8 +650,8 @@ describe('EditHistory.vue actions', () => {
         expect(w.find('[data-testid="history-loaded-0"]').exists()).toBe(false)
       )
       expect(w.find('[data-testid="history-loaded-1"]').exists()).toBe(true)
-      // The marker clears before undo's deferred replay runs. Drain it here or
-      // it fires in the next test against a series with no `undo`.
+      // Drain undo's deferred replay here or it fires in the next test
+      // against a series with no `undo`.
       await vi.waitFor(() => expect(isUpdating.value).toBe(false))
       expect(undo).toHaveBeenCalled()
     })
@@ -620,8 +664,8 @@ describe('EditHistory.vue actions', () => {
         makeEntry('DELETE_POINTS', [], { durationMs: 20 }),
       ]
       editHistory.value = history
-      const reloadHistory = vi.fn(async () => [])
-      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const previewHistory = previewing()
+      selectedSeries.value = { data: { history, redoStack: [], previewHistory } }
 
       const w = createWrapper()
       await flushPromises()
@@ -631,7 +675,7 @@ describe('EditHistory.vue actions', () => {
       await vi.waitFor(() => expect(isUpdating.value).toBe(false))
       await flushPromises()
 
-      expect(reloadHistory).toHaveBeenCalledWith(-1)
+      expect(previewHistory).toHaveBeenCalledWith(-1)
       // Nothing is applied at the baseline, so no row reports a run.
       expect(w.find('[data-testid="history-duration-0"]').exists()).toBe(false)
       expect(w.find('[data-testid="history-duration-1"]').exists()).toBe(false)
@@ -640,8 +684,8 @@ describe('EditHistory.vue actions', () => {
     it('marks the Data loaded row as the one being shown', async () => {
       const history = [makeEntry('SELECTION', [], { durationMs: 10 })]
       editHistory.value = history
-      const reloadHistory = vi.fn(async () => [])
-      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const previewHistory = previewing()
+      selectedSeries.value = { data: { history, redoStack: [], previewHistory } }
 
       const w = createWrapper()
       await flushPromises()
@@ -658,8 +702,8 @@ describe('EditHistory.vue actions', () => {
 
     it('makes no row clickable when there is nothing to step back from', async () => {
       editHistory.value = []
-      const reloadHistory = vi.fn(async () => [])
-      selectedSeries.value = makeSeries({ reloadHistory })
+      const previewHistory = previewing()
+      selectedSeries.value = makeSeries({ previewHistory })
 
       const w = createWrapper()
       await flushPromises()
@@ -670,7 +714,7 @@ describe('EditHistory.vue actions', () => {
         w.find('[data-testid="history-step-btn-baseline"]').attributes('disabled')
       ).toBeDefined()
       await baseline.trigger('click')
-      expect(reloadHistory).not.toHaveBeenCalled()
+      expect(previewHistory).not.toHaveBeenCalled()
     })
 
     // Reloading from a step un-applies everything below it. A committed
@@ -683,8 +727,8 @@ describe('EditHistory.vue actions', () => {
         makeEntry('INTERPOLATE', [], { durationMs: 30 }),
       ]
       editHistory.value = history
-      const reloadHistory = vi.fn(async () => [])
-      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const previewHistory = previewing()
+      selectedSeries.value = { data: { history, redoStack: [], previewHistory } }
 
       const w = createWrapper()
       await flushPromises()
@@ -787,8 +831,8 @@ describe('EditHistory.vue actions', () => {
         { method: 'INTERPOLATE', args: [], execution: {} },
       ]
       editHistory.value = history
-      const reloadHistory = vi.fn(async () => [])
-      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const previewHistory = previewing()
+      selectedSeries.value = { data: { history, redoStack: [], previewHistory } }
       const w = createWrapper()
       await flushPromises()
 
@@ -828,16 +872,16 @@ describe('EditHistory.vue actions', () => {
     it('replays to its step once', async () => {
       const history = [makeEntry('SELECTION'), makeEntry('DELETE_POINTS')]
       editHistory.value = history
-      const reloadHistory = vi.fn(async () => [])
-      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const previewHistory = previewing()
+      selectedSeries.value = { data: { history, redoStack: [], previewHistory } }
       const w = createWrapper()
       await flushPromises()
 
       await w.find('[data-testid="history-step-btn-0"]').trigger('click')
       await vi.waitFor(() => expect(isUpdating.value).toBe(false))
       await flushPromises()
-      expect(reloadHistory).toHaveBeenCalledTimes(1)
-      expect(reloadHistory).toHaveBeenCalledWith(0)
+      expect(previewHistory).toHaveBeenCalledTimes(1)
+      expect(previewHistory).toHaveBeenCalledWith(0)
     })
 
     it('is disabled while a dispatch is running', async () => {
@@ -859,8 +903,8 @@ describe('EditHistory.vue actions', () => {
     it('expanding the args drawer does not step back', async () => {
       const history = [makeEntry('SELECTION'), makeEntry('DELETE_POINTS')]
       editHistory.value = history
-      const reloadHistory = vi.fn(async () => [])
-      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const previewHistory = previewing()
+      selectedSeries.value = { data: { history, redoStack: [], previewHistory } }
       const w = createWrapper()
       await flushPromises()
 
@@ -868,30 +912,30 @@ describe('EditHistory.vue actions', () => {
         .find('[data-testid="history-item-0"] .edit-history__expand')
         .trigger('click')
       expect(w.text()).toContain('Arguments')
-      expect(reloadHistory).not.toHaveBeenCalled()
+      expect(previewHistory).not.toHaveBeenCalled()
     })
 
     it('plotting a comparison line does not step back', async () => {
       const history = [makeEntry('SELECTION'), makeEntry('DELETE_POINTS')]
       editHistory.value = history
-      const reloadHistory = vi.fn(async () => [])
-      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const previewHistory = previewing()
+      selectedSeries.value = { data: { history, redoStack: [], previewHistory } }
       const w = createWrapper()
       await flushPromises()
 
       await w.find('[data-testid="history-snapshot-0"]').trigger('click')
-      expect(reloadHistory).not.toHaveBeenCalled()
+      expect(previewHistory).not.toHaveBeenCalled()
     })
 
     it('reload-from-server does not step back', async () => {
       const history = [makeEntry('SELECTION')]
       editHistory.value = history
-      const reloadHistory = vi.fn(async () => [])
+      const previewHistory = previewing()
       selectedSeries.value = {
         data: {
           history,
           redoStack: [],
-          reloadHistory,
+          previewHistory,
           reload: vi.fn().mockResolvedValue(undefined),
         },
       }
@@ -904,7 +948,7 @@ describe('EditHistory.vue actions', () => {
       await w.find('[data-testid="history-reload-btn"]').trigger('click')
       await vi.runAllTimersAsync()
       vi.useRealTimers()
-      expect(reloadHistory).not.toHaveBeenCalled()
+      expect(previewHistory).not.toHaveBeenCalled()
     })
   })
 
@@ -916,9 +960,9 @@ describe('EditHistory.vue actions', () => {
         { method: 'INTERPOLATE', args: [], execution: {} },
       ]
       editHistory.value = history
-      const reloadHistory = vi.fn(async () => [])
+      const previewHistory = previewing()
       selectedSeries.value = {
-        data: { history, redoStack: [], reloadHistory },
+        data: { history, redoStack: [], previewHistory },
       }
       return history
     }
@@ -996,13 +1040,13 @@ describe('EditHistory.vue actions', () => {
         { method: 'DELETE_POINTS', args: [], execution: {} },
       ]
       editHistory.value = history
-      const reloadHistory = vi.fn(async () => returned as any)
-      selectedSeries.value = { data: { history, redoStack: [], reloadHistory } }
+      const previewHistory = vi.fn(async () => returned as any)
+      selectedSeries.value = { data: { history, redoStack: [], previewHistory } }
       const w = createWrapper()
       await flushPromises()
       // Item 0 isn't the trailing entry, so its buttons are expand + reload.
       await w.find('[data-testid="history-item-0"] .edit-history__row').trigger('click')
-      await vi.waitFor(() => expect(reloadHistory).toHaveBeenCalledWith(0))
+      await vi.waitFor(() => expect(previewHistory).toHaveBeenCalledWith(0))
       await vi.waitFor(() => expect(isUpdating.value).toBe(false))
       await flushPromises()
     }
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditTargetPanel.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditTargetPanel.spec.ts
index 8512d8f1..fc754d52 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditTargetPanel.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditTargetPanel.spec.ts
@@ -79,7 +79,7 @@ describe('EditTargetPanel', () => {
 
   it('goes back to the editor without touching the session', async () => {
     const wrapper = mountPanel()
-    await wrapper.find('[data-testid="back-to-editor-btn"]').trigger('click')
+    await wrapper.find('[data-testid="open-editor-btn"]').trigger('click')
     expect(openEditor).toHaveBeenCalledTimes(1)
   })
 
diff --git a/apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts
index 4764e454..71938c7a 100644
--- a/apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts
@@ -83,7 +83,12 @@ describe('LeaveSessionDialog', () => {
   })
 
   it('offers save, discard and cancel for unsaved edits', async () => {
-    leavePrompt.value = { kind: 'unsaved', unsavedCount: 2, canSave: true }
+    leavePrompt.value = {
+      datastreamName: 'Temp (QC)',
+      kind: 'unsaved',
+      unsavedCount: 2,
+      canSave: true,
+    }
     const wrapper = mountDialog()
     await wrapper.vm.$nextTick()
 
@@ -103,7 +108,12 @@ describe('LeaveSessionDialog', () => {
   })
 
   it('disables saving when no session is open', async () => {
-    leavePrompt.value = { kind: 'unsaved', unsavedCount: 1, canSave: false }
+    leavePrompt.value = {
+      datastreamName: 'Temp (QC)',
+      kind: 'unsaved',
+      unsavedCount: 1,
+      canSave: false,
+    }
     const wrapper = mountDialog()
     await wrapper.vm.$nextTick()
 
@@ -112,11 +122,16 @@ describe('LeaveSessionDialog', () => {
   })
 
   it('offers keep or discard for a session with no edits', async () => {
-    leavePrompt.value = { kind: 'empty', unsavedCount: 0, canSave: true }
+    leavePrompt.value = {
+      datastreamName: 'Temp (QC)',
+      kind: 'empty',
+      unsavedCount: 0,
+      canSave: true,
+    }
     const wrapper = mountDialog()
     await wrapper.vm.$nextTick()
 
-    expect(dialogText()).toContain('no edits')
+    expect(dialogText()).toContain('Your session on Temp (QC) has no edits')
     expect(button('leave-save-btn')).toBeNull()
 
     button('leave-keep-btn')?.click()
@@ -126,7 +141,12 @@ describe('LeaveSessionDialog', () => {
   })
 
   it('says the session stays in progress when everything is saved', async () => {
-    leavePrompt.value = { kind: 'saved', unsavedCount: 0, canSave: true }
+    leavePrompt.value = {
+      datastreamName: 'Temp (QC)',
+      kind: 'saved',
+      unsavedCount: 0,
+      canSave: true,
+    }
     const wrapper = mountDialog()
     await wrapper.vm.$nextTick()
 
@@ -138,7 +158,12 @@ describe('LeaveSessionDialog', () => {
   })
 
   it('locks the choices while one is running', async () => {
-    leavePrompt.value = { kind: 'unsaved', unsavedCount: 1, canSave: true }
+    leavePrompt.value = {
+      datastreamName: 'Temp (QC)',
+      kind: 'unsaved',
+      unsavedCount: 1,
+      canSave: true,
+    }
     leaveWork.value = 'save'
     const wrapper = mountDialog()
     await wrapper.vm.$nextTick()
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
index d5cab844..3f3710ff 100644
--- a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
@@ -197,8 +197,10 @@ const present = (testId: string) =>
   !!document.querySelector(`[data-testid="${testId}"]`)
 
 // Dialog order: chooser, create form, session window.
-const dialogOpen = (wrapper: ReturnType<typeof mountFlow>['wrapper'], i: number) =>
-  wrapper.findAllComponents(VDialog)[i].props('modelValue')
+const dialogOpen = (
+  wrapper: ReturnType<typeof mountFlow>['wrapper'],
+  i: number
+) => wrapper.findAllComponents(VDialog)[i].props('modelValue')
 
 enableAutoUnmount(afterEach)
 afterEach(() => {
@@ -247,7 +249,8 @@ describe('StartEditingFlow', () => {
     wrapper.findComponent(ChooserStub).vm.$emit('edit', option)
     await flushPromises()
     expect(enterEdit).toHaveBeenCalledTimes(1)
-    expect(enterEdit.mock.calls[0]).toEqual(['mgd', undefined, undefined])
+    // Picked from a row, so it opens in the Select view's preview.
+    expect(enterEdit.mock.calls[0]).toEqual(['mgd', undefined, 'Select'])
     expect(present('window-stub')).toBe(false)
   })
 
@@ -264,7 +267,7 @@ describe('StartEditingFlow', () => {
 
     wrapper.findComponent(WindowStub).vm.$emit('confirm', window)
     await flushPromises()
-    expect(enterEdit).toHaveBeenCalledWith('mgd', window, undefined)
+    expect(enterEdit).toHaveBeenCalledWith('mgd', window, 'Select')
     expect(present('window-stub')).toBe(false)
   })
 
@@ -465,6 +468,47 @@ describe('StartEditingFlow', () => {
     expect(dialogOpen(wrapper, 0)).toBe(true)
   })
 
+  it('asks about the open session when another datastream is picked, before the window step', async () => {
+    const option = optionWith([committed])
+    loadForSource.mockResolvedValue([option])
+    qcDatastream.value = { id: 'other', name: 'Other (QC)' }
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    wrapper.findComponent(ChooserStub).vm.$emit('edit', option)
+    await flushPromises()
+    expect(closeEditor).toHaveBeenCalled()
+    expect(present('window-stub')).toBe(true)
+  })
+
+  it('returns to the chooser when a pick keeps the open session', async () => {
+    const option = optionWith([committed])
+    loadForSource.mockResolvedValue([option])
+    closeEditor.mockResolvedValue(false)
+    qcDatastream.value = { id: 'other', name: 'Other (QC)' }
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    wrapper.findComponent(ChooserStub).vm.$emit('edit', option)
+    await flushPromises()
+    expect(present('window-stub')).toBe(false)
+    expect(enterEdit).not.toHaveBeenCalled()
+    expect(dialogOpen(wrapper, 0)).toBe(true)
+  })
+
+  it('does not ask when the open datastream is picked again', async () => {
+    const option = optionWith([inProgress])
+    loadForSource.mockResolvedValue([option])
+    qcDatastream.value = { id: option.managed.id, name: option.managed.name }
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    wrapper.findComponent(ChooserStub).vm.$emit('edit', option)
+    await flushPromises()
+    expect(closeEditor).not.toHaveBeenCalled()
+    expect(enterEdit).toHaveBeenCalled()
+  })
+
   it('does not ask when nothing is being edited', async () => {
     loadForSource.mockResolvedValue([])
     const { flow } = mountFlow()
@@ -484,14 +528,12 @@ describe('StartEditingFlow', () => {
     const { wrapper, flow } = mountFlow()
     await flow.openFor(source)
     await flushPromises()
-    wrapper
-      .findComponent(CreateStub)
-      .vm.$emit('confirm', {
-        source,
-        processingLevelId: 'pl',
-        description: 'Cleaned series',
-        sensorId: 'sn-1',
-      })
+    wrapper.findComponent(CreateStub).vm.$emit('confirm', {
+      source,
+      processingLevelId: 'pl',
+      description: 'Cleaned series',
+      sensorId: 'sn-1',
+    })
     await flushPromises()
 
     expect(addQcHistory).toHaveBeenCalledWith({ id: 'h-new' })
@@ -503,6 +545,6 @@ describe('StartEditingFlow', () => {
 
     wrapper.findComponent(WindowStub).vm.$emit('confirm', window)
     await flushPromises()
-    expect(enterEdit).toHaveBeenCalledWith('mgd-new', window, undefined)
+    expect(enterEdit).toHaveBeenCalledWith('mgd-new', window, 'Select')
   })
 })
diff --git a/apps/qc-app/src/components/Navigation/SelectDrawer.vue b/apps/qc-app/src/components/Navigation/SelectDrawer.vue
index 3c1ebc17..fbef8c11 100644
--- a/apps/qc-app/src/components/Navigation/SelectDrawer.vue
+++ b/apps/qc-app/src/components/Navigation/SelectDrawer.vue
@@ -35,6 +35,8 @@
 
       <v-divider />
 
+      <!-- While editing, the Context menu above the plot sets the range. -->
+      <template v-if="!qcDatastream">
       <div
         class="select-drawer__section-header d-flex align-center ga-1 px-3 py-1 cursor-pointer"
         role="button"
@@ -55,6 +57,7 @@
       </div>
 
       <v-divider />
+      </template>
 
       <div
         class="select-drawer__section-header d-flex align-center ga-1 px-3 py-1 cursor-pointer"
@@ -93,6 +96,10 @@
 import DataVisTimeFilters from '@/components/VisualizeData/DataVisTimeFilters.vue'
 import DatastreamFilters from '@/components/VisualizeData/DatastreamFilters.vue'
 import { useResizable, usePersistedFlag } from '@/composables/useResizable'
+import { storeToRefs } from 'pinia'
+import { useDataVisStore } from '@/store/dataVisualization'
+
+const { qcDatastream } = storeToRefs(useDataVisStore())
 
 const { size: drawerWidth, onStart: startDrag, dragging } = useResizable({
   initial: 320,
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index ac6ff0a9..cb1c8dc8 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -47,12 +47,9 @@
             class="select-view__side d-flex flex-column flex-grow-0 flex-shrink-0 overflow-hidden"
           >
             <EditTargetPanel v-if="qcDatastream" />
-            <v-divider v-if="qcDatastream && plottedDatastreams.length" />
-            <div
-              v-if="plottedDatastreams.length"
-              class="select-view__plotted-body flex-grow-1 overflow-y-auto"
-            >
-              <PlottedDatastreams />
+            <v-divider v-if="qcDatastream" />
+            <div class="select-view__plotted-body flex-grow-1 overflow-y-auto">
+              <PlottedDatastreams clearable />
             </div>
           </div>
         </template>
@@ -488,6 +485,8 @@ const {
   beginDate,
   endDate,
   selectedDateBtnId,
+  contextPresetId,
+  activePresetId,
   selectedThings,
   selectedObservedPropertyNames,
   selectedProcessingLevelNames,
@@ -789,10 +788,12 @@ const hydrateFromUrl = () => {
   // A preset resolves against the plotted data during that load. An
   // unknown id (a share link built by a newer version, or hand-edited)
   // is not accepted as a preset.
+  // A link with an edit target carries the editor's Context range.
+  const presetId = state.editDatastreamId ? contextPresetId : selectedDateBtnId
   if (state.datePresetId != null && findPreset(state.datePresetId)) {
-    selectedDateBtnId.value = state.datePresetId
+    presetId.value = state.datePresetId
   } else if (state.beginMs != null || state.endMs != null) {
-    selectedDateBtnId.value = CUSTOM_PRESET_ID
+    presetId.value = CUSTOM_PRESET_ID
     if (state.beginMs != null) beginDate.value = new Date(state.beginMs)
     if (state.endMs != null) endDate.value = new Date(state.endMs)
   }
@@ -908,7 +909,7 @@ watch(
     activeTab,
     beginDate,
     endDate,
-    selectedDateBtnId,
+    activePresetId,
     selectedThings,
     selectedObservedPropertyNames,
     selectedProcessingLevelNames,
@@ -939,8 +940,8 @@ watch(
       tableTab: activeTab.value === 'table',
       datastreamIds: ids,
       snapshots,
-      datePresetId: Number.isFinite(selectedDateBtnId.value)
-        ? selectedDateBtnId.value
+      datePresetId: Number.isFinite(activePresetId.value)
+        ? activePresetId.value
         : null,
       beginMs: beginDate.value ? beginDate.value.getTime() : null,
       endMs: endDate.value ? endDate.value.getTime() : null,
diff --git a/apps/qc-app/src/components/VisualizeData/ContextPlot.vue b/apps/qc-app/src/components/VisualizeData/ContextPlot.vue
index d95b385a..8af5ba96 100644
--- a/apps/qc-app/src/components/VisualizeData/ContextPlot.vue
+++ b/apps/qc-app/src/components/VisualizeData/ContextPlot.vue
@@ -40,7 +40,13 @@ import Plotly from 'plotly.js-dist'
 import type { Layout, LayoutAxis, PlotlyHTMLElement } from 'plotly.js-dist'
 import { usePlotlyStore } from '@/store/plotly'
 import { useDataVisStore } from '@/store/dataVisualization'
-import { COLORS, SOURCE_CONTEXT_COLOR, type AppPlotlyTrace } from '@/utils/plotting/plotly'
+import {
+  COLORS,
+  SOURCE_CONTEXT_COLOR,
+  splitAroundWindow,
+  toEditWindow,
+  type AppPlotlyTrace,
+} from '@/utils/plotting/plotly'
 import type { GraphSeries } from '@/types'
 
 const TARGET_POINTS = 2000
@@ -55,7 +61,8 @@ type PlotlyEventEmitter = {
 
 const { plotlyRef, graphSeriesArray, mainPlotEpoch } =
   storeToRefs(usePlotlyStore())
-const { qcDatastream, sourceContextDatastream } = storeToRefs(useDataVisStore())
+const { qcDatastream, sourceContextDatastream, editSessionWindow } =
+  storeToRefs(useDataVisStore())
 
 const rootEl = ref<HTMLDivElement>()
 const plotEl = ref<HTMLDivElement>()
@@ -120,6 +127,7 @@ function buildContextTraces(series: GraphSeries[]): {
 } {
   const qcId = qcDatastream.value?.id
   const sourceId = sourceContextDatastream.value?.id
+  const editWindow = toEditWindow(editSessionWindow.value)
   const traces: AppPlotlyTrace[] = []
   let xMin = Infinity
   let xMax = -Infinity
@@ -135,16 +143,28 @@ function buildContextTraces(series: GraphSeries[]): {
     const yNorm = normalize(sampled.y)
     const isQc = !!qcId && s.id === qcId
     const isSource = !!sourceId && s.id === sourceId
-    const color = isQc ? COLORS[0] : isSource ? SOURCE_CONTEXT_COLOR : (s.color ?? COLORS[1])
-    traces.push({
-      x: sampled.x,
-      y: yNorm,
-      type: 'scattergl',
-      mode: 'lines',
-      line: { color, width: 1 },
-      hoverinfo: 'skip',
-      showlegend: false,
-    })
+    const pushLine = (
+      line: { x: ArrayLike<number>; y: ArrayLike<number> },
+      color: string | undefined
+    ) =>
+      traces.push({
+        x: line.x as number[],
+        y: line.y as number[],
+        type: 'scattergl',
+        mode: 'lines',
+        line: { color, width: 1 },
+        hoverinfo: 'skip',
+        showlegend: false,
+      })
+    const line = { x: sampled.x, y: yNorm }
+    if (isSource) {
+      // Same gap over the session window as the main plot.
+      const { before, after } = splitAroundWindow(line, editWindow)
+      pushLine(before, SOURCE_CONTEXT_COLOR)
+      pushLine(after, SOURCE_CONTEXT_COLOR)
+    } else {
+      pushLine(line, isQc ? COLORS[0] : (s.color ?? COLORS[1]))
+    }
   }
 
   // Mirror main plot draw order: seriesArray[0] paints on top.
@@ -415,10 +435,12 @@ async function buildOrUpdate() {
   syncBrushFromMain()
 }
 
-// Refresh only on series identity / length / color / QC target shifts so
-// per-cell y-edits stay cheap; overall shape barely changes anyway.
+// Refresh only on series identity / length / color / QC target / session
+// window shifts so per-cell y-edits stay cheap; overall shape barely
+// changes anyway.
 const rebuildSignature = computed(() => {
-  const qid = qcDatastream.value?.id ?? ''
+  const w = toEditWindow(editSessionWindow.value)
+  const qid = (qcDatastream.value?.id ?? '') + (w ? `@${w.begin}-${w.end}` : '')
   const sid = sourceContextDatastream.value?.id ?? ''
   const parts = graphSeriesArray.value.map(
     (s) => `${s.id}:${s.data?.dataX?.length ?? 0}:${s.color}`
diff --git a/apps/qc-app/src/components/VisualizeData/DataTable.vue b/apps/qc-app/src/components/VisualizeData/DataTable.vue
index 11e0e4ce..ed3db127 100644
--- a/apps/qc-app/src/components/VisualizeData/DataTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataTable.vue
@@ -52,7 +52,12 @@
         </v-btn>
 
         <v-btn
-          :disabled="!pendingEditCount || isUpdating"
+          :disabled="!pendingEditCount || isUpdating || previewIndex !== null"
+          :title="
+            previewIndex !== null
+              ? 'Go back to the latest history step to save'
+              : undefined
+          "
           :loading="isSaving"
           color="primary"
           variant="flat"
@@ -168,7 +173,7 @@ import { useDataSelection } from '@/composables/useDataSelection'
 import { useQualifierStore } from '@/store/qualifiers'
 import EditableCell from '@/components/VisualizeData/EditableCell.vue'
 
-const { isUpdating, selectedSeries, tableScrollRequest } = storeToRefs(
+const { isUpdating, selectedSeries, tableScrollRequest, previewIndex } = storeToRefs(
   usePlotlyStore()
 )
 const { redraw } = usePlotlyStore()
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
index 29b5fd1c..8e6f6d25 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
@@ -178,7 +178,22 @@
         </template>
 
         <template #item.edit="{ item }">
+          <!-- The row being edited ends its session here instead. -->
+          <span v-if="isEditing(item)" @click.stop>
+            <v-btn
+              prepend-icon="mdi-close"
+              size="small"
+              variant="tonal"
+              density="comfortable"
+              :data-testid="`close-datastream-${item.id}`"
+              :aria-label="`Close ${qcDatastream?.name ?? item.name}`"
+              @click.stop="closeEditor()"
+            >
+              Close
+            </v-btn>
+          </span>
           <v-tooltip
+            v-else
             location="top"
             :text="
               canEditWorkspace
@@ -208,7 +223,22 @@
         </template>
 
         <template #item.name="{ item }">
-          <span class="name-cell" :title="item.name">{{ item.name || '-' }}</span>
+          <div class="d-flex align-center ga-2" style="min-width: 0">
+            <v-chip
+              v-if="isEditing(item)"
+              data-testid="editing-chip"
+              size="x-small"
+              color="primary"
+              variant="flat"
+              label
+              prepend-icon="mdi-pencil"
+              class="flex-shrink-0"
+              :title="`Editing ${qcDatastream?.name ?? item.name}`"
+            >
+              Editing
+            </v-chip>
+            <span class="name-cell" :title="item.name">{{ item.name || '-' }}</span>
+          </div>
         </template>
 
         <template #item.siteCodeName="{ item }">
@@ -272,6 +302,7 @@
         :plotted-ids="plotDialogSelected"
         :loading="plotDialogLoading"
         :slots-left="plotDialogSlots"
+        :editing-id="qcDatastream?.id"
         @apply="onPlotApply"
         @cancel="plotDialogSource = null"
       />
@@ -294,6 +325,7 @@ import {
 } from '@/composables/useManagedDatastreams'
 import { Snackbar } from '@uwrl/qc-utils'
 import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
+import { useEditEntry } from '@/composables/useEditEntry'
 
 /** Datastreams the user can check. The fifth plot slot is kept for the
  *  datastream being edited. */
@@ -307,8 +339,13 @@ const { canEdit, roleName } = useWorkspacePermissions()
 const canEditWorkspace = computed(() => canEdit())
 const workspaceRole = computed(() => roleName())
 
-const { filteredDatastreams, plottedDatastreams, historiesBySource } =
-  storeToRefs(useDataVisStore())
+const {
+  filteredDatastreams,
+  plottedDatastreams,
+  historiesBySource,
+  qcDatastream,
+  editSourceDatastream,
+} = storeToRefs(useDataVisStore())
 const {
   toggleDatastream,
   clearPlottedDatastreams,
@@ -316,6 +353,7 @@ const {
   plotSourceSelection,
 } = useDataVisStore()
 const { loadForSource } = useManagedDatastreams()
+const { closeEditor } = useEditEntry()
 
 const showOnlySelected = ref(false)
 const openInfoCard = ref(false)
@@ -421,6 +459,12 @@ const isPartial = (item: Datastream) =>
 const managedCount = (item: Datastream) =>
   historiesBySource.value.get(item.id)?.length ?? 0
 
+// Rows are sources, so the edit target shows on the row it derives from.
+const isEditing = (item: Datastream) =>
+  !!qcDatastream.value &&
+  (item.id === editSourceDatastream.value?.id ||
+    item.id === qcDatastream.value.id)
+
 const isAtCap = (item: Datastream) =>
   plottedDatastreams.value.length >= PLOT_CAP && !isChecked(item)
 
@@ -484,6 +528,7 @@ const getRowProps = ({ item }: { item: Datastream }) => ({
   class: {
     'datasets-table__row--at-cap': isAtCap(item),
     'datasets-table__row--plotted': isChecked(item),
+    'datasets-table__row--editing': isEditing(item),
   },
 })
 
@@ -574,6 +619,10 @@ const resetSort = () => {
   box-shadow: inset 3px 0 0 rgba(var(--v-theme-primary), 0.45);
 }
 
+:deep(tbody tr.datasets-table__row--editing > td:first-child) {
+  box-shadow: inset 4px 0 0 rgb(var(--v-theme-primary));
+}
+
 :deep(tbody tr:hover > td) {
   background-color: rgba(var(--v-theme-primary), 0.08) !important;
   cursor: pointer;
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue b/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
index 45cc3f2d..7158f746 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisTimeFilters.vue
@@ -46,7 +46,7 @@
         {{ option.label }}
       </v-chip>
       <v-chip
-        v-if="selectedDateBtnId === CUSTOM_PRESET_ID"
+        v-if="activePresetId === CUSTOM_PRESET_ID"
         data-testid="date-preset-custom"
         color="secondary"
         variant="tonal"
@@ -82,10 +82,10 @@ const props = withDefaults(
 )
 
 const { setDateRange, onDateBtnClick } = useDataVisStore()
-const { beginDate, endDate, selectedDateBtnId } = storeToRefs(useDataVisStore())
+const { beginDate, endDate, activePresetId } = storeToRefs(useDataVisStore())
 
 const shownId = computed(() =>
-  shownPresetId(selectedDateBtnId.value, props.presets)
+  shownPresetId(activePresetId.value, props.presets)
 )
 </script>
 
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index d600ce2e..5eddd389 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -30,20 +30,30 @@
           class="ml-1"
         />
 
-        <v-chip
+        <div
           v-if="(selectedData?.length || hasSelectionShape) && !isPlotPreview"
-          class="plot-toolbar__selection flex-grow-0 flex-shrink-0"
-          size="small"
-          color="red"
-          variant="tonal"
-          prepend-icon="mdi-selection-drag"
-          closable
-          close-icon="mdi-close"
-          @click:close="clearSelected()"
+          class="plot-toolbar__selection d-inline-flex align-stretch rounded-lg ml-1"
         >
-          <b class="mr-1">{{ selectedData?.length ?? 0 }}</b>
-          point{{ (selectedData?.length ?? 0) === 1 ? '' : 's' }} selected
-        </v-chip>
+          <div
+            class="plot-toolbar__selection-count d-inline-flex align-center ga-1"
+            aria-live="polite"
+          >
+            <v-icon icon="mdi-selection-drag" size="16" />
+            <b>{{ selectedCount.toLocaleString() }}</b>
+            <span>point{{ selectedCount === 1 ? '' : 's' }} selected</span>
+          </div>
+          <span class="plot-toolbar__selection-divider" aria-hidden="true" />
+          <button
+            type="button"
+            data-testid="clear-selection-btn"
+            class="plot-toolbar__selection-clear d-inline-flex align-center ga-1 cursor-pointer"
+            title="Clear selection"
+            @click="clearSelected()"
+          >
+            <v-icon icon="mdi-close" size="14" />
+            Clear
+          </button>
+        </div>
 
         <v-spacer />
 
@@ -276,15 +286,43 @@
             <v-btn
               v-bind="menuProps"
               size="small"
-              variant="text"
-              prepend-icon="mdi-calendar-range"
+              :variant="showSourceContext ? 'tonal' : 'text'"
+              :color="showSourceContext ? 'primary' : undefined"
+              :prepend-icon="
+                showSourceContext ? 'mdi-calendar-range' : 'mdi-eye-off-outline'
+              "
               data-testid="context-range-btn"
-              title="Context range"
+              :data-context-on="showSourceContext"
+              :title="
+                showSourceContext
+                  ? `Context range: ${contextRangeLabel} around the session`
+                  : `Context range: ${contextRangeLabel} around the session. The grey source context is hidden.`
+              "
             >
-              Context
+              Context &middot; {{ contextRangeLabel }}
             </v-btn>
           </template>
           <v-card width="300" class="pa-3" data-testid="context-range-menu">
+            <v-switch
+              data-testid="context-toggle"
+              :model-value="showSourceContext"
+              color="primary"
+              density="compact"
+              hide-details
+              inset
+              class="mb-2"
+              @update:model-value="setShowSourceContext(!!$event)"
+            >
+              <template #label>
+                <span class="d-inline-flex align-center ga-2">
+                  <span
+                    class="context-swatch"
+                    :style="{ backgroundColor: SOURCE_CONTEXT_COLOR }"
+                  />
+                  Show source context
+                </span>
+              </template>
+            </v-switch>
             <DataVisTimeFilters
               :presets="EDITOR_PRESETS"
               description="How much of the source and plotted datastreams to show before and after the session window. Your edits are not reloaded."
@@ -412,7 +450,12 @@ import {
 import DataTable from '@/components/VisualizeData/DataTable.vue'
 import ContextPlot from '@/components/VisualizeData/ContextPlot.vue'
 import DataVisTimeFilters from '@/components/VisualizeData/DataVisTimeFilters.vue'
-import { EDITOR_PRESETS } from '@/utils/timeRangePresets'
+import {
+  CUSTOM_PRESET_ID,
+  EDITOR_PRESETS,
+  findPreset,
+} from '@/utils/timeRangePresets'
+import { SOURCE_CONTEXT_COLOR } from '@/utils/plotting/plotly'
 import { useDataSelection } from '@/composables/useDataSelection'
 import { useBufferedNumber } from '@/composables/useBufferedNumber'
 import { usePersistedFlag } from '@/composables/useResizable'
@@ -440,14 +483,27 @@ const {
   pendingShareZoom,
   shareZoomEditTarget,
 } = storeToRefs(usePlotlyStore())
-const { selectedData, hasSelectionShape, qcDatastream } =
-  storeToRefs(useDataVisStore())
-const { trackPlotWork } = useDataVisStore()
+const {
+  selectedData,
+  hasSelectionShape,
+  qcDatastream,
+  contextPresetId,
+  showSourceContext,
+} = storeToRefs(useDataVisStore())
+const { trackPlotWork, setShowSourceContext } = useDataVisStore()
+
+const contextRangeLabel = computed(() =>
+  contextPresetId.value === CUSTOM_PRESET_ID
+    ? 'Custom'
+    : (findPreset(contextPresetId.value)?.label ?? 'Custom')
+)
 const { viewedSession, inProgressSession } = storeToRefs(useQcSessionStore())
 // The Select view previews the plot only when nothing is being edited; an
 // open session keeps the full chrome in both views.
 const { isPlotPreview } = storeToRefs(useUIStore())
 
+const selectedCount = computed(() => selectedData.value?.length ?? 0)
+
 const tooltipsAutoDisabled = computed(
   () =>
     tooltipsMode.value === 'auto' &&
@@ -777,6 +833,13 @@ const onTabChange = () => {
 </script>
 
 <style scoped>
+.context-swatch {
+  display: inline-block;
+  width: 14px;
+  height: 3px;
+  border-radius: 2px;
+}
+
 .plot-root {
   min-height: 0;
 }
@@ -821,8 +884,51 @@ const onTabChange = () => {
   color: rgb(var(--v-theme-on-surface));
 }
 
+/* Same chrome as the data-points combo so the toolbar reads as one set. */
 .plot-toolbar__selection {
+  height: 28px;
+  font-size: 0.8rem;
+  line-height: 1;
+  white-space: nowrap;
   font-variant-numeric: tabular-nums;
+  color: rgb(var(--v-theme-error));
+  background-color: rgba(var(--v-theme-error), 0.08);
+  border: 1px solid rgba(var(--v-theme-error), 0.24);
+}
+
+.plot-toolbar__selection-count {
+  padding: 0 10px 0 8px;
+}
+
+.plot-toolbar__selection-count b {
+  font-weight: 600;
+}
+
+.plot-toolbar__selection-divider {
+  width: 1px;
+  align-self: stretch;
+  background-color: rgba(var(--v-theme-error), 0.24);
+}
+
+.plot-toolbar__selection-clear {
+  padding: 0 10px 0 8px;
+  background: transparent;
+  border: none;
+  color: inherit;
+  font: inherit;
+  font-weight: 500;
+  border-top-right-radius: 7px;
+  border-bottom-right-radius: 7px;
+  transition: background-color 120ms ease;
+}
+
+.plot-toolbar__selection-clear:hover {
+  background-color: rgba(var(--v-theme-error), 0.14);
+}
+
+.plot-toolbar__selection-clear:focus-visible {
+  outline: 2px solid rgb(var(--v-theme-error));
+  outline-offset: -2px;
 }
 
 /* Axis-title chips. CSS vars set per-element on the chip. Default
diff --git a/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue b/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
index 23df6e7e..c08a2e3a 100644
--- a/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
+++ b/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
@@ -43,8 +43,19 @@
                   size="18"
                 />
                 <div class="d-flex flex-column" style="min-width: 0">
-                  <span class="text-body-medium font-weight-medium">
+                  <span class="text-body-medium font-weight-medium d-flex align-center ga-1">
                     {{ row.name }}
+                    <v-chip
+                      v-if="isEditing(row.id)"
+                      size="x-small"
+                      color="primary"
+                      variant="flat"
+                      label
+                      prepend-icon="mdi-pencil"
+                      :data-testid="`plot-option-editing-${row.id}`"
+                    >
+                      Editing
+                    </v-chip>
                   </span>
                   <span class="text-body-small text-medium-emphasis">
                     {{ row.summary }}
@@ -97,6 +108,8 @@ const props = defineProps<{
   loading?: boolean
   /** Free slots under the 4-datastream plot cap, ignoring this source's group. */
   slotsLeft: number
+  /** The datastream being edited: already on the plot, so not offered. */
+  editingId?: string | null
 }>()
 
 const emit = defineEmits<{
@@ -141,8 +154,10 @@ const rows = computed<Row[]>(() => [
 
 const slotsLeftForNew = computed(() => props.slotsLeft - checked.value.length)
 
+const isEditing = (id: string) => !!props.editingId && id === props.editingId
+
 const isDisabled = (id: string) =>
-  !checked.value.includes(id) && slotsLeftForNew.value <= 0
+  isEditing(id) || (!checked.value.includes(id) && slotsLeftForNew.value <= 0)
 
 function toggle(id: string) {
   if (checked.value.includes(id)) {
diff --git a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
index 8f2ac3a7..024bd0eb 100644
--- a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
+++ b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
@@ -1,29 +1,34 @@
 <template>
   <div class="plotted-wrapper d-flex flex-column">
     <div
-      v-if="!qcDatastream"
+      v-if="clearable"
       class="plotted-toolbar d-flex align-center px-3 py-2"
     >
       <v-spacer></v-spacer>
       <v-btn
-        :disabled="!plottedDatastreams.length"
+        data-testid="clear-plot-btn"
+        :disabled="!plottedDatastreams.length && !qcDatastream"
         size="x-small"
         variant="text"
         prepend-icon="mdi-close-circle-outline"
-        @click="clearAll"
+        :title="
+          qcDatastream
+            ? 'Close the datastream being edited and unplot everything'
+            : 'Unplot everything'
+        "
+        @click="clearPlot"
       >
-        Unplot all
+        Clear plot
       </v-btn>
     </div>
     <v-divider />
     <ul class="plotted-list pa-0 ma-0">
       <li
-        v-for="(datastream, index) of seriesDatastreams"
+        v-for="(datastream, index) of listedDatastreams"
         :key="datastream.id"
         class="plotted-item"
         :class="{
           'plotted-item--qc': isEdit(datastream),
-          'plotted-item--source': isSource(datastream),
           'plotted-item--hidden': visibleDict[datastream.id] === false,
           'plotted-item--drop-before':
             dragIndex !== null &&
@@ -122,15 +127,6 @@
               class="flex-shrink-0"
             />
             <span>{{ datastream.name }}</span>
-            <v-chip
-              v-if="isSource(datastream)"
-              size="x-small"
-              variant="tonal"
-              label
-              class="flex-shrink-0"
-            >
-              raw source
-            </v-chip>
             <v-chip
               v-if="snapshotFor(datastream.id)"
               size="x-small"
@@ -207,6 +203,12 @@ import { usePlotlyStore } from '@/store/plotly'
 import { ref, computed } from 'vue'
 import { Datastream } from '@hydroserver/client'
 import { formatDayStamp } from '@/utils/time'
+import { useEditEntry } from '@/composables/useEditEntry'
+
+/** `clearable`: show the Clear plot toolbar (the Select view's list). */
+defineProps<{ clearable?: boolean }>()
+
+const { closeEditor } = useEditEntry()
 
 const { updateOptions, labelColorForDatastream } = usePlotlyStore()
 const {
@@ -234,6 +236,11 @@ const isPinned = (ds: Datastream) => isEdit(ds) || isSource(ds)
 const isPrimary = (ds: Datastream, index: number) =>
   isPinned(ds) || (!qcDatastream.value && index === 0)
 
+// The source is context, switched on and off from the Context menu.
+const listedDatastreams = computed(() =>
+  seriesDatastreams.value.filter((ds) => !isSource(ds))
+)
+
 const contextCount = computed(
   () => seriesDatastreams.value.filter((ds) => !isPinned(ds)).length
 )
@@ -293,7 +300,9 @@ const snapshotSubtitle = (id: string): string => {
   return parts.join(' - ')
 }
 
-async function clearAll() {
+// Ending the edit session asks first; staying keeps the plot as it is.
+async function clearPlot() {
+  if (qcDatastream.value && !(await closeEditor())) return
   hiddenTraceIds.value = new Set()
   await clearPlottedDatastreams()
 }
@@ -317,23 +326,24 @@ const toggleVisibility = async (datastream: Datastream) => {
   else next.add(datastream.id)
   hiddenTraceIds.value = next
 
-  // Gap overlays carry only `_gapOverlayFor`; toggle alongside the main
+  // Gap overlays carry only `_partOf`; toggle alongside the main
   // trace so hiding a datastream removes both its line and its markers.
   for (let i = 0; i < traces.length; i++) {
     const t = traces[i] as AppPlotlyTrace
-    if (i === mainIndex || t._gapOverlayFor === datastream.id) {
+    if (i === mainIndex || t._partOf === datastream.id) {
       await toggleTraceVisibility(plotlyRef.value, i, nextVisible)
     }
   }
 }
 
-// Drag indices are row positions in `seriesDatastreams`. Firefox needs
+// Drag indices are row positions in `listedDatastreams`. Firefox needs
 // `setData` for a drag to actually start, hence the payload.
 const dragIndex = ref<number | null>(null)
 const dropIndex = ref<number | null>(null)
 
+// Row indices count the listed rows, which leave the source out.
 const isPinnedAt = (index: number) => {
-  const ds = seriesDatastreams.value[index]
+  const ds = listedDatastreams.value[index]
   return !ds || isPinned(ds)
 }
 
@@ -374,8 +384,8 @@ function onDragEnd() {
 // then re-sorts `graphSeriesArray` so trace order (and the colours derived
 // from it) follows `seriesDatastreams`.
 function reorder(from: number, to: number): boolean {
-  const movedId = seriesDatastreams.value[from]?.id
-  const targetId = seriesDatastreams.value[to]?.id
+  const movedId = listedDatastreams.value[from]?.id
+  const targetId = listedDatastreams.value[to]?.id
   const list = plottedDatastreams.value
   const fromPos = list.findIndex((d) => d.id === movedId)
   const toPos = list.findIndex((d) => d.id === targetId)
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
index bb427b1d..f67522f7 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
@@ -33,6 +33,7 @@ vi.mock('@/store/plotly', () => ({
   usePlotlyStore: () => ({
     isUpdating,
     selectedSeries,
+    previewIndex: ref(null),
     redraw,
     tableScrollRequest,
   }),
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
index 55cfb97e..bde759d0 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
@@ -25,6 +25,8 @@ const lonely = { id: 'solo', name: 'Solo', valueCount: 5, thing: { id: 't-1' } }
 
 const filteredDatastreams = ref<any[]>([raw, lonely])
 const plottedDatastreams = ref<any[]>([])
+const qcDatastream = ref<any>(null)
+const editSourceDatastream = ref<any>(null)
 const historiesBySource = ref(
   new Map<string, any[]>([
     ['src', [{ id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'src' }]],
@@ -41,6 +43,8 @@ vi.mock('@/store/dataVisualization', () => ({
     filteredDatastreams,
     plottedDatastreams,
     historiesBySource,
+    qcDatastream,
+    editSourceDatastream,
     toggleDatastream,
     clearPlottedDatastreams,
     sourceGroupIds,
@@ -65,6 +69,11 @@ vi.mock('@/composables/useManagedDatastreams', () => ({
   useManagedDatastreams: () => ({ loadForSource }),
 }))
 
+const closeEditor = vi.fn().mockResolvedValue(true)
+vi.mock('@/composables/useEditEntry', () => ({
+  useEditEntry: () => ({ closeEditor }),
+}))
+
 vi.mock('@/utils/csvExport', () => ({
   downloadDatastreamsCsvZip: vi.fn().mockResolvedValue(undefined),
 }))
@@ -79,6 +88,8 @@ beforeEach(() => {
   vi.clearAllMocks()
   canEdit.mockReturnValue(true)
   plottedDatastreams.value = []
+  qcDatastream.value = null
+  editSourceDatastream.value = null
 })
 
 const mountTable = async () => {
@@ -247,3 +258,38 @@ describe('DataVisDatasetsTable edit button', () => {
     expect(wrapper.text()).not.toContain('QC target')
   })
 })
+
+describe('DataVisDatasetsTable editing indicator', () => {
+  const rowOf = (wrapper: any, id: string) =>
+    wrapper.find(`[data-testid="plot-checkbox-${id}"]`).element.closest('tr')
+
+  it('marks the source row of the datastream being edited', async () => {
+    qcDatastream.value = { id: 'mgd', name: 'Raw (QC)' }
+    editSourceDatastream.value = raw
+    const wrapper = await mountTable()
+
+    const chips = wrapper.findAll('[data-testid="editing-chip"]')
+    expect(chips).toHaveLength(1)
+    expect(chips[0].attributes('title')).toBe('Editing Raw (QC)')
+    expect(rowOf(wrapper, 'src').classList).toContain('datasets-table__row--editing')
+    expect(rowOf(wrapper, 'solo').classList).not.toContain(
+      'datasets-table__row--editing'
+    )
+  })
+
+  it('swaps Edit for Close on the edited row, which ends the session', async () => {
+    qcDatastream.value = { id: 'mgd', name: 'Raw (QC)' }
+    editSourceDatastream.value = raw
+    const wrapper = await mountTable()
+
+    expect(wrapper.find('[data-testid="edit-datastream-src"]').exists()).toBe(false)
+    expect(wrapper.find('[data-testid="edit-datastream-solo"]').exists()).toBe(true)
+    await wrapper.find('[data-testid="close-datastream-src"]').trigger('click')
+    expect(closeEditor).toHaveBeenCalledTimes(1)
+  })
+
+  it('marks nothing when not editing', async () => {
+    const wrapper = await mountTable()
+    expect(wrapper.findAll('[data-testid="editing-chip"]')).toHaveLength(0)
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
index d3c9dd49..b6ed22ed 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisTimeFilters.spec.ts
@@ -6,7 +6,7 @@ import { createTestVuetify } from '@/utils/test/vuetify'
 
 const beginDate = ref<string | null>(null)
 const endDate = ref<string | null>(null)
-const selectedDateBtnId = ref<number | null>(null)
+const activePresetId = ref<number | null>(null)
 const setDateRange = () => {}
 const onDateBtnClick = () => {}
 
@@ -14,7 +14,7 @@ vi.mock('@/store/dataVisualization', () => ({
   useDataVisStore: () => ({
     beginDate,
     endDate,
-    selectedDateBtnId,
+    activePresetId,
     setDateRange,
     onDateBtnClick,
   }),
@@ -65,15 +65,15 @@ describe('DataVisTimeFilters.vue', () => {
   })
 
   it('highlights All for a saved YTD preference with the editor presets, keeping the preference', () => {
-    selectedDateBtnId.value = 3
+    activePresetId.value = 3
     const w = mountIt({ presets: EDITOR_PRESETS })
     expect(w.find('[data-testid="date-preset-All"]').classes()).toContain('v-chip--variant-tonal')
     expect(w.find('[data-testid="date-preset-1w"]').classes()).toContain('v-chip--variant-outlined')
-    expect(selectedDateBtnId.value).toBe(3)
+    expect(activePresetId.value).toBe(3)
   })
 
   it('highlights YTD itself where it is offered', () => {
-    selectedDateBtnId.value = 3
+    activePresetId.value = 3
     const w = mountIt()
     expect(w.find('[data-testid="date-preset-YTD"]').classes()).toContain('v-chip--variant-tonal')
     expect(w.find('[data-testid="date-preset-All"]').classes()).toContain('v-chip--variant-outlined')
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
index 4b33f51e..3adf770e 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
@@ -47,6 +47,9 @@ vi.mock('@/store/dataVisualization', () => ({
       hasSelectionShape: ref(false),
       qcDatastream,
       trackPlotWork,
+      contextPresetId: ref(1),
+      showSourceContext: ref(true),
+      setShowSourceContext: vi.fn(),
     }),
 }))
 
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
index 45646c9f..5d35e543 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
@@ -74,11 +74,17 @@ vi.mock('@/utils/plotting/plotly', () => ({
   toggleTraceVisibility: vi.fn().mockResolvedValue(undefined),
 }))
 
+const closeEditor = vi.fn().mockResolvedValue(true)
+vi.mock('@/composables/useEditEntry', () => ({
+  useEditEntry: () => ({ closeEditor }),
+}))
+
 import PlottedDatastreams from '@/components/VisualizeData/PlottedDatastreams.vue'
 import { handleNewPlot } from '@/utils/plotting/plotly'
 
-function mountIt() {
+function mountIt(props: { clearable?: boolean } = {}) {
   return mount(PlottedDatastreams, {
+    props,
     global: {
       plugins: [createTestPinia(), createTestVuetify()],
     },
@@ -224,7 +230,7 @@ describe('PlottedDatastreams.vue: load status', () => {
     plotlyOptions.value = {
       traces: [
         { id: datastreamA.id, x: new Array(6) },
-        { _isGapOverlay: true, _gapOverlayFor: datastreamA.id, x: new Array(50) },
+        { _isGapOverlay: true, _partOf: datastreamA.id, x: new Array(50) },
       ],
     }
     const wrapper = mountIt()
@@ -329,29 +335,31 @@ describe('PlottedDatastreams row kinds', () => {
     wrapper.find(`[data-testid="plotted-item-${id}"]`)
   const plottedIds = () => plottedDatastreams.value.map((d) => d.id)
 
-  it('lists the edit target, its source, then the plotted context', () => {
+  it('lists the edit target, then the plotted context, leaving the source context out', () => {
     startEditing()
     const wrapper = mountIt()
     const ids = wrapper
       .findAll('.plotted-item')
       .map((r) => r.attributes('data-testid'))
 
+    // The source context is switched from the Context menu, not listed.
     expect(ids).toEqual([
       'plotted-item-mgd',
-      'plotted-item-src',
       'plotted-item-ctx',
       'plotted-item-ctx2',
     ])
   })
 
-  it('gives the source row no remove and no axis toggle', () => {
-    startEditing()
+  it('lists a source the user plotted as an ordinary row', () => {
+    qcDatastream.value = edit
+    // Plotted by the user, so the store draws it whole, not as context.
+    sourceContextDatastream.value = null
+    plottedDatastreams.value = [source, ctx]
     const r = row(mountIt(), 'src')
 
-    expect(r.classes()).toContain('plotted-item--source')
-    expect(r.text()).toContain('raw source')
-    expect(r.find('.plotted-item__close').exists()).toBe(false)
-    expect(r.find('.plotted-item__axis-toggle').exists()).toBe(false)
+    expect(r.exists()).toBe(true)
+    expect(r.find('.plotted-item__close').exists()).toBe(true)
+    expect(r.attributes('draggable')).toBe('true')
   })
 
   it('gives the edit row no remove and no axis toggle', () => {
@@ -387,10 +395,29 @@ describe('PlottedDatastreams row kinds', () => {
     expect(mountIt().find('.plotted-item__dot').exists()).toBe(false)
   })
 
-  it('shows Unplot all only when nothing is being edited', () => {
-    expect(mountIt().text()).toContain('Unplot all')
+  it('shows Clear plot only where asked, editing or not', () => {
+    expect(mountIt().find('[data-testid="clear-plot-btn"]').exists()).toBe(false)
+    expect(mountIt({ clearable: true }).find('[data-testid="clear-plot-btn"]').exists()).toBe(true)
     startEditing()
-    expect(mountIt().text()).not.toContain('Unplot all')
+    expect(mountIt({ clearable: true }).find('[data-testid="clear-plot-btn"]').exists()).toBe(true)
+  })
+
+  it('closes the edited datastream before clearing the plot', async () => {
+    startEditing()
+    const wrapper = mountIt({ clearable: true })
+    await wrapper.find('[data-testid="clear-plot-btn"]').trigger('click')
+    await flushPromises()
+    expect(closeEditor).toHaveBeenCalledTimes(1)
+    expect(clearPlottedDatastreams).toHaveBeenCalledTimes(1)
+  })
+
+  it('keeps the plot when the user stays in the session', async () => {
+    startEditing()
+    closeEditor.mockResolvedValueOnce(false)
+    const wrapper = mountIt({ clearable: true })
+    await wrapper.find('[data-testid="clear-plot-btn"]').trigger('click')
+    await flushPromises()
+    expect(clearPlottedDatastreams).not.toHaveBeenCalled()
   })
 
   it('makes only context rows draggable', () => {
@@ -398,7 +425,6 @@ describe('PlottedDatastreams row kinds', () => {
     const wrapper = mountIt()
 
     expect(row(wrapper, 'mgd').attributes('draggable')).toBe('false')
-    expect(row(wrapper, 'src').attributes('draggable')).toBe('false')
     expect(row(wrapper, 'ctx').attributes('draggable')).toBe('true')
   })
 
diff --git a/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts b/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts
index 6649689f..650940e2 100644
--- a/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts
@@ -23,6 +23,7 @@ vi.mock('@/store/plotly', () => {
       plotlyRef,
       selectedSeries,
       suppressedEchoSelection,
+      previewIndex: ref(null),
       graphSeriesArray,
       updateOptions: vi.fn(),
       clearChartState: vi.fn(),
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 0f1c4c06..290fa342 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -30,9 +30,9 @@ vi.mock('@/store/hydroserver', () => ({
   useHydroServer: () => ({ hs }),
 }))
 
-const fetchObservationsInRange = vi.fn()
+const fetchDetachedRecord = vi.fn()
 vi.mock('@/store/observations', () => ({
-  useObservationStore: () => ({ fetchObservationsInRange }),
+  useObservationStore: () => ({ fetchDetachedRecord }),
 }))
 
 const wcRebuild = vi.fn()
@@ -61,7 +61,7 @@ const { snackbarWarn, ObservationRecordDouble } = vi.hoisted(() => {
     reload = vi.fn(async () => {})
     // `discardUnsavedEdits` calls this on the working copy; mirror the
     // truncate-in-place semantics the other test double in this file uses.
-    reloadHistory = vi.fn(async function (this: ObservationRecordDouble, index: number) {
+    truncateHistory = vi.fn(async function (this: ObservationRecordDouble, index: number) {
       this.history.splice(index + 1)
       return []
     })
@@ -108,7 +108,7 @@ const makeRecord = (history: any[] = []) => ({
   dataY: [10],
   reload: vi.fn(async () => {}),
   // Stands in for qc-utils: truncate to `0..index` and hand back a selection.
-  reloadHistory: vi.fn(async function (this: any, index: number) {
+  truncateHistory: vi.fn(async function (this: any, index: number) {
     history.splice(index + 1)
     return []
   }),
@@ -134,7 +134,7 @@ beforeEach(() => {
   selectedSeries.value = { data: makeRecord() }
   getItem.mockResolvedValue({ id: 's-1', name: 'Source' })
   createObservations.mockResolvedValue(undefined)
-  fetchObservationsInRange.mockResolvedValue(makeRecord())
+  fetchDetachedRecord.mockResolvedValue(makeRecord())
   wcRebuild.mockResolvedValue({
     sessionId: 'x',
     record: makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }]),
@@ -353,7 +353,7 @@ describe('useEditSession', () => {
   it('startSession loads the managed datastream as the working base', async () => {
     await seedHistory()
     const managedBase = makeRecord()
-    fetchObservationsInRange.mockResolvedValue(managedBase)
+    fetchDetachedRecord.mockResolvedValue(managedBase)
     const { useEditSession } = await import('@/composables/useEditSession')
     const session = useEditSession()
     await session.beginEditing()
@@ -361,7 +361,7 @@ describe('useEditSession', () => {
     expect(session.needsSession.value).toBe(false)
     expect(useQcSessionStore().inProgressSession?.description).toBe('Jan')
     // Working copy comes from the managed datastream (latest committed state).
-    expect(fetchObservationsInRange.mock.calls[0]?.[0].id).toBe('m-1')
+    expect(fetchDetachedRecord.mock.calls[0]?.[0].id).toBe('m-1')
     expect(Array.from(selectedSeries.value.data.dataX)).toEqual(
       Array.from(managedBase.dataX)
     )
@@ -559,7 +559,7 @@ describe('useEditSession', () => {
       })
     )
     await qc.sessions.create(h.id, WIN)
-    fetchObservationsInRange.mockResolvedValue(
+    fetchDetachedRecord.mockResolvedValue(
       makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }])
     )
     const { useEditSession } = await import('@/composables/useEditSession')
@@ -605,7 +605,7 @@ describe('useEditSession', () => {
     const store = useQcSessionStore()
     expect(store.historyId).toBeNull()
     expect(store.sessions).toEqual([])
-    expect(fetchObservationsInRange).not.toHaveBeenCalled()
+    expect(fetchDetachedRecord).not.toHaveBeenCalled()
   })
 
   it('commit completes without writing the session store when the target changes while sessions reload', async () => {
@@ -807,7 +807,7 @@ describe('useEditSession.viewSession', () => {
     await qc.sessions.create(h.id, WIN)
 
     const viewed = makeRecord([])
-    fetchObservationsInRange.mockResolvedValue(viewed)
+    fetchDetachedRecord.mockResolvedValue(viewed)
 
     const { useEditSession } = await import('@/composables/useEditSession')
     const store = useQcSessionStore()
@@ -843,7 +843,7 @@ describe('useEditSession.viewSession', () => {
     await session.beginEditing()
 
     const seen: Array<{ viewed: string | null; switching: boolean }> = []
-    fetchObservationsInRange.mockImplementation(async () => {
+    fetchDetachedRecord.mockImplementation(async () => {
       seen.push({
         viewed: store.viewedSessionId,
         switching: store.isSwitchingSession,
@@ -875,7 +875,7 @@ describe('useEditSession.viewSession', () => {
     await session.beginEditing()
     expect(store.viewedSessionId).toBe(inProgress.id)
 
-    fetchObservationsInRange.mockRejectedValueOnce(new Error('network'))
+    fetchDetachedRecord.mockRejectedValueOnce(new Error('network'))
     await expect(session.viewSession(committed.id)).rejects.toThrow('network')
 
     expect(store.viewedSessionId).toBe(inProgress.id)
@@ -902,7 +902,7 @@ describe('useEditSession.viewSession', () => {
 
     // View the committed session read-only.
     const viewed = makeRecord([])
-    fetchObservationsInRange.mockResolvedValue(viewed)
+    fetchDetachedRecord.mockResolvedValue(viewed)
     await session.viewSession(committed.id)
     expect(store.viewedSessionId).toBe(committed.id)
     expect(store.isReadOnly).toBe(true)
@@ -943,7 +943,7 @@ describe('useEditSession.viewSession', () => {
     setEditRecord.mockClear()
 
     // The user leaves and enters another target mid-reconstruction.
-    fetchObservationsInRange.mockImplementationOnce(async () => {
+    fetchDetachedRecord.mockImplementationOnce(async () => {
       qcDatastream.value = { id: 'm-2' }
       return makeRecord([])
     })
diff --git a/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts b/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
index 4835843a..c5edee1e 100644
--- a/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
@@ -22,11 +22,11 @@ const {
 } = vi.hoisted(() => {
   const { ref: r } = require('vue') as typeof import('vue')
   return {
-    qcDatastream: r<{ id: string } | null>(null),
+    qcDatastream: r<{ id: string; name?: string } | null>(null),
     resumeDatastreamId: r<string | null>(null),
     historyId: r<string | null>('h-1'),
     sessions: r<Array<{ id: string; dependencyIds?: string[] }>>([]),
-    inProgressSession: r<{ id: string } | null>(null),
+    inProgressSession: r<{ id: string; name?: string } | null>(null),
     isReadOnly: r(false),
     hasSessionOperations: r(false),
     hasUnsavedChanges: r(false),
@@ -92,7 +92,7 @@ const tick = () => new Promise((resolve) => setTimeout(resolve, 0))
 
 /** Nothing saved, nothing unsaved: an untouched in-progress session. */
 function emptySession() {
-  qcDatastream.value = { id: 'mgd-1' }
+  qcDatastream.value = { id: 'mgd-1', name: 'Temp (QC)' }
   inProgressSession.value = { id: 'qcs-1' }
   sessions.value = [{ id: 'qcs-1' }]
   hasSessionOperations.value = false
@@ -102,7 +102,7 @@ function emptySession() {
 beforeEach(() => {
   createTestPinia()
   vi.clearAllMocks()
-  qcDatastream.value = { id: 'mgd-1' }
+  qcDatastream.value = { id: 'mgd-1', name: 'Temp (QC)' }
   resumeDatastreamId.value = 'mgd-1'
   historyId.value = 'h-1'
   sessions.value = []
@@ -166,6 +166,7 @@ describe('requestLeave', () => {
 
     expect(leavePrompt.value).toEqual({
       kind: 'unsaved',
+      datastreamName: 'Temp (QC)',
       unsavedCount: 3,
       canSave: true,
     })
diff --git a/apps/qc-app/src/composables/useDataSelection.ts b/apps/qc-app/src/composables/useDataSelection.ts
index f505ae99..c3f0c9d3 100644
--- a/apps/qc-app/src/composables/useDataSelection.ts
+++ b/apps/qc-app/src/composables/useDataSelection.ts
@@ -20,7 +20,7 @@ export function useDataSelection() {
   const { plotlyRef, suppressedEchoSelection } = storeToRefs(
     usePlotlyStore()
   )
-  const { selectedSeries } = storeToRefs(usePlotlyStore())
+  const { selectedSeries, previewIndex } = storeToRefs(usePlotlyStore())
   const { selectedData } = storeToRefs(useDataVisStore())
 
   /**
@@ -115,7 +115,8 @@ export function useDataSelection() {
     selectedData.value = []
     hasSelectionShape.value = false
 
-    if (recordHistory) {
+    // A previewed step records nothing; the clear only shows.
+    if (recordHistory && previewIndex.value === null) {
       // Explicitly log the cleared state so qc-utils' `_selection`
       // empty-case logic (pop self, optionally pop the underlying
       // filter that drove the now-cleared selection) runs even though
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index 31a90675..d5e18d95 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -81,7 +81,9 @@ export function useEditSession() {
   const { replaceDatastream, setEditRecord } = useDataVisStore()
   const { selectedSeries } = storeToRefs(usePlotlyStore())
   const { hs } = storeToRefs(useHydroServer())
-  const { fetchObservationsInRange } = useObservationStore()
+  // Working copies build on records of their own, so the plotted source
+  // keeps its context window.
+  const { fetchDetachedRecord } = useObservationStore()
   const sessionStore = useQcSessionStore()
   const workingCopies = useWorkingCopiesStore()
 
@@ -240,7 +242,7 @@ export function useEditSession() {
         {
           qcSessions: hs.value.qualityControlSessions,
           qcOperations: hs.value.qualityControlOperations,
-          fetchInRange: fetchObservationsInRange,
+          fetchInRange: fetchDetachedRecord,
           applyHistory,
         },
         source,
@@ -284,7 +286,7 @@ export function useEditSession() {
     // Start from the latest committed state (the managed datastream), or the
     // raw source when nothing has been committed yet.
     const base = await loadLatestBase(
-      fetchObservationsInRange,
+      fetchDetachedRecord,
       managed,
       source,
       new Date(session.phenomenonTimeStart),
@@ -314,7 +316,7 @@ export function useEditSession() {
   async function discardUnsavedEdits(): Promise<number[] | undefined> {
     const record = selectedSeries.value?.data as ObservationRecord | undefined
     if (!record) return
-    const selection = await record.reloadHistory(savedEdits.value.length - 1)
+    const selection = await record.truncateHistory(savedEdits.value.length - 1)
     record.history.forEach((item, i) => {
       item.comment = savedComments.value[i] || undefined
     })
diff --git a/apps/qc-app/src/composables/useHistorySnapshots.ts b/apps/qc-app/src/composables/useHistorySnapshots.ts
index c220bd8f..43863a94 100644
--- a/apps/qc-app/src/composables/useHistorySnapshots.ts
+++ b/apps/qc-app/src/composables/useHistorySnapshots.ts
@@ -5,11 +5,11 @@
 
 import { ref } from 'vue'
 import { storeToRefs } from 'pinia'
-import { ObservationRecord, Snackbar, applyHistory } from '@uwrl/qc-utils'
-import type { Datastream, QualityControlSession } from '@hydroserver/client'
+import { Snackbar, applyHistory } from '@uwrl/qc-utils'
+import type { QualityControlSession } from '@hydroserver/client'
 import { useHydroServer } from '@/store/hydroserver'
 import { useDataVisStore } from '@/store/dataVisualization'
-import { useObservationStore, type ObservationData } from '@/store/observations'
+import { useObservationStore } from '@/store/observations'
 import { usePlotlyStore } from '@/store/plotly'
 import { useQcSessionStore } from '@/store/qcSession'
 import { buildSnapshotRecord } from '@/services/qualityControl/buildSnapshot'
@@ -30,23 +30,8 @@ export function useHistorySnapshots() {
   const { plottedDatastreams } = storeToRefs(dataVis)
   const { selectedSeries } = storeToRefs(usePlotlyStore())
   const { hs } = storeToRefs(useHydroServer())
-  const { observationsRaw } = storeToRefs(useObservationStore())
-  const { fetchObservationsInRange } = useObservationStore()
-
-  /**
-   * The observation store hands back one shared record per datastream, and the
-   * replay mutates whatever it is given, so build on an independent record.
-   */
-  const fetchDetached = async (ds: Datastream, start: Date, end: Date) => {
-    await fetchObservationsInRange(ds, start, end)
-    const raw: ObservationData = observationsRaw.value[ds.id] ?? {
-      datetimes: new Float64Array(0),
-      dataValues: new Float32Array(0),
-    }
-    const record = new ObservationRecord(raw)
-    await record.applyWindow(start.getTime(), end.getTime())
-    return record
-  }
+  // The replay mutates whatever it is given, so build on a record of its own.
+  const { fetchDetachedRecord } = useObservationStore()
 
   const isSnapshotPlotted = (sessionId: string, opIndex: number): boolean => {
     const id = makeSnapshotId(sessionId, opIndex)
@@ -81,7 +66,7 @@ export function useHistorySnapshots() {
         {
           qcSessions: hs.value.qualityControlSessions,
           qcOperations: hs.value.qualityControlOperations,
-          fetchInRange: fetchDetached,
+          fetchInRange: fetchDetachedRecord,
           applyHistory,
         },
         { historyId, session, source, managed, opIndex, liveHistory }
diff --git a/apps/qc-app/src/composables/useLeaveSession.ts b/apps/qc-app/src/composables/useLeaveSession.ts
index de12439a..be019dff 100644
--- a/apps/qc-app/src/composables/useLeaveSession.ts
+++ b/apps/qc-app/src/composables/useLeaveSession.ts
@@ -37,6 +37,8 @@ export type LeaveCase =
 
 export interface LeavePrompt {
   kind: Exclude<LeaveCase, 'none'>
+  /** The datastream being left, so the prompt never reads as the next one. */
+  datastreamName: string
   /** Edits made since the last save. */
   unsavedCount: number
   /** False when no session is open, so there is nowhere to save to. */
@@ -68,6 +70,7 @@ function leaveCase(): LeaveCase {
 function show(kind: Exclude<LeaveCase, 'none'>): void {
   leavePrompt.value = {
     kind,
+    datastreamName: useDataVisStore().qcDatastream?.name ?? '',
     unsavedCount: useEditSession().unsavedEditCount.value,
     canSave: !!useQcSessionStore().inProgressSession,
   }
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index 07a225b4..663b431d 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -12,7 +12,9 @@ const mockGraphSeriesArray = shallowRef<any[]>([])
 const mockUpdateOptions = vi.fn()
 const mockClearChartState = vi.fn()
 const mockClearZoomHistory = vi.fn()
-const mockFetchGraphSeries = vi.fn(async (ds: any) => ({ id: ds.id, name: ds.name, data: {} }))
+const mockFetchGraphSeries = vi.fn(
+  async (ds: any, _start?: Date, _end?: Date, _exclude?: unknown, _fetchAs?: any) => ({ id: ds.id, name: ds.name, data: {} })
+)
 const mockBuildGraphSeries = vi.fn((ds: any, data: any) => ({ id: ds.id, name: ds.name, data }))
 const mockAssignSeriesColors = vi.fn()
 const mockRedraw = vi.fn()
@@ -263,6 +265,33 @@ const managedPair = async () => {
   return { store, source, managed, other }
 }
 
+const WIN_START = '2025-06-01T00:00:00Z'
+const WIN_END = '2025-06-10T00:00:00Z'
+const WINDOW = { begin: new Date(WIN_START), end: new Date(WIN_END) }
+
+/** Load an in-progress session over `WINDOW` for the edit target. */
+async function loadSession() {
+  const { useQcSessionStore } = await import('@/store/qcSession')
+  useQcSessionStore().applySessions('h1', [
+    {
+      id: 's1',
+      status: 'in_progress',
+      phenomenonTimeStart: WIN_START,
+      phenomenonTimeEnd: WIN_END,
+    },
+  ] as any)
+  await nextTick()
+}
+
+const settle = () => new Promise((r) => setTimeout(r, 0))
+
+/** Edit `mgd` with its session window known and the context loaded. */
+async function editWithWindow(store: any) {
+  await store.setEditTarget('mgd')
+  await loadSession()
+  await settle()
+}
+
 describe('useDataVisStore edit target', () => {
   it('plotting never picks an edit target', async () => {
     const { store, other } = await managedPair()
@@ -282,23 +311,62 @@ describe('useDataVisStore edit target', () => {
     const { store, other } = await managedPair()
     await store.plotDatastream(other as any)
     await store.setEditTarget('mgd')
-    expect(store.sourceContextDatastream?.id).toBe('src')
-    expect(store.seriesDatastreams.map((d) => d.id)).toEqual(['mgd', 'src', 'other'])
+    // The grey context has an id of its own; it reads the source's data.
+    expect(store.sourceContextDatastream?.id).toBe('ctx:src')
+    expect(store.seriesDatastreams.map((d) => d.id)).toEqual(['mgd', 'ctx:src', 'other'])
   })
 
-  it('does not duplicate a source the user already plotted', async () => {
+  it('keeps the grey context beside a source the user plotted', async () => {
     const { store, source, other } = await managedPair()
     await store.plotDatastream(other as any)
     await store.plotDatastream(source as any)
     await store.setEditTarget('mgd')
-    expect(store.seriesDatastreams.map((d) => d.id)).toEqual(['mgd', 'src', 'other'])
+    expect(store.editSourceDatastream?.id).toBe('src')
+    expect(store.seriesDatastreams.map((d) => d.id)).toEqual([
+      'mgd',
+      'ctx:src',
+      'other',
+      'src',
+    ])
   })
 
-  it('never fetches the edit target when refreshing', async () => {
+  it('fills the context series with the source data', async () => {
+    const { store } = await managedPair()
+    await editWithWindow(store)
+    const call = mockFetchGraphSeries.mock.calls.find((c) => c[0].id === 'ctx:src')
+    expect(call?.[4]?.id).toBe('src')
+  })
+
+  it('drops the source context when it is switched off', async () => {
     const { store } = await managedPair()
     await store.setEditTarget('mgd')
-    // `setEditTarget` already loaded `src` as context, so this refresh
-    // updates it in place via `fetchObservationsInRange`, not a fresh fetch.
+    await store.setShowSourceContext(false)
+    expect(store.sourceContextDatastream).toBeNull()
+    expect(store.seriesDatastreams.map((d) => d.id)).toEqual(['mgd'])
+    await store.setShowSourceContext(true)
+    expect(store.seriesDatastreams.map((d) => d.id)).toEqual(['mgd', 'ctx:src'])
+  })
+
+  it('fetches a source the user plotted whole, without waiting for the window', async () => {
+    const { store, source } = await managedPair()
+    await store.plotDatastream(source as any)
+    mockFetchGraphSeries.mockClear()
+    mockFetchObservationsInRange.mockClear()
+    await store.setEditTarget('mgd')
+    await store.refreshGraphSeriesArray()
+    const calls = [
+      ...mockFetchGraphSeries.mock.calls,
+      ...mockFetchObservationsInRange.mock.calls,
+    ].filter((c) => c[0].id === 'src')
+    expect(calls.length).toBeGreaterThan(0)
+    expect(calls.every((c) => c[3] === undefined)).toBe(true)
+  })
+
+  it('never fetches the edit target when refreshing', async () => {
+    const { store } = await managedPair()
+    await editWithWindow(store)
+    // The source is already loaded as context, so this refresh updates it
+    // in place via `fetchObservationsInRange`, not a fresh fetch.
     mockFetchGraphSeries.mockClear()
     mockFetchObservationsInRange.mockClear()
     await store.refreshGraphSeriesArray()
@@ -306,10 +374,35 @@ describe('useDataVisStore edit target', () => {
     expect(mockFetchGraphSeries).not.toHaveBeenCalled()
   })
 
-  it('setEditTarget with nothing plotted still fetches the source as context', async () => {
+  it('waits for the session window before fetching the source', async () => {
     const { store } = await managedPair()
     await store.setEditTarget('mgd')
-    expect(mockFetchGraphSeries.mock.calls.map((c) => c[0].id)).toEqual(['src'])
+    expect(mockFetchGraphSeries).not.toHaveBeenCalled()
+
+    await loadSession()
+    await settle()
+
+    expect(
+      mockFetchGraphSeries.mock.calls.map((c) => [c[0].id, c[3], c[4]?.id])
+    ).toEqual([['ctx:src', WINDOW, 'src']])
+  })
+
+  it('never fetches the session window for the source', async () => {
+    const { store } = await managedPair()
+    await editWithWindow(store)
+    mockFetchObservationsInRange.mockClear()
+    await store.refreshGraphSeriesArray()
+    expect(mockFetchObservationsInRange.mock.calls[0]?.[3]).toEqual(WINDOW)
+  })
+
+  it('fetches other plotted datastreams whole', async () => {
+    const { store, other } = await managedPair()
+    await store.plotDatastream(other as any)
+    await editWithWindow(store)
+    mockFetchObservationsInRange.mockClear()
+    await store.refreshGraphSeriesArray()
+    const call = mockFetchObservationsInRange.mock.calls.find((c) => c[0].id === 'other')
+    expect(call?.[3]).toBeUndefined()
   })
 
   it('unplotting the last plotted datastream while editing keeps the edit series', async () => {
@@ -502,9 +595,9 @@ describe('useDataVisStore.setDateRange', () => {
 
   it('refreshes context when only the edit target is set, nothing plotted', async () => {
     const { store } = await managedPair()
-    await store.setEditTarget('mgd')
-    // `setEditTarget` already loaded `src` as context, so the date change
-    // updates it in place via `fetchObservationsInRange`.
+    await editWithWindow(store)
+    // The source is already loaded as context, so the date change updates
+    // it in place via `fetchObservationsInRange`.
     mockFetchObservationsInRange.mockClear()
     mockRedraw.mockClear()
     mockClearZoomHistory.mockClear()
@@ -534,7 +627,7 @@ describe('useDataVisStore overlapping plot loads', () => {
 
   it('drops a context response whose range was superseded', async () => {
     const { store } = await managedPair()
-    await store.setEditTarget('mgd')
+    await editWithWindow(store)
     mockRedraw.mockClear()
     const first = deferred<any>()
     const second = deferred<any>()
@@ -549,20 +642,20 @@ describe('useDataVisStore overlapping plot loads', () => {
     second.resolve({ id: 'apr' })
     await Promise.all([older, newer])
 
-    const src = mockGraphSeriesArray.value.find((s) => s.id === 'src')
+    const src = mockGraphSeriesArray.value.find((s) => s.id === 'ctx:src')
     expect(src.data).toEqual({ id: 'apr' })
-    expect(store.loadingStates.get('src')).toBe(false)
+    expect(store.loadingStates.get('ctx:src')).toBe(false)
     expect(mockRedraw).toHaveBeenCalledTimes(1)
   })
 
   it('a rebuild whose range moves while loading draws the new range once', async () => {
     const { store } = await managedPair()
-    await store.setEditTarget('mgd')
+    await editWithWindow(store)
     mockPlotlyRef.value = {}
     const { handleNewPlot } = await import('@/utils/plotting/plotly')
     const drawn: unknown[] = []
     vi.mocked(handleNewPlot).mockImplementationOnce(async () => {
-      drawn.push(mockGraphSeriesArray.value.find((s) => s.id === 'src')?.data)
+      drawn.push(mockGraphSeriesArray.value.find((s) => s.id === 'ctx:src')?.data)
     })
     mockFetchObservationsInRange.mockClear()
     mockRedraw.mockClear()
@@ -590,7 +683,7 @@ describe('useDataVisStore overlapping plot loads', () => {
 
   it('a rebuild requested while a context reload loads waits for it', async () => {
     const { store, other } = await managedPair()
-    await store.setEditTarget('mgd')
+    await editWithWindow(store)
     mockFetchObservationsInRange.mockClear()
     mockFetchGraphSeries.mockClear()
     const pending = deferred<any>()
@@ -612,7 +705,7 @@ describe('useDataVisStore overlapping plot loads', () => {
 
   it('a context reload requested while a rebuild is queued joins it', async () => {
     const { store } = await managedPair()
-    await store.setEditTarget('mgd')
+    await editWithWindow(store)
     mockPlotlyRef.value = {}
     mockRedraw.mockClear()
     const pending = deferred<any>()
@@ -638,7 +731,7 @@ describe('useDataVisStore overlapping plot loads', () => {
   it('does not start a second load when one is requested as a load settles', async () => {
     const runScenario = async (hops: number) => {
       const { store } = await managedPair()
-      await store.setEditTarget('mgd')
+      await editWithWindow(store)
       mockPlotlyRef.value = {}
       let active = 0
       let maxActive = 0
@@ -721,21 +814,30 @@ describe('useDataVisStore overlapping plot loads', () => {
 })
 
 describe('useDataVisStore context range around the session window', () => {
-  const WIN_START = '2025-06-01T00:00:00Z'
-  const WIN_END = '2025-06-10T00:00:00Z'
-  const loadSession = async () => {
-    const { useQcSessionStore } = await import('@/store/qcSession')
-    useQcSessionStore().applySessions('h1', [
-      {
-        id: 's1',
-        status: 'in_progress',
-        phenomenonTimeStart: WIN_START,
-        phenomenonTimeEnd: WIN_END,
-      },
-    ] as any)
-    await nextTick()
-  }
-  const settle = () => new Promise((r) => setTimeout(r, 0))
+
+  it('keeps the Context preset apart from the Select view Time range', async () => {
+    const { store } = await managedPair()
+    store.selectedDateBtnId = 2
+    await store.setEditTarget('mgd')
+    await loadSession()
+    await settle()
+
+    await store.onDateBtnClick(0)
+    expect(store.contextPresetId).toBe(0)
+    expect(store.selectedDateBtnId).toBe(2)
+
+    await store.clearEditTarget()
+    expect(store.activePresetId).toBe(2)
+  })
+
+  it('records a custom range on the preset in use', async () => {
+    const { store } = await managedPair()
+    store.selectedDateBtnId = 2
+    await store.setEditTarget('mgd')
+    await store.setDateRange({ begin: new Date('2025-02-01T00:00:00Z'), end: new Date('2025-03-01T00:00:00Z') })
+    expect(store.contextPresetId).toBe(-1)
+    expect(store.selectedDateBtnId).toBe(2)
+  })
 
   it('the editor counts presets out from the session window', async () => {
     const { store } = await managedPair()
@@ -785,9 +887,10 @@ describe('useDataVisStore context range around the session window', () => {
     const { store } = await managedPair()
     await store.setEditTarget('mgd')
     expect(store.endDate.toISOString()).toBe('2025-12-31T00:00:00.000Z')
-    mockFetchObservationsInRange.mockClear()
+    mockPlotlyRef.value = {}
+    const { handleNewPlot } = await import('@/utils/plotting/plotly')
+    vi.mocked(handleNewPlot).mockClear()
     mockFetchGraphSeries.mockClear()
-    mockRedraw.mockClear()
     mockClearZoomHistory.mockClear()
 
     await loadSession()
@@ -795,10 +898,10 @@ describe('useDataVisStore context range around the session window', () => {
 
     expect(store.beginDate.getTime()).toBe(subtractMonths(new Date(WIN_START), 1).getTime())
     expect(store.endDate.getTime()).toBe(subtractMonths(new Date(WIN_END), -1).getTime())
-    expect(mockFetchObservationsInRange.mock.calls.map((c) => c[0].id)).toEqual(['src'])
-    expect(mockFetchGraphSeries).not.toHaveBeenCalled()
-    expect(mockRedraw).toHaveBeenCalledTimes(1)
-    expect(mockRedraw).toHaveBeenCalledWith(false, true)
+    // The source's traces first appear now, so it is a rebuild.
+    expect(mockFetchGraphSeries.mock.calls.map((c) => c[0].id)).toEqual(['ctx:src'])
+    expect(handleNewPlot).toHaveBeenCalledTimes(1)
+    expect(handleNewPlot).toHaveBeenCalledWith(undefined, { preserveZoom: true })
     expect(mockClearZoomHistory).not.toHaveBeenCalled()
   })
 
@@ -808,8 +911,8 @@ describe('useDataVisStore context range around the session window', () => {
     await store.setEditRecord({ dataX: [1], history: [] } as any)
     expect(store.isEditorReady).toBe(false)
     let resolveFetch!: (v: unknown) => void
-    mockFetchObservationsInRange.mockImplementationOnce(
-      () => new Promise((r) => (resolveFetch = r))
+    mockFetchGraphSeries.mockImplementationOnce(
+      () => new Promise((r) => (resolveFetch = r)) as any
     )
 
     await loadSession()
@@ -817,7 +920,7 @@ describe('useDataVisStore context range around the session window', () => {
     expect(store.beginDate.getTime()).toBe(subtractMonths(new Date(WIN_START), 1).getTime())
     expect(store.isEditorReady).toBe(false)
 
-    resolveFetch({ id: 'stub', data: {} })
+    resolveFetch({ id: 'src', data: {} })
     await settle()
     expect(store.isEditorReady).toBe(true)
   })
@@ -1028,7 +1131,9 @@ describe('useDataVisStore time range presets', () => {
     expect(mockFetchGraphSeries).toHaveBeenCalledWith(
       expect.objectContaining({ id: 'old' }),
       store.beginDate,
-      store.endDate
+      store.endDate,
+      undefined,
+      expect.objectContaining({ id: 'old' })
     )
   })
 
@@ -1370,7 +1475,9 @@ describe('useDataVisStore managed datastream working copy', () => {
     expect(mockFetchGraphSeries).toHaveBeenCalledWith(
       expect.objectContaining({ id: 'mgd' }),
       expect.any(Date),
-      expect.any(Date)
+      expect.any(Date),
+      undefined,
+      expect.objectContaining({ id: 'mgd' })
     )
   })
 
diff --git a/apps/qc-app/src/store/__tests__/observations.spec.ts b/apps/qc-app/src/store/__tests__/observations.spec.ts
index 4c3a591f..2a9fe618 100644
--- a/apps/qc-app/src/store/__tests__/observations.spec.ts
+++ b/apps/qc-app/src/store/__tests__/observations.spec.ts
@@ -158,3 +158,70 @@ describe('useObservationStore.fetchObservationsInRange overlapping requests', ()
     expect((await next).dataX.length).toBe(5)
   })
 })
+
+describe('useObservationStore.fetchObservationsInRange coverage', () => {
+  const requested = () =>
+    (fetchObservationsSync as any).mock.calls.map(
+      ([, b, e]: [unknown, Date, Date]) => [b.getTime(), e.getTime()]
+    )
+
+  it('skips the excluded stretch and leaves a gap in the record', async () => {
+    const store = useObservationStore()
+    const rec = await store.fetchObservationsInRange(
+      datastream,
+      new Date(0),
+      new Date(10),
+      { begin: new Date(3), end: new Date(6) }
+    )
+    expect(requested()).toEqual([
+      [0, 2],
+      [7, 10],
+    ])
+    expect(Array.from(rec.dataX)).toEqual([0, 1, 2, 7, 8, 9, 10])
+  })
+
+  it('fetches only the gap once it is asked for', async () => {
+    const store = useObservationStore()
+    await store.fetchObservationsInRange(datastream, new Date(0), new Date(10), {
+      begin: new Date(3),
+      end: new Date(6),
+    })
+    ;(fetchObservationsSync as any).mockClear()
+    const rec = await store.fetchObservationsInRange(
+      datastream,
+      new Date(0),
+      new Date(10)
+    )
+    expect(requested()).toEqual([[3, 6]])
+    expect(Array.from(rec.dataX)).toEqual(ALL_TIMES)
+  })
+
+  it('does not ask again for a range that held no data', async () => {
+    const store = useObservationStore()
+    await store.fetchObservationsInRange(datastream, new Date(100), new Date(200))
+    ;(fetchObservationsSync as any).mockClear()
+    await store.fetchObservationsInRange(datastream, new Date(100), new Date(200))
+    expect(fetchObservationsSync).not.toHaveBeenCalled()
+  })
+
+  it('keeps one copy of a timestamp two ranges both returned', async () => {
+    ;(fetchObservationsSync as any)
+      .mockImplementationOnce(async () => ({ datetimes: [0, 1, 2], dataValues: [0, 10, 20] }))
+      .mockImplementationOnce(async () => ({ datetimes: [2, 3], dataValues: [99, 30] }))
+    const store = useObservationStore()
+    await store.fetchObservationsInRange(datastream, new Date(0), new Date(2))
+    const rec = await store.fetchObservationsInRange(datastream, new Date(0), new Date(3))
+    expect(Array.from(rec.dataX)).toEqual([0, 1, 2, 3])
+    expect(Array.from(rec.dataY)).toEqual([0, 10, 20, 30])
+  })
+
+  it('asks again for a range whose fetch failed', async () => {
+    ;(fetchObservationsSync as any).mockRejectedValueOnce(new Error('offline'))
+    const store = useObservationStore()
+    await expect(
+      store.fetchObservationsInRange(datastream, new Date(0), new Date(10))
+    ).rejects.toThrow('offline')
+    const rec = await store.fetchObservationsInRange(datastream, new Date(0), new Date(10))
+    expect(rec.dataX.length).toBe(11)
+  })
+})
diff --git a/apps/qc-app/src/store/__tests__/plotly.spec.ts b/apps/qc-app/src/store/__tests__/plotly.spec.ts
index 502e18f0..ced16dbf 100644
--- a/apps/qc-app/src/store/__tests__/plotly.spec.ts
+++ b/apps/qc-app/src/store/__tests__/plotly.spec.ts
@@ -38,7 +38,7 @@ vi.mock('@/store/observations', () => ({
 }))
 
 // Inline constants to break circular dep: options.ts → usePlotlyStore → plotly barrel.
-const SOURCE_CONTEXT_COLOR = '#9e9e9e'
+const SOURCE_CONTEXT_COLOR = '#cfcfcf'
 const SOURCE_CONTEXT_LABEL_COLOR = '#616161'
 
 vi.mock('@/utils/plotting/plotly', () => ({
@@ -589,7 +589,7 @@ describe('usePlotlyStore.editHistory', () => {
   })
 })
 
-describe('usePlotlyStore.redraw shapes', () => {
+describe('usePlotlyStore.redraw', () => {
   beforeEach(() => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
@@ -597,38 +597,15 @@ describe('usePlotlyStore.redraw shapes', () => {
     sourceId.value = null
   })
 
-  it('keeps a live stage shape and replaces the edit-window shape', async () => {
+  it('leaves the live shapes alone', async () => {
     const mod = await import('@/utils/plotting/plotly')
-    const freshWindow = { name: 'edit-window', x0: 300, x1: 400 }
     const { usePlotlyStore } = await import('@/store/plotly')
     const store = usePlotlyStore()
-    ;(mod.createPlotlyOption as any).mockReturnValueOnce({
-      traces: [],
-      layout: { shapes: [freshWindow] },
-    })
-    const stage = { name: 'stage', x0: 10, x1: 20 }
-    store.plotlyRef = {
-      layout: { shapes: [stage, { name: 'edit-window', x0: 100, x1: 200 }] },
-    } as any
-
-    await store.redraw()
-
-    const layout = (mod.applyTraceUpdate as any).mock.calls.at(-1)[2]
-    expect(layout.shapes).toEqual([stage, freshWindow])
-  })
-
-  it('drops a stale edit-window shape once the layout has none', async () => {
-    const mod = await import('@/utils/plotting/plotly')
-    const { usePlotlyStore } = await import('@/store/plotly')
-    const store = usePlotlyStore()
-    const stage = { name: 'stage', x0: 10, x1: 20 }
-    store.plotlyRef = {
-      layout: { shapes: [stage, { name: 'edit-window', x0: 100, x1: 200 }] },
-    } as any
+    store.plotlyRef = { layout: { shapes: [{ name: 'stage' }] } } as any
 
     await store.redraw()
 
     const layout = (mod.applyTraceUpdate as any).mock.calls.at(-1)[2]
-    expect(layout.shapes).toEqual([stage])
+    expect(layout).not.toHaveProperty('shapes')
   })
 })
diff --git a/apps/qc-app/src/store/__tests__/workingCopies.spec.ts b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
index 0b5face4..fc727544 100644
--- a/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
+++ b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
@@ -7,9 +7,9 @@ import { unwrap } from '@/services/qualityControl/unwrap'
 const hs = ref<any>(null)
 vi.mock('@/store/hydroserver', () => ({ useHydroServer: () => ({ hs }) }))
 
-const fetchObservationsInRange = vi.fn()
+const fetchDetachedRecord = vi.fn()
 vi.mock('@/store/observations', () => ({
-  useObservationStore: () => ({ fetchObservationsInRange }),
+  useObservationStore: () => ({ fetchDetachedRecord }),
 }))
 
 const applyHistory = vi.fn(async () => ({ applied: 0, failed: [] }))
@@ -41,7 +41,7 @@ function deferred<T>() {
 }
 
 /**
- * Gates every `fetchObservationsInRange` call behind `gate`, and resolves
+ * Gates every `fetchDetachedRecord` call behind `gate`, and resolves
  * `started` the first time a call is made, so a test can wait for a
  * build to have actually begun (past its generation bump) before racing
  * another store call against it.
@@ -49,7 +49,7 @@ function deferred<T>() {
 function gateFetch() {
   const gate = deferred<void>()
   const started = deferred<void>()
-  fetchObservationsInRange.mockImplementation(async (ds: any) => {
+  fetchDetachedRecord.mockImplementation(async (ds: any) => {
     started.resolve()
     await gate.promise
     return ds.id === 'm-1' ? rec([]) : rec([1, 2, 3])
@@ -83,7 +83,7 @@ beforeEach(async () => {
     await qc.histories.create({ managedDatastreamId: 'm-1', sourceDatastreamId: 's-1' })
   ).id
   // Managed datastream empty, source has data.
-  fetchObservationsInRange.mockImplementation(async (ds: any) =>
+  fetchDetachedRecord.mockImplementation(async (ds: any) =>
     ds.id === 'm-1' ? rec([]) : rec([1, 2, 3])
   )
 })
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index ff13921e..014ef9ba 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -15,6 +15,7 @@ import {
   type TimeWindow,
 } from '@/utils/timeRangePresets'
 import { isSnapshotId } from '@/utils/snapshotId'
+import { isContextId, makeContextId } from '@/utils/contextSeriesId'
 import { useWorkingCopiesStore } from '@/store/workingCopies'
 import { useQcSessionStore } from '@/store/qcSession'
 import type { SnapshotMeta } from '@/types'
@@ -161,14 +162,36 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
       : null
   )
 
-  /** The edit target's source, drawn behind it as context. */
-  const sourceContextDatastream = computed(() =>
+  /** The edit target's source datastream. */
+  const editSourceDatastream = computed(() =>
     qcDatastreamId.value
       ? (managedContext(qcDatastreamId.value)?.source ?? null)
       : null
   )
 
-  /** What the plot draws, in order: edit target, its source, then plotted. */
+  /** Whether the edit target's source is drawn around it as context. A
+   *  user preference, like the context range. */
+  const showSourceContext = ref(true)
+
+  /** Turn the source context on or off, redrawing when editing. */
+  async function setShowSourceContext(show: boolean) {
+    if (show === showSourceContext.value) return
+    showSourceContext.value = show
+    if (qcDatastreamId.value) await rebuildPlot()
+  }
+
+  /** The grey source context drawn around the edit target, cut around the
+   *  session window: the source under a context id of its own (see
+   *  `makeContextId`), so the user can also plot the source itself as an
+   *  ordinary series. Both read the source's data. Null when context is off. */
+  const sourceContextDatastream = computed<Datastream | null>(() => {
+    const source = editSourceDatastream.value
+    if (!source || !showSourceContext.value) return null
+    return { ...source, id: makeContextId(source.id) } as Datastream
+  })
+
+  /** What the plot draws, in order: edit target, its source context, then
+   *  plotted. */
   const seriesDatastreams = computed<Datastream[]>(() => {
     const edit = qcDatastream.value
     if (!edit) return plottedDatastreams.value
@@ -210,7 +233,21 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   // data to anchor a preset to, so the window is a placeholder.
   const endDate = ref<Date>(new Date())
   const beginDate = ref<Date>(subtractMonths(endDate.value, 1))
+  /** The Select view's Time range preset. */
   const selectedDateBtnId = ref(DEFAULT_PRESET_ID)
+  /** The editor's Context range preset, remembered apart from the Select
+   *  view's so neither moves the other. */
+  const contextPresetId = ref(DEFAULT_PRESET_ID)
+  /** The preset the loaded range follows: Context while an edit target is
+   *  set, else the Select view's Time range. */
+  const activePresetId = computed({
+    get: () =>
+      qcDatastreamId.value ? contextPresetId.value : selectedDateBtnId.value,
+    set: (id: number) => {
+      if (qcDatastreamId.value) contextPresetId.value = id
+      else selectedDateBtnId.value = id
+    },
+  })
 
   /** The edit session's window, while an edit target has one. */
   const editSessionWindow = computed<TimeWindow | null>(() => {
@@ -230,7 +267,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
    *  the session window while editing one, otherwise back from the context
    *  data's end. */
   function resolvePresetWindow() {
-    if (selectedDateBtnId.value === CUSTOM_PRESET_ID) return null
+    if (activePresetId.value === CUSTOM_PRESET_ID) return null
     const workingCopies = useWorkingCopiesStore()
     const context = seriesDatastreams.value.filter(
       (d) => d.id !== qcDatastreamId.value && !isSnapshotId(d.id)
@@ -241,9 +278,9 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     ])
     const sessionWindow = editSessionWindow.value
     if (sessionWindow) {
-      return presetAroundWindow(selectedDateBtnId.value, sessionWindow, extent)
+      return presetAroundWindow(activePresetId.value, sessionWindow, extent)
     }
-    return extent ? presetWindow(selectedDateBtnId.value, extent) : null
+    return extent ? presetWindow(activePresetId.value, extent) : null
   }
 
   function resetState() {
@@ -448,12 +485,22 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   const MAX_RANGE_PASSES = 5
   let loadInFlight: Promise<void> | null = null
   let queuedLoad: { kind: PlotLoadKind; promise: Promise<void> } | null = null
-  /** The range the series were last loaded for, as `begin-end` ms. */
+  /** The `loadKey` the series were last loaded for. */
   let loadedRangeKey: string | null = null
 
   const rangeKey = (begin: Date, end: Date) =>
     `${begin.getTime()}-${end.getTime()}`
 
+  /** What a load covers: the range, plus the session window the source's
+   *  context is cut around. */
+  const loadKey = () => {
+    const w = editSessionWindow.value
+    return (
+      rangeKey(beginDate.value, endDate.value) +
+      (w ? `|${rangeKey(w.begin, w.end)}` : '')
+    )
+  }
+
   function queuePlotLoad(kind: PlotLoadKind): Promise<void> {
     return trackPlotWork(() => nextPlotLoad(kind))
   }
@@ -498,9 +545,10 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     for (let pass = 0; pass < MAX_RANGE_PASSES; pass++) {
       const begin = beginDate.value
       const end = endDate.value
+      const key = loadKey()
       await refreshGraphSeriesArray()
       if (isCurrentRange(begin, end)) {
-        loadedRangeKey = rangeKey(begin, end)
+        loadedRangeKey = key
         return
       }
     }
@@ -541,7 +589,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
    *  load already caught up with the range. */
   async function doReloadRange(): Promise<void> {
     if (!seriesDatastreams.value.length) return
-    if (loadedRangeKey === rangeKey(beginDate.value, endDate.value)) return
+    if (loadedRangeKey === loadKey()) return
     await loadCurrentRange()
     const { redraw, clearZoomHistory } = usePlotlyStore()
     if (qcDatastreamId.value) {
@@ -628,7 +676,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
 
     if (begin) beginDate.value = begin
     if (end) endDate.value = end
-    if (custom) selectedDateBtnId.value = CUSTOM_PRESET_ID
+    if (custom) activePresetId.value = CUSTOM_PRESET_ID
 
     if (update && seriesDatastreams.value.length) {
       await queuePlotLoad('range')
@@ -646,23 +694,25 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
 
   const onDateBtnClick = async (selectedId: number) => {
     if (!findPreset(selectedId)) return
-    selectedDateBtnId.value = selectedId
+    activePresetId.value = selectedId
     await applyActivePreset()
   }
 
   // The editor loads its context before the session is known, so re-anchor
-  // the preset once the window arrives or changes. Like a Context change, this
-  // reloads context series only and keeps the zoom. A watch rather than a call
-  // at each session store write: the window follows sessions being applied, a
-  // session viewed, a return to the current one and a failed view reverted.
-  // It queues behind any rebuild, and joins a queued one.
+  // the preset once the window arrives or changes. The source waits for the
+  // window (its context is cut around it), so its series can appear here, and
+  // only a rebuild adds traces. The rebuild resolves the preset itself, never
+  // fetches the edit target, keeps the zoom, and reloads only what the cache
+  // lacks. A watch rather than a call at each session store write: the window
+  // follows sessions being applied, a session viewed, a return to the current
+  // one and a failed view reverted. It queues behind any load.
   watch(
     () =>
       editSessionWindow.value &&
       rangeKey(editSessionWindow.value.begin, editSessionWindow.value.end),
     (key) => {
       if (!key) return
-      applyActivePreset().catch((error) => {
+      rebuildPlot().catch((error) => {
         console.error('Failed to reload the context range:', error)
         Snackbar.error('Could not reload the context range')
       })
@@ -690,10 +740,15 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   const updateOrFetchGraphSeries = async (
     datastream: Datastream,
     start: Date,
-    end: Date
+    end: Date,
+    exclude?: TimeWindow
   ) => {
     const load = {}
     latestLoads.set(datastream.id, load)
+    // The source context series reads the source's own data.
+    const fetchDs = isContextId(datastream.id)
+      ? (editSourceDatastream.value ?? datastream)
+      : datastream
     try {
       // A managed datastream with a session in progress plots its working
       // copy: it spans the session window and is never re-windowed, since a
@@ -713,9 +768,10 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
       if (seriesIndex >= 0) {
         // Update the existing graph series with new data
         const obsRecord = await fetchObservationsInRange(
-          datastream,
+          fetchDs,
           start,
-          end
+          end,
+          exclude
         ).catch((error) => {
           Snackbar.error('Failed to fetch observations')
           console.error('Failed to fetch observations:', error)
@@ -727,7 +783,13 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
         const series = graphSeriesArray.value.find((s) => s.id === datastream.id)
         if (obsRecord && series) series.data = obsRecord
       } else {
-        const newSeries = await fetchGraphSeries(datastream, start, end)
+        const newSeries = await fetchGraphSeries(
+          datastream,
+          start,
+          end,
+          exclude,
+          fetchDs
+        )
         if (!isCurrentRange(start, end)) return
         // Callers of `refreshGraphSeriesArray` outside the plot load queue
         // (the edit history reload) may have pushed this series meanwhile. A
@@ -774,12 +836,22 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
       currentIds.has(s.id)
     )
 
+    // The source is drawn only around the session window, so it waits for
+    // the window and never fetches what lies inside it.
+    const sourceId = sourceContextDatastream.value?.id
+    const sessionWindow = editSessionWindow.value
     const updateOrFetchPromises = seriesDatastreams.value
       // Snapshots have no server datastream; the edit target's data belongs to the session.
       .filter((ds) => !isSnapshotId(ds.id) && ds.id !== qcDatastreamId.value)
+      .filter((ds) => ds.id !== sourceId || !!sessionWindow)
       .map(async (ds) => {
         loadingStates.value.set(ds.id, true)
-        return updateOrFetchGraphSeries(ds, beginDate.value, endDate.value)
+        return updateOrFetchGraphSeries(
+          ds,
+          beginDate.value,
+          endDate.value,
+          ds.id === sourceId ? (sessionWindow ?? undefined) : undefined
+        )
       })
 
     const results = await Promise.all(updateOrFetchPromises)
@@ -856,8 +928,14 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     isEditorReady,
     trackPlotWork,
     selectedDateBtnId,
+    contextPresetId,
+    activePresetId,
+    editSessionWindow,
+    showSourceContext,
+    setShowSourceContext,
     qcDatastream,
     qcDatastreamId,
+    editSourceDatastream,
     sourceContextDatastream,
     seriesDatastreams,
     qualifierSet,
@@ -890,15 +968,21 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   // Persist only the user's preset choice. Catalogs, loading maps and
   // filters refetch cleanly on every load; the window resolves from the data.
   persist: {
-    pick: ['selectedDateBtnId'],
+    pick: ['selectedDateBtnId', 'contextPresetId', 'showSourceContext'],
     // A persisted Custom id (or a stale/unknown one) comes back with no
     // window to resolve against, so the placeholder range would apply
     // instead. Only a real preset survives hydration.
     afterHydrate: (ctx) => {
-      const store = ctx.store as unknown as { selectedDateBtnId: number }
+      const store = ctx.store as unknown as {
+        selectedDateBtnId: number
+        contextPresetId: number
+      }
       if (!findPreset(store.selectedDateBtnId)) {
         store.selectedDateBtnId = DEFAULT_PRESET_ID
       }
+      if (!findPreset(store.contextPresetId)) {
+        store.contextPresetId = DEFAULT_PRESET_ID
+      }
     },
   },
 })
diff --git a/apps/qc-app/src/store/observations.ts b/apps/qc-app/src/store/observations.ts
index 2f39f82e..c52168a0 100644
--- a/apps/qc-app/src/store/observations.ts
+++ b/apps/qc-app/src/store/observations.ts
@@ -3,12 +3,55 @@ import { ref } from 'vue'
 import { fetchObservationsSync } from '@/utils/observations'
 import { ObservationRecord } from "@uwrl/qc-utils"
 import { Datastream } from '@hydroserver/client'
+import {
+  mergeIntervals,
+  subtractIntervals,
+  type Interval,
+} from '@/utils/timeIntervals'
 
 export type ObservationData = {
   datetimes: Float64Array<ArrayBuffer>
   dataValues: Float32Array<ArrayBuffer>
 }
 
+type FetchedChunk = { datetimes: number[]; dataValues: number[] }
+
+/**
+ * The cache with the fetched `chunks` merged in, in time order. Chunks come
+ * from disjoint ranges in ascending order; a timestamp the cache already
+ * holds (a range endpoint fetched twice) keeps the cached value.
+ */
+function mergeObservations(
+  cached: ObservationData,
+  chunks: FetchedChunk[]
+): ObservationData {
+  const addedX = chunks.flatMap((c) => c.datetimes)
+  const addedY = chunks.flatMap((c) => c.dataValues)
+  const oldX = cached.datetimes
+  const oldY = cached.dataValues
+  const x = new Float64Array(oldX.length + addedX.length)
+  const y = new Float32Array(oldY.length + addedY.length)
+  let i = 0
+  let j = 0
+  let n = 0
+  while (i < oldX.length || j < addedX.length) {
+    const takeOld =
+      j >= addedX.length ||
+      (i < oldX.length && (oldX[i] as number) <= (addedX[j] as number))
+    if (takeOld) {
+      if (j < addedX.length && oldX[i] === addedX[j]) j++
+      x[n] = oldX[i] as number
+      y[n++] = oldY[i++] as number
+    } else {
+      x[n] = addedX[j] as number
+      y[n++] = addedY[j++] as number
+    }
+  }
+  return n === x.length
+    ? { datetimes: x, dataValues: y }
+    : { datetimes: x.slice(0, n), dataValues: y.slice(0, n) }
+}
+
 export const useObservationStore = defineStore(
   'observations',
   () => {
@@ -30,35 +73,30 @@ export const useObservationStore = defineStore(
     // }
 
     /** The last request queued per datastream. */
-    const queued = new Map<
-      string,
-      { begin: number; end: number; promise: Promise<ObservationRecord> }
-    >()
+    const queued = new Map<string, { key: string; promise: Promise<unknown> }>()
+
+    /** The ranges already asked of the server per datastream, found or not. */
+    const covered = new Map<string, Interval[]>()
+
+    type Exclude = { begin: Date; end: Date }
 
     /**
-     * Fetches requested observations that aren't currently in the pinia store,
-     * updates the store, then returns the corresponding `ObservationRecord`.
-     *
-     * Requests for one datastream run in order: each fills the cache from what
-     * the previous one left, and the record ends on the latest requested
-     * window. A request matching the last queued range shares it.
+     * Run `task` after every request already queued for the datastream, so
+     * each fills the cache from what the previous one left and the shared
+     * record ends on the latest requested window. A request matching the
+     * last queued one (same `key`) shares it.
      */
-    const fetchObservationsInRange = (
-      datastream: Datastream,
-      beginTime: Date,
-      endTime: Date
-    ): Promise<ObservationRecord> => {
-      const id = datastream.id
-      const begin = beginTime.getTime()
-      const end = endTime.getTime()
+    const enqueue = <T>(
+      id: string,
+      key: string,
+      task: () => Promise<T>
+    ): Promise<T> => {
       const last = queued.get(id)
-      if (last && last.begin === begin && last.end === end) return last.promise
+      if (last?.key === key) return last.promise as Promise<T>
 
       const previous = last?.promise.catch(() => undefined) ?? Promise.resolve()
-      const promise = previous.then(() =>
-        loadRange(datastream, beginTime, endTime)
-      )
-      const entry = { begin, end, promise }
+      const promise = previous.then(task)
+      const entry = { key, promise }
       queued.set(id, entry)
       const release = () => {
         if (queued.get(id) === entry) queued.delete(id)
@@ -67,126 +105,106 @@ export const useObservationStore = defineStore(
       return promise
     }
 
-    const loadRange = async (
+    const requestKey = (
+      kind: string,
+      beginTime: Date,
+      endTime: Date,
+      exclude?: Exclude
+    ) =>
+      [
+        kind,
+        beginTime.getTime(),
+        endTime.getTime(),
+        exclude?.begin.getTime(),
+        exclude?.end.getTime(),
+      ].join(':')
+
+    /** Fill the cache for `[beginTime, endTime]`, never requesting `exclude`,
+     *  and resolve with the whole cache. */
+    const loadMissing = async (
       datastream: Datastream,
       beginTime: Date,
-      endTime: Date
-    ): Promise<ObservationRecord> => {
+      endTime: Date,
+      exclude?: Exclude
+    ): Promise<ObservationData> => {
       const id = datastream.id
-
-      let beginDataPromise: Promise<{
-        datetimes: number[]
-        dataValues: number[]
-      }> = Promise.resolve({ datetimes: [], dataValues: [] })
-      let endDataPromise: Promise<{
-        datetimes: number[]
-        dataValues: number[]
-      }> = Promise.resolve({ datetimes: [], dataValues: [] })
-
-      if (observationsRaw.value[id]?.dataValues.length) {
-        const rawBeginDatetime = new Date(
-          observationsRaw.value[id].datetimes[0] as number
-        )
-
-        // Strict `<`: skip the request entirely when the requested
-        // begin is at or inside the cached window. `<=` used to fire a
-        // 1-second range request on exact matches (e.g. re-clicking
-        // the same preset), which is pure waste.
-        if (beginTime < rawBeginDatetime) {
-          // Results in range will be inclusive, so we need to offset by 1
-          rawBeginDatetime.setSeconds(rawBeginDatetime.getSeconds() - 1)
-          beginDataPromise = fetchObservationsSync(
-            datastream,
-            beginTime,
-            rawBeginDatetime
-          )
-        }
-
-        const rawEndDatetime = new Date(
-          observationsRaw.value[id].datetimes[
-          observationsRaw.value[id].datetimes.length - 1
-          ] as number
+      const wanted = subtractIntervals(
+        [[beginTime.getTime(), endTime.getTime()]],
+        exclude ? [[exclude.begin.getTime(), exclude.end.getTime()]] : []
+      )
+      const missing = subtractIntervals(wanted, covered.get(id) ?? [])
+      const chunks = await Promise.all(
+        missing.map(([start, end]) =>
+          fetchObservationsSync(datastream, new Date(start), new Date(end))
         )
+      )
+      covered.set(id, mergeIntervals([...(covered.get(id) ?? []), ...missing]))
 
-        // Same note as above: only fetch the trailing segment when the
-        // requested end is *past* what we've already cached.
-        if (endTime > rawEndDatetime) {
-          rawEndDatetime.setSeconds(rawEndDatetime.getSeconds() + 1)
-          endDataPromise = fetchObservationsSync(
-            datastream,
-            rawEndDatetime,
-            endTime
-          )
-        }
-      } else {
-        beginDataPromise = fetchObservationsSync(datastream, beginTime, endTime)
+      const cached = observationsRaw.value[id] ?? {
+        datetimes: new Float64Array(0),
+        dataValues: new Float32Array(0),
       }
+      observationsRaw.value[id] = chunks.some((c) => c.dataValues.length)
+        ? mergeObservations(cached, chunks)
+        : cached
+      return observationsRaw.value[id]
+    }
 
-      const [beginData, endData] = await Promise.all([
-        beginDataPromise,
-        endDataPromise,
-      ])
-
-      if (!observationsRaw.value[id]) {
-        observationsRaw.value[id] = {
-          datetimes: new Float64Array(0),
-          dataValues: new Float32Array(0),
+    /**
+     * The shared record for a datastream, windowed to `[beginTime, endTime]`.
+     * The plot draws it, so windowing it moves what the plot shows. `exclude`
+     * is a stretch inside the window not to fetch; whatever the cache already
+     * holds there stays in the record.
+     */
+    const fetchObservationsInRange = (
+      datastream: Datastream,
+      beginTime: Date,
+      endTime: Date,
+      exclude?: Exclude
+    ): Promise<ObservationRecord> => {
+      const id = datastream.id
+      return enqueue(
+        id,
+        requestKey('shared', beginTime, endTime, exclude),
+        async () => {
+          const raw = await loadMissing(datastream, beginTime, endTime, exclude)
+          if (!observations.value[id]) {
+            observations.value[id] = new ObservationRecord(raw)
+          }
+          const obsRecord = observations.value[id] as ObservationRecord
+          // A no-op when neither the window nor the cache changed, so
+          // unrelated replots keep their edits/history.
+          await obsRecord.applyWindow(beginTime.getTime(), endTime.getTime(), raw)
+          return obsRecord
         }
-      }
-
-      if (beginData.dataValues.length > 0 || endData.dataValues.length > 0) {
-        const newLength =
-          beginData.dataValues.length +
-          endData.dataValues.length +
-          observationsRaw.value[id].dataValues.length
-        const newBufferX = new ArrayBuffer(
-          newLength * Float64Array.BYTES_PER_ELEMENT
-        )
-        const newBufferY = new ArrayBuffer(
-          newLength * Float32Array.BYTES_PER_ELEMENT
-        )
-
-        const newArrayX = new Float64Array(newBufferX)
-        const newArrayY = new Float32Array(newBufferY)
-
-        // Begin data
-        let offset = 0
-        newArrayX.set(beginData.datetimes, offset)
-        newArrayY.set(beginData.dataValues, offset)
-
-        // Previous data
-        offset += beginData.datetimes.length
-        newArrayX.set(observationsRaw.value[id].datetimes, offset)
-        newArrayY.set(observationsRaw.value[id].dataValues, offset)
-
-        // End data
-        offset += endData.datetimes.length
-        newArrayX.set(endData.datetimes, offset)
-        newArrayY.set(endData.dataValues, offset)
-
-        observationsRaw.value[id].datetimes = newArrayX
-        observationsRaw.value[id].dataValues = newArrayY
-      }
-
-      // If nothing is stored yet, create a new record
-      if (!observations.value[id] && observationsRaw.value[datastream.id]) {
-        observations.value[id] = new ObservationRecord(observationsRaw.value[datastream.id] as ObservationData)
-      }
-
-      const obsRecord = observations.value[id] as ObservationRecord
-      // `rawData` is the full accumulated cache; the record slices it to the
-      // selected window (`applyWindow` no-ops when the window is unchanged,
-      // so unrelated replots keep their edits/history).
-      obsRecord.rawData = observationsRaw.value[id]
-      await obsRecord.applyWindow(beginTime.getTime(), endTime.getTime())
+      )
+    }
 
-      return obsRecord
+    /**
+     * A record of its own over `[beginTime, endTime]`, filled from the shared
+     * cache. For work that builds on the data (a working copy, a snapshot):
+     * it never re-windows the shared record the plot draws.
+     */
+    const fetchDetachedRecord = async (
+      datastream: Datastream,
+      beginTime: Date,
+      endTime: Date
+    ): Promise<ObservationRecord> => {
+      const raw = await enqueue(
+        datastream.id,
+        requestKey('detached', beginTime, endTime),
+        () => loadMissing(datastream, beginTime, endTime)
+      )
+      const record = new ObservationRecord(raw)
+      await record.applyWindow(beginTime.getTime(), endTime.getTime())
+      return record
     }
 
     return {
       observations,
       observationsRaw,
       fetchObservationsInRange,
+      fetchDetachedRecord,
     }
   },
   {
diff --git a/apps/qc-app/src/store/plotly.ts b/apps/qc-app/src/store/plotly.ts
index e0b817a6..66399c2c 100644
--- a/apps/qc-app/src/store/plotly.ts
+++ b/apps/qc-app/src/store/plotly.ts
@@ -16,7 +16,6 @@ import {
   SOURCE_CONTEXT_COLOR,
   SOURCE_CONTEXT_LABEL_COLOR,
 } from '@/utils/plotting/plotly'
-import { withLiveShapes } from '@/utils/plotting/shapes'
 import type {
   AppPlotlyHTMLElement,
   AppPlotlyTrace,
@@ -185,6 +184,15 @@ export const usePlotlyStore = defineStore('Plotly', () => {
     () => selectedSeries.value?.data?.history ?? []
   )
 
+  /**
+   * The earlier history step the edit target shows, or null when it shows
+   * the whole history. Previewing only shows a step, so edits wait until
+   * the user is back on the latest one.
+   */
+  const previewIndex = computed<number | null>(
+    () => selectedSeries.value?.data?.previewIndex ?? null
+  )
+
   // Initialize to an empty-trace PlotlyChartOptions so consumers can read
   // `plotlyOptions.value.traces` etc. without null-guards. 
   const plotlyOptions: Ref<PlotlyChartOptions> = ref(createPlotlyOption([]))
@@ -332,9 +340,17 @@ export const usePlotlyStore = defineStore('Plotly', () => {
         | undefined)
       : undefined
     if (liveLayout) {
+      // A y axis with no points drawn sits on Plotly's default range, not a
+      // view the user chose, so it autoranges to the new data instead.
+      const drawnAxes = new Set(
+        ((plotlyRef.value?.data ?? []) as AppPlotlyTrace[])
+          .filter((t) => (t.x as ArrayLike<unknown> | undefined)?.length)
+          .map((t) => `yaxis${String(t.yaxis ?? 'y').slice(1)}`)
+      )
       const layoutRecord = opts.layout as Record<string, unknown>
       for (const key of Object.keys(layoutRecord)) {
         if (key !== 'xaxis' && !key.startsWith('yaxis')) continue
+        if (key.startsWith('yaxis') && !drawnAxes.has(key)) continue
         const nextAxis = layoutRecord[key] as Partial<LayoutAxis> | undefined
         const liveAxis = liveLayout[key] as Partial<LayoutAxis> | undefined
         const liveRange = liveAxis?.range as
@@ -358,8 +374,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
         x: opts.traces.map((t) => (t as AppPlotlyTrace).x),
         y: opts.traces.map((t) => (t as AppPlotlyTrace).y),
       } as unknown as Partial<PlotData>,
-      // `Plotly.update` replaces the whole shapes array; keep the stage band.
-      withLiveShapes(opts.layout, plotlyRef.value?.layout)
+      opts.layout
     )
 
     if (recomputeXaxisRange) {
@@ -406,13 +421,17 @@ export const usePlotlyStore = defineStore('Plotly', () => {
     } as GraphSeries
   }
 
+  /** A series for `datastream`, filled with `fetchAs`'s observations (the
+   *  source context series reads the source's). */
   const fetchGraphSeries = async (
     datastream: Datastream,
     start: Date,
-    end: Date
+    end: Date,
+    exclude?: { begin: Date; end: Date },
+    fetchAs: Datastream = datastream
   ): Promise<GraphSeries> => {
     const { fetchObservationsInRange } = useObservationStore()
-    const data = await fetchObservationsInRange(datastream, start, end)
+    const data = await fetchObservationsInRange(fetchAs, start, end, exclude)
     if (!data.dataset.source.x) {
       await data.reload()
     }
@@ -534,6 +553,7 @@ export const usePlotlyStore = defineStore('Plotly', () => {
     selectedSeriesIndex,
     selectedSeries,
     editHistory,
+    previewIndex,
     suppressedEchoSelection,
     updateOptions,
     redraw,
diff --git a/apps/qc-app/src/store/workingCopies.ts b/apps/qc-app/src/store/workingCopies.ts
index 454cbcc1..fb4da27c 100644
--- a/apps/qc-app/src/store/workingCopies.ts
+++ b/apps/qc-app/src/store/workingCopies.ts
@@ -89,12 +89,12 @@ export const useWorkingCopiesStore = defineStore('workingCopies', () => {
     generation: number
   ): Promise<WorkingCopy | null> {
     const { hs } = storeToRefs(useHydroServer())
-    const { fetchObservationsInRange } = useObservationStore()
+    const { fetchDetachedRecord } = useObservationStore()
     const { record } = await reconstructSession(
       {
         qcSessions: hs.value.qualityControlSessions,
         qcOperations: hs.value.qualityControlOperations,
-        fetchInRange: fetchObservationsInRange,
+        fetchInRange: fetchDetachedRecord,
         applyHistory,
       },
       managed,
diff --git a/apps/qc-app/src/utils/__tests__/timeIntervals.spec.ts b/apps/qc-app/src/utils/__tests__/timeIntervals.spec.ts
new file mode 100644
index 00000000..53542ff6
--- /dev/null
+++ b/apps/qc-app/src/utils/__tests__/timeIntervals.spec.ts
@@ -0,0 +1,39 @@
+import { describe, it, expect } from 'vitest'
+import { mergeIntervals, subtractIntervals } from '../timeIntervals'
+
+describe('mergeIntervals', () => {
+  it('joins overlapping and touching intervals, in order', () => {
+    expect(mergeIntervals([[20, 30], [0, 10], [11, 15], [5, 8]])).toEqual([
+      [0, 15],
+      [20, 30],
+    ])
+  })
+
+  it('keeps separated intervals apart', () => {
+    expect(mergeIntervals([[0, 10], [12, 20]])).toEqual([[0, 10], [12, 20]])
+  })
+})
+
+describe('subtractIntervals', () => {
+  it('cuts a hole out of an interval', () => {
+    expect(subtractIntervals([[0, 100]], [[40, 60]])).toEqual([
+      [0, 39],
+      [61, 100],
+    ])
+  })
+
+  it('trims either end and drops what is fully covered', () => {
+    expect(subtractIntervals([[0, 10], [20, 30], [40, 50]], [[5, 25], [35, 60]])).toEqual([
+      [0, 4],
+      [26, 30],
+    ])
+  })
+
+  it('returns the input when nothing is removed', () => {
+    expect(subtractIntervals([[0, 10]], [])).toEqual([[0, 10]])
+  })
+
+  it('returns nothing when everything is removed', () => {
+    expect(subtractIntervals([[0, 10]], [[0, 10]])).toEqual([])
+  })
+})
diff --git a/apps/qc-app/src/utils/contextSeriesId.ts b/apps/qc-app/src/utils/contextSeriesId.ts
new file mode 100644
index 00000000..8199a36f
--- /dev/null
+++ b/apps/qc-app/src/utils/contextSeriesId.ts
@@ -0,0 +1,15 @@
+/**
+ * Synthetic id for the grey source context drawn around the edit target. It
+ * reads the source's own data, but needs an id of its own so the source can
+ * also be plotted as an ordinary series beside it.
+ */
+
+export const CONTEXT_PREFIX = 'ctx:'
+
+export function makeContextId(sourceId: string): string {
+  return `${CONTEXT_PREFIX}${sourceId}`
+}
+
+export function isContextId(id: string | null | undefined): boolean {
+  return !!id && id.startsWith(CONTEXT_PREFIX)
+}
diff --git a/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
index c4b2939f..35ba166f 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
@@ -80,39 +80,26 @@ describe('handleNewPlot', () => {
     expect(typeof handleNewPlot).toBe('function')
   })
 
-  it('carries other writers shapes through a re-plot', async () => {
+  it('carries the live stage band through a re-plot', async () => {
     const { handleNewPlot } = await import('@/utils/plotting/events')
     plotlyRef.value = fakeGraphDiv({
-      shapes: [
-        { name: 'stage', type: 'rect' },
-        { name: 'edit-window', type: 'rect', x0: 1 },
-      ],
+      shapes: [{ name: 'stage', type: 'rect' }],
     })
-    plotlyOptions.value = {
-      traces: [],
-      layout: { shapes: [{ name: 'edit-window', type: 'rect', x0: 2 }] },
-      config: {},
-    }
+    plotlyOptions.value = { traces: [], layout: {}, config: {} }
 
     await handleNewPlot()
 
     const layout = newPlot.mock.calls[0]?.[2] as { shapes: any[] }
-    // The stage band survives; the window band is replaced by the fresh one.
-    expect(layout.shapes.map((s) => s.name)).toEqual(['stage', 'edit-window'])
-    expect(layout.shapes.find((s) => s.name === 'edit-window').x0).toBe(2)
+    expect(layout.shapes.map((s) => s.name)).toEqual(['stage'])
   })
 
   it('draws a first mount with the layout it was given', async () => {
     const { handleNewPlot } = await import('@/utils/plotting/events')
-    plotlyOptions.value = {
-      traces: [],
-      layout: { shapes: [{ name: 'edit-window', type: 'rect' }] },
-      config: {},
-    }
+    const layout = { dragmode: 'pan' }
+    plotlyOptions.value = { traces: [], layout, config: {} }
 
     await handleNewPlot(fakeGraphDiv({}) as unknown as HTMLElement)
 
-    const layout = newPlot.mock.calls[0]?.[2] as { shapes: any[] }
-    expect(layout.shapes.map((s) => s.name)).toEqual(['edit-window'])
+    expect(newPlot.mock.calls[0]?.[2]).toStrictEqual(layout)
   })
 })
diff --git a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
index 3917f11d..3ac1ef87 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
@@ -16,6 +16,16 @@ vi.mock('@uwrl/qc-utils', () => ({
     }
     return lo
   },
+  findLastLessOrEqual: (arr: number[], target: number) => {
+    let lo = 0
+    let hi = arr.length
+    while (lo < hi) {
+      const mid = (lo + hi) >>> 1
+      if (arr[mid] > target) hi = mid
+      else lo = mid + 1
+    }
+    return lo - 1
+  },
   EnumFilterOperations: { SELECTION: 'SELECTION' },
 }))
 
@@ -24,6 +34,7 @@ const sourceContextDatastream = ref<{ id: string } | null>(null)
 const beginDate = ref<Date | null>(null)
 const endDate = ref<Date | null>(null)
 const selectedData = ref<number[] | null>(null)
+const editSessionWindow = ref<{ begin: Date; end: Date } | null>(null)
 
 // `vi.fn()`-wrapped so individual tests can override the return value for
 // one call (`mockReturnValueOnce`) to simulate `createPlotlyOption` being
@@ -36,16 +47,7 @@ vi.mock('@/store/dataVisualization', () => ({
     beginDate,
     endDate,
     selectedData,
-  })),
-}))
-
-const viewedSession = ref<{ phenomenonTimeStart: string; phenomenonTimeEnd: string } | null>(null)
-const inProgressSession = ref<{ phenomenonTimeStart: string; phenomenonTimeEnd: string } | null>(null)
-
-vi.mock('@/store/qcSession', () => ({
-  useQcSessionStore: vi.fn(() => ({
-    viewedSession,
-    inProgressSession,
+    editSessionWindow,
   })),
 }))
 
@@ -99,9 +101,10 @@ import {
   createPlotlyOption,
   findGapIndices,
   insertGapBreaks,
+  splitAroundWindow,
+  bridgeAcrossWindow,
 } from '../options'
 import { useDataVisStore } from '@/store/dataVisualization'
-import { useQcSessionStore } from '@/store/qcSession'
 
 const makeSeries = (overrides: Partial<Record<string, unknown>> = {}) => {
   const x = new Float64Array([1, 2, 3, 4, 5])
@@ -192,8 +195,7 @@ describe('createPlotlyOption', () => {
     setActivePinia(createPinia())
     qcDatastream.value = null
     sourceContextDatastream.value = null
-    viewedSession.value = null
-    inProgressSession.value = null
+    editSessionWindow.value = null
     beginDate.value = null
     endDate.value = null
     isPlotPreview.value = false
@@ -255,12 +257,11 @@ describe('createPlotlyOption', () => {
     // overlay's x/y match the main trace verbatim, but the trace
     // pair is still emitted so the line-drawing path is exercised
     // regardless of whether real gaps are present.
-    qcDatastream.value = { id: 'qc' }
     const qc = makeSeries({ id: 'qc', intendedSpacingMs: 1.5 })
     const opts = createPlotlyOption([qc])
     const main = opts.traces.find((t: any) => t.id === 'qc') as any
     const overlay = opts.traces.find(
-      (t: any) => t._gapOverlayFor === 'qc'
+      (t: any) => t._partOf === 'qc'
     ) as any
     expect(main.mode).toBe('markers')
     expect(overlay).toBeDefined()
@@ -274,7 +275,6 @@ describe('createPlotlyOption', () => {
   })
 
   it('inserts NaN-y break points in the overlay where gaps exceed the spacing', () => {
-    qcDatastream.value = { id: 'qc' }
     const x = new Float64Array([0, 10, 20, 200, 210])
     const y = new Float64Array([1, 2, 3, 4, 5])
     const qc = makeSeries({
@@ -284,7 +284,7 @@ describe('createPlotlyOption', () => {
     })
     const opts = createPlotlyOption([qc])
     const overlay = opts.traces.find(
-      (t: any) => t._gapOverlayFor === 'qc'
+      (t: any) => t._partOf === 'qc'
     ) as any
     const oy = Array.from(overlay.y as Float64Array)
     // Exactly one gap (20 → 200), so exactly one NaN injected.
@@ -383,43 +383,70 @@ describe('createPlotlyOption', () => {
     expect(ids.indexOf('src')).toBeLessThan(ids.indexOf('qc'))
   })
 
-  it('shades the session window while editing', () => {
+  it('draws the edit target as one piece in the QC colour', () => {
     qcDatastream.value = { id: 'qc' }
-    inProgressSession.value = {
-      phenomenonTimeStart: '2025-02-01T00:00:00Z',
-      phenomenonTimeEnd: '2025-03-01T00:00:00Z',
-    }
-    const opts = createPlotlyOption([makeSeries({ id: 'qc' })])
-    const shape = ((opts.layout as any).shapes ?? []).find((s: any) => s.name === 'edit-window')
-    expect(shape.x0).toBe(Date.parse('2025-02-01T00:00:00Z'))
-    expect(shape.x1).toBe(Date.parse('2025-03-01T00:00:00Z'))
-    expect(shape.yref).toBe('paper')
+    editSessionWindow.value = { begin: new Date(2), end: new Date(4) }
+    const opts = createPlotlyOption([makeSeries({ id: 'qc', intendedSpacingMs: 1.5 })])
+    const main = opts.traces.find((t: any) => t.id === 'qc') as any
+    const overlays = opts.traces.filter((t: any) => t._partOf === 'qc') as any[]
+    expect(main.marker.color).toBe(COLORS[0])
+    expect(Array.from(main.x)).toEqual([1, 2, 3, 4, 5])
+    expect(overlays.map((t) => t.line.color)).toEqual([COLORS[0]])
   })
 
-  it('prefers the viewed session window over the in-progress one', () => {
+  it('draws the source around the session window, leaving a gap', () => {
     qcDatastream.value = { id: 'qc' }
-    inProgressSession.value = { phenomenonTimeStart: '2025-02-01T00:00:00Z', phenomenonTimeEnd: '2025-03-01T00:00:00Z' }
-    viewedSession.value = { phenomenonTimeStart: '2025-01-01T00:00:00Z', phenomenonTimeEnd: '2025-01-15T00:00:00Z' }
-    const opts = createPlotlyOption([makeSeries({ id: 'qc' })])
-    const shape = (opts.layout as any).shapes.find((s: any) => s.name === 'edit-window')
-    expect(shape.x0).toBe(Date.parse('2025-01-01T00:00:00Z'))
+    sourceContextDatastream.value = { id: 'src' }
+    editSessionWindow.value = { begin: new Date(2), end: new Date(4) }
+    const src = makeSeries({ id: 'src', intendedSpacingMs: 1.5 })
+    const opts = createPlotlyOption([makeSeries({ id: 'qc' }), src])
+    const main = opts.traces.find((t: any) => t.id === 'src') as any
+    const parts = opts.traces.filter((t: any) => t._partOf === 'src') as any[]
+    const points = parts.filter((t) => !t._isGapOverlay)
+    const lines = parts.filter((t) => t._isGapOverlay)
+
+    expect(Array.from(main.x)).toEqual([1])
+    expect(points.map((t) => Array.from(t.x))).toEqual([[5]])
+    expect(points[0].id).toBeUndefined()
+    expect(points[0].marker.color).toBe(SOURCE_CONTEXT_COLOR)
+    // Reversed: the bridge to the edit target (whose points 1..5 sit a
+    // step from each piece), then the after and before pieces.
+    expect(lines.map((t) => Array.from(t.x))).toEqual([[1, 1, 3, 5, 5], [5], [1]])
+    expect(lines.every((t) => t.line.color === SOURCE_CONTEXT_COLOR)).toBe(true)
+    // Views of the record, not copies.
+    expect(main.x.buffer).toBe(src.data.dataX.buffer)
+    expect(points[0].y.buffer).toBe(src.data.dataY.buffer)
+  })
+
+  it('keeps the source trace count when there is no window yet', () => {
+    qcDatastream.value = { id: 'qc' }
+    sourceContextDatastream.value = { id: 'src' }
+    const opts = createPlotlyOption([
+      makeSeries({ id: 'qc' }),
+      makeSeries({ id: 'src', intendedSpacingMs: 1.5 }),
+    ])
+    const main = opts.traces.find((t: any) => t.id === 'src') as any
+    const parts = opts.traces.filter((t: any) => t._partOf === 'src') as any[]
+    expect(Array.from(main.x)).toEqual([1, 2, 3, 4, 5])
+    // Bridge, after line, before line, after points.
+    expect(parts.map((t) => t.x.length)).toEqual([2, 0, 5, 0])
   })
 
-  it('draws no window without an edit target', () => {
-    inProgressSession.value = { phenomenonTimeStart: '2025-02-01T00:00:00Z', phenomenonTimeEnd: '2025-03-01T00:00:00Z' }
-    const opts = createPlotlyOption([makeSeries({ id: 'a' })])
-    expect(((opts.layout as any).shapes ?? []).some((s: any) => s.name === 'edit-window')).toBe(false)
+  it('draws no shapes', () => {
+    qcDatastream.value = { id: 'qc' }
+    editSessionWindow.value = { begin: new Date(2), end: new Date(4) }
+    const opts = createPlotlyOption([makeSeries({ id: 'qc' })])
+    expect((opts.layout as any).shapes).toBeUndefined()
   })
 
-  it('does not throw when called during store setup, before the data-vis and qcSession refs exist', () => {
+  it('does not throw when called during store setup, before the data-vis refs exist', () => {
     // Regression: `usePlotlyStore` seeds `plotlyOptions` with
     // `createPlotlyOption([])` while its own store setup runs, at which
-    // point `useDataVisStore()` (and `useQcSessionStore()`) can still be
-    // under construction. `storeToRefs` on that in-progress store yields
-    // an object with no properties, so every destructured ref is
-    // `undefined` rather than a ref holding `null`.
+    // point `useDataVisStore()` can still be under construction.
+    // `storeToRefs` on that in-progress store yields an object with no
+    // properties, so every destructured ref is `undefined` rather than a
+    // ref holding `null`.
     vi.mocked(useDataVisStore).mockReturnValueOnce({} as any)
-    vi.mocked(useQcSessionStore).mockReturnValueOnce({} as any)
 
     let opts: ReturnType<typeof createPlotlyOption> | undefined
     expect(() => {
@@ -429,9 +456,6 @@ describe('createPlotlyOption', () => {
     expect(opts!.traces).toEqual([])
     expect(opts!.layout).toBeDefined()
     expect(opts!.config).toBeDefined()
-    expect(
-      ((opts!.layout as any).shapes ?? []).some((s: any) => s.name === 'edit-window')
-    ).toBe(false)
   })
 })
 
@@ -553,3 +577,64 @@ describe('insertGapBreaks', () => {
     expect(ys.filter((v) => Number.isNaN(v)).length).toBe(2)
   })
 })
+
+describe('splitAroundWindow', () => {
+  const line = {
+    x: new Float64Array([1, 2, 3, 4, 5]),
+    y: new Float64Array([10, 20, 30, 40, 50]),
+  }
+  const xs = (l: { x: ArrayLike<number> }) => Array.from(l.x)
+
+  it('drops the points inside the window, bounds included', () => {
+    const { before, after } = splitAroundWindow(line, { begin: 2, end: 4 })
+    expect([xs(before), xs(after)]).toEqual([[1], [5]])
+  })
+
+  it('leaves before empty when the window starts the data', () => {
+    const { before, after } = splitAroundWindow(line, { begin: 0, end: 3 })
+    expect([xs(before), xs(after)]).toEqual([[], [4, 5]])
+  })
+
+  it('leaves after empty when the window ends the data', () => {
+    const { before, after } = splitAroundWindow(line, { begin: 3, end: 9 })
+    expect([xs(before), xs(after)]).toEqual([[1, 2], []])
+  })
+
+  it('keeps every point around a window that holds none', () => {
+    const { before, after } = splitAroundWindow(line, { begin: 3.2, end: 3.8 })
+    expect([xs(before), xs(after)]).toEqual([[1, 2, 3], [4, 5]])
+  })
+
+  it('keeps the whole line in before without a window', () => {
+    const { before, after } = splitAroundWindow(line, null)
+    expect([xs(before), xs(after)]).toEqual([[1, 2, 3, 4, 5], []])
+  })
+
+  it('slices plain arrays too', () => {
+    const { after } = splitAroundWindow({ x: [1, 2, 3], y: [4, 5, 6] }, { begin: 1, end: 2 })
+    expect(Array.from(after.y)).toEqual([6])
+  })
+})
+
+describe('bridgeAcrossWindow', () => {
+  const before = { x: [1, 2], y: [10, 20] }
+  const after = { x: [8, 9], y: [80, 90] }
+  const target = { x: [3, 7], y: [31, 71] }
+
+  it('joins the context to the edit target at both window edges', () => {
+    const { x, y } = bridgeAcrossWindow(before, after, target, 1)
+    expect(x).toEqual([2, 3, 5, 7, 8])
+    expect(y.slice(0, 2)).toEqual([20, 31])
+    expect(Number.isNaN(y[2])).toBe(true)
+    expect(y.slice(3)).toEqual([71, 80])
+  })
+
+  it('leaves a step wider than the cadence open', () => {
+    expect(bridgeAcrossWindow(before, after, { x: [5, 7], y: [0, 0] }, 1).x).toEqual([7, 8])
+  })
+
+  it('draws nothing without an edit target or context', () => {
+    expect(bridgeAcrossWindow(before, after, { x: [], y: [] }, 1).x).toEqual([])
+    expect(bridgeAcrossWindow({ x: [], y: [] }, { x: [], y: [] }, target, 1).x).toEqual([])
+  })
+})
diff --git a/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
index 6d4cfe5b..63dbc06f 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
@@ -272,7 +272,7 @@ const makeStub = (overrides?: {
       marker: { opacity: number }
       hoverinfo: string
       _isGapOverlay: boolean
-      _gapOverlayFor: string
+      _partOf: string
     }>
   >
 }) => ({
@@ -426,7 +426,7 @@ describe('handleRelayout', () => {
           marker: { opacity: 1 },
           hoverinfo: 'skip',
           _isGapOverlay: true,
-          _gapOverlayFor: 'qc-target',
+          _partOf: 'qc-target',
         },
       ],
     })
diff --git a/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts
index 548ec78f..8b21b779 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts
@@ -12,6 +12,7 @@ const selectedSeries = ref<any>(null)
 const isUpdating = ref(false)
 const suppressedEchoSelection = ref<number[] | null>(null)
 
+const previewIndex = ref<number | null>(null)
 vi.mock('@/store/plotly', () => ({
   usePlotlyStore: () => ({
     plotlyRef,
@@ -19,6 +20,7 @@ vi.mock('@/store/plotly', () => ({
     isUpdating,
     editHistory: ref([]),
     suppressedEchoSelection,
+    previewIndex,
   }),
 }))
 
@@ -149,4 +151,19 @@ describe('handleSelected', () => {
     await handleSelected({ points: [] } as any)
     expect(dispatchFilter).toHaveBeenCalledWith('SELECTION', [102])
   })
+
+describe('handleSelected while previewing a history step', () => {
+  it('highlights without recording a SELECTION', async () => {
+    previewIndex.value = 1
+    try {
+      const { handleSelected } = await import('../selected')
+      const dispatchFilter = vi.fn()
+      selectedSeries.value = { data: { dispatchFilter } } as any
+      await handleSelected({ points: [{ pointIndex: 1, curveNumber: 0 }] } as any)
+      expect(dispatchFilter).not.toHaveBeenCalled()
+    } finally {
+      previewIndex.value = null
+    }
+  })
+})
 })
diff --git a/apps/qc-app/src/utils/plotting/__tests__/shapes.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/shapes.spec.ts
index 202b0d97..06dc82a5 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/shapes.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/shapes.spec.ts
@@ -1,41 +1,19 @@
 import { describe, it, expect } from 'vitest'
-import { composeShapes, withLiveShapes } from '../shapes'
+import { withLiveShapes } from '../shapes'
 
 const stage = { name: 'stage', x0: 10, x1: 20 }
-const oldWindow = { name: 'edit-window', x0: 100, x1: 200 }
-const newWindow = { name: 'edit-window', x0: 300, x1: 400 }
-
-describe('composeShapes', () => {
-  it('replaces only the named shapes, after the others by default', () => {
-    expect(composeShapes([stage, oldWindow], 'edit-window', [newWindow])).toEqual([
-      stage,
-      newWindow,
-    ])
-  })
-
-  it('puts its own shapes first when asked', () => {
-    expect(
-      composeShapes([oldWindow, stage], 'stage', [stage], { first: true })
-    ).toEqual([stage, oldWindow])
-  })
-
-  it('handles a plot with no shapes', () => {
-    expect(composeShapes(undefined, 'stage', [])).toEqual([])
-  })
-})
 
 describe('withLiveShapes', () => {
-  it('keeps the live stage shape and takes the fresh edit window', () => {
-    const layout = { dragmode: 'pan', shapes: [newWindow] } as any
-    const merged = withLiveShapes(layout, { shapes: [stage, oldWindow] } as any)
-    expect(merged.shapes).toEqual([stage, newWindow])
+  it('carries the live shapes onto a fresh layout', () => {
+    const layout = { dragmode: 'pan' } as any
+    const merged = withLiveShapes(layout, { shapes: [stage] } as any)
+    expect(merged.shapes).toEqual([stage])
     expect(merged.dragmode).toBe('pan')
     // The stored layout is not mutated.
-    expect(layout.shapes).toEqual([newWindow])
+    expect(layout.shapes).toBeUndefined()
   })
 
-  it('removes the live edit window when the fresh layout has none', () => {
-    const merged = withLiveShapes({}, { shapes: [stage, oldWindow] } as any)
-    expect(merged.shapes).toEqual([stage])
+  it('handles a plot with no shapes', () => {
+    expect(withLiveShapes({}, undefined).shapes).toEqual([])
   })
 })
diff --git a/apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts
index 5fbfaa2b..ffc82ee8 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts
@@ -37,41 +37,29 @@ const applyRelayout = (root: any, update: any) => {
   return Promise.resolve()
 }
 
-describe('staging shape merging', () => {
+describe('staging shapes', () => {
   beforeEach(() => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
     plotlyMock.relayout.mockImplementation(applyRelayout)
     stagePanMode.value = true
-    plotlyRef.value = {
-      layout: {
-        dragmode: 'pan',
-        shapes: [{ name: 'edit-window', x0: 100, x1: 200, yref: 'paper' }],
-      },
-    }
+    plotlyRef.value = { layout: { dragmode: 'pan' } }
   })
 
-  it('keeps the edit-window shape alongside the stage shape when showing the stage band', async () => {
+  it('writes the stage band as the only shape', async () => {
     await setStageShape(10, 20)
 
     const lastCall = plotlyMock.relayout.mock.calls.at(-1)
     const shapes = lastCall![1].shapes as any[]
-    expect(shapes.some((s) => s.name === 'edit-window')).toBe(true)
-    expect(shapes.some((s) => s.name === 'stage')).toBe(true)
-    // Untouched by the merge.
-    const editWindow = shapes.find((s) => s.name === 'edit-window')
-    expect(editWindow.x0).toBe(100)
-    expect(editWindow.x1).toBe(200)
+    expect(shapes.map((s) => s.name)).toEqual(['stage'])
   })
 
-  it('keeps the edit-window shape after clearing the stage shape', async () => {
+  it('clears the shapes when the stage band is removed', async () => {
     await setStageShape(10, 20)
     await clearStageShape()
 
     const lastCall = plotlyMock.relayout.mock.calls.at(-1)
-    const shapes = lastCall![1].shapes as any[]
-    expect(shapes.some((s) => s.name === 'edit-window')).toBe(true)
-    expect(shapes.some((s) => s.name === 'stage')).toBe(false)
+    expect(lastCall![1].shapes).toEqual([])
   })
 
   it('does not duplicate the stage shape across repeated setStageShape calls', async () => {
diff --git a/apps/qc-app/src/utils/plotting/events.ts b/apps/qc-app/src/utils/plotting/events.ts
index 8bf620cb..57efbbeb 100644
--- a/apps/qc-app/src/utils/plotting/events.ts
+++ b/apps/qc-app/src/utils/plotting/events.ts
@@ -83,11 +83,11 @@ export const handleNewPlot = async (
     }
     for (const trace of plotlyOptions.value.traces) {
       const t = trace as AppPlotlyTrace
-      // Gap overlays expose `_gapOverlayFor` instead of `id` so the
+      // Gap overlays expose `_partOf` instead of `id` so the
       // selection-by-id lookup keeps targeting the main trace; we still
       // want them to inherit the user's hide/show choice so the line
       // disappears with the markers it shadows.
-      const lookupId = t.id ?? t._gapOverlayFor
+      const lookupId = t.id ?? t._partOf
       if (!lookupId) continue
       const carried = visibleBySeriesId[lookupId]
       if (carried !== undefined) {
@@ -119,7 +119,9 @@ export const handleNewPlot = async (
     const yRangesBySeriesId: Record<string, Array<string | number>> = {}
     for (const trace of plotlyRef.value.data) {
       const t = trace as AppPlotlyTrace
-      if (!t.id) continue
+      // An empty trace's axis sits on Plotly's default range, not a view the
+      // user chose; carrying it would push the new data off the plot.
+      if (!t.id || !(t.x as ArrayLike<unknown> | undefined)?.length) continue
       const key = yAxisKey(t.yaxis as string | undefined)
       const range = (oldLayout[key] as Partial<LayoutAxis> | undefined)
         ?.range as Array<string | number> | undefined
@@ -140,9 +142,8 @@ export const handleNewPlot = async (
     }
   }
 
-  // A re-plot keeps the shapes other writers own (the staged range band);
-  // `createPlotlyOption` only rebuilds the session window band. A first
-  // mount has no live plot to read them from.
+  // A re-plot keeps the live staged range band, which `createPlotlyOption`
+  // doesn't build. A first mount has no live plot to read it from.
   const layout = element
     ? plotlyOptions.value.layout
     : withLiveShapes(plotlyOptions.value.layout, plotlyRef.value?.layout)
diff --git a/apps/qc-app/src/utils/plotting/interaction.ts b/apps/qc-app/src/utils/plotting/interaction.ts
index a3e3c137..aa945b02 100644
--- a/apps/qc-app/src/utils/plotting/interaction.ts
+++ b/apps/qc-app/src/utils/plotting/interaction.ts
@@ -391,6 +391,7 @@ export const updateAxisChips = (gd: PlotlyHTMLElement | null): void => {
   const graphWidth = (gd as HTMLElement).clientWidth
   let leftIdx = 0
   let rightIdx = 0
+  const seenAxes = new Set<string>()
   // Iterate `graphSeriesArray`: it's the authoritative sidebar
   // order; `gd.data` gets reversed so traces paint top-over-bottom
   // (see `createPlotlyOption`), which would flip the stacked chips.
@@ -404,6 +405,9 @@ export const updateAxisChips = (gd: PlotlyHTMLElement | null): void => {
     // on `y2`/`y3`/...
     const isPrimary = axisRef === 'y'
     const axisKey = isPrimary ? 'yaxis' : `yaxis${axisRef.slice(1)}`
+    // Series sharing an axis (the edit target and its source) share its chip.
+    if (seenAxes.has(axisKey)) continue
+    seenAxes.add(axisKey)
     const ax = fl[axisKey] as
       | { visible?: boolean; _mainLinePosition?: number; _shift?: number }
       | undefined
diff --git a/apps/qc-app/src/utils/plotting/operations.ts b/apps/qc-app/src/utils/plotting/operations.ts
index 6b101f7d..14abb240 100644
--- a/apps/qc-app/src/utils/plotting/operations.ts
+++ b/apps/qc-app/src/utils/plotting/operations.ts
@@ -11,7 +11,6 @@ import { storeToRefs } from 'pinia'
 import { findFirstGreaterOrEqual } from '@uwrl/qc-utils'
 import { handleNewPlot } from './events'
 import { traceXAsNumbers } from './internal'
-import { withLiveShapes } from './shapes'
 import type { AppPlotlyTrace } from './options'
 
 /**
@@ -162,10 +161,7 @@ export const cropXaxisRange = async () => {
 
   setTimeout(async () => {
     try {
-      const layoutUpdates = withLiveShapes(
-        plotlyOptions.value.layout,
-        plotlyRef.value?.layout
-      )
+      const layoutUpdates = plotlyOptions.value.layout
       const xAxis = layoutUpdates.xaxis as Partial<LayoutAxis> | undefined
       const xRange = xAxis?.range as Array<string | number> | undefined
       // Plotly will rewrite timestamps as datestrings. We need to convert them back to timestamps.
diff --git a/apps/qc-app/src/utils/plotting/options.ts b/apps/qc-app/src/utils/plotting/options.ts
index c88b6de6..38b9986e 100644
--- a/apps/qc-app/src/utils/plotting/options.ts
+++ b/apps/qc-app/src/utils/plotting/options.ts
@@ -8,18 +8,15 @@ import type {
   LayoutAxis,
   PlotData,
   PlotlyHTMLElement,
-  Shape,
 } from 'plotly.js-dist'
 import { storeToRefs } from 'pinia'
 import { useDataVisStore } from '@/store/dataVisualization'
-import { useQcSessionStore } from '@/store/qcSession'
 import { useQualifierStore } from '@/store/qualifiers'
 import { useUIStore } from '@/store/userInterface'
-import { findFirstGreaterOrEqual } from '@uwrl/qc-utils'
+import { findFirstGreaterOrEqual, findLastLessOrEqual } from '@uwrl/qc-utils'
 import { DENSITY_HIDE_MARKERS, Y_AXIS_KEY_RE } from './internal'
 import { undoZoom, redoZoom } from './zoom'
 import { fitXaxisToVisible, fitYaxisToVisible } from './operations'
-import { EDIT_WINDOW_SHAPE_NAME } from './shapes'
 
 /**
  * Return type of `createPlotlyOption`. Plan 03-03 will reuse this shape
@@ -55,11 +52,12 @@ export type AppPlotlyTrace = Partial<PlotData> & {
    */
   _isGapOverlay?: boolean
   /**
-   * Datastream id of the main trace this gap overlay belongs to. Lets
-   * visibility toggles and replot-state carries route to both traces
-   * even though only the main trace exposes the public `id`.
+   * Datastream id of the series a companion trace belongs to: a gap
+   * overlay, or the points of the source's piece after the session window.
+   * Lets visibility toggles and replot-state carries route to every trace
+   * of the series even though only the main trace exposes the public `id`.
    */
-  _gapOverlayFor?: string
+  _partOf?: string
 }
 
 /**
@@ -114,10 +112,89 @@ export const LABEL_COLORS = [
   '#117a85', // cyan
 ]
 
-// Read-only raw-source context drawn behind the edit target while editing.
-export const SOURCE_CONTEXT_COLOR = '#9e9e9e'
+// Read-only raw-source context drawn around the edit target while editing.
+export const SOURCE_CONTEXT_COLOR = '#cfcfcf'
 export const SOURCE_CONTEXT_LABEL_COLOR = '#616161'
 
+export type EditWindow = { begin: number; end: number }
+
+export const toEditWindow = (
+  w: { begin: Date; end: Date } | null | undefined
+): EditWindow | null =>
+  w ? { begin: w.begin.getTime(), end: w.end.getTime() } : null
+
+export type Line = { x: ArrayLike<number>; y: ArrayLike<number> }
+
+const view = (a: ArrayLike<number>, start: number, stop: number) =>
+  ArrayBuffer.isView(a)
+    ? (a as Float64Array).subarray(start, stop)
+    : Array.prototype.slice.call(a, start, stop)
+
+/**
+ * The parts of a sorted line before and after the window, as views of the
+ * same buffers, leaving a gap where the window is. With no window `before`
+ * is the whole line and `after` is empty.
+ */
+export const splitAroundWindow = (
+  line: Line,
+  w: EditWindow | null
+): { before: Line; after: Line } => {
+  const n = line.x.length
+  const xs = line.x as number[]
+  const start = w ? findFirstGreaterOrEqual(xs, w.begin) : n
+  const stop = w ? findLastLessOrEqual(xs, w.end) + 1 : n
+  const slice = (from: number, to: number): Line => ({
+    x: view(line.x, from, to),
+    y: view(line.y, from, to),
+  })
+  return { before: slice(0, start), after: slice(Math.max(stop, start), n) }
+}
+
+/**
+ * The segments joining the source's context to the edit target across the
+ * window edges: before's last point to the target's first, and the target's
+ * last to after's first. A step wider than `maxGapMs` is a real gap and
+ * stays open, as it does inside the lines. NaN-y breaks keep the two
+ * segments apart.
+ */
+export const bridgeAcrossWindow = (
+  before: Line,
+  after: Line,
+  target: Line,
+  maxGapMs: number
+): { x: number[]; y: number[] } => {
+  const x: number[] = []
+  const y: number[] = []
+  const n = target.x.length
+  const join = (x0: number, y0: number, x1: number, y1: number) => {
+    if (x1 - x0 > maxGapMs) return
+    if (x.length) {
+      x.push((x[x.length - 1]! + x0) / 2)
+      y.push(NaN)
+    }
+    x.push(x0, x1)
+    y.push(y0, y1)
+  }
+  const b = before.x.length - 1
+  if (b >= 0 && n) {
+    join(
+      before.x[b] as number,
+      before.y[b] as number,
+      target.x[0] as number,
+      target.y[0] as number
+    )
+  }
+  if (after.x.length && n) {
+    join(
+      target.x[n - 1] as number,
+      target.y[n - 1] as number,
+      after.x[0] as number,
+      after.y[0] as number
+    )
+  }
+  return { x, y }
+}
+
 /** Companion text colour for a `COLORS[i]` line; falls back to QC grey. */
 export const labelColorFor = (lineColor: string): string => {
   const idx = COLORS.indexOf(lineColor)
@@ -409,12 +486,16 @@ export { buildQualifierBand }
 export const createPlotlyOption = (
   seriesArray: GraphSeries[]
 ): PlotlyChartOptions => {
-  const { qcDatastream, sourceContextDatastream, beginDate, endDate } =
-    storeToRefs(useDataVisStore())
+  const {
+    qcDatastream,
+    sourceContextDatastream,
+    beginDate,
+    endDate,
+    editSessionWindow,
+  } = storeToRefs(useDataVisStore())
   const { hiddenAxisIds, hiddenTraceIds, plotlyRef } = storeToRefs(
     usePlotlyStore()
   )
-  const { viewedSession, inProgressSession } = storeToRefs(useQcSessionStore())
   const { isPlotPreview } = storeToRefs(useUIStore())
   const isPreview = isPlotPreview?.value ?? false
   const hiddenAxes = hiddenAxisIds?.value ?? new Set<string>()
@@ -441,6 +522,8 @@ export const createPlotlyOption = (
   const densityRangeValid =
     Number.isFinite(densityStart) && Number.isFinite(densityEnd)
 
+  const editWindow = toEditWindow(editSessionWindow?.value)
+
   const traces: AppPlotlyTrace[] = []
   const yaxis: Partial<Layout> = {}
 
@@ -506,10 +589,18 @@ export const createPlotlyOption = (
     // with NaN-y breaks at gaps; otherwise the series renders as a pure
     // scatter plot. Either way the main trace owns selection and point
     // indices, so its `line` attribute would be dead config, so it is omitted.
+    // The source is drawn only around the session window: its points
+    // before and after it, as views of the record, with a gap in between.
+    // Every other series is one piece.
+    const whole = { x: xData ?? [], y: yData ?? [] } as Line
+    const pieces: Line[] = isSource
+      ? Object.values(splitAroundWindow(whole, editWindow))
+      : [whole]
+
     const trace: AppPlotlyTrace = {
       id: s.id,
-      x: xData,
-      y: yData,
+      x: (isSource ? pieces[0]!.x : xData) as PlotData['x'],
+      y: (isSource ? pieces[0]!.y : yData) as PlotData['y'],
       yaxis: axisRef,
       type: 'scattergl',
       mode: 'markers',
@@ -534,8 +625,8 @@ export const createPlotlyOption = (
       )
 
     } else if (isSource) {
-      // Read-only raw-source context: grey, shares the edit target's axis,
-      // never gets its own axis entry or selection styling.
+      // Read-only raw-source context: light grey, shares the edit target's
+      // axis, never gets its own axis entry or selection styling.
       trace.marker = { ...(trace.marker ?? {}), color: SOURCE_CONTEXT_COLOR }
       trace.unselected = { marker: { opacity: markerOpacity } }
 
@@ -598,6 +689,17 @@ export const createPlotlyOption = (
     }
 
     traces.push(trace)
+    // The pieces after the first draw their points as companions of the
+    // main trace, which keeps the series id.
+    for (const piece of pieces.slice(1)) {
+      const { id: _id, ...rest } = trace
+      traces.push({
+        ...rest,
+        x: piece.x as PlotData['x'],
+        y: piece.y as PlotData['y'],
+        _partOf: s.id,
+      })
+    }
 
     // Gap-aware line rendering. When the source datastream declares an
     // intended cadence, a sibling overlay trace draws the connecting
@@ -608,30 +710,49 @@ export const createPlotlyOption = (
     // an overlay is enough. The overlay carries no `id`, so
     // selection/lookup logic that finds traces by datastream id keeps
     // targeting the main trace and its stable point indices.
+    // One overlay per piece, so no line crosses the source's window gap.
+    // Pieces without a gap stay views of the record.
     if (hasLineFallback && spacingMs && xData?.length && yData?.length) {
-      const gaps = findGapIndices(xData as ArrayLike<number>, spacingMs)
-      const broken = insertGapBreaks(
-        xData as ArrayLike<number>,
-        yData as ArrayLike<number>,
-        gaps
+      const lineColor = isQc
+        ? COLORS[0]
+        : isSource
+          ? SOURCE_CONTEXT_COLOR
+          : color
+      // The source also joins the edit target at the window edges, so the
+      // line runs on into it. Always present so the trace count holds.
+      const edit = seriesArray.find((e) => e.id === editId)?.data
+      const lines: Line[] = pieces.map((piece) =>
+        insertGapBreaks(piece.x, piece.y, findGapIndices(piece.x, spacingMs))
       )
-      const overlay: AppPlotlyTrace = {
-        x: broken.x as unknown as PlotData['x'],
-        y: broken.y as unknown as PlotData['y'],
-        yaxis: axisRef,
-        type: 'scattergl',
-        mode: 'lines',
-        hoverinfo: 'skip',
-        showLegend: false,
-        line: { color: isQc ? COLORS[0] : isSource ? SOURCE_CONTEXT_COLOR : color },
-        _isGapOverlay: true,
-        _gapOverlayFor: s.id,
+      if (isSource) {
+        lines.push(
+          bridgeAcrossWindow(
+            pieces[0]!,
+            pieces[1]!,
+            { x: edit?.dataX ?? [], y: edit?.dataY ?? [] } as Line,
+            spacingMs
+          )
+        )
+      }
+      for (const line of lines) {
+        const overlay: AppPlotlyTrace = {
+          x: line.x as unknown as PlotData['x'],
+          y: line.y as unknown as PlotData['y'],
+          yaxis: axisRef,
+          type: 'scattergl',
+          mode: 'lines',
+          hoverinfo: 'skip',
+          showLegend: false,
+          line: { color: lineColor },
+          _isGapOverlay: true,
+          _partOf: s.id,
+        }
+        // The eye toggle hides marker + gap overlay together; mirror
+        // the main trace's visibility flag on the overlay so a hidden
+        // series doesn't leak its connecting line through.
+        if (!isTraceVisible) overlay.visible = false
+        traces.push(overlay)
       }
-      // The eye toggle hides marker + gap overlay together; mirror
-      // the main trace's visibility flag on the overlay so a hidden
-      // series doesn't leak its connecting line through.
-      if (!isTraceVisible) overlay.visible = false
-      traces.push(overlay)
     }
   })
 
@@ -697,31 +818,6 @@ export const createPlotlyOption = (
     showlegend: false,
   }
 
-  // Shade the session window while editing: the viewed session takes
-  // precedence over the in-progress one (viewing history overrides the
-  // live session's own window).
-  const editWindow = qcDatastream?.value
-    ? (viewedSession?.value ?? inProgressSession?.value)
-    : null
-  if (editWindow) {
-    layout.shapes = [
-      {
-        name: EDIT_WINDOW_SHAPE_NAME,
-        type: 'rect',
-        xref: 'x',
-        yref: 'paper',
-        x0: Date.parse(editWindow.phenomenonTimeStart),
-        x1: Date.parse(editWindow.phenomenonTimeEnd),
-        y0: 0,
-        y1: 1,
-        fillcolor: 'rgba(25, 118, 210, 0.07)',
-        line: { width: 0 },
-        layer: 'below',
-        editable: false,
-      } as Partial<Shape>,
-    ]
-  }
-
   // Modebar buttons. `isPreview` drops select/lasso and the Fit buttons.
   const undoZoomButton = {
     name: 'Undo zoom',
diff --git a/apps/qc-app/src/utils/plotting/relayout.ts b/apps/qc-app/src/utils/plotting/relayout.ts
index 1a130636..6e97005d 100644
--- a/apps/qc-app/src/utils/plotting/relayout.ts
+++ b/apps/qc-app/src/utils/plotting/relayout.ts
@@ -227,20 +227,20 @@ export const handleRelayout = async (
       // Scatter-only series (datastreams without a declared
       // `intendedTimeSpacing`) ship no gap-overlay line, so fading
       // their markers would leave nothing on screen. Identify them
-      // by the absence of a companion `_gapOverlayFor` trace and
-      // pin their opacity at 1.
+      // by the absence of a gap overlay `_partOf` them and pin their
+      // opacity at 1. Companion point traces count as their series.
       const tooltipsWillRun = areTooltipsEnabled.value
       const { qcDatastream } = storeToRefs(useDataVisStore())
       const qcId = qcDatastream.value?.id
       const traces = liveTraces
       const lineBackedIds = new Set<string>()
       for (const t of traces) {
-        if (t._gapOverlayFor) lineBackedIds.add(t._gapOverlayFor)
+        if (t._isGapOverlay && t._partOf) lineBackedIds.add(t._partOf)
       }
       const perTraceOpacity = perTraceVisible.map((n, i) => {
         if (tooltipsWillRun) return 1
         const trace = traces[i] as AppPlotlyTrace | undefined
-        const id = trace?.id
+        const id = trace?.id ?? trace?._partOf
         if (id && !trace?._isGapOverlay && !lineBackedIds.has(id)) return 1
         if (tooltipsMode.value === 'manual') return 0
         return n > DENSITY_HIDE_MARKERS ? 0 : 1
diff --git a/apps/qc-app/src/utils/plotting/selected.ts b/apps/qc-app/src/utils/plotting/selected.ts
index 51dd1394..6a234477 100644
--- a/apps/qc-app/src/utils/plotting/selected.ts
+++ b/apps/qc-app/src/utils/plotting/selected.ts
@@ -32,8 +32,13 @@ export const handleSelected = async (
   eventData?: PlotMouseEvent | PlotRelayoutEvent | PlotSelectionEvent | null,
   opts: { fromRelayout?: boolean } = {}
 ) => {
-  const { plotlyRef, selectedSeries, isUpdating, suppressedEchoSelection } =
-    storeToRefs(usePlotlyStore())
+  const {
+    plotlyRef,
+    selectedSeries,
+    isUpdating,
+    suppressedEchoSelection,
+    previewIndex,
+  } = storeToRefs(usePlotlyStore())
   const { selectedData } = storeToRefs(useDataVisStore())
   const { qcDatastream } = storeToRefs(useDataVisStore())
 
@@ -79,7 +84,8 @@ export const handleSelected = async (
     const current = selectedData.value ?? []
     if (sameSelection(expected, current)) return
   }
-  if (eventData && !isUpdating.value) {
+  // A previewed step only shows data: the selection highlights, unrecorded.
+  if (eventData && !isUpdating.value && previewIndex.value === null) {
     await selectedSeries.value?.data.dispatchFilter(
       EnumFilterOperations.SELECTION,
       selectedData.value ?? []
diff --git a/apps/qc-app/src/utils/plotting/shapes.ts b/apps/qc-app/src/utils/plotting/shapes.ts
index a20a43d6..a0709dd9 100644
--- a/apps/qc-app/src/utils/plotting/shapes.ts
+++ b/apps/qc-app/src/utils/plotting/shapes.ts
@@ -1,8 +1,7 @@
 /**
- * `layout.shapes` has several writers, and each owns only the shapes it
- * names: `createPlotlyOption` owns the session window band, the staging
- * helpers own the stage band. A write replaces its own shapes and carries
- * everything else on the live plot through unchanged.
+ * `layout.shapes` belongs to the staging helpers (the stage band).
+ * `createPlotlyOption` builds layouts without shapes, so a full re-plot
+ * carries the live ones over.
  */
 
 import type { Layout } from 'plotly.js-dist'
@@ -12,30 +11,13 @@ export type PlotlyShape = Partial<NonNullable<Layout['shapes']>[number]> & {
   name?: string
 }
 
-export const EDIT_WINDOW_SHAPE_NAME = 'edit-window'
 export const STAGE_SHAPE_NAME = 'stage'
 
-/** `live` with the shapes named `name` replaced by `own`. `first` puts
- *  `own` ahead of the other writers' shapes instead of after them. */
-export function composeShapes(
-  live: readonly PlotlyShape[] | undefined,
-  name: string,
-  own: readonly PlotlyShape[],
-  { first = false }: { first?: boolean } = {}
-): PlotlyShape[] {
-  const others = (live ?? []).filter((shape) => shape?.name !== name)
-  return first ? [...own, ...others] : [...others, ...own]
-}
-
-/** A copy of a fresh `createPlotlyOption` layout whose shapes keep what
- *  other writers have on the live plot. */
+/** A copy of a fresh `createPlotlyOption` layout carrying the live plot's
+ *  shapes. */
 export function withLiveShapes(
   layout: Partial<Layout>,
   liveLayout: Partial<Layout> | undefined
 ): Partial<Layout> {
-  const own = ((layout.shapes ?? []) as PlotlyShape[]).filter(
-    (shape) => shape.name === EDIT_WINDOW_SHAPE_NAME
-  )
-  const live = liveLayout?.shapes as PlotlyShape[] | undefined
-  return { ...layout, shapes: composeShapes(live, EDIT_WINDOW_SHAPE_NAME, own) }
+  return { ...layout, shapes: [...(liveLayout?.shapes ?? [])] }
 }
diff --git a/apps/qc-app/src/utils/plotting/staging.ts b/apps/qc-app/src/utils/plotting/staging.ts
index 01f66421..5d2c3721 100644
--- a/apps/qc-app/src/utils/plotting/staging.ts
+++ b/apps/qc-app/src/utils/plotting/staging.ts
@@ -6,9 +6,8 @@
  * helper tolerates being called when the plot isn't mounted yet so
  * callers can fire-and-forget from setup/unmount hooks.
  *
- * The stage band is the only shape this module owns. Every flush
- * rebuilds the live `layout.shapes` through `composeShapes`, so shapes
- * other writers own (the session window band) are carried through.
+ * The stage band is the only shape on the plot, so every flush writes
+ * `layout.shapes` whole.
  *
  * Gap bands used to live here as secondary red rectangles marking
  * detected gaps. They were removed because `edits.shapePosition:
@@ -25,7 +24,7 @@ import type { Layout } from 'plotly.js-dist'
 import { ref } from 'vue'
 import { usePlotlyStore } from '@/store/plotly'
 import { storeToRefs } from 'pinia'
-import { composeShapes, STAGE_SHAPE_NAME, type PlotlyShape } from './shapes'
+import { STAGE_SHAPE_NAME, type PlotlyShape } from './shapes'
 
 const GHOST_TRACE_NAME = 'qc-ghost-fills'
 
@@ -51,14 +50,9 @@ function getRoot(): HTMLElement | null {
 async function flushShapes() {
   const root = getRoot()
   if (!root) return
-  const live = (root as unknown as { layout?: { shapes?: PlotlyShape[] } })
-    .layout?.shapes
   // Drop the stage shape outside pan mode so zoom / select / lasso
-  // gestures aren't captured by the shape-edit hit-tester. Stage stays
-  // at index 0 when present so `onStageDrag`'s `shapes[0].*` event keys
-  // keep pointing at it.
-  const own = stageShape && stagePanMode.value ? [stageShape] : []
-  const shapes = composeShapes(live, STAGE_SHAPE_NAME, own, { first: true })
+  // gestures aren't captured by the shape-edit hit-tester.
+  const shapes = stageShape && stagePanMode.value ? [stageShape] : []
   await Plotly.relayout(root, { shapes } as unknown as Partial<Layout>)
 }
 
@@ -219,14 +213,11 @@ export function onStageDrag(
       return
     }
 
-    // The stage shape is always prepended first in `flushShapes`, so
-    // its index in the layout shapes array is always 0 regardless of
-    // how many other shapes (e.g. the edit-window band) are present.
-    const stageIdx = 0
-    const x0Key = `shapes[${stageIdx}].x0`
-    const x1Key = `shapes[${stageIdx}].x1`
-    const y0Key = `shapes[${stageIdx}].y0`
-    const y1Key = `shapes[${stageIdx}].y1`
+    // The stage band is the only shape, so it is always `shapes[0]`.
+    const x0Key = 'shapes[0].x0'
+    const x1Key = 'shapes[0].x1'
+    const y0Key = 'shapes[0].y0'
+    const y1Key = 'shapes[0].y1'
     const touchedX0 = Object.prototype.hasOwnProperty.call(evt, x0Key)
     const touchedX1 = Object.prototype.hasOwnProperty.call(evt, x1Key)
     const touchedY0 = Object.prototype.hasOwnProperty.call(evt, y0Key)
diff --git a/apps/qc-app/src/utils/timeIntervals.ts b/apps/qc-app/src/utils/timeIntervals.ts
new file mode 100644
index 00000000..8e8f29ac
--- /dev/null
+++ b/apps/qc-app/src/utils/timeIntervals.ts
@@ -0,0 +1,36 @@
+/** Closed `[start, end]` epoch-ms intervals. Timestamps are whole
+ *  milliseconds, so neighbouring intervals meet at `end + 1`. */
+export type Interval = [number, number]
+
+/** Sorted, non-overlapping union of `intervals`. Touching ones join. */
+export function mergeIntervals(intervals: readonly Interval[]): Interval[] {
+  const sorted = [...intervals].sort((a, b) => a[0] - b[0])
+  const out: Interval[] = []
+  for (const [start, end] of sorted) {
+    const last = out[out.length - 1]
+    if (last && start <= last[1] + 1) last[1] = Math.max(last[1], end)
+    else out.push([start, end])
+  }
+  return out
+}
+
+/** The parts of `from` that no interval in `remove` covers. */
+export function subtractIntervals(
+  from: readonly Interval[],
+  remove: readonly Interval[]
+): Interval[] {
+  const cuts = mergeIntervals(remove)
+  const out: Interval[] = []
+  for (const [start, end] of from) {
+    let cursor = start
+    for (const [cutStart, cutEnd] of cuts) {
+      if (cutEnd < cursor) continue
+      if (cutStart > end) break
+      if (cutStart > cursor) out.push([cursor, cutStart - 1])
+      cursor = cutEnd + 1
+      if (cursor > end) break
+    }
+    if (cursor <= end) out.push([cursor, end])
+  }
+  return out
+}
diff --git a/packages/qc-utils/docs/API_REFERENCE.md b/packages/qc-utils/docs/API_REFERENCE.md
index 445c2ef6..0a977d80 100644
--- a/packages/qc-utils/docs/API_REFERENCE.md
+++ b/packages/qc-utils/docs/API_REFERENCE.md
@@ -89,9 +89,12 @@ call `reload()` once construction is done to initialize.
 | `dispatchFilter(op: EnumFilterOperations, ...args)` | `Promise<void>` | Run one filter op. Produces a selection.                                             |
 | `undo()`                                     | `Promise<void>`  | Pop the last history entry; replay the rest from a fresh `reload()`.                       |
 | `redo()`                                     | `Promise<void>`  | Replay the most recently undone entry.                                                     |
-| `applyWindow(begin, end)`                    | `Promise<void>`  | Materialize the inclusive epoch-ms window `[begin, end]` of `rawData` into `dataX`/`dataY`. Clears history on a real change; no-op when the window is unchanged. |
+| `applyWindow(begin, end, rawData?)`          | `Promise<void>`  | Materialize the inclusive epoch-ms window `[begin, end]` of `rawData` into `dataX`/`dataY`. A passed `rawData` replaces the full series first (e.g. after a cache filled a gap). Clears history on a real change; no-op when neither the window nor the data changed. |
 | `reload()`                                   | `Promise<void>`  | Re-initialize the typed arrays from `rawData`, sliced to the current window; clear history. |
-| `reloadHistory()`                            | `Promise<void>`  | Replay current history against a fresh `reload()`.                                         |
+| `previewHistory(index)`                      | `Promise<number[]>` | Show the data as of step `index` (`-1` for the starting state) and keep every later step listed, unapplied. Sets `previewIndex`; edits throw `HistoryPreviewError` until `exitPreview`. Previewing the last step is `exitPreview`. Returns the shown step's selection. |
+| `exitPreview()`                              | `Promise<number[]>` | Replay the whole history after a preview. A no-op when nothing is previewed. |
+| `truncateHistory(index)`                     | `Promise<number[]>` | Drop every step after `index` for good (discarding unsaved edits), clear the redo stack and replay the rest. |
+| `previewIndex`                               | `number \| null`    | The step being previewed, or null when the data reflects the whole history. `undo` ends a preview; `redo` ends it first, then redoes. |
 | `removeHistoryItem(index: number)`           | `Promise<void>`  | Drop a specific entry; replay the rest.                                                    |
 
 The op handlers themselves are private — dispatch by enum.
diff --git a/packages/qc-utils/docs/ARCHITECTURE.md b/packages/qc-utils/docs/ARCHITECTURE.md
index 7b86b0b2..ff7c685c 100644
--- a/packages/qc-utils/docs/ARCHITECTURE.md
+++ b/packages/qc-utils/docs/ARCHITECTURE.md
@@ -89,8 +89,9 @@ unit tests.
 
 The record holds the full series in `rawData`, but `dataX` / `dataY` (the
 operation surface) carry only the active **window**. `applyWindow(begin,
-end)` slices `rawData` into `dataX` / `dataY`; `reload()` restores that
-windowed baseline. A window change clears history — operations only ever see
+end, rawData?)` slices `rawData` into `dataX` / `dataY`, optionally taking
+new `rawData` first (a cache that grew); `reload()` restores that windowed
+baseline. A window or data change clears history — operations only ever see
 data inside the current window, so a new window starts from a fresh QC
 baseline.
 
diff --git a/packages/qc-utils/docs/QUALITY.md b/packages/qc-utils/docs/QUALITY.md
index 5fbb5d1b..8a021827 100644
--- a/packages/qc-utils/docs/QUALITY.md
+++ b/packages/qc-utils/docs/QUALITY.md
@@ -126,8 +126,10 @@ is the obvious next step if/when a non-browser consumer appears.
 ## Where the code is solid
 
 - The dispatch surface is small (`dispatch`, `dispatchAction`,
-  `dispatchFilter`, `undo`, `redo`, `reload`, `reloadHistory`,
-  `removeHistoryItem`) and consistently the only mutation path. No
+  `dispatchFilter`, `undo`, `redo`, `reload`, `truncateHistory`,
+  `removeHistoryItem`) and consistently the only mutation path.
+  `previewHistory` / `exitPreview` only replay; edits are refused while a
+  step is previewed. No
   sneaky escape hatches.
 - The two-flavor kernel pattern is consistent across every op. Reading
   one is enough to understand all of them.
diff --git a/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
index 6f85fabf..e6c308b7 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
@@ -16,7 +16,11 @@ vi.mock('../value-threshold.worker?worker&inline', () => import('./workerMocks')
 vi.mock('../change-values.worker?worker&inline', () => import('./workerMocks').then(m => ({ default: m.MockChangeValuesWorker })));
 
 import { mockDatastream } from './mock';
-import { ObservationRecord, INCREASE_AMOUNT } from '../observation-record';
+import {
+  ObservationRecord,
+  INCREASE_AMOUNT,
+  HistoryPreviewError,
+} from '../observation-record';
 import {
   EnumEditOperations,
   EnumFilterOperations,
@@ -161,6 +165,24 @@ describe('ObservationRecord', () => {
       expect(rec.history.length).toBe(histLen);
     });
 
+    it('re-materializes an unchanged window over new rawData', async () => {
+      const rec = makeRec();
+      await rec.applyWindow(t(0), t(9));
+      const full = {
+        datetimes: Float64Array.from(grid.datetimes),
+        dataValues: Float32Array.from(grid.dataValues),
+      };
+      const gapped = {
+        datetimes: full.datetimes.filter((_, i) => i < 3 || i > 6),
+        dataValues: full.dataValues.filter((_, i) => i < 3 || i > 6),
+      };
+      await rec.applyWindow(t(0), t(9), gapped);
+      expect(rec.dataX.length).toBe(6);
+      await rec.applyWindow(t(0), t(9), full);
+      expect(rec.dataX.length).toBe(10);
+      expect(rec.rawData).toBe(full);
+    });
+
     it('reload() restores the windowed baseline, not the full series', async () => {
       const rec = makeRec();
       await rec.applyWindow(t(4), t(7)); // 4 points
@@ -745,22 +767,109 @@ describe('ObservationRecord', () => {
       expect(rec.history).toEqual([]);
     });
 
-    it('reloadHistory replays entries up to a given index', async () => {
-      await rec.dispatch([
-        [EnumFilterOperations.SELECTION, [0, 1]],
-        [EnumEditOperations.DELETE_POINTS],
-        [EnumFilterOperations.SELECTION, [0]],
-        [EnumEditOperations.DELETE_POINTS],
-      ]);
-      const lenAfterTwo = rec.dataX.length;
+    describe('previewHistory', () => {
+      const twoDeletes = () =>
+        rec.dispatch([
+          [EnumFilterOperations.SELECTION, [0, 1]],
+          [EnumEditOperations.DELETE_POINTS],
+          [EnumFilterOperations.SELECTION, [0]],
+          [EnumEditOperations.DELETE_POINTS],
+        ]);
+
+      it('shows an earlier step and keeps the later ones listed', async () => {
+        await twoDeletes();
+        const later = rec.history.slice(2);
+
+        // Only the first SELECTION + DELETE pair (entries 0 + 1) applies.
+        await rec.previewHistory(1);
+
+        expect(rec.dataX.length).toBe(mockRawData.datetimes.length - 2);
+        expect(rec.history).toHaveLength(4);
+        expect(rec.history.slice(2)).toEqual(later);
+        expect(rec.previewIndex).toBe(1);
+      });
+
+      it('previews the untouched data at -1', async () => {
+        await twoDeletes();
+        await rec.previewHistory(-1);
+        expect(rec.dataX.length).toBe(mockRawData.datetimes.length);
+        expect(rec.history).toHaveLength(4);
+        expect(rec.previewIndex).toBe(-1);
+      });
+
+      it('returns to the whole history', async () => {
+        await twoDeletes();
+        const len = rec.dataX.length;
+        await rec.previewHistory(1);
 
-      // Replay only up through the first SELECTION + DELETE pair (entries 0 + 1).
-      await rec.reloadHistory(1);
-      expect(rec.dataX.length).toBe(mockRawData.datetimes.length - 2);
-      expect(rec.dataX.length).toBeGreaterThan(lenAfterTwo);
+        await rec.exitPreview();
+
+        expect(rec.dataX.length).toBe(len);
+        expect(rec.history).toHaveLength(4);
+        expect(rec.previewIndex).toBeNull();
+      });
+
+      it('treats previewing the last step as returning', async () => {
+        await twoDeletes();
+        await rec.previewHistory(1);
+        await rec.previewHistory(3);
+        expect(rec.previewIndex).toBeNull();
+        expect(rec.dataX.length).toBe(mockRawData.datetimes.length - 3);
+      });
+
+      it('refuses edits while previewing', async () => {
+        await twoDeletes();
+        await rec.previewHistory(1);
+        await expect(
+          rec.dispatch(EnumFilterOperations.SELECTION, [0])
+        ).rejects.toBeInstanceOf(HistoryPreviewError);
+        await expect(
+          rec.dispatch(EnumEditOperations.ADD_POINTS, [[1, 1]])
+        ).rejects.toBeInstanceOf(HistoryPreviewError);
+        expect(rec.history).toHaveLength(4);
+      });
+
+      it('undoes the last step of the whole history, ending the preview', async () => {
+        await twoDeletes();
+        await rec.previewHistory(0);
+
+        await rec.undo();
+
+        expect(rec.previewIndex).toBeNull();
+        expect(rec.history).toHaveLength(3);
+        expect(rec.redoStack).toHaveLength(1);
+      });
+
+      it('redoes on top of the whole history, ending the preview', async () => {
+        await twoDeletes();
+        await rec.undo();
+        await rec.previewHistory(-1);
+
+        await rec.redo();
+
+        expect(rec.previewIndex).toBeNull();
+        expect(rec.history).toHaveLength(4);
+        expect(rec.dataX.length).toBe(mockRawData.datetimes.length - 3);
+      });
+
+      it('keeps the redo stack while previewing', async () => {
+        await twoDeletes();
+        await rec.undo();
+        await rec.previewHistory(0);
+        expect(rec.redoStack).toHaveLength(1);
+        await rec.exitPreview();
+        expect(rec.redoStack).toHaveLength(1);
+      });
+
+      it('ends on a reload', async () => {
+        await twoDeletes();
+        await rec.previewHistory(1);
+        await rec.reload();
+        expect(rec.previewIndex).toBeNull();
+      });
     });
 
-    it('keeps comment and attribution across a step reload', async () => {
+    it('keeps comment and attribution across a step preview', async () => {
       await rec.dispatch([
         [EnumFilterOperations.SELECTION, [0, 1]],
         [EnumEditOperations.DELETE_POINTS],
@@ -771,7 +880,7 @@ describe('ObservationRecord', () => {
       rec.history[0]!.comment = 'Dropped the spike';
       rec.history[1]!.performedBy = 'Grace Hopper';
 
-      await rec.reloadHistory(1);
+      await rec.previewHistory(1);
 
       expect(rec.history[0]!.performedBy).toBe('Ada Lovelace');
       expect(rec.history[0]!.comment).toBe('Dropped the spike');
@@ -786,7 +895,8 @@ describe('ObservationRecord', () => {
       rec.history[0]!.performedBy = 'Ada Lovelace';
       rec.history[0]!.execution!.durationMs = 9999;
 
-      await rec.reloadHistory(1);
+      // Step 0 is replayed; previewing the last step would replay nothing.
+      await rec.previewHistory(0);
 
       expect(rec.history[0]!.performedBy).toBe('Ada Lovelace');
       // The timing describes the run that just happened, not the old one.
diff --git a/packages/qc-utils/src/utils/plotting/observation-record.ts b/packages/qc-utils/src/utils/plotting/observation-record.ts
index 39e635fe..0593a797 100644
--- a/packages/qc-utils/src/utils/plotting/observation-record.ts
+++ b/packages/qc-utils/src/utils/plotting/observation-record.ts
@@ -145,6 +145,15 @@ function consumesPrecedingSelection(
   }
 }
 
+/** Thrown by an edit dispatched while an earlier history step is previewed.
+ *  Previewing only shows a step; return to the latest step to edit. */
+export class HistoryPreviewError extends Error {
+  constructor() {
+    super("An earlier step is being previewed. Return to the latest step to edit.");
+    this.name = "HistoryPreviewError";
+  }
+}
+
 export class ObservationRecord {
   /** The generated dataset to be used for plotting */
   dataset: {
@@ -183,6 +192,14 @@ export class ObservationRecord {
    * dispatch, so the stack survives the internal replay.
    */
   private _isReplaying: boolean = false;
+  /**
+   * The history step the data currently shows, when it is an earlier one
+   * than the last. `history` still lists every step; those after this one
+   * are not applied. Null when the data reflects the whole history. Set by
+   * `previewHistory`, cleared by `exitPreview`, `undo`, `redo` and any
+   * reload of the data.
+   */
+  previewIndex: number | null = null;
   /**
    * Set by each operation handler (or any of its subroutines) to record
    * whether a worker was actually spawned during this dispatch. Sticky-
@@ -239,17 +256,31 @@ export class ObservationRecord {
     this.loadingTime = measurement.duration;
 
     this.history.length = 0;
+    this.previewIndex = null;
     this.isLoading = false;
   }
 
   /**
    * Materialize the inclusive epoch-ms window `[begin, end]` of `rawData`
-   * into `dataset.source`. A real window change clears history — the new
-   * window is a fresh QC baseline. An unchanged window is a no-op so an
-   * unrelated reload doesn't discard in-flight edits.
+   * into `dataset.source`. Passing `rawData` replaces the full series first,
+   * e.g. after a cache filled a gap. A real window or data change clears
+   * history, since it is a fresh QC baseline. An unchanged window over the
+   * same data is a no-op so an unrelated reload doesn't discard in-flight
+   * edits.
    */
-  async applyWindow(begin: number, end: number) {
-    if (begin === this.windowBegin && end === this.windowEnd) return;
+  async applyWindow(
+    begin: number,
+    end: number,
+    rawData: ObservationRecord["rawData"] = this.rawData,
+  ) {
+    if (
+      begin === this.windowBegin &&
+      end === this.windowEnd &&
+      rawData === this.rawData
+    ) {
+      return;
+    }
+    this.rawData = rawData;
     this.windowBegin = begin;
     this.windowEnd = end;
     await this.loadData(this._windowedRaw());
@@ -359,14 +390,10 @@ export class ObservationRecord {
     this.loadingTime = null;
     this.isLoading = true;
     this.history.length = 0;
+    this.previewIndex = null;
     await this.loadData(this._windowedRaw());
   }
 
-  /**
-   * Truncate history at `index` (inclusive), reload from raw, and
-   * replay the surviving entries. Used by the "Reload from this step"
-   * button in EditHistory.
-   */
   /**
    * Re-stamp the metadata a replay cannot reproduce. `dispatch` rebuilds
    * each entry from `[method, ...args]` alone, so the operator's comment
@@ -389,21 +416,65 @@ export class ObservationRecord {
     }
   }
 
-  async reloadHistory(index: number): Promise<number[]> {
-    const newHistory = this.history.slice(0, index + 1);
-    this.redoStack.length = 0;
+  /** Reload from raw and replay `steps`, keeping the redo stack. */
+  private async _replay(steps: HistoryItem[]): Promise<number[]> {
     await this.reload();
+    this._isReplaying = true;
+    try {
+      const selection = await this.dispatch(
+        steps.map((h) => [h.method, ...(h.args || [])]),
+      );
+      this._restoreReplayedMeta(steps);
+      return selection;
+    } finally {
+      this._isReplaying = false;
+    }
+  }
 
-    const selection = await this.dispatch(
-      newHistory.map((h) => [h.method, ...(h.args || [])]),
-    );
-    this._restoreReplayedMeta(newHistory);
+  /**
+   * Show the data as of history step `index` (-1 for the untouched data)
+   * without dropping the steps after it: they stay listed in `history`,
+   * unapplied, until `exitPreview`. Edits are refused meanwhile. Previewing
+   * the last step is the same as `exitPreview`. Returns the selection the
+   * shown step leaves.
+   */
+  async previewHistory(index: number): Promise<number[]> {
+    const steps = [...this.history];
+    if (index >= steps.length - 1) return this.exitPreview();
+    const shown = steps.slice(0, index + 1);
+    const selection = await this._replay(shown);
+    const tail = steps.slice(index + 1);
+    this.history.push(...tail);
+    this.previewIndex = this.history.length - tail.length - 1;
     return selection;
   }
 
+  /**
+   * Drop every step after `index` for good (discarding unsaved edits):
+   * reload from raw and replay the steps kept. Clears the redo stack and
+   * ends a preview. To look at an earlier step, use `previewHistory`.
+   */
+  async truncateHistory(index: number): Promise<number[]> {
+    const kept = this.history.slice(0, index + 1);
+    this.redoStack.length = 0;
+    return this._replay(kept);
+  }
+
+  /** Apply the whole history again after `previewHistory`. A no-op
+   *  returning no selection when nothing is previewed. */
+  async exitPreview(): Promise<number[]> {
+    if (this.previewIndex === null) return [];
+    return this._replay([...this.history]);
+  }
+
+  private _refuseWhilePreviewing() {
+    if (this.previewIndex !== null) throw new HistoryPreviewError();
+  }
+
   /** Splice the history entry at `index`, reload from raw, and replay
    *  the survivors. */
   async removeHistoryItem(index: number): Promise<number[]> {
+    this._refuseWhilePreviewing();
     const newHistory = [...this.history];
     newHistory.splice(index, 1);
     this.redoStack.length = 0;
@@ -418,24 +489,15 @@ export class ObservationRecord {
   /**
    * Undo the most recent history entry. Pushes it onto `redoStack` so a
    * subsequent `redo()` can re-apply it, then reloads from the raw
-   * dataset and replays the remaining history in order.
+   * dataset and replays the remaining history in order. Ends a preview:
+   * it acts on the whole history, not on the step shown.
    */
   async undo(): Promise<number[]> {
     if (!this.history.length) return [];
     const popped = this.history[this.history.length - 1];
     const newHistory = this.history.slice(0, -1);
-    await this.reload();
     this.redoStack.push(popped);
-    this._isReplaying = true;
-    try {
-      const selection = await this.dispatch(
-        newHistory.map((h) => [h.method, ...(h.args || [])]),
-      );
-      this._restoreReplayedMeta(newHistory);
-      return selection;
-    } finally {
-      this._isReplaying = false;
-    }
+    return this._replay(newHistory);
   }
 
   /**
@@ -446,6 +508,8 @@ export class ObservationRecord {
    */
   async redo(): Promise<number[]> {
     if (!this.redoStack.length) return [];
+    // Redo builds on the whole history, so a preview ends first.
+    await this.exitPreview();
     const item = this.redoStack.pop()!;
     this._isReplaying = true;
     try {
@@ -491,6 +555,7 @@ export class ObservationRecord {
    * can pass locally-computed indices without going through history.
    */
   async dispatchAction(action: EnumEditOperations, ...args: any) {
+    this._refuseWhilePreviewing();
     const actions: EnumDictionary<EnumEditOperations, Function> = {
       [EnumEditOperations.ADD_POINTS]: this._addDataPoints,
       [EnumEditOperations.CHANGE_VALUES]: this._changeValues,
@@ -641,6 +706,7 @@ export class ObservationRecord {
 
   /** Filter operations do not transform the data and return a selection */
   async dispatchFilter(action: EnumFilterOperations, ...args: any): Promise<number[]> {
+    this._refuseWhilePreviewing();
     const filters: EnumDictionary<EnumFilterOperations, Function> = {
       [EnumFilterOperations.FIND_GAPS]: this._findGaps,
       [EnumFilterOperations.VALUE_THRESHOLD]: this._valueThreshold,

From 9ab68df17d6af03d4022f6253dab382146a6a5b7 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Tue, 22 Sep 2026 10:15:30 -0600
Subject: [PATCH 68/93] feat: move the time range control to the plot toolbar

---
 apps/qc-app/docs/PLOTTING.md                  |  14 +-
 apps/qc-app/docs/USER_GUIDE.md                |  21 ++-
 apps/qc-app/e2e/edit-selection.spec.ts        |  24 +--
 apps/qc-app/e2e/time-range.spec.ts            |  45 ++++-
 .../components/Navigation/SelectDrawer.vue    |  33 ----
 .../VisualizeData/DataVisualization.vue       | 168 ++++++++++--------
 .../src/components/VisualizeData/Plot.vue     |  88 +--------
 .../VisualizeData/TimeRangeMenu.vue           | 109 ++++++++++++
 .../__tests__/DataVisualization.spec.ts       |   7 +
 .../VisualizeData/__tests__/Plot.spec.ts      |   5 +-
 .../__tests__/TimeRangeMenu.spec.ts           | 108 +++++++++++
 11 files changed, 394 insertions(+), 228 deletions(-)
 create mode 100644 apps/qc-app/src/components/VisualizeData/TimeRangeMenu.vue
 create mode 100644 apps/qc-app/src/components/VisualizeData/__tests__/TimeRangeMenu.spec.ts

diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index c69a1f25..2f70920e 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -218,12 +218,14 @@ calls it instead of assigning `selectedSeries.value.data` directly.
 
 ## Context range
 
-The editor toolbar's **Context** menu reuses `DataVisTimeFilters` (with
-`EDITOR_PRESETS`, which leaves out YTD and highlights All for a persisted
-YTD through `shownPresetId`, and the same From / To pickers as
-the Select view) bound to the same `beginDate` / `endDate` store range. The
-Select view's own Time range is hidden while an edit target is set, and each
-remembers its own preset: `activePresetId` is `contextPresetId` while editing
+The plot toolbar's `TimeRangeMenu` is the one range control. With no edit
+target it is the Select view's **Time range** (every preset); with one it is
+the **Context** menu, which adds the source context switch and passes
+`DataVisTimeFilters` the `EDITOR_PRESETS` (YTD left out, All highlighted for
+a persisted YTD through `shownPresetId`). Both bind the same `beginDate` /
+`endDate` store range. When the plot is not mounted (nothing plotted, or no
+observations in the range), `DataVisualization` shows the menu in a toolbar
+row of its own, so a window can be set before any data is pulled. Each mode remembers its own preset: `activePresetId` is `contextPresetId` while editing
 and `selectedDateBtnId` otherwise, and every range action goes through it.
 Picking a preset or a custom date calls `setDateRange`, which, while an
 edit target is set, reloads only the context series
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 8ee3e715..d4c4bd04 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -79,15 +79,14 @@ The select view is the default landing surface after picking a workspace. The le
 
 ![Select view (Home)](./images/home-select.png)
 
-The filter drawer has two collapsible sections:
-
-- **Time range**: the loaded time window for the plotted datastreams. The two date pickers (`From` / `To`) show the window. The preset chips below them, **1w**, **1m**, **6m**, **1y**, **YTD**, **All**, count back from the last observation of the plotted datastreams (the newest one when several are plotted), so a preset always lands on data, even for datastreams whose observations are years old. That last observation is the one known when the workspace loaded, so data that arrives later only shows up after a reload. **1m** is the default, and the last preset you pick is remembered. Adding or removing a datastream re-anchors the preset. A `Custom` chip appears when you change a date in `From` or `To`; a custom window stays fixed while you add or remove datastreams. While you are editing, this section is hidden: the **Context** menu above the plot sets the range instead, with presets that count out from the edit session's window. The two remember their own preset, so neither changes the other.
-
-- **Datastream filters**: These filters allow you control the list of datastreams shown in the datastreams table by picking the site, observed property, and/or processing level. The list of matching datastreams updates live in the datastreams table.
+The filter drawer holds the **Datastream filters**. They let you control the list of datastreams shown in the datastreams table by picking the site, observed property, and/or processing level. The list of matching datastreams updates live in the datastreams table.
 
 The main area to the right of the window is split top/bottom:
 
 - **Top card** carries the preview plot and a header showing how many datastreams are plotted ("N datastreams plotted" or "No datastream plotted"). The right pane of the card lists currently plotted datastreams, and, while you are editing, an **Editing** panel above it.
+
+  The plot toolbar's **Time range** button (reading **Time range · 1m**, the active preset) opens the loaded time window for the plotted datastreams. The two date pickers (`From` / `To`) show the window. The preset chips below them, **1w**, **1m**, **6m**, **1y**, **YTD**, **All**, count back from the last observation of the plotted datastreams (the newest one when several are plotted), so a preset always lands on data, even for datastreams whose observations are years old. That last observation is the one known when the workspace loaded, so data that arrives later only shows up after a reload. **1m** is the default, and the last preset you pick is remembered. Adding or removing a datastream re-anchors the preset. A `Custom` chip appears when you change a date in `From` or `To`; a custom window stays fixed while you add or remove datastreams. The button is there before anything is plotted too, so you can set the window before pulling data: dates you pick are kept as a `Custom` window when the data loads, and a preset applies as soon as there is data to count back from. It also stays when the window holds no observations, so you can widen it. While you are editing, the same button becomes the **Context** menu, with presets that count out from the edit session's window. The two remember their own preset, so neither changes the other.
+
 - **Bottom card** is the **Datastreams selection table**: listing every datastream the filters match. Each row has a **Plot** toggle (check box) and an **Edit** column (Edit button). See [Datastreams with quality-controlled versions](#datastreams-with-quality-controlled-versions) below for what the Edit button does.
 
 ![Select view with one datastream plotted](./images/home-plotted.png)
@@ -123,9 +122,9 @@ What changes:
   versions: a source you plot this way draws in full, in its own colour, like
   any plotted datastream. The edit target itself is marked **Editing** in the
   chooser and cannot be picked, since it is already on the plot.
-- The filter drawer's **Time range** is hidden. The **Context** menu above the
-  plot sets the range around the session, with presets counting out from the
-  session window. Your edits are never reloaded by it.
+- The plot's **Time range** button becomes the **Context** menu, which sets
+  the range around the session, with presets counting out from the session
+  window. Your edits are never reloaded by it.
 - The rail's **Edit** button and the panel's **Open editor** button both
   open the editor with the session, your zoom and any staged range intact.
 - The row **Edit** button on a different datastream starts the usual entry
@@ -241,7 +240,7 @@ Between the help menu and the right edge is the **Context** button. It reads
 **Context off** when it is not. Its menu starts with a **Show source
 context** switch: off, the raw source leaves the plot entirely. Below it are
 preset chips and From / To pickers like the Select view's Time range
-filter, but here they control how much of the raw source and any plotted
+menu, but here they control how much of the raw source and any plotted
 datastreams load around your edit, not what you're editing. The raw source
 is never loaded inside the session window. The presets
 count out from the edit session's window instead of back from the last
@@ -326,7 +325,7 @@ The right-hand list (visible on both Select and Edit views) is the roster of eve
 - The datastream name and a subtitle showing the number of points loaded **in the current time window**, e.g. `1,248 pts loaded`. While the fetch is still in flight, the subtitle reads `loading…`.
 - An `×` button to unplot the row. The edit target and its source can't be unplotted this way. Leave the editor to drop them.
 
-If a plotted datastream has no observations in the current window (either because the dataset is empty there or because the chosen time range doesn't cover its data), the row title shows a small warning-tinted database icon. Hover it for the tooltip "No observations in the current time window". Widening the time range (or clicking **All** in the Time range / Context section) usually clears it.
+If a plotted datastream has no observations in the current window (either because the dataset is empty there or because the chosen time range doesn't cover its data), the row title shows a small warning-tinted database icon. Hover it for the tooltip "No observations in the current time window". Widening the time range (or clicking **All** in the Time range / Context menu) usually clears it.
 
 While editing, the edit target's row shows the session's working data: committed data where it exists, otherwise the raw datastream, over the session's window, with saved draft edits applied. Its line always covers the whole session window.
 
@@ -764,7 +763,7 @@ Click **Workspaces** in the nav rail and **Select** another, or **Continue** to
 | Symptom | Likely cause | Fix |
 |---------|--------------|-----|
 | Blank page on load | Wrong API URL or `localhost` vs `127.0.0.1` mismatch. | See [DEPLOYMENT.md](./DEPLOYMENT.md). |
-| Plot stays empty after picking a datastream | A `Custom` time range that doesn't overlap the datastream's observations, or the datastream has no observations. The plotted row shows a database-off icon and the subtitle reads `0 pts loaded`. | Click a preset such as **1m** or **All** in Time range. |
+| Plot stays empty after picking a datastream | A `Custom` time range that doesn't overlap the datastream's observations, or the datastream has no observations. The plotted row shows a database-off icon and the subtitle reads `0 pts loaded`. | Click a preset such as **1m** or **All** in the plot's Time range menu. |
 | The Edit rail item is greyed out | You are not editing. | Click a row's Edit button. |
 | The Select view shows an "Editing" panel | A session is open, or you just picked a datastream to edit. | That is expected: click **Open editor** to edit it, or **Close** on its row (or in the editor) to end it. |
 | I closed the editor and cannot find my session | It is still in progress, just not open. | Click the Edit button on that datastream's row to resume it. Sessions you discarded on the way out are gone for good. |
diff --git a/apps/qc-app/e2e/edit-selection.spec.ts b/apps/qc-app/e2e/edit-selection.spec.ts
index ea14d1d9..d8ec5704 100644
--- a/apps/qc-app/e2e/edit-selection.spec.ts
+++ b/apps/qc-app/e2e/edit-selection.spec.ts
@@ -247,9 +247,9 @@ test.describe('edit selection', () => {
         ])
       )
 
-    await page.getByTestId('context-range-btn').click()
+    await page.getByTestId('time-range-btn').click()
     await page
-      .getByTestId('context-range-menu')
+      .getByTestId('time-range-menu')
       .getByTestId('date-preset-All')
       .click()
     await expect(page.getByTestId('exit-save-btn')).toBeVisible()
@@ -341,8 +341,8 @@ test.describe('edit selection', () => {
       w.__updates = 0
       gd.on('plotly_update', () => w.__updates++)
     })
-    await page.getByTestId('context-range-btn').click()
-    const menu = page.getByTestId('context-range-menu')
+    await page.getByTestId('time-range-btn').click()
+    const menu = page.getByTestId('time-range-menu')
     const allActive = /v-chip--variant-tonal/.test(
       (await menu.getByTestId('date-preset-All').getAttribute('class')) ?? ''
     )
@@ -360,7 +360,9 @@ test.describe('edit selection', () => {
   test('the editor opens zoomed to the session window', async ({ page }) => {
     await gotoHome(page)
     await plotDatastreamById(page, DATASTREAM_ID_B)
-    await page.getByTestId('date-preset-All').click()
+    await page.getByTestId('time-range-btn').click()
+    await page.getByTestId('time-range-menu').getByTestId('date-preset-All').click()
+    await page.keyboard.press('Escape')
 
     // Start the session halfway through the source, so the window is
     // narrower than the context around it.
@@ -388,7 +390,9 @@ test.describe('edit selection', () => {
   test('changing the Context range keeps the user zoom', async ({ page }) => {
     await gotoHome(page)
     await plotDatastreamById(page, DATASTREAM_ID_B)
-    await page.getByTestId('date-preset-All').click()
+    await page.getByTestId('time-range-btn').click()
+    await page.getByTestId('time-range-menu').getByTestId('date-preset-All').click()
+    await page.keyboard.press('Escape')
     await startSessionFromRow(page)
     await expect.poll(() => xRange(page)).not.toBeNull()
     const opened = (await xRange(page))!
@@ -405,8 +409,8 @@ test.describe('edit selection', () => {
       .toBe(true)
     const zoomed = (await xRange(page))!
 
-    await page.getByTestId('context-range-btn').click()
-    const menu = page.getByTestId('context-range-menu')
+    await page.getByTestId('time-range-btn').click()
+    const menu = page.getByTestId('time-range-menu')
 
     // Record every redraw from here on, in page time, so none is missed.
     await page.evaluate(() => {
@@ -483,8 +487,8 @@ test.describe('edit selection', () => {
     await page.getByTestId('session-window-start').click()
     await openEditor(page)
 
-    await page.getByTestId('context-range-btn').click()
-    const menu = page.getByTestId('context-range-menu')
+    await page.getByTestId('time-range-btn').click()
+    const menu = page.getByTestId('time-range-menu')
     await expect(menu.getByTestId('date-preset-YTD')).toHaveCount(0)
     await menu.getByTestId('date-preset-1w').click()
     await expect(menu.getByTestId('date-preset-1w')).toHaveClass(
diff --git a/apps/qc-app/e2e/time-range.spec.ts b/apps/qc-app/e2e/time-range.spec.ts
index 56cf45f7..10269b34 100644
--- a/apps/qc-app/e2e/time-range.spec.ts
+++ b/apps/qc-app/e2e/time-range.spec.ts
@@ -41,19 +41,54 @@ test.describe('time range presets', () => {
     await expect(page.locator('.plotted-item__subtitle').first()).toHaveText(
       `${FIXTURE_OBS_COUNT} pts loaded`
     )
-    await expect(page.getByTestId('date-preset-1m')).toHaveClass(
+    await expect(page.getByTestId('time-range-btn')).toContainText('1m')
+    await page.getByTestId('time-range-btn').click()
+    const menu = page.getByTestId('time-range-menu')
+    await expect(menu.getByTestId('date-preset-1m')).toHaveClass(
       /v-chip--variant-tonal/
     )
-    await expect(page.getByTestId('date-preset-custom')).toHaveCount(0)
+    await expect(menu.getByTestId('date-preset-custom')).toHaveCount(0)
   })
 
   test('leaving a date field untouched keeps the preset', async ({ page }) => {
     await plotFirstDatastream(page)
-    await page.getByTestId('date-range-from').locator('input').first().click()
+    await page.getByTestId('time-range-btn').click()
+    const menu = page.getByTestId('time-range-menu')
+    await menu.getByTestId('date-range-from').locator('input').first().click()
     await page.keyboard.press('Tab')
-    await expect(page.getByTestId('date-preset-custom')).toHaveCount(0)
-    await expect(page.getByTestId('date-preset-1m')).toHaveClass(
+    await expect(menu.getByTestId('date-preset-custom')).toHaveCount(0)
+    await expect(menu.getByTestId('date-preset-1m')).toHaveClass(
       /v-chip--variant-tonal/
     )
   })
+
+  test('the Select view sets the range from the plot toolbar', async ({
+    page,
+  }) => {
+    await plotFirstDatastream(page)
+    await page.getByTestId('time-range-btn').click()
+    await page
+      .getByTestId('time-range-menu')
+      .getByTestId('date-preset-All')
+      .click()
+    await expect(page.getByTestId('time-range-btn')).toContainText('All')
+    await expect(page.getByTestId('context-toggle')).toHaveCount(0)
+  })
+
+  test('a range can be set before anything is plotted', async ({ page }) => {
+    await expect(page.getByText('Find a datastream')).toBeVisible()
+    await page.getByTestId('time-range-btn').click()
+    await page
+      .getByTestId('time-range-menu')
+      .getByTestId('date-preset-All')
+      .click()
+    await page.keyboard.press('Escape')
+    await expect(page.getByTestId('time-range-btn')).toContainText('All')
+
+    await plotFirstDatastream(page)
+    await expect(page.locator('.plotted-item__subtitle').first()).toHaveText(
+      `${FIXTURE_OBS_COUNT} pts loaded`
+    )
+    await expect(page.getByTestId('time-range-btn')).toContainText('All')
+  })
 })
diff --git a/apps/qc-app/src/components/Navigation/SelectDrawer.vue b/apps/qc-app/src/components/Navigation/SelectDrawer.vue
index fbef8c11..1d718cbf 100644
--- a/apps/qc-app/src/components/Navigation/SelectDrawer.vue
+++ b/apps/qc-app/src/components/Navigation/SelectDrawer.vue
@@ -35,30 +35,6 @@
 
       <v-divider />
 
-      <!-- While editing, the Context menu above the plot sets the range. -->
-      <template v-if="!qcDatastream">
-      <div
-        class="select-drawer__section-header d-flex align-center ga-1 px-3 py-1 cursor-pointer"
-        role="button"
-        tabindex="0"
-        @click="timeCollapsed = !timeCollapsed"
-        @keydown.enter.prevent="timeCollapsed = !timeCollapsed"
-        @keydown.space.prevent="timeCollapsed = !timeCollapsed"
-      >
-        <v-icon
-          size="16"
-          :icon="timeCollapsed ? 'mdi-chevron-right' : 'mdi-chevron-down'"
-        />
-        <v-icon icon="mdi-calendar-range" color="primary" size="16" />
-        <span class="text-body-small font-weight-medium">Time range</span>
-      </div>
-      <div v-show="!timeCollapsed" class="px-3 pt-2 pb-3" style="min-width: 0">
-        <DataVisTimeFilters />
-      </div>
-
-      <v-divider />
-      </template>
-
       <div
         class="select-drawer__section-header d-flex align-center ga-1 px-3 py-1 cursor-pointer"
         role="button"
@@ -93,13 +69,8 @@
 </template>
 
 <script setup lang="ts">
-import DataVisTimeFilters from '@/components/VisualizeData/DataVisTimeFilters.vue'
 import DatastreamFilters from '@/components/VisualizeData/DatastreamFilters.vue'
 import { useResizable, usePersistedFlag } from '@/composables/useResizable'
-import { storeToRefs } from 'pinia'
-import { useDataVisStore } from '@/store/dataVisualization'
-
-const { qcDatastream } = storeToRefs(useDataVisStore())
 
 const { size: drawerWidth, onStart: startDrag, dragging } = useResizable({
   initial: 320,
@@ -111,10 +82,6 @@ const drawerCollapsed = usePersistedFlag(
   'qc:selectLayout:drawerCollapsed',
   false
 )
-const timeCollapsed = usePersistedFlag(
-  'qc:selectLayout:timeCollapsed',
-  false
-)
 const filtersCollapsed = usePersistedFlag(
   'qc:selectLayout:filtersCollapsed',
   false
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisualization.vue b/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
index ff3c445d..a5106d33 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
@@ -26,94 +26,104 @@
     </template>
   </Plot>
 
-  <div
-    v-else-if="seriesDatastreams.length"
-    class="data-vis-state fill-height d-flex flex-column align-center justify-center pa-6 text-center"
-  >
-    <v-icon
-      icon="mdi-calendar-remove-outline"
-      size="56"
-      color="warning"
-      class="mb-3"
-    />
-    <div class="text-title-medium font-weight-bold mb-1">
-      No observations in this range
+  <!-- Nothing drawn yet: keep the range control where the plot toolbar puts
+       it, so a window can be set before any data is pulled. -->
+  <div v-else class="fill-height d-flex flex-column">
+    <div class="data-vis-toolbar d-flex align-center px-3 py-1">
+      <v-spacer />
+      <TimeRangeMenu />
     </div>
-    <div class="text-body-medium text-medium-emphasis" style="max-width: 360px">
-      The selected datastream{{ seriesDatastreams.length === 1 ? '' : 's' }}
-      returned no data for the current time window. Try a different range from
-      the drawer on the left.
+    <v-divider />
+
+    <div
+      v-if="seriesDatastreams.length"
+      class="data-vis-state flex-grow-1 d-flex flex-column align-center justify-center pa-6 text-center"
+    >
+      <v-icon
+        icon="mdi-calendar-remove-outline"
+        size="56"
+        color="warning"
+        class="mb-3"
+      />
+      <div class="text-title-medium font-weight-bold mb-1">
+        No observations in this range
+      </div>
+      <div class="text-body-medium text-medium-emphasis" style="max-width: 360px">
+        The selected datastream{{ seriesDatastreams.length === 1 ? '' : 's' }}
+        returned no data for the current time window. Try a different range
+        from the <b>Time range</b> menu above.
+      </div>
     </div>
-  </div>
 
-  <div
-    v-else
-    class="data-vis-state fill-height d-flex flex-column justify-center align-center pa-6"
-  >
-    <div class="data-vis-state__steps">
-      <div class="data-vis-state__step">
-        <div class="data-vis-state__step-num d-inline-flex align-center justify-center rounded-pill text-white">1</div>
-        <v-icon
-          icon="mdi-filter-variant"
-          size="24"
-          color="primary"
-          class="data-vis-state__step-icon"
-        />
-        <div class="data-vis-state__step-body">
-          <div class="text-title-small font-weight-bold">Find a datastream</div>
-          <div class="text-body-small text-medium-emphasis">
-            Use the filters on the left drawer and the search bar at the top of
-            the table to narrow the list.
+    <div
+      v-else
+      class="data-vis-state flex-grow-1 d-flex flex-column justify-center align-center pa-6"
+    >
+      <div class="data-vis-state__steps">
+        <div class="data-vis-state__step">
+          <div class="data-vis-state__step-num d-inline-flex align-center justify-center rounded-pill text-white">1</div>
+          <v-icon
+            icon="mdi-filter-variant"
+            size="24"
+            color="primary"
+            class="data-vis-state__step-icon"
+          />
+          <div class="data-vis-state__step-body">
+            <div class="text-title-small font-weight-bold">Find a datastream</div>
+            <div class="text-body-small text-medium-emphasis">
+              Use the filters on the left drawer and the search bar at the top of
+              the table to narrow the list.
+            </div>
           </div>
         </div>
-      </div>
 
-      <div class="data-vis-state__step">
-        <div class="data-vis-state__step-num d-inline-flex align-center justify-center rounded-pill text-white">2</div>
-        <v-icon
-          icon="mdi-checkbox-marked-outline"
-          size="24"
-          color="primary"
-          class="data-vis-state__step-icon"
-        />
-        <div class="data-vis-state__step-body">
-          <div class="text-title-small font-weight-bold">Plot datastreams</div>
-          <div class="text-body-small text-medium-emphasis">
-            Click the <b>Plot</b> checkbox on rows to preview them together.
+        <div class="data-vis-state__step">
+          <div class="data-vis-state__step-num d-inline-flex align-center justify-center rounded-pill text-white">2</div>
+          <v-icon
+            icon="mdi-checkbox-marked-outline"
+            size="24"
+            color="primary"
+            class="data-vis-state__step-icon"
+          />
+          <div class="data-vis-state__step-body">
+            <div class="text-title-small font-weight-bold">Plot datastreams</div>
+            <div class="text-body-small text-medium-emphasis">
+              Click the <b>Plot</b> checkbox on rows to preview them together.
+            </div>
           </div>
         </div>
-      </div>
 
-      <div class="data-vis-state__step">
-        <div class="data-vis-state__step-num d-inline-flex align-center justify-center rounded-pill text-white">3</div>
-        <v-icon
-          icon="mdi-calendar-range"
-          size="24"
-          color="primary"
-          class="data-vis-state__step-icon"
-        />
-        <div class="data-vis-state__step-body">
-          <div class="text-title-small font-weight-bold">Set the time range</div>
-          <div class="text-body-small text-medium-emphasis">
-            Adjust <b>Time filters</b> from the left drawer to cover the period
-            you want to inspect.
+        <div class="data-vis-state__step">
+          <div class="data-vis-state__step-num d-inline-flex align-center justify-center rounded-pill text-white">3</div>
+          <v-icon
+            icon="mdi-calendar-range"
+            size="24"
+            color="primary"
+            class="data-vis-state__step-icon"
+          />
+          <div class="data-vis-state__step-body">
+            <div class="text-title-small font-weight-bold">Set the time range</div>
+            <div class="text-body-small text-medium-emphasis">
+              Pick the period to inspect from the <b>Time range</b> menu
+              above, before or after plotting.
+            </div>
           </div>
         </div>
-      </div>
 
-      <div class="data-vis-state__step">
-        <div class="data-vis-state__step-num d-inline-flex align-center justify-center rounded-pill text-white">4</div>
-        <v-icon
-          icon="mdi-pencil"
-          size="24"
-          color="primary"
-          class="data-vis-state__step-icon"
-        />
-        <div class="data-vis-state__step-body">
-          <div class="text-title-small font-weight-bold">Edit one</div>
-          <div class="text-body-small text-medium-emphasis">
-            Click the <b>pencil</b> on a row to pick or create its QC datastream
-            and start a session.
+        <div class="data-vis-state__step">
+          <div class="data-vis-state__step-num d-inline-flex align-center justify-center rounded-pill text-white">4</div>
+          <v-icon
+            icon="mdi-pencil"
+            size="24"
+            color="primary"
+            class="data-vis-state__step-icon"
+          />
+          <div class="data-vis-state__step-body">
+            <div class="text-title-small font-weight-bold">Edit one</div>
+            <div class="text-body-small text-medium-emphasis">
+              Click the <b>pencil</b> on a row to pick or create its QC datastream
+              and start a session.
+            </div>
           </div>
         </div>
       </div>
@@ -127,6 +137,7 @@ import { computed } from 'vue'
 import { storeToRefs } from 'pinia'
 import { usePlotlyStore } from '@/store/plotly'
 import Plot from '@/components/VisualizeData/Plot.vue'
+import TimeRangeMenu from '@/components/VisualizeData/TimeRangeMenu.vue'
 
 const { plotlyOptions } = storeToRefs(usePlotlyStore())
 
@@ -151,6 +162,11 @@ const isDataAvailable = computed(() => {
   background-color: rgba(var(--v-theme-surface), 0.75);
 }
 
+.data-vis-toolbar {
+  background-color: rgba(var(--v-theme-primary), 0.02);
+  min-height: 40px;
+}
+
 .data-vis-state {
   background-color: rgba(var(--v-theme-primary), 0.02);
 }
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index 5eddd389..e8316c45 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -275,60 +275,7 @@
           </v-card>
         </v-menu>
 
-        <v-menu
-          v-if="!isPlotPreview"
-          v-model="contextRangeOpen"
-          :close-on-content-click="false"
-          location="bottom end"
-          offset="6"
-        >
-          <template #activator="{ props: menuProps }">
-            <v-btn
-              v-bind="menuProps"
-              size="small"
-              :variant="showSourceContext ? 'tonal' : 'text'"
-              :color="showSourceContext ? 'primary' : undefined"
-              :prepend-icon="
-                showSourceContext ? 'mdi-calendar-range' : 'mdi-eye-off-outline'
-              "
-              data-testid="context-range-btn"
-              :data-context-on="showSourceContext"
-              :title="
-                showSourceContext
-                  ? `Context range: ${contextRangeLabel} around the session`
-                  : `Context range: ${contextRangeLabel} around the session. The grey source context is hidden.`
-              "
-            >
-              Context &middot; {{ contextRangeLabel }}
-            </v-btn>
-          </template>
-          <v-card width="300" class="pa-3" data-testid="context-range-menu">
-            <v-switch
-              data-testid="context-toggle"
-              :model-value="showSourceContext"
-              color="primary"
-              density="compact"
-              hide-details
-              inset
-              class="mb-2"
-              @update:model-value="setShowSourceContext(!!$event)"
-            >
-              <template #label>
-                <span class="d-inline-flex align-center ga-2">
-                  <span
-                    class="context-swatch"
-                    :style="{ backgroundColor: SOURCE_CONTEXT_COLOR }"
-                  />
-                  Show source context
-                </span>
-              </template>
-            </v-switch>
-            <DataVisTimeFilters
-              :presets="EDITOR_PRESETS"
-              description="How much of the source and plotted datastreams to show before and after the session window. Your edits are not reloaded."
-            />
-          </v-card>
-        </v-menu>
+        <TimeRangeMenu />
       </div>
     </div>
 
@@ -449,13 +396,7 @@ import {
 } from '@/utils/plotting/plotly'
 import DataTable from '@/components/VisualizeData/DataTable.vue'
 import ContextPlot from '@/components/VisualizeData/ContextPlot.vue'
-import DataVisTimeFilters from '@/components/VisualizeData/DataVisTimeFilters.vue'
-import {
-  CUSTOM_PRESET_ID,
-  EDITOR_PRESETS,
-  findPreset,
-} from '@/utils/timeRangePresets'
-import { SOURCE_CONTEXT_COLOR } from '@/utils/plotting/plotly'
+import TimeRangeMenu from '@/components/VisualizeData/TimeRangeMenu.vue'
 import { useDataSelection } from '@/composables/useDataSelection'
 import { useBufferedNumber } from '@/composables/useBufferedNumber'
 import { usePersistedFlag } from '@/composables/useResizable'
@@ -483,20 +424,10 @@ const {
   pendingShareZoom,
   shareZoomEditTarget,
 } = storeToRefs(usePlotlyStore())
-const {
-  selectedData,
-  hasSelectionShape,
-  qcDatastream,
-  contextPresetId,
-  showSourceContext,
-} = storeToRefs(useDataVisStore())
-const { trackPlotWork, setShowSourceContext } = useDataVisStore()
-
-const contextRangeLabel = computed(() =>
-  contextPresetId.value === CUSTOM_PRESET_ID
-    ? 'Custom'
-    : (findPreset(contextPresetId.value)?.label ?? 'Custom')
+const { selectedData, hasSelectionShape, qcDatastream } = storeToRefs(
+  useDataVisStore()
 )
+const { trackPlotWork } = useDataVisStore()
 const { viewedSession, inProgressSession } = storeToRefs(useQcSessionStore())
 // The Select view previews the plot only when nothing is being edited; an
 // open session keeps the full chrome in both views.
@@ -623,8 +554,6 @@ async function copyShareableLink() {
   }
 }
 
-const contextRangeOpen = ref(false)
-
 const editWindow = computed(() => {
   const s = viewedSession.value ?? inProgressSession.value
   return s
@@ -833,13 +762,6 @@ const onTabChange = () => {
 </script>
 
 <style scoped>
-.context-swatch {
-  display: inline-block;
-  width: 14px;
-  height: 3px;
-  border-radius: 2px;
-}
-
 .plot-root {
   min-height: 0;
 }
diff --git a/apps/qc-app/src/components/VisualizeData/TimeRangeMenu.vue b/apps/qc-app/src/components/VisualizeData/TimeRangeMenu.vue
new file mode 100644
index 00000000..b38563ab
--- /dev/null
+++ b/apps/qc-app/src/components/VisualizeData/TimeRangeMenu.vue
@@ -0,0 +1,109 @@
+<template>
+  <v-menu
+    v-model="open"
+    :close-on-content-click="false"
+    location="bottom end"
+    offset="6"
+  >
+    <template #activator="{ props: menuProps }">
+      <v-btn
+        v-if="isEditing"
+        v-bind="menuProps"
+        size="small"
+        :variant="showSourceContext ? 'tonal' : 'text'"
+        :color="showSourceContext ? 'primary' : undefined"
+        :prepend-icon="
+          showSourceContext ? 'mdi-calendar-range' : 'mdi-eye-off-outline'
+        "
+        data-testid="time-range-btn"
+        :data-context-on="showSourceContext"
+        :title="
+          showSourceContext
+            ? `Context range: ${rangeLabel} around the session`
+            : `Context range: ${rangeLabel} around the session. The grey source context is hidden.`
+        "
+      >
+        Context &middot; {{ rangeLabel }}
+      </v-btn>
+      <v-btn
+        v-else
+        v-bind="menuProps"
+        size="small"
+        variant="tonal"
+        color="primary"
+        prepend-icon="mdi-calendar-range"
+        data-testid="time-range-btn"
+        :title="`Time range: ${rangeLabel}`"
+      >
+        Time range &middot; {{ rangeLabel }}
+      </v-btn>
+    </template>
+    <v-card width="300" class="pa-3" data-testid="time-range-menu">
+      <v-switch
+        v-if="isEditing"
+        data-testid="context-toggle"
+        :model-value="showSourceContext"
+        color="primary"
+        density="compact"
+        hide-details
+        inset
+        class="mb-2"
+        @update:model-value="setShowSourceContext(!!$event)"
+      >
+        <template #label>
+          <span class="d-inline-flex align-center ga-2">
+            <span
+              class="context-swatch"
+              :style="{ backgroundColor: SOURCE_CONTEXT_COLOR }"
+            />
+            Show source context
+          </span>
+        </template>
+      </v-switch>
+      <DataVisTimeFilters
+        v-if="isEditing"
+        :presets="EDITOR_PRESETS"
+        description="How much of the source and plotted datastreams to show before and after the session window. Your edits are not reloaded."
+      />
+      <DataVisTimeFilters v-else />
+    </v-card>
+  </v-menu>
+</template>
+
+<script setup lang="ts">
+import DataVisTimeFilters from '@/components/VisualizeData/DataVisTimeFilters.vue'
+import { useDataVisStore } from '@/store/dataVisualization'
+import { SOURCE_CONTEXT_COLOR } from '@/utils/plotting/plotly'
+import {
+  CUSTOM_PRESET_ID,
+  EDITOR_PRESETS,
+  findPreset,
+} from '@/utils/timeRangePresets'
+import { storeToRefs } from 'pinia'
+import { computed, ref } from 'vue'
+
+const { qcDatastream, activePresetId, showSourceContext } = storeToRefs(
+  useDataVisStore()
+)
+const { setShowSourceContext } = useDataVisStore()
+
+// With an edit target the range is the Context around the session.
+const isEditing = computed(() => !!qcDatastream.value)
+
+const rangeLabel = computed(() =>
+  activePresetId.value === CUSTOM_PRESET_ID
+    ? 'Custom'
+    : (findPreset(activePresetId.value)?.label ?? 'Custom')
+)
+
+const open = ref(false)
+</script>
+
+<style scoped>
+.context-swatch {
+  display: inline-block;
+  width: 14px;
+  height: 3px;
+  border-radius: 2px;
+}
+</style>
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts
index 436a950d..4cf0c308 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisualization.spec.ts
@@ -46,6 +46,10 @@ vi.mock('@/components/VisualizeData/Plot.vue', () => ({
   }),
 }))
 
+vi.mock('@/components/VisualizeData/TimeRangeMenu.vue', () => ({
+  default: { name: 'TimeRangeMenu', render: () => null },
+}))
+
 import DataVisualization from '@/components/VisualizeData/DataVisualization.vue'
 
 function mountIt() {
@@ -131,6 +135,7 @@ describe('DataVisualization.vue', () => {
     const wrapper = mountIt()
     expect(wrapper.text()).toContain('No observations in this range')
     expect(wrapper.findComponent({ name: 'PlotStub' }).exists()).toBe(false)
+    expect(wrapper.findComponent({ name: 'TimeRangeMenu' }).exists()).toBe(true)
   })
 
   it('shows the getting-started steps when nothing is plotted', () => {
@@ -139,5 +144,7 @@ describe('DataVisualization.vue', () => {
     const wrapper = mountIt()
     expect(wrapper.text()).toContain('Find a datastream')
     expect(wrapper.findComponent({ name: 'PlotStub' }).exists()).toBe(false)
+    // A window can be set before anything is plotted.
+    expect(wrapper.findComponent({ name: 'TimeRangeMenu' }).exists()).toBe(true)
   })
 })
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
index 3adf770e..3c057f5f 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
@@ -47,9 +47,6 @@ vi.mock('@/store/dataVisualization', () => ({
       hasSelectionShape: ref(false),
       qcDatastream,
       trackPlotWork,
-      contextPresetId: ref(1),
-      showSourceContext: ref(true),
-      setShowSourceContext: vi.fn(),
     }),
 }))
 
@@ -98,7 +95,7 @@ function mountIt(preview = true, slots: Record<string, string> = {}) {
     slots,
     global: {
       plugins: [createTestPinia(), createTestVuetify()],
-      stubs: { ContextPlot: true, DataTable: true, DataVisTimeFilters: true },
+      stubs: { ContextPlot: true, DataTable: true, TimeRangeMenu: true },
     },
   })
 }
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/TimeRangeMenu.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/TimeRangeMenu.spec.ts
new file mode 100644
index 00000000..3a792dea
--- /dev/null
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/TimeRangeMenu.spec.ts
@@ -0,0 +1,108 @@
+import { mount } from '@vue/test-utils'
+import { reactive, ref } from 'vue'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import { CUSTOM_PRESET_ID, EDITOR_PRESETS } from '@/utils/timeRangePresets'
+;(globalThis as any).ResizeObserver ||= class {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+// jsdom has no visualViewport; Vuetify's overlay positioning reads it.
+;(globalThis as any).visualViewport ||= {
+  addEventListener() {},
+  removeEventListener() {},
+  offsetLeft: 0,
+  offsetTop: 0,
+  width: 1024,
+  height: 768,
+  scale: 1,
+}
+
+const qcDatastream = ref<{ id: string } | null>(null)
+const activePresetId = ref(1)
+const showSourceContext = ref(true)
+
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () =>
+    reactive({
+      qcDatastream,
+      activePresetId,
+      showSourceContext,
+      setShowSourceContext: vi.fn(),
+    }),
+}))
+
+vi.mock('@/utils/plotting/plotly', () => ({ SOURCE_CONTEXT_COLOR: '#999' }))
+
+vi.mock('@/components/VisualizeData/DataVisTimeFilters.vue', () => ({
+  default: {
+    name: 'DataVisTimeFilters',
+    props: ['presets', 'description'],
+    render: () => null,
+  },
+}))
+
+import TimeRangeMenu from '@/components/VisualizeData/TimeRangeMenu.vue'
+
+function mountIt() {
+  return mount(TimeRangeMenu, {
+    attachTo: document.body,
+    global: { plugins: [createTestPinia(), createTestVuetify()] },
+  })
+}
+
+async function openMenu(w: ReturnType<typeof mountIt>) {
+  await w.find('[data-testid="time-range-btn"]').trigger('click')
+  await new Promise((r) => setTimeout(r))
+}
+
+describe('TimeRangeMenu.vue', () => {
+  beforeEach(() => {
+    qcDatastream.value = null
+    activePresetId.value = 1
+    showSourceContext.value = true
+    document.body.innerHTML = ''
+  })
+
+  it('labels the Select view range with the active preset', () => {
+    activePresetId.value = 5
+    const w = mountIt()
+    expect(w.find('[data-testid="time-range-btn"]').text()).toContain(
+      'Time range · All'
+    )
+    w.unmount()
+  })
+
+  it('labels a manual range as Custom', () => {
+    activePresetId.value = CUSTOM_PRESET_ID
+    const w = mountIt()
+    expect(w.find('[data-testid="time-range-btn"]').text()).toContain('Custom')
+    w.unmount()
+  })
+
+  it('offers every preset and no context switch in the Select view', async () => {
+    const w = mountIt()
+    await openMenu(w)
+    const filters = w.findComponent({ name: 'DataVisTimeFilters' })
+    expect(filters.props('presets')).toBeUndefined()
+    expect(document.querySelector('[data-testid="context-toggle"]')).toBeNull()
+    w.unmount()
+  })
+
+  it('becomes the Context menu while editing', async () => {
+    qcDatastream.value = { id: 'qc-1' }
+    const w = mountIt()
+    expect(w.find('[data-testid="time-range-btn"]').text()).toContain(
+      'Context · 1m'
+    )
+    await openMenu(w)
+    const filters = w.findComponent({ name: 'DataVisTimeFilters' })
+    expect(filters.props('presets')).toBe(EDITOR_PRESETS)
+    expect(
+      document.querySelector('[data-testid="context-toggle"]')
+    ).not.toBeNull()
+    w.unmount()
+  })
+})

From 0f2e9a376271b24c98150b5cfd3e2248a1d6dee4 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Tue, 22 Sep 2026 10:15:33 -0600
Subject: [PATCH 69/93] fix: show the overview strip only in the Edit view

---
 apps/qc-app/docs/ARCHITECTURE.md              |  1 +
 apps/qc-app/docs/PLOTTING.md                  |  4 ++-
 apps/qc-app/docs/USER_GUIDE.md                |  3 ++-
 apps/qc-app/e2e/select-while-editing.spec.ts  | 23 +++++++++++++++++
 .../src/components/VisualizeData/Plot.vue     | 15 ++++++++---
 .../VisualizeData/__tests__/Plot.spec.ts      | 25 +++++++++++++++++--
 6 files changed, 63 insertions(+), 8 deletions(-)

diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 6ca38ae8..645e38ec 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -211,6 +211,7 @@ Consequences of that split:
 - `isPlotPreview` (Select view, nothing being edited) drives the plot's
   preview chrome. An edit target keeps the full plot in both views, so the
   Select view shows the same series, the session band and the Context control.
+  The overview strip is the one piece kept to the Edit view.
   The flag only flips together with a rebuild (`setEditTarget` /
   `clearEditTarget`), so no extra redraw is needed for it.
 
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index 2f70920e..f23f14e4 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -325,7 +325,9 @@ trip moves the Plotly graph div instead of rebuilding it: the zoom, the live
 `layout.shapes` and the WebGL traces all stay. `userInterface.isPlotPreview`
 (Select view, nothing being edited) drives the preview chrome; with an edit
 target the full plot shows in both views, and the flag only flips alongside a
-rebuild (`setEditTarget` / `clearEditTarget`).
+rebuild (`setEditTarget` / `clearEditTarget`). The overview strip
+(`ContextPlot`) is the exception: it mounts only in the Edit view, and
+draws from the current series when it does.
 
 A share link's zoom (`pendingShareZoom`) is an explicit viewport, so it beats
 the editor's default "open on the session window" for the session that link
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index d4c4bd04..d39e274c 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -110,6 +110,7 @@ What changes:
   you in.
 - The preview plot draws the same series the editor does: the edit target,
   and its raw source around the session window.
+  The overview strip under the plot stays in the editor.
 - **Clear plot**, above the plotted list in the right pane, empties the
   plot. While a datastream is being edited it closes it first, with the
   usual question about its session; choosing to stay keeps everything as it
@@ -355,7 +356,7 @@ Once a datastream is on the plot, the **plotted datastreams list** on the right
 The Plotly toolbar at the top of the chart drives the X-axis gestures (Zoom, Pan, Reset). For Y axes, the gestures are slightly different and worth knowing about when you're juggling several datastreams:
 
 - **X-axis zoom**: drag a horizontal box on the time grid (Zoom tool), or scroll over the plot. Every axis stays time-synchronized.
-- **X-axis pan**: enable Pan in the toolbar, then drag the plot body. The context plot at the bottom is also draggable. It acts as an overview thumbnail.
+- **X-axis pan**: enable Pan in the toolbar, then drag the plot body. In the Edit view, the overview strip at the bottom is also draggable. It acts as a thumbnail of the loaded range.
 - **Per-axis zoom**: hover near the ends of any axis and drag. Only that axis rescales. Sibling axes keep their current view. This is how you "compare units": drop the temperature axis to a tight window without touching streamflow.
 - **Per-axis pan**: hover the middle of an axis and drag.
 - **Reset**: the home icon on the Plotly toolbar returns to the default zoom (does not change the begin/end dates in the
diff --git a/apps/qc-app/e2e/select-while-editing.spec.ts b/apps/qc-app/e2e/select-while-editing.spec.ts
index 1b7ec5d2..4ce2cdbf 100644
--- a/apps/qc-app/e2e/select-while-editing.spec.ts
+++ b/apps/qc-app/e2e/select-while-editing.spec.ts
@@ -164,6 +164,29 @@ test.describe('Select while editing', () => {
     await expect(fromField).toHaveValue(stagedFrom)
   })
 
+  test('keeps the overview strip to the editor', async ({ page }) => {
+    await setupEditView(page)
+    const overview = page.locator('.plot-context')
+    await expect(overview).toBeVisible()
+
+    await goToSelect(page)
+    await expect(overview).toHaveCount(0)
+
+    // Remounted on the way back, it draws from the series already loaded.
+    await goToEditor(page)
+    await expect(overview).toBeVisible()
+    await expect
+      .poll(() =>
+        page.evaluate(() => {
+          const gd = document.querySelector('.plot-context__plot') as
+            | (HTMLElement & { data?: unknown[] })
+            | null
+          return gd?.data?.length ?? 0
+        })
+      )
+      .toBeGreaterThan(0)
+  })
+
   test('plots context around the edit target from the table', async ({
     page,
   }) => {
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index e8316c45..eaec4bdc 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -291,7 +291,7 @@
               class="flex-fill"
               style="min-height: 0"
             ></div>
-            <template v-if="!isPlotPreview">
+            <template v-if="showOverview">
               <div
                 class="plot-context-strip d-flex align-center justify-center cursor-pointer user-select-none"
                 :title="
@@ -403,7 +403,7 @@ import { usePersistedFlag } from '@/composables/useResizable'
 import { formatDate, Snackbar } from '@uwrl/qc-utils'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { useQcSessionStore } from '@/store/qcSession'
-import { useUIStore } from '@/store/userInterface'
+import { DrawerType, useUIStore } from '@/store/userInterface'
 
 const { setPlotSelection, clearSelected } = useDataSelection()
 const { updateOptions, requestTableScroll } = usePlotlyStore()
@@ -431,7 +431,9 @@ const { trackPlotWork } = useDataVisStore()
 const { viewedSession, inProgressSession } = storeToRefs(useQcSessionStore())
 // The Select view previews the plot only when nothing is being edited; an
 // open session keeps the full chrome in both views.
-const { isPlotPreview } = storeToRefs(useUIStore())
+const { isPlotPreview, currentView } = storeToRefs(useUIStore())
+// The overview strip belongs to the editor alone.
+const showOverview = computed(() => currentView.value === DrawerType.Edit)
 
 const selectedCount = computed(() => selectedData.value?.length ?? 0)
 
@@ -611,7 +613,7 @@ const applyThreshold = () => {
   }
 }
 
-const gestures = [
+const allGestures = [
   {
     icon: 'mdi-cursor-default-click-outline',
     title: 'Click a point to toggle it',
@@ -636,6 +638,7 @@ const gestures = [
     icon: 'mdi-chart-areaspline',
     title: 'Use the overview strip',
     desc: 'Drag the band on the bottom mini-plot to set the visible time window.',
+    overview: true,
   },
   {
     icon: 'mdi-cursor-move',
@@ -644,6 +647,10 @@ const gestures = [
   },
 ]
 
+const gestures = computed(() =>
+  allGestures.filter((g) => !g.overview || showOverview.value)
+)
+
 const keyboardShortcuts = [
   {
     icon: 'mdi-undo-variant',
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
index 3c057f5f..c9a7a0cb 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
@@ -65,9 +65,11 @@ vi.mock('@/composables/useDataSelection', () => ({
 }))
 
 const isPlotPreview = ref(true)
+const currentView = ref<'Edit' | 'Select'>('Select')
 
-vi.mock('@/store/userInterface', () => ({
-  useUIStore: () => reactive({ isPlotPreview }),
+vi.mock('@/store/userInterface', async (importOriginal) => ({
+  ...(await importOriginal<typeof import('@/store/userInterface')>()),
+  useUIStore: () => reactive({ isPlotPreview, currentView }),
 }))
 
 vi.mock('@/composables/useResizable', () => ({
@@ -277,6 +279,25 @@ describe('Plot.vue delayed mount', () => {
     })
   })
 
+  describe('the overview strip', () => {
+    afterEach(() => {
+      currentView.value = 'Select'
+    })
+
+    it('shows in the Edit view', () => {
+      currentView.value = 'Edit'
+      const wrapper = mountIt(false)
+      expect(wrapper.find('.plot-context-strip').exists()).toBe(true)
+      wrapper.unmount()
+    })
+
+    it('stays out of the Select view, even with a session open', () => {
+      const wrapper = mountIt(false)
+      expect(wrapper.find('.plot-context-strip').exists()).toBe(false)
+      wrapper.unmount()
+    })
+  })
+
   it('renders the body overlay under the toolbar, not over it', () => {
     const wrapper = mountIt(false, {
       'body-overlay': '<div data-testid="overlay-probe" />',

From b783d21ea3882648835a814d3be1ca3a12bd2925 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Tue, 29 Sep 2026 10:06:39 -0600
Subject: [PATCH 70/93] fix: block edits while a committed session is on screen

---
 apps/qc-app/docs/API_REFERENCE.md             | 13 +++-
 apps/qc-app/docs/USER_GUIDE.md                | 14 ++++
 .../components/EditData/OperationPanel.vue    | 18 ++++-
 .../EditData/__tests__/OperationPanel.spec.ts | 70 +++++++++++++++++
 .../src/components/Navigation/EditDrawer.vue  | 22 +++++-
 .../Navigation/__tests__/EditDrawer.spec.ts   | 75 +++++++++++++++++++
 .../components/VisualizeData/DataTable.vue    | 17 ++++-
 .../components/VisualizeData/EditableCell.vue | 22 +++++-
 .../VisualizeData/__tests__/DataTable.spec.ts | 58 +++++++++++++-
 .../__tests__/EditableCell.spec.ts            | 40 ++++++++++
 .../composables/__tests__/useEditLock.spec.ts | 57 ++++++++++++++
 .../src/composables/useDataSelection.ts       |  9 ++-
 apps/qc-app/src/composables/useEditLock.ts    | 26 +++++++
 .../src/store/__tests__/qcSession.spec.ts     | 19 +++++
 apps/qc-app/src/store/qcSession.ts            |  6 +-
 .../utils/plotting/__tests__/selected.spec.ts | 21 ++++++
 apps/qc-app/src/utils/plotting/selected.ts    |  8 +-
 17 files changed, 475 insertions(+), 20 deletions(-)
 create mode 100644 apps/qc-app/src/components/EditData/__tests__/OperationPanel.spec.ts
 create mode 100644 apps/qc-app/src/components/Navigation/__tests__/EditDrawer.spec.ts
 create mode 100644 apps/qc-app/src/components/VisualizeData/__tests__/EditableCell.spec.ts
 create mode 100644 apps/qc-app/src/composables/__tests__/useEditLock.spec.ts
 create mode 100644 apps/qc-app/src/composables/useEditLock.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 5997073e..09fbab86 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -255,6 +255,17 @@ const report = await importHistory(file)
 into the active datastream **before** replay (selection-coupled ops
 reference indices against this windowed dataset).
 
+### `useEditLock()`
+
+`src/composables/useEditLock.ts`. The one answer to "can the edit record take
+a new operation right now?", read by every surface that starts an edit: the
+Operations drawer, `OperationPanel`, plot selections (`handleSelected`,
+`clearSelected`) and the data table's cells and Save button.
+
+| Name       | Kind     | Type                                  | Notes |
+|------------|----------|---------------------------------------|-------|
+| `editLock` | computed | `'readOnly' \| 'preview' \| null` | `readOnly` while a committed session is on screen (`useQcSessionStore().isReadOnly`), else `preview` while an earlier history step is shown (`usePlotlyStore().previewIndex`), else `null`. |
+
 ### `useEditSession()`
 
 Orchestrates the server-backed QC session workflow against the
@@ -722,7 +733,7 @@ puts the editor in read-only mode.
 | `viewedSession`     | computed | `QualityControlSession \| null`         | The session for `viewedSessionId`. |
 | `hasSessionOperations` | computed | `boolean`                            | True when the in-progress session holds any work: the operations the server returned with it, plus anything saved since (a save leaves them in `savedEdits` before the sessions are re-fetched). The leave flow tells an untouched session from one worth keeping with it. |
 | `fetchSessions`     | action   | `(historyId: string) => Promise<QualityControlSession[]>` | Fetch a history's sessions with their operations without writing any state, so a caller can drop a result that went stale (see `useEditSession`). |
-| `applySessions`     | action   | `(historyId: string, sessions: QualityControlSession[]) => void` | Adopt fetched sessions; default the view to the in-progress one, else the latest committed. |
+| `applySessions`     | action   | `(historyId: string, sessions: QualityControlSession[]) => void` | Adopt fetched sessions; default the view to the in-progress one, else the one committed last (by `committedAt`), so a commit leaves the session it just made on screen. |
 | `viewSession`       | action   | `(sessionId: string) => void`           | View a session read-only (no-op for an unknown id). |
 | `returnToCurrent`   | action   | `() => void`                            | Return to the editable in-progress session. |
 | `reset`             | action   | `() => void`                            | Clear all state. |
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index d39e274c..53834f65 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -682,6 +682,20 @@ The dialog lets you add an optional session description. Once you confirm:
 2. The session is committed and becomes read-only. The Snackbar shows "Session committed." and the footer swaps Save and Commit for **New session**.
 3. On failure, the Snackbar shows the backend's error message verbatim. Show that to your administrator if you need help.
 
+### Committed sessions are read-only
+
+A committed session can be looked at but not changed. This applies right after
+a commit, and whenever you pick an earlier session in the session list. While a
+committed session is on screen:
+
+- The Operations drawer disables every operation and says why.
+- Plot selections highlight points but are not recorded in the history.
+- Table cells show their values without offering to edit them.
+- Undo, redo, loading a QC history and step undo are unavailable.
+
+To keep editing, click **Return to current** (when a session is in progress) or
+**New session**.
+
 ## Leaving a session
 
 You never leave an edit session by accident. Anything that ends it, the
diff --git a/apps/qc-app/src/components/EditData/OperationPanel.vue b/apps/qc-app/src/components/EditData/OperationPanel.vue
index 9009a8e8..e003cfd4 100644
--- a/apps/qc-app/src/components/EditData/OperationPanel.vue
+++ b/apps/qc-app/src/components/EditData/OperationPanel.vue
@@ -37,7 +37,19 @@
     <v-divider />
 
     <div
-      v-if="previewIndex !== null"
+      v-if="editLock === 'readOnly'"
+      class="px-3 py-4 d-flex flex-column align-center text-center"
+      data-testid="operation-read-only-blocked"
+    >
+      <v-icon icon="mdi-lock-outline" size="28" color="primary" class="mb-2" />
+      <div class="text-body-small text-medium-emphasis">
+        This session is committed and read-only. Return to the current
+        session, or start a new one, to keep editing.
+      </div>
+    </div>
+
+    <div
+      v-else-if="editLock === 'preview'"
       class="px-3 py-4 d-flex flex-column align-center text-center"
       data-testid="operation-preview-blocked"
     >
@@ -118,17 +130,17 @@
 </template>
 
 <script setup lang="ts">
-import { usePlotlyStore } from '@/store/plotly'
 import { computed } from 'vue'
 import { storeToRefs } from 'pinia'
 import { useUIStore } from '@/store/userInterface'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { operationsById } from './operations'
 import FilterRangePanel from '@/components/FilterPoints/FilterRangePanel.vue'
+import { useEditLock } from '@/composables/useEditLock'
 
 const { selectedOperation, filterRangeActive } = storeToRefs(useUIStore())
 const { selectedData } = storeToRefs(useDataVisStore())
-const { previewIndex } = storeToRefs(usePlotlyStore())
+const { editLock } = useEditLock()
 
 const op = computed(() =>
   selectedOperation.value ? operationsById[selectedOperation.value] : null
diff --git a/apps/qc-app/src/components/EditData/__tests__/OperationPanel.spec.ts b/apps/qc-app/src/components/EditData/__tests__/OperationPanel.spec.ts
new file mode 100644
index 00000000..488c23a8
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/__tests__/OperationPanel.spec.ts
@@ -0,0 +1,70 @@
+import { mount, flushPromises } from '@vue/test-utils'
+import { ref } from 'vue'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+import { createTestVuetify } from '@/utils/test/vuetify'
+
+const editLock = ref<'readOnly' | 'preview' | null>(null)
+vi.mock('@/composables/useEditLock', () => ({
+  useEditLock: () => ({ editLock }),
+}))
+
+const selectedOperation = ref<string | null>('changeValues')
+const filterRangeActive = ref(false)
+vi.mock('@/store/userInterface', () => ({
+  useUIStore: () => ({ selectedOperation, filterRangeActive }),
+}))
+
+const selectedData = ref<number[] | null>([1, 2])
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () => ({ selectedData }),
+}))
+
+vi.mock('../operations', () => ({
+  operationsById: {
+    changeValues: {
+      id: 'changeValues',
+      title: 'Change values',
+      description: '',
+      icon: 'mdi-pencil',
+      group: 'edit',
+      requiresSelection: true,
+      component: { template: '<div data-testid="op-body" />' },
+    },
+  },
+}))
+
+import OperationPanel from '@/components/EditData/OperationPanel.vue'
+
+const mountPanel = () =>
+  mount(OperationPanel, {
+    global: { plugins: [createTestPinia(), createTestVuetify()] },
+  })
+
+beforeEach(() => {
+  editLock.value = null
+})
+
+describe('OperationPanel edit lock', () => {
+  it('renders the operation when unlocked', async () => {
+    const w = mountPanel()
+    await flushPromises()
+    expect(w.find('[data-testid="op-body"]').exists()).toBe(true)
+  })
+
+  it('blocks the operation on a committed session', async () => {
+    editLock.value = 'readOnly'
+    const w = mountPanel()
+    await flushPromises()
+    expect(w.find('[data-testid="operation-read-only-blocked"]').exists()).toBe(true)
+    expect(w.find('[data-testid="op-body"]').exists()).toBe(false)
+  })
+
+  it('blocks the operation while previewing', async () => {
+    editLock.value = 'preview'
+    const w = mountPanel()
+    await flushPromises()
+    expect(w.find('[data-testid="operation-preview-blocked"]').exists()).toBe(true)
+    expect(w.find('[data-testid="op-body"]').exists()).toBe(false)
+  })
+})
diff --git a/apps/qc-app/src/components/Navigation/EditDrawer.vue b/apps/qc-app/src/components/Navigation/EditDrawer.vue
index b85c841c..7b956450 100644
--- a/apps/qc-app/src/components/Navigation/EditDrawer.vue
+++ b/apps/qc-app/src/components/Navigation/EditDrawer.vue
@@ -1,6 +1,17 @@
 <template>
   <div class="edit-drawer d-flex flex-column fill-height">
     <div class="flex-grow-1 overflow-y-auto" style="min-height: 0">
+      <div
+        v-if="isReadOnly"
+        class="edit-drawer__locked d-flex align-start ga-2 px-3 py-2 text-body-small text-medium-emphasis"
+        data-testid="edit-drawer-read-only"
+      >
+        <v-icon icon="mdi-lock-outline" size="16" class="mt-1" />
+        <span>
+          Viewing a committed session. Operations are unavailable until you
+          return to the current session or start a new one.
+        </span>
+      </div>
       <div
         class="edit-drawer__section-header d-flex align-center ga-1 px-3 py-1 cursor-pointer"
         role="button"
@@ -23,6 +34,7 @@
           rounded="lg"
           class="mb-1"
           :active="selectedOperation === item.id"
+          :disabled="isReadOnly"
           :data-testid="`op-${item.id}`"
           @click="selectOperation(item.id)"
         >
@@ -64,7 +76,7 @@
           rounded="lg"
           class="mb-1"
           :active="selectedOperation === item.id"
-          :disabled="item.requiresSelection && !selectedData?.length"
+          :disabled="isReadOnly || (item.requiresSelection && !selectedData?.length)"
           :data-testid="`op-${item.id}`"
           @click="selectOperation(item.id)"
         >
@@ -106,6 +118,7 @@
           rounded="lg"
           class="mb-1"
           :active="selectedOperation === item.id"
+          :disabled="isReadOnly"
           :data-testid="`op-${item.id}`"
           @click="selectOperation(item.id)"
         >
@@ -127,6 +140,7 @@
 </template>
 
 <script setup lang="ts">
+import { computed } from 'vue'
 import { storeToRefs } from 'pinia'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { useUIStore } from '@/store/userInterface'
@@ -135,9 +149,12 @@ import {
   colorForOperation,
 } from '@/components/EditData/operations'
 import { usePersistedFlag } from '@/composables/useResizable'
+import { useEditLock } from '@/composables/useEditLock'
 
 const { selectedData } = storeToRefs(useDataVisStore())
+const { editLock } = useEditLock()
 const { selectedOperation } = storeToRefs(useUIStore())
+const isReadOnly = computed(() => editLock.value === 'readOnly')
 
 const filterPoints = operationsByGroup.filter
 const editData = operationsByGroup.edit
@@ -163,6 +180,9 @@ function selectOperation(id: string) {
   background-color: rgba(var(--v-theme-primary), 0.04);
   min-height: 28px;
 }
+.edit-drawer__locked {
+  background-color: rgba(var(--v-theme-primary), 0.06);
+}
 .edit-drawer__section-header:hover,
 .edit-drawer__section-header:focus {
   background-color: rgba(var(--v-theme-primary), 0.08);
diff --git a/apps/qc-app/src/components/Navigation/__tests__/EditDrawer.spec.ts b/apps/qc-app/src/components/Navigation/__tests__/EditDrawer.spec.ts
new file mode 100644
index 00000000..2e284ff5
--- /dev/null
+++ b/apps/qc-app/src/components/Navigation/__tests__/EditDrawer.spec.ts
@@ -0,0 +1,75 @@
+import { mount, flushPromises } from '@vue/test-utils'
+import { ref } from 'vue'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { createTestPinia } from '@/utils/test/pinia'
+import { createTestVuetify } from '@/utils/test/vuetify'
+
+const editLock = ref<'readOnly' | 'preview' | null>(null)
+vi.mock('@/composables/useEditLock', () => ({
+  useEditLock: () => ({ editLock }),
+}))
+
+const selectedOperation = ref<string | null>(null)
+vi.mock('@/store/userInterface', () => ({
+  useUIStore: () => ({ selectedOperation }),
+}))
+
+const selectedData = ref<number[] | null>([1])
+vi.mock('@/store/dataVisualization', () => ({
+  useDataVisStore: () => ({ selectedData }),
+}))
+
+vi.mock('@/components/EditData/operations', () => {
+  const op = (id: string, group: string) => ({
+    id,
+    title: id,
+    description: '',
+    icon: 'mdi-x',
+    group,
+    requiresSelection: group === 'edit',
+  })
+  return {
+    operationsByGroup: {
+      filter: [op('valueThreshold', 'filter')],
+      edit: [op('changeValues', 'edit')],
+      add: [op('addPoints', 'add')],
+    },
+    colorForOperation: () => 'primary',
+  }
+})
+
+import EditDrawer from '@/components/Navigation/EditDrawer.vue'
+
+const mountDrawer = () =>
+  mount(EditDrawer, {
+    global: { plugins: [createTestPinia(), createTestVuetify()] },
+  })
+
+const isDisabled = (w: ReturnType<typeof mountDrawer>, id: string) =>
+  w.find(`[data-testid="op-${id}"]`).classes().includes('v-list-item--disabled')
+
+beforeEach(() => {
+  editLock.value = null
+  selectedOperation.value = null
+})
+
+describe('EditDrawer edit lock', () => {
+  it('enables every operation when unlocked', async () => {
+    const w = mountDrawer()
+    await flushPromises()
+    expect(w.find('[data-testid="edit-drawer-read-only"]').exists()).toBe(false)
+    for (const id of ['valueThreshold', 'changeValues', 'addPoints']) {
+      expect(isDisabled(w, id)).toBe(false)
+    }
+  })
+
+  it('disables every operation and explains why on a committed session', async () => {
+    editLock.value = 'readOnly'
+    const w = mountDrawer()
+    await flushPromises()
+    expect(w.find('[data-testid="edit-drawer-read-only"]').exists()).toBe(true)
+    for (const id of ['valueThreshold', 'changeValues', 'addPoints']) {
+      expect(isDisabled(w, id)).toBe(true)
+    }
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData/DataTable.vue b/apps/qc-app/src/components/VisualizeData/DataTable.vue
index ed3db127..ba8b85bb 100644
--- a/apps/qc-app/src/components/VisualizeData/DataTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataTable.vue
@@ -8,7 +8,12 @@
             Observations
           </span>
           <span class="text-body-small text-medium-emphasis lh-1 mt-1">
-            Click <b>Datetime</b> or <b>Value</b> cells to edit
+            <template v-if="editLock === 'readOnly'">
+              Committed session, read-only
+            </template>
+            <template v-else>
+              Click <b>Datetime</b> or <b>Value</b> cells to edit
+            </template>
           </span>
         </div>
       </div>
@@ -52,9 +57,9 @@
         </v-btn>
 
         <v-btn
-          :disabled="!pendingEditCount || isUpdating || previewIndex !== null"
+          :disabled="!pendingEditCount || isUpdating || editLock !== null"
           :title="
-            previewIndex !== null
+            editLock === 'preview'
               ? 'Go back to the latest history step to save'
               : undefined
           "
@@ -110,6 +115,7 @@
                 : ''
             "
             input-type="datetime-local"
+            :readonly="editLock === 'readOnly'"
             @save="onDatetimeSave(index, $event)"
             @clear="clearDatetimeEdit(index)"
           />
@@ -126,6 +132,7 @@
             "
             input-type="number"
             align="end"
+            :readonly="editLock === 'readOnly'"
             @save="onValueSave(index, $event)"
             @clear="clearValueEdit(index)"
           />
@@ -172,10 +179,12 @@ import {
 import { useDataSelection } from '@/composables/useDataSelection'
 import { useQualifierStore } from '@/store/qualifiers'
 import EditableCell from '@/components/VisualizeData/EditableCell.vue'
+import { useEditLock } from '@/composables/useEditLock'
 
-const { isUpdating, selectedSeries, tableScrollRequest, previewIndex } = storeToRefs(
+const { isUpdating, selectedSeries, tableScrollRequest } = storeToRefs(
   usePlotlyStore()
 )
+const { editLock } = useEditLock()
 const { redraw } = usePlotlyStore()
 const { selectedData, qcDatastream } = storeToRefs(useDataVisStore())
 const { clearSelected } = useDataSelection()
diff --git a/apps/qc-app/src/components/VisualizeData/EditableCell.vue b/apps/qc-app/src/components/VisualizeData/EditableCell.vue
index 84badb01..560ab9b1 100644
--- a/apps/qc-app/src/components/VisualizeData/EditableCell.vue
+++ b/apps/qc-app/src/components/VisualizeData/EditableCell.vue
@@ -43,6 +43,12 @@
         <v-icon icon="mdi-close" size="16" />
       </button>
     </div>
+    <span
+      v-else-if="readonly"
+      class="editable-cell__display editable-cell__display--readonly d-inline-flex align-center rounded-sm"
+    >
+      {{ display }}
+    </span>
     <template v-else>
       <button
         type="button"
@@ -75,7 +81,7 @@
 </template>
 
 <script setup lang="ts">
-import { nextTick, ref } from 'vue'
+import { nextTick, ref, watch } from 'vue'
 
 const props = defineProps<{
   value: string
@@ -85,6 +91,8 @@ const props = defineProps<{
   editedDisplay: string
   inputType: 'number' | 'datetime-local'
   align?: 'start' | 'end'
+  /** Show the value without offering to edit it. */
+  readonly?: boolean
 }>()
 
 const emit = defineEmits<{
@@ -113,6 +121,13 @@ function commit() {
 function cancel() {
   isEditing.value = false
 }
+
+watch(
+  () => props.readonly,
+  (readonly) => {
+    if (readonly) cancel()
+  }
+)
 </script>
 
 <style scoped>
@@ -130,6 +145,11 @@ function cancel() {
   background: transparent;
 }
 
+.editable-cell__display--readonly:hover {
+  border-color: transparent;
+  background: transparent;
+}
+
 .editable-cell__display:hover {
   border-color: rgba(var(--v-theme-primary), 0.45);
   background-color: rgba(var(--v-theme-primary), 0.06);
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
index f67522f7..d1639471 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
@@ -26,6 +26,11 @@ const tableScrollRequest = ref<{ time: number; seq: number } | null>(null)
 const selectedData = ref<number[] | null>(null)
 const qcDatastream = ref<any>({ id: 'ds-1' })
 
+const editLock = ref<'readOnly' | 'preview' | null>(null)
+vi.mock('@/composables/useEditLock', () => ({
+  useEditLock: () => ({ editLock }),
+}))
+
 const qualifierById = ref<Record<string, any>>({})
 const applied = ref<Record<string, any>>({})
 
@@ -33,7 +38,6 @@ vi.mock('@/store/plotly', () => ({
   usePlotlyStore: () => ({
     isUpdating,
     selectedSeries,
-    previewIndex: ref(null),
     redraw,
     tableScrollRequest,
   }),
@@ -69,7 +73,7 @@ vi.mock('@uwrl/qc-utils', () => ({
 vi.mock('@/components/VisualizeData/EditableCell.vue', () => ({
   default: {
     name: 'EditableCell',
-    props: ['value', 'display', 'edited', 'originalDisplay', 'editedDisplay', 'inputType', 'align'],
+    props: ['value', 'display', 'edited', 'originalDisplay', 'editedDisplay', 'inputType', 'align', 'readonly'],
     emits: ['save', 'clear'],
     template: '<div class="editable-cell-stub" />',
   },
@@ -153,6 +157,7 @@ function createWrapperWithSlots() {
 
 afterEach(() => {
   while (openWrappers.length) openWrappers.pop()!.unmount()
+  editLock.value = null
 })
 
 describe('DataTable.vue', () => {
@@ -990,3 +995,52 @@ describe('DataTable.vue scroll requests', () => {
     expect(scrollToIndexCalls.at(-1)).toBe(0)
   })
 })
+
+describe('DataTable.vue edit lock', () => {
+  beforeEach(() => {
+    isUpdating.value = false
+    selectedSeries.value = {
+      data: {
+        dataX: [1000, 2000],
+        dataY: [10, 20],
+        dispatch: vi.fn().mockResolvedValue(undefined),
+      },
+    }
+    selectedData.value = null
+    qcDatastream.value = { id: 'ds-1' }
+    qualifierById.value = {}
+    applied.value = {}
+  })
+
+  it('makes cells read-only and blocks saving on a committed session', async () => {
+    const wrapper = createWrapperWithSlots()
+    await flushPromises()
+    valueCell(wrapper, 0).vm.$emit('save', '99')
+    await flushPromises()
+
+    editLock.value = 'readOnly'
+    await flushPromises()
+
+    expect(valueCell(wrapper, 0).props('readonly')).toBe(true)
+    expect(datetimeCell(wrapper, 0).props('readonly')).toBe(true)
+    expect(wrapper.text()).toContain('Committed session, read-only')
+    const save = wrapper
+      .findAll('button')
+      .find((b) => b.text().includes('Save changes'))!
+    expect(save.attributes('disabled')).toBeDefined()
+  })
+
+  it('keeps cells editable while previewing, but blocks saving', async () => {
+    const wrapper = createWrapperWithSlots()
+    await flushPromises()
+    valueCell(wrapper, 0).vm.$emit('save', '99')
+    editLock.value = 'preview'
+    await flushPromises()
+
+    expect(valueCell(wrapper, 0).props('readonly')).toBe(false)
+    const save = wrapper
+      .findAll('button')
+      .find((b) => b.text().includes('Save changes'))!
+    expect(save.attributes('disabled')).toBeDefined()
+  })
+})
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/EditableCell.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/EditableCell.spec.ts
new file mode 100644
index 00000000..5a29ebf2
--- /dev/null
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/EditableCell.spec.ts
@@ -0,0 +1,40 @@
+import { mount } from '@vue/test-utils'
+import { describe, it, expect } from 'vitest'
+import { createTestVuetify } from '@/utils/test/vuetify'
+import EditableCell from '@/components/VisualizeData/EditableCell.vue'
+
+const props = {
+  value: '10',
+  display: '10',
+  edited: false,
+  originalDisplay: '10',
+  editedDisplay: '',
+  inputType: 'number' as const,
+}
+
+const mountCell = (extra: Record<string, unknown> = {}) =>
+  mount(EditableCell, {
+    props: { ...props, ...extra },
+    global: { plugins: [createTestVuetify()] },
+  })
+
+describe('EditableCell readonly', () => {
+  it('opens an input on click when editable', async () => {
+    const w = mountCell()
+    await w.find('button.editable-cell__display').trigger('click')
+    expect(w.find('input').exists()).toBe(true)
+  })
+
+  it('shows the value with no way to edit when readonly', () => {
+    const w = mountCell({ readonly: true })
+    expect(w.text()).toContain('10')
+    expect(w.find('button').exists()).toBe(false)
+  })
+
+  it('closes an open input when it becomes readonly', async () => {
+    const w = mountCell()
+    await w.find('button.editable-cell__display').trigger('click')
+    await w.setProps({ readonly: true })
+    expect(w.find('input').exists()).toBe(false)
+  })
+})
diff --git a/apps/qc-app/src/composables/__tests__/useEditLock.spec.ts b/apps/qc-app/src/composables/__tests__/useEditLock.spec.ts
new file mode 100644
index 00000000..97e838de
--- /dev/null
+++ b/apps/qc-app/src/composables/__tests__/useEditLock.spec.ts
@@ -0,0 +1,57 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { ref } from 'vue'
+import { createTestPinia } from '@/utils/test/pinia'
+import { useQcSessionStore } from '@/store/qcSession'
+import { useEditLock } from '@/composables/useEditLock'
+
+const previewIndex = ref<number | null>(null)
+vi.mock('@/store/plotly', () => ({
+  usePlotlyStore: () => ({ previewIndex }),
+}))
+
+const session = (id: string, status: 'committed' | 'in_progress') =>
+  ({
+    id,
+    status,
+    phenomenonTimeStart: '2025-01-01T00:00:00Z',
+    phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+    createdAt: '2025-01-01T00:00:00Z',
+    committedAt: status === 'committed' ? '2025-02-01T00:00:00Z' : null,
+  }) as any
+
+beforeEach(() => {
+  createTestPinia()
+  previewIndex.value = null
+})
+
+describe('useEditLock', () => {
+  it('is unlocked on the in-progress session', () => {
+    useQcSessionStore().applySessions('h', [session('a', 'in_progress')])
+    expect(useEditLock().editLock.value).toBeNull()
+  })
+
+  it('is unlocked outside the session workflow', () => {
+    expect(useEditLock().editLock.value).toBeNull()
+  })
+
+  it('locks as preview while an earlier step is shown', () => {
+    previewIndex.value = 0
+    expect(useEditLock().editLock.value).toBe('preview')
+  })
+
+  it('locks as read-only once the only session is committed', () => {
+    useQcSessionStore().applySessions('h', [session('a', 'committed')])
+    expect(useEditLock().editLock.value).toBe('readOnly')
+  })
+
+  it('locks as read-only when viewing a committed session, even mid-preview', () => {
+    const store = useQcSessionStore()
+    store.applySessions('h', [
+      session('a', 'committed'),
+      session('b', 'in_progress'),
+    ])
+    store.viewSession('a')
+    previewIndex.value = 0
+    expect(useEditLock().editLock.value).toBe('readOnly')
+  })
+})
diff --git a/apps/qc-app/src/composables/useDataSelection.ts b/apps/qc-app/src/composables/useDataSelection.ts
index c3f0c9d3..f4dea043 100644
--- a/apps/qc-app/src/composables/useDataSelection.ts
+++ b/apps/qc-app/src/composables/useDataSelection.ts
@@ -13,6 +13,7 @@ import {
 import type { AppPlotlyTrace } from '@/utils/plotting/plotly'
 import type { PlotData } from 'plotly.js-dist'
 import { storeToRefs } from 'pinia'
+import { useEditLock } from '@/composables/useEditLock'
 
 import { computed } from 'vue'
 
@@ -20,7 +21,8 @@ export function useDataSelection() {
   const { plotlyRef, suppressedEchoSelection } = storeToRefs(
     usePlotlyStore()
   )
-  const { selectedSeries, previewIndex } = storeToRefs(usePlotlyStore())
+  const { selectedSeries } = storeToRefs(usePlotlyStore())
+  const { editLock } = useEditLock()
   const { selectedData } = storeToRefs(useDataVisStore())
 
   /**
@@ -115,8 +117,9 @@ export function useDataSelection() {
     selectedData.value = []
     hasSelectionShape.value = false
 
-    // A previewed step records nothing; the clear only shows.
-    if (recordHistory && previewIndex.value === null) {
+    // A previewed step or a committed session records nothing; the clear
+    // only shows.
+    if (recordHistory && editLock.value === null) {
       // Explicitly log the cleared state so qc-utils' `_selection`
       // empty-case logic (pop self, optionally pop the underlying
       // filter that drove the now-cleared selection) runs even though
diff --git a/apps/qc-app/src/composables/useEditLock.ts b/apps/qc-app/src/composables/useEditLock.ts
new file mode 100644
index 00000000..03d034c7
--- /dev/null
+++ b/apps/qc-app/src/composables/useEditLock.ts
@@ -0,0 +1,26 @@
+import { computed } from 'vue'
+import { storeToRefs } from 'pinia'
+import { usePlotlyStore } from '@/store/plotly'
+import { useQcSessionStore } from '@/store/qcSession'
+
+/**
+ * Why the edit record can't take new operations right now, or null when it
+ * can. `readOnly`: a committed session is on screen (after a commit, or
+ * picked from the session list). `preview`: an earlier history step is shown.
+ * Every surface that starts an edit (operation drawer and panels, plot
+ * selections, table cells) checks this one value.
+ */
+export type EditLock = 'readOnly' | 'preview' | null
+
+export function useEditLock() {
+  const { previewIndex } = storeToRefs(usePlotlyStore())
+  const { isReadOnly } = storeToRefs(useQcSessionStore())
+
+  const editLock = computed<EditLock>(() => {
+    if (isReadOnly.value) return 'readOnly'
+    if (previewIndex.value !== null) return 'preview'
+    return null
+  })
+
+  return { editLock }
+}
diff --git a/apps/qc-app/src/store/__tests__/qcSession.spec.ts b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
index 971a2488..8f99d0a6 100644
--- a/apps/qc-app/src/store/__tests__/qcSession.spec.ts
+++ b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
@@ -116,6 +116,25 @@ describe('useQcSessionStore', () => {
     expect(store.isReadOnly).toBe(true)
   })
 
+  it('views the session committed last, even when an older one starts later', () => {
+    const session = (id: string, start: string, committedAt: string) =>
+      ({
+        id,
+        status: 'committed',
+        ...win(start, start),
+        createdAt: committedAt,
+        committedAt,
+      }) as any
+
+    const store = useQcSessionStore()
+    store.applySessions('h-1', [
+      session('later-window', '2025-06-01T00:00:00Z', '2025-07-01T00:00:00Z'),
+      session('just-committed', '2025-01-01T00:00:00Z', '2025-08-01T00:00:00Z'),
+    ])
+
+    expect(store.viewedSessionId).toBe('just-committed')
+  })
+
   it('keeps the resume pointer out of reset, so a reload can still reopen', async () => {
     const h = unwrap(
       await qc.histories.create({
diff --git a/apps/qc-app/src/store/qcSession.ts b/apps/qc-app/src/store/qcSession.ts
index afce0870..ed8dafdc 100644
--- a/apps/qc-app/src/store/qcSession.ts
+++ b/apps/qc-app/src/store/qcSession.ts
@@ -123,10 +123,12 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     sessions.value = list
     const inProgress = list.find((s) => s.status === 'in_progress') ?? null
     currentSessionId.value = inProgress?.id ?? null
-    // Default view: the editable session, else the latest committed.
+    // Default view: the editable session, else the last one committed (the
+    // one a commit just made).
+    const commitOrder = (s: QualityControlSession) => s.committedAt ?? s.createdAt
     const latestCommitted = [...list]
       .filter((s) => s.status === 'committed')
-      .sort((a, b) => b.phenomenonTimeStart.localeCompare(a.phenomenonTimeStart))[0]
+      .sort((a, b) => commitOrder(b).localeCompare(commitOrder(a)))[0]
     viewedSessionId.value = inProgress?.id ?? latestCommitted?.id ?? null
   }
 
diff --git a/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts
index 8b21b779..96520b7c 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/selected.spec.ts
@@ -152,6 +152,27 @@ describe('handleSelected', () => {
     expect(dispatchFilter).toHaveBeenCalledWith('SELECTION', [102])
   })
 
+describe('handleSelected on a committed session', () => {
+  it('highlights without recording a SELECTION', async () => {
+    const { useQcSessionStore } = await import('@/store/qcSession')
+    useQcSessionStore().applySessions('h', [
+      {
+        id: 'a',
+        status: 'committed',
+        phenomenonTimeStart: '2025-01-01T00:00:00Z',
+        phenomenonTimeEnd: '2025-02-01T00:00:00Z',
+        createdAt: '2025-01-01T00:00:00Z',
+        committedAt: '2025-02-01T00:00:00Z',
+      } as any,
+    ])
+    const { handleSelected } = await import('../selected')
+    const dispatchFilter = vi.fn()
+    selectedSeries.value = { data: { dispatchFilter } } as any
+    await handleSelected({ points: [{ pointIndex: 1, curveNumber: 0 }] } as any)
+    expect(dispatchFilter).not.toHaveBeenCalled()
+  })
+})
+
 describe('handleSelected while previewing a history step', () => {
   it('highlights without recording a SELECTION', async () => {
     previewIndex.value = 1
diff --git a/apps/qc-app/src/utils/plotting/selected.ts b/apps/qc-app/src/utils/plotting/selected.ts
index 6a234477..4cf96ece 100644
--- a/apps/qc-app/src/utils/plotting/selected.ts
+++ b/apps/qc-app/src/utils/plotting/selected.ts
@@ -1,6 +1,7 @@
 import { usePlotlyStore } from '@/store/plotly'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { storeToRefs } from 'pinia'
+import { useEditLock } from '@/composables/useEditLock'
 import type {
   PlotMouseEvent,
   PlotRelayoutEvent,
@@ -37,8 +38,8 @@ export const handleSelected = async (
     selectedSeries,
     isUpdating,
     suppressedEchoSelection,
-    previewIndex,
   } = storeToRefs(usePlotlyStore())
+  const { editLock } = useEditLock()
   const { selectedData } = storeToRefs(useDataVisStore())
   const { qcDatastream } = storeToRefs(useDataVisStore())
 
@@ -84,8 +85,9 @@ export const handleSelected = async (
     const current = selectedData.value ?? []
     if (sameSelection(expected, current)) return
   }
-  // A previewed step only shows data: the selection highlights, unrecorded.
-  if (eventData && !isUpdating.value && previewIndex.value === null) {
+  // A previewed step or a committed session only shows data: the selection
+  // highlights, unrecorded.
+  if (eventData && !isUpdating.value && editLock.value === null) {
     await selectedSeries.value?.data.dispatchFilter(
       EnumFilterOperations.SELECTION,
       selectedData.value ?? []

From 9a056721cdd71371ea2720c8a039b391f01cf745 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Tue, 29 Sep 2026 10:06:56 -0600
Subject: [PATCH 71/93] fix: refit the y axis when a series gets new data

---
 apps/qc-app/docs/API_REFERENCE.md             |  2 +-
 apps/qc-app/docs/PLOTTING.md                  |  7 +-
 apps/qc-app/docs/TESTING.md                   |  4 +-
 apps/qc-app/e2e/editor-y-range.spec.ts        | 90 +++++++++++++++++++
 .../store/__tests__/dataVisualization.spec.ts | 14 +++
 apps/qc-app/src/store/dataVisualization.ts    | 17 ++--
 .../utils/plotting/__tests__/events.spec.ts   | 76 ++++++++++++++++
 apps/qc-app/src/utils/plotting/events.ts      | 25 +++++-
 8 files changed, 223 insertions(+), 12 deletions(-)
 create mode 100644 apps/qc-app/e2e/editor-y-range.spec.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 09fbab86..13f34960 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -476,7 +476,7 @@ on boot.
 | `removeSnapshotSeries`              | action   | `(id: string) => Promise<void>`                   | Drop one snapshot line. Leaves the edit target alone. |
 | `setPlottedDatastreams`             | action   | `(items: Datastream[]) => Promise<void>`          | Replace the plotted set wholesale (used by URL hydration). Doesn't touch the edit target. Hydrate that separately with `setEditTarget`. |
 | `setEditTarget`                     | action   | `(managedId: string) => Promise<void>`            | Begin editing `managedId`: drops snapshots, resets the `qcSession` store (not `resumeDatastreamId`) and clears zoom history when the target changes, sets `qcDatastreamId`, rebuilds the plot. Its data arrives later via `setEditRecord`. This action doesn't fetch it. |
-| `setEditRecord`                     | action   | `(record: ObservationRecord) => Promise<void>`    | Put the edit target's record on the plot: updates its graph series in place if one exists, otherwise adds it (`buildGraphSeries`) and redraws. The only path by which the edit target's data reaches the plot. Counted as plot work until drawn. |
+| `setEditRecord`                     | action   | `(record: ObservationRecord) => Promise<void>`    | Put the edit target's record on the plot: updates its graph series in place if one exists, otherwise adds it (`buildGraphSeries`), then re-plots keeping the x view and refitting the target's y axis (`handleNewPlot` with `refitSeriesIds`). The only path by which the edit target's data reaches the plot. Counted as plot work until drawn. |
 | `clearEditTarget`                   | action   | `() => Promise<void>`                             | Stop editing: drops snapshots, invalidates the edit target's working copy, clears `qcDatastreamId`, removes its graph series, and rebuilds. Plotted datastreams are left exactly as they were. |
 | `rebuildPlot`                       | action   | `() => Promise<void>`                             | Rebuild (refresh series, regenerate options, render) as a plot load. Plot loads (rebuilds and `setDateRange` range reloads) run one at a time; requests made while one runs share one queued follow-up, which is a rebuild if any of them was, and every caller resolves only after a load that started after its change. A load whose range moves while it loads drops those responses and loads again before drawing. Drops zoom history in the Select view; keeps the zoom while an edit target is set. |
 
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index f23f14e4..234803bc 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -300,7 +300,12 @@ previous target's window is never applied to the new one.
   source down to the window, leaving no context.
 - **Axes**: a re-plot (`handleNewPlot`) or `redraw` keeps the live y range
   only for an axis that had points drawn; an empty axis sits on Plotly's
-  default range, and carrying it would put the new data off the plot. Series
+  default range, and carrying it would put the new data off the plot. A
+  re-plot also refits an axis whose content is new: one that gains a series
+  with points, or holds a series passed in `refitSeriesIds`. `setEditRecord`
+  passes the edit target, whose record it just replaced. Opening a session
+  draws the source context alone first, then adds the working copy to the
+  same axis; keeping the context's range there clipped the edit target. Series
   sharing an axis (the edit target and its source) share one axis chip. The window watch runs a rebuild,
   since the source's traces first appear then and only a rebuild adds
   traces. The window is part of the `loadKey` a range reload checks.
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 50627e31..91bc61f7 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -12,8 +12,8 @@ the operator's manual; QUALITY.md is the policy.
 
 | Layer      | Runner                          | Where                        | Count    |
 | ---------- | ------------------------------- | ---------------------------- | -------- |
-| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 75 files |
-| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 34 files |
+| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 81 files |
+| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 35 files |
 
 There is no separate "integration" tier; component tests live in the
 unit tier and mount real Vue components with the Vue Test Utils
diff --git a/apps/qc-app/e2e/editor-y-range.spec.ts b/apps/qc-app/e2e/editor-y-range.spec.ts
new file mode 100644
index 00000000..2e1d2a99
--- /dev/null
+++ b/apps/qc-app/e2e/editor-y-range.spec.ts
@@ -0,0 +1,90 @@
+/**
+ * Opening a session fits the plot's y axis to the edit target's data. The
+ * editor first draws the grey source context alone, then adds the working
+ * copy on the same axis; the axis must refit to it rather than keep the range
+ * fitted to the context, which clipped the series.
+ */
+
+import { expect, test } from '@playwright/test'
+import { installMocks, type MockQcSession } from './support/mocks'
+import { gotoHome } from './support/app'
+import {
+  DATASTREAM_ID,
+  FIXTURE_OBS_END_ISO,
+  FIXTURE_OBS_SPACING_MS,
+  FIXTURE_OBS_START_MS,
+  MANAGED_DATASTREAM_ID,
+  QC_HISTORY_ID,
+  QC_SOURCE_CHECKSUM,
+} from './support/fixtures'
+
+const at = (i: number) =>
+  new Date(FIXTURE_OBS_START_MS + i * FIXTURE_OBS_SPACING_MS).toISOString()
+
+const series = (from: number, to: number, value: (i: number) => number) => {
+  const idx = Array.from({ length: to - from }, (_, k) => from + k)
+  return { phenomenonTime: idx.map(at), result: idx.map(value) }
+}
+
+// The context before the window sits near 10; the session's data reaches 500.
+const WINDOW_START = 40
+const PEAK = 500
+
+const session: MockQcSession = {
+  id: 'qcs-open',
+  historyId: QC_HISTORY_ID,
+  status: 'in_progress',
+  description: null,
+  phenomenonTimeStart: at(WINDOW_START),
+  phenomenonTimeEnd: FIXTURE_OBS_END_ISO,
+  sourceChecksum: QC_SOURCE_CHECKSUM,
+  createdAt: at(WINDOW_START),
+  committedAt: null,
+  createdBy: { name: 'Test User', email: 'test@example.com' },
+  dependencyIds: [],
+  operations: [],
+}
+
+test('the edit target is not clipped by a range fitted to other data', async ({
+  page,
+}) => {
+  await installMocks(page, {
+    qcHistories: true,
+    qcSessionState: [session],
+    observationsById: {
+      [DATASTREAM_ID]: series(0, 120, (i) => (i < WINDOW_START ? 10 : PEAK)),
+      [MANAGED_DATASTREAM_ID]: series(0, WINDOW_START, () => 10),
+    },
+  })
+  await gotoHome(page)
+
+  // The raw source is already plotted, zoomed onto its low start.
+  await page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`).click()
+  await page.getByTestId(`plot-option-${DATASTREAM_ID}`).click()
+  await page.getByTestId('plot-source-apply').click()
+  await page
+    .getByTestId('data-loading-indicator')
+    .waitFor({ state: 'hidden', timeout: 30_000 })
+  const plotArea = page.locator('.js-plotly-plot .nsewdrag').first()
+  const box = (await plotArea.boundingBox())!
+  await page.mouse.move(box.x + box.width * 0.1, box.y + box.height / 2)
+  for (let k = 0; k < 6; k++) await page.mouse.wheel(0, -300)
+
+  await page.getByTestId(`edit-datastream-${DATASTREAM_ID}`).click()
+  await page.getByRole('button', { name: /continue/i }).first().click()
+  await expect(page.getByTestId('open-editor-btn')).toBeVisible({
+    timeout: 30_000,
+  })
+
+  await expect
+    .poll(
+      () =>
+        page.evaluate((id) => {
+          const gd = document.querySelector('.js-plotly-plot') as any
+          const edit = gd?.data?.find((t: any) => t.id === id)
+          return edit?.y?.length ? (gd.layout.yaxis.range[1] as number) : null
+        }, MANAGED_DATASTREAM_ID),
+      { timeout: 15_000 }
+    )
+    .toBeGreaterThanOrEqual(PEAK)
+})
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index 663b431d..88cd1454 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -431,6 +431,20 @@ describe('useDataVisStore edit target', () => {
     expect(edits[0].data).toBe(second)
   })
 
+  it('setEditRecord keeps the x view and refits the edit series y axis', async () => {
+    const { store } = await managedPair()
+    await store.setEditTarget('mgd')
+    mockPlotlyRef.value = {}
+    const { handleNewPlot } = await import('@/utils/plotting/plotly')
+    vi.mocked(handleNewPlot).mockClear()
+    await store.setEditRecord({ dataX: [1], history: [] } as any)
+    await store.setEditRecord({ dataX: [2], history: [] } as any)
+    expect(vi.mocked(handleNewPlot).mock.calls).toEqual([
+      [undefined, { preserveZoom: true, refitSeriesIds: ['mgd'] }],
+      [undefined, { preserveZoom: true, refitSeriesIds: ['mgd'] }],
+    ])
+  })
+
   it('clearEditTarget keeps the plotted datastreams and drops edit series', async () => {
     const { store, other } = await managedPair()
     await store.plotDatastream(other as any)
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index 014ef9ba..97080203 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -442,7 +442,8 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     await rebuildPlot()
   }
 
-  /** Put the edit target's record on the plot, adding its series if needed. */
+  /** Put the edit target's record on the plot, adding its series if needed.
+   *  The x view stays; the target's y axis refits to the new record. */
   function setEditRecord(record: ObservationRecord): Promise<void> {
     return trackPlotWork(async () => {
       const edit = qcDatastream.value
@@ -450,14 +451,18 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
       const existing = graphSeriesArray.value.find((s) => s.id === edit.id)
       if (existing) {
         existing.data = record
-        await usePlotlyStore().redraw()
-        return
+      } else {
+        graphSeriesArray.value.push(buildGraphSeries(edit, record))
+        orderAndColorSeries()
       }
-      graphSeriesArray.value.push(buildGraphSeries(edit, record))
-      orderAndColorSeries()
       updateOptions()
       const { plotlyRef } = storeToRefs(usePlotlyStore())
-      if (plotlyRef.value) await handleNewPlot(undefined, { preserveZoom: true })
+      if (plotlyRef.value) {
+        await handleNewPlot(undefined, {
+          preserveZoom: true,
+          refitSeriesIds: [edit.id],
+        })
+      }
     })
   }
 
diff --git a/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
index 35ba166f..f4775230 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
@@ -102,4 +102,80 @@ describe('handleNewPlot', () => {
 
     expect(newPlot.mock.calls[0]?.[2]).toStrictEqual(layout)
   })
+
+  describe('preserveZoom y ranges', () => {
+    const trace = (id: string, n: number, yaxis = 'y') => ({
+      id,
+      yaxis,
+      x: Array.from({ length: n }, (_, i) => i),
+      y: Array.from({ length: n }, (_, i) => i),
+    })
+
+    /** Live plot: `ctx` alone on the primary axis, `other` on yaxis2. */
+    function livePlot() {
+      const gd = fakeGraphDiv({
+        xaxis: { range: [0, 10] },
+        yaxis: { range: [9, 11] },
+        yaxis2: { range: [100, 200] },
+      })
+      gd.data = [trace('ctx', 5), trace('other', 5, 'y2')]
+      plotlyRef.value = gd
+    }
+
+    const drawnLayout = () =>
+      newPlot.mock.calls.at(-1)?.[2] as Record<string, { range?: unknown }>
+
+    it('keeps every axis when the same series redraw', async () => {
+      const { handleNewPlot } = await import('@/utils/plotting/events')
+      livePlot()
+      plotlyOptions.value = {
+        traces: [trace('ctx', 8), trace('other', 5, 'y2')],
+        layout: { xaxis: {}, yaxis: {}, yaxis2: {} },
+        config: {},
+      }
+
+      await handleNewPlot(undefined, { preserveZoom: true })
+
+      expect(drawnLayout().yaxis!.range).toEqual([9, 11])
+      expect(drawnLayout().yaxis2!.range).toEqual([100, 200])
+    })
+
+    it('refits an axis that gains a series with points', async () => {
+      const { handleNewPlot } = await import('@/utils/plotting/events')
+      livePlot()
+      plotlyOptions.value = {
+        traces: [
+          trace('ctx', 5),
+          trace('edit', 50),
+          trace('other', 5, 'y2'),
+        ],
+        layout: { xaxis: {}, yaxis: {}, yaxis2: {} },
+        config: {},
+      }
+
+      await handleNewPlot(undefined, { preserveZoom: true })
+
+      expect(drawnLayout().yaxis!.range).toBeUndefined()
+      expect(drawnLayout().yaxis2!.range).toEqual([100, 200])
+      expect(drawnLayout().xaxis!.range).toEqual([0, 10])
+    })
+
+    it('refits the axis of a series whose data was replaced', async () => {
+      const { handleNewPlot } = await import('@/utils/plotting/events')
+      livePlot()
+      plotlyOptions.value = {
+        traces: [trace('ctx', 5), trace('other', 5, 'y2')],
+        layout: { xaxis: {}, yaxis: {}, yaxis2: {} },
+        config: {},
+      }
+
+      await handleNewPlot(undefined, {
+        preserveZoom: true,
+        refitSeriesIds: ['ctx'],
+      })
+
+      expect(drawnLayout().yaxis!.range).toBeUndefined()
+      expect(drawnLayout().yaxis2!.range).toEqual([100, 200])
+    })
+  })
 })
diff --git a/apps/qc-app/src/utils/plotting/events.ts b/apps/qc-app/src/utils/plotting/events.ts
index 57efbbeb..b0e48b48 100644
--- a/apps/qc-app/src/utils/plotting/events.ts
+++ b/apps/qc-app/src/utils/plotting/events.ts
@@ -62,9 +62,16 @@ const handleClick = async (eventData: PlotMouseEvent) => {
   }
 }
 
+/**
+ * Draw `plotlyOptions` onto the plot. With `preserveZoom`, the live x range
+ * and each series' y range carry over, except on an axis whose content is
+ * new: one that gains a series with points, or holds a series listed in
+ * `refitSeriesIds` (its data was replaced). Such an axis was fitted to other
+ * data, so it autoranges instead of clipping what arrived.
+ */
 export const handleNewPlot = async (
   element?: HTMLElement,
-  opts?: { preserveZoom?: boolean }
+  opts?: { preserveZoom?: boolean; refitSeriesIds?: string[] }
 ) => {
   const { plotlyOptions, plotlyRef, mainPlotEpoch, pendingShareZoom } =
     storeToRefs(usePlotlyStore())
@@ -116,24 +123,38 @@ export const handleNewPlot = async (
     const yAxisKey = (yref: string | undefined) =>
       `yaxis${(yref ?? 'y').slice(1)}`
 
+    const hasPoints = (t: AppPlotlyTrace) =>
+      !!(t.x as ArrayLike<unknown> | undefined)?.length
+
     const yRangesBySeriesId: Record<string, Array<string | number>> = {}
     for (const trace of plotlyRef.value.data) {
       const t = trace as AppPlotlyTrace
       // An empty trace's axis sits on Plotly's default range, not a view the
       // user chose; carrying it would push the new data off the plot.
-      if (!t.id || !(t.x as ArrayLike<unknown> | undefined)?.length) continue
+      if (!t.id || !hasPoints(t)) continue
       const key = yAxisKey(t.yaxis as string | undefined)
       const range = (oldLayout[key] as Partial<LayoutAxis> | undefined)
         ?.range as Array<string | number> | undefined
       if (range) yRangesBySeriesId[t.id] = range
     }
 
+    const refit = new Set(opts.refitSeriesIds ?? [])
+    const refitAxes = new Set<string>()
+    for (const trace of plotlyOptions.value.traces) {
+      const t = trace as AppPlotlyTrace
+      if (!t.id || !hasPoints(t)) continue
+      if (refit.has(t.id) || !yRangesBySeriesId[t.id]) {
+        refitAxes.add(yAxisKey(t.yaxis as string | undefined))
+      }
+    }
+
     for (const trace of plotlyOptions.value.traces) {
       const t = trace as AppPlotlyTrace
       if (!t.id) continue
       const oldRange = yRangesBySeriesId[t.id]
       if (!oldRange) continue
       const key = yAxisKey(t.yaxis as string | undefined)
+      if (refitAxes.has(key)) continue
       const nextAxis = newLayout[key] as Partial<LayoutAxis> | undefined
       if (nextAxis) {
         nextAxis.range = [...oldRange]

From 7e3d5a25c0f99853691ab645783e70142918954e Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Tue, 29 Sep 2026 10:07:05 -0600
Subject: [PATCH 72/93] feat: show the local time zone on date and time inputs

---
 apps/qc-app/docs/USER_GUIDE.md                |  5 +++
 .../VisualizeData/DatePickerField.vue         | 27 ++++++++++++++--
 .../VisualizeData/TimeRangeMenu.vue           |  2 +-
 .../__tests__/DatePickerField.spec.ts         | 12 +++++++
 apps/qc-app/src/utils/__tests__/time.spec.ts  | 31 +++++++++++++++++++
 apps/qc-app/src/utils/time.ts                 | 20 ++++++++++++
 6 files changed, 93 insertions(+), 4 deletions(-)

diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 53834f65..c2f2327a 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -50,6 +50,11 @@ Everything runs in your web browser. The backend never sees your edit history un
 
 4. On **Home**, the left filter drawer is open. Pick a time range and filter the datastream list by site / observed property / processing level. Click a row to plot it.
 
+All dates and times in the app, in the pickers, the plot axis, the table and
+tooltips, are in your browser's local time zone. Every time input names that
+zone at its end (for example `MDT`, or `MST` for a date in winter); hover it
+for the full zone name and its UTC offset.
+
 If the screen ever stays blank with a console error like `Failed to fetch app settings`, ask your administrator to check the API URL and COOP/COEP configuration ([DEPLOYMENT.md](./DEPLOYMENT.md) covers this).
 
 ## The QC App user interface, top to bottom
diff --git a/apps/qc-app/src/components/VisualizeData/DatePickerField.vue b/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
index 333675ae..c711b851 100644
--- a/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
+++ b/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
@@ -1,7 +1,8 @@
 <template>
-  <div class="d-flex align-center ga-1" v-bind="$attrs">
+  <div class="d-flex flex-wrap align-center ga-1" v-bind="$attrs">
     <v-text-field
       ref="dateField"
+      class="date-picker-field__date"
       :model-value="dateInput"
       @blur="handleDateBlur"
       placeholder="MM/DD/YYYY"
@@ -17,9 +18,12 @@
       @blur="handleTimeBlur"
       :placeholder="seconds ? 'HH:MM:SS' : 'HH:MM'"
       prepend-inner-icon="mdi-clock-outline"
+      :suffix="zone"
+      :title="`Local time: ${zoneDescription}`"
       hide-details
       density="compact"
-      :style="{ 'max-width': seconds ? '9rem' : '7rem' }"
+      :style="{ flex: '0 0 auto', width: seconds ? '10.5rem' : '8.75rem' }"
+      data-testid="date-picker-time"
     />
   </div>
 
@@ -50,7 +54,15 @@
 </template>
 
 <script setup lang="ts">
-import { nextTick, onBeforeUnmount, onMounted, ref, watch } from 'vue'
+import {
+  computed,
+  nextTick,
+  onBeforeUnmount,
+  onMounted,
+  ref,
+  watch,
+} from 'vue'
+import { timeZoneAbbreviation, timeZoneDescription } from '@/utils/time'
 
 defineOptions({ inheritAttrs: false })
 
@@ -125,6 +137,10 @@ const toMidnight = (date: Date) => {
 
 const showDateDialog = ref(false)
 
+// Inputs read and write the browser's local time; say which one.
+const zone = computed(() => timeZoneAbbreviation(props.modelValue))
+const zoneDescription = computed(() => timeZoneDescription(props.modelValue))
+
 const dateInput = ref(formatDateStr(props.modelValue))
 const timeInput = ref(formatTimeStr(props.modelValue))
 const pickerDate = ref<Date>(toMidnight(props.modelValue))
@@ -429,6 +445,11 @@ onBeforeUnmount(() => {
 </script>
 
 <style scoped>
+/* Wide enough for MM/DD/YYYY; the time field wraps below it instead. */
+.date-picker-field__date {
+  min-width: 9.25rem;
+}
+
 .date-picker-card :deep(.v-date-picker-controls) {
   justify-content: center;
   gap: 0.25rem;
diff --git a/apps/qc-app/src/components/VisualizeData/TimeRangeMenu.vue b/apps/qc-app/src/components/VisualizeData/TimeRangeMenu.vue
index b38563ab..07db6e76 100644
--- a/apps/qc-app/src/components/VisualizeData/TimeRangeMenu.vue
+++ b/apps/qc-app/src/components/VisualizeData/TimeRangeMenu.vue
@@ -38,7 +38,7 @@
         Time range &middot; {{ rangeLabel }}
       </v-btn>
     </template>
-    <v-card width="300" class="pa-3" data-testid="time-range-menu">
+    <v-card width="330" class="pa-3" data-testid="time-range-menu">
       <v-switch
         v-if="isEditing"
         data-testid="context-toggle"
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts
index 429b4f25..fa940d87 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts
@@ -7,6 +7,7 @@ import { mount } from '@vue/test-utils'
 import { describe, it, expect } from 'vitest'
 import { createTestVuetify } from '@/utils/test/vuetify'
 import DatePickerField from '../DatePickerField.vue'
+import { timeZoneAbbreviation, timeZoneDescription } from '@/utils/time'
 ;(globalThis as any).ResizeObserver ||= class {
   observe() {}
   unobserve() {}
@@ -52,4 +53,15 @@ describe('DatePickerField', () => {
     )
     wrapper.unmount()
   })
+
+  it('labels the time input with the local zone of the shown date', () => {
+    const value = new Date(2026, 6, 15, 12)
+    const wrapper = mountField(value)
+    const field = wrapper.find('[data-testid="date-picker-time"]')
+    expect(field.text()).toContain(timeZoneAbbreviation(value))
+    expect(wrapper.find('[title^="Local time"]').attributes('title')).toBe(
+      `Local time: ${timeZoneDescription(value)}`
+    )
+    wrapper.unmount()
+  })
 })
diff --git a/apps/qc-app/src/utils/__tests__/time.spec.ts b/apps/qc-app/src/utils/__tests__/time.spec.ts
index b1ae345d..49e5561f 100644
--- a/apps/qc-app/src/utils/__tests__/time.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/time.spec.ts
@@ -4,6 +4,8 @@ import {
   formatDateRange,
   formatDayStamp,
   formatStamp,
+  timeZoneAbbreviation,
+  timeZoneDescription,
 } from '../time'
 
 // Datetime strings without a timezone offset are parsed as local time, so
@@ -91,3 +93,32 @@ describe('formatStamp', () => {
     expect(formatStamp(new Date(NaN))).toBe('–')
   })
 })
+
+describe('time zone labels', () => {
+  const winter = new Date(2026, 0, 15, 12)
+  const summer = new Date(2026, 6, 15, 12)
+  const zone = Intl.DateTimeFormat().resolvedOptions().timeZone
+
+  const offsetOf = (d: Date) => {
+    const m = -d.getTimezoneOffset()
+    const sign = m < 0 ? '-' : '+'
+    const hh = String(Math.floor(Math.abs(m) / 60)).padStart(2, '0')
+    const mm = String(Math.abs(m) % 60).padStart(2, '0')
+    return `UTC${sign}${hh}:${mm}`
+  }
+
+  it('abbreviates the browser zone on the given date', () => {
+    for (const d of [winter, summer]) {
+      const expected = new Intl.DateTimeFormat('en-US', { timeZoneName: 'short' })
+        .formatToParts(d)
+        .find((p) => p.type === 'timeZoneName')!.value
+      expect(timeZoneAbbreviation(d)).toBe(expected)
+      expect(timeZoneAbbreviation(d)).not.toBe('')
+    }
+  })
+
+  it('names the zone and its UTC offset on the given date', () => {
+    expect(timeZoneDescription(winter)).toBe(`${zone}, ${offsetOf(winter)}`)
+    expect(timeZoneDescription(summer)).toBe(`${zone}, ${offsetOf(summer)}`)
+  })
+})
diff --git a/apps/qc-app/src/utils/time.ts b/apps/qc-app/src/utils/time.ts
index 41ec8113..c41acc39 100644
--- a/apps/qc-app/src/utils/time.ts
+++ b/apps/qc-app/src/utils/time.ts
@@ -111,6 +111,26 @@ function getLocalTimeZone(): string {
   return Intl.DateTimeFormat().resolvedOptions().timeZone || '–'
 }
 
+const ZONE_NAME = new Intl.DateTimeFormat('en-US', { timeZoneName: 'short' })
+
+/** The browser's zone abbreviation on `date` (`MDT`, `MST`, `GMT+2`), which
+ *  tracks daylight saving. Every date the app shows or takes is in it. */
+export function timeZoneAbbreviation(date: Date): string {
+  return (
+    ZONE_NAME.formatToParts(date).find((p) => p.type === 'timeZoneName')
+      ?.value ?? ''
+  )
+}
+
+/** `America/Denver, UTC-06:00` for `date`. */
+export function timeZoneDescription(date: Date): string {
+  const offset = -date.getTimezoneOffset()
+  const sign = offset < 0 ? '-' : '+'
+  const hh = String(Math.floor(Math.abs(offset) / 60)).padStart(2, '0')
+  const mm = String(Math.abs(offset) % 60).padStart(2, '0')
+  return `${getLocalTimeZone()}, UTC${sign}${hh}:${mm}`
+}
+
 export function formatTimeWithZone(time?: string | null) {
   if (!time) return '–'
   return `${formatTime(time)} (${getLocalTimeZone()})`

From b75f7204e8ecd9cb7a9523ae87c81e762d605406 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Tue, 29 Sep 2026 10:07:18 -0600
Subject: [PATCH 73/93] feat: show the period each history step touched

---
 apps/qc-app/docs/USER_GUIDE.md                |  6 ++
 .../src/components/EditData/EditHistory.vue   | 61 ++++++++++----
 .../EditData/__tests__/EditHistory.spec.ts    | 25 ++++++
 packages/qc-utils/docs/API_REFERENCE.md       | 11 ++-
 packages/qc-utils/docs/ARCHITECTURE.md        |  1 +
 packages/qc-utils/docs/QC_HISTORY.md          |  2 +-
 packages/qc-utils/src/types/index.ts          | 13 ++-
 .../__tests__/observation-record.spec.ts      | 84 +++++++++++++++++++
 .../src/utils/plotting/observation-record.ts  | 47 ++++++++---
 9 files changed, 219 insertions(+), 31 deletions(-)

diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index c2f2327a..18d758f4 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -553,6 +553,12 @@ The body shows:
 - A baseline **Data loaded** row at the top, carrying a plot-this-step button and a **discard-edits-and-reload-from-server** button (cloud icon). Clicking the row returns the plot to the state the session started from. The cloud button is hidden on a committed session, where there are no edits to discard.
 - One row per history entry, each with:
   - The operation icon and Title-Case name.
+  - Under the name, the period the step touched and how many points, for
+    example `Mar 3, 12:05 PM – Apr 1, 2026, 8:00 AM · 1,200 pts`. For a
+    filter or selection that is the points it picked; for an edit, the points
+    it changed, as they were before it ran; for Add points and Fill gaps, the
+    points it inserted. The dates are recorded when the step runs, so later
+    edits that move or remove points don't change them.
   - A failure badge (red `!`) if the op threw at author time. Common after a QC history import that references something missing in this datastream.
   - A duration badge.
   - In dev mode, a small chip showing whether the op ran inline or on a worker.
diff --git a/apps/qc-app/src/components/EditData/EditHistory.vue b/apps/qc-app/src/components/EditData/EditHistory.vue
index 69221951..9e233cc8 100644
--- a/apps/qc-app/src/components/EditData/EditHistory.vue
+++ b/apps/qc-app/src/components/EditData/EditHistory.vue
@@ -329,28 +329,38 @@
             <!-- Grows as one unit so the badge sits against the title text. -->
             <button
               type="button"
-              class="edit-history__step edit-history__title flex-grow-1 d-flex align-center ga-1"
+              class="edit-history__step edit-history__title flex-grow-1 d-flex flex-column align-start"
               :data-testid="`history-step-btn-${index}`"
               :disabled="!canStepTo || entry.execution?.inFlight"
               :title="stepTitle(index)"
               @click.stop="onRowReload(index)"
             >
-              <span class="edit-history__method text-truncate font-weight-medium">
-                {{ formatMethod(entry.method) }}
+              <span class="d-flex align-center ga-1 w-100">
+                <span class="edit-history__method text-truncate font-weight-medium">
+                  {{ formatMethod(entry.method) }}
+                </span>
+
+                <v-tooltip v-if="entry.comment" location="start" :text="entry.comment">
+                  <template #activator="{ props: tp }">
+                    <v-icon
+                      v-bind="tp"
+                      :data-testid="`history-comment-badge-${index}`"
+                      icon="mdi-comment-text-outline"
+                      size="14"
+                      color="primary"
+                      class="flex-shrink-0"
+                    />
+                  </template>
+                </v-tooltip>
               </span>
 
-              <v-tooltip v-if="entry.comment" location="start" :text="entry.comment">
-                <template #activator="{ props: tp }">
-                  <v-icon
-                    v-bind="tp"
-                    :data-testid="`history-comment-badge-${index}`"
-                    icon="mdi-comment-text-outline"
-                    size="14"
-                    color="primary"
-                    class="flex-shrink-0"
-                  />
-                </template>
-              </v-tooltip>
+              <span
+                v-if="stepExtent(entry)"
+                class="edit-history__extent text-medium-emphasis text-truncate w-100"
+                :data-testid="`history-extent-${index}`"
+              >
+                {{ stepExtent(entry) }}
+              </span>
             </button>
 
             <v-chip
@@ -563,6 +573,7 @@ import { useHistorySnapshots } from '@/composables/useHistorySnapshots'
 import { SNAPSHOT_BASELINE_INDEX } from '@/utils/snapshotId'
 import { Snackbar } from '@uwrl/qc-utils'
 import type { HistoryItem } from '@uwrl/qc-utils'
+import { formatDateRange, formatStamp } from '@/utils/time'
 
 const props = withDefaults(
   defineProps<{
@@ -677,6 +688,21 @@ const stepTitle = (index: number) =>
     ? 'Preview this step'
     : 'Not applied in the step currently shown. Click to preview it.'
 
+/** The period a step touched and how many points, from the datetimes qc-utils
+ *  read when it ran. Indices would drift with later edits. */
+function stepExtent(entry: HistoryItem): string {
+  const extent = entry.execution?.extent
+  if (!extent) return ''
+  const begin = new Date(extent.begin)
+  const end = new Date(extent.end)
+  const period =
+    extent.begin === extent.end
+      ? formatStamp(begin)
+      : formatDateRange(begin.toISOString(), end.toISOString())
+  const n = entry.execution?.selectionSize
+  return n ? `${period} · ${n.toLocaleString()} pt${n === 1 ? '' : 's'}` : period
+}
+
 function formatMethod(method: string) {
   if (!method) return ''
   return method
@@ -970,6 +996,11 @@ onBeforeUnmount(() => window.removeEventListener('keydown', onKeydown))
   min-width: 0;
 }
 
+.edit-history__extent {
+  font-size: 0.6875rem;
+  line-height: 1.2;
+}
+
 .edit-history__args {
   background-color: rgba(var(--v-theme-primary), 0.03);
   border-left: 2px solid rgb(var(--v-theme-primary));
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
index cd816a29..053af458 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
@@ -156,6 +156,7 @@ function makeEntry(
     mode: 'worker' | 'inline'
     datasetSize: number
     selectionSize: number
+    extent: { begin: number; end: number }
   }> = {},
 ) {
   return {
@@ -188,6 +189,30 @@ describe('EditHistory.vue', () => {
     vi.clearAllMocks()
   })
 
+  it('shows the period and point count a step touched', async () => {
+    const begin = new Date(2026, 2, 3, 12, 5)
+    const end = new Date(2026, 3, 1, 8, 0)
+    editHistory.value = [
+      makeEntry('INTERPOLATE', [], {
+        extent: { begin: begin.getTime(), end: end.getTime() },
+        selectionSize: 1200,
+      }),
+      makeEntry('ADD_POINTS', [], {
+        extent: { begin: end.getTime(), end: end.getTime() },
+      }),
+      makeEntry('FIND_GAPS'),
+    ]
+    const wrapper = createWrapper()
+    await flushPromises()
+    expect(wrapper.find('[data-testid="history-extent-0"]').text()).toBe(
+      'Mar 3, 12:05 PM – Apr 1, 2026, 8:00 AM · 1,200 pts'
+    )
+    expect(wrapper.find('[data-testid="history-extent-1"]').text()).toBe(
+      'Apr 1, 2026, 8:00 AM'
+    )
+    expect(wrapper.find('[data-testid="history-extent-2"]').exists()).toBe(false)
+  })
+
   it('disables undo/redo when history is empty', () => {
     const wrapper = createWrapper()
     expect(wrapper.find('[data-testid="history-undo-btn"]').attributes('disabled')).toBeDefined()
diff --git a/packages/qc-utils/docs/API_REFERENCE.md b/packages/qc-utils/docs/API_REFERENCE.md
index 0a977d80..3dee2bfb 100644
--- a/packages/qc-utils/docs/API_REFERENCE.md
+++ b/packages/qc-utils/docs/API_REFERENCE.md
@@ -366,6 +366,7 @@ runtime fact. See below for the field-by-field contract.
   mode?: 'worker' | 'inline',          // routing decision the calibration layer made
   datasetSize?: number,                // observation count at dispatch time
   selectionSize?: number,              // indices the op acted on
+  extent?: { begin: number, end: number }, // epoch-ms of the first / last point acted on
 }
 ```
 
@@ -373,10 +374,16 @@ runtime fact. See below for the field-by-field contract.
 
 - **Push time** (synchronous, before the handler runs): `startedAt`,
   `inFlight: true`, `datasetSize`, and (for selection-consuming
-  edits) `selectionSize`.
+  edits) `selectionSize` and `extent`, read from the preceding entry's
+  selection (a SELECTION or a filter result) before the edit moves it.
 - **Resolve time** (after the handler returns or throws): `status`,
   `durationMs`, `mode`, `inFlight: false`, and (for filters)
-  `selectionSize` (populated from the produced selection).
+  `selectionSize` and `extent` from the produced selection. ADD_POINTS
+  and FILL_GAPS get `extent` here from the points they inserted.
+
+`extent` names the period a step touched in datetimes, which later edits
+don't shift the way they shift indices. It is runtime-only, like
+`selected`: replay recomputes it and `serializeHistory` leaves it out.
 
 The qc-app reads this object to drive the EditHistory UI (per-row
 spinner via `inFlight`, failure badge via `status`, duration text
diff --git a/packages/qc-utils/docs/ARCHITECTURE.md b/packages/qc-utils/docs/ARCHITECTURE.md
index ff7c685c..4c89690d 100644
--- a/packages/qc-utils/docs/ARCHITECTURE.md
+++ b/packages/qc-utils/docs/ARCHITECTURE.md
@@ -240,6 +240,7 @@ last `reload()`. Each `HistoryItem`:
     mode?: 'worker' | 'inline', // calibration routing decision
     datasetSize?: number,       // observation count at dispatch time
     selectionSize?: number,     // indices the op acted on
+    extent?: { begin: number, end: number }, // datetimes of the first / last point acted on
   },
 }
 ```
diff --git a/packages/qc-utils/docs/QC_HISTORY.md b/packages/qc-utils/docs/QC_HISTORY.md
index 5fbd9970..6bbe8fa7 100644
--- a/packages/qc-utils/docs/QC_HISTORY.md
+++ b/packages/qc-utils/docs/QC_HISTORY.md
@@ -168,7 +168,7 @@ The shape mirrors `ObservationRecord.dispatch`'s tuple form:
 | `durationMs`    | number  | Wall-clock duration of the handler. Useful for retrospective perf review.   |
 | `mode`          | string  | `"worker"` or `"inline"` — the calibration layer's routing decision.        |
 | `datasetSize`   | number  | Observation count at dispatch time. Reflects the pre-edit shape.            |
-| `selectionSize` | number  | Indices the op acted on (filter's produced selection, or preceding SELECTION for selection-consuming edits). |
+| `selectionSize` | number  | Indices the op acted on (filter's produced selection, or the preceding entry's selection for selection-consuming edits). |
 
 Every sub-field is optional; the loader validates the type of any
 present value (finite number / `"success"` \| `"failed"` /
diff --git a/packages/qc-utils/src/types/index.ts b/packages/qc-utils/src/types/index.ts
index 5d81c764..789c9ac3 100644
--- a/packages/qc-utils/src/types/index.ts
+++ b/packages/qc-utils/src/types/index.ts
@@ -133,10 +133,21 @@ export interface HistoryExecution {
    * Number of indices the op acted on. For filters this is the
    * size of the resulting selection (populated at resolve time);
    * for selection-consuming edits it's the size of the preceding
-   * SELECTION (populated at push time). `undefined` when not
+   * entry's selection, a SELECTION or a filter result (populated at
+   * push time). `undefined` when not
    * applicable (e.g. ADD_POINTS, which is datetime-addressed).
    */
   selectionSize?: number;
+  /**
+   * Epoch-ms datetimes of the first and last points the op acted on, read
+   * from the data as it stood when the op ran. Indices shift with later
+   * edits; these don't, so a consumer can say which period a step touched.
+   * Filters: their resulting selection. Selection-consuming edits: the
+   * preceding entry's selection, before the edit. ADD_POINTS and FILL_GAPS: the
+   * points they inserted. `undefined` when the op touched no points.
+   * Runtime-only like `selected`: recomputed on replay, never serialized.
+   */
+  extent?: { begin: number; end: number };
 }
 
 export type HistoryItem = {
diff --git a/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
index e6c308b7..e52d7942 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
@@ -955,6 +955,90 @@ describe('ObservationRecord', () => {
     });
   });
 
+  describe('execution extent', () => {
+    const { datetimes, dataValues, startMs, spacingMs } = buildUniformData(20);
+    const at = (i: number) => startMs + i * spacingMs;
+    let rec: ObservationRecord;
+
+    beforeEach(async () => {
+      rec = new ObservationRecord({ datetimes, dataValues });
+      await rec.reload();
+    });
+
+    it('spans the points a selection picked', async () => {
+      await rec.dispatchFilter(EnumFilterOperations.SELECTION, [7, 3, 5]);
+      expect(rec.history[0]!.execution.extent).toEqual({
+        begin: at(3),
+        end: at(7),
+      });
+    });
+
+    it('spans a filter result', async () => {
+      await rec.dispatchFilter(EnumFilterOperations.VALUE_THRESHOLD, {
+        [FilterOperation.GT]: 15,
+      });
+      expect(rec.history[0]!.execution.extent).toEqual({
+        begin: at(16),
+        end: at(19),
+      });
+    });
+
+    it('keeps the datetimes a delete removed, not the indices left behind', async () => {
+      await rec.dispatch([
+        [EnumFilterOperations.SELECTION, [0, 1]],
+        [EnumEditOperations.DELETE_POINTS],
+        [EnumFilterOperations.SELECTION, [0, 1]],
+        [EnumEditOperations.DELETE_POINTS],
+      ]);
+      expect(rec.history[1]!.execution.extent).toEqual({
+        begin: at(0),
+        end: at(1),
+      });
+      // Same indices, but by then they held the next two points.
+      expect(rec.history[3]!.execution.extent).toEqual({
+        begin: at(2),
+        end: at(3),
+      });
+    });
+
+    it('spans the selection an edit took from a filter', async () => {
+      await rec.dispatchFilter(EnumFilterOperations.VALUE_THRESHOLD, {
+        [FilterOperation.GT]: 17,
+      });
+      await rec.dispatchAction(EnumEditOperations.DELETE_POINTS);
+      expect(rec.history[1]!.execution.selectionSize).toBe(2);
+      expect(rec.history[1]!.execution.extent).toEqual({
+        begin: at(18),
+        end: at(19),
+      });
+    });
+
+    it('spans the points an insert added', async () => {
+      await rec.dispatchAction(EnumEditOperations.ADD_POINTS, [
+        [at(30), 1],
+        [at(25), 1],
+      ]);
+      expect(rec.history[0]!.execution.extent).toEqual({
+        begin: at(25),
+        end: at(30),
+      });
+    });
+
+    it('is recomputed on replay', async () => {
+      await rec.dispatch([
+        [EnumFilterOperations.SELECTION, [2, 4]],
+        [EnumEditOperations.DELETE_POINTS],
+      ]);
+      rec.history[1]!.execution.extent = undefined;
+      await rec.undo();
+      await rec.redo();
+      expect(rec.history[1]!.execution.extent).toEqual({
+        begin: at(2),
+        end: at(4),
+      });
+    });
+  });
+
   describe('dispatch', () => {
     it('runs a batched sequence and returns the last response', async () => {
       const rec = new ObservationRecord(mockRawData);
diff --git a/packages/qc-utils/src/utils/plotting/observation-record.ts b/packages/qc-utils/src/utils/plotting/observation-record.ts
index 0593a797..fbe8df88 100644
--- a/packages/qc-utils/src/utils/plotting/observation-record.ts
+++ b/packages/qc-utils/src/utils/plotting/observation-record.ts
@@ -467,6 +467,23 @@ export class ObservationRecord {
     return this._replay([...this.history]);
   }
 
+  /** First and last datetime among `indices`, or undefined if none are in
+   *  range. Indices need not be sorted. */
+  private _extentOf(
+    indices: ArrayLike<number>,
+  ): { begin: number; end: number } | undefined {
+    const xs = this.dataX;
+    let begin = Infinity;
+    let end = -Infinity;
+    for (let k = 0; k < indices.length; k++) {
+      const t = xs[indices[k]];
+      if (t === undefined || !Number.isFinite(t)) continue;
+      if (t < begin) begin = t;
+      if (t > end) end = t;
+    }
+    return begin <= end ? { begin, end } : undefined;
+  }
+
   private _refuseWhilePreviewing() {
     if (this.previewIndex !== null) throw new HistoryPreviewError();
   }
@@ -581,17 +598,14 @@ export class ObservationRecord {
       // survives the internal re-dispatch.
       if (!this._isReplaying) this.redoStack.length = 0;
 
-      // Selection-consuming edits read the preceding SELECTION's
-      // size at push time so the audit record reflects what the
-      // handler will operate on. `consumesPrecedingSelection` gates
-      // the read so non-selection edits (ADD_POINTS, FILL_GAPS)
-      // don't pick up a stale value.
-      const prevSelLen =
-        consumesPrecedingSelection(action, args) &&
-          this.history[this.history.length - 1]?.method ===
-          EnumFilterOperations.SELECTION
-          ? this.history[this.history.length - 1].selected?.length
-          : undefined;
+      // Selection-consuming edits read the preceding entry's selection
+      // (a SELECTION or a filter's result, as the handlers do) at push
+      // time so the audit record reflects what the handler will operate
+      // on. `consumesPrecedingSelection` gates the read so non-selection
+      // edits (ADD_POINTS, FILL_GAPS) don't pick up a stale value.
+      const prevSelected = consumesPrecedingSelection(action, args)
+        ? this.history[this.history.length - 1]?.selected
+        : undefined;
 
       historyItem = {
         method: action,
@@ -600,7 +614,9 @@ export class ObservationRecord {
           startedAt: Date.now(),
           inFlight: true,
           datasetSize: this.dataset.source.x?.length ?? 0,
-          selectionSize: prevSelLen,
+          selectionSize: prevSelected?.length,
+          // Read before the edit moves or removes those points.
+          extent: prevSelected && this._extentOf(prevSelected),
         },
       };
       this.history.push(historyItem);
@@ -642,6 +658,10 @@ export class ObservationRecord {
           durationMs: measurement.duration,
           mode: this._pendingExecutionMode,
           inFlight: false,
+          // Inserting ops have no prior selection; their result is it.
+          extent:
+            stored.execution.extent ??
+            (newSelection?.length ? this._extentOf(newSelection) : undefined),
         };
       }
     } catch (e) {
@@ -821,6 +841,9 @@ export class ObservationRecord {
           durationMs: measurement.duration,
           mode: this._pendingExecutionMode,
           selectionSize: measurement.response?.length,
+          extent: measurement.response?.length
+            ? this._extentOf(measurement.response)
+            : undefined,
           inFlight: false,
         };
       }

From 244be060344b888404dc92d769d1879e62e9a089 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Tue, 29 Sep 2026 10:07:26 -0600
Subject: [PATCH 74/93] feat: start new sessions where the last commit ended

---
 apps/qc-app/docs/API_REFERENCE.md             | 12 ++++---
 apps/qc-app/docs/USER_GUIDE.md                | 10 +++---
 .../EditData/SessionWindowDialog.vue          |  8 ++---
 .../__tests__/SessionWindowDialog.spec.ts     | 23 +++++++++-----
 .../src/utils/__tests__/sessionWindow.spec.ts | 31 ++++++++++++++-----
 apps/qc-app/src/utils/sessionWindow.ts        | 24 +++++++++++---
 6 files changed, 77 insertions(+), 31 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 13f34960..f7852334 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -798,10 +798,14 @@ and their pure resolution:
 Pure validation for the session-window step (`SessionWindowDialog.vue`,
 opened from the row Edit chooser or the editor footer's **New session**):
 
-- `defaultSessionWindow(source)`. The step's default window: the source's
-  full extent, begin to end. `null` when the source has no observations.
-  The rules below accept it whenever the committed history lies inside that
-  extent; the dialog reports the rare case where it does not.
+- `defaultSessionWindow(source, sessions)`. The step's default window,
+  where the last session left off: from the committed history's end to the
+  source's end (the **Since commit** preset). The source's full extent when
+  nothing is committed, when no data arrived after the history, or when the
+  history ends outside the source. `null` when the source has no
+  observations. The rules below accept it whenever the committed history
+  lies inside the source's extent; the dialog reports the rare case where it
+  does not.
 - `sessionWindowIssue(window, source, sessions)`: `null` when `window` is
   valid, otherwise `{ kind, message, fix }` for the first broken rule. The
   window must lie inside the source's extent, and it can't leave a gap
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 18d758f4..8dac60bc 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -187,10 +187,12 @@ need to.
 #### The session window
 
 The session window step shows the source's data extent and, if the managed
-datastream already has committed history, that history's extent too. The
-default window is the source's own begin and end datetimes, so a new session
-covers the whole record and overlaps whatever is already committed. Narrow it
-whenever you only want to work on part of the record.
+datastream already has committed history, that history's extent too, so you
+can see where the last session left off. The default window picks up from
+there: it starts where the committed history ends and runs to the end of the
+source record (the **Since commit** preset). With nothing committed yet, or no
+new data since the last commit, it covers the whole record instead. Widen or
+narrow it as you need; the **All** preset goes back to the whole record.
 
 You can adjust the From / To pickers, but two rules apply:
 
diff --git a/apps/qc-app/src/components/EditData/SessionWindowDialog.vue b/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
index 3defe949..b6635b0e 100644
--- a/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
+++ b/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
@@ -5,9 +5,9 @@
         New session on "{{ managedName }}"
       </div>
       <div class="text-body-small text-medium-emphasis mt-1">
-        The window starts out covering the whole source record. Narrow it to
-        the period you want to edit: it can overlap committed sessions but
-        cannot leave a gap before or after them.
+        The window starts where the committed history ends and runs to the end
+        of the source record. Adjust it to the period you want to edit: it can
+        overlap committed sessions but cannot leave a gap before or after them.
       </div>
     </div>
 
@@ -147,7 +147,7 @@ const emit = defineEmits<{
   (e: 'cancel'): void
 }>()
 
-const initial = defaultSessionWindow(props.source)
+const initial = defaultSessionWindow(props.source, props.sessions)
 const begin = ref<Date | null>(initial?.begin ?? null)
 const end = ref<Date | null>(initial?.end ?? null)
 
diff --git a/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
index cf891103..919cf01d 100644
--- a/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/SessionWindowDialog.spec.ts
@@ -64,14 +64,21 @@ const pickers = (w: ReturnType<typeof mount>) =>
   w.findAllComponents(DatePickerStub)
 
 describe('SessionWindowDialog', () => {
-  it('prefills the window with the whole source extent', () => {
+  it('prefills the window from where the committed history ends', () => {
     const w = mountDialog()
     const [from, to] = pickers(w)
-    expect((from!.props('modelValue') as Date).toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect((from!.props('modelValue') as Date).toISOString()).toBe('2025-05-01T00:00:00.000Z')
     expect((to!.props('modelValue') as Date).toISOString()).toBe('2025-12-31T00:00:00.000Z')
     expect(w.find('[data-testid="session-window-error"]').exists()).toBe(false)
   })
 
+  it('prefills the whole source extent when nothing is committed', () => {
+    const w = mountDialog({ sessions: [] })
+    const [from, to] = pickers(w)
+    expect((from!.props('modelValue') as Date).toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect((to!.props('modelValue') as Date).toISOString()).toBe('2025-12-31T00:00:00.000Z')
+  })
+
   it('shows the committed history range', () => {
     const w = mountDialog()
     expect(w.find('[data-testid="session-window-committed"]').text()).not.toMatch(/Nothing committed/)
@@ -86,7 +93,7 @@ describe('SessionWindowDialog', () => {
     const w = mountDialog()
     await w.find('[data-testid="session-window-start"]').trigger('click')
     const [window] = w.emitted('confirm')![0] as [{ begin: Date; end: Date }]
-    expect(window.begin.toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect(window.begin.toISOString()).toBe('2025-05-01T00:00:00.000Z')
     expect(window.end.toISOString()).toBe('2025-12-31T00:00:00.000Z')
   })
 
@@ -157,11 +164,11 @@ describe('SessionWindowDialog presets', () => {
 
   it('marks the preset matching the current window', async () => {
     const w = mountDialog()
-    expect(preset(w, 'all').attributes('aria-pressed')).toBe('true')
-    await preset(w, 'since').trigger('click')
-    await flushPromises()
     expect(preset(w, 'since').attributes('aria-pressed')).toBe('true')
-    expect(preset(w, 'all').attributes('aria-pressed')).toBe('false')
+    await preset(w, 'all').trigger('click')
+    await flushPromises()
+    expect(preset(w, 'all').attributes('aria-pressed')).toBe('true')
+    expect(preset(w, 'since').attributes('aria-pressed')).toBe('false')
   })
 })
 
@@ -193,7 +200,7 @@ describe('SessionWindowDialog one-click fix', () => {
     await fix(w).trigger('click')
     await flushPromises()
     const [from, to] = pickers(w)
-    expect((from!.props('modelValue') as Date).toISOString()).toBe('2025-01-01T00:00:00.000Z')
+    expect((from!.props('modelValue') as Date).toISOString()).toBe('2025-05-01T00:00:00.000Z')
     expect((to!.props('modelValue') as Date).toISOString()).toBe('2025-12-31T00:00:00.000Z')
     expect(w.find('[data-testid="session-window-error"]').exists()).toBe(false)
   })
diff --git a/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts b/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
index 09ac78ec..1a632f62 100644
--- a/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/sessionWindow.spec.ts
@@ -44,26 +44,43 @@ describe('committedExtent', () => {
 
 describe('defaultSessionWindow', () => {
   it('covers the whole source when nothing is committed', () => {
-    expect(defaultSessionWindow(source)).toEqual(
+    expect(defaultSessionWindow(source, [])).toEqual(
+      win('2025-01-01T00:00:00Z', '2025-12-31T00:00:00Z')
+    )
+  })
+
+  it('starts where the committed history ends', () => {
+    const sessions = [
+      committed('2025-01-01T00:00:00Z', '2025-03-01T00:00:00Z'),
+      committed('2025-03-01T00:00:00Z', '2025-05-01T00:00:00Z'),
+    ]
+    expect(defaultSessionWindow(source, sessions)).toEqual(
+      win('2025-05-01T00:00:00Z', '2025-12-31T00:00:00Z')
+    )
+  })
+
+  it('covers the whole source when nothing arrived after the history', () => {
+    const sessions = [committed('2025-01-01T00:00:00Z', '2025-12-31T00:00:00Z')]
+    expect(defaultSessionWindow(source, sessions)).toEqual(
       win('2025-01-01T00:00:00Z', '2025-12-31T00:00:00Z')
     )
   })
 
   it('is null when the source has no observations', () => {
-    expect(defaultSessionWindow({})).toBeNull()
+    expect(defaultSessionWindow({}, [])).toBeNull()
   })
 })
 
 describe('the default window against the rules', () => {
   it('is valid while committed history sits inside the source', () => {
     const sessions = [committed('2025-01-01T00:00:00Z', '2025-05-01T00:00:00Z')]
-    const window = defaultSessionWindow(source)!
+    const window = defaultSessionWindow(source, sessions)!
     expect(sessionWindowIssue(window, source, sessions)).toBeNull()
   })
 
   it('is valid when the history already covers the whole source', () => {
     const sessions = [committed('2025-01-01T00:00:00Z', '2025-12-31T00:00:00Z')]
-    const window = defaultSessionWindow(source)!
+    const window = defaultSessionWindow(source, sessions)!
     expect(sessionWindowIssue(window, source, sessions)).toBeNull()
   })
 
@@ -71,7 +88,7 @@ describe('the default window against the rules', () => {
   // then even the full extent leaves a gap. The dialog reports it.
   it('is rejected when the committed history sits outside the source', () => {
     const sessions = [committed('2026-03-01T00:00:00Z', '2026-04-01T00:00:00Z')]
-    const window = defaultSessionWindow(source)!
+    const window = defaultSessionWindow(source, sessions)!
     expect(message(window, sessions)).toMatch(/gap before/)
   })
 })
@@ -206,7 +223,7 @@ describe('the one-click fix', () => {
   // The history sits outside the source, so closing the gap would push the
   // end past the source and swap one error for another.
   it('offers nothing when the correction is itself invalid', () => {
-    const issue = sessionWindowIssue(defaultSessionWindow(source)!, source, [
+    const issue = sessionWindowIssue(defaultSessionWindow(source, [])!, source, [
       committed('2026-03-01T00:00:00Z', '2026-04-01T00:00:00Z'),
     ])
     expect(issue?.kind).toBe('gap-before-history')
@@ -229,7 +246,7 @@ describe('sessionWindowPresets', () => {
     expect(ids([])).toEqual(['all', '1y', '6m', '1m'])
     const presets = sessionWindowPresets(source, [])
     const all = presets.find((p) => p.id === 'all')!
-    expect(all.window).toEqual(defaultSessionWindow(source))
+    expect(all.window).toEqual(defaultSessionWindow(source, []))
     const month = presets.find((p) => p.id === '1m')!
     const recordEnd = new Date(source.phenomenonEndTime)
     expect(month.window.end.getTime()).toBe(recordEnd.getTime())
diff --git a/apps/qc-app/src/utils/sessionWindow.ts b/apps/qc-app/src/utils/sessionWindow.ts
index 7b776c50..d7195e42 100644
--- a/apps/qc-app/src/utils/sessionWindow.ts
+++ b/apps/qc-app/src/utils/sessionWindow.ts
@@ -63,10 +63,26 @@ export function committedExtent(
   return { begin: new Date(begin), end: new Date(end) }
 }
 
-/** The source's own extent, which the rules above always accept while the
- *  committed history lies inside it. */
-export function defaultSessionWindow(source: SourceExtent): TimeWindow | null {
-  return dataExtent([source])
+/** Where the last session left off: from the end of the committed history
+ *  to the end of the source. The whole source when nothing is committed, or
+ *  when nothing new arrived after the history (or it lies outside the source).
+ *  The rules above accept it while the committed history lies inside the
+ *  source. */
+export function defaultSessionWindow(
+  source: SourceExtent,
+  sessions: readonly SessionRange[]
+): TimeWindow | null {
+  const extent = dataExtent([source])
+  if (!extent) return null
+  const history = committedExtent(sessions)
+  if (
+    history &&
+    history.end.getTime() >= extent.begin.getTime() &&
+    history.end.getTime() < extent.end.getTime()
+  ) {
+    return { begin: new Date(history.end), end: new Date(extent.end) }
+  }
+  return extent
 }
 
 export const NO_SOURCE_DATA_ISSUE: SessionWindowIssue = {

From ea63c3642ad0a47922c48da20ec7bcb751b1038f Mon Sep 17 00:00:00 2001
From: Ken Lippold <klippold@Kens-MacBook-Pro-2.local>
Date: Tue, 29 Sep 2026 11:44:43 -0700
Subject: [PATCH 75/93] Updated version

---
 django/contracts/openapi/auth.openapi.json | 2 +-
 django/contracts/openapi/data.openapi.json | 2 +-
 django/hydroserver/__init__.py             | 2 +-
 packages/hydroserverpy/setup.cfg           | 2 +-
 4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/django/contracts/openapi/auth.openapi.json b/django/contracts/openapi/auth.openapi.json
index 5015d836..510af9db 100644
--- a/django/contracts/openapi/auth.openapi.json
+++ b/django/contracts/openapi/auth.openapi.json
@@ -678,7 +678,7 @@
   "info": {
     "description": "",
     "title": "HydroServer User Authentication API",
-    "version": "1.12.0b5"
+    "version": "1.12.0b6"
   },
   "openapi": "3.1.0",
   "paths": {
diff --git a/django/contracts/openapi/data.openapi.json b/django/contracts/openapi/data.openapi.json
index 70ec5c89..40489ab5 100644
--- a/django/contracts/openapi/data.openapi.json
+++ b/django/contracts/openapi/data.openapi.json
@@ -11613,7 +11613,7 @@
   "info": {
     "description": "",
     "title": "HydroServer Data Management API",
-    "version": "1.12.0b5"
+    "version": "1.12.0b6"
   },
   "openapi": "3.1.0",
   "paths": {
diff --git a/django/hydroserver/__init__.py b/django/hydroserver/__init__.py
index 63954708..9411ef63 100644
--- a/django/hydroserver/__init__.py
+++ b/django/hydroserver/__init__.py
@@ -1,4 +1,4 @@
 from .celery import app as celery_app
 
-__version__ = "1.12.0b5"
+__version__ = "1.12.0b6"
 __all__ = ("celery_app", "__version__",)
diff --git a/packages/hydroserverpy/setup.cfg b/packages/hydroserverpy/setup.cfg
index 08ec1ace..52683afe 100644
--- a/packages/hydroserverpy/setup.cfg
+++ b/packages/hydroserverpy/setup.cfg
@@ -1,6 +1,6 @@
 [metadata]
 name = hydroserverpy
-version = 1.12.0b5
+version = 1.12.0b6
 description = A Python client for managing HydroServer data
 long_description = file: README.md
 long_description_content_type = text/markdown

From dcdb34381ca47768a5fc676fc66bef9754e87587 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 05:34:58 -0600
Subject: [PATCH 76/93] feat: replace observations over an explicit time range

---
 django/contracts/openapi/data.openapi.json    | 82 ++++++++++++++++-
 django/core/sta/services/observation.py       | 29 +++++-
 .../interfaces/api/schemas/sta/observation.py | 11 ++-
 .../interfaces/api/views/sta/observation.py   |  2 +
 django/tests/sta/services/test_observation.py | 88 +++++++++++++++++++
 .../src/generated/data.types.d.ts             | 18 +++-
 6 files changed, 220 insertions(+), 10 deletions(-)

diff --git a/django/contracts/openapi/data.openapi.json b/django/contracts/openapi/data.openapi.json
index 40489ab5..90bf72ae 100644
--- a/django/contracts/openapi/data.openapi.json
+++ b/django/contracts/openapi/data.openapi.json
@@ -5861,8 +5861,40 @@
                 "type": "null"
               }
             ],
-            "description": "Specifies how new observations are added to the datastream. `insert` allows observations at any timestamp. `append` adds only future observations (after the latest existing timestamp). `backfill` adds only historical observations (before the earliest existing timestamp). `replace` deletes all observations in the range of provided observations before inserting new ones.",
+            "description": "Specifies how new observations are added to the datastream. `insert` allows observations at any timestamp. `append` adds only future observations (after the latest existing timestamp). `backfill` adds only historical observations (before the earliest existing timestamp). `replace` deletes all observations in the range of provided observations before inserting new ones, or in `phenomenon_time_start` to `phenomenon_time_end` when given.",
             "title": "Mode"
+          },
+          "phenomenon_time_end": {
+            "anyOf": [
+              {
+                "type": "string"
+              },
+              {
+                "format": "date-time",
+                "type": "string"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "description": "End of the range `replace` deletes. Requires `phenomenon_time_start`.",
+            "title": "Phenomenon Time End"
+          },
+          "phenomenon_time_start": {
+            "anyOf": [
+              {
+                "type": "string"
+              },
+              {
+                "format": "date-time",
+                "type": "string"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "description": "Start of the range `replace` deletes. Requires `phenomenon_time_end`.",
+            "title": "Phenomenon Time Start"
           }
         },
         "title": "ObservationBulkPostQueryParameters",
@@ -14008,7 +14040,7 @@
             }
           },
           {
-            "description": "Specifies how new observations are added to the datastream. `insert` allows observations at any timestamp. `append` adds only future observations (after the latest existing timestamp). `backfill` adds only historical observations (before the earliest existing timestamp). `replace` deletes all observations in the range of provided observations before inserting new ones.",
+            "description": "Specifies how new observations are added to the datastream. `insert` allows observations at any timestamp. `append` adds only future observations (after the latest existing timestamp). `backfill` adds only historical observations (before the earliest existing timestamp). `replace` deletes all observations in the range of provided observations before inserting new ones, or in `phenomenon_time_start` to `phenomenon_time_end` when given.",
             "in": "query",
             "name": "mode",
             "required": false,
@@ -14027,9 +14059,53 @@
                   "type": "null"
                 }
               ],
-              "description": "Specifies how new observations are added to the datastream. `insert` allows observations at any timestamp. `append` adds only future observations (after the latest existing timestamp). `backfill` adds only historical observations (before the earliest existing timestamp). `replace` deletes all observations in the range of provided observations before inserting new ones.",
+              "description": "Specifies how new observations are added to the datastream. `insert` allows observations at any timestamp. `append` adds only future observations (after the latest existing timestamp). `backfill` adds only historical observations (before the earliest existing timestamp). `replace` deletes all observations in the range of provided observations before inserting new ones, or in `phenomenon_time_start` to `phenomenon_time_end` when given.",
               "title": "Mode"
             }
+          },
+          {
+            "description": "Start of the range `replace` deletes. Requires `phenomenon_time_end`.",
+            "in": "query",
+            "name": "phenomenon_time_start",
+            "required": false,
+            "schema": {
+              "anyOf": [
+                {
+                  "type": "string"
+                },
+                {
+                  "format": "date-time",
+                  "type": "string"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "description": "Start of the range `replace` deletes. Requires `phenomenon_time_end`.",
+              "title": "Phenomenon Time Start"
+            }
+          },
+          {
+            "description": "End of the range `replace` deletes. Requires `phenomenon_time_start`.",
+            "in": "query",
+            "name": "phenomenon_time_end",
+            "required": false,
+            "schema": {
+              "anyOf": [
+                {
+                  "type": "string"
+                },
+                {
+                  "format": "date-time",
+                  "type": "string"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "description": "End of the range `replace` deletes. Requires `phenomenon_time_start`.",
+              "title": "Phenomenon Time End"
+            }
           }
         ],
         "requestBody": {
diff --git a/django/core/sta/services/observation.py b/django/core/sta/services/observation.py
index 3d6151e2..2b76435e 100644
--- a/django/core/sta/services/observation.py
+++ b/django/core/sta/services/observation.py
@@ -296,7 +296,17 @@ def bulk_create(
         datastream_id: uuid.UUID,
         mode: Literal["insert", "append", "backfill", "replace"],
         update_datastream_statistics: bool = True,
+        phenomenon_time_start: Optional[datetime] = None,
+        phenomenon_time_end: Optional[datetime] = None,
     ):
+        has_range = phenomenon_time_start is not None or phenomenon_time_end is not None
+        if has_range and mode != "replace":
+            raise HttpError(400, "A phenomenon time range only applies to replace mode")
+        if (phenomenon_time_start is None) != (phenomenon_time_end is None):
+            raise HttpError(
+                400, "Give both phenomenon_time_start and phenomenon_time_end, or neither"
+            )
+
         datastream = datastream_service.get_datastream_for_action(
             principal, datastream_id, action="edit"
         )
@@ -379,13 +389,24 @@ def bulk_create(
                 )
 
         elif mode == "replace":
-            start_time = min(obs.phenomenon_time for obs in observation_records)
-            end_time = max(obs.phenomenon_time for obs in observation_records)
+            # Without an explicit range, replace the span the upload covers.
+            if phenomenon_time_start is None:
+                if not observation_records:
+                    raise HttpError(
+                        400,
+                        "Replace mode needs observations or a phenomenon time range",
+                    )
+                phenomenon_time_start = min(
+                    obs.phenomenon_time for obs in observation_records
+                )
+                phenomenon_time_end = max(
+                    obs.phenomenon_time for obs in observation_records
+                )
             self.bulk_delete(
                 principal=principal,
                 data=ObservationBulkDeleteBody(
-                    phenomenon_time_start=start_time,
-                    phenomenon_time_end=end_time,
+                    phenomenon_time_start=phenomenon_time_start,
+                    phenomenon_time_end=phenomenon_time_end,
                 ),
                 datastream_id=datastream_id,
                 update_datastream_statistics=False,
diff --git a/django/interfaces/api/schemas/sta/observation.py b/django/interfaces/api/schemas/sta/observation.py
index e35e30f9..0304bff9 100644
--- a/django/interfaces/api/schemas/sta/observation.py
+++ b/django/interfaces/api/schemas/sta/observation.py
@@ -96,9 +96,18 @@ class ObservationBulkPostQueryParameters(Schema):
             "`insert` allows observations at any timestamp. "
             "`append` adds only future observations (after the latest existing timestamp). "
             "`backfill` adds only historical observations (before the earliest existing timestamp). "
-            "`replace` deletes all observations in the range of provided observations before inserting new ones."
+            "`replace` deletes all observations in the range of provided observations before inserting new ones, "
+            "or in `phenomenon_time_start` to `phenomenon_time_end` when given."
         ),
     )
+    phenomenon_time_start: Optional[ISODatetime] = Query(
+        None,
+        description="Start of the range `replace` deletes. Requires `phenomenon_time_end`.",
+    )
+    phenomenon_time_end: Optional[ISODatetime] = Query(
+        None,
+        description="End of the range `replace` deletes. Requires `phenomenon_time_start`.",
+    )
 
 
 class ObservationBulkPostBody(BasePostBody):
diff --git a/django/interfaces/api/views/sta/observation.py b/django/interfaces/api/views/sta/observation.py
index e2eaa2be..386010ef 100644
--- a/django/interfaces/api/views/sta/observation.py
+++ b/django/interfaces/api/views/sta/observation.py
@@ -111,6 +111,8 @@ def insert_observations(
         data=data,
         datastream_id=datastream_id,
         mode=query.mode or "append",
+        phenomenon_time_start=query.phenomenon_time_start,
+        phenomenon_time_end=query.phenomenon_time_end,
     )
 
 
diff --git a/django/tests/sta/services/test_observation.py b/django/tests/sta/services/test_observation.py
index 8f523113..1aaa1225 100644
--- a/django/tests/sta/services/test_observation.py
+++ b/django/tests/sta/services/test_observation.py
@@ -235,6 +235,94 @@ def test_create_observations(
     )
 
 
+REPLACE_DATASTREAM_ID = uuid.UUID("27c70b41-e845-40ea-8cc7-d1b40f89816b")
+
+
+def replace_observations(get_principal, rows, **kwargs):
+    observation_service.bulk_create(
+        principal=get_principal("owner"),
+        datastream_id=REPLACE_DATASTREAM_ID,
+        data=ObservationBulkPostBody(fields=["phenomenonTime", "result"], data=rows),
+        mode="replace",
+        **kwargs,
+    )
+    return sorted(
+        Observation.objects.filter(datastream_id=REPLACE_DATASTREAM_ID).values_list(
+            "result", flat=True
+        )
+    )
+
+
+def test_replace_observations_deletes_the_given_range(get_principal):
+    # The existing points (08:00Z, 09:00Z) sit outside the upload's own span.
+    results = replace_observations(
+        get_principal,
+        [["2025-02-10T08:30:00Z", 5.5]],
+        phenomenon_time_start="2025-02-10T07:00:00Z",
+        phenomenon_time_end="2025-02-10T10:00:00Z",
+    )
+
+    assert results == [5.5]
+
+
+def test_replace_observations_with_an_empty_upload_clears_the_range(get_principal):
+    results = replace_observations(
+        get_principal,
+        [],
+        phenomenon_time_start="2025-02-10T07:00:00Z",
+        phenomenon_time_end="2025-02-10T10:00:00Z",
+    )
+
+    assert results == []
+
+
+def test_replace_observations_without_a_range_uses_the_upload_span(get_principal):
+    results = replace_observations(get_principal, [["2025-02-10T08:30:00Z", 5.5]])
+
+    assert results == [1.1, 3.1, 5.5]
+
+
+def test_replace_observations_rejects_an_empty_upload_without_a_range(get_principal):
+    from ninja.errors import HttpError
+
+    with pytest.raises(HttpError) as exc_info:
+        replace_observations(get_principal, [])
+
+    assert exc_info.value.status_code == 400
+
+
+def test_replace_observations_rejects_half_a_range(get_principal):
+    from ninja.errors import HttpError
+
+    with pytest.raises(HttpError) as exc_info:
+        replace_observations(
+            get_principal,
+            [["2025-02-10T08:30:00Z", 5.5]],
+            phenomenon_time_start="2025-02-10T07:00:00Z",
+        )
+
+    assert exc_info.value.status_code == 400
+
+
+def test_create_observations_rejects_a_range_outside_replace_mode(get_principal):
+    from ninja.errors import HttpError
+
+    with pytest.raises(HttpError) as exc_info:
+        observation_service.bulk_create(
+            principal=get_principal("owner"),
+            datastream_id=REPLACE_DATASTREAM_ID,
+            data=ObservationBulkPostBody(
+                fields=["phenomenonTime", "result"],
+                data=[["2025-03-10T01:00:00Z", 9.1]],
+            ),
+            mode="insert",
+            phenomenon_time_start="2025-03-10T00:00:00Z",
+            phenomenon_time_end="2025-03-10T02:00:00Z",
+        )
+
+    assert exc_info.value.status_code == 400
+
+
 def test_delete_observations(
     django_assert_max_num_queries,
     get_principal,
diff --git a/packages/hydroserver-ts/src/generated/data.types.d.ts b/packages/hydroserver-ts/src/generated/data.types.d.ts
index a4721843..aefddb14 100644
--- a/packages/hydroserver-ts/src/generated/data.types.d.ts
+++ b/packages/hydroserver-ts/src/generated/data.types.d.ts
@@ -3955,9 +3955,19 @@ export interface components {
         ObservationBulkPostQueryParameters: {
             /**
              * Mode
-             * @description Specifies how new observations are added to the datastream. `insert` allows observations at any timestamp. `append` adds only future observations (after the latest existing timestamp). `backfill` adds only historical observations (before the earliest existing timestamp). `replace` deletes all observations in the range of provided observations before inserting new ones.
+             * @description Specifies how new observations are added to the datastream. `insert` allows observations at any timestamp. `append` adds only future observations (after the latest existing timestamp). `backfill` adds only historical observations (before the earliest existing timestamp). `replace` deletes all observations in the range of provided observations before inserting new ones, or in `phenomenon_time_start` to `phenomenon_time_end` when given.
              */
             mode?: ("insert" | "append" | "backfill" | "replace") | null;
+            /**
+             * Phenomenon Time End
+             * @description End of the range `replace` deletes. Requires `phenomenon_time_start`.
+             */
+            phenomenon_time_end?: string | null;
+            /**
+             * Phenomenon Time Start
+             * @description Start of the range `replace` deletes. Requires `phenomenon_time_end`.
+             */
+            phenomenon_time_start?: string | null;
         };
         /** ObservationColumnarResponse */
         ObservationColumnarResponse: {
@@ -6829,8 +6839,12 @@ export interface operations {
     interfaces_api_views_sta_observation_insert_observations: {
         parameters: {
             query?: {
-                /** @description Specifies how new observations are added to the datastream. `insert` allows observations at any timestamp. `append` adds only future observations (after the latest existing timestamp). `backfill` adds only historical observations (before the earliest existing timestamp). `replace` deletes all observations in the range of provided observations before inserting new ones. */
+                /** @description Specifies how new observations are added to the datastream. `insert` allows observations at any timestamp. `append` adds only future observations (after the latest existing timestamp). `backfill` adds only historical observations (before the earliest existing timestamp). `replace` deletes all observations in the range of provided observations before inserting new ones, or in `phenomenon_time_start` to `phenomenon_time_end` when given. */
                 mode?: ("insert" | "append" | "backfill" | "replace") | null;
+                /** @description Start of the range `replace` deletes. Requires `phenomenon_time_end`. */
+                phenomenon_time_start?: string | null;
+                /** @description End of the range `replace` deletes. Requires `phenomenon_time_start`. */
+                phenomenon_time_end?: string | null;
             };
             header?: never;
             path: {

From 40af26a6cc3b2c23fd41e52c4c78d3819e091d2f Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 05:34:59 -0600
Subject: [PATCH 77/93] feat: read the checksum of an observation range

---
 django/hydroserver/settings.py                |  1 +
 django/tests/sta/test_observation_headers.py  | 16 +++++
 .../thing-datastream.service.spec.ts          | 66 +++++++++++++++++++
 packages/hydroserver-ts/src/api/apiMethods.ts | 13 ++++
 .../src/api/services/datastream.service.ts    | 10 +++
 5 files changed, 106 insertions(+)
 create mode 100644 django/tests/sta/test_observation_headers.py

diff --git a/django/hydroserver/settings.py b/django/hydroserver/settings.py
index ffa4fb98..1ed2db2b 100644
--- a/django/hydroserver/settings.py
+++ b/django/hydroserver/settings.py
@@ -76,6 +76,7 @@
 CORS_EXPOSE_HEADERS = [
     "X-Total-Pages",
     "X-Total-Count",
+    "X-Checksum",
 ]
 
 # Celery
diff --git a/django/tests/sta/test_observation_headers.py b/django/tests/sta/test_observation_headers.py
new file mode 100644
index 00000000..643ff960
--- /dev/null
+++ b/django/tests/sta/test_observation_headers.py
@@ -0,0 +1,16 @@
+import pytest
+from django.test import override_settings
+
+
+@pytest.mark.django_db
+@override_settings(CORS_ORIGIN_ALLOW_ALL=True)
+def test_observation_checksum_header_is_readable_cross_origin(client):
+    response = client.get(
+        "/api/data/datastreams/27c70b41-e845-40ea-8cc7-d1b40f89816b/observations",
+        HTTP_ORIGIN="http://qc.example.org",
+    )
+
+    assert response.status_code == 200
+    assert response["X-Checksum"]
+    exposed = response["Access-Control-Expose-Headers"].lower().split(", ")
+    assert "x-checksum" in exposed
diff --git a/packages/hydroserver-ts/src/api/__tests__/thing-datastream.service.spec.ts b/packages/hydroserver-ts/src/api/__tests__/thing-datastream.service.spec.ts
index 1b942280..3d825a85 100644
--- a/packages/hydroserver-ts/src/api/__tests__/thing-datastream.service.spec.ts
+++ b/packages/hydroserver-ts/src/api/__tests__/thing-datastream.service.spec.ts
@@ -120,6 +120,72 @@ describe('DatastreamService', () => {
     })
   })
 
+  describe('createObservations', () => {
+    it('sends the replace range as query params', async () => {
+      vi.stubGlobal('fetch', vi.fn().mockResolvedValue(jsonResponse(null, 201)))
+
+      await client.datastreams.createObservations(
+        'ds-1',
+        { fields: ['phenomenonTime', 'result'], data: [] },
+        {
+          mode: 'replace',
+          phenomenon_time_start: '2025-01-01T00:00:00Z',
+          phenomenon_time_end: '2025-02-01T00:00:00Z',
+        }
+      )
+
+      const [url] = (fetch as any).mock.calls[0]
+      const parsed = new URL(url)
+      expect(parsed.pathname).toMatch(/\/datastreams\/ds-1\/observations\/bulk-create$/)
+      expect(parsed.searchParams.get('mode')).toBe('replace')
+      expect(parsed.searchParams.get('phenomenon_time_start')).toBe('2025-01-01T00:00:00Z')
+      expect(parsed.searchParams.get('phenomenon_time_end')).toBe('2025-02-01T00:00:00Z')
+    })
+  })
+
+  describe('getObservationsChecksum', () => {
+    const start = new Date('2025-01-01T00:00:00Z')
+    const end = new Date('2025-02-01T00:00:00Z')
+
+    it('reads the X-Checksum header for the window', async () => {
+      vi.stubGlobal(
+        'fetch',
+        vi.fn().mockResolvedValue(
+          new Response(JSON.stringify({ phenomenonTime: [], result: [] }), {
+            status: 200,
+            headers: { 'Content-Type': 'application/json', 'X-Checksum': 'abc123' },
+          })
+        )
+      )
+
+      const res = await client.datastreams.getObservationsChecksum('ds-1', start, end)
+
+      expect(res).toMatchObject({ ok: true, data: 'abc123' })
+      const [url] = (fetch as any).mock.calls[0]
+      const parsed = new URL(url)
+      expect(parsed.pathname).toMatch(/\/datastreams\/ds-1\/observations$/)
+      expect(parsed.searchParams.get('phenomenon_time_min')).toBe(start.toISOString())
+      expect(parsed.searchParams.get('phenomenon_time_max')).toBe(end.toISOString())
+      expect(parsed.searchParams.get('page_size')).toBe('1')
+    })
+
+    it('fails when the response has no checksum', async () => {
+      vi.stubGlobal('fetch', vi.fn().mockResolvedValue(jsonResponse({}, 200)))
+
+      const res = await client.datastreams.getObservationsChecksum('ds-1', start, end)
+
+      expect(res.ok).toBe(false)
+    })
+
+    it('returns ok:false on a failed request', async () => {
+      vi.stubGlobal('fetch', vi.fn().mockResolvedValue(jsonResponse({ detail: 'nope' }, 404)))
+
+      const res = await client.datastreams.getObservationsChecksum('ds-1', start, end)
+
+      expect(res.ok).toBe(false)
+    })
+  })
+
   describe('getVisualizationBootstrap', () => {
     it('maps bootstrap payloads into model instances and resolves workspaceId', async () => {
       vi.stubGlobal(
diff --git a/packages/hydroserver-ts/src/api/apiMethods.ts b/packages/hydroserver-ts/src/api/apiMethods.ts
index 2c98f947..dfa32d34 100644
--- a/packages/hydroserver-ts/src/api/apiMethods.ts
+++ b/packages/hydroserver-ts/src/api/apiMethods.ts
@@ -79,6 +79,19 @@ export const apiMethods = {
     return await limit(() => interceptedFetch<T>(endpoint, options))
   },
 
+  /** GET `endpoint` and return the value of response header `name`. */
+  async fetchHeader(endpoint: string, name: string): Promise<ApiResponse<string>> {
+    const opts = requestInterceptor({ method: 'GET' })
+    const response = await limit(() => fetch(endpoint, opts))
+    const res = await responseInterceptor<unknown>(response)
+    if (!res.ok) return res
+    const value = response.headers.get(name)
+    if (value === null) {
+      return { ok: false, status: res.status, message: `Missing ${name} header.` }
+    }
+    return { ok: true, status: res.status, data: value }
+  },
+
   async paginatedFetch<T>(base: string): Promise<ApiResponse<T>> {
     const url = new URL(String(base), globalThis.location?.origin ?? undefined)
     const urlAlreadyHasPage = url.searchParams.has('page')
diff --git a/packages/hydroserver-ts/src/api/services/datastream.service.ts b/packages/hydroserver-ts/src/api/services/datastream.service.ts
index 6c7fa943..13abab46 100644
--- a/packages/hydroserver-ts/src/api/services/datastream.service.ts
+++ b/packages/hydroserver-ts/src/api/services/datastream.service.ts
@@ -163,6 +163,16 @@ export class DatastreamService extends HydroServerBaseService<typeof C, M> {
     return apiMethods.paginatedFetch<ObservationListResponse>(url)
   }
 
+  /** The checksum of the datastream's observations in `[start, end]`. */
+  getObservationsChecksum(datastreamId: string, start: Date, end: Date) {
+    const url = this.withQuery(`${this._route}/${datastreamId}/observations`, {
+      phenomenon_time_min: start.toISOString(),
+      phenomenon_time_max: end.toISOString(),
+      page_size: 1,
+    })
+    return apiMethods.fetchHeader(url, 'X-Checksum')
+  }
+
   createObservation(datastreamId: string, body: ObservationPostBody) {
     const url = `${this._route}/${datastreamId}/observations`
     return apiMethods.post<ObservationResponse>(url, body)

From b254033171fe0f8b23ba1aa920140494991e43b3 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 05:35:00 -0600
Subject: [PATCH 78/93] fix: keep step comments through replays and skip
 missing delete indices

---
 packages/qc-utils/README.md                   |   8 +-
 packages/qc-utils/docs/API_REFERENCE.md       |  28 +++--
 packages/qc-utils/docs/ARCHITECTURE.md        |  14 ++-
 packages/qc-utils/docs/QUALITY.md             |   2 +-
 packages/qc-utils/src/types/index.ts          |   7 +-
 .../utils/plotting/__tests__/history.spec.ts  |  21 +++-
 .../__tests__/observation-record.spec.ts      | 111 ++++++++++++++++++
 .../qc-utils/src/utils/plotting/history.ts    |  14 +--
 .../src/utils/plotting/observation-record.ts  |  82 +++++++------
 9 files changed, 222 insertions(+), 65 deletions(-)

diff --git a/packages/qc-utils/README.md b/packages/qc-utils/README.md
index 3e4e3242..e3410de1 100644
--- a/packages/qc-utils/README.md
+++ b/packages/qc-utils/README.md
@@ -64,8 +64,12 @@ The single state container. Holds:
 - `redoStack` — items popped by `undo()`, ready for `redo()`.
 
 Mutations only happen through `dispatch` / `dispatchAction` /
-`dispatchFilter` / `undo` / `redo` / `reload` / `reloadHistory` /
-`removeHistoryItem`. The handlers themselves are private — operations
+`dispatchFilter` / `dispatchStep` / `undo` / `redo` / `reload` /
+`truncateHistory` / `restoreHistory` / `removeHistoryItem` /
+`applyWindow(begin, end, rawData?)`. `previewHistory(index)` shows an
+earlier step without dropping the later ones (`previewIndex` says which);
+edits throw `HistoryPreviewError` until `exitPreview()`. The handlers
+themselves are private, so operations
 are driven by enum + args so the same call shape works at runtime, on
 replay from a saved QC history, and in unit tests.
 
diff --git a/packages/qc-utils/docs/API_REFERENCE.md b/packages/qc-utils/docs/API_REFERENCE.md
index 3dee2bfb..5d151ced 100644
--- a/packages/qc-utils/docs/API_REFERENCE.md
+++ b/packages/qc-utils/docs/API_REFERENCE.md
@@ -84,18 +84,20 @@ call `reload()` once construction is done to initialize.
 
 | Method                                       | Returns          | Effect                                                                                     |
 |----------------------------------------------|------------------|--------------------------------------------------------------------------------------------|
-| `dispatch(ops: Array<[Enum, ...args]>)`      | `Promise<void>`  | Run a chain of operations atomically. Each op appends a `HistoryItem`.                     |
-| `dispatchAction(op: EnumEditOperations, ...args)` | `Promise<void>` | Run one edit op.                                                                      |
-| `dispatchFilter(op: EnumFilterOperations, ...args)` | `Promise<void>` | Run one filter op. Produces a selection.                                             |
-| `undo()`                                     | `Promise<void>`  | Pop the last history entry; replay the rest from a fresh `reload()`.                       |
-| `redo()`                                     | `Promise<void>`  | Replay the most recently undone entry.                                                     |
+| `dispatch(ops: Array<[Enum, ...args]>)`      | `Promise<number[]>` | Run a chain of operations in order. Each op appends a `HistoryItem` (a filter can replace the one before it). Returns the last op's selection. |
+| `dispatchStep(step)`                         | `Promise<number[]>` | Run one recorded step (`method`, `args`) and carry its `comment` and `performedBy` onto the entry it produces. Every replay goes through it, so these survive undo, redo, removal and `applyHistory` even when replay merges entries. |
+| `dispatchAction(op: EnumEditOperations, ...args)` | `Promise<number[]>` | Run one edit op. Returns the selection it leaves.                                  |
+| `dispatchFilter(op: EnumFilterOperations, ...args)` | `Promise<number[]>` | Run one filter op. Returns its selection.                                        |
+| `undo()`                                     | `Promise<number[]>` | Pop the last history entry; replay the rest from a fresh `reload()`. Returns the selection left. |
+| `redo()`                                     | `Promise<number[]>` | Replay the most recently undone entry. Returns its selection.                           |
 | `applyWindow(begin, end, rawData?)`          | `Promise<void>`  | Materialize the inclusive epoch-ms window `[begin, end]` of `rawData` into `dataX`/`dataY`. A passed `rawData` replaces the full series first (e.g. after a cache filled a gap). Clears history on a real change; no-op when neither the window nor the data changed. |
 | `reload()`                                   | `Promise<void>`  | Re-initialize the typed arrays from `rawData`, sliced to the current window; clear history. |
 | `previewHistory(index)`                      | `Promise<number[]>` | Show the data as of step `index` (`-1` for the starting state) and keep every later step listed, unapplied. Sets `previewIndex`; edits throw `HistoryPreviewError` until `exitPreview`. Previewing the last step is `exitPreview`. Returns the shown step's selection. |
 | `exitPreview()`                              | `Promise<number[]>` | Replay the whole history after a preview. A no-op when nothing is previewed. |
-| `truncateHistory(index)`                     | `Promise<number[]>` | Drop every step after `index` for good (discarding unsaved edits), clear the redo stack and replay the rest. |
+| `truncateHistory(index)`                     | `Promise<number[]>` | Drop every step after `index` for good, clear the redo stack and replay the rest. |
+| `restoreHistory(steps)`                      | `Promise<number[]>` | Replace the history with `steps` (keeping their comment and attribution), clear the redo stack and replay from raw. For going back to a saved history that undo and new edits have diverged from. |
 | `previewIndex`                               | `number \| null`    | The step being previewed, or null when the data reflects the whole history. `undo` ends a preview; `redo` ends it first, then redoes. |
-| `removeHistoryItem(index: number)`           | `Promise<void>`  | Drop a specific entry; replay the rest.                                                    |
+| `removeHistoryItem(index: number)`           | `Promise<number[]>` | Drop a specific entry and clear the redo stack; replay the rest. Returns the selection left. |
 
 The op handlers themselves are private — dispatch by enum.
 
@@ -116,7 +118,7 @@ matters; lower means smaller idle memory, more frequent grow / copy.
 | `CHANGE_VALUES`        | `(operator: Operator, value: number, [range?])` — applies at prior selection.|
 | `ASSIGN_VALUES_BULK`   | `(indices: number[], values: number[])` — parallel arrays. No workers.       |
 | `ASSIGN_DATETIMES_BULK`| `(indices: number[], datetimes: number[])` — combined delete + add.          |
-| `DELETE_POINTS`        | `(indices?: number[])` — defaults to the prior selection.                    |
+| `DELETE_POINTS`        | `(indices?: number[])`, defaulting to the prior selection. Indices past the end and repeats are ignored. |
 | `INTERPOLATE`          | `()` — linear interpolation per consecutive group in the prior selection.    |
 | `SHIFT_DATETIMES`      | `(amount: number, unit: TimeUnit)`                                           |
 | `DRIFT_CORRECTION`     | `(value: number)` — linear drift across each consecutive group.              |
@@ -193,6 +195,8 @@ are captured in the report but do not abort.
 {
   method: EnumEditOperations | EnumFilterOperations,
   args: any[],
+  comment?: string,                    // the operator's note, trimmed; absent when blank
+  performedBy?: string,                // who applied it, for display; audit only
   execution?: QcHistoryExecution,
 }
 ```
@@ -348,10 +352,18 @@ Wrap any thunk with wall-clock measurement. Used by dispatch to fill
   method: EnumEditOperations | EnumFilterOperations,
   args?: any[],
   selected?: number[],
+  comment?: string,                    // authored; survives every replay
+  performedBy?: string,                // server provenance; survives every replay
   execution: HistoryExecution,
 }
 ```
 
+### `class HistoryPreviewError`
+
+Thrown by any edit dispatched while `previewIndex` is set, that is while
+`previewHistory` shows an earlier step. `exitPreview()` (or previewing the
+last step) ends the preview.
+
 `execution` is always present and carries every per-dispatch
 runtime fact. See below for the field-by-field contract.
 
diff --git a/packages/qc-utils/docs/ARCHITECTURE.md b/packages/qc-utils/docs/ARCHITECTURE.md
index 4c89690d..340512d2 100644
--- a/packages/qc-utils/docs/ARCHITECTURE.md
+++ b/packages/qc-utils/docs/ARCHITECTURE.md
@@ -91,7 +91,7 @@ The record holds the full series in `rawData`, but `dataX` / `dataY` (the
 operation surface) carry only the active **window**. `applyWindow(begin,
 end, rawData?)` slices `rawData` into `dataX` / `dataY`, optionally taking
 new `rawData` first (a cache that grew); `reload()` restores that windowed
-baseline. A window or data change clears history — operations only ever see
+baseline. A window or data change clears history: operations only ever see
 data inside the current window, so a new window starts from a fresh QC
 baseline.
 
@@ -247,7 +247,17 @@ last `reload()`. Each `HistoryItem`:
 
 `undo()` truncates the last entry, pushes it onto `redoStack`, and
 replays the remaining history from scratch against the freshly
-`reload()`-ed dataset. `redo()` is the inverse. This is conservative
+`reload()`-ed dataset. `redo()` is the inverse. Every replay runs each
+step through `dispatchStep`, which carries the step's `comment` and
+`performedBy` onto the entry it produces, so they survive even when a
+replay merges entries (two filters left side by side collapse into one).
+
+`previewHistory(index)` replays only up to `index` and leaves the later
+steps listed but unapplied, with `previewIndex` marking the step shown.
+Edits throw `HistoryPreviewError` until `exitPreview()` replays the whole
+history again. `truncateHistory(index)` drops the later steps for good,
+and `restoreHistory(steps)` replaces the history wholesale, for going back
+to a saved one. This is conservative
 (every undo is O(history-length)) but correctness is straightforward —
 no rollback / inverse-op machinery to maintain. For typical QC sessions
 (<100 ops) it's instant.
diff --git a/packages/qc-utils/docs/QUALITY.md b/packages/qc-utils/docs/QUALITY.md
index 8a021827..7c030249 100644
--- a/packages/qc-utils/docs/QUALITY.md
+++ b/packages/qc-utils/docs/QUALITY.md
@@ -126,7 +126,7 @@ is the obvious next step if/when a non-browser consumer appears.
 ## Where the code is solid
 
 - The dispatch surface is small (`dispatch`, `dispatchAction`,
-  `dispatchFilter`, `undo`, `redo`, `reload`, `truncateHistory`,
+  `dispatchFilter`, `undo`, `redo`, `reload`, `truncateHistory`, `restoreHistory`,
   `removeHistoryItem`) and consistently the only mutation path.
   `previewHistory` / `exitPreview` only replay; edits are refused while a
   step is previewed. No
diff --git a/packages/qc-utils/src/types/index.ts b/packages/qc-utils/src/types/index.ts
index 789c9ac3..c1ba1b55 100644
--- a/packages/qc-utils/src/types/index.ts
+++ b/packages/qc-utils/src/types/index.ts
@@ -143,7 +143,8 @@ export interface HistoryExecution {
    * from the data as it stood when the op ran. Indices shift with later
    * edits; these don't, so a consumer can say which period a step touched.
    * Filters: their resulting selection. Selection-consuming edits: the
-   * preceding entry's selection, before the edit. ADD_POINTS and FILL_GAPS: the
+   * preceding entry's selection, before the edit, so SHIFT_DATETIMES reports
+   * where the points were, not where they moved. ADD_POINTS and FILL_GAPS: the
    * points they inserted. `undefined` when the op touched no points.
    * Runtime-only like `selected`: recomputed on replay, never serialized.
    */
@@ -162,8 +163,8 @@ export type HistoryItem = {
   comment?: string;
   /**
    * Who applied this operation, for display. Server-assigned provenance
-   * rather than authored input: the consumer stamps it when replaying a
-   * stored history, and it is never sent back as part of an operation.
+   * rather than authored input: `applyHistory` and every replay carry it
+   * from the stored operation, and it is never sent back as part of one.
    */
   performedBy?: string;
   /**
diff --git a/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts
index 35f2170f..63a6f59f 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts
@@ -213,6 +213,21 @@ describe('parseHistory', () => {
     expect(without.operations[0].comment).toBeUndefined();
   });
 
+  it('trims a parsed comment and author, and drops blank ones', () => {
+    const parsed = parseHistory({
+      version: '1',
+      createdAt: '2024-01-01T00:00:00.000Z',
+      window: SAMPLE_WINDOW,
+      operations: [
+        { method: 'VALUE_THRESHOLD', args: [], comment: '  fouling  ', performedBy: ' Ada ' },
+        { method: 'VALUE_THRESHOLD', args: [], comment: '   ', performedBy: '' },
+      ],
+    });
+    expect(parsed.operations[0]).toMatchObject({ comment: 'fouling', performedBy: 'Ada' });
+    expect(parsed.operations[1].comment).toBeUndefined();
+    expect(parsed.operations[1].performedBy).toBeUndefined();
+  });
+
   it('round-trips performedBy and rejects a non-string one', () => {
     const base = {
       version: '1',
@@ -721,7 +736,7 @@ describe('applyHistory — round-trip', () => {
       history: [],
       redoStack: [],
       reload: async () => { },
-      dispatch: async () => { throw new Error('boom-with-stack'); },
+      dispatchStep: async () => { throw new Error('boom-with-stack'); },
     } as unknown as ObservationRecord;
 
     const history = parseHistory({
@@ -738,7 +753,7 @@ describe('applyHistory — round-trip', () => {
   it('catches non-Error throws from dispatch and stringifies the value into the report', async () => {
     // `dispatchAction` / `dispatchFilter` normally swallow handler
     // errors themselves, but the catch in `applyHistory` is defensive
-    // and handles a bare throw too. Stub `record.dispatch` so it
+    // and handles a bare throw too. Stub `record.dispatchStep` so it
     // throws a plain string — the catch path must stringify it
     // (line 220's `e instanceof Error ? e.message : String(e)`
     // ternary) and record it in `report.failed[].error` without
@@ -747,7 +762,7 @@ describe('applyHistory — round-trip', () => {
       history: [],
       redoStack: [],
       reload: async () => { },
-      dispatch: async () => { throw 'plain-string-failure'; },
+      dispatchStep: async () => { throw 'plain-string-failure'; },
     } as unknown as ObservationRecord;
 
     const history = parseHistory({
diff --git a/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
index e52d7942..f807c4a0 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
@@ -87,6 +87,14 @@ describe('ObservationRecord', () => {
       expect(rec.dataX.length).toBe(mockRawData.datetimes.length);
     });
 
+    it('holds no points when loaded with none', async () => {
+      const rec = new ObservationRecord({ datetimes: [], dataValues: [] });
+      await rec.reload();
+
+      expect(rec.dataX.length).toBe(0);
+      expect(rec.dataY.length).toBe(0);
+    });
+
     it('grows the shared buffer when raw data exceeds INCREASE_AMOUNT', async () => {
       const big = buildUniformData(INCREASE_AMOUNT + 1234);
       const rec = new ObservationRecord({
@@ -255,6 +263,48 @@ describe('ObservationRecord', () => {
       expect(rec.dataX[rec.dataX.length - 1]).toBe(keptX[keptX.length - 1]);
     });
 
+    describe('DELETE_POINTS with indices that are not all points', () => {
+      const small = async (n: number) => {
+        const r = new ObservationRecord({
+          datetimes: Array.from({ length: n }, (_, i) => i + 1),
+          dataValues: Array.from({ length: n }, (_, i) => (i + 1) * 10),
+        });
+        await r.reload();
+        return r;
+      };
+      const deleteAt = (r: ObservationRecord, indices: number[]) =>
+        r.dispatch([
+          [EnumFilterOperations.SELECTION, indices],
+          [EnumEditOperations.DELETE_POINTS],
+        ]);
+
+      it('leaves an empty record empty', async () => {
+        const r = await small(0);
+        await deleteAt(r, [1, 2, 3]);
+        expect(r.dataX.length).toBe(0);
+        expect(r.dataY.length).toBe(0);
+      });
+
+      it('skips indices past the end', async () => {
+        const r = await small(5);
+        await deleteAt(r, [3, 4, 7, 9]);
+        expect(Array.from(r.dataX)).toEqual([1, 2, 3]);
+        expect(Array.from(r.dataY)).toEqual([10, 20, 30]);
+      });
+
+      it('deletes a repeated index once', async () => {
+        const r = await small(5);
+        await deleteAt(r, [1, 1, 2]);
+        expect(Array.from(r.dataX)).toEqual([1, 4, 5]);
+      });
+
+      it('deletes indices given out of order', async () => {
+        const r = await small(5);
+        await deleteAt(r, [3, 0]);
+        expect(Array.from(r.dataX)).toEqual([2, 3, 5]);
+      });
+    });
+
     it('CHANGE_VALUES applies operator to the prior SELECTION indexes', async () => {
       const indexes = distinctSortedInts(25, 0, rec.dataX.length - 1);
       const originalY = indexes.map((i) => rec.dataY[i]);
@@ -953,6 +1003,67 @@ describe('ObservationRecord', () => {
       await rec.removeHistoryItem(0);
       expect(rec.dataX.length).toBe(originalLen - 1);
     });
+
+    // Removing the ADD_POINTS leaves the two thresholds side by side, so the
+    // replay merges them into one entry and every later entry moves up.
+    it('keeps each comment on its own operation when replay merges entries', async () => {
+      await rec.dispatch([
+        [EnumFilterOperations.VALUE_THRESHOLD, { 'Greater than': 1 }],
+        [EnumEditOperations.ADD_POINTS, [[rec.dataX[1]! + 1, 5]]],
+        [EnumFilterOperations.VALUE_THRESHOLD, { 'Greater than': 2 }],
+        [EnumEditOperations.DELETE_POINTS],
+      ]);
+      rec.history.forEach((item, i) => {
+        item.comment = `step ${i}`;
+        item.performedBy = `user ${i}`;
+      });
+
+      await rec.removeHistoryItem(1);
+
+      expect(rec.history.map((h) => h.method)).toEqual([
+        EnumFilterOperations.VALUE_THRESHOLD,
+        EnumEditOperations.DELETE_POINTS,
+      ]);
+      expect(rec.history.map((h) => h.comment)).toEqual(['step 2', 'step 3']);
+      expect(rec.history.map((h) => h.performedBy)).toEqual(['user 2', 'user 3']);
+    });
+
+    describe('restoreHistory', () => {
+      it('puts back steps that were undone and replaced', async () => {
+        const originalLen = rec.dataX.length;
+        await rec.dispatch([
+          [EnumFilterOperations.SELECTION, [0, 1]],
+          [EnumEditOperations.DELETE_POINTS],
+        ]);
+        const saved = [...rec.history];
+        await rec.undo();
+        await rec.dispatch(EnumEditOperations.CHANGE_VALUES, Operator.ADD, 1);
+
+        await rec.restoreHistory(saved);
+
+        expect(rec.history.map((h) => h.method)).toEqual([
+          EnumFilterOperations.SELECTION,
+          EnumEditOperations.DELETE_POINTS,
+        ]);
+        expect(rec.dataX.length).toBe(originalLen - 2);
+        expect(rec.redoStack).toHaveLength(0);
+      });
+
+      it('keeps comment and attribution', async () => {
+        await rec.dispatch([
+          [EnumFilterOperations.SELECTION, [0, 1]],
+          [EnumEditOperations.DELETE_POINTS],
+        ]);
+        rec.history[1]!.comment = 'Removed the pair';
+        rec.history[1]!.performedBy = 'Grace Hopper';
+        const saved = [...rec.history];
+
+        await rec.restoreHistory(saved);
+
+        expect(rec.history[1]!.comment).toBe('Removed the pair');
+        expect(rec.history[1]!.performedBy).toBe('Grace Hopper');
+      });
+    });
   });
 
   describe('execution extent', () => {
diff --git a/packages/qc-utils/src/utils/plotting/history.ts b/packages/qc-utils/src/utils/plotting/history.ts
index 15f47a30..aff494d3 100644
--- a/packages/qc-utils/src/utils/plotting/history.ts
+++ b/packages/qc-utils/src/utils/plotting/history.ts
@@ -234,8 +234,11 @@ export function parseHistory(json: unknown): QcHistory {
       method: o.method as EnumEditOperations | EnumFilterOperations,
       args: [...o.args],
     };
-    if (o.comment) op.comment = o.comment as string;
-    if (o.performedBy) op.performedBy = o.performedBy as string;
+    // Trimmed and dropped when blank, as `serializeHistory` writes them.
+    const comment = (o.comment as string | undefined)?.trim();
+    if (comment) op.comment = comment;
+    const performedBy = (o.performedBy as string | undefined)?.trim();
+    if (performedBy) op.performedBy = performedBy;
     const exec = parseExecution(o.execution, i);
     if (exec) op.execution = exec;
     return op;
@@ -288,16 +291,11 @@ export async function applyHistory(
     try {
       // dispatch handles routing to dispatchAction / dispatchFilter
       // based on whether the method is in EnumFilterOperations.
-      await record.dispatch(op.method, ...op.args);
+      await record.dispatchStep(op);
       // The dispatch path's catch block writes
       // `historyItem.execution.status = "failed"` on throw. Read it
       // back to decide whether to count as applied.
       const last = record.history[record.history.length - 1];
-      // Unlike `execution` (runtime telemetry, re-stamped per dispatch),
-      // the comment is authored intent and belongs to the operation, so
-      // it is carried onto the replayed entry.
-      if (last && op.comment) last.comment = op.comment;
-      if (last && op.performedBy) last.performedBy = op.performedBy;
       if (last?.execution.status === "failed") {
         report.failed.push({
           index: i,
diff --git a/packages/qc-utils/src/utils/plotting/observation-record.ts b/packages/qc-utils/src/utils/plotting/observation-record.ts
index fbe8df88..a781f651 100644
--- a/packages/qc-utils/src/utils/plotting/observation-record.ts
+++ b/packages/qc-utils/src/utils/plotting/observation-record.ts
@@ -89,6 +89,15 @@ function growBuffer(buffer: ArrayBufferLike, newByteLength: number): void {
   }
 }
 
+/** The distinct indices in `[0, length)`, ascending. Both delete paths
+ *  size their output by this count. */
+function existingIndices(indices: ArrayLike<number>, length: number): number[] {
+  const sorted = Array.from(indices)
+    .filter((i) => i >= 0 && i < length)
+    .sort((a, b) => a - b);
+  return sorted.filter((i, k) => k === 0 || i !== sorted[k - 1]);
+}
+
 /**
  * Shallow per-index equality for two ascending index arrays. Used by
  * `_selection` to detect SELECTION dispatches that are echoes of the
@@ -395,25 +404,27 @@ export class ObservationRecord {
   }
 
   /**
-   * Re-stamp the metadata a replay cannot reproduce. `dispatch` rebuilds
-   * each entry from `[method, ...args]` alone, so the operator's comment
-   * and the server's attribution would be dropped on every undo, redo, or
-   * step reload. Execution timings are deliberately left alone: those
-   * describe the run that just happened, not the operation.
-   *
-   * Paired positionally and only when the method still matches, so a
-   * replay that lands differently leaves entries unstamped rather than
-   * attributing an operation to the wrong person.
+   * Dispatch one recorded step and carry its comment and `performedBy`
+   * onto the entry it produces. `dispatch` rebuilds an entry from
+   * `[method, ...args]` alone, so without this every undo, redo or replay
+   * would drop them. The entry is the one this step pushed or replaced;
+   * a step that merges into nothing new (a selection echo) stamps nothing.
+   * Execution timings are not carried: they describe the run that just
+   * happened.
    */
-  private _restoreReplayedMeta(preserved: HistoryItem[]) {
-    const len = Math.min(preserved.length, this.history.length);
-    for (let i = 0; i < len; i++) {
-      const from = preserved[i];
-      const to = this.history[i];
-      if (!from || !to || from.method !== to.method) continue;
-      if (from.comment !== undefined) to.comment = from.comment;
-      if (from.performedBy !== undefined) to.performedBy = from.performedBy;
+  async dispatchStep(
+    step: Pick<HistoryItem, "method" | "args" | "comment" | "performedBy">,
+  ): Promise<number[]> {
+    const before = this.history[this.history.length - 1];
+    const selection = await this.dispatch([[step.method, ...(step.args || [])]]);
+    const produced = this.history[this.history.length - 1];
+    if (produced && produced !== before && produced.method === step.method) {
+      if (step.comment !== undefined) produced.comment = step.comment;
+      if (step.performedBy !== undefined) {
+        produced.performedBy = step.performedBy;
+      }
     }
+    return selection;
   }
 
   /** Reload from raw and replay `steps`, keeping the redo stack. */
@@ -421,10 +432,8 @@ export class ObservationRecord {
     await this.reload();
     this._isReplaying = true;
     try {
-      const selection = await this.dispatch(
-        steps.map((h) => [h.method, ...(h.args || [])]),
-      );
-      this._restoreReplayedMeta(steps);
+      let selection: number[] = [];
+      for (const step of steps) selection = await this.dispatchStep(step);
       return selection;
     } finally {
       this._isReplaying = false;
@@ -460,6 +469,14 @@ export class ObservationRecord {
     return this._replay(kept);
   }
 
+  /** Reload from raw and replay `steps` as the whole history, dropping the
+   *  redo stack. For putting back a saved history that undo and new edits
+   *  have diverged from. */
+  async restoreHistory(steps: HistoryItem[]): Promise<number[]> {
+    this.redoStack.length = 0;
+    return this._replay([...steps]);
+  }
+
   /** Apply the whole history again after `previewHistory`. A no-op
    *  returning no selection when nothing is previewed. */
   async exitPreview(): Promise<number[]> {
@@ -495,12 +512,7 @@ export class ObservationRecord {
     const newHistory = [...this.history];
     newHistory.splice(index, 1);
     this.redoStack.length = 0;
-    await this.reload();
-    const selection = await this.dispatch(
-      newHistory.map((h) => [h.method, ...(h.args || [])]),
-    );
-    this._restoreReplayedMeta(newHistory);
-    return selection;
+    return this._replay(newHistory);
   }
 
   /**
@@ -530,15 +542,7 @@ export class ObservationRecord {
     const item = this.redoStack.pop()!;
     this._isReplaying = true;
     try {
-      const selection = await this.dispatch([[item.method, ...(item.args || [])]]);
-      const replayed = this.history[this.history.length - 1];
-      if (replayed && replayed.method === item.method) {
-        if (item.comment !== undefined) replayed.comment = item.comment;
-        if (item.performedBy !== undefined) {
-          replayed.performedBy = item.performedBy;
-        }
-      }
-      return selection;
+      return await this.dispatchStep(item);
     } finally {
       this._isReplaying = false;
     }
@@ -1419,15 +1423,17 @@ export class ObservationRecord {
   }
 
   /**
-   * Delete points by ascending `deleteIndices` from x/y. Inline path
+   * Delete the points at `indices` from x/y. Indices past the end and
+   * repeats are ignored, and order doesn't matter. Inline path
    * runs a single skip-on-delete copy on the main thread; worker path:
    *  1. Main thread splits the original array into equal segments.
    *  2. Per-segment binary search locates the indexes to delete (deleteSegment) for efficient lookups.
    *  3. Cumulative deletions before each segment give each worker's output startTarget so segments don't overlap.
    *  4. Each worker walks its segment linearly, skipping deletions and copying kept elements into place.
    */
-  private async _deleteDataPoints(deleteIndices: number[]) {
+  private async _deleteDataPoints(indices: number[]) {
     const oldLen = this.dataX.length;
+    const deleteIndices = existingIndices(indices, oldLen);
     const newLength = oldLen - deleteIndices.length;
 
     // Inline fast path: skip segmentation + worker spawns; allocate

From a201aae902a38094cb2088a7cebd940367b604b2 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 05:35:17 -0600
Subject: [PATCH 79/93] fix: QC session review fixes

- Save reconciles from the first changed operation, and discard restores the saved history
- A session's base uses committed data inside committed windows and source data elsewhere
- Past sessions replay each ancestor on its own window, in commit order
- Commit checks the source checksum and fails on a failed push
- The observation cache forgets a datastream after a commit and never marks skipped ranges
- Save follows unsaved changes, and Commit needs permission to create observations
- Deleting the open datastream or session closes the editor
- The leave prompt runs after the redirecting guards
- The create form stays open until the create finishes
- Session rows are keyboard buttons, and viewing over unsaved edits asks in a dialog
- History file import removed, with dead code, duplication and band-aids cleaned up
- Docs and tests updated
---
 apps/qc-app/README.md                         |  19 +-
 apps/qc-app/docs/API_REFERENCE.md             | 113 +--
 apps/qc-app/docs/ARCHITECTURE.md              | 121 ++-
 apps/qc-app/docs/DEPLOYMENT.md                |  13 +-
 apps/qc-app/docs/ONBOARDING.md                |   5 +-
 apps/qc-app/docs/PERFORMANCE.md               |  10 +-
 apps/qc-app/docs/PLOTTING.md                  |  18 +-
 apps/qc-app/docs/QUALITY.md                   |  17 +-
 apps/qc-app/docs/TESTING.md                   |  33 +-
 apps/qc-app/docs/USER_GUIDE.md                | 127 +--
 apps/qc-app/e2e/data-table.spec.ts            |  61 ++
 apps/qc-app/e2e/edit-selection.spec.ts        |  23 +-
 apps/qc-app/e2e/editor-y-range.spec.ts        |  19 +-
 apps/qc-app/e2e/history.spec.ts               |   2 +-
 apps/qc-app/e2e/leave-session.spec.ts         |  26 +-
 apps/qc-app/e2e/managed-preview.spec.ts       |   1 +
 apps/qc-app/e2e/navigation.spec.ts            |  16 +-
 apps/qc-app/e2e/qc-golden-path.spec.ts        |  11 -
 apps/qc-app/e2e/qc-history.spec.ts            |  68 +-
 apps/qc-app/e2e/select-while-editing.spec.ts  |  16 +-
 apps/qc-app/e2e/session-list.spec.ts          |  43 +
 apps/qc-app/e2e/submit.spec.ts                |  23 +
 apps/qc-app/e2e/support/app.ts                |  11 +
 apps/qc-app/e2e/support/fixtures.ts           |   5 +-
 apps/qc-app/e2e/support/mocks.ts              |  76 +-
 apps/qc-app/e2e/support/ops.ts                |  13 +
 .../EditData/CreateDatastreamForm.vue         |  32 +-
 .../src/components/EditData/EditHistory.vue   | 735 +++++++-----------
 .../EditData/EditHistoryStepDetails.vue       | 126 +++
 .../EditData/LeaveSessionDialog.vue           |   3 +-
 .../src/components/EditData/SessionList.vue   |  42 +-
 .../EditData/SessionTimelineItem.vue          | 104 +++
 .../EditData/SessionWindowDialog.vue          |   6 -
 .../EditData/StartEditingDialog.vue           | 154 ++--
 .../components/EditData/StartEditingFlow.vue  |  30 +-
 .../__tests__/CreateDatastreamForm.spec.ts    |  12 +
 .../EditData/__tests__/EditHistory.spec.ts    |   3 -
 .../__tests__/LeaveSessionDialog.spec.ts      |   5 +-
 .../EditData/__tests__/SessionList.spec.ts    |   9 +
 .../__tests__/StartEditingDialog.spec.ts      |   9 +-
 .../__tests__/StartEditingFlow.spec.ts        | 116 ++-
 .../__tests__/NavigationRail.spec.ts          |   2 -
 apps/qc-app/src/components/VisualizeData.vue  | 130 ++--
 .../VisualizeData/DataVisDatasetsTable.vue    |  52 +-
 .../VisualizeData/DataVisualization.vue       |   4 +-
 .../components/VisualizeData/EditableCell.vue |   7 +-
 .../src/components/VisualizeData/Plot.vue     |  35 +-
 .../VisualizeData/PlotSourceDialog.vue        |   6 +-
 .../VisualizeData/PlottedDatastreams.vue      |  14 +-
 .../__tests__/DataVisDatasetsTable.spec.ts    |  85 +-
 .../__tests__/PlottedDatastreams.spec.ts      |   4 -
 .../useCreateManagedDatastream.spec.ts        |  13 +-
 .../__tests__/useEditEntry.spec.ts            |  80 +-
 .../__tests__/useEditSession.spec.ts          | 188 ++++-
 .../__tests__/useLeaveSession.spec.ts         |   4 +-
 .../__tests__/useManagedDatastreams.spec.ts   |   5 +-
 .../__tests__/useWorkspacePermissions.spec.ts |  52 +-
 apps/qc-app/src/composables/useClearPlot.ts   |  29 +
 apps/qc-app/src/composables/useEditEntry.ts   |  19 +-
 apps/qc-app/src/composables/useEditSession.ts | 123 ++-
 .../src/composables/useHistorySnapshots.ts    |  20 +-
 .../qc-app/src/composables/useLeaveSession.ts |  10 +-
 .../src/composables/useManagedDatastreams.ts  |  10 +-
 apps/qc-app/src/composables/useQcHistory.ts   |  66 +-
 .../src/composables/useResumeEditSession.ts   |  16 +-
 .../composables/useWorkspacePermissions.ts    |  56 +-
 .../src/router/__tests__/guards.spec.ts       |  49 +-
 apps/qc-app/src/router/guards.ts              | 122 +--
 apps/qc-app/src/router/router.ts              |   9 +-
 .../__tests__/buildSnapshot.spec.ts           |  10 +-
 .../__tests__/cloneRecord.spec.ts             |  23 -
 .../__tests__/commitSession.spec.ts           |   6 +-
 .../__tests__/createManagedDatastream.spec.ts |  16 +-
 .../__tests__/persistOperations.spec.ts       |  74 +-
 .../qualityControl/__tests__/qcServiceFake.ts |   4 +-
 .../__tests__/reconstructSession.spec.ts      | 399 +++-------
 .../__tests__/recordFrom.spec.ts              |  17 +
 .../qualityControl/__tests__/session.spec.ts  | 129 +--
 .../services/qualityControl/buildSnapshot.ts  |   8 +-
 .../services/qualityControl/cloneRecord.ts    |  18 -
 .../services/qualityControl/commitSession.ts  |  22 +-
 .../qualityControl/createManagedDatastream.ts |  13 +-
 .../src/services/qualityControl/index.ts      |   2 +-
 .../qualityControl/observationsBody.ts        |   2 +-
 .../qualityControl/persistOperations.ts       |  62 +-
 .../qualityControl/reconstructSession.ts      | 151 ++--
 .../src/services/qualityControl/recordFrom.ts |  19 +
 .../src/services/qualityControl/session.ts    | 132 +++-
 .../store/__tests__/dataVisualization.spec.ts |   4 +-
 .../src/store/__tests__/observations.spec.ts  |  56 +-
 .../src/store/__tests__/qcSession.spec.ts     |  46 +-
 .../src/store/__tests__/workingCopies.spec.ts |   8 +-
 .../src/store/__tests__/workspaces.spec.ts    |  80 --
 apps/qc-app/src/store/dataVisualization.ts    |  63 +-
 apps/qc-app/src/store/observations.ts         |  26 +-
 apps/qc-app/src/store/qcSession.ts            |  52 +-
 apps/qc-app/src/store/workspaces.ts           |  22 -
 apps/qc-app/src/styles/global.scss            |  65 --
 .../src/utils/__tests__/observations.spec.ts  |  62 ++
 .../src/utils/__tests__/qcHistory.spec.ts     |  38 +
 apps/qc-app/src/utils/__tests__/time.spec.ts  |  51 --
 .../src/utils/__tests__/timeZone.spec.ts      |  30 +
 apps/qc-app/src/utils/observations.ts         |  66 +-
 apps/qc-app/src/utils/plotCap.ts              |   3 +
 .../utils/plotting/__tests__/events.spec.ts   |  18 +
 apps/qc-app/src/utils/plotting/events.ts      |   4 +-
 apps/qc-app/src/utils/plotting/options.ts     |   2 +-
 apps/qc-app/src/utils/qcHistory.ts            |  15 +
 apps/qc-app/src/utils/sessionGraph.ts         |   6 +
 apps/qc-app/src/utils/sessionWindow.ts        |  18 +-
 apps/qc-app/src/utils/share.ts                |  24 +-
 apps/qc-app/src/utils/snapshotId.ts           |  35 +-
 apps/qc-app/src/utils/time.ts                 |  12 -
 113 files changed, 2914 insertions(+), 2485 deletions(-)
 create mode 100644 apps/qc-app/e2e/data-table.spec.ts
 create mode 100644 apps/qc-app/e2e/session-list.spec.ts
 create mode 100644 apps/qc-app/src/components/EditData/EditHistoryStepDetails.vue
 create mode 100644 apps/qc-app/src/components/EditData/SessionTimelineItem.vue
 create mode 100644 apps/qc-app/src/composables/useClearPlot.ts
 delete mode 100644 apps/qc-app/src/services/qualityControl/__tests__/cloneRecord.spec.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/__tests__/recordFrom.spec.ts
 delete mode 100644 apps/qc-app/src/services/qualityControl/cloneRecord.ts
 create mode 100644 apps/qc-app/src/services/qualityControl/recordFrom.ts
 create mode 100644 apps/qc-app/src/utils/__tests__/observations.spec.ts
 create mode 100644 apps/qc-app/src/utils/__tests__/qcHistory.spec.ts
 create mode 100644 apps/qc-app/src/utils/__tests__/timeZone.spec.ts
 create mode 100644 apps/qc-app/src/utils/plotCap.ts
 create mode 100644 apps/qc-app/src/utils/qcHistory.ts

diff --git a/apps/qc-app/README.md b/apps/qc-app/README.md
index 2e7bc029..e947938c 100644
--- a/apps/qc-app/README.md
+++ b/apps/qc-app/README.md
@@ -14,8 +14,8 @@ This web app facilitates QC/QA for time series observations stored in a HydroSer
 
 The app is the operator's view of HydroServer's QC pipeline:
 
-1. **Browse**: pick a workspace, filter datastreams by site / observed property / processing level, and plot up to five at once on a synchronized multi-axis chart.
-2. **QC one stream at a time**: the pencil button on a datastream row picks what to edit, separately from the plot checkboxes. The editor draws that stream over its raw source, with the plotted datastreams as read-only context.
+1. **Browse**: pick a workspace, filter datastreams by site / observed property / processing level, and plot up to four at once on a synchronized multi-axis chart, plus the one you edit.
+2. **QC one stream at a time**: the Edit button on a datastream row picks what to edit, separately from the plot checkboxes. The editor draws that stream over its raw source, with the plotted datastreams as read-only context.
 3. **Filter / edit / add**: every operation (Value Threshold, Find Gaps, Persistence, Interpolate, Drift Correction, Fill Gaps, Add Points, etc.) commits a `HistoryItem` to a replayable edit history backed by [`@uwrl/qc-utils`](https://www.npmjs.com/package/@uwrl/qc-utils).
 4. **Save / load a QC History**: export the history as a JSON document, replay it on the same datastream a week later, or templatize across stations.
 5. **Save and commit**: save the edits to a QC session as a draft, then commit the session to push the quality-controlled observations to its managed datastream in `replace` mode.
@@ -48,7 +48,7 @@ Cross-origin isolation is on by default (`vite.config.ts` sends `Cross-Origin-Op
 
 ```
 src/
-├─ pages/                       Top-level routed views (Visualize, Edit, Submit, Auth).
+├─ pages/                       Routed views: Home (the Select and Edit views) and Workspaces.
 ├─ components/
 │  ├─ Navigation/               NavigationRail, EditDrawer, SelectDrawer.
 │  ├─ FilterPoints/             One panel per filter op (ValueThreshold, GapFinder, …).
@@ -57,22 +57,29 @@ src/
 ├─ composables/
 │  ├─ useDataSelection.ts       The bridge between Plotly's selectedpoints and the Pinia store.
 │  ├─ useFilterDispatch.ts      Shared "open panel → run op → highlight result" sequence.
-│  └─ useQcHistory.ts            Wiring around qc-utils' serializeHistory / applyHistory.
+│  ├─ useEditEntry.ts           Entering and leaving the editor.
+│  ├─ useEditSession.ts         Open, save and commit a QC session.
+│  ├─ useLeaveSession.ts        The one question asked before any exit.
+│  └─ useQcHistory.ts           Download the QC history (qc-utils' serializeHistory).
+├─ services/qualityControl/     The QC API workflow: sessions, saving, commit, past-session replay.
 ├─ store/
 │  ├─ plotly.ts                 Plot ref, edit history, redraw, suppressedEchoSelection sentinel.
 │  ├─ dataVisualization.ts      Selected datastream, plotted streams, selectedData.
 │  ├─ userInterface.ts          Drawer state, operator + filter inputs, persisted prefs.
+│  ├─ qcSession.ts              The open history's sessions, which one is viewed, saved baseline.
+│  ├─ workingCopies.ts          Each in-progress session's replayed record, shared by both views.
+│  ├─ observations.ts           Per-datastream observation cache.
 │  └─ workspaces.ts             Workspace list + selected workspace (persisted).
 ├─ utils/plotting/
 │  ├─ events.ts                 plotly_click / plotly_relayout / mousemove handlers.
 │  ├─ relayout.ts               Debounced viewport recomputation, tick alignment.
 │  ├─ selected.ts               handleSelected: translates Plotly selection into a SELECTION dispatch.
 │  ├─ staging.ts                Ghost-fill markers + drag-resizable stage shape.
-│  └─ plotly.ts                 Trace builders + low-level setSelectedPoints / clearSelection.
+│  └─ plotly.ts                 Barrel over the modules above, plus options (trace builders) and operations.
 └─ router/                      vue-router 5 setup with workspace + auth guards.
 ```
 
-The data flow for an edit is always: panel collects args → `useFilterDispatch` / `useQcHistory` calls `selectedSeries.data.dispatch(...)` (qc-utils) → qc-utils mutates typed arrays + appends a `HistoryItem` → `redraw()` pushes the new x / y into Plotly. The UI never touches the typed arrays directly.
+The data flow for an edit is always: panel collects args → `useFilterDispatch` calls `selectedSeries.data.dispatch(...)` (qc-utils) → qc-utils mutates typed arrays + appends a `HistoryItem` → `redraw()` pushes the new x / y into Plotly. The UI never touches the typed arrays directly.
 
 ## Working with `@uwrl/qc-utils` locally
 
diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index f7852334..f63603ad 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -70,13 +70,20 @@ hs.datastreams.createObservations(
     fields: ['phenomenonTime', 'result'],
     data: dataX.map((ts, i) => [new Date(ts).toISOString(), dataY[i]]),
   },
-  { mode: 'replace' }
+  {
+    mode: 'replace',
+    phenomenon_time_start: session.phenomenonTimeStart,
+    phenomenon_time_end: session.phenomenonTimeEnd,
+  }
 )
 ```
 
-Replace mode tells HydroServer to overwrite any observation inside the
-posted window. The QC App always submits the full edited window;
-incremental submission is not implemented today.
+Replace mode deletes every observation between `phenomenon_time_start` and
+`phenomenon_time_end`, then inserts the posted ones. The QC App always sends
+the session window as that range, so points deleted at the window's edges are
+removed too, and a session that deleted every point commits an empty body.
+Without the range, the backend falls back to the span of the posted
+observations. Incremental submission is not implemented today.
 
 ### Result qualifier codes
 
@@ -141,6 +148,10 @@ below), and share-link hydration (the `ed` query param).
   `StartEditingFlow.vue` asks earlier, through `closeEditor()`: a created
   managed datastream is always a new target, so the takeover is certain and
   the question must come before anything reaches the server.
+  When the load is superseded it returns `'superseded'`. It shows the
+  `ResumeSupersededError` message only when the same target is still open; a
+  newer entry taking over, or the editor being closed mid-load, ends it
+  quietly, since the user asked for that.
 - `startSessionOver(window)`: starts a new session on the current edit
   target over `window` (a `utils/timeRangePresets.ts` `TimeWindow`); backs
   **Start new session** and the editor footer's **New session**.
@@ -191,11 +202,11 @@ and `LeaveSessionDialog.vue`, mounted once in `App.vue`, shows it.
   Saving is refused with no session open. Discarding that empties the session
   falls through to the `'empty'` prompt rather than leaving silently.
 - `keepSession()` / `discardSessionAndLeave()`: the `'empty'` answers.
+  `keepSession()` is also the `'saved'` answer, keeping the session as it is.
   Discarding deletes the session through
   `useManagedDatastreams().deleteSession`, and refuses if another
   session was somehow built on it. A failed delete keeps the user in the
   session.
-- `closeSession()`: the `'saved'` answer, keeping the session as it is.
 - `cancelLeave()`: stay, with the zoom, staged band and unsaved edits intact.
 - `forgetSession()`: drop the resume pointer only. For exits that unmount the
   editor, where clearing the edit target would have the editor's URL writer
@@ -207,7 +218,7 @@ and `LeaveSessionDialog.vue`, mounted once in `App.vue`, shows it.
 ### `useResumeEditSession()`
 
 ```ts
-const { resume } = useResumeEditSession(async (id) => {
+useResumeEditSession(async (id) => {
   await startEditing.value?.resume(id) // StartEditingFlow
 })
 ```
@@ -243,17 +254,14 @@ interacted with the page.
 ### `useQcHistory()`
 
 ```ts
-const { exportHistory, importHistory } = useQcHistory()
+const { exportHistory } = useQcHistory()
 
-await exportHistory()             // downloads qc-history-<datastream>-<isoTimestamp>.json
-const report = await importHistory(file)
-// report = { applied: 12, failed: [{ index, method, error }, ...] }
+await exportHistory() // downloads qc-history-<datastream>-<isoTimestamp>.json
 ```
 
-`exportHistory` reads the current wall-clock window from
-`useDataVisStore`. `importHistory` fetches the script's authored window
-into the active datastream **before** replay (selection-coupled ops
-reference indices against this windowed dataset).
+Serializes the edit record's history with the session window (the viewed or
+in-progress session, or the loaded range when there is none). Download only;
+the app has no import.
 
 ### `useEditLock()`
 
@@ -274,16 +282,17 @@ Orchestrates the server-backed QC session workflow against the
 ```ts
 const {
   beginEditing, startSession, saveDraft, discardUnsavedEdits, commit,
-  needsSession, needsHistory, hasUnsavedChanges, unsavedEditCount,
+  hasUnsavedChanges, unsavedEditCount,
 } = useEditSession()
 ```
 
 - `beginEditing()`: resolves the QC history for the QC datastream, loads
-  its sessions, resumes the in-progress one (or sets `needsSession`); sets
-  `needsHistory` when the datastream isn't QC-managed yet. Resuming rebuilds
-  the shared working copy; if that rebuild is superseded it throws
-  `ResumeSupersededError`, leaves `needsSession` false, and `enterEdit`
-  returns to the Select view. Entries can overlap, so it also throws
+  its sessions and resumes the in-progress one. Returns `'resumed'`,
+  `'needs-session'` when there is none to resume, or `'not-managed'` when
+  the datastream isn't QC-managed yet. Resuming rebuilds the shared working
+  copy; if that rebuild is superseded it throws `ResumeSupersededError`
+  rather than asking for a session, and `enterEdit` returns to the Select
+  view. Entries can overlap, so it also throws
   `ResumeSupersededError` when the edit target changes during any of its
   fetches, and it never writes the session store (source, sessions) for a
   target it no longer owns: sessions are fetched with `fetchSessions` and
@@ -297,8 +306,11 @@ const {
   (append-only reconcile).
 - `discardUnsavedEdits()`: drops edits made since the last save and restores
   edited comments.
-- `commit()`: saves, verifies checksum C, pushes observations
-  (`mode: 'replace'`), then locks the session and reloads the sessions. If
+- `commit()`: saves, checks that the source window's checksum
+  (`datastreams.getObservationsChecksum`) still matches the one the session
+  started with, pushes observations (`mode: 'replace'` over the session
+  window), then locks the session and reloads the sessions. A failed push
+  throws before the session is locked, so it stays in progress. If
   another target took over the editor while they reloaded, it leaves the
   session store and the saved-edits baseline alone and still resolves, since
   the commit itself went through.
@@ -375,16 +387,23 @@ have a null role; `accountType === 'admin'` overrides), so no separate
 endpoint is needed.
 
 ```ts
-const { canEdit, canCreateDatastream, roleName, isOwner, can } =
+const { canEdit, canCommit, canCreateDatastream, roleName } =
   useWorkspacePermissions()
-canEdit()                 // selected workspace: can run the QC edit flow?
-canCreateDatastream(ws)   // can create the managed datastream here?
+canEdit()                 // selected workspace: can work in QC sessions?
+canCommit()               // ...and commit them?
+canCreateDatastream(ws)   // can create a managed datastream here?
 roleName(ws)              // 'Owner' | <collaborator role> | 'Admin' | 'Read-only'
 ```
 
-Used to disable the row Edit button, the editor's Save / Commit controls,
-and the create-datastream form, and to mark each workspace's role on the
-picker.
+Each check is what the API enforces for that step:
+
+| Check | Needs | Gates |
+|---|---|---|
+| `canEdit` | edit Datastream (QC sessions and operations) | row Edit, New session, Save |
+| `canCommit` | `canEdit` and create Observation (the push to the managed datastream) | Commit |
+| `canCreateDatastream` | `canEdit` and create Datastream (its QC history needs edit) | the create-datastream form |
+
+`roleName` labels each workspace's role on the picker.
 
 ### `useResizable()`
 
@@ -442,7 +461,7 @@ on boot.
 | `plottedDatastreams`                | state    | `Datastream[]`                                    | Up to 4 streams the user chose to plot. Editing never adds to or removes from it (snapshots are the exception, dropped on leave); the 4-stream cap doesn't count the edit target or its source, so the plot holds at most 6 series while editing. |
 | `qcDatastreamId`                    | state    | `string \| null`                                  | The edit target's id. Set only by the edit flow (`setEditTarget` / `clearEditTarget`); null in the Select view. |
 | `qcDatastream`                      | computed | `Datastream \| null`                              | Live catalog lookup of `qcDatastreamId` in `datastreams`, not `plottedDatastreams`, since the edit target isn't a plotted entry. |
-| `sourceContextDatastream`           | computed | `Datastream \| null`                              | The source drawn around the edit target as context (light grey, cut around the session window). Null without an edit target, with context switched off, or when the user plotted the source, which then draws as an ordinary series. |
+| `sourceContextDatastream`           | computed | `Datastream \| null`                              | The source drawn around the edit target as context (light grey, cut around the session window), with a `ctx:` id so the user can also plot the source as an ordinary series beside it. Null without an edit target or with context switched off. |
 | `seriesDatastreams`                 | computed | `Datastream[]`                                    | What the plot actually draws, in order: `[edit target, its source, ...plotted minus those]` while editing, otherwise `plottedDatastreams` unchanged. Refresh, colour assignment, series ordering, working-copy invalidation, `PlottedDatastreams`, the share watcher, and snapshots all iterate this instead of `plottedDatastreams`. |
 | `qualifierSet`                      | state    | `Set<string>`                                     | Qualifier codes seen on the edit target's loaded points. |
 | `selectedQualifier`                 | state    | `string`                                          | Active qualifier in the picker. |
@@ -563,7 +582,7 @@ handles, live chart caches).
 | `fetchGraphSeries`         | action   | `(ds, start: Date, end: Date) => Promise<GraphSeries>` | Fetch observations for `ds` over `[start, end]` and build a `GraphSeries` via `buildGraphSeries`. |
 | `buildGraphSeries`         | action   | `(ds: Datastream, data: ObservationRecord) => GraphSeries` | Build a `GraphSeries` from an already-loaded record, no fetch. Used directly for a managed datastream's working copy. |
 | `assignSeriesColors`       | action   | `(orderedIds: string[]) => void`                  | Stable per-id colour assignment over the legend order. |
-| `colorForDatastream`       | action   | `(id?: string) => string`                         | Resolve the line colour for a datastream (QC is always black). |
+| `colorForDatastream`       | action   | `(id?: string) => string`                         | Resolve the line colour for a datastream (QC is always dark grey, `#3f3f3f`). |
 | `labelColorForDatastream`  | action   | `(id?: string) => string`                         | Darker companion for legend text. |
 | `clearZoomHistory`         | action   | `() => void`                                      | Empty both stacks. |
 | `pushZoomState`            | action   | `(state: ZoomState) => void`                      | Called by the debounced recorder in `utils/plotting/zoom.ts`. |
@@ -579,10 +598,11 @@ Fetches + caches observation windows and inflates them into
 | `observationsRaw`          | state    | `Record<string, ObservationData>`                 | Typed-array cache (`Float64Array` datetimes + `Float32Array` values). |
 | `fetchObservationsInRange` | action   | `(ds: Datastream, b: Date, e: Date, exclude?: { begin: Date; end: Date }) => Promise<ObservationRecord>` | Fetches only the parts of `[b, e]` never asked for before, skipping `exclude`, and returns the shared record windowed to `[b, e]`. Requests for one datastream run in order, so the record ends on the latest requested window; a request matching the last queued one shares its promise. |
 | `fetchDetachedRecord`      | action   | `(ds: Datastream, b: Date, e: Date) => Promise<ObservationRecord>` | Fills the same cache through the same per-datastream queue, but returns a record of its own windowed to `[b, e]`. For working copies and snapshots: never re-windows the shared record the plot draws. |
+| `forget`                   | action   | `(datastreamId: string) => void`                                   | Drops a datastream's cached observations so the next load asks the server again. Called by `commit()` after its upload; a load already in flight discards its result and retries. |
 
 ### `useWorkspaceStore()` (`src/store/workspaces.ts`)
 
-Workspace selection + role-derived edit permission.
+Workspace selection. Permissions come from `useWorkspacePermissions()`.
 
 Persistence: `selectedWorkspace` only (key
 `qc:selectedWorkspace:v1`) so the router's `hasWorkspaceGuard` sees a
@@ -595,7 +615,6 @@ restored selection on the first navigation.
 | `isLoading`                | state    | `boolean`                                         | True while `loadWorkspaces` is in flight. |
 | `selectedWorkspaceId`      | computed | `string \| null`                                  | Shortcut for `selectedWorkspace?.id`. |
 | `hasSelection`             | computed | `boolean`                                         | True iff `selectedWorkspace` is non-null. |
-| `canEditSelected`          | computed | `boolean`                                         | True for workspace owners; for collaborators, true when their role includes an Observation create/edit permission. |
 | `loadWorkspaces`           | action   | `() => Promise<Workspace[]>`                      | Refetch + reconcile against the stored selection (drops the selection if the user lost access). |
 | `selectWorkspace`          | action   | `(id: string \| null) => Workspace \| null`       | Pick by id from `availableWorkspaces`. |
 | `applyWorkspaceById`       | action   | `(id: string) => Workspace \| null`               | Falls back to a placeholder `{ id }` when the list isn't loaded yet. Used by URL hydration. |
@@ -729,7 +748,6 @@ puts the editor in read-only mode.
 | `savedComments`     | state    | `string[]`                              | Comment text of `savedEdits`, since comments are edited in place. |
 | `isReadOnly`        | computed | `boolean`                               | True when sessions exist and the viewed one isn't the in-progress session. Guarded on `sessions.length` so plain editing outside the session workflow isn't treated as read-only. |
 | `inProgressSession` | computed | `QualityControlSession \| null`         | The editable session, if any. |
-| `committedSessions` | computed | `QualityControlSession[]`               | Sessions with status `committed`. |
 | `viewedSession`     | computed | `QualityControlSession \| null`         | The session for `viewedSessionId`. |
 | `hasSessionOperations` | computed | `boolean`                            | True when the in-progress session holds any work: the operations the server returned with it, plus anything saved since (a save leaves them in `savedEdits` before the sessions are re-fetched). The leave flow tells an untouched session from one worth keeping with it. |
 | `fetchSessions`     | action   | `(historyId: string) => Promise<QualityControlSession[]>` | Fetch a history's sessions with their operations without writing any state, so a caller can drop a result that went stale (see `useEditSession`). |
@@ -751,15 +769,18 @@ Persisted QC editing preferences. Persistence: key `qc:preferences:v1`,
 
 ### `src/utils/plotting/plotly.ts` (barrel)
 
-Re-exports:
+Re-exports `options`, `zoom`, `relayout`, `selected`, `events`,
+`interaction` and `operations`. `internal.ts` stays private. The ones most
+callers reach for:
+
+| Function                 | From            | Purpose                                                          |
+|--------------------------|-----------------|------------------------------------------------------------------|
+| `handleNewPlot(...)`     | `events.ts`     | First draw: build traces, wire events, set the initial range.    |
+| `createPlotlyOption(...)`| `options.ts`    | Build the traces and layout for the plotted series.              |
+| `setSelectedPoints(...)` | `operations.ts` | Programmatic selection update via `Plotly.restyle`.              |
+| `clearSelection(...)`    | `operations.ts` | Drop the selection shape + restyle.                              |
 
-| Function                 | Purpose                                                          |
-|--------------------------|------------------------------------------------------------------|
-| `handleNewPlot(...)`     | First-mount: build traces, wire events, set initial range.       |
-| `setSelectedPoints(...)` | Programmatic selection update via `Plotly.restyle`.              |
-| `clearSelection(...)`    | Drop the selection shape + restyle.                              |
-| `redrawTraces(...)`      | Push new typed-array x/y into all plotted traces.                |
-| `updateOptions(...)`     | Apply axis label / range / tick-formatting changes.              |
+`updateOptions()` is on the plotly store (`store/plotly.ts`), not here.
 
 ### `src/utils/observations.ts`
 
@@ -809,7 +830,7 @@ opened from the row Edit chooser or the editor footer's **New session**):
 - `sessionWindowIssue(window, source, sessions)`: `null` when `window` is
   valid, otherwise `{ kind, message, fix }` for the first broken rule. The
   window must lie inside the source's extent, and it can't leave a gap
-  before or after the committed history (history spec 7.2.3 / 7.2.4).
+  before or after the committed history.
   Touching an edge or overlapping is fine. `fix` carries only the
   endpoints at fault, set to the nearest valid value (source start or end,
   committed end for a gap after, committed start for a gap before), plus
@@ -818,9 +839,11 @@ opened from the row Edit chooser or the editor footer's **New session**):
   shows it as a button on the warning.
 - `NO_SOURCE_DATA_ISSUE`: the issue for a source without observations.
 - `sessionWindowPresets(source, sessions)`: the dialog's preset chips.
-  `All` (the whole record, the default), then `1y`, `6m`, `1m` counting back
-  from the source's last observation through `presetWindow`, and, when
-  something is committed, `Since commit` (committed end to source end).
+  `All` (the whole record), then `1y`, `6m`, `1m` counting back from the
+  source's last observation through `presetWindow`, and, when something is
+  committed, `Since commit` (committed end to source end). The default
+  window is `defaultSessionWindow` above: `Since commit` when there is
+  committed work after which data arrived, else `All`.
   Each carries its `window` and a short `disabledReason` when that window
   breaks a rule; the dialog disables it and shows the reason as its
   tooltip. Empty when the source has no observations.
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 645e38ec..8e8290e6 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -12,7 +12,7 @@ the end-user perspective, see [USER_GUIDE.md](./USER_GUIDE.md).
 | UI framework     | Vue 3 (Composition API, `<script setup>`)    | Same framework as `hydroserver-data-management-app`; lets HydroServer maintainers move between apps without re-learning the runtime. Composition API + `<script setup>` keeps components small and testable. |
 | UI kit           | Vuetify 3 + MDI icons                        | Material Design, accessible primitives, theming, and a ready-made navigation drawer pattern. Cuts custom CSS to a minimum. |
 | State            | Pinia (+ `pinia-plugin-persistedstate`)      | Pinia is the official Vue 3 store; tree-shakeable, fully typed, no boilerplate. The persistence plugin handles `localStorage` rehydration for workspace + UI prefs. |
-| Routing          | vue-router 5                                 | Only three routes; vue-router is the standard.                                  |
+| Routing          | vue-router 5                                 | Only two routes; vue-router is the standard.                                    |
 | Charting         | Plotly.js (`plotly.js-dist`)                 | Multi-axis synchronized plotting, lasso + box selection, programmatic restyle, mature in the science community. ECharts and uPlot were prototyped; Plotly's mature selection model was the deciding factor. |
 | QC engine        | `@uwrl/qc-utils`                             | Worker-parallelized typed-array kernels with a replayable history. Lives in `packages/qc-utils/` so non-Vue consumers can reuse it. |
 | Backend client   | `@hydroserver/client`                        | Generated typed REST client around the HydroServer Django + django-ninja API. Keeps endpoint shapes in sync with the backend. |
@@ -37,9 +37,11 @@ lives in three places, in increasing order of authority:
    operator inputs (`pinia-plugin-persistedstate`). Per-browser, never
    sent to the backend. Use only for ephemeral UI prefs; **never** persist
    observation data here.
-3. **HydroServer backend**: the system of record. The QC App reads
-   observations and writes edited observations back via the
-   `@hydroserver/client`'s `replace`-mode bulk POST. The backend stores
+3. **HydroServer backend**: the system of record. It holds the QC work
+   itself: each managed datastream's QC history, its sessions, and every
+   saved operation with who applied it. A commit writes the session's
+   observations to the managed datastream with the `@hydroserver/client`'s
+   `replace`-mode bulk POST. The backend stores
    everything in HydroServer's Postgres-compatible store, and is the
    place to look for archival, ACID, retention, and migration concerns.
    The QC App does not own any of that.
@@ -75,13 +77,14 @@ always be recovered by a hard refresh.
                 │  Django + django-ninja  ·  Postgres      │
                 │  Workspaces · Things · Datastreams       │
                 │  Observations (bulk read / replace)      │
+                │  QC histories · sessions · operations    │
                 │  Result qualifiers · Auth (AllAuth)      │
                 └──────────────────────────────────────────┘
 ```
 
-All QC computation runs in-browser. The backend never sees the intermediate
-edit state; it only sees the final observations the operator chooses to
-submit.
+All QC computation runs in-browser. The backend stores the operations, not
+the edited data: Save writes a session's operations as a draft, and Commit
+writes the observations they produce to the managed datastream.
 
 ## Source layout
 
@@ -93,21 +96,37 @@ src/
 │  ├─ Navigation/               NavigationRail, SelectDrawer, EditDrawer, PerformanceCalibration.
 │  ├─ VisualizeData/            DataVisualization (Plotly host), data table, info card, filters.
 │  ├─ FilterPoints/             One panel per filter op (ValueThreshold, GapFinder, Persistence, …).
-│  ├─ EditData/                 One panel per edit op (FillGaps, ChangeValues, Interpolate, …)
-│  │                            plus EditHistory + the operation metadata registry.
+│  ├─ EditData/                 One panel per edit op (FillGaps, ChangeValues, Interpolate, …),
+│  │                            EditHistory and its session list, the Start editing chooser,
+│  │                            the session window and leave dialogs, and the operation registry.
 │  └─ base/                     Generic Vuetify wrappers (FullScreenLoader, Notifications).
 ├─ composables/
 │  ├─ useDataSelection.ts       Bridges Plotly's selectedpoints into the Pinia store.
 │  ├─ useFilterDispatch.ts      Shared "open panel → dispatch op → highlight result" flow.
-│  ├─ useQcHistory.ts            Save / load QC Historys (calls qc-utils' serializeHistory / applyHistory).
+│  ├─ useEditEntry.ts           Entering and leaving the editor.
+│  ├─ useEditSession.ts         Open, save and commit a QC session.
+│  ├─ useLeaveSession.ts        The one question asked before any exit.
+│  ├─ useManagedDatastreams.ts  The chooser's managed datastreams, and deleting them.
+│  ├─ useHistorySnapshots.ts    History snapshots plotted as comparison lines.
+│  ├─ useWorkspacePermissions.ts Role checks that gate editing, Save and Commit.
+│  ├─ useQcHistory.ts           Download the QC history (qc-utils' serializeHistory).
 │  ├─ useResizable.ts           Generic drag-to-resize hook used by drawers + the plot.
 │  └─ useBufferedNumber.ts      Debounced numeric input wrapper for filter panels.
+├─ services/qualityControl/     The QC API workflow, with dependencies passed in so it tests
+│                               without Pinia: creating a managed datastream, a session's base,
+│                               saving operations, commit, and replaying a past session.
 ├─ store/                       Pinia stores. See "State stores" below.
 ├─ utils/
 │  ├─ plotting/                 Plotly integration (trace builders, event handlers, selection, staging).
 │  ├─ dateMath.ts               Time-range arithmetic for presets ("1w", "1m", "All", …).
 │  ├─ timeRangePresets.ts       Preset definitions, resolved back from the context data's end, or around the session window while editing.
 │  ├─ observations.ts           Observation fetch helpers (paged columnar fetch).
+│  ├─ sessionWindow.ts          Rules and presets for a new session's window.
+│  ├─ sessionGraph.ts           Session order and dependencies.
+│  ├─ share.ts                  Share-link encoding.
+│  ├─ snapshotId.ts, contextSeriesId.ts  Ids for series that are not datastreams.
+│  ├─ qcHistory.ts              Typed reads off QC history and session shapes.
+│  ├─ plotCap.ts                How many datastreams can be plotted.
 │  ├─ rules.ts                  Vuetify form validation rules.
 │  └─ time.ts                   Time unit conversions.
 ├─ router/                      vue-router setup, auth + workspace guards.
@@ -134,6 +153,8 @@ goes one direction.
 | `operationParams.ts`  | Per-operation form inputs, persisted so they survive panel re-opens.  |
 | `uiLayout.ts`         | Drawer widths, table heights: persisted UI geometry.                  |
 | `workingCopies.ts`    | Working copy per managed datastream, keyed by its in-progress session.|
+| `qcSession.ts`        | The open history's sessions, which one is current and which viewed, the saved-edits baseline, and the resume pointer. |
+| `qcPreferences.ts`    | Last-used processing level for the create form (persisted).           |
 
 The persisted stores use `pinia-plugin-persistedstate` with explicit
 `storage: localStorage` and an explicit `paths` list. **Never** persist
@@ -154,7 +175,7 @@ current plot ref or fetched observations are not; they belong in memory).
  └──────────────┬───────────────────────┘
                 ▼
  ┌──────────────────────────────────────┐
- │ useFilterDispatch / useQcHistory      │
+ │ useFilterDispatch                    │
  │   selectedSeries.data.dispatch(...)  │  ──►  qc-utils ObservationRecord
  └──────────────┬───────────────────────┘            · routes inline vs worker (calibration)
                 │                                    · mutates typed arrays
@@ -222,7 +243,7 @@ in tests.
 
 | File             | Concern                                                                |
 |------------------|------------------------------------------------------------------------|
-| `plotly.ts`      | Barrel + low-level trace builders, `setSelectedPoints`, `clearSelection`. |
+| `plotly.ts`      | Barrel over the modules below (`internal.ts` stays private).           |
 | `events.ts`      | `plotly_click` / `plotly_relayout` / `mousemove` handlers.             |
 | `relayout.ts`    | Debounced viewport recomputation, tick alignment.                      |
 | `selected.ts`    | Translates Plotly selection events into a `SELECTION` dispatch.        |
@@ -293,10 +314,12 @@ Two contract notes worth keeping in mind:
 
 - **The backend stores the operation DAG as metadata only; it never replays
   operations.** The app applies ops locally (qc-utils), pushes the edited
-  series to the managed datastream via `bulk-create` (replace mode), then calls
-  `/commit`, which only records checksums and extends the history window.
-  Checksum verification (source/managed) is the client's responsibility;
-  `/commit` performs none.
+  series to the managed datastream via `bulk-create` (replace mode over the
+  session window), then calls `/commit`, which only records checksums and
+  extends the history window. `/commit` verifies nothing; that is the
+  client's job. Before pushing, `commit()` reads the source window's current
+  `X-Checksum` and refuses to commit if it differs from the session's
+  `sourceChecksum`. The managed-side check is not implemented yet.
 - **Vocabulary differs across the boundary.** qc-utils serializes operations as
   `{ method, args }`; the QC API speaks `{ operationType, arguments, order }`.
   The enum values are identical, so `persistOperations`/`reconstructSession`
@@ -310,13 +333,15 @@ Two contract notes worth keeping in mind:
   refuses updates on committed sessions, so the panel renders their comments
   read-only.
 - **Sessions start/resume from the latest committed state, not the raw source.**
-  Because each commit replays its session into the managed datastream (in-range
-  `replace`), the managed datastream's observations already carry every
-  committed session. `startSession`/`reconstructSession` therefore load the
-  managed datastream as the working base (via `loadLatestBase`, falling back to
-  the source only when nothing has been committed yet) and replay just the
-  current session's own draft operations on top. `loadLatestBase` hands back a
-  standalone copy (`cloneRecord`), not the observation store's cached record,
+  Each commit replaces its window in the managed datastream, so the managed
+  datastream carries every committed session, but only over the ranges those
+  sessions covered. `loadLatestBase` builds a window's base from the managed
+  datastream inside committed session windows and from the source everywhere
+  else, so a window that reaches past the last commit still gets the new
+  source data, and points deleted inside a committed range stay deleted.
+  `startSession`/`reconstructSession` replay just the current session's own
+  draft operations on top. `loadLatestBase` hands back a standalone record,
+  not the observation store's cached record,
   so editing, resuming, and viewing a committed session never modify the
   store's cached records. The raw line stays unedited regardless of what a
   session does to its working copy.
@@ -329,8 +354,8 @@ Two contract notes worth keeping in mind:
   pointer, so only an interrupted session reopens.
 - **The editor and the Select-view plot share one working copy per managed
   datastream**, cached in `workingCopies` (`store/workingCopies.ts`). It is
-  built on a copy of the base (`reconstructSession`/`loadLatestBase`'s
-  `cloneRecord`), never the observation store's cached record, so the raw
+  built on a copy of the base (`loadLatestBase`), never the observation
+  store's cached record, so the raw
   datastream's cached observations are never edited. It is invalidated when
   the edit target is cleared (`dataVisualization.clearEditTarget`, which every
   exit reaches through `useEditEntry.leaveEdit`), on commit, when its session
@@ -351,16 +376,19 @@ Two contract notes worth keeping in mind:
   The managed datastream carries every commit, so it cannot be the base for a
   historical view: replaying an older session's operations on top of it would
   reproduce the final state. `reconstructCommittedSession` instead fetches the
-  ancestor closure (`ancestor_of`), loads the raw source over the union of
-  every window in the chain, and replays the chain in **commit order**
-  (`committedAt`, falling back to `createdAt`): committing is what writes
-  observations into the managed datastream, so it is commit order, not
-  authoring order, that decides what a later session built on. The union
-  window matters because operations replay against array indices: loading
-  only the viewed session's window would misalign a wider ancestor's
-  selections and corrupt the result silently. The panel then shows just the
-  viewed session's own operations; the ancestors produced the data, but the
-  history is about what this session did.
+  ancestor closure (`ancestor_of`), loads the raw source once over the chain's
+  windows, and replays the chain in **commit order** (`committedAt`, falling
+  back to `createdAt`): committing is what writes observations into the
+  managed datastream, so it is commit order, not authoring order, that decides
+  what a later session built on. Each session replays on the base its own
+  window had when it was edited (`composeBase`: the simulated managed data
+  where an earlier session committed, source elsewhere), and its result then
+  replaces its window in the simulated managed data (`replaceWindow`), as a
+  commit does. Operations replay against array indices, so any other base
+  would misalign a session's selections and corrupt the result silently.
+  The panel then shows just the viewed session's own operations; the
+  ancestors produced the data, but the history is about what this session
+  did.
 - **Operations are attributed by the server.** Every `QCOperation` carries a
   `created_by`, stamped from the authenticated user on create, and the
   response resolves a deleted account to a placeholder contact rather than
@@ -398,8 +426,10 @@ package, not the app.
 **Permission gating.** QC editing writes to the source datastream's workspace
 (creates the managed datastream, pushes observations), so the editor's entry
 points are gated on the signed-in user's workspace role via
-`useWorkspacePermissions()`: a read-only collaborator sees a disabled row
-Edit button, and disabled Save / Commit controls, with an explanation instead of a mid-flow 403,
+`useWorkspacePermissions()`, which checks what the API enforces for each
+step: datastream edit for sessions and Save, plus observation create for
+Commit, plus datastream create for the create form. A collaborator without
+them sees disabled controls with an explanation instead of a mid-flow 403,
 and each workspace's role is marked on the picker. The role rides along on the
 `Workspace` object (`collaboratorRole.permissions`; owners have a null role;
 admins override), so no extra request is needed.
@@ -432,16 +462,23 @@ Three constraints shape the implementation:
 
 ## Routing and auth
 
-vue-router 5, two routes (Home, Workspaces). Three guards run on
-every navigation:
+vue-router 5, two routes (Home, Workspaces). Guards run on every navigation,
+in this order:
 
-- **`leaveSessionGuard`**: a change of page ends an open edit session, so it
-  runs the leave flow first and cancels the navigation if the user stays.
 - **`hasAuthGuard`**: redirects unauthenticated users to the
   data-management app's `/login` route and remembers the intended QC
   destination.
 - **`hasWorkspaceGuard`**: redirects users without a selected workspace
-  to `/workspaces`.
+  to `/workspaces`. The picker itself sends a user who already has one on to
+  the page they asked for.
+- **`leaveSessionGuard`**: a change of page ends an open edit session, so it
+  runs the leave flow and cancels the navigation if the user stays. It is
+  the last guard, so it runs after the redirects above and a navigation they
+  send back to the editor never asks. Asking first could end the session for
+  a navigation that lands back in the editor.
+
+The page title and meta tags update in `afterEach` (`updateHead`), so a
+cancelled navigation leaves them as they were.
 
 The nav rail's "Edit" entry is enabled only while an edit target is set: it
 returns to the open editor and never picks one itself.
diff --git a/apps/qc-app/docs/DEPLOYMENT.md b/apps/qc-app/docs/DEPLOYMENT.md
index 6a8a8063..45f805e0 100644
--- a/apps/qc-app/docs/DEPLOYMENT.md
+++ b/apps/qc-app/docs/DEPLOYMENT.md
@@ -142,7 +142,10 @@ The QC App is browser-side software, so the observability story is
 
 - **In-app Snackbar notifications** (`Snackbar` from `qc-utils`) surface
   successes, warnings, and failures for every user-driven action: load
-  failures, submit results, script import reports, etc.
+  failures, save and commit results, etc.
+- **An audit trail of QC work** in HydroServer: every saved operation
+  records who applied it, and committed sessions stay in the managed
+  datastream's QC history.
 - **Browser console** carries qc-utils dispatch logs and any unhandled
   promise rejections. The history panel in dev mode displays per-entry
   `inline` vs `worker` badges when `import.meta.env.DEV` is truthy.
@@ -159,8 +162,6 @@ The QC App is browser-side software, so the observability story is
   no Datadog RUM, no Google Analytics). The team has consciously kept
   the SPA telemetry-free; adding it is an opt-in deployment decision.
 - **No server-side metrics**, because there is no server-side runtime.
-- **No audit log** of QC edits. The QC History file is the closest
-  equivalent (export before submitting to keep a replayable record).
 - **No alerting** beyond what your CDN / object-store provider offers.
 
 If you need richer observability, the natural insertion point is `main.ts`
@@ -225,8 +226,10 @@ matters here is **how the QC App weathers them**:
   `packages/hydroserver-ts`. When the backend ships a schema change,
   update that client, run `npx vue-tsc --noEmit` to see the breakage, and
   patch the call sites.
-- The observation upload path uses `mode: 'replace'` on the bulk POST.
-  Replace semantics are stable across HydroServer versions. If you ever
+- The observation upload path uses `mode: 'replace'` on the bulk POST, with
+  `phenomenon_time_start`/`phenomenon_time_end` set to the session window.
+  Backends without those params replace only the span of the posted
+  observations, which leaves points deleted at the window's edges in place. If you ever
   need to change to append-only or upsert semantics, that's a coordinated
   release between the QC App and the backend.
 
diff --git a/apps/qc-app/docs/ONBOARDING.md b/apps/qc-app/docs/ONBOARDING.md
index 553d7045..818749a8 100644
--- a/apps/qc-app/docs/ONBOARDING.md
+++ b/apps/qc-app/docs/ONBOARDING.md
@@ -107,8 +107,9 @@ you need to verify the published package artifacts.
 - Pinia stores for cross-component state; component-local state stays in
   `ref` / `reactive` inside the component.
 - Comments only for the non-obvious _why_. Don't restate the code.
-- Commit format: `{type}({scope}): {description}`: `feat`, `fix`,
-  `test`, `refactor`, `perf`, `docs`, `style`, `chore`.
+- Commit format: `{type}: {description}`, with no scope (the QC app is
+  implied): `feat`, `fix`, `test`, `refactor`, `perf`, `docs`, `style`,
+  `chore`.
 
 ## Documentation gaps
 
diff --git a/apps/qc-app/docs/PERFORMANCE.md b/apps/qc-app/docs/PERFORMANCE.md
index 42ea5625..b86e01e7 100644
--- a/apps/qc-app/docs/PERFORMANCE.md
+++ b/apps/qc-app/docs/PERFORMANCE.md
@@ -101,7 +101,7 @@ UI freeze on operations that take >300 ms inline.
 
 Observation fetches use HydroServer's columnar format (`format=column`)
 because the row format times out on ~35k-point ranges (see comments in
-`src/utils/observations.ts:24`). The QC App paginates client-side at
+`src/utils/observations.ts`). The QC App paginates client-side at
 50,000 obs / page and merges into the cache.
 
 Key fetch optimizations already in place:
@@ -116,6 +116,14 @@ Key fetch optimizations already in place:
   `applyWindow(begin, end, rawData)`, which re-slices when either the window
   or the cache changed, so the plot, table and counts only ever touch the
   current window.
+
+  A range only counts as asked once a request for it succeeded. A failed
+  page throws instead of returning what it has, and a datastream with no
+  observations yet is never marked, since it may gain some. Paging stops at a
+  short page rather than trusting the datastream's `valueCount`, which can be
+  out of date. A commit calls `forget(id)` on the managed datastream after
+  its upload, which drops its cache, so the next load asks the server again;
+  a load still in flight at that point discards its result and retries.
 - **One plot load at a time.** Rebuilds and context range reloads are
   serialized in `useDataVisStore`. A range reload that lands while a rebuild
   is queued joins it, and one that finds its range already loaded skips, so
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index 234803bc..cdabc9bf 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -39,11 +39,11 @@ Take "user opens Find Gaps, picks a threshold, the plot updates". The
 sequence is:
 
 ```
-panel (FindGaps.vue)
+panel (GapFinder.vue)
    │
    │  validates inputs, sets store state
    ▼
-useFilterDispatch / useQcHistory
+useFilterDispatch
    │
    │  selectedSeries.data.dispatchFilter(FIND_GAPS, args)
    ▼
@@ -141,7 +141,7 @@ stack.
 The fix lives in two places:
 
 1. **The writers** (`setSelectedPoints` in `operations.ts`,
-   `clearSelected` in the dataVisualization store) arm
+   `clearSelected` in `useDataSelection`) arm
    `plotly.suppressedEchoSelection` with the payload they're about
    to write. One-shot, cleared on read.
 
@@ -202,7 +202,7 @@ target, so this order only matters for the legend and axis stacking.
 `createPlotlyOption` reads roles off the stores, not a field on `GraphSeries`:
 
 - **Edit target** (`useDataVisStore().qcDatastream`): primary axis `y`,
-  black, selectable.
+  dark grey (`#3f3f3f`), selectable.
 - **Source context** (`sourceContextDatastream`): shares `y` with the edit
   target, grey, drawn underneath it, read-only (no selection styling, no
   own axis).
@@ -271,10 +271,12 @@ instead. The edit target is one trace in QC grey (`COLORS[0]`), and its
 working copy spans only the session window. The source is the context around
 it, drawn light grey (`SOURCE_CONTEXT_COLOR`) and only outside
 `editSessionWindow` (the viewed session's window, else the in-progress one).
-`sourceContextDatastream` is that drawn source: null when the Context menu's
-switch is off (`showSourceContext`), or when the user plotted the source
-themselves, which then draws whole as an ordinary series and counts toward
-the 4. `editSourceDatastream` is the edit target's source either way.
+`sourceContextDatastream` is that drawn source, under its own `ctx:` id
+(`utils/contextSeriesId.ts`) so the user can also plot the source as an
+ordinary series beside it, which draws whole and counts toward the 4. It is
+null without an edit target or when the Context menu's switch is off
+(`showSourceContext`). `editSourceDatastream` is the edit target's source
+either way.
 `setEditTarget` resets the session store when the target changes, so the
 previous target's window is never applied to the new one.
 
diff --git a/apps/qc-app/docs/QUALITY.md b/apps/qc-app/docs/QUALITY.md
index be05bf02..c360a188 100644
--- a/apps/qc-app/docs/QUALITY.md
+++ b/apps/qc-app/docs/QUALITY.md
@@ -40,12 +40,16 @@ signal.
 - All composables under `src/composables/__tests__/`.
 - The plotting utility layer: `events`, `selected`, `zoom`, `internal`,
   `options`, `operations`, `interaction`.
-- Three substantial SFCs under `src/components/`: `EditHistory.vue`,
-  `DataTable.vue`, `DatastreamFilters.vue`.
+- The SFCs with real behaviour of their own: the editor's session and
+  history components (`EditHistory`, `SessionList`, `StartEditingFlow`, the
+  dialogs) and the plot and table components (`DataVisualization`,
+  `PlottedDatastreams`, `DataTable`, `DataVisDatasetsTable`, …), with specs
+  beside each folder in `__tests__/`. Some of them are still in the
+  coverage excludes below, which predate their specs.
 
 ### What is intentionally excluded from coverage
 
-The blanket coverage excludes are listed inline in `vite.config.ts:74-128`
+The blanket coverage excludes are listed inline in `vite.config.ts` (`test.coverage.exclude`)
 with rationale. The high-level groups:
 
 | Group                                                  | Rationale                                                                |
@@ -90,11 +94,10 @@ HydroServer API team either making the columnar response carry
 qualifiers via opt-in (`include=resultQualifierCodes`) or speeding up
 the row mode. Tracked inline as a TODO.
 
-### 2. Three large SFCs not yet unit-tested
+### 2. A large SFC not yet unit-tested
 
-`DataVisualization.vue`, `PlottedDatastreams.vue`, and `FilterPanel.vue`
-are the three highest-complexity SFCs without unit tests. They are
-exercised by E2E specs but lack the per-branch coverage the rest of the
+`FilterPanel.vue` is the highest-complexity SFC without unit tests. It is
+exercised by E2E specs but lacks the per-branch coverage the rest of the
 project has.
 
 ### 3. Documentation gaps
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 91bc61f7..6fd7260e 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -12,8 +12,8 @@ the operator's manual; QUALITY.md is the policy.
 
 | Layer      | Runner                          | Where                        | Count    |
 | ---------- | ------------------------------- | ---------------------------- | -------- |
-| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 81 files |
-| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 35 files |
+| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 84 files |
+| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 37 files |
 
 There is no separate "integration" tier; component tests live in the
 unit tier and mount real Vue components with the Vue Test Utils
@@ -159,7 +159,7 @@ undefined (reading 'value')` from inside a `setTimeout` (the
 
 ### Coverage thresholds
 
-Configured in [`vite.config.ts:130-140`](../vite.config.ts). The
+Configured in [`vite.config.ts`](../vite.config.ts) (`test.coverage.thresholds`). The
 gate is:
 
 - Lines / statements / functions: **80%**
@@ -226,7 +226,7 @@ The CI gate prints uncovered line numbers per file. Common causes:
 - `projects: chromium, firefox` only. WebKit is **excluded on
   purpose**: `SharedArrayBuffer` + COOP/COEP behavior diverges in
   Safari and would need its own validation pass.
-- `baseURL: http://127.0.0.1:5173`: **never `localhost`**. The
+- `baseURL: http://127.0.0.1:15173` (`E2E_APP_HOST` / `E2E_APP_PORT`): **never `localhost`**. The
   backend (`playground.hydroserver.org`) CORS-allowlists
   the `127.0.0.1` origins only; using `localhost` makes API requests fail
   with `net::ERR_FAILED` and the app never mounts.
@@ -300,6 +300,7 @@ await installMocks(page, {
   authenticated: false,                 // simulate signed-out
   qcHistories: true,                    // give DATASTREAM_ID a managed datastream
   qcSessionState: sessions,             // live QC sessions, for assertions
+  qcCommittedSession: false,            // leave out the fixture's committed session
 })
 ```
 
@@ -312,7 +313,23 @@ straight from its check box, which is what the other specs assume.
 The QC session and operation routes are stateful. They start from the
 session fixtures and apply the app's creates, saves, commits and deletes, so
 a spec can pass a `qcSessionState` array and assert on what was persisted
-(see `submit.spec.ts`).
+(see `submit.spec.ts`). They refuse what the API refuses, with the same 400:
+a second in-progress session, commits and operation changes on a committed
+session, and deleting a session another one depends on. A new session
+depends on the committed sessions its window overlaps, and `ancestor_of`
+lists those dependencies transitively, as the API does.
+
+The committed session's id is `COMMITTED_SESSION_ID` in
+`support/fixtures.ts`. Shared steps live in `support/app.ts` (`goToSelect`,
+`leaveDialog`, the entry helpers) and `support/ops.ts` (`selectAllPoints`,
+`applyChangeValues`).
+
+The fixtures include a committed session over the whole observation window,
+and the app treats the managed datastream as the truth inside committed
+windows. A spec whose managed observations don't match that (nothing
+committed yet, or a commit over only part of the window) sets
+`qcCommittedSession: false` and passes the sessions it means in
+`qcSessionState` (see `managed-preview.spec.ts`, `editor-y-range.spec.ts`).
 
 Save and Commit only appear once a session is open. `setupEditView` enters
 through a row's Edit button rather than the nav rail, so it needs
@@ -499,8 +516,10 @@ targeting `main`. The job:
 1. Add the implementation in `src/testHooks.ts` under the
    `installTestHooks()` function, same registration pattern as
    `waitForSelectedData`.
-2. Update the `Window['__vbwTestHooks']` interface in
-   `e2e/support/app.ts` so spec callers get type-checking.
+2. Update the `Window['__vbwTestHooks']` interface in both
+   `src/testHooks.ts` and `e2e/support/app.ts` (the app and the e2e specs
+   are separate TypeScript projects), so callers on both sides get
+   type-checking.
 3. Hooks ship only when `import.meta.env.DEV` or
    `VITE_APP_E2E_HOOKS` is set; production builds never expose
    them.
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 8dac60bc..69ba57a7 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -1,10 +1,10 @@
 # Quality Controlling Data with the QC App
 
 :::warning
-The HydroServer Quality Control (QC) App is currently under active development. We hope to make a first public release during the summer of 2026. The user documentation provided here is intended to be a description of what the QC App can do. Some of this functionality is still under construction and may not work correctly yet. A demo of the QC App is available for the HydroServer Playground instance at [https://playground.hydroserver.org/quality-control-demo/](https://playground.hydroserver.org/quality-control-demo/). If you want to try it out, you should create a workspace on the Playground instance, load some data, and then select that workspace with the QC App. We update the Playground instance periodically with the latest functionality, but the version deployed there might not match the latest functionality under development.
+The HydroServer Quality Control (QC) App is currently under active development. We are working toward a first public release. The user documentation provided here is intended to be a description of what the QC App can do. Some of this functionality is still under construction and may not work correctly yet. A demo of the QC App is available for the HydroServer Playground instance at [https://playground.hydroserver.org/quality-control-demo/](https://playground.hydroserver.org/quality-control-demo/). If you want to try it out, you should create a workspace on the Playground instance, load some data, and then select that workspace with the QC App. We update the Playground instance periodically with the latest functionality, but the version deployed there might not match the latest functionality under development.
 :::
 
-This guide is for the operator of the QC App: Usually a hydrologist or data technician who picks a datastream, flags, edits, and/or corrects the bad or erroneous points, and then submits the cleaned datastream back to HydroServer. It walks through every feature of the QC App and the shortest path to common QC tasks.
+This guide is for the operator of the QC App: Usually a hydrologist or data technician who picks a datastream, flags, edits, and/or corrects the bad or erroneous points, and then commits the cleaned data to a quality-controlled datastream in HydroServer. It walks through every feature of the QC App and the shortest path to common QC tasks.
 
 If you are looking for developer / deployment docs for the QC App, start with [ARCHITECTURE.md](./ARCHITECTURE.md) instead.
 
@@ -17,10 +17,10 @@ The QC App is the operator's view of HydroServer's quality control pipeline. Wit
 3. **Pick one datastream to edit with its row's Edit button.** Plotted datastreams are read-only context.
 4. **Filter** suspicious points using value thresholds, time windows, change detection, rate-of-change limits, gap detection, or persistence runs.
 5. **Edit** the selected points: change values, interpolate, drift-correct, shift datetimes, delete, fill gaps, add points, attach qualifier flags.
-6. **Save your edits as a QC history** (a JSON file you can replay on the same datastream later).
-7. **Submit** the cleaned observations back to HydroServer to save them.
+6. **Save** your work to the session as you go. Saved operations are stored in HydroServer with who applied them, so anyone with access can pick the session up.
+7. **Commit** the session to write the cleaned observations to the managed datastream. The source datastream is never changed.
 
-Everything runs in your web browser. The backend never sees your edit history until you press Save.
+The edits themselves run in your web browser. HydroServer stores the operations you save and the observations you commit.
 
 ## Glossary
 
@@ -34,8 +34,11 @@ Everything runs in your web browser. The backend never sees your edit history un
 | **Context traces** | The raw source, drawn around the session window, and any plotted datastreams shown around it. |
 | **History** | The ordered list of filters + selections + edits you've applied in the current edit session. Undo / redo / save / load all operate on this list. |
 | **Selection** | The set of point indices a filter (or your click / lasso) produced. Edits operate on the current selection. |
-| **QC history** | A JSON file holding your editing history for a datastream. This is the canonical save format. |
-| **Submit / Save** | POST the cleaned observations back to HydroServer to either overwrite the existing observations in the window you have plotted or to be saved as a new datastream, depending on your versioning scheme. |
+| **Managed datastream** | The quality-controlled copy of a source datastream. You edit it; the source stays as recorded. |
+| **Session** | One stretch of QC work on a managed datastream, over a time window. It is in progress until you commit it. |
+| **Save** | Store the session's operations in HydroServer as a draft. Nothing is written to the managed datastream yet. |
+| **Commit** | Write the session's result to the managed datastream and lock the session. The next session starts from it. |
+| **QC history** | A managed datastream's sessions, in the order they were committed. **Save QC history** downloads the open session as a JSON record. |
 
 ## First-time setup
 
@@ -48,7 +51,7 @@ Everything runs in your web browser. The backend never sees your edit history un
 
    ![Workspaces picker](./images/workspaces.png)
 
-4. On **Home**, the left filter drawer is open. Pick a time range and filter the datastream list by site / observed property / processing level. Click a row to plot it.
+4. On **Home**, the left filter drawer is open. Filter the datastream list by site / observed property / processing level there, and click a row to plot it. The time range is set from the **Time range** button on the plot toolbar.
 
 All dates and times in the app, in the pickers, the plot axis, the table and
 tooltips, are in your browser's local time zone. Every time input names that
@@ -161,9 +164,17 @@ datastream is plotted, and shows a partial mark when only managed versions are.
 
 The row's **Edit** button is separate from plotting: it opens a
 chooser of that source's managed datastreams, or, the first time, skips
-straight to the create-datastream form. Pick a managed datastream to
-continue its in-progress session, or **Start new session** to open the
-session window step.
+straight to the create-datastream form. Each managed datastream shows its
+sessions as a timeline, oldest first:
+
+- A session in progress has a **Continue** button that reopens it, saved
+  operations and all.
+- With nothing in progress, the timeline is headed by **Start new session**,
+  which opens the session window step. It reads **Continue from the latest
+  commit** once something is committed, since the new session starts from
+  what that commit left.
+- **Create new managed datastream** at the bottom sets up another
+  quality-controlled version of the same source.
 
 #### Creating a managed datastream
 
@@ -184,6 +195,10 @@ property, unit, medium, aggregation) is copied from the source and cannot be
 changed here; edit the datastream in the Data Management app afterwards if you
 need to.
 
+The form stays open while the datastream is being created, with a spinner on
+**Create datastream**. If the create fails, you see why and the form keeps
+what you entered, so you can fix it and try again.
+
 #### The session window
 
 The session window step shows the source's data extent and, if the managed
@@ -244,8 +259,8 @@ See the [Pan and zoom across axes](#pan-and-zoom-across-axes) section below for
 Hover any toolbar icon to see its name. The left side of the toolbar flips between **Plot** and **Table** views (see below); the right side carries the **data points toggle**, a share-link button, and the `?` help menu.
 
 Between the help menu and the right edge is the **Context** button. It reads
-**Context · 1m** (the active range) while the source context is shown, and
-**Context off** when it is not. Its menu starts with a **Show source
+**Context · 1m** (the active range), and switches to an eye-off icon while
+the source context is hidden. Its menu starts with a **Show source
 context** switch: off, the raw source leaves the plot entirely. Below it are
 preset chips and From / To pickers like the Select view's Time range
 menu, but here they control how much of the raw source and any plotted
@@ -265,7 +280,7 @@ never reloaded or re-windowed. The editor opens zoomed to the edit session's
 own window; open the Context menu (or click **All**) to see more of the
 surrounding data.
 
-Clicking a single point on the plot selects just that point. Clicking the empty plot area clears the selection.
+Clicking a point on the plot adds it to the selection, or takes it out if it is already selected. Clicking the empty plot area clears the selection.
 
 ### Plot vs Table tabs
 
@@ -329,19 +344,19 @@ The right-hand list (visible on both Select and Edit views) is the roster of eve
 
 - A drag handle to reorder the list (the line colors track the order). While editing, the **edit target** is pinned at the top and can't be reordered or unplotted; only the plotted datastreams can be dragged. The raw source context is not listed: it is switched from the **Context** menu.
 - An **eye** toggle that hides the trace from the plot without unplotting it. Hidden rows render with a strikethrough.
-- A **Y-axis** toggle (non-primary rows only) that collapses that datastream's secondary axis to provide more horizontal space for the plot. The edit target and its source share the primary axis, so neither row has this toggle.
+- A **Y-axis** toggle (non-primary rows only) that collapses that datastream's secondary axis to provide more horizontal space for the plot. The edit target shares the primary axis with the raw source, so its row has no toggle.
 - The datastream name and a subtitle showing the number of points loaded **in the current time window**, e.g. `1,248 pts loaded`. While the fetch is still in flight, the subtitle reads `loading…`.
-- An `×` button to unplot the row. The edit target and its source can't be unplotted this way. Leave the editor to drop them.
+- An `×` button to unplot the row. The edit target can't be unplotted this way. Leave the editor to drop it.
 
 If a plotted datastream has no observations in the current window (either because the dataset is empty there or because the chosen time range doesn't cover its data), the row title shows a small warning-tinted database icon. Hover it for the tooltip "No observations in the current time window". Widening the time range (or clicking **All** in the Time range / Context menu) usually clears it.
 
-While editing, the edit target's row shows the session's working data: committed data where it exists, otherwise the raw datastream, over the session's window, with saved draft edits applied. Its line always covers the whole session window.
+While editing, the edit target's row shows the session's working data over the session's window: committed data where a session was committed, the raw datastream everywhere else, with saved draft edits applied. Its line always covers the whole session window.
 
 ![Plotted datastreams list with two rows](./images/plotted-datastreams-list.png)
 
 ### Plotting multiple datastreams
 
-You can check up to **4 datastreams at a time**. The fifth plot slot is kept for the datastream you edit, so there is always room for it. The plotted count and cap are surfaced in the Datastreams table toolbar as a chip ("`N/4 plotted`"). Once you hit the cap, the unchecked rows disable their plot toggles and a tooltip explains why. Unplot a row from either the table or the list to free up a slot. While editing, the edit target and its raw source context are shown in addition to the 4 plotted datastreams. They don't count against the cap; a source you plot yourself does.
+You can check up to **4 datastreams at a time**. The fifth plot slot is kept for the datastream you edit, so there is always room for it. The plotted count and cap are surfaced in the Datastreams table toolbar as a chip ("`N/4 plotted`"). Once you hit the cap, the unchecked rows disable their plot toggles and a tooltip explains why. Unplot a row from either the table or the list to free up a slot. While editing, the edit target and its raw source context are shown in addition to the 4 plotted datastreams. They don't count against the cap; a source you plot yourself does. History comparison lines (see [Comparing against a point in history](#comparing-against-a-point-in-history)) don't count either, and aren't included in **Download selected**.
 
 ![Two datastreams on independent y-axes](./images/home-multi-datastreams.png)
 
@@ -355,7 +370,7 @@ Once a datastream is on the plot, the **plotted datastreams list** on the right
 
 - **Eye toggle**: hide / show a trace on the plot without unplotting it. Useful when one series is visually crowding the others. Hidden rows render with a strikethrough; their axis stays on the plot so the scale doesn't jump.
 - **Y-axis toggle** (non-primary rows only): collapse just the secondary axis without removing the trace. Reach for it when the extra axes start eating horizontal room and you don't actually need the numeric scale.
-- **Drag handle**: drag a row up or down to reorder the legend. The plot redraws so the trace colors track the new order. The edit target and its source are pinned and can't be reordered.
+- **Drag handle**: drag a row up or down to reorder the legend. The plot redraws so the trace colors track the new order. The edit target is pinned and can't be reordered.
 - **× button**: unplot the row entirely.
 
 ### Pan and zoom across axes
@@ -548,7 +563,7 @@ Every filter, edit, and add operation appends a row to **Edit history** in the r
 
 ![Edit history with one applied operation](./images/edit-history.png)
 
-The header carries the count chip and four icon buttons (left to right): **undo**, **redo**, **save QC history** (tray-arrow-down), **load QC history** (tray-arrow-up), and **open in window** (the pop-out icon, which reopens the same panel inside a modal). Keyboard shortcuts: `Ctrl+Z` to undo, `Ctrl+Y` or `Ctrl+Shift+Z` to redo.
+The header carries the count chip and four icon buttons (left to right): **undo**, **redo**, **save QC history** (tray-arrow-down), and **open in window** (the pop-out icon, which reopens the same panel inside a modal). Keyboard shortcuts: `Ctrl+Z` to undo, `Ctrl+Y` or `Ctrl+Shift+Z` to redo.
 
 The body shows:
 
@@ -561,13 +576,25 @@ The body shows:
     it changed, as they were before it ran; for Add points and Fill gaps, the
     points it inserted. The dates are recorded when the step runs, so later
     edits that move or remove points don't change them.
-  - A failure badge (red `!`) if the op threw at author time. Common after a QC history import that references something missing in this datastream.
+  - A failure badge (red `!`) if the op threw at author time.
   - A duration badge.
   - In dev mode, a small chip showing whether the op ran inline or on a worker.
   - A **plot-this-step** button that adds that point in history to the plot as a comparison line.
   - Clicking the row previews the data as it was at that step; a selection step also selects its points on the plot. Every step stays in the history: steps after the one on screen are dimmed, since they are recorded but not reflected in the plot. A banner reads **Previewing step N of M** with a **Back to latest** button (clicking the last step works too). While previewing, editing waits: operation panels, plot selections and table saves hold until you are back on the latest step. Replaying re-measures each step's duration but keeps its comment and attribution.
   - An **undo** button on the trailing entry only. Undo and redo are the only ways to change the history; from a preview, undo acts on the whole history and ends the preview.
-- A chevron toggles an inline "Arguments" drawer that shows the raw qc-utils call arguments.
+- A chevron toggles an inline drawer under the row. It shows the raw qc-utils call arguments, **Applied by** with the name of whoever applied the step (once it is saved), and a **Comment** box for why the step was applied. A comment is saved with the operation, and a row that has one shows a comment icon next to its name. On a committed session the comment is shown but can't be changed.
+
+### The sessions timeline
+
+Above the history sits **Sessions**: every session of the managed datastream
+you are editing, oldest first. The one you are working in is marked
+**Editing**; the others are marked **View**. Click a session to look at the
+data as it left it, with its own operations in the history below. Viewing a
+committed session is read-only (see [Committed sessions are read-only](#committed-sessions-are-read-only)),
+and **Return to current** takes you back to the session in progress. If you
+have unsaved edits, the app asks before switching, since viewing another
+session drops them. The session rows work from the keyboard too: Tab to one
+and press Enter.
 
 ### Preview a step vs. reload from server
 
@@ -634,17 +661,19 @@ delete the newest session, then the next, one at a time.
 The confirmation dialog names the session and says its operations go with it.
 Earlier sessions are untouched. **This cannot be undone.**
 
-If the server rejects the delete, the chooser reloads from the server so the
-list reflects what actually survived.
+If the server rejects the delete, you see why, and the chooser reloads from
+the server so the list reflects what actually survived.
 
 To remove a managed datastream and all of its sessions at once, use the trash
-icon on the datastream's own header row instead.
-
-## Save / load a QC history
+icon on the datastream's own header row instead. The same kind of dialog asks
+first, naming the datastream and how many sessions go with it. The source
+datastream is never touched.
 
-The QC history is the canonical save format. It's a JSON file you can keep, re-apply, share, or version-control.
+Deleting the datastream you are editing, or one of its sessions, closes the
+editor once the delete goes through, since what it had open is gone. If the
+delete fails, the editor stays as it was.
 
-### Save
+## Save a QC history
 
 In the Edit history header, click the tray-arrow-down icon ("Save QC history"). The browser downloads a file named like:
 
@@ -652,39 +681,24 @@ In the Edit history header, click the tray-arrow-down icon ("Save QC history").
 qc-history-<datastream-name>-<isoTimestamp>.json
 ```
 
-The file contains:
-
-- The wall-clock window of the plotted data.
-- Every operation in the history, in order, with its args.
-
-A Snackbar confirms "QC history saved."
-
-### Load
-
-Click the tray-arrow-up icon ("Load QC history") and pick a JSON file. The app will:
-
-1. Fetch the QC history's authored window into your current QC datastream (the indices in selection-coupled ops reference the *windowed* dataset, so the window has to match).
-2. Replay each operation in order.
-3. Show a Snackbar with `Loaded N operations`, plus a warning if any ops failed.
-
-Per-op failures do not abort the replay. The app keeps going. If your QC history targets columns that don't exist in the new datastream (e.g., a qualifier code that isn't registered), that specific op fails and shows a red `!` badge on its history row, but the rest still run.
-
-### When to use it
-
-- **Repeatable QC.** Apply the same QC routine to your raw dataset to generate the quality controlled dataset.
-- **Audit trail.** Save the QC history before submitting, so you have a record of every transformation you applied.
-- **Iterate offline.** Edit the QC history's JSON if you want to tweak a threshold without re-clicking through the panels.
+The file contains the session window and every operation in the history, in order, with its args and comments. A Snackbar confirms "QC history saved." The file is a record of the session; it can't be loaded back into the app.
 
 ## Submit (Save / Commit / Close)
 
 When you're satisfied with the edits, hit one of the action buttons at the bottom of the Edit history panel:
 
-- **Save**: writes the session's operations to the backend as a draft and keeps you in the Edit view.
+- **Save**: writes the session's operations to the backend as a draft and keeps you in the Edit view. Enabled only when there is something unsaved, including undoing an edit you already saved.
 - **Commit**: materializes the session into the managed datastream and locks it into the history.
 - **Discard**: drops every edit made since the last save, returning the session to its last saved state. Edits already saved to the session stay. Disabled when there is nothing unsaved, and it asks for confirmation first.
 - **New session**: replaces Save and Commit once the session is committed. Opens the session window step, starting from the state the last commit left behind.
 - **Close**: leaves the editor, after asking what should happen to the session. See "Leaving a session" below.
 
+What you can do depends on your role in the workspace. Editing and saving
+need permission to edit datastreams. Commit also needs permission to create
+observations, and creating a managed datastream needs permission to create
+datastreams. A button your role can't use is disabled, and hovering it says
+why.
+
 Clicking Commit opens a confirmation dialog so a misclick won't push data to the server.
 
 ![Commit confirmation dialog](./images/commit-dialog.png)
@@ -704,7 +718,7 @@ committed session is on screen:
 - The Operations drawer disables every operation and says why.
 - Plot selections highlight points but are not recorded in the history.
 - Table cells show their values without offering to edit them.
-- Undo, redo, loading a QC history and step undo are unavailable.
+- Undo, redo and step undo are unavailable.
 
 To keep editing, click **Return to current** (when a session is in progress) or
 **New session**.
@@ -775,13 +789,6 @@ See [PERFORMANCE.md](./PERFORMANCE.md) for the envelope details.
 4. Inspect the result on the chart. Use the undo button in Edit history if it's wrong.
 5. Click **Save** when satisfied.
 
-### "I want to replay last week's QC on this week's data."
-
-1. Load the new week of data on the same datastream.
-2. Open the Edit history header → click the tray-arrow-up icon → pick last week's JSON.
-3. The QC history's authored window may differ from this week's; the app will fetch the QC history's window. To re-apply against the new window instead, save the new window first, edit the QC history's `window` field in a text editor, then re-import.
-4. Review the history. Click **Save**.
-
 ### "I picked the wrong workspace."
 
 Click **Workspaces** in the nav rail and **Select** another, or **Continue** to stay in the current one. If an edit session is open, the app asks what should happen to it first.
diff --git a/apps/qc-app/e2e/data-table.spec.ts b/apps/qc-app/e2e/data-table.spec.ts
new file mode 100644
index 00000000..236d9065
--- /dev/null
+++ b/apps/qc-app/e2e/data-table.spec.ts
@@ -0,0 +1,61 @@
+/**
+ * Hovering a data table cell draws the edit affordance only when the cell
+ * can be edited. While a committed session is on screen its cells are
+ * read-only. Switching back to the plot keeps the selection.
+ */
+
+import { expect, test } from '@playwright/test'
+import { installMocks } from './support/mocks'
+import { COMMITTED_SESSION_ID } from './support/fixtures'
+import { setupEditView } from './support/app'
+import { selectAllPoints } from './support/ops'
+
+test('a read-only cell does not look editable on hover', async ({ page }) => {
+  await installMocks(page, { qcHistories: true })
+  await setupEditView(page)
+  await page.getByTestId(`session-header-${COMMITTED_SESSION_ID}`).click()
+  await expect(page.getByTestId('session-return-current')).toBeVisible()
+  await page.getByRole('button', { name: 'Table' }).click()
+
+  const cell = page.locator('.editable-cell__display--readonly').first()
+  await cell.hover()
+
+  await expect(cell).toHaveCSS('border-top-color', 'rgba(0, 0, 0, 0)')
+  await expect(cell).toHaveCSS('background-color', 'rgba(0, 0, 0, 0)')
+})
+
+test('an editable cell shows the edit affordance on hover', async ({ page }) => {
+  await installMocks(page, { qcHistories: true })
+  await setupEditView(page)
+  await page.getByRole('button', { name: 'Table' }).click()
+
+  const cell = page
+    .locator('.editable-cell__display:not(.editable-cell__display--readonly)')
+    .first()
+  await cell.hover()
+
+  await expect(cell).not.toHaveCSS('border-top-color', 'rgba(0, 0, 0, 0)')
+})
+
+test('the selection is still shown after a trip to the table', async ({
+  page,
+}) => {
+  await installMocks(page, { qcHistories: true })
+  await setupEditView(page)
+  await selectAllPoints(page)
+  const selectedOnPlot = () =>
+    page.evaluate(() => {
+      const gd = document.querySelector('[data-testid="main-plot"]') as any
+      return Math.max(
+        0,
+        ...(gd?.data ?? []).map((t: any) => t.selectedpoints?.length ?? 0)
+      )
+    })
+  const before = await selectedOnPlot()
+  expect(before).toBeGreaterThan(0)
+
+  await page.getByRole('button', { name: 'Table' }).click()
+  await page.getByRole('button', { name: 'Plot', exact: true }).click()
+
+  await expect.poll(selectedOnPlot).toBe(before)
+})
diff --git a/apps/qc-app/e2e/edit-selection.spec.ts b/apps/qc-app/e2e/edit-selection.spec.ts
index d8ec5704..c9ec9f91 100644
--- a/apps/qc-app/e2e/edit-selection.spec.ts
+++ b/apps/qc-app/e2e/edit-selection.spec.ts
@@ -23,7 +23,7 @@ import {
   plotDatastreamById,
   setupEditView,
   startSessionFromRow,
-  waitForEditorReady,
+  waitForSelection,
 } from './support/app'
 import {
   DATASTREAM_ID,
@@ -247,12 +247,23 @@ test.describe('edit selection', () => {
         ])
       )
 
+    // A Context change reloads around the session and keeps all three.
     await page.getByTestId('time-range-btn').click()
     await page
       .getByTestId('time-range-menu')
       .getByTestId('date-preset-All')
       .click()
-    await expect(page.getByTestId('exit-save-btn')).toBeVisible()
+    await page.keyboard.press('Escape')
+    await expect(page.getByTestId('time-range-btn')).toContainText('All')
+    await expect
+      .poll(() => traceIds(page))
+      .toEqual(
+        expect.arrayContaining([
+          MANAGED_DATASTREAM_ID,
+          `ctx:${DATASTREAM_ID}`,
+          DATASTREAM_ID_B,
+        ])
+      )
   })
 
   test('closing the editor keeps the plotted datastreams', async ({ page }) => {
@@ -319,6 +330,7 @@ test.describe('edit selection', () => {
       steps: 10,
     })
     await page.mouse.up()
+    await waitForSelection(page, 1)
 
     const clear = page.getByTestId('clear-selection-btn')
     await expect(clear).toBeVisible()
@@ -341,12 +353,11 @@ test.describe('edit selection', () => {
       w.__updates = 0
       gd.on('plotly_update', () => w.__updates++)
     })
+    // The Context range starts on 1m, so All is a change.
     await page.getByTestId('time-range-btn').click()
     const menu = page.getByTestId('time-range-menu')
-    const allActive = /v-chip--variant-tonal/.test(
-      (await menu.getByTestId('date-preset-All').getAttribute('class')) ?? ''
-    )
-    await menu.getByTestId(allActive ? 'date-preset-1w' : 'date-preset-All').click()
+    await expect(page.getByTestId('time-range-btn')).toContainText('1m')
+    await menu.getByTestId('date-preset-All').click()
     await page.waitForFunction(
       () => (window as unknown as { __updates: number }).__updates > 0
     )
diff --git a/apps/qc-app/e2e/editor-y-range.spec.ts b/apps/qc-app/e2e/editor-y-range.spec.ts
index 2e1d2a99..2e860178 100644
--- a/apps/qc-app/e2e/editor-y-range.spec.ts
+++ b/apps/qc-app/e2e/editor-y-range.spec.ts
@@ -30,6 +30,22 @@ const series = (from: number, to: number, value: (i: number) => number) => {
 const WINDOW_START = 40
 const PEAK = 500
 
+// Committed up to the window, which the managed datastream holds.
+const committed: MockQcSession = {
+  id: 'qcs-before',
+  historyId: QC_HISTORY_ID,
+  status: 'committed',
+  description: null,
+  phenomenonTimeStart: at(0),
+  phenomenonTimeEnd: at(WINDOW_START - 1),
+  sourceChecksum: QC_SOURCE_CHECKSUM,
+  createdAt: at(0),
+  committedAt: at(0),
+  createdBy: { name: 'Test User', email: 'test@example.com' },
+  dependencyIds: [],
+  operations: [],
+}
+
 const session: MockQcSession = {
   id: 'qcs-open',
   historyId: QC_HISTORY_ID,
@@ -50,7 +66,8 @@ test('the edit target is not clipped by a range fitted to other data', async ({
 }) => {
   await installMocks(page, {
     qcHistories: true,
-    qcSessionState: [session],
+    qcCommittedSession: false,
+    qcSessionState: [committed, session],
     observationsById: {
       [DATASTREAM_ID]: series(0, 120, (i) => (i < WINDOW_START ? 10 : PEAK)),
       [MANAGED_DATASTREAM_ID]: series(0, WINDOW_START, () => 10),
diff --git a/apps/qc-app/e2e/history.spec.ts b/apps/qc-app/e2e/history.spec.ts
index 16b9fa3a..cd793b96 100644
--- a/apps/qc-app/e2e/history.spec.ts
+++ b/apps/qc-app/e2e/history.spec.ts
@@ -3,7 +3,7 @@
  *   - Undo (toolbar + Ctrl+Z) pops the trailing edit and repopulates
  *     the redoStack; Redo re-applies it.
  *   - Expand chevron on a history row opens the Arguments panel.
- *   - "Reload from this step" reloads to a specific history index.
+ *   - The undo button on the trailing entry rolls it back.
  *
  * We exercise the flow end-to-end: one filter + one edit, then undo /
  * redo the edit and check the row count.
diff --git a/apps/qc-app/e2e/leave-session.spec.ts b/apps/qc-app/e2e/leave-session.spec.ts
index 21f7dfcb..07221040 100644
--- a/apps/qc-app/e2e/leave-session.spec.ts
+++ b/apps/qc-app/e2e/leave-session.spec.ts
@@ -11,35 +11,19 @@
 import { expect, test, type Page } from '@playwright/test'
 import { installMocks, type MockQcSession } from './support/mocks'
 import {
-  gotoHome,
-  openOp,
+  goToSelect,
+  leaveDialog,
   setupEditView,
   waitForEditorReady,
 } from './support/app'
-import { expectHistoryContains, selectAllPoints } from './support/ops'
-import { DATASTREAM_ID_B } from './support/fixtures'
+import { applyChangeValues } from './support/ops'
+import { COMMITTED_SESSION_ID, DATASTREAM_ID_B } from './support/fixtures'
 
-const leaveDialog = (page: Page) => page.getByTestId('leave-session-dialog')
 const editPanel = (page: Page) => page.getByTestId('edit-target-panel')
 
 const inProgress = (sessions: MockQcSession[]) =>
   sessions.find((s) => s.status === 'in_progress')
 
-async function applyChangeValues(page: Page) {
-  await selectAllPoints(page)
-  await openOp(page, 'changeValues')
-  await page.getByLabel('Value').fill('1')
-  await page.getByRole('button', { name: 'Apply' }).click()
-  await expectHistoryContains(page, 'Change Values')
-}
-
-async function goToSelect(page: Page) {
-  await page.getByTestId('nav-rail-item-select').click()
-  await expect(page.getByTestId('datastreams-table')).toBeVisible({
-    timeout: 30_000,
-  })
-}
-
 /** The editor is closed: the Select view is showing with nothing being edited. */
 async function expectClosed(page: Page) {
   await expect(page.getByTestId('datastreams-table')).toBeVisible({
@@ -142,7 +126,7 @@ test.describe('leaving a session', () => {
   test('viewing committed history closes without a question', async ({
     page,
   }) => {
-    await page.getByTestId('session-header-qcs-e2e-1').click()
+    await page.getByTestId(`session-header-${COMMITTED_SESSION_ID}`).click()
     await expect(page.getByTestId('session-return-current')).toBeVisible()
 
     await page.getByTestId('exit-close-btn').click()
diff --git a/apps/qc-app/e2e/managed-preview.spec.ts b/apps/qc-app/e2e/managed-preview.spec.ts
index 66aafa0d..2105fc11 100644
--- a/apps/qc-app/e2e/managed-preview.spec.ts
+++ b/apps/qc-app/e2e/managed-preview.spec.ts
@@ -55,6 +55,7 @@ test.describe('managed datastream preview', () => {
   test.beforeEach(async ({ page }) => {
     await installMocks(page, {
       qcHistories: true,
+      qcCommittedSession: false,
       qcSessionState: [inProgressSession()],
       observationsById: {
         [MANAGED_DATASTREAM_ID]: { phenomenonTime: [], result: [] },
diff --git a/apps/qc-app/e2e/navigation.spec.ts b/apps/qc-app/e2e/navigation.spec.ts
index f22cc848..55971dcc 100644
--- a/apps/qc-app/e2e/navigation.spec.ts
+++ b/apps/qc-app/e2e/navigation.spec.ts
@@ -12,26 +12,14 @@ import { expect, test, type Page } from '@playwright/test'
 import { installMocks, type MockQcSession } from './support/mocks'
 import {
   gotoHome,
-  openOp,
+  leaveDialog,
   plotDatastreamById,
   setupEditView,
   startSessionFromRow,
 } from './support/app'
-import { expectHistoryContains, selectAllPoints } from './support/ops'
+import { applyChangeValues, expectHistoryContains } from './support/ops'
 import { DATASTREAM_ID_B, WORKSPACE_ID } from './support/fixtures'
 
-async function applyChangeValues(page: Page) {
-  await selectAllPoints(page)
-  await openOp(page, 'changeValues')
-  await page.getByLabel('Value').fill('1')
-  await page.getByRole('button', { name: 'Apply' }).click()
-  await expectHistoryContains(page, 'Change Values')
-}
-
-function leaveDialog(page: Page) {
-  return page.getByTestId('leave-session-dialog')
-}
-
 function expectWorkspacePicker(page: Page) {
   return expect(page.getByTestId(`workspace-pick-${WORKSPACE_ID}`)).toBeVisible({
     timeout: 30_000,
diff --git a/apps/qc-app/e2e/qc-golden-path.spec.ts b/apps/qc-app/e2e/qc-golden-path.spec.ts
index 3fa0e2d7..9d804b84 100644
--- a/apps/qc-app/e2e/qc-golden-path.spec.ts
+++ b/apps/qc-app/e2e/qc-golden-path.spec.ts
@@ -3,17 +3,6 @@ import { config as loadEnv } from 'dotenv'
 
 loadEnv({ path: '.env.local' })
 
-declare global {
-  interface Window {
-    __vbwTestHooks?: {
-      waitForSelectedData: (
-        minLength?: number,
-        timeoutMs?: number
-      ) => Promise<number>
-    }
-  }
-}
-
 const QC_ENTRYPOINT = process.env.QC_E2E_BASE_URL || 'http://127.0.0.1:1203/qc/'
 const E2E_EMAIL = process.env.HYDROSERVER_E2E_EMAIL
 const E2E_PASSWORD = process.env.HYDROSERVER_E2E_PASSWORD
diff --git a/apps/qc-app/e2e/qc-history.spec.ts b/apps/qc-app/e2e/qc-history.spec.ts
index a37310ac..240e442c 100644
--- a/apps/qc-app/e2e/qc-history.spec.ts
+++ b/apps/qc-app/e2e/qc-history.spec.ts
@@ -1,14 +1,7 @@
 /**
- * QC History: round-trip e2e.
- *
- * Apply two operations (a value-threshold filter + a delete edit
- * that consumes its selection), use the EditHistory header's
- * Save button to download the QC history, then load that same file
- * back via the Load button and assert the history matches.
- *
- * The save side is exercised against a Playwright-wired download
- * promise; the load side feeds the captured payload back through
- * the hidden file input by way of `setInputFiles`.
+ * QC History: export e2e. Apply a filter and a delete that consumes its
+ * selection, save the history from the EditHistory header, and check the
+ * downloaded payload.
  */
 
 import { expect, test } from '@playwright/test'
@@ -17,15 +10,13 @@ import { installMocks } from './support/mocks'
 import { openOp, setupEditView } from './support/app'
 import { expectHistoryContains, selectAllPoints } from './support/ops'
 
-test.describe('QC history: save / load round-trip', () => {
+test.describe('QC history: export', () => {
   test.beforeEach(async ({ page }) => {
     await installMocks(page, { qcHistories: true })
     await setupEditView(page)
   })
 
-  test('exports a QC history and re-applies it through the Load button', async ({
-    page,
-  }) => {
+  test('saves the history with its window and operations', async ({ page }) => {
     // --- Author a couple of operations ---------------------------
     // First a filter that selects a useful subset of the fixture
     // sine wave (y > 0 hits all points). selectAllPoints() does
@@ -44,14 +35,6 @@ test.describe('QC history: save / load round-trip', () => {
     const beforeCount = await beforeRows.count()
     expect(beforeCount).toBeGreaterThanOrEqual(2)
 
-    // Snapshot just the method-name spans (`.edit-history__method`).
-    // Pulling whole-row text would also capture the duration and
-    // execution-mode chips, which legitimately differ across the
-    // round-trip (the loaded run remeasures and re-tags them).
-    const beforeMethodLabels = await beforeRows
-      .locator('.edit-history__method')
-      .evaluateAll((spans) => spans.map((s) => s.textContent?.trim() ?? ''))
-
     // --- Save: trigger the Save button and capture the download --
     const saveBtn = page.getByTestId('history-save-btn')
     await expect(saveBtn).toBeEnabled()
@@ -61,8 +44,7 @@ test.describe('QC history: save / load round-trip', () => {
     const path = await download.path()
     expect(path).toBeTruthy()
     const buf = await readFile(path!)
-    const historyText = buf.toString('utf-8')
-    const history = JSON.parse(historyText)
+    const history = JSON.parse(buf.toString('utf-8'))
 
     expect(history.version).toBe('1')
     expect(history.window).toBeDefined()
@@ -70,43 +52,5 @@ test.describe('QC history: save / load round-trip', () => {
     expect(typeof history.window.endDate).toBe('string')
     expect(Array.isArray(history.operations)).toBe(true)
     expect(history.operations.length).toBe(beforeCount)
-
-    // --- Reset history before loading ----------------------------
-    // Discard via the Reload button on the baseline row, which
-    // clears history in-place (preserving the editHistory ref).
-    // Easier: re-enter the page is overkill; just undo back to 0.
-    while (
-      await page.getByTestId('history-undo-btn').isEnabled().catch(() => false)
-    ) {
-      await page.getByTestId('history-undo-btn').click()
-      // Brief settle so the undo's redraw + selection sync fire.
-      await page.waitForTimeout(150)
-    }
-    await expect(
-      page.locator('[data-testid^="history-item-"]')
-    ).toHaveCount(0)
-
-    // --- Load: feed the captured QC history through the hidden input
-    const fileInput = page.locator('input[type="file"][accept*="json"]')
-    await fileInput.setInputFiles({
-      name: 'round-trip.json',
-      mimeType: 'application/json',
-      buffer: buf,
-    })
-
-    // The composable does the reload + dispatch; wait for the row
-    // count to repopulate.
-    await expect(
-      page.locator('[data-testid^="history-item-"]')
-    ).toHaveCount(beforeCount, { timeout: 10_000 })
-
-    // Compare method-label arrays directly.
-    const afterRows = page.locator('[data-testid^="history-item-"]')
-    const afterMethodLabels = await afterRows
-      .locator('.edit-history__method')
-      .evaluateAll((spans) =>
-        spans.map((s) => s.textContent?.trim() ?? '')
-      )
-    expect(afterMethodLabels).toEqual(beforeMethodLabels)
   })
 })
diff --git a/apps/qc-app/e2e/select-while-editing.spec.ts b/apps/qc-app/e2e/select-while-editing.spec.ts
index 4ce2cdbf..f56bc829 100644
--- a/apps/qc-app/e2e/select-while-editing.spec.ts
+++ b/apps/qc-app/e2e/select-while-editing.spec.ts
@@ -12,13 +12,14 @@
 import { expect, test, type Page } from '@playwright/test'
 import { installMocks } from './support/mocks'
 import {
+  goToSelect,
   gotoHome,
   openOp,
   setupEditView,
   startSessionFromRow,
   waitForEditorReady,
 } from './support/app'
-import { expectHistoryContains, selectAllPoints } from './support/ops'
+import { applyChangeValues, expectHistoryContains } from './support/ops'
 import {
   DATASTREAM_ID,
   DATASTREAM_ID_B,
@@ -32,13 +33,6 @@ const editPanel = (page: Page) => page.getByTestId('edit-target-panel')
  *  own plotted list carries the same test ids. */
 const sidePanel = (page: Page) => page.getByTestId('select-side-panel')
 
-async function goToSelect(page: Page) {
-  await page.getByTestId('nav-rail-item-select').click()
-  await expect(page.getByTestId('datastreams-table')).toBeVisible({
-    timeout: 30_000,
-  })
-}
-
 async function goToEditor(page: Page) {
   await page.getByTestId('nav-rail-item-edit').click()
   await waitForEditorReady(page)
@@ -108,11 +102,7 @@ test.describe('Select while editing', () => {
 
   test('keeps unsaved edits and leads back to the editor', async ({ page }) => {
     await setupEditView(page)
-    await selectAllPoints(page)
-    await openOp(page, 'changeValues')
-    await page.getByLabel('Value').fill('1')
-    await page.getByRole('button', { name: 'Apply' }).click()
-    await expectHistoryContains(page, 'Change Values')
+    await applyChangeValues(page)
 
     await goToSelect(page)
 
diff --git a/apps/qc-app/e2e/session-list.spec.ts b/apps/qc-app/e2e/session-list.spec.ts
new file mode 100644
index 00000000..3840367c
--- /dev/null
+++ b/apps/qc-app/e2e/session-list.spec.ts
@@ -0,0 +1,43 @@
+/**
+ * The sessions timeline in the editor: each session row is a button the
+ * keyboard can reach, and opening another session over unsaved edits asks
+ * first.
+ */
+
+import { expect, test } from '@playwright/test'
+import { installMocks } from './support/mocks'
+import { COMMITTED_SESSION_ID } from './support/fixtures'
+import { setupEditView } from './support/app'
+import { applyChangeValues } from './support/ops'
+
+test('opens a session from the keyboard', async ({ page }) => {
+  await installMocks(page, { qcHistories: true })
+  await setupEditView(page)
+
+  const header = page.getByTestId(`session-header-${COMMITTED_SESSION_ID}`)
+  await expect(header).toHaveRole('button')
+  await header.focus()
+  await page.keyboard.press('Enter')
+
+  await expect(page.getByTestId('session-return-current')).toBeVisible()
+})
+
+test('asks before viewing another session over unsaved edits', async ({
+  page,
+}) => {
+  await installMocks(page, { qcHistories: true })
+  await setupEditView(page)
+  await applyChangeValues(page)
+
+  const header = page.getByTestId(`session-header-${COMMITTED_SESSION_ID}`)
+  const confirm = page.getByTestId('view-session-confirm')
+  await header.click()
+  await expect(confirm).toBeVisible()
+  await confirm.getByRole('button', { name: 'Cancel' }).click()
+  await expect(confirm).toBeHidden()
+  await expect(page.getByTestId('session-return-current')).toBeHidden()
+
+  await header.click()
+  await page.getByTestId('confirm-view-session-btn').click()
+  await expect(page.getByTestId('session-return-current')).toBeVisible()
+})
diff --git a/apps/qc-app/e2e/submit.spec.ts b/apps/qc-app/e2e/submit.spec.ts
index 1f01fc1a..ca18cb41 100644
--- a/apps/qc-app/e2e/submit.spec.ts
+++ b/apps/qc-app/e2e/submit.spec.ts
@@ -12,6 +12,29 @@ import { openOp, setupEditView } from './support/app'
 import { expectHistoryContains, selectAllPoints } from './support/ops'
 
 test.describe('commit a QC session', () => {
+  test('Save is enabled only while there are unsaved changes', async ({ page }) => {
+    await installMocks(page, { qcHistories: true })
+    await setupEditView(page)
+    const save = page.getByTestId('exit-save-btn')
+    await expect(save).toBeDisabled()
+
+    await selectAllPoints(page)
+    await openOp(page, 'changeValues')
+    const value = page.getByLabel('Value')
+    await value.fill('1')
+    await value.press('Enter')
+    await expectHistoryContains(page, 'Change Values')
+    await expect(save).toBeEnabled()
+
+    await save.click()
+    await expect(page.getByText('Draft saved.')).toBeVisible()
+    await expect(save).toBeDisabled()
+
+    // Undoing a saved edit is a change that still needs saving.
+    await page.getByTestId('history-undo-btn').click()
+    await expect(save).toBeEnabled()
+  })
+
   test('save then commit pushes a replace-mode payload', async ({ page }) => {
     const submissions: Array<{ mode: string | null; body: any }> = []
     const sessions: MockQcSession[] = []
diff --git a/apps/qc-app/e2e/support/app.ts b/apps/qc-app/e2e/support/app.ts
index 09f6b1bd..ad8bb2f2 100644
--- a/apps/qc-app/e2e/support/app.ts
+++ b/apps/qc-app/e2e/support/app.ts
@@ -165,6 +165,17 @@ export async function waitForEditorReady(page: Page): Promise<void> {
   )
 }
 
+/** Show the Select view from the nav rail. */
+export async function goToSelect(page: Page): Promise<void> {
+  await page.getByTestId('nav-rail-item-select').click()
+  await expect(page.getByTestId('datastreams-table')).toBeVisible({
+    timeout: 30_000,
+  })
+}
+
+/** The dialog asking what should happen to the session being left. */
+export const leaveDialog = (page: Page) => page.getByTestId('leave-session-dialog')
+
 /** Open an operation panel in the edit drawer by id (see operations.ts). */
 export async function openOp(page: Page, id: string): Promise<void> {
   await page.getByTestId(`op-${id}`).click()
diff --git a/apps/qc-app/e2e/support/fixtures.ts b/apps/qc-app/e2e/support/fixtures.ts
index ae3fb98d..d862de0b 100644
--- a/apps/qc-app/e2e/support/fixtures.ts
+++ b/apps/qc-app/e2e/support/fixtures.ts
@@ -329,9 +329,12 @@ export const QC_SESSION_AUTHOR = {
 }
 export const QC_SOURCE_CHECKSUM = 'e2e-source-checksum'
 
+/** The committed session every QC history fixture starts with. */
+export const COMMITTED_SESSION_ID = 'qcs-e2e-1'
+
 export const qcSessions = [
   {
-    id: 'qcs-e2e-1',
+    id: COMMITTED_SESSION_ID,
     historyId: QC_HISTORY_ID,
     status: 'committed' as const,
     description: 'First pass',
diff --git a/apps/qc-app/e2e/support/mocks.ts b/apps/qc-app/e2e/support/mocks.ts
index 83e88603..94da3fd0 100644
--- a/apps/qc-app/e2e/support/mocks.ts
+++ b/apps/qc-app/e2e/support/mocks.ts
@@ -85,6 +85,11 @@ export interface MockOptions {
    * it, so pass an array to assert on what the app persisted.
    */
   qcSessionState?: MockQcSession[]
+  /**
+   * Seed the fixture's committed session over the whole observation window.
+   * Defaults to true; turn it off for a history with nothing committed.
+   */
+  qcCommittedSession?: boolean
   /** Set to false to mark the session as unauthenticated. */
   authenticated?: boolean
 }
@@ -96,6 +101,7 @@ export interface MockQcOperation {
   comment?: string | null
   order: number
   createdAt: string
+  createdBy?: { name: string; email: string }
 }
 
 export interface MockQcSession {
@@ -123,7 +129,7 @@ function corsHeaders(route: Route): Record<string, string> {
     'Access-Control-Allow-Credentials': 'true',
     'Access-Control-Allow-Methods': 'GET,POST,PATCH,DELETE,OPTIONS',
     'Access-Control-Allow-Headers': '*',
-    'Access-Control-Expose-Headers': 'X-Total-Pages,X-Total-Count',
+    'Access-Control-Expose-Headers': 'X-Total-Pages,X-Total-Count,X-Checksum',
   }
 }
 
@@ -160,7 +166,7 @@ export async function installMocks(
   const datastreamCreates = options.datastreamCreates ?? []
   const withQcHistories = options.qcHistories ?? false
   const sessionState = options.qcSessionState ?? []
-  if (withQcHistories) {
+  if (withQcHistories && options.qcCommittedSession !== false) {
     sessionState.push(...qcSessions.map((s) => ({ ...s, operations: [] })))
   }
   // The managed datastream only exists for specs that opted into histories.
@@ -238,7 +244,11 @@ export async function installMocks(
       // Only the first page carries data; subsequent pages are empty
       // so the client's pagination loop terminates.
       const data = page === 1 ? sliced : { phenomenonTime: [], result: [] }
-      return json(route, { data }, 200, { 'X-Total-Pages': '1' })
+      // Every mocked session starts from this checksum, so the source never changes.
+      return json(route, { data }, 200, {
+        'X-Total-Pages': '1',
+        'X-Checksum': QC_SOURCE_CHECKSUM,
+      })
     }
 
     // --- Units ---
@@ -352,7 +362,9 @@ export async function installMocks(
 
 /**
  * Stateful stand-in for the QC session and operation endpoints, covering
- * what the editor calls to start, save, commit, and delete a session.
+ * what the editor calls to start, save, commit, and delete a session. It
+ * refuses what the API refuses, so a test can't pass on a request the real
+ * server would reject.
  */
 async function handleQcSessions(
   route: Route,
@@ -364,17 +376,26 @@ async function handleQcSessions(
   const now = new Date().toISOString()
   const noContent = () =>
     route.fulfill({ status: 204, headers: corsHeaders(route) })
+  const refuse = (detail: string) => json(route, { detail }, 400)
+  const inHistory = state.filter((s) => s.historyId === historyId)
 
   if (!sessionId) {
     if (method === 'GET') {
-      const status = new URL(request.url()).searchParams.get('status')
+      const params = new URL(request.url()).searchParams
+      const status = params.get('status')
+      const ancestorOf = params.get('ancestor_of')
+      const ancestors = ancestorOf ? ancestorIds(inHistory, ancestorOf) : null
       return json(route, {
-        data: state.filter(
-          (s) => s.historyId === historyId && (!status || s.status === status)
+        data: inHistory.filter(
+          (s) =>
+            (!status || s.status === status) && (!ancestors || ancestors.has(s.id))
         ),
       })
     }
     if (method === 'POST') {
+      if (inHistory.some((s) => s.status === 'in_progress')) {
+        return refuse('This history already has an in-progress session.')
+      }
       const body = await safeJson(request)
       const session: MockQcSession = {
         id: `qcs-e2e-new-${state.length + 1}`,
@@ -387,7 +408,15 @@ async function handleQcSessions(
         createdAt: now,
         committedAt: null,
         createdBy: QC_SESSION_AUTHOR,
-        dependencyIds: [],
+        // Like the API: every committed session the new window overlaps.
+        dependencyIds: inHistory
+          .filter(
+            (s) =>
+              s.status === 'committed' &&
+              s.phenomenonTimeStart < body?.phenomenonTimeEnd &&
+              s.phenomenonTimeEnd > body?.phenomenonTimeStart
+          )
+          .map((s) => s.id),
         operations: [],
       }
       state.push(session)
@@ -399,8 +428,10 @@ async function handleQcSessions(
     (s) => s.id === sessionId && s.historyId === historyId
   )
   if (!session) return json(route, { detail: 'Session not found.' }, 404)
+  const inProgress = session.status === 'in_progress'
 
   if (action === 'commit' && method === 'POST') {
+    if (!inProgress) return refuse('Only in-progress sessions can be committed.')
     session.status = 'committed'
     session.committedAt = now
     return json(route, { data: session })
@@ -409,6 +440,9 @@ async function handleQcSessions(
   if (action === 'operations' && !operationId) {
     if (method === 'GET') return json(route, { data: session.operations })
     if (method === 'POST') {
+      if (!inProgress) {
+        return refuse('Operations can only be added to an in-progress session.')
+      }
       const bodies = ((await safeJson(request)) ?? []) as Array<
         Omit<MockQcOperation, 'id' | 'createdAt'>
       >
@@ -416,6 +450,7 @@ async function handleQcSessions(
         ...b,
         id: `${session.id}-op-${b.order}`,
         createdAt: now,
+        createdBy: QC_SESSION_AUTHOR,
       }))
       session.operations.push(...created)
       return json(route, { data: created }, 201)
@@ -425,6 +460,9 @@ async function handleQcSessions(
   if (action === 'operations' && operationId) {
     const index = session.operations.findIndex((o) => o.id === operationId)
     if (index < 0) return json(route, { detail: 'Operation not found.' }, 404)
+    if ((method === 'PATCH' || method === 'DELETE') && !inProgress) {
+      return refuse('Operations can only be changed in an in-progress session.')
+    }
     if (method === 'PATCH') {
       const body = await safeJson(request)
       session.operations[index]!.comment = body?.comment ?? null
@@ -439,11 +477,17 @@ async function handleQcSessions(
   if (!action) {
     if (method === 'GET') return json(route, { data: session })
     if (method === 'PATCH') {
+      if (!inProgress) return refuse('Only in-progress sessions can be updated.')
       const body = await safeJson(request)
       if (body && 'description' in body) session.description = body.description
       return json(route, { data: session })
     }
     if (method === 'DELETE') {
+      if (inHistory.some((s) => s.dependencyIds.includes(session.id))) {
+        return refuse(
+          'This session cannot be deleted because other sessions depend on it.'
+        )
+      }
       state.splice(state.indexOf(session), 1)
       return noContent()
     }
@@ -452,6 +496,22 @@ async function handleQcSessions(
   return json(route, { detail: `Unmocked ${method} ${pathOf(request.url())}` }, 405)
 }
 
+/** Every session `id` depends on, directly or through others. */
+function ancestorIds(sessions: MockQcSession[], id: string): Set<string> {
+  const found = new Set<string>()
+  const frontier = [id]
+  while (frontier.length) {
+    const s = sessions.find((x) => x.id === frontier.pop())
+    for (const dep of s?.dependencyIds ?? []) {
+      if (!found.has(dep)) {
+        found.add(dep)
+        frontier.push(dep)
+      }
+    }
+  }
+  return found
+}
+
 async function safeJson(request: ReturnType<Page['request']> | any): Promise<any> {
   try {
     return JSON.parse(request.postData() ?? 'null')
diff --git a/apps/qc-app/e2e/support/ops.ts b/apps/qc-app/e2e/support/ops.ts
index 4a71c808..6c1a5a0d 100644
--- a/apps/qc-app/e2e/support/ops.ts
+++ b/apps/qc-app/e2e/support/ops.ts
@@ -32,6 +32,19 @@ export async function selectAllPoints(page: Page): Promise<void> {
   await waitForSelection(page, 1)
 }
 
+/**
+ * Select every point and change their values: one unsaved edit. Enter on
+ * the field, not a click on Apply, for the same reason as `selectAllPoints`.
+ */
+export async function applyChangeValues(page: Page): Promise<void> {
+  await selectAllPoints(page)
+  await openOp(page, 'changeValues')
+  const value = page.getByLabel('Value')
+  await value.fill('1')
+  await value.press('Enter')
+  await expectHistoryContains(page, 'Change Values')
+}
+
 /**
  * Wait for a history entry matching `methodText` (the formatted
  * Title-Case rendering of the op's method name, e.g. "Change Values").
diff --git a/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue b/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
index 5622ef77..62f5665f 100644
--- a/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
+++ b/apps/qc-app/src/components/EditData/CreateDatastreamForm.vue
@@ -1,11 +1,11 @@
 <template>
   <div class="qc-create-datastream-form pa-4">
-    <div class="text-body-1 font-weight-medium mb-1">
+    <div class="text-title-medium font-weight-bold mb-1">
       Set up QC editing for "{{ source.name }}"
     </div>
     <div class="text-body-small text-medium-emphasis mb-3">
-      Creates an empty managed datastream from this source, links a QC history,
-      and tags the source.
+      Creates an empty managed datastream from this source and links a QC
+      history to it.
     </div>
 
     <v-alert
@@ -157,7 +157,12 @@
     />
 
     <div class="d-flex justify-end ga-2">
-      <v-btn data-testid="create-cancel" variant="text" @click="emit('cancel')">
+      <v-btn
+        data-testid="create-cancel"
+        variant="text"
+        :disabled="loading"
+        @click="emit('cancel')"
+      >
         Cancel
       </v-btn>
       <v-btn
@@ -165,6 +170,7 @@
         color="primary"
         variant="flat"
         :disabled="!isValid"
+        :loading="loading"
         @click="onConfirm"
       >
         Create datastream
@@ -176,6 +182,7 @@
 <script setup lang="ts">
 import { computed, ref } from 'vue'
 import type { Datastream } from '@hydroserver/client'
+import type { CreateManagedDatastreamSpec } from '@/composables/useCreateManagedDatastream'
 
 interface ProcessingLevelOption {
   id: string
@@ -188,17 +195,6 @@ interface SensorOption {
   name: string
 }
 
-export interface CreateDatastreamSpec {
-  source: Datastream
-  processingLevelId: string
-  name?: string
-  description: string
-  /** Left out when the user clears it: the field is nullable. */
-  status?: string
-  /** A datastream's method is its sensor. */
-  sensorId: string
-}
-
 const props = defineProps<{
   source: Datastream
   processingLevels: ProcessingLevelOption[]
@@ -211,6 +207,8 @@ const props = defineProps<{
   /** When set, the user can't create datastreams here: shown as a warning
    *  and the confirm button is disabled. */
   permissionError?: string
+  /** True while the datastream is being created. */
+  loading?: boolean
   /** Creates a processing level in the active workspace and resolves with the
    *  new level (or null on failure). When provided, an inline "Add processing
    *  level" affordance is shown so users don't have to leave for the
@@ -225,7 +223,7 @@ const props = defineProps<{
 
 const emit = defineEmits<{
   (e: 'cancel'): void
-  (e: 'confirm', spec: CreateDatastreamSpec): void
+  (e: 'confirm', spec: CreateManagedDatastreamSpec): void
 }>()
 
 const knownLevelIds = computed(
@@ -312,7 +310,7 @@ const isValid = computed(
 )
 
 function onConfirm(): void {
-  if (!isValid.value || !processingLevelId.value) return
+  if (props.loading || !isValid.value || !processingLevelId.value) return
   emit('confirm', {
     source: props.source,
     processingLevelId: processingLevelId.value,
diff --git a/apps/qc-app/src/components/EditData/EditHistory.vue b/apps/qc-app/src/components/EditData/EditHistory.vue
index 9e233cc8..ae518085 100644
--- a/apps/qc-app/src/components/EditData/EditHistory.vue
+++ b/apps/qc-app/src/components/EditData/EditHistory.vue
@@ -76,30 +76,6 @@
         </template>
       </v-tooltip>
 
-      <v-tooltip location="bottom" text="Load QC History">
-        <template #activator="{ props: tp }">
-          <v-btn
-            v-bind="tp"
-            data-testid="history-load-btn"
-            aria-label="Load QC History"
-            size="x-small"
-            variant="text"
-            density="comfortable"
-            icon="mdi-tray-arrow-up"
-            :disabled="isUpdating || isReadOnly"
-            @click.stop="onLoadHistoryClick"
-          />
-        </template>
-      </v-tooltip>
-      <input
-        ref="fileInputRef"
-        type="file"
-        accept="application/json,.json"
-        class="d-none"
-        @click.stop
-        @change="onLoadHistoryFile"
-      />
-
       <v-tooltip v-if="popOutEnabled" location="bottom" text="Open in window">
         <template #activator="{ props: tp }">
           <v-btn
@@ -125,416 +101,356 @@
       <SessionList @view="emit('view-session', $event)">
         <template #operations>
           <div class="rounded border bg-surface overflow-hidden">
-      <!-- Row clicks are a mouse shortcut. The step button is the real control,
-           so the row's other buttons are not nested inside a button. -->
-      <div
-        class="edit-history__row edit-history__row--baseline px-3 py-2 d-flex align-center"
-        :class="{ 'edit-history__row--clickable': canStepTo }"
-        data-testid="history-reload-step-baseline"
-        :title="canStepTo ? `Preview the session's starting state` : undefined"
-        @click="onRowReload(SNAPSHOT_BASELINE_INDEX)"
-      >
-        <v-icon
-          :icon="
-            selectedSeries?.data.isLoading
-              ? 'mdi-progress-download'
-              : 'mdi-database-check'
-          "
-          size="16"
-          :color="selectedSeries?.data.isLoading ? 'grey' : 'success'"
-          class="mr-2"
-        />
-        <button
-          type="button"
-          class="edit-history__step text-body-small font-weight-medium flex-grow-1 text-truncate"
-          data-testid="history-step-btn-baseline"
-          :disabled="!canStepTo"
-          :title="canStepTo ? `Preview the session's starting state` : undefined"
-          @click.stop="onRowReload(SNAPSHOT_BASELINE_INDEX)"
-        >
-          {{ selectedSeries?.data.isLoading ? 'Loading data…' : 'Data loaded' }}
-        </button>
-        <v-chip
-          v-if="shownStepIndex === SNAPSHOT_BASELINE_INDEX"
-          size="x-small"
-          color="primary"
-          variant="tonal"
-          label
-          class="mr-1 flex-shrink-0"
-          data-testid="history-loaded-baseline"
-        >
-          Showing
-        </v-chip>
-
-        <span
-          v-if="selectedSeries?.data.loadingTime"
-          class="text-body-small text-medium-emphasis mr-1 flex-shrink-0"
-        >
-          {{ formatDuration(selectedSeries?.data.loadingTime) }}
-        </span>
-        <v-progress-circular
-          v-if="selectedSeries?.data.isLoading"
-          size="14"
-          width="2"
-          color="primary"
-          indeterminate
-        />
-        <v-tooltip
-          location="start"
-          :text="
-            snapshotShown(SNAPSHOT_BASELINE_INDEX)
-              ? 'Remove this comparison line'
-              : `Plot the session's starting state`
-          "
-        >
-          <template #activator="{ props: tp }">
-            <v-btn
-              v-bind="tp"
-              data-testid="history-snapshot-baseline"
-              aria-label="Plot the session's starting state"
-              size="x-small"
-              variant="text"
-              density="comfortable"
-              :icon="
-                snapshotShown(SNAPSHOT_BASELINE_INDEX)
-                  ? 'mdi-chart-line-variant'
-                  : 'mdi-chart-line'
-              "
-              :color="
-                snapshotShown(SNAPSHOT_BASELINE_INDEX) ? 'primary' : undefined
-              "
-              :disabled="isBuilding"
-              @click.stop="onToggleSnapshot(SNAPSHOT_BASELINE_INDEX)"
-            />
-          </template>
-        </v-tooltip>
-
-        <!-- Refetches from the server; the step reload above replays the
-             in-memory raw. Hidden on a committed session. -->
-        <v-tooltip
-          v-if="!selectedSeries?.data.isLoading && !isReadOnly"
-          location="start"
-          text="Discard edits and reload from server"
-        >
-          <template #activator="{ props: tp }">
-            <v-btn
-              v-bind="tp"
-              data-testid="history-reload-btn"
-              aria-label="Discard edits and reload from server"
-              size="x-small"
-              variant="text"
-              density="comfortable"
-              icon="mdi-cloud-download-outline"
-              :disabled="isUpdating"
-              @click.stop="onReload"
-            />
-          </template>
-        </v-tooltip>
-      </div>
-
-      <v-divider />
-
-      <div
-        v-if="previewIndex !== null && !isSwitchingSession"
-        class="edit-history__preview d-flex align-center ga-2 px-3 py-2"
-        data-testid="history-preview-banner"
-      >
-        <v-icon icon="mdi-eye-outline" size="16" color="primary" />
-        <span class="text-body-small flex-grow-1">
-          Previewing
-          {{
-            previewIndex < 0
-              ? 'the starting state'
-              : `step ${previewIndex + 1} of ${editCount}`
-          }}. Editing waits until you are back on the latest step.
-        </span>
-        <v-btn
-          data-testid="history-back-to-latest-btn"
-          size="small"
-          variant="flat"
-          color="primary"
-          :disabled="isUpdating"
-          @click.stop="onBackToLatest"
-        >
-          Back to latest
-        </v-btn>
-      </div>
-
-      <!-- Or the outgoing session's operations linger as if they were these. -->
-      <div
-        v-if="isSwitchingSession"
-        class="pa-4 text-center"
-        data-testid="history-loading"
-      >
-        <v-progress-circular indeterminate color="primary" size="24" />
-        <div class="text-body-small text-medium-emphasis mt-2">
-          Loading session…
-        </div>
-      </div>
-
-      <div v-else-if="editCount === 0" class="pa-4 text-center">
-        <v-icon icon="mdi-clock-outline" size="28" color="grey" class="mb-2" />
-        <div class="text-body-small text-medium-emphasis">
-          Edit operations will appear here.
-        </div>
-      </div>
-
-      <div v-else>
-        <div
-          v-for="(entry, index) of editHistory"
-          :key="index"
-          :data-testid="`history-item-${index}`"
-        >
-          <div
-            class="edit-history__row px-3 py-1 d-flex align-center"
-            :class="{
-              'edit-history__row--loading': entry.execution?.inFlight,
-              'edit-history__row--open': openIndex === index,
-              'edit-history__row--loaded': shownStepIndex === index,
-              'edit-history__row--unapplied': !isApplied(index),
-              'edit-history__row--clickable': canStepTo,
-            }"
-            :title="stepTitle(index)"
-            @click="onRowReload(index)"
-          >
-            <button
-              type="button"
-              class="edit-history__expand mr-1 d-inline-flex align-center justify-center cursor-pointer rounded-sm"
-              :title="openIndex === index ? 'Collapse' : 'Expand arguments'"
-              :aria-label="
-                openIndex === index ? 'Collapse' : 'Expand arguments'
-              "
-              :aria-expanded="openIndex === index"
-              @click.stop="toggle(index)"
+            <!-- Row clicks are a mouse shortcut. The step button is the real control,
+                 so the row's other buttons are not nested inside a button. -->
+            <div
+              class="edit-history__row edit-history__row--baseline px-3 py-2 d-flex align-center"
+              :class="{ 'edit-history__row--clickable': canStepTo }"
+              data-testid="history-reload-step-baseline"
+              :title="canStepTo ? `Preview the session's starting state` : undefined"
+              @click="onRowReload(SNAPSHOT_BASELINE_INDEX)"
             >
               <v-icon
                 :icon="
-                  openIndex === index ? 'mdi-chevron-down' : 'mdi-chevron-right'
+                  selectedSeries?.data.isLoading
+                    ? 'mdi-progress-download'
+                    : 'mdi-database-check'
                 "
                 size="16"
+                :color="selectedSeries?.data.isLoading ? 'grey' : 'success'"
+                class="mr-2"
               />
-            </button>
-
-            <v-icon
-              :icon="iconForMethod(entry.method)"
-              size="16"
-              :color="
-                entry.execution?.status === 'failed'
-                  ? 'error'
-                  : colorForMethod(entry.method)
-              "
-              class="mr-2"
-            />
-
-            <!-- Grows as one unit so the badge sits against the title text. -->
-            <button
-              type="button"
-              class="edit-history__step edit-history__title flex-grow-1 d-flex flex-column align-start"
-              :data-testid="`history-step-btn-${index}`"
-              :disabled="!canStepTo || entry.execution?.inFlight"
-              :title="stepTitle(index)"
-              @click.stop="onRowReload(index)"
-            >
-              <span class="d-flex align-center ga-1 w-100">
-                <span class="edit-history__method text-truncate font-weight-medium">
-                  {{ formatMethod(entry.method) }}
-                </span>
-
-                <v-tooltip v-if="entry.comment" location="start" :text="entry.comment">
-                  <template #activator="{ props: tp }">
-                    <v-icon
-                      v-bind="tp"
-                      :data-testid="`history-comment-badge-${index}`"
-                      icon="mdi-comment-text-outline"
-                      size="14"
-                      color="primary"
-                      class="flex-shrink-0"
-                    />
-                  </template>
-                </v-tooltip>
-              </span>
-
-              <span
-                v-if="stepExtent(entry)"
-                class="edit-history__extent text-medium-emphasis text-truncate w-100"
-                :data-testid="`history-extent-${index}`"
+              <button
+                type="button"
+                class="edit-history__step text-body-small font-weight-medium flex-grow-1 text-truncate"
+                data-testid="history-step-btn-baseline"
+                :disabled="!canStepTo"
+                :title="canStepTo ? `Preview the session's starting state` : undefined"
+                @click.stop="onRowReload(SNAPSHOT_BASELINE_INDEX)"
               >
-                {{ stepExtent(entry) }}
-              </span>
-            </button>
-
-            <v-chip
-              v-if="shownStepIndex === index"
-              size="x-small"
-              color="primary"
-              variant="tonal"
-              label
-              class="mr-1 flex-shrink-0"
-              :data-testid="`history-loaded-${index}`"
-            >
-              Showing
-            </v-chip>
-
-            <div class="d-flex align-center ga-2 flex-shrink-0">
-              <v-tooltip
-                v-if="isApplied(index) && entry.execution?.status === 'failed'"
-                location="start"
-                text="Operation failed: see console for details"
+                {{ selectedSeries?.data.isLoading ? 'Loading data…' : 'Data loaded' }}
+              </button>
+              <v-chip
+                v-if="shownStepIndex === SNAPSHOT_BASELINE_INDEX"
+                size="x-small"
+                color="primary"
+                variant="tonal"
+                label
+                class="mr-1 flex-shrink-0"
+                data-testid="history-loaded-baseline"
               >
-                <template #activator="{ props: tp }">
-                  <v-icon
-                    v-bind="tp"
-                    :data-testid="`history-failed-${index}`"
-                    icon="mdi-alert-circle"
-                    size="14"
-                    color="error"
-                  />
-                </template>
-              </v-tooltip>
+                Showing
+              </v-chip>
 
-              <!-- `!= null` so a replay that measures 0ms still reads as
-                   having run. -->
               <span
-                v-if="isApplied(index) && entry.execution?.durationMs != null"
-                :data-testid="`history-duration-${index}`"
-                class="text-body-small text-medium-emphasis"
+                v-if="selectedSeries?.data.loadingTime"
+                class="text-body-small text-medium-emphasis mr-1 flex-shrink-0"
               >
-                {{ formatDuration(entry.execution.durationMs) }}
+                {{ formatDuration(selectedSeries?.data.loadingTime) }}
               </span>
-
               <v-progress-circular
-                v-if="entry.execution?.inFlight"
+                v-if="selectedSeries?.data.isLoading"
                 size="14"
                 width="2"
                 color="primary"
                 indeterminate
               />
-
               <v-tooltip
                 location="start"
                 :text="
-                  snapshotShown(index)
+                  snapshotShown(SNAPSHOT_BASELINE_INDEX)
                     ? 'Remove this comparison line'
-                    : 'Plot this step as a comparison line'
+                    : `Plot the session's starting state`
                 "
               >
                 <template #activator="{ props: tp }">
                   <v-btn
                     v-bind="tp"
-                    :data-testid="`history-snapshot-${index}`"
-                    aria-label="Plot this step as a comparison line"
+                    data-testid="history-snapshot-baseline"
+                    aria-label="Plot the session's starting state"
                     size="x-small"
                     variant="text"
                     density="comfortable"
                     :icon="
-                      snapshotShown(index)
+                      snapshotShown(SNAPSHOT_BASELINE_INDEX)
                         ? 'mdi-chart-line-variant'
                         : 'mdi-chart-line'
                     "
-                    :color="snapshotShown(index) ? 'primary' : undefined"
-                    :disabled="isBuilding || entry.execution?.inFlight"
-                    @click.stop="onToggleSnapshot(index)"
+                    :color="
+                      snapshotShown(SNAPSHOT_BASELINE_INDEX) ? 'primary' : undefined
+                    "
+                    :disabled="isBuilding"
+                    @click.stop="onToggleSnapshot(SNAPSHOT_BASELINE_INDEX)"
                   />
                 </template>
               </v-tooltip>
 
-              <!-- Trailing entry only; middle entries use "Reload from this
-                   step". Never on a committed session. -->
+              <!-- Refetches from the server; the step reload above replays the
+                   in-memory raw. Hidden on a committed session. -->
               <v-tooltip
-                v-if="index === editHistory.length - 1 && !isReadOnly"
+                v-if="!selectedSeries?.data.isLoading && !isReadOnly"
                 location="start"
-                text="Undo this step"
+                text="Discard edits and reload from server"
               >
                 <template #activator="{ props: tp }">
                   <v-btn
                     v-bind="tp"
-                    :data-testid="`history-undo-${index}`"
-                    aria-label="Undo this step"
+                    data-testid="history-reload-btn"
+                    aria-label="Discard edits and reload from server"
                     size="x-small"
                     variant="text"
                     density="comfortable"
-                    icon="mdi-undo-variant"
-                    color="error"
+                    icon="mdi-cloud-download-outline"
                     :disabled="isUpdating"
-                    @click.stop="onUndo"
+                    @click.stop="onReload"
                   />
                 </template>
               </v-tooltip>
             </div>
-          </div>
 
-          <div v-if="openIndex === index" class="edit-history__args px-3 py-2">
-            <div class="text-body-small text-medium-emphasis mb-1">Arguments</div>
-            <ul class="edit-history__args-list pa-0 ma-0 overflow-y-auto">
-              <li
-                v-for="(arg, argIdx) of entry.args"
-                :key="argIdx"
-                class="text-body-small px-1 py-1"
-                style="word-break: break-all"
-              >
-                <code class="text-body-small">{{ formatArg(arg) }}</code>
-              </li>
-            </ul>
+            <v-divider />
 
             <div
-              v-if="isApplied(index) && isDev && entry.execution?.mode"
-              class="text-body-small text-medium-emphasis mt-3 d-flex align-center ga-2"
-              :data-testid="`history-execution-${index}`"
+              v-if="previewIndex !== null && !isSwitchingSession"
+              class="edit-history__preview d-flex align-center ga-2 px-3 py-2"
+              data-testid="history-preview-banner"
             >
-              <!-- Dev-only: whether the dispatch ran on a worker or inline. -->
-              <v-chip
-                v-if="isDev && entry.execution?.mode"
-                size="x-small"
-                variant="tonal"
-                :color="entry.execution.mode === 'inline' ? 'success' : 'primary'"
-                class="edit-history__mode-chip"
-                :title="
-                  entry.execution.mode === 'inline'
-                    ? 'Ran on the main thread (inline)'
-                    : 'Ran on a web worker'
-                "
+              <v-icon icon="mdi-eye-outline" size="16" color="primary" />
+              <span class="text-body-small flex-grow-1">
+                Previewing
+                {{
+                  previewIndex < 0
+                    ? 'the starting state'
+                    : `step ${previewIndex + 1} of ${editCount}`
+                }}. Editing waits until you are back on the latest step.
+              </span>
+              <v-btn
+                data-testid="history-back-to-latest-btn"
+                size="small"
+                variant="flat"
+                color="primary"
+                :disabled="isUpdating"
+                @click.stop="onBackToLatest"
               >
-                {{ entry.execution.mode }}
-              </v-chip>
+                Back to latest
+              </v-btn>
             </div>
 
+            <!-- Or the outgoing session's operations linger as if they were these. -->
             <div
-              v-if="entry.performedBy"
-              class="text-body-small text-medium-emphasis mt-3"
-              :data-testid="`history-author-detail-${index}`"
+              v-if="isSwitchingSession"
+              class="pa-4 text-center"
+              data-testid="history-loading"
             >
-              Applied by {{ entry.performedBy }}
+              <v-progress-circular indeterminate color="primary" size="24" />
+              <div class="text-body-small text-medium-emphasis mt-2">
+                Loading session…
+              </div>
             </div>
 
-            <div class="text-body-small text-medium-emphasis mt-3 mb-1">
-              Comment
+            <div v-else-if="editCount === 0" class="pa-4 text-center">
+              <v-icon icon="mdi-clock-outline" size="28" color="grey" class="mb-2" />
+              <div class="text-body-small text-medium-emphasis">
+                Edit operations will appear here.
+              </div>
             </div>
-            <v-textarea
-              v-if="!isReadOnly"
-              :model-value="entry.comment ?? ''"
-              :data-testid="`history-comment-${index}`"
-              placeholder="Why was this operation applied?"
-              variant="outlined"
-              density="compact"
-              rows="2"
-              auto-grow
-              hide-details
-              class="text-body-small"
-              @update:model-value="setComment(entry, $event)"
-            />
-            <div
-              v-else
-              class="text-body-small"
-              :class="{ 'text-medium-emphasis font-italic': !entry.comment }"
-              :data-testid="`history-comment-readonly-${index}`"
-            >
-              {{ entry.comment || 'No comment.' }}
-            </div>
-          </div>
 
-          <v-divider />
-        </div>
-      </div>
+            <div v-else>
+              <div
+                v-for="(entry, index) of editHistory"
+                :key="index"
+                :data-testid="`history-item-${index}`"
+              >
+                <div
+                  class="edit-history__row px-3 py-1 d-flex align-center"
+                  :class="{
+                    'edit-history__row--loading': entry.execution?.inFlight,
+                    'edit-history__row--open': openIndex === index,
+                    'edit-history__row--loaded': shownStepIndex === index,
+                    'edit-history__row--unapplied': !isApplied(index),
+                    'edit-history__row--clickable': canStepTo,
+                  }"
+                  :title="stepTitle(index)"
+                  @click="onRowReload(index)"
+                >
+                  <button
+                    type="button"
+                    class="edit-history__expand mr-1 d-inline-flex align-center justify-center cursor-pointer rounded-sm"
+                    :title="openIndex === index ? 'Collapse' : 'Expand arguments'"
+                    :aria-label="
+                      openIndex === index ? 'Collapse' : 'Expand arguments'
+                    "
+                    :aria-expanded="openIndex === index"
+                    @click.stop="toggle(index)"
+                  >
+                    <v-icon
+                      :icon="
+                        openIndex === index ? 'mdi-chevron-down' : 'mdi-chevron-right'
+                      "
+                      size="16"
+                    />
+                  </button>
+
+                  <v-icon
+                    :icon="iconForMethod(entry.method)"
+                    size="16"
+                    :color="
+                      entry.execution?.status === 'failed'
+                        ? 'error'
+                        : colorForMethod(entry.method)
+                    "
+                    class="mr-2"
+                  />
+
+                  <!-- Grows as one unit so the badge sits against the title text. -->
+                  <button
+                    type="button"
+                    class="edit-history__step edit-history__title flex-grow-1 d-flex flex-column align-start"
+                    :data-testid="`history-step-btn-${index}`"
+                    :disabled="!canStepTo || entry.execution?.inFlight"
+                    :title="stepTitle(index)"
+                    @click.stop="onRowReload(index)"
+                  >
+                    <span class="d-flex align-center ga-1 w-100">
+                      <span class="edit-history__method text-truncate font-weight-medium">
+                        {{ formatMethod(entry.method) }}
+                      </span>
+
+                      <v-tooltip v-if="entry.comment" location="start" :text="entry.comment">
+                        <template #activator="{ props: tp }">
+                          <v-icon
+                            v-bind="tp"
+                            :data-testid="`history-comment-badge-${index}`"
+                            icon="mdi-comment-text-outline"
+                            size="14"
+                            color="primary"
+                            class="flex-shrink-0"
+                          />
+                        </template>
+                      </v-tooltip>
+                    </span>
+
+                    <span
+                      v-if="stepExtent(entry)"
+                      class="edit-history__extent text-medium-emphasis text-truncate w-100"
+                      :data-testid="`history-extent-${index}`"
+                    >
+                      {{ stepExtent(entry) }}
+                    </span>
+                  </button>
+
+                  <v-chip
+                    v-if="shownStepIndex === index"
+                    size="x-small"
+                    color="primary"
+                    variant="tonal"
+                    label
+                    class="mr-1 flex-shrink-0"
+                    :data-testid="`history-loaded-${index}`"
+                  >
+                    Showing
+                  </v-chip>
+
+                  <div class="d-flex align-center ga-2 flex-shrink-0">
+                    <v-tooltip
+                      v-if="isApplied(index) && entry.execution?.status === 'failed'"
+                      location="start"
+                      text="Operation failed: see console for details"
+                    >
+                      <template #activator="{ props: tp }">
+                        <v-icon
+                          v-bind="tp"
+                          :data-testid="`history-failed-${index}`"
+                          icon="mdi-alert-circle"
+                          size="14"
+                          color="error"
+                        />
+                      </template>
+                    </v-tooltip>
+
+                    <!-- `!= null` so a replay that measures 0ms still reads as
+                         having run. -->
+                    <span
+                      v-if="isApplied(index) && entry.execution?.durationMs != null"
+                      :data-testid="`history-duration-${index}`"
+                      class="text-body-small text-medium-emphasis"
+                    >
+                      {{ formatDuration(entry.execution.durationMs) }}
+                    </span>
+
+                    <v-progress-circular
+                      v-if="entry.execution?.inFlight"
+                      size="14"
+                      width="2"
+                      color="primary"
+                      indeterminate
+                    />
+
+                    <v-tooltip
+                      location="start"
+                      :text="
+                        snapshotShown(index)
+                          ? 'Remove this comparison line'
+                          : 'Plot this step as a comparison line'
+                      "
+                    >
+                      <template #activator="{ props: tp }">
+                        <v-btn
+                          v-bind="tp"
+                          :data-testid="`history-snapshot-${index}`"
+                          aria-label="Plot this step as a comparison line"
+                          size="x-small"
+                          variant="text"
+                          density="comfortable"
+                          :icon="
+                            snapshotShown(index)
+                              ? 'mdi-chart-line-variant'
+                              : 'mdi-chart-line'
+                          "
+                          :color="snapshotShown(index) ? 'primary' : undefined"
+                          :disabled="isBuilding || entry.execution?.inFlight"
+                          @click.stop="onToggleSnapshot(index)"
+                        />
+                      </template>
+                    </v-tooltip>
+
+                    <!-- Trailing entry only: earlier steps are previewed, not
+                         undone. Never on a committed session. -->
+                    <v-tooltip
+                      v-if="index === editHistory.length - 1 && !isReadOnly"
+                      location="start"
+                      text="Undo this step"
+                    >
+                      <template #activator="{ props: tp }">
+                        <v-btn
+                          v-bind="tp"
+                          :data-testid="`history-undo-${index}`"
+                          aria-label="Undo this step"
+                          size="x-small"
+                          variant="text"
+                          density="comfortable"
+                          icon="mdi-undo-variant"
+                          color="error"
+                          :disabled="isUpdating"
+                          @click.stop="onUndo"
+                        />
+                      </template>
+                    </v-tooltip>
+                  </div>
+                </div>
+
+                <EditHistoryStepDetails
+                  v-if="openIndex === index"
+                  :entry="entry"
+                  :index="index"
+                  :applied="isApplied(index)"
+                  :read-only="isReadOnly"
+                  @update:comment="entry.comment = $event"
+                />
+
+                <v-divider />
+              </div>
+            </div>
           </div>
         </template>
       </SessionList>
@@ -567,6 +483,7 @@ import { useDataVisStore } from '@/store/dataVisualization'
 import { useUIStore } from '@/store/userInterface'
 import { iconForMethod, colorForMethod } from '@/components/EditData/operations'
 import SessionList from '@/components/EditData/SessionList.vue'
+import EditHistoryStepDetails from '@/components/EditData/EditHistoryStepDetails.vue'
 import { useQcHistory } from '@/composables/useQcHistory'
 import { useQcSessionStore } from '@/store/qcSession'
 import { useHistorySnapshots } from '@/composables/useHistorySnapshots'
@@ -620,8 +537,7 @@ const { editHistory, selectedSeries, isUpdating, previewIndex } =
 const { selectedOperation } = storeToRefs(useUIStore())
 const { redraw } = usePlotlyStore()
 const { clearSelected, setPlotSelection } = useDataSelection()
-const { exportHistory, importHistory } = useQcHistory()
-const fileInputRef = ref<HTMLInputElement | null>(null)
+const { exportHistory } = useQcHistory()
 
 const openIndex = ref<number | null>(null)
 
@@ -649,12 +565,6 @@ const onToggleSnapshot = async (opIndex: number) => {
 const snapshotShown = (opIndex: number) =>
   !!viewedSessionId.value && isSnapshotPlotted(viewedSessionId.value, opIndex)
 
-function setComment(entry: HistoryItem, value: string) {
-  entry.comment = value
-}
-
-const isDev = import.meta.env.DEV
-
 const editCount = computed(() => editHistory.value?.length ?? 0)
 
 const canUndo = computed(
@@ -712,26 +622,6 @@ function formatMethod(method: string) {
     .join(' ')
 }
 
-function formatArg(arg: unknown): string {
-  if (Array.isArray(arg)) {
-    const len = arg.length
-    if (!len) return '[]'
-    const preview = arg
-      .slice(0, 5)
-      .map((v) => (typeof v === 'number' ? v : JSON.stringify(v)))
-      .join(', ')
-    return len <= 5 ? `[${preview}]` : `[${preview}, … (${len} items)]`
-  }
-  if (arg && typeof arg === 'object') {
-    try {
-      return JSON.stringify(arg)
-    } catch {
-      return String(arg)
-    }
-  }
-  return String(arg)
-}
-
 const onReload = async () => {
   if (isReadOnly.value || isUpdating.value) return
   isUpdating.value = true
@@ -788,39 +678,6 @@ const onSaveHistory = async () => {
   }
 }
 
-const onLoadHistoryClick = () => {
-  fileInputRef.value?.click()
-}
-
-const onLoadHistoryFile = async (e: Event) => {
-  const input = e.target as HTMLInputElement
-  const file = input.files?.[0]
-  // Reset early so re-picking the same file fires change again.
-  input.value = ''
-  if (!file) return
-
-  isUpdating.value = true
-  try {
-    const report = await importHistory(file)
-    if (report.failed.length === 0) {
-      Snackbar.success(
-        `Loaded ${report.applied} operation${report.applied === 1 ? '' : 's'}.`
-      )
-    } else {
-      Snackbar.warn(
-        `Loaded ${report.applied} operation${report.applied === 1 ? '' : 's'}; ` +
-          `${report.failed.length} failed (see history badges).`
-      )
-    }
-    await redraw()
-  } catch (err) {
-    const msg = err instanceof Error ? err.message : String(err)
-    Snackbar.error(`Couldn't load QC history: ${msg}`)
-  } finally {
-    isUpdating.value = false
-  }
-}
-
 // Pass recordHistory: false on the clear path because the replay is
 // authoritative; dispatching an empty SELECTION could pop a filter the
 // replay just restored.
@@ -1000,22 +857,4 @@ onBeforeUnmount(() => window.removeEventListener('keydown', onKeydown))
   font-size: 0.6875rem;
   line-height: 1.2;
 }
-
-.edit-history__args {
-  background-color: rgba(var(--v-theme-primary), 0.03);
-  border-left: 2px solid rgb(var(--v-theme-primary));
-}
-
-.edit-history__args-list {
-  list-style: none;
-  max-height: 12rem;
-}
-
-.edit-history__mode-chip {
-  font-size: 0.625rem;
-  letter-spacing: 0.05em;
-  text-transform: uppercase;
-  height: 16px;
-  padding-inline: 6px;
-}
 </style>
diff --git a/apps/qc-app/src/components/EditData/EditHistoryStepDetails.vue b/apps/qc-app/src/components/EditData/EditHistoryStepDetails.vue
new file mode 100644
index 00000000..a5a9d64f
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/EditHistoryStepDetails.vue
@@ -0,0 +1,126 @@
+<template>
+  <div class="edit-history-step-details px-3 py-2">
+    <div class="text-body-small text-medium-emphasis mb-1">Arguments</div>
+    <ul class="edit-history-step-details__args pa-0 ma-0 overflow-y-auto">
+      <li
+        v-for="(arg, argIdx) of entry.args"
+        :key="argIdx"
+        class="text-body-small px-1 py-1"
+        style="word-break: break-all"
+      >
+        <code class="text-body-small">{{ formatArg(arg) }}</code>
+      </li>
+    </ul>
+
+    <!-- Dev-only: whether the dispatch ran on a worker or inline. -->
+    <div
+      v-if="applied && isDev && entry.execution?.mode"
+      class="text-body-small text-medium-emphasis mt-3 d-flex align-center ga-2"
+      :data-testid="`history-execution-${index}`"
+    >
+      <v-chip
+        size="x-small"
+        variant="tonal"
+        :color="entry.execution.mode === 'inline' ? 'success' : 'primary'"
+        class="edit-history-step-details__mode"
+        :title="
+          entry.execution.mode === 'inline'
+            ? 'Ran on the main thread (inline)'
+            : 'Ran on a web worker'
+        "
+      >
+        {{ entry.execution.mode }}
+      </v-chip>
+    </div>
+
+    <div
+      v-if="entry.performedBy"
+      class="text-body-small text-medium-emphasis mt-3"
+      :data-testid="`history-author-detail-${index}`"
+    >
+      Applied by {{ entry.performedBy }}
+    </div>
+
+    <div class="text-body-small text-medium-emphasis mt-3 mb-1">Comment</div>
+    <v-textarea
+      v-if="!readOnly"
+      :model-value="entry.comment ?? ''"
+      :data-testid="`history-comment-${index}`"
+      placeholder="Why was this operation applied?"
+      variant="outlined"
+      density="compact"
+      rows="2"
+      auto-grow
+      hide-details
+      class="text-body-small"
+      @update:model-value="emit('update:comment', $event)"
+    />
+    <div
+      v-else
+      class="text-body-small"
+      :class="{ 'text-medium-emphasis font-italic': !entry.comment }"
+      :data-testid="`history-comment-readonly-${index}`"
+    >
+      {{ entry.comment || 'No comment.' }}
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+/** The expanded part of an edit history step: its arguments, who applied
+ *  it, and its comment. */
+
+import type { HistoryItem } from '@uwrl/qc-utils'
+
+defineProps<{
+  entry: HistoryItem
+  index: number
+  /** False for a step past the one on screen, whose run no longer holds. */
+  applied: boolean
+  readOnly: boolean
+}>()
+
+const emit = defineEmits<{ (e: 'update:comment', value: string): void }>()
+
+const isDev = import.meta.env.DEV
+
+function formatArg(arg: unknown): string {
+  if (Array.isArray(arg)) {
+    const len = arg.length
+    if (!len) return '[]'
+    const preview = arg
+      .slice(0, 5)
+      .map((v) => (typeof v === 'number' ? v : JSON.stringify(v)))
+      .join(', ')
+    return len <= 5 ? `[${preview}]` : `[${preview}, … (${len} items)]`
+  }
+  if (arg && typeof arg === 'object') {
+    try {
+      return JSON.stringify(arg)
+    } catch {
+      return String(arg)
+    }
+  }
+  return String(arg)
+}
+</script>
+
+<style scoped>
+.edit-history-step-details {
+  background-color: rgba(var(--v-theme-primary), 0.03);
+  border-left: 2px solid rgb(var(--v-theme-primary));
+}
+
+.edit-history-step-details__args {
+  list-style: none;
+  max-height: 12rem;
+}
+
+.edit-history-step-details__mode {
+  font-size: 0.625rem;
+  letter-spacing: 0.05em;
+  text-transform: uppercase;
+  height: 16px;
+  padding-inline: 6px;
+}
+</style>
diff --git a/apps/qc-app/src/components/EditData/LeaveSessionDialog.vue b/apps/qc-app/src/components/EditData/LeaveSessionDialog.vue
index 214f9115..9a413409 100644
--- a/apps/qc-app/src/components/EditData/LeaveSessionDialog.vue
+++ b/apps/qc-app/src/components/EditData/LeaveSessionDialog.vue
@@ -86,7 +86,7 @@
             prepend-icon="mdi-close"
             data-testid="leave-close-btn"
             :disabled="!!leaveWork"
-            @click="closeSession"
+            @click="keepSession"
           >
             Close
           </v-btn>
@@ -111,7 +111,6 @@ const {
   leaveWork,
   cancelLeave,
   keepSession,
-  closeSession,
   saveAndLeave,
   discardEditsAndLeave,
   discardSessionAndLeave,
diff --git a/apps/qc-app/src/components/EditData/SessionList.vue b/apps/qc-app/src/components/EditData/SessionList.vue
index c3caf847..929d0450 100644
--- a/apps/qc-app/src/components/EditData/SessionList.vue
+++ b/apps/qc-app/src/components/EditData/SessionList.vue
@@ -32,26 +32,17 @@
       line-thickness="2"
       class="qc-session-list__timeline px-2 py-2"
     >
-      <v-timeline-item
+      <SessionTimelineItem
         v-for="session of orderedSessions"
         :key="session.id"
-        fill-dot
-        size="x-small"
-        :icon="session.status === 'in_progress' ? 'mdi-pencil' : 'mdi-check'"
-        :icon-color="session.status === 'in_progress' ? 'warning' : 'success'"
-        width="100%"
+        :kind="session.status"
+        :viewed="session.id === viewedSessionId"
         :data-testid="`session-${session.id}`"
-        class="qc-session-list__item qc-timeline__item"
-        :class="[
-          session.status === 'in_progress'
-            ? 'qc-timeline__item--active'
-            : 'qc-timeline__item--done',
-          { 'qc-timeline__item--viewed': session.id === viewedSessionId },
-        ]"
       >
         <!-- Only the header selects; clicks inside the nested panel must not. -->
-        <div
-          class="d-flex align-center ga-2 cursor-pointer"
+        <button
+          type="button"
+          class="qc-session-list__header-btn d-flex align-center ga-2 w-100 cursor-pointer"
           :data-testid="`session-header-${session.id}`"
           @click="onSelect(session)"
         >
@@ -78,7 +69,7 @@
           <v-chip v-else size="x-small" color="grey" variant="tonal" label>
             View
           </v-chip>
-        </div>
+        </button>
 
         <div v-if="session.id === viewedSessionId" class="mt-2">
           <slot name="operations" />
@@ -98,7 +89,7 @@
         >
           No operations.
         </div>
-      </v-timeline-item>
+      </SessionTimelineItem>
     </v-timeline>
 
     <div v-if="!viewedSessionId" class="pa-2">
@@ -111,7 +102,9 @@
 import { computed } from 'vue'
 import { storeToRefs } from 'pinia'
 import { useQcSessionStore } from '@/store/qcSession'
+import SessionTimelineItem from '@/components/EditData/SessionTimelineItem.vue'
 import { formatDateRange } from '@/utils/time'
+import { sessionOperations } from '@/utils/qcHistory'
 import type { QualityControlSession } from '@hydroserver/client'
 
 const store = useQcSessionStore()
@@ -128,7 +121,7 @@ const sessionPeriod = (session: QualityControlSession): string =>
   formatDateRange(session.phenomenonTimeStart, session.phenomenonTimeEnd)
 
 const operationCount = (session: QualityControlSession): number =>
-  (session as { operations?: unknown[] }).operations?.length ?? 0
+  sessionOperations(session).length
 
 function previewLabel(session: QualityControlSession): string {
   const total = operationCount(session)
@@ -155,4 +148,17 @@ function returnToCurrent(): void {
   min-height: 28px;
 }
 
+.qc-session-list__header-btn {
+  padding: 0;
+  border: 0;
+  background: none;
+  color: inherit;
+  font: inherit;
+  text-align: start;
+}
+
+.qc-session-list__header-btn:focus-visible {
+  outline: 2px solid rgb(var(--v-theme-primary));
+  outline-offset: 2px;
+}
 </style>
diff --git a/apps/qc-app/src/components/EditData/SessionTimelineItem.vue b/apps/qc-app/src/components/EditData/SessionTimelineItem.vue
new file mode 100644
index 00000000..c4696d90
--- /dev/null
+++ b/apps/qc-app/src/components/EditData/SessionTimelineItem.vue
@@ -0,0 +1,104 @@
+<template>
+  <v-timeline-item
+    fill-dot
+    size="x-small"
+    width="100%"
+    :icon="MARKERS[kind].icon"
+    :icon-color="MARKERS[kind].color"
+    class="qc-timeline__item"
+    :class="[MARKERS[kind].modifier, { 'qc-timeline__item--viewed': viewed }]"
+  >
+    <slot />
+  </v-timeline-item>
+</template>
+
+<script setup lang="ts">
+/**
+ * One node of a session timeline: a session, or the "start a new session"
+ * action that heads the chooser's timeline. Shared by the editor's session
+ * list and the "Start editing" chooser.
+ */
+
+defineProps<{
+  kind: 'in_progress' | 'committed' | 'new'
+  /** The session on screen. */
+  viewed?: boolean
+}>()
+
+const MARKERS = {
+  in_progress: {
+    icon: 'mdi-pencil',
+    color: 'warning',
+    modifier: 'qc-timeline__item--active',
+  },
+  committed: {
+    icon: 'mdi-check',
+    color: 'success',
+    modifier: 'qc-timeline__item--done',
+  },
+  new: { icon: 'mdi-plus', color: 'primary', modifier: 'qc-timeline__item--new' },
+}
+</script>
+
+<style scoped>
+/* Markers: Vuetify's is a saturated disc ringed in surface-light with the
+   glyph flush to its edge. Flatten it to a tonal marker with room for the
+   icon, nudged onto the title's row. */
+.qc-timeline__item :deep(.v-timeline-divider__dot) {
+  width: 20px;
+  height: 20px;
+  margin-block-start: 4px;
+  background: rgb(var(--v-theme-surface));
+  box-shadow: none;
+}
+
+/* `.v-icon` declares the multiplier on itself, so it has to be set there. */
+.qc-timeline__item :deep(.v-timeline-divider__inner-dot .v-icon) {
+  --v-icon-size-multiplier: 0.75;
+}
+
+.qc-timeline__item--active :deep(.v-timeline-divider__inner-dot) {
+  background: rgba(var(--v-theme-warning), 0.16);
+  box-shadow: inset 0 0 0 1px rgba(var(--v-theme-warning), 0.35);
+}
+
+.qc-timeline__item--done :deep(.v-timeline-divider__inner-dot) {
+  background: rgba(var(--v-theme-success), 0.16);
+  box-shadow: inset 0 0 0 1px rgba(var(--v-theme-success), 0.35);
+}
+
+.qc-timeline__item--new :deep(.v-timeline-divider__inner-dot) {
+  background: rgba(var(--v-theme-primary), 0.16);
+  box-shadow: inset 0 0 0 1px rgba(var(--v-theme-primary), 0.35);
+}
+
+/* Every session reads as its own container, so the unselected ones don't
+   float loose against the timeline rail. The card padding replaces Vuetify's
+   body inset, so the gap to the rail comes back as a margin. */
+.qc-timeline__item :deep(.v-timeline-item__body) {
+  margin-inline-start: 10px;
+  border-radius: 6px;
+  padding: 6px 8px;
+  border: thin solid rgba(var(--v-border-color), var(--v-border-opacity));
+  background-color: rgba(var(--v-theme-on-surface), 0.02);
+  transition:
+    background-color 120ms ease,
+    border-color 120ms ease;
+}
+
+.qc-timeline__item:hover :deep(.v-timeline-item__body) {
+  background-color: rgba(var(--v-theme-primary), 0.06);
+}
+
+.qc-timeline__item--viewed :deep(.v-timeline-item__body) {
+  background-color: rgba(var(--v-theme-primary), 0.12);
+  border-color: rgba(var(--v-theme-primary), 0.4);
+}
+
+/* The action node heads the timeline rather than recording a session, so it
+   stays flat instead of taking the session card treatment. */
+.qc-timeline__item--new :deep(.v-timeline-item__body) {
+  border-style: dashed;
+  background-color: transparent;
+}
+</style>
diff --git a/apps/qc-app/src/components/EditData/SessionWindowDialog.vue b/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
index b6635b0e..345d4cb8 100644
--- a/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
+++ b/apps/qc-app/src/components/EditData/SessionWindowDialog.vue
@@ -44,7 +44,6 @@
               role="button"
               :aria-pressed="String(activePresetId === preset.id)"
               :disabled="!!preset.disabledReason"
-              class="session-window__preset-chip"
               @click="applyPreset(preset)"
             >
               {{ preset.label }}
@@ -197,9 +196,4 @@ function onStart() {
   flex-wrap: wrap;
   gap: 4px;
 }
-
-.session-window__preset-chip {
-  font-size: 0.75rem !important;
-  height: 26px !important;
-}
 </style>
diff --git a/apps/qc-app/src/components/EditData/StartEditingDialog.vue b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
index 7f6646da..461b4628 100644
--- a/apps/qc-app/src/components/EditData/StartEditingDialog.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingDialog.vue
@@ -2,11 +2,11 @@
   <v-card rounded="lg">
     <div class="px-4 pt-4 pb-2">
       <div class="text-title-medium font-weight-bold">
-        Quality-control "{{ source.name }}"
+        Quality control for "{{ source.name }}"
       </div>
       <div class="text-body-small text-medium-emphasis mt-1">
-        Continue an in-progress session, start a new one over the current time
-        range, or set up a new managed datastream.
+        Continue an in-progress session, start a new one, or set up a new
+        managed datastream.
       </div>
     </div>
 
@@ -49,44 +49,10 @@
               color="error"
               :data-testid="`delete-managed-${opt.managed.id}`"
               title="Delete this managed datastream"
-              @click="confirmingDeleteId = opt.historyId"
+              @click="pendingDelete = { option: opt }"
             />
           </div>
 
-          <v-alert
-            v-if="confirmingDeleteId === opt.historyId"
-            type="warning"
-            variant="tonal"
-            density="compact"
-            class="mx-3 mb-2"
-          >
-            <div class="d-flex align-center flex-wrap ga-2">
-              <span class="text-body-small">
-                Delete "{{ opt.managed.name }}" and its
-                {{ opt.sessions.length }} session{{
-                  opt.sessions.length === 1 ? '' : 's'
-                }}? This can't be undone.
-              </span>
-              <v-spacer />
-              <v-btn
-                size="x-small"
-                variant="text"
-                @click="confirmingDeleteId = null"
-              >
-                Cancel
-              </v-btn>
-              <v-btn
-                size="x-small"
-                color="error"
-                variant="flat"
-                :data-testid="`confirm-delete-${opt.managed.id}`"
-                @click="onDelete(opt)"
-              >
-                Delete
-              </v-btn>
-            </div>
-          </v-alert>
-
           <v-divider />
 
           <v-timeline
@@ -99,15 +65,7 @@
           >
             <!-- Heads the timeline as its own node so starting a session
                  reads as part of the history rather than a header action. -->
-            <v-timeline-item
-              v-if="!hasInProgress(opt)"
-              fill-dot
-              size="x-small"
-              width="100%"
-              icon="mdi-plus"
-              icon-color="primary"
-              class="qc-timeline__item qc-timeline__item--new"
-            >
+            <SessionTimelineItem v-if="!hasInProgress(opt)" kind="new">
               <div class="d-flex align-center ga-2">
                 <div class="flex-grow-1 text-body-small text-medium-emphasis">
                   {{
@@ -127,23 +85,13 @@
                   Start new session
                 </v-btn>
               </div>
-            </v-timeline-item>
+            </SessionTimelineItem>
 
-            <v-timeline-item
+            <SessionTimelineItem
               v-for="(s, i) in orderedSessions(opt.sessions)"
               :key="s.id"
-              fill-dot
-              size="x-small"
-              width="100%"
-              :icon="s.status === 'in_progress' ? 'mdi-pencil' : 'mdi-check'"
-              :icon-color="s.status === 'in_progress' ? 'warning' : 'success'"
+              :kind="s.status"
               :data-testid="`chooser-session-${s.id}`"
-              class="qc-timeline__item"
-              :class="
-                s.status === 'in_progress'
-                  ? 'qc-timeline__item--active'
-                  : 'qc-timeline__item--done'
-              "
             >
               <div class="d-flex align-center ga-2">
                 <div class="flex-grow-1" style="min-width: 0">
@@ -196,10 +144,10 @@
                   class="flex-shrink-0"
                   :data-testid="`delete-session-${s.id}`"
                   title="Delete this session"
-                  @click="openConfirm(opt, s)"
+                  @click="pendingDelete = { option: opt, session: s }"
                 />
               </div>
-            </v-timeline-item>
+            </SessionTimelineItem>
           </v-timeline>
         </v-card>
 
@@ -227,14 +175,24 @@
   </v-card>
 
   <v-dialog v-model="confirmOpen" max-width="480" persistent>
-    <v-card rounded="lg" data-testid="delete-session-dialog">
+    <v-card
+      v-if="pendingDelete"
+      rounded="lg"
+      :data-testid="
+        pendingDelete.session ? 'delete-session-dialog' : 'delete-managed-dialog'
+      "
+    >
       <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
         <v-avatar color="error" variant="tonal" size="40">
           <v-icon icon="mdi-delete-alert-outline" size="22" />
         </v-avatar>
         <div class="d-flex flex-column">
           <div class="text-title-large font-weight-bold">
-            Delete this session?
+            {{
+              pendingDelete.session
+                ? 'Delete this session?'
+                : 'Delete this managed datastream?'
+            }}
           </div>
           <div class="text-body-small text-medium-emphasis">
             This permanently removes quality control work
@@ -242,11 +200,18 @@
         </div>
       </div>
 
-      <v-card-text v-if="confirmingSession" class="pt-2 pb-4 px-6">
-        <p class="text-body-medium mb-3">
-          "{{ sessionLabel(confirmingSession) }}" and the record of its
+      <v-card-text class="pt-2 pb-4 px-6">
+        <p v-if="pendingDelete.session" class="text-body-medium mb-3">
+          "{{ sessionLabel(pendingDelete.session) }}" and the record of its
           operations are removed. Earlier sessions are untouched.
         </p>
+        <p v-else class="text-body-medium mb-3">
+          "{{ pendingDelete.option.managed.name }}", its observations and its
+          {{ pendingDelete.option.sessions.length }} session{{
+            pendingDelete.option.sessions.length === 1 ? '' : 's'
+          }}
+          are removed. The source datastream is untouched.
+        </p>
 
         <v-alert type="error" variant="tonal" density="compact">
           <span class="text-body-small">This cannot be undone.</span>
@@ -255,7 +220,7 @@
 
       <v-divider />
       <v-card-actions class="d-flex align-center ga-2 px-4 py-3">
-        <v-btn variant="text" data-testid="cancel-delete-session" @click="closeConfirm">
+        <v-btn variant="text" data-testid="cancel-delete" @click="pendingDelete = null">
           Cancel
         </v-btn>
         <v-spacer />
@@ -263,10 +228,14 @@
           color="error"
           variant="flat"
           prepend-icon="mdi-delete-outline"
-          :data-testid="`confirm-delete-session-${confirmingSession?.id}`"
-          @click="onDeleteSession"
+          :data-testid="
+            pendingDelete.session
+              ? `confirm-delete-session-${pendingDelete.session.id}`
+              : `confirm-delete-${pendingDelete.option.managed.id}`
+          "
+          @click="onConfirmDelete"
         >
-          Delete session
+          {{ pendingDelete.session ? 'Delete session' : 'Delete datastream' }}
         </v-btn>
       </v-card-actions>
     </v-card>
@@ -279,6 +248,7 @@ import type { Datastream, QualityControlSession } from '@hydroserver/client'
 import type { ManagedDatastreamOption } from '@/composables/useManagedDatastreams'
 import { formatDateRange } from '@/utils/time'
 import { datastreamSummary } from '@/utils/datastreamSummary'
+import SessionTimelineItem from '@/components/EditData/SessionTimelineItem.vue'
 
 defineProps<{
   source: Datastream
@@ -294,40 +264,26 @@ const emit = defineEmits<{
   (e: 'cancel'): void
 }>()
 
-// historyId of the managed datastream whose delete is awaiting confirmation.
-const confirmingDeleteId = ref<string | null>(null)
-// The session whose delete is awaiting confirmation, and the option it
-// belongs to, so the emit can name both.
-const confirmingSession = ref<QualityControlSession | null>(null)
-const confirmingOption = ref<ManagedDatastreamOption | null>(null)
+// The delete awaiting confirmation: a whole managed datastream, or one of
+// its sessions.
+const pendingDelete = ref<{
+  option: ManagedDatastreamOption
+  session?: QualityControlSession
+} | null>(null)
 
 const confirmOpen = computed({
-  get: () => !!confirmingSession.value,
+  get: () => !!pendingDelete.value,
   set: (open: boolean) => {
-    if (!open) closeConfirm()
+    if (!open) pendingDelete.value = null
   },
 })
 
-function openConfirm(opt: ManagedDatastreamOption, s: QualityControlSession) {
-  confirmingOption.value = opt
-  confirmingSession.value = s
-}
-
-function closeConfirm() {
-  confirmingSession.value = null
-  confirmingOption.value = null
-}
-
-function onDelete(opt: ManagedDatastreamOption) {
-  confirmingDeleteId.value = null
-  emit('delete', opt)
-}
-
-function onDeleteSession() {
-  const opt = confirmingOption.value
-  const sessionId = confirmingSession.value?.id
-  closeConfirm()
-  if (opt && sessionId) emit('deleteSession', opt, sessionId)
+function onConfirmDelete() {
+  const pending = pendingDelete.value
+  pendingDelete.value = null
+  if (!pending) return
+  if (pending.session) emit('deleteSession', pending.option, pending.session.id)
+  else emit('delete', pending.option)
 }
 
 const summary = (opt: ManagedDatastreamOption) =>
diff --git a/apps/qc-app/src/components/EditData/StartEditingFlow.vue b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
index 40c1f2c0..ea45ae14 100644
--- a/apps/qc-app/src/components/EditData/StartEditingFlow.vue
+++ b/apps/qc-app/src/components/EditData/StartEditingFlow.vue
@@ -24,6 +24,7 @@
         :default-processing-level-id="qcPreferences.processingLevelId"
         :on-create-processing-level="onCreateProcessingLevel"
         :permission-error="createPermissionError"
+        :loading="isCreatingDatastream"
         @cancel="onCreateCancel"
         @confirm="onCreateDatastream"
       />
@@ -111,7 +112,7 @@ const { enterEdit, startSessionOver, leaveEdit, closeEditor } = useEditEntry()
 const createPermissionError = computed(() =>
   canCreateDatastream()
     ? ''
-    : `Your role on this workspace (${roleName()}) can't create datastreams. Ask a workspace owner for an editor role.`
+    : `Your role on this workspace (${roleName()}) can't create and edit datastreams. Ask a workspace owner for an editor role.`
 )
 
 const showChooser = ref(false)
@@ -119,6 +120,7 @@ const chooserLoading = ref(false)
 const chooserOptions = ref<ManagedDatastreamOption[]>([])
 const chooserSource = ref<Datastream | null>(null)
 const showCreateDatastream = ref(false)
+const isCreatingDatastream = ref(false)
 
 // The form's method and status lists are only needed once it opens.
 watch(showCreateDatastream, (open) => {
@@ -298,15 +300,18 @@ async function onCreateProcessingLevel(input: {
   }
 }
 
+// The form stays open until the create finishes, so a failure keeps what
+// the user entered.
 async function onCreateDatastream(spec: CreateManagedDatastreamSpec) {
-  showCreateDatastream.value = false
   qcPreferences.processingLevelId = spec.processingLevelId
+  isCreatingDatastream.value = true
   try {
     const { managedDatastream, history } = await createManaged(spec)
     // Register the new history and datastream so it's hidden from the
     // catalog and resolvable as an edit target without a reload.
     addQcHistory(history)
     datastreams.value = [...datastreams.value, managedDatastream]
+    showCreateDatastream.value = false
     Snackbar.success('Managed datastream created.')
     openWindow({
       managedId: managedDatastream.id,
@@ -319,12 +324,20 @@ async function onCreateDatastream(spec: CreateManagedDatastreamSpec) {
     Snackbar.error(
       e instanceof Error ? e.message : 'Could not create the datastream.'
     )
+  } finally {
+    isCreatingDatastream.value = false
   }
 }
 
+// The chooser already confirmed the delete, so leaving needs no question.
+async function leaveIfEditing(managedId: string) {
+  if (qcDatastream.value?.id === managedId) await leaveEdit()
+}
+
 async function onChooserDelete(option: ManagedDatastreamOption) {
   try {
     await deleteManaged(option.historyId, option.managed.id)
+    await leaveIfEditing(option.managed.id)
     removeManagedDatastream(option.historyId, option.managed.id)
     workingCopies.invalidate(option.managed.id)
     chooserOptions.value = chooserOptions.value.filter(
@@ -346,6 +359,7 @@ async function onChooserDeleteSession(
 ) {
   try {
     await deleteSession(option.historyId, sessionId)
+    await leaveIfEditing(option.managed.id)
     workingCopies.invalidate(option.managed.id)
     chooserOptions.value = chooserOptions.value.map((o) =>
       o.historyId === option.historyId
@@ -354,13 +368,17 @@ async function onChooserDeleteSession(
     )
     Snackbar.success('Session deleted.')
   } catch (e) {
-    // The server may have changed under the chooser, so reload its copy.
-    if (chooserSource.value) {
-      chooserOptions.value = await loadForSource(chooserSource.value.id)
-    }
     Snackbar.error(
       e instanceof Error ? e.message : 'Could not delete the session.'
     )
+    // The server may have changed under the chooser, so reload its copy.
+    if (chooserSource.value) {
+      try {
+        chooserOptions.value = await loadForSource(chooserSource.value.id)
+      } catch {
+        // The delete error above is what the user needs to see.
+      }
+    }
   }
 }
 
diff --git a/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts b/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
index 2ea5fcbb..a471c063 100644
--- a/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/CreateDatastreamForm.spec.ts
@@ -177,6 +177,18 @@ describe('CreateDatastreamForm', () => {
     expect(spec.processingLevelId).toBe('pl-new')
   })
 
+  it('shows progress and blocks both buttons while creating', async () => {
+    const w = mountForm({ defaultProcessingLevelId: 'pl-qc', loading: true })
+    const confirm = w.find('[data-testid="create-confirm"]')
+    expect(confirm.classes()).toContain('v-btn--loading')
+    expect(
+      w.find('[data-testid="create-cancel"]').attributes('disabled')
+    ).toBeDefined()
+
+    await field(w, 'create-confirm').vm.$emit('click')
+    expect(w.emitted('confirm')).toBeUndefined()
+  })
+
   it('blocks create and shows a warning when permissionError is set', async () => {
     const w = mountForm({
       defaultProcessingLevelId: 'pl-qc', // otherwise valid
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
index 053af458..6d4fc2e6 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
@@ -558,9 +558,6 @@ describe('EditHistory.vue actions', () => {
       expect(
         w.find('[data-testid="history-redo-btn"]').attributes('disabled')
       ).toBeDefined()
-      expect(
-        w.find('[data-testid="history-load-btn"]').attributes('disabled')
-      ).toBeDefined()
     })
 
     it('hides reload-from-server, which would wipe the record', async () => {
diff --git a/apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts
index 71938c7a..842058f4 100644
--- a/apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/LeaveSessionDialog.spec.ts
@@ -23,7 +23,6 @@ const {
   leaveWork,
   cancelLeave,
   keepSession,
-  closeSession,
   saveAndLeave,
   discardEditsAndLeave,
   discardSessionAndLeave,
@@ -34,7 +33,6 @@ const {
     leaveWork: r<string | null>(null),
     cancelLeave: vi.fn(),
     keepSession: vi.fn(),
-    closeSession: vi.fn(),
     saveAndLeave: vi.fn(),
     discardEditsAndLeave: vi.fn(),
     discardSessionAndLeave: vi.fn(),
@@ -47,7 +45,6 @@ vi.mock('@/composables/useLeaveSession', () => ({
     leaveWork,
     cancelLeave,
     keepSession,
-    closeSession,
     saveAndLeave,
     discardEditsAndLeave,
     discardSessionAndLeave,
@@ -154,7 +151,7 @@ describe('LeaveSessionDialog', () => {
     expect(button('leave-discard-session-btn')).toBeNull()
 
     button('leave-close-btn')?.click()
-    expect(closeSession).toHaveBeenCalled()
+    expect(keepSession).toHaveBeenCalled()
   })
 
   it('locks the choices while one is running', async () => {
diff --git a/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts b/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
index f170d3e8..b870cdec 100644
--- a/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/SessionList.spec.ts
@@ -128,6 +128,15 @@ describe('SessionList', () => {
     expect(wrapper.emitted('view')![0]).toEqual([committedId])
   })
 
+  it('makes each session row a button, so the keyboard can reach it', async () => {
+    const { committedId } = await seedAndLoad()
+    const wrapper = mountList()
+    await flushPromises()
+    const header = wrapper.find(`[data-testid="session-header-${committedId}"]`)
+    expect(header.element.tagName).toBe('BUTTON')
+    expect(header.attributes('type')).toBe('button')
+  })
+
   it('emits view for the in-progress session too', async () => {
     const { inProgressId } = await seedAndLoad()
     const wrapper = mountList()
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
index 0667680b..9905447c 100644
--- a/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingDialog.spec.ts
@@ -159,7 +159,7 @@ describe('StartEditingDialog', () => {
   it('cancelling the confirmation emits nothing', async () => {
     const w = mountDialog()
     await w.find('[data-testid="delete-session-s-1"]').trigger('click')
-    await clickInDialog('cancel-delete-session')
+    await clickInDialog('cancel-delete')
     expect(w.emitted('deleteSession')).toBeUndefined()
   })
 
@@ -198,9 +198,10 @@ describe('StartEditingDialog', () => {
   it('asks for confirmation, then emits delete with the chosen option', async () => {
     const w = mountDialog()
     await w.find('[data-testid="delete-managed-mgd-1"]').trigger('click')
-    const confirm = w.find('[data-testid="confirm-delete-mgd-1"]')
-    expect(confirm.exists()).toBe(true)
-    await confirm.trigger('click')
+    await flushPromises()
+    expect(inDialog('delete-managed-dialog')?.textContent).toContain('Temp (QC)')
+    expect(w.emitted('delete')).toBeUndefined()
+    await clickInDialog('confirm-delete-mgd-1')
     expect(w.emitted('delete')![0][0]).toMatchObject({ historyId: 'h-1' })
   })
 
diff --git a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
index 3f3710ff..62026720 100644
--- a/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/StartEditingFlow.spec.ts
@@ -139,10 +139,12 @@ const ChooserStub = stub('StartEditingDialog', 'chooser-stub', [
   'create',
   'cancel',
 ])
-const CreateStub = stub('CreateDatastreamForm', 'create-stub', [
-  'cancel',
-  'confirm',
-])
+const CreateStub = defineComponent({
+  name: 'CreateDatastreamForm',
+  emits: ['cancel', 'confirm'],
+  props: { loading: Boolean },
+  setup: () => () => h('div', { 'data-testid': 'create-stub' }),
+})
 const WindowStub = stub('SessionWindowDialog', 'window-stub', [
   'confirm',
   'cancel',
@@ -547,4 +549,110 @@ describe('StartEditingFlow', () => {
     await flushPromises()
     expect(enterEdit).toHaveBeenCalledWith('mgd-new', window, 'Select')
   })
+
+  it('keeps the create form open and busy until the create finishes', async () => {
+    loadForSource.mockResolvedValue([])
+    let finish!: (value: unknown) => void
+    createManaged.mockReturnValue(new Promise((resolve) => (finish = resolve)))
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    const form = wrapper.findComponent(CreateStub)
+    form.vm.$emit('confirm', { source, processingLevelId: 'pl' })
+    await flushPromises()
+
+    expect(dialogOpen(wrapper, 1)).toBe(true)
+    expect(form.props('loading')).toBe(true)
+
+    finish({ managedDatastream: { id: 'mgd-new', name: 'New' }, history: {} })
+    await flushPromises()
+    expect(dialogOpen(wrapper, 1)).toBe(false)
+    expect(form.props('loading')).toBe(false)
+  })
+
+  it('keeps the create form open when the create fails', async () => {
+    loadForSource.mockResolvedValue([])
+    createManaged.mockRejectedValue(new Error('Name already taken'))
+    const { wrapper, flow } = mountFlow()
+    await flow.openFor(source)
+    await flushPromises()
+    const form = wrapper.findComponent(CreateStub)
+    form.vm.$emit('confirm', { source, processingLevelId: 'pl' })
+    await flushPromises()
+
+    expect(error).toHaveBeenCalledWith('Name already taken')
+    expect(dialogOpen(wrapper, 1)).toBe(true)
+    expect(form.props('loading')).toBe(false)
+    expect(present('window-stub')).toBe(false)
+  })
+})
+
+describe('StartEditingFlow deletes', () => {
+  const openChooser = async (option: unknown) => {
+    loadForSource.mockResolvedValue([option])
+    const mounted = mountFlow()
+    await mounted.flow.openFor(source)
+    await flushPromises()
+    return mounted.wrapper.findComponent(ChooserStub)
+  }
+
+  it('leaves the editor after deleting the managed datastream it has open', async () => {
+    qcDatastream.value = { id: 'mgd' }
+    deleteManaged.mockResolvedValue(undefined)
+    const chooser = await openChooser(optionWith([committed]))
+
+    chooser.vm.$emit('delete', optionWith([committed]))
+    await flushPromises()
+
+    expect(removeManagedDatastream).toHaveBeenCalledWith('h', 'mgd')
+    expect(leaveEdit).toHaveBeenCalledTimes(1)
+  })
+
+  it('keeps the editor when another managed datastream is deleted', async () => {
+    qcDatastream.value = { id: 'other' }
+    deleteManaged.mockResolvedValue(undefined)
+    const chooser = await openChooser(optionWith([committed]))
+
+    chooser.vm.$emit('delete', optionWith([committed]))
+    await flushPromises()
+
+    expect(leaveEdit).not.toHaveBeenCalled()
+  })
+
+  it('leaves the editor after deleting a session of the datastream it has open', async () => {
+    qcDatastream.value = { id: 'mgd' }
+    deleteSession.mockResolvedValue(undefined)
+    const option = optionWith([committed, inProgress])
+    const chooser = await openChooser(option)
+
+    chooser.vm.$emit('deleteSession', option, 's-1')
+    await flushPromises()
+
+    expect(deleteSession).toHaveBeenCalledWith('h', 's-1')
+    expect(leaveEdit).toHaveBeenCalledTimes(1)
+  })
+
+  it('keeps the editor when the delete fails', async () => {
+    qcDatastream.value = { id: 'mgd' }
+    deleteManaged.mockRejectedValue(new Error('Forbidden'))
+    const chooser = await openChooser(optionWith([committed]))
+
+    chooser.vm.$emit('delete', optionWith([committed]))
+    await flushPromises()
+
+    expect(error).toHaveBeenCalledWith('Forbidden')
+    expect(leaveEdit).not.toHaveBeenCalled()
+  })
+
+  it('shows a failed session delete even when reloading the chooser fails too', async () => {
+    deleteSession.mockRejectedValue(new Error('Has dependents'))
+    const option = optionWith([committed, inProgress])
+    const chooser = await openChooser(option)
+    loadForSource.mockRejectedValue(new Error('Offline'))
+
+    chooser.vm.$emit('deleteSession', option, 's-1')
+    await flushPromises()
+
+    expect(error).toHaveBeenCalledWith('Has dependents')
+  })
 })
diff --git a/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts b/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
index 5209c2aa..c3dcdc63 100644
--- a/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
+++ b/apps/qc-app/src/components/Navigation/__tests__/NavigationRail.spec.ts
@@ -141,8 +141,6 @@ vi.mock('@/composables/useEditSession', async () => {
       discardUnsavedEdits: vi.fn(),
       beginEditing: vi.fn(),
       startSession: vi.fn(),
-      needsSession: ref(false),
-      needsHistory: ref(false),
     }),
   }
 })
diff --git a/apps/qc-app/src/components/VisualizeData.vue b/apps/qc-app/src/components/VisualizeData.vue
index cb1c8dc8..66caaf65 100644
--- a/apps/qc-app/src/components/VisualizeData.vue
+++ b/apps/qc-app/src/components/VisualizeData.vue
@@ -22,7 +22,7 @@
               Choose what to plot around the session you are editing
             </template>
             <template v-else-if="plottedDatastreams.length">
-              Use the pencil button on a row to edit a datastream
+              Use the Edit button on a row to edit a datastream
             </template>
             <template v-else>
               Select datastreams from the table below to preview their data
@@ -264,6 +264,11 @@
                     prepend-icon="mdi-cloud-check-outline"
                     :disabled="commitDisabled"
                     :loading="isCommitting"
+                    :title="
+                      canCommitWorkspace
+                        ? undefined
+                        : `Your role on this workspace (${workspaceRole}) can't write observations, so it can't commit.`
+                    "
                     @click="openCommit"
                   >
                     Commit
@@ -384,6 +389,40 @@
       </v-card>
     </v-dialog>
 
+    <v-dialog v-model="showViewConfirm" max-width="520">
+      <v-card rounded="lg" data-testid="view-session-confirm">
+        <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
+          <v-avatar color="warning" variant="tonal" size="40">
+            <v-icon icon="mdi-content-save-alert-outline" size="22" />
+          </v-avatar>
+          <div class="d-flex flex-column">
+            <div class="text-title-large font-weight-bold">Discard unsaved edits?</div>
+            <div class="text-body-small text-medium-emphasis">
+              {{ unsavedEditCount }} edit{{ unsavedEditCount === 1 ? '' : 's' }}
+              since the last save
+            </div>
+          </div>
+        </div>
+        <v-card-text class="text-body-medium pt-2 pb-4 px-6">
+          Viewing another session drops the edits made since the last save.
+          Save first to keep them.
+        </v-card-text>
+        <v-divider />
+        <v-card-actions class="d-flex align-center ga-2 px-4 py-3">
+          <v-btn variant="text" @click="showViewConfirm = false">Cancel</v-btn>
+          <v-spacer />
+          <v-btn
+            color="warning"
+            variant="flat"
+            data-testid="confirm-view-session-btn"
+            @click="onConfirmViewSession"
+          >
+            Discard and view
+          </v-btn>
+        </v-card-actions>
+      </v-card>
+    </v-dialog>
+
     <v-dialog v-model="showDiscardConfirm" max-width="520">
       <v-card rounded="lg">
         <div class="d-flex align-center ga-3 px-6 pt-5 pb-2">
@@ -423,9 +462,8 @@
 
   <!-- One plot for both layouts. Moving it instead of rebuilding it is what
        keeps the zoom, the session band and a staged range across a view
-       switch. The hosts only exist once this component is in the document,
-       hence the mounted gate. -->
-  <Teleport v-if="isMounted" :to="plotHost">
+       switch. Deferred, since the hosts are rendered by this component. -->
+  <Teleport defer :to="plotHost">
     <DataVisualization />
   </Teleport>
 
@@ -495,7 +533,6 @@ const { currentView, selectedDrawer, selectedOperation } =
   storeToRefs(useUIStore())
 const { selectedWorkspaceId } = storeToRefs(useWorkspaceStore())
 const {
-  editHistory,
   isUpdating,
   activeTab,
   hiddenTraceIds,
@@ -521,18 +558,26 @@ const {
 useUnsavedChangesWarning(hasUnsavedChanges)
 
 // Selecting a session in the list loads the data as that session left it,
-// plus its own operations. Guarded so unsaved edits aren't dropped silently.
+// plus its own operations. Unsaved edits are not dropped without asking.
 const isViewingSession = ref(false)
-async function onViewSession(sessionId: string) {
+const pendingViewSessionId = ref<string | null>(null)
+const showViewConfirm = computed({
+  get: () => !!pendingViewSessionId.value,
+  set: (open) => {
+    if (!open) pendingViewSessionId.value = null
+  },
+})
+function onViewSession(sessionId: string) {
   if (isViewingSession.value) return
-  if (
-    hasUnsavedChanges.value &&
-    !window.confirm(
-      'You have unsaved edits. Viewing another session will discard them. Continue?'
-    )
-  ) {
-    return
-  }
+  if (hasUnsavedChanges.value) pendingViewSessionId.value = sessionId
+  else void openSession(sessionId)
+}
+function onConfirmViewSession() {
+  const sessionId = pendingViewSessionId.value
+  pendingViewSessionId.value = null
+  if (sessionId) void openSession(sessionId)
+}
+async function openSession(sessionId: string) {
   isViewingSession.value = true
   try {
     await viewSession(sessionId)
@@ -546,12 +591,13 @@ async function onViewSession(sessionId: string) {
 }
 const { isReadOnly, inProgressSession, viewedSession, resumeDatastreamId } =
   storeToRefs(useQcSessionStore())
-const { canEdit, roleName } = useWorkspacePermissions()
+const { canEdit, canCommit, roleName } = useWorkspacePermissions()
 const { closeEditor } = useEditEntry()
 
 // Gate the editor footer so a read-only collaborator sees a disabled state
 // instead of a 403 mid-flow.
 const canEditWorkspace = computed(() => canEdit())
+const canCommitWorkspace = computed(() => canCommit())
 const workspaceRole = computed(() => roleName())
 
 // The Edit layout needs a resolved target to render; without one the Select
@@ -560,12 +606,6 @@ const isEditLayout = computed(
   () => currentView.value === DrawerType.Edit && !!qcDatastream.value
 )
 
-// Teleport resolves its target with `document.querySelector`, and this
-// component's own DOM is still detached while it first mounts.
-const isMounted = ref(false)
-onMounted(() => {
-  isMounted.value = true
-})
 const plotHost = computed(() =>
   isEditLayout.value ? '#qc-plot-host-edit' : '#qc-plot-host-select'
 )
@@ -597,7 +637,6 @@ useResumeEditSession(async (id) => {
   await startEditing.value?.resume(id, initialView)
 })
 
-const editCount = computed(() => editHistory.value?.length ?? 0)
 const showCommitConfirm = ref(false)
 const commitDescription = ref('')
 const isSavingDraft = ref(false)
@@ -610,14 +649,14 @@ const saveDisabled = computed(
     !canEditWorkspace.value ||
     isReadOnly.value ||
     !inProgressSession.value ||
-    !editCount.value ||
+    !hasUnsavedChanges.value ||
     isUpdating.value ||
     isSavingDraft.value ||
     isCommitting.value
 )
 const commitDisabled = computed(
   () =>
-    !canEditWorkspace.value ||
+    !canCommitWorkspace.value ||
     isReadOnly.value ||
     !inProgressSession.value ||
     isUpdating.value ||
@@ -625,10 +664,7 @@ const commitDisabled = computed(
     isCommitting.value
 )
 
-// --- Editor layout: sidebar sizes + collapse flags ------------------
-// Persisted to localStorage so the user's preferred layout survives
-// reloads. Widths / percentages share the same `qc:editorLayout`
-// namespace; flags are boolean keys alongside.
+// Editor layout, kept across reloads under `qc:editorLayout`.
 const {
   size: drawerWidth,
   onStart: startDrawerDrag,
@@ -653,10 +689,8 @@ const {
   invert: true,
   storageKey: 'qc:editorLayout:auxWidth',
 })
-// Template ref used by `useResizable` to convert pixel deltas into
-// percent-of-container during the History / OperationPanel split
-// drag. Without the conversion a small pointer move would add raw
-// pixels onto the percent value, making the panel lunge.
+// The History / OperationPanel split is a percentage, so the drag converts
+// pixels against this container.
 const auxBodyEl = useTemplateRef<HTMLElement>('auxBodyEl')
 const {
   size: historyPercent,
@@ -784,11 +818,9 @@ const hydrateFromUrl = () => {
     .map((id) => datastreams.value.find((ds) => ds.id === id))
     .filter((ds): ds is NonNullable<typeof ds> => !!ds)
 
-  // Set the window BEFORE loading datastreams so the first fetch uses it.
-  // A preset resolves against the plotted data during that load. An
-  // unknown id (a share link built by a newer version, or hand-edited)
-  // is not accepted as a preset.
-  // A link with an edit target carries the editor's Context range.
+  // Set the window before loading datastreams so the first fetch uses it.
+  // An unknown preset id is ignored. A link with an edit target carries the
+  // editor's Context range.
   const presetId = state.editDatastreamId ? contextPresetId : selectedDateBtnId
   if (state.datePresetId != null && findPreset(state.datePresetId)) {
     presetId.value = state.datePresetId
@@ -829,11 +861,8 @@ const hydrateFromUrl = () => {
     tooltipsMaxDataPoints.value = state.dataPointsThreshold
   }
 
-  // Park the URL-supplied zoom so the Plot's mount hook can apply it
-  // after `handleNewPlot` finishes the default-fit render.
-  // Carry `source: 'user'` so the zoom-history recorder treats this
-  // as a deliberate viewport choice (URL share is an intentional user
-  // action), matching the ZoomState contract.
+  // Applied once the first draw finishes. A shared zoom is the user's
+  // choice, so it goes into the zoom history as one.
   pendingShareZoom.value = state.zoom
     ? {
         xRange: state.zoom.xRange,
@@ -850,11 +879,9 @@ const hydrateFromUrl = () => {
   // before the resume hook is registered. Nothing to do here for it.
   void setPlottedDatastreams(resolved)
 
-  // Snapshots replay against the session store, which the resume hook loads
-  // asynchronously and independently of the plot. Wait for the editor to
-  // actually be open on the `ed` target (an in-progress or viewed session
-  // means its sessions were applied) rather than chaining off
-  // `setPlottedDatastreams`, which only settles the plot.
+  // Snapshots replay against the session store, which the resume loads on
+  // its own, so wait for the editor to be open on the `ed` target rather
+  // than for the plot.
   const snapshots = state.snapshots ?? []
   const editTargetId = state.editDatastreamId
   // No `immediate`: resuming the editor always takes at least one await, so
@@ -891,11 +918,8 @@ if (datastreams.value.length) {
   )
 }
 
-// Push URL updates whenever any piece of share-relevant state moves.
-// `router.replace` keeps the browser history clean (no entry per
-// click). Heavy lifting (key choice, default-elision, compaction)
-// lives in `share.ts` so this watcher reads as a plain assembly of
-// inputs.
+// Keep the URL in step with the share state. `replace`, so the browser
+// history doesn't get an entry per click.
 const SHARE_KEYS = [
   'ws', 'm', 'ed', 'tab', 'ds', 'snap', 'r', 'from', 'to',
   't', 'op', 'pl', 'h', 'ya', 'z', 'yz', 'dp', 'th',
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
index 8e6f6d25..c523b34a 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
@@ -6,11 +6,11 @@
         <span class="text-body-medium font-weight-bold">Datastreams</span>
         <v-chip
           size="x-small"
-          :color="plottedDatastreams.length ? 'primary' : undefined"
-          :variant="plottedDatastreams.length ? 'tonal' : 'outlined'"
+          :color="plottedSeries.length ? 'primary' : undefined"
+          :variant="plottedSeries.length ? 'tonal' : 'outlined'"
           label
         >
-          {{ plottedDatastreams.length }}/{{ PLOT_CAP }} plotted
+          {{ plottedSeries.length }}/{{ PLOT_CAP }} plotted
         </v-chip>
       </div>
 
@@ -41,10 +41,10 @@
           <v-list density="compact" nav>
             <v-list-item
               prepend-icon="mdi-close-circle-outline"
-              :disabled="!plottedDatastreams.length"
-              @click="clearSelected"
+              :disabled="!canClearPlot"
+              @click="onClearPlot"
             >
-              <v-list-item-title>Clear selected</v-list-item-title>
+              <v-list-item-title>Clear plot</v-list-item-title>
             </v-list-item>
 
             <v-list-item
@@ -60,8 +60,8 @@
 
             <v-list-item
               prepend-icon="mdi-download"
-              :disabled="!plottedDatastreams.length || downloading"
-              @click="downloadSelected(plottedDatastreams)"
+              :disabled="!plottedSeries.length || downloading"
+              @click="downloadSelected(plottedSeries)"
             >
               <v-list-item-title>
                 {{ downloading ? 'Downloading…' : 'Download selected' }}
@@ -326,10 +326,9 @@ import {
 import { Snackbar } from '@uwrl/qc-utils'
 import { useWorkspacePermissions } from '@/composables/useWorkspacePermissions'
 import { useEditEntry } from '@/composables/useEditEntry'
-
-/** Datastreams the user can check. The fifth plot slot is kept for the
- *  datastream being edited. */
-const PLOT_CAP = 4
+import { useClearPlot } from '@/composables/useClearPlot'
+import { isSnapshotId } from '@/utils/snapshotId'
+import { PLOT_CAP } from '@/utils/plotCap'
 
 const emit = defineEmits<{
   (e: 'edit', datastream: Datastream & DatastreamExtended): void
@@ -346,9 +345,12 @@ const {
   qcDatastream,
   editSourceDatastream,
 } = storeToRefs(useDataVisStore())
+// History comparison lines are plotted too, but take no slot.
+const plottedSeries = computed(() =>
+  plottedDatastreams.value.filter((d) => !isSnapshotId(d.id))
+)
 const {
   toggleDatastream,
-  clearPlottedDatastreams,
   sourceGroupIds,
   plotSourceSelection,
 } = useDataVisStore()
@@ -436,9 +438,10 @@ const formatTableDate = (raw: unknown): string => {
   return Number.isNaN(d.getTime()) ? '-' : DATE_FORMATTER.format(d)
 }
 
-const clearSelected = () => {
+const { canClearPlot, clearPlot } = useClearPlot()
+const onClearPlot = async () => {
   showOnlySelected.value = false
-  void clearPlottedDatastreams()
+  await clearPlot()
 }
 
 const plottedIds = computed(
@@ -466,7 +469,7 @@ const isEditing = (item: Datastream) =>
     item.id === qcDatastream.value.id)
 
 const isAtCap = (item: Datastream) =>
-  plottedDatastreams.value.length >= PLOT_CAP && !isChecked(item)
+  plottedSeries.value.length >= PLOT_CAP && !isChecked(item)
 
 // --- Plot selection ---------------------------------------------------------
 // Sources with managed datastreams can't be a single toggle: the click opens
@@ -494,7 +497,7 @@ const plotDialogSlots = computed(() => {
   const source = plotDialogSource.value
   if (!source) return 0
   const group = new Set(sourceGroupIds(source.id))
-  const others = plottedDatastreams.value.filter((d) => !group.has(d.id)).length
+  const others = plottedSeries.value.filter((d) => !group.has(d.id)).length
   return Math.max(PLOT_CAP - others, 0)
 })
 
@@ -506,15 +509,20 @@ async function onPlotClick(item: Datastream & DatastreamExtended) {
   plotDialogSource.value = item
   plotDialogOptions.value = []
   plotDialogLoading.value = true
+  // The dialog may have closed, or moved to another source, meanwhile.
+  const stillOpen = () => plotDialogSource.value?.id === item.id
   try {
-    plotDialogOptions.value = await loadForSource(item.id)
+    const options = await loadForSource(item.id)
+    if (stillOpen()) plotDialogOptions.value = options
   } catch (e) {
     // The raw option stays selectable, so the click isn't a dead end.
-    Snackbar.error(
-      e instanceof Error ? e.message : 'Could not load QC datastreams.'
-    )
+    if (stillOpen()) {
+      Snackbar.error(
+        e instanceof Error ? e.message : 'Could not load QC datastreams.'
+      )
+    }
   } finally {
-    plotDialogLoading.value = false
+    if (stillOpen()) plotDialogLoading.value = false
   }
 }
 
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisualization.vue b/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
index a5106d33..fbe5a640 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisualization.vue
@@ -121,8 +121,8 @@
           <div class="data-vis-state__step-body">
             <div class="text-title-small font-weight-bold">Edit one</div>
             <div class="text-body-small text-medium-emphasis">
-              Click the <b>pencil</b> on a row to pick or create its QC datastream
-              and start a session.
+              Click <b>Edit</b> on a row to pick or create its QC datastream and
+              start a session.
             </div>
           </div>
         </div>
diff --git a/apps/qc-app/src/components/VisualizeData/EditableCell.vue b/apps/qc-app/src/components/VisualizeData/EditableCell.vue
index 560ab9b1..60e6e787 100644
--- a/apps/qc-app/src/components/VisualizeData/EditableCell.vue
+++ b/apps/qc-app/src/components/VisualizeData/EditableCell.vue
@@ -145,12 +145,7 @@ watch(
   background: transparent;
 }
 
-.editable-cell__display--readonly:hover {
-  border-color: transparent;
-  background: transparent;
-}
-
-.editable-cell__display:hover {
+.editable-cell__display:not(.editable-cell__display--readonly):hover {
   border-color: rgba(var(--v-theme-primary), 0.45);
   background-color: rgba(var(--v-theme-primary), 0.06);
 }
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index eaec4bdc..20798532 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -668,7 +668,6 @@ const keyboardShortcuts = [
 
 let plotResizeObserver: ResizeObserver | null = null
 let pendingResizeFrame: number | null = null
-let cancelFirstDraw: (() => void) | null = null
 // Deferred work checks this so it never touches a detached plot.
 let isUnmounted = false
 
@@ -686,24 +685,12 @@ watch(
   { flush: 'post' }
 )
 
-// Plot work from the moment the element appears, so the editor is not
-// reported ready before the plot the user sees is drawn.
+// Plot work, so the editor is not reported ready before the plot the user
+// sees is drawn. A container still growing (the view-switch animation) is
+// caught by the size observer.
 function scheduleFirstDraw(target: HTMLDivElement) {
-  cancelFirstDraw?.()
   const drawn = trackPlotWork(async () => {
-    // Wait for the view-switch animation to expand the container.
-    let cancel!: () => void
-    const due = await new Promise<boolean>((resolve) => {
-      const timer = setTimeout(() => resolve(true), 200)
-      cancel = () => {
-        clearTimeout(timer)
-        resolve(false)
-      }
-      cancelFirstDraw = cancel
-    })
-    // A newer schedule owns the flag by now, so only clear our own.
-    if (cancelFirstDraw === cancel) cancelFirstDraw = null
-    if (!due || isUnmounted) return
+    if (isUnmounted) return
     updateOptions()
     // A share-URL zoom, if any, is applied and cleared inside
     // handleNewPlot itself, so capture it before that happens: it
@@ -747,7 +734,6 @@ function observePlotSize(target: HTMLDivElement) {
 
 onBeforeUnmount(() => {
   isUnmounted = true
-  cancelFirstDraw?.()
   if (pendingResizeFrame != null) {
     cancelAnimationFrame(pendingResizeFrame)
     pendingResizeFrame = null
@@ -758,13 +744,12 @@ onBeforeUnmount(() => {
   }
 })
 
-const onTabChange = () => {
-  if (tab.value === 'plot') {
-    setTimeout(() => {
-      if (isUnmounted) return
-      setPlotSelection(selectedData.value || [])
-    })
-  }
+// The plot tab is back in the DOM after the next render.
+const onTabChange = async () => {
+  if (tab.value !== 'plot') return
+  await nextTick()
+  if (isUnmounted) return
+  setPlotSelection(selectedData.value || [])
 }
 </script>
 
diff --git a/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue b/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
index c08a2e3a..abb29e72 100644
--- a/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
+++ b/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
@@ -71,7 +71,8 @@
           class="text-body-small text-medium-emphasis"
           data-testid="plot-source-cap"
         >
-          Maximum of 4 datastreams plotted; remove one to add another.
+          Maximum of {{ PLOT_CAP }} datastreams plotted; remove one to add
+          another.
         </div>
       </template>
     </div>
@@ -100,13 +101,14 @@ import { computed, ref, watch } from 'vue'
 import type { Datastream, DatastreamExtended } from '@hydroserver/client'
 import type { ManagedDatastreamOption } from '@/composables/useManagedDatastreams'
 import { datastreamSummary } from '@/utils/datastreamSummary'
+import { PLOT_CAP } from '@/utils/plotCap'
 
 const props = defineProps<{
   source: Datastream & Partial<DatastreamExtended>
   options: ManagedDatastreamOption[]
   plottedIds: string[]
   loading?: boolean
-  /** Free slots under the 4-datastream plot cap, ignoring this source's group. */
+  /** Free slots under the plot cap, ignoring this source's group. */
   slotsLeft: number
   /** The datastream being edited: already on the plot, so not offered. */
   editingId?: string | null
diff --git a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
index 024bd0eb..ace77dde 100644
--- a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
+++ b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
@@ -7,7 +7,7 @@
       <v-spacer></v-spacer>
       <v-btn
         data-testid="clear-plot-btn"
-        :disabled="!plottedDatastreams.length && !qcDatastream"
+        :disabled="!canClearPlot"
         size="x-small"
         variant="text"
         prepend-icon="mdi-close-circle-outline"
@@ -203,12 +203,12 @@ import { usePlotlyStore } from '@/store/plotly'
 import { ref, computed } from 'vue'
 import { Datastream } from '@hydroserver/client'
 import { formatDayStamp } from '@/utils/time'
-import { useEditEntry } from '@/composables/useEditEntry'
+import { useClearPlot } from '@/composables/useClearPlot'
 
 /** `clearable`: show the Clear plot toolbar (the Select view's list). */
 defineProps<{ clearable?: boolean }>()
 
-const { closeEditor } = useEditEntry()
+const { canClearPlot, clearPlot } = useClearPlot()
 
 const { updateOptions, labelColorForDatastream } = usePlotlyStore()
 const {
@@ -226,7 +226,7 @@ const {
   sourceContextDatastream,
   loadingStates,
 } = storeToRefs(useDataVisStore())
-const { toggleDatastream, clearPlottedDatastreams } = useDataVisStore()
+const { toggleDatastream } = useDataVisStore()
 
 const isEdit = (ds: Datastream) => qcDatastream.value?.id === ds.id
 const isSource = (ds: Datastream) =>
@@ -301,12 +301,6 @@ const snapshotSubtitle = (id: string): string => {
 }
 
 // Ending the edit session asks first; staying keeps the plot as it is.
-async function clearPlot() {
-  if (qcDatastream.value && !(await closeEditor())) return
-  hiddenTraceIds.value = new Set()
-  await clearPlottedDatastreams()
-}
-
 const toggleVisibility = async (datastream: Datastream) => {
   const traces = plotlyRef.value?.data ?? []
   const mainIndex = traces.findIndex(
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
index bde759d0..6b02f830 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
@@ -5,8 +5,8 @@
  */
 
 import { mount, flushPromises } from '@vue/test-utils'
-import { ref } from 'vue'
-import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { ref, type Component } from 'vue'
+import { describe, it, expect, beforeAll, beforeEach, vi } from 'vitest'
 import { createTestVuetify } from '@/utils/test/vuetify'
 import PlotSourceDialog from '../PlotSourceDialog.vue'
 import DatastreamInformationCard from '../DatastreamInformationCard.vue'
@@ -27,11 +27,11 @@ const filteredDatastreams = ref<any[]>([raw, lonely])
 const plottedDatastreams = ref<any[]>([])
 const qcDatastream = ref<any>(null)
 const editSourceDatastream = ref<any>(null)
-const historiesBySource = ref(
+const srcHistories = () =>
   new Map<string, any[]>([
     ['src', [{ id: 'h-1', managedDatastreamId: 'mgd', sourceDatastreamId: 'src' }]],
   ])
-)
+const historiesBySource = ref(srcHistories())
 
 const toggleDatastream = vi.fn().mockResolvedValue(undefined)
 const clearPlottedDatastreams = vi.fn().mockResolvedValue(undefined)
@@ -52,6 +52,10 @@ vi.mock('@/store/dataVisualization', () => ({
   }),
 }))
 
+vi.mock('@/store/plotly', () => ({
+  usePlotlyStore: () => ({ hiddenTraceIds: ref(new Set()) }),
+}))
+
 const canEdit = vi.fn(() => true)
 vi.mock('@/composables/useWorkspacePermissions', () => ({
   useWorkspacePermissions: () => ({ canEdit, roleName: () => 'owner' }),
@@ -87,14 +91,21 @@ vi.mock('@uwrl/qc-utils', async (importOriginal) => {
 beforeEach(() => {
   vi.clearAllMocks()
   canEdit.mockReturnValue(true)
+  filteredDatastreams.value = [raw, lonely]
+  historiesBySource.value = srcHistories()
   plottedDatastreams.value = []
   qcDatastream.value = null
   editSourceDatastream.value = null
 })
 
+// Imported after the mocks, which use the variables above. Once, so the
+// first test doesn't pay for compiling the component.
+let DataVisDatasetsTable: Component
+beforeAll(async () => {
+  DataVisDatasetsTable = (await import('../DataVisDatasetsTable.vue')).default
+}, 60_000)
+
 const mountTable = async () => {
-  const DataVisDatasetsTable = (await import('../DataVisDatasetsTable.vue'))
-    .default
   const wrapper = mount(DataVisDatasetsTable, {
     global: {
       plugins: [createTestVuetify()],
@@ -143,6 +154,33 @@ describe('DataVisDatasetsTable plot checkbox', () => {
     expect(dialog.props('options')).toEqual([])
   })
 
+  it('ignores a chooser load that finishes after another source was opened', async () => {
+    const other = { id: 'src2', name: 'Other', valueCount: 3, thing: { id: 't-1' } }
+    filteredDatastreams.value = [raw, lonely, other]
+    historiesBySource.value.set('src2', [
+      { id: 'h-2', managedDatastreamId: 'mgd2', sourceDatastreamId: 'src2' },
+    ])
+    let finishFirst!: (options: unknown[]) => void
+    loadForSource
+      .mockImplementationOnce(() => new Promise((resolve) => (finishFirst = resolve)))
+      .mockImplementationOnce(() => new Promise(() => {}))
+    const wrapper = await mountTable()
+
+    await checkbox(wrapper, 'src').trigger('click')
+    await flushPromises()
+    wrapper.findComponent(PlotSourceDialog).vm.$emit('cancel')
+    await flushPromises()
+    await checkbox(wrapper, 'src2').trigger('click')
+    await flushPromises()
+    finishFirst([{ historyId: 'h-1', managed: { id: 'mgd' }, sessions: [] }])
+    await flushPromises()
+
+    const dialog = wrapper.findComponent(PlotSourceDialog)
+    expect(dialog.props('source').id).toBe('src2')
+    expect(dialog.props('options')).toEqual([])
+    expect(dialog.props('loading')).toBe(true)
+  })
+
   it('applies the chooser selection through the batched store action', async () => {
     const wrapper = await mountTable()
     await checkbox(wrapper, 'src').trigger('click')
@@ -170,6 +208,32 @@ describe('DataVisDatasetsTable plot checkbox', () => {
     expect(toggleDatastream).not.toHaveBeenCalled()
   })
 
+  // History comparison lines ride along in the plotted list but take no slot.
+  it('leaves comparison lines out of the plot limit', async () => {
+    plottedDatastreams.value = [
+      { id: 'a' },
+      { id: 'b' },
+      { id: 'c' },
+      { id: 'snap:s-1:0' },
+      { id: 'snap:s-1:1' },
+    ]
+    const wrapper = await mountTable()
+    expect(wrapper.text()).toContain('3/4 plotted')
+
+    await checkbox(wrapper, 'solo').trigger('click')
+    await flushPromises()
+    expect(toggleDatastream).toHaveBeenCalledWith(expect.objectContaining({ id: 'solo' }))
+  })
+
+  it('leaves comparison lines out of the chooser slots', async () => {
+    plottedDatastreams.value = [{ id: 'a' }, { id: 'snap:s-1:0' }, { id: 'snap:s-1:1' }]
+    const wrapper = await mountTable()
+    await checkbox(wrapper, 'src').trigger('click')
+    await flushPromises()
+
+    expect(wrapper.findComponent(PlotSourceDialog).props('slotsLeft')).toBe(3)
+  })
+
   it('shows a checked box when the raw datastream is plotted', async () => {
     plottedDatastreams.value = [raw]
     const wrapper = await mountTable()
@@ -248,15 +312,6 @@ describe('DataVisDatasetsTable edit button', () => {
       false
     )
   })
-
-  it('no longer marks a QC target row', async () => {
-    plottedDatastreams.value = [lonely]
-    const wrapper = await mountTable()
-
-    expect(wrapper.find('.qc-pill').exists()).toBe(false)
-    expect(wrapper.find('.datasets-table__row--qc').exists()).toBe(false)
-    expect(wrapper.text()).not.toContain('QC target')
-  })
 })
 
 describe('DataVisDatasetsTable editing indicator', () => {
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
index 5d35e543..2e03855b 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
@@ -391,10 +391,6 @@ describe('PlottedDatastreams row kinds', () => {
     ).toBe(true)
   })
 
-  it('has no QC target radio', () => {
-    expect(mountIt().find('.plotted-item__dot').exists()).toBe(false)
-  })
-
   it('shows Clear plot only where asked, editing or not', () => {
     expect(mountIt().find('[data-testid="clear-plot-btn"]').exists()).toBe(false)
     expect(mountIt({ clearable: true }).find('[data-testid="clear-plot-btn"]').exists()).toBe(true)
diff --git a/apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts b/apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts
index e6f0f40c..f2bfc6ab 100644
--- a/apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useCreateManagedDatastream.spec.ts
@@ -5,8 +5,7 @@ import { makeQcFake } from '@/services/qualityControl/__tests__/qcServiceFake'
 import { unwrap } from '@/services/qualityControl/unwrap'
 
 const hsCreate = vi.fn()
-const createTag = vi.fn()
-const hs = ref<any>({ datastreams: { create: hsCreate, createTag } })
+const hs = ref<any>({ datastreams: { create: hsCreate } })
 vi.mock('@/store/hydroserver', () => ({
   useHydroServer: () => ({ hs }),
 }))
@@ -30,7 +29,7 @@ beforeEach(() => {
   vi.clearAllMocks()
   qc = makeQcFake()
   hs.value = {
-    datastreams: { create: hsCreate, createTag },
+    datastreams: { create: hsCreate },
     qualityControlHistories: qc.histories,
     qualityControlSessions: qc.sessions,
     qualityControlOperations: qc.operations,
@@ -41,11 +40,10 @@ beforeEach(() => {
     status: 201,
     message: '',
   })
-  createTag.mockResolvedValue({})
 })
 
 describe('useCreateManagedDatastream', () => {
-  it('creates the datastream, the history, and tags the source', async () => {
+  it('creates the datastream and the history', async () => {
     const { useCreateManagedDatastream } = await import(
       '@/composables/useCreateManagedDatastream'
     )
@@ -69,11 +67,6 @@ describe('useCreateManagedDatastream', () => {
 
     expect(result.history.managedDatastream.id).toBe('managed-1')
     expect(unwrap(await qc.histories.list())).toHaveLength(1)
-
-    expect(createTag).toHaveBeenCalledWith(
-      'source-1',
-      expect.objectContaining({ value: 'managed-1' })
-    )
   })
 
   it('sends no status when the spec leaves it out', async () => {
diff --git a/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts b/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
index 0ef62f30..69a4cc5f 100644
--- a/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditEntry.spec.ts
@@ -9,8 +9,6 @@ const {
   clearEditTarget,
   beginEditing,
   startSession,
-  needsSession,
-  needsHistory,
   currentView,
   selectedDrawer,
   isDrawerOpen,
@@ -35,8 +33,6 @@ const {
     clearEditTarget: vi.fn(),
     beginEditing: vi.fn(),
     startSession: vi.fn(),
-    needsSession: r(false),
-    needsHistory: r(false),
     currentView: r('Select'),
     selectedDrawer: r('Select'),
     isDrawerOpen: r(false),
@@ -96,8 +92,6 @@ vi.mock('@/composables/useEditSession', () => ({
   useEditSession: () => ({
     beginEditing,
     startSession,
-    needsSession,
-    needsHistory,
   }),
 }))
 
@@ -114,8 +108,6 @@ const window = {
 beforeEach(() => {
   createTestPinia()
   vi.clearAllMocks()
-  needsSession.value = false
-  needsHistory.value = false
   currentView.value = 'Select'
   selectedDrawer.value = 'Select'
   isDrawerOpen.value = false
@@ -127,14 +119,13 @@ beforeEach(() => {
   clearEditTarget.mockImplementation(async () => {
     qcDatastream.value = null
   })
-  beginEditing.mockResolvedValue(true)
+  beginEditing.mockResolvedValue('resumed')
   startSession.mockResolvedValue(undefined)
   requestLeave.mockResolvedValue(true)
 })
 
 describe('useEditEntry', () => {
   it('resumes into the editor when a session is in progress', async () => {
-    needsSession.value = false
     const result = await useEditEntry().enterEdit('mgd')
     expect(setEditTarget).toHaveBeenCalledWith('mgd')
     expect(result).toBe('editing')
@@ -143,19 +134,13 @@ describe('useEditEntry', () => {
   })
 
   it('asks for a window when there is no session and none was given', async () => {
-    beginEditing.mockImplementation(async () => {
-      needsSession.value = true
-      return false
-    })
+    beginEditing.mockResolvedValue('needs-session')
     expect(await useEditEntry().enterEdit('mgd')).toBe('needs-window')
     expect(startSession).not.toHaveBeenCalled()
   })
 
   it('starts a session over the given window', async () => {
-    beginEditing.mockImplementation(async () => {
-      needsSession.value = true
-      return false
-    })
+    beginEditing.mockResolvedValue('needs-session')
     expect(await useEditEntry().enterEdit('mgd', window)).toBe('editing')
     expect(startSession).toHaveBeenCalledWith({
       phenomenonTimeStart: '2025-01-01T00:00:00.000Z',
@@ -164,10 +149,7 @@ describe('useEditEntry', () => {
   })
 
   it('keeps the editor open when starting the session fails', async () => {
-    beginEditing.mockImplementation(async () => {
-      needsSession.value = true
-      return false
-    })
+    beginEditing.mockResolvedValue('needs-session')
     startSession.mockRejectedValueOnce(new Error('bad window'))
     expect(await useEditEntry().enterEdit('mgd', window)).toBe('needs-window')
     expect(error).toHaveBeenCalledWith('bad window')
@@ -176,10 +158,7 @@ describe('useEditEntry', () => {
   })
 
   it('leaves the editor when the new session was superseded', async () => {
-    beginEditing.mockImplementation(async () => {
-      needsSession.value = true
-      return false
-    })
+    beginEditing.mockResolvedValue('needs-session')
     startSession.mockRejectedValueOnce(new ResumeSupersededError())
     expect(await useEditEntry().enterEdit('mgd', window)).toBe('superseded')
     expect(currentView.value).toBe('Select')
@@ -194,6 +173,27 @@ describe('useEditEntry', () => {
     expect(resumeDatastreamId.value).toBeNull()
   })
 
+  it('says nothing when the editor was closed while the session opened', async () => {
+    beginEditing.mockImplementationOnce(async () => {
+      qcDatastream.value = null
+      throw new ResumeSupersededError()
+    })
+    expect(await useEditEntry().enterEdit('mgd')).toBe('superseded')
+    expect(error).not.toHaveBeenCalled()
+    expect(currentView.value).toBe('Select')
+  })
+
+  it('says nothing when the editor was closed while a new session started', async () => {
+    beginEditing.mockResolvedValue('needs-session')
+    startSession.mockImplementationOnce(async () => {
+      qcDatastream.value = null
+      throw new ResumeSupersededError()
+    })
+    expect(await useEditEntry().enterEdit('mgd', window)).toBe('superseded')
+    expect(error).not.toHaveBeenCalled()
+    expect(currentView.value).toBe('Select')
+  })
+
   it('leaves the editor and rethrows any other resume failure', async () => {
     beginEditing.mockRejectedValueOnce(new Error('offline'))
     await expect(useEditEntry().enterEdit('mgd')).rejects.toThrow('offline')
@@ -217,19 +217,16 @@ describe('useEditEntry', () => {
   })
 
   // A newer entry took over while this one awaited; it owns the view state.
-  const takeOverDuringBegin = (flags: () => void) =>
+  const takeOverDuringBegin = (outcome: string) =>
     beginEditing.mockImplementation(async () => {
       qcDatastream.value = { id: 'newer' }
       resumeDatastreamId.value = 'newer'
       currentView.value = 'Edit'
-      flags()
-      return false
+      return outcome
     })
 
   it('stands down when the target changes before a not-managed result', async () => {
-    takeOverDuringBegin(() => {
-      needsHistory.value = true
-    })
+    takeOverDuringBegin('not-managed')
     expect(await useEditEntry().enterEdit('mgd')).toBe('superseded')
     expect(clearEditTarget).not.toHaveBeenCalled()
     expect(resumeDatastreamId.value).toBe('newer')
@@ -238,9 +235,7 @@ describe('useEditEntry', () => {
   })
 
   it('stands down when the target changes before starting a session', async () => {
-    takeOverDuringBegin(() => {
-      needsSession.value = true
-    })
+    takeOverDuringBegin('needs-session')
     expect(await useEditEntry().enterEdit('mgd', window)).toBe('superseded')
     expect(startSession).not.toHaveBeenCalled()
     expect(clearEditTarget).not.toHaveBeenCalled()
@@ -248,10 +243,7 @@ describe('useEditEntry', () => {
   })
 
   it('does not leave a newer target when its own start is superseded', async () => {
-    beginEditing.mockImplementation(async () => {
-      needsSession.value = true
-      return false
-    })
+    beginEditing.mockResolvedValue('needs-session')
     startSession.mockImplementationOnce(async () => {
       qcDatastream.value = { id: 'newer' }
       resumeDatastreamId.value = 'newer'
@@ -263,10 +255,7 @@ describe('useEditEntry', () => {
   })
 
   it('backs out when the datastream has no QC history', async () => {
-    beginEditing.mockImplementation(async () => {
-      needsHistory.value = true
-      return false
-    })
+    beginEditing.mockResolvedValue('not-managed')
     expect(await useEditEntry().enterEdit('mgd')).toBe('not-managed')
     expect(clearEditTarget).toHaveBeenCalled()
     expect(currentView.value).toBe('Select')
@@ -303,10 +292,7 @@ describe('useEditEntry', () => {
 
   it('still starts a session over a window on its own target', async () => {
     qcDatastream.value = { id: 'mgd' }
-    beginEditing.mockImplementation(async () => {
-      needsSession.value = true
-      return false
-    })
+    beginEditing.mockResolvedValue('needs-session')
     expect(await useEditEntry().enterEdit('mgd', window)).toBe('editing')
     expect(startSession).toHaveBeenCalled()
   })
diff --git a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
index 290fa342..a10c1f7f 100644
--- a/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useEditSession.spec.ts
@@ -25,14 +25,18 @@ vi.mock('@/store/plotly', () => ({
 
 const getItem = vi.fn()
 const createObservations = vi.fn()
-const hs = ref<any>({ datastreams: { getItem, createObservations } })
+const getObservationsChecksum = vi.fn()
+const hs = ref<any>({
+  datastreams: { getItem, createObservations, getObservationsChecksum },
+})
 vi.mock('@/store/hydroserver', () => ({
   useHydroServer: () => ({ hs }),
 }))
 
 const fetchDetachedRecord = vi.fn()
+const forget = vi.fn()
 vi.mock('@/store/observations', () => ({
-  useObservationStore: () => ({ fetchDetachedRecord }),
+  useObservationStore: () => ({ fetchDetachedRecord, forget }),
 }))
 
 const wcRebuild = vi.fn()
@@ -59,10 +63,10 @@ const { snackbarWarn, ObservationRecordDouble } = vi.hoisted(() => {
     history: any[] = []
     redoStack: any[] = []
     reload = vi.fn(async () => {})
-    // `discardUnsavedEdits` calls this on the working copy; mirror the
-    // truncate-in-place semantics the other test double in this file uses.
-    truncateHistory = vi.fn(async function (this: ObservationRecordDouble, index: number) {
-      this.history.splice(index + 1)
+    // `discardUnsavedEdits` calls this on the working copy; replay builds
+    // fresh entries, as the real record does.
+    restoreHistory = vi.fn(async function (this: ObservationRecordDouble, steps: any[]) {
+      this.history.splice(0, this.history.length, ...steps.map((h) => ({ ...h })))
       return []
     })
     constructor({ datetimes, dataValues }: { datetimes: number[]; dataValues: number[] }) {
@@ -107,9 +111,9 @@ const makeRecord = (history: any[] = []) => ({
   dataX: [Date.UTC(2025, 0, 1)],
   dataY: [10],
   reload: vi.fn(async () => {}),
-  // Stands in for qc-utils: truncate to `0..index` and hand back a selection.
-  truncateHistory: vi.fn(async function (this: any, index: number) {
-    history.splice(index + 1)
+  // Replay builds fresh entries, as the real record does.
+  restoreHistory: vi.fn(async (steps: any[]) => {
+    history.splice(0, history.length, ...steps.map((h) => ({ ...h })))
     return []
   }),
 })
@@ -118,7 +122,7 @@ let qc: ReturnType<typeof makeQcFake>
 
 const wireHs = () => {
   hs.value = {
-    datastreams: { getItem, createObservations },
+    datastreams: { getItem, createObservations, getObservationsChecksum },
     qualityControlHistories: qc.histories,
     qualityControlSessions: qc.sessions,
     qualityControlOperations: qc.operations,
@@ -133,7 +137,13 @@ beforeEach(() => {
   qcDatastream.value = { id: 'm-1' }
   selectedSeries.value = { data: makeRecord() }
   getItem.mockResolvedValue({ id: 's-1', name: 'Source' })
-  createObservations.mockResolvedValue(undefined)
+  createObservations.mockResolvedValue({ ok: true, status: 201, data: null })
+  // The source is unchanged unless a test says otherwise.
+  getObservationsChecksum.mockImplementation(async () => ({
+    ok: true,
+    status: 200,
+    data: useQcSessionStore().inProgressSession?.sourceChecksum,
+  }))
   fetchDetachedRecord.mockResolvedValue(makeRecord())
   wcRebuild.mockResolvedValue({
     sessionId: 'x',
@@ -154,19 +164,16 @@ describe('useEditSession', () => {
   it('beginEditing resolves the history and needs a session when none is in progress', async () => {
     await seedHistory()
     const { useEditSession } = await import('@/composables/useEditSession')
-    const { beginEditing, needsSession, sourceDatastream } = useEditSession()
-    await beginEditing()
+    const { beginEditing, sourceDatastream } = useEditSession()
+    expect(await beginEditing()).toBe('needs-session')
     expect(sourceDatastream.value?.id).toBe('s-1')
-    expect(needsSession.value).toBe(true)
     expect(useQcSessionStore().historyId).toBeTruthy()
   })
 
-  it('beginEditing flags needsHistory when the datastream is not a managed one', async () => {
+  it('beginEditing reports a datastream that is not a managed one', async () => {
     const { useEditSession } = await import('@/composables/useEditSession')
-    const { beginEditing, needsHistory, needsSession } = useEditSession()
-    await beginEditing()
-    expect(needsHistory.value).toBe(true)
-    expect(needsSession.value).toBe(false)
+    const { beginEditing } = useEditSession()
+    expect(await beginEditing()).toBe('not-managed')
   })
 
   it('beginEditing resumes an in-progress session from the shared working copy', async () => {
@@ -187,9 +194,8 @@ describe('useEditSession', () => {
       end: new Date(1),
     })
     const { useEditSession } = await import('@/composables/useEditSession')
-    const { beginEditing, needsSession } = useEditSession()
-    await beginEditing()
-    expect(needsSession.value).toBe(false)
+    const { beginEditing } = useEditSession()
+    expect(await beginEditing()).toBe('resumed')
     expect(wcRebuild).toHaveBeenCalledWith(
       expect.objectContaining({ id: 'm-1' }),
       expect.objectContaining({ id: 's-1' }),
@@ -224,7 +230,7 @@ describe('useEditSession', () => {
     const { useEditSession } = await import('@/composables/useEditSession')
     const { beginEditing } = useEditSession()
 
-    await expect(beginEditing()).resolves.toBe(true)
+    await expect(beginEditing()).resolves.toBe('resumed')
 
     expect(setEditRecord).toHaveBeenCalledWith(rebuiltRecord)
   })
@@ -244,13 +250,11 @@ describe('useEditSession', () => {
     const { useEditSession, ResumeSupersededError } = await import(
       '@/composables/useEditSession'
     )
-    const { beginEditing, needsSession } = useEditSession()
-    needsSession.value = true
+    const { beginEditing } = useEditSession()
 
+    // A throw, not 'needs-session': starting a session here would edit a
+    // bare base under the saved draft.
     await expect(beginEditing()).rejects.toBeInstanceOf(ResumeSupersededError)
-
-    // Starting a session here would edit a bare base under the saved draft.
-    expect(needsSession.value).toBe(false)
     expect(selectedSeries.value.data).toBe(original)
     expect(useQcSessionStore().savedEdits).toEqual([])
     expect(setEditRecord).not.toHaveBeenCalled()
@@ -350,17 +354,23 @@ describe('useEditSession', () => {
     expect(wcRebuild).not.toHaveBeenCalled()
   })
 
-  it('startSession loads the managed datastream as the working base', async () => {
-    await seedHistory()
+  it('startSession bases a committed window on the managed datastream', async () => {
+    const h = unwrap(
+      await qc.histories.create({
+        managedDatastreamId: 'm-1',
+        sourceDatastreamId: 's-1',
+      })
+    )
+    const earlier = unwrap(await qc.sessions.create(h.id, WIN))
+    await qc.sessions.commit(h.id, earlier.id)
     const managedBase = makeRecord()
     fetchDetachedRecord.mockResolvedValue(managedBase)
     const { useEditSession } = await import('@/composables/useEditSession')
     const session = useEditSession()
     await session.beginEditing()
     await session.startSession({ ...WIN, description: 'Jan' })
-    expect(session.needsSession.value).toBe(false)
     expect(useQcSessionStore().inProgressSession?.description).toBe('Jan')
-    // Working copy comes from the managed datastream (latest committed state).
+    expect(fetchDetachedRecord).toHaveBeenCalledTimes(1)
     expect(fetchDetachedRecord.mock.calls[0]?.[0].id).toBe('m-1')
     expect(Array.from(selectedSeries.value.data.dataX)).toEqual(
       Array.from(managedBase.dataX)
@@ -413,7 +423,72 @@ describe('useEditSession', () => {
     ])
   })
 
-  it('commit pushes observations in replace mode and locks the session', async () => {
+  it('commit checks the source window has not changed', async () => {
+    await seedHistory()
+    selectedSeries.value = { data: makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }]) }
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    await session.commit()
+
+    expect(getObservationsChecksum).toHaveBeenCalledWith(
+      's-1',
+      new Date(WIN.phenomenonTimeStart),
+      new Date(WIN.phenomenonTimeEnd)
+    )
+  })
+
+  it('commit stops when the source window changed since the session started', async () => {
+    await seedHistory()
+    selectedSeries.value = { data: makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }]) }
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    getObservationsChecksum.mockResolvedValue({ ok: true, status: 200, data: 'changed' })
+
+    await expect(session.commit()).rejects.toThrow(/source data changed/i)
+
+    expect(createObservations).not.toHaveBeenCalled()
+    expect(useQcSessionStore().inProgressSession).not.toBeNull()
+  })
+
+  it('commit drops the cached managed observations it replaced', async () => {
+    await seedHistory()
+    selectedSeries.value = { data: makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }]) }
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+
+    await session.commit()
+
+    expect(forget).toHaveBeenCalledWith('m-1')
+  })
+
+  it('commit leaves the session open when the upload fails', async () => {
+    await seedHistory()
+    selectedSeries.value = { data: makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }]) }
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    await session.beginEditing()
+    await session.startSession(WIN)
+    createObservations.mockResolvedValue({
+      ok: false,
+      status: 500,
+      message: 'Server error',
+    })
+    const commit = vi.spyOn(qc.sessions, 'commit')
+
+    await expect(session.commit()).rejects.toThrow('Server error')
+
+    expect(commit).not.toHaveBeenCalled()
+    expect(forget).not.toHaveBeenCalled()
+    expect(useQcSessionStore().inProgressSession).not.toBeNull()
+  })
+
+  it('commit replaces the session window and locks the session', async () => {
     await seedHistory()
     selectedSeries.value = { data: makeRecord([{ method: 'VALUE_THRESHOLD', args: [] }]) }
     const { useEditSession } = await import('@/composables/useEditSession')
@@ -425,10 +500,14 @@ describe('useEditSession', () => {
     expect(createObservations).toHaveBeenCalledWith(
       'm-1',
       expect.objectContaining({ fields: ['phenomenonTime', 'result'] }),
-      { mode: 'replace' }
+      {
+        mode: 'replace',
+        phenomenon_time_start: new Date(WIN.phenomenonTimeStart).toISOString(),
+        phenomenon_time_end: new Date(WIN.phenomenonTimeEnd).toISOString(),
+      }
     )
     const store = useQcSessionStore()
-    expect(store.committedSessions.length).toBe(1)
+    expect(store.sessions.filter((s) => s.status === 'committed')).toHaveLength(1)
     expect(store.inProgressSession).toBeNull()
   })
 
@@ -461,7 +540,7 @@ describe('useEditSession', () => {
     await session.commit()
 
     const store = useQcSessionStore()
-    expect(store.committedSessions.length).toBe(1)
+    expect(store.sessions.filter((s) => s.status === 'committed')).toHaveLength(1)
     expect(store.inProgressSession).toBeNull()
     expect(replaceDatastream).not.toHaveBeenCalled()
     expect(snackbarWarn).toHaveBeenCalledWith(
@@ -482,7 +561,7 @@ describe('useEditSession', () => {
     await session.commit()
 
     const store = useQcSessionStore()
-    expect(store.committedSessions.length).toBe(1)
+    expect(store.sessions.filter((s) => s.status === 'committed')).toHaveLength(1)
     expect(store.inProgressSession).toBeNull()
     expect(replaceDatastream).not.toHaveBeenCalled()
     expect(snackbarWarn).toHaveBeenCalledWith(
@@ -581,7 +660,9 @@ describe('useEditSession', () => {
     await session.commit('Reviewed January spike')
 
     const store = useQcSessionStore()
-    expect(store.committedSessions[0]?.description).toBe('Reviewed January spike')
+    expect(store.sessions.find((s) => s.status === 'committed')?.description).toBe(
+      'Reviewed January spike'
+    )
   })
 
   it('startSession rejects without writing the session store when the target changes while sessions load', async () => {
@@ -662,8 +743,7 @@ describe('useEditSession', () => {
     )
     const { useEditSession } = await import('@/composables/useEditSession')
     const session = useEditSession()
-    await session.beginEditing()
-    expect(session.needsSession.value).toBe(true)
+    expect(await session.beginEditing()).toBe('needs-session')
 
     // Someone else starts the session and saves a draft first.
     const existing = unwrap(await qc.sessions.create(h.id, WIN))
@@ -695,7 +775,6 @@ describe('useEditSession', () => {
     expect(toRaw(selectedSeries.value.data)).toBe(replayed)
     expect(useQcSessionStore().savedEdits).toHaveLength(2)
     expect(session.hasUnsavedChanges.value).toBe(false)
-    expect(session.needsSession.value).toBe(false)
 
     // Saving keeps the server's operations instead of reconciling them away.
     await session.saveDraft()
@@ -789,6 +868,32 @@ describe('useEditSession.discardUnsavedEdits', () => {
     expect(record.history[0].comment).toBe('the saved reason')
     expect(hasUnsavedChanges.value).toBe(false)
   })
+
+  it('puts back a saved edit that was undone and replaced', async () => {
+    await seedHistory()
+    const { useEditSession } = await import('@/composables/useEditSession')
+    const session = useEditSession()
+    const { saveDraft, discardUnsavedEdits, hasUnsavedChanges } = session
+    await session.beginEditing()
+    await session.startSession(WIN)
+
+    const record = selectedSeries.value.data
+    record.history.push(
+      { method: 'SELECTION', args: [[0]] },
+      { method: 'DELETE_POINTS', args: [] }
+    )
+    await saveDraft()
+    record.redoStack.push(record.history.pop())
+    record.history.push({ method: 'INTERPOLATE', args: [] })
+
+    await discardUnsavedEdits()
+
+    expect(record.history.map((h: any) => h.method)).toEqual([
+      'SELECTION',
+      'DELETE_POINTS',
+    ])
+    expect(hasUnsavedChanges.value).toBe(false)
+  })
 })
 
 describe('useEditSession.viewSession', () => {
@@ -920,7 +1025,6 @@ describe('useEditSession.viewSession', () => {
     expect(store.viewedSessionId).toBe(committed.id)
     expect(store.isReadOnly).toBe(true)
     expect(selectedSeries.value.data).toBe(plottedBeforeReturn)
-    expect(session.needsSession.value).toBe(false)
   })
 
   it('rejects when the edit target changes while a committed session loads, without calling setEditRecord', async () => {
diff --git a/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts b/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
index c5edee1e..c8f00423 100644
--- a/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useLeaveSession.spec.ts
@@ -327,11 +327,11 @@ describe('requestLeave', () => {
   it('asks before leaving a session whose edits are all saved', async () => {
     emptySession()
     hasSessionOperations.value = true
-    const { requestLeave, leavePrompt, closeSession } = useLeaveSession()
+    const { requestLeave, leavePrompt, keepSession } = useLeaveSession()
     const pending = requestLeave()
 
     expect(leavePrompt.value?.kind).toBe('saved')
-    closeSession()
+    keepSession()
 
     expect(await pending).toBe(true)
     expect(deleteSession).not.toHaveBeenCalled()
diff --git a/apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts b/apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts
index 2d2c80a2..22c70028 100644
--- a/apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useManagedDatastreams.spec.ts
@@ -65,9 +65,8 @@ describe('useManagedDatastreams.loadForSource', () => {
     historiesBySource.set('src-2', [
       {
         id: 'h-9',
-        managedDatastreamId: 'mgd-missing',
-        managedDatastream: { name: 'Orphan (QC)' },
-        sourceDatastreamId: 'src-2',
+        managedDatastream: { id: 'mgd-missing', name: 'Orphan (QC)' },
+        sourceDatastream: { id: 'src-2' },
       },
     ])
     listAllItems.mockResolvedValue([])
diff --git a/apps/qc-app/src/composables/__tests__/useWorkspacePermissions.spec.ts b/apps/qc-app/src/composables/__tests__/useWorkspacePermissions.spec.ts
index 310eed3d..1109c031 100644
--- a/apps/qc-app/src/composables/__tests__/useWorkspacePermissions.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useWorkspacePermissions.spec.ts
@@ -30,7 +30,22 @@ const editorWs = ws({
   owner: { email: 'other@x.org' } as any,
   collaboratorRole: role('Editor', [
     { action: PermissionAction.Create, resource: PermissionResource.Datastream },
-    { action: PermissionAction.Edit, resource: PermissionResource.Observation },
+    { action: PermissionAction.Edit, resource: PermissionResource.Datastream },
+    { action: PermissionAction.Create, resource: PermissionResource.Observation },
+  ]),
+})
+const creatorWs = ws({
+  id: 'cr',
+  owner: { email: 'other@x.org' } as any,
+  collaboratorRole: role('Creator', [
+    { action: PermissionAction.Create, resource: PermissionResource.Datastream },
+  ]),
+})
+const draftWs = ws({
+  id: 'dr',
+  owner: { email: 'other@x.org' } as any,
+  collaboratorRole: role('Drafter', [
+    { action: PermissionAction.Edit, resource: PermissionResource.Datastream },
   ]),
 })
 const viewerWs = ws({
@@ -57,22 +72,38 @@ describe('useWorkspacePermissions', () => {
 
   it('owner: can edit and create, role is Owner', () => {
     setUser('me@x.org')
-    const { isOwner, canEdit, canCreateDatastream, roleName } =
-      useWorkspacePermissions()
-    expect(isOwner(ownerWs)).toBe(true)
+    const { canEdit, canCreateDatastream, roleName } = useWorkspacePermissions()
     expect(canEdit(ownerWs)).toBe(true)
     expect(canCreateDatastream(ownerWs)).toBe(true)
     expect(roleName(ownerWs)).toBe('Owner')
   })
 
-  it('editor role: can edit and create datastreams', () => {
+  it('editor role: can edit, commit and create datastreams', () => {
     setUser('me@x.org')
-    const { canEdit, canCreateDatastream, roleName } = useWorkspacePermissions()
+    const { canEdit, canCommit, canCreateDatastream, roleName } =
+      useWorkspacePermissions()
     expect(canEdit(editorWs)).toBe(true)
+    expect(canCommit(editorWs)).toBe(true)
     expect(canCreateDatastream(editorWs)).toBe(true)
     expect(roleName(editorWs)).toBe('Editor')
   })
 
+  // The API needs datastream edit for the QC history and its sessions.
+  it('creating datastreams alone allows no QC work', () => {
+    setUser('me@x.org')
+    const { canEdit, canCommit, canCreateDatastream } = useWorkspacePermissions()
+    expect(canEdit(creatorWs)).toBe(false)
+    expect(canCommit(creatorWs)).toBe(false)
+    expect(canCreateDatastream(creatorWs)).toBe(false)
+  })
+
+  it('without creating observations, sessions can be saved but not committed', () => {
+    setUser('me@x.org')
+    const { canEdit, canCommit } = useWorkspacePermissions()
+    expect(canEdit(draftWs)).toBe(true)
+    expect(canCommit(draftWs)).toBe(false)
+  })
+
   it('viewer role: cannot edit or create', () => {
     setUser('me@x.org')
     const { canEdit, canCreateDatastream, roleName } = useWorkspacePermissions()
@@ -102,13 +133,4 @@ describe('useWorkspacePermissions', () => {
     expect(canEdit()).toBe(true)
     expect(roleName()).toBe('Editor')
   })
-
-  it('workspaceById resolves from availableWorkspaces', () => {
-    setUser('me@x.org')
-    useWorkspaceStore().availableWorkspaces = [editorWs, viewerWs]
-    const { workspaceById } = useWorkspacePermissions()
-    expect(workspaceById('ed')?.id).toBe('ed')
-    expect(workspaceById('missing')).toBeNull()
-    expect(workspaceById(null)).toBeNull()
-  })
 })
diff --git a/apps/qc-app/src/composables/useClearPlot.ts b/apps/qc-app/src/composables/useClearPlot.ts
new file mode 100644
index 00000000..29dd0b2b
--- /dev/null
+++ b/apps/qc-app/src/composables/useClearPlot.ts
@@ -0,0 +1,29 @@
+/**
+ * Unplot everything. The datastream being edited is on the plot too, so the
+ * editor closes first, asking about the session the way any exit does.
+ */
+
+import { computed } from 'vue'
+import { storeToRefs } from 'pinia'
+import { useDataVisStore } from '@/store/dataVisualization'
+import { usePlotlyStore } from '@/store/plotly'
+import { useEditEntry } from '@/composables/useEditEntry'
+
+export function useClearPlot() {
+  const dataVis = useDataVisStore()
+  const { plottedDatastreams, qcDatastream } = storeToRefs(dataVis)
+  const { hiddenTraceIds } = storeToRefs(usePlotlyStore())
+  const { closeEditor } = useEditEntry()
+
+  const canClearPlot = computed(
+    () => !!plottedDatastreams.value.length || !!qcDatastream.value
+  )
+
+  async function clearPlot() {
+    if (qcDatastream.value && !(await closeEditor())) return
+    hiddenTraceIds.value = new Set()
+    await dataVis.clearPlottedDatastreams()
+  }
+
+  return { canClearPlot, clearPlot }
+}
diff --git a/apps/qc-app/src/composables/useEditEntry.ts b/apps/qc-app/src/composables/useEditEntry.ts
index 2aa6587d..76ef66e6 100644
--- a/apps/qc-app/src/composables/useEditEntry.ts
+++ b/apps/qc-app/src/composables/useEditEntry.ts
@@ -40,14 +40,16 @@ export function useEditEntry() {
   const { setEditTarget, clearEditTarget } = dataVis
   const { showView } = useUIStore()
   const { resumeDatastreamId } = storeToRefs(useQcSessionStore())
-  const { beginEditing, startSession, needsSession, needsHistory } =
-    useEditSession()
+  const { beginEditing, startSession } = useEditSession()
   const { requestLeave, forgetSession } = useLeaveSession()
 
   const owns = (id: string | undefined) => qcDatastream.value?.id === id
   // A cleared target is not a takeover: leaving again is harmless.
   const takenOver = (id: string | undefined) =>
     !!qcDatastream.value && !owns(id)
+  // No target means the user closed the editor mid-load, so a superseded
+  // load has nothing to report.
+  const closed = () => !qcDatastream.value
 
   /** Show the editor on the current target. Nothing about the session, the
    *  working copy or the staged edits changes. */
@@ -86,8 +88,9 @@ export function useEditEntry() {
     } catch (e) {
       if (takenOver(owner)) return 'superseded'
       if (e instanceof ResumeSupersededError) {
+        const wasClosed = closed()
         await leaveEdit()
-        Snackbar.error(e.message)
+        if (!wasClosed) Snackbar.error(e.message)
         return 'superseded'
       }
       Snackbar.error(
@@ -115,25 +118,27 @@ export function useEditEntry() {
       return 'editing'
     }
     if (takenOver(managedId) && !(await requestLeave())) return 'kept'
+    let begun
     try {
       await setEditTarget(managedId)
-      await beginEditing()
+      begun = await beginEditing()
     } catch (e) {
       if (takenOver(managedId)) return 'superseded'
+      const wasClosed = closed()
       await leaveEdit()
       if (!(e instanceof ResumeSupersededError)) throw e
-      Snackbar.error(e.message)
+      if (!wasClosed) Snackbar.error(e.message)
       return 'superseded'
     }
     if (!owns(managedId)) return 'superseded'
-    if (needsHistory.value) {
+    if (begun === 'not-managed') {
       await leaveEdit()
       Snackbar.error('This datastream is not set up for QC editing.')
       return 'not-managed'
     }
     showView(view)
     resumeDatastreamId.value = managedId
-    if (!needsSession.value) return 'editing'
+    if (begun === 'resumed') return 'editing'
     if (!window) return 'needs-window'
     const outcome = await tryStartSession(window)
     if (outcome === 'started') return 'editing'
diff --git a/apps/qc-app/src/composables/useEditSession.ts b/apps/qc-app/src/composables/useEditSession.ts
index d5e18d95..c1d36e65 100644
--- a/apps/qc-app/src/composables/useEditSession.ts
+++ b/apps/qc-app/src/composables/useEditSession.ts
@@ -1,20 +1,11 @@
 /**
- * Orchestrates the session-based editing workflow (spec section 5-9),
- * wiring the QC service layer to the app's stores:
- *   - beginEditing: resolve the managed datastream's history, load its
- *     sessions, and resume the in-progress one (or signal that a session
- *     must be started); reports whether it actually resumed,
- *   - startSession: create a session and copy the source window in, or
- *     resume the in-progress one if it already exists,
- *   - saveDraft: persist the record's edit operations to the session
- *     (append-only, so each user's operations keep their creator),
- *   - commit: push the final observations (replace) and lock the session.
- *
- * The QC API client comes from `hs.qualityControl*`; the server stamps every
- * operation/session with the authenticated user.
+ * The session-based editing workflow, wiring the QC service layer to the
+ * app's stores: open or resume a session, save its operations as a draft,
+ * and commit it. The server stamps every session and operation with the
+ * signed-in user.
  */
 
-import { ref, computed } from 'vue'
+import { computed } from 'vue'
 import { storeToRefs } from 'pinia'
 import { serializeHistory, applyHistory, Snackbar } from '@uwrl/qc-utils'
 import type { Datastream, DatastreamExtended, QualityControlSessionContract } from '@hydroserver/client'
@@ -29,22 +20,19 @@ import {
   findHistoryForDatastream,
   startOrResumeSession,
   loadLatestBase,
+  committedWindows,
   persistSessionOperations,
   commitQcSession,
   reconstructCommittedSession,
   observationsBulkBody,
+  unwrap,
 } from '@/services/qualityControl'
 
 type QcSessionPostBody = QualityControlSessionContract.PostBody
 
-/**
- * Keep a new session's window within the source datastream's observed extent.
- * The window now comes from the session window dialog, so it is normally
- * already valid; this clamp is a backstop for the backend's end-time rule
- * (`phenomenon_time_end cannot extend past the source datastream's current
- * end time`). Resume ignores the spec, so this only shapes freshly-created
- * sessions.
- */
+/** Keep a new session's window within the source's observed extent. The
+ *  window dialog already checks it; this backs up the API's rule that a
+ *  session can't end past the source's last observation. */
 function clampSpecToSource(
   spec: QcSessionPostBody,
   source: Pick<Datastream, 'phenomenonBeginTime' | 'phenomenonEndTime'>
@@ -76,6 +64,8 @@ export class ResumeSupersededError extends Error {
   }
 }
 
+export type BeginOutcome = 'resumed' | 'needs-session' | 'not-managed'
+
 export function useEditSession() {
   const { qcDatastream } = storeToRefs(useDataVisStore())
   const { replaceDatastream, setEditRecord } = useDataVisStore()
@@ -83,7 +73,7 @@ export function useEditSession() {
   const { hs } = storeToRefs(useHydroServer())
   // Working copies build on records of their own, so the plotted source
   // keeps its context window.
-  const { fetchDetachedRecord } = useObservationStore()
+  const { fetchDetachedRecord, forget } = useObservationStore()
   const sessionStore = useQcSessionStore()
   const workingCopies = useWorkingCopiesStore()
 
@@ -91,10 +81,6 @@ export function useEditSession() {
   // footer and the leave flow) sees the same unsaved state.
   const { sourceDatastream, savedEdits, savedComments } =
     storeToRefs(sessionStore)
-  /** True when the managed datastream has no in-progress session to resume. */
-  const needsSession = ref(false)
-  /** True when the selected datastream has no QC history (not a managed datastream). */
-  const needsHistory = ref(false)
 
   const currentEdits = (): HistoryItem[] =>
     (selectedSeries.value?.data as ObservationRecord | undefined)?.history ?? []
@@ -143,15 +129,15 @@ export function useEditSession() {
     await setEditRecord(built.record)
     // The replayed draft operations are the saved baseline.
     snapshotSavedEdits()
-    needsSession.value = false
   }
 
-  /** Returns true when an in-progress session's working copy was wired into
-   *  the plot, i.e. the editor is now genuinely editable over it. */
-  async function beginEditing(): Promise<boolean> {
+  /** `'resumed'` once the in-progress session's working copy is on the plot,
+   *  so the editor is editable over it. `'needs-session'` when there is none
+   *  to resume, and `'not-managed'` when the target has no QC history (the
+   *  caller can offer to create one with it as the source). */
+  async function beginEditing(): Promise<BeginOutcome> {
     const managed = qcDatastream.value
-    if (!managed) return false
-    needsHistory.value = false
+    if (!managed) throw new ResumeSupersededError()
 
     // Entries can overlap; never write the session store for a target
     // another entry has since taken over.
@@ -164,12 +150,7 @@ export function useEditSession() {
       managed.id
     )
     stillOwner()
-    if (!history) {
-      // Not a managed datastream: the caller should offer to create one
-      // from it (with this datastream as the source).
-      needsHistory.value = true
-      return false
-    }
+    if (!history) return 'not-managed'
     const source =
       (await hs.value.datastreams.getItem(history.sourceDatastream.id)) ?? null
     stillOwner()
@@ -179,20 +160,11 @@ export function useEditSession() {
     sessionStore.applySessions(history.id, sessions)
 
     const inProgress = sessionStore.inProgressSession
-    if (inProgress && sourceDatastream.value) {
-      // A session exists, so a failed resume must never ask to start one.
-      needsSession.value = false
-      await resumeWorkingCopy(
-        managed,
-        sourceDatastream.value,
-        history.id,
-        inProgress
-      )
-      return true
-    } else {
-      needsSession.value = true
-      return false
-    }
+    // A session exists, so a failed resume throws rather than asking to
+    // start one.
+    if (!inProgress || !sourceDatastream.value) return 'needs-session'
+    await resumeWorkingCopy(managed, sourceDatastream.value, history.id, inProgress)
+    return 'resumed'
   }
 
   /**
@@ -219,10 +191,9 @@ export function useEditSession() {
       sessionStore.returnToCurrent()
       sessionStore.isSwitchingSession = true
       try {
-        const resumed = await beginEditing()
-        if (!resumed && previousSessionId) {
+        const outcome = await beginEditing()
+        if (outcome !== 'resumed' && previousSessionId) {
           sessionStore.viewSession(previousSessionId)
-          needsSession.value = false
         }
       } catch (e) {
         if (previousSessionId) sessionStore.viewSession(previousSessionId)
@@ -283,14 +254,13 @@ export function useEditSession() {
       await resumeWorkingCopy(managed, source, historyId, session)
       return
     }
-    // Start from the latest committed state (the managed datastream), or the
-    // raw source when nothing has been committed yet.
     const base = await loadLatestBase(
       fetchDetachedRecord,
       managed,
       source,
       new Date(session.phenomenonTimeStart),
-      new Date(session.phenomenonTimeEnd)
+      new Date(session.phenomenonTimeEnd),
+      committedWindows(sessions)
     )
     // The edit target may have changed under this await; same reasoning as
     // the check in `resumeWorkingCopy`.
@@ -303,20 +273,18 @@ export function useEditSession() {
       new Date(session.phenomenonTimeStart),
       new Date(session.phenomenonTimeEnd)
     )
-    needsSession.value = false
     // Fresh session: the loaded working copy is the saved baseline.
     snapshotSavedEdits()
   }
 
   /**
-   * Drop everything added since the last save: replay the saved prefix,
-   * then put the saved comment text back, since a comment edited in place
-   * on a surviving entry survives the replay.
+   * Go back to the last save: replay the saved edits, then put the saved
+   * comment text back, since comments are edited in place on the entries.
    */
   async function discardUnsavedEdits(): Promise<number[] | undefined> {
     const record = selectedSeries.value?.data as ObservationRecord | undefined
     if (!record) return
-    const selection = await record.truncateHistory(savedEdits.value.length - 1)
+    const selection = await record.restoreHistory(savedEdits.value)
     record.history.forEach((item, i) => {
       item.comment = savedComments.value[i] || undefined
     })
@@ -349,7 +317,8 @@ export function useEditSession() {
     const historyId = sessionStore.historyId
     const session = sessionStore.inProgressSession
     const record = selectedSeries.value?.data
-    if (!managed || !historyId || !session || !record) {
+    const source = sourceDatastream.value
+    if (!managed || !historyId || !session || !record || !source) {
       throw new Error('No active edit session to commit.')
     }
     await saveDraft()
@@ -373,13 +342,23 @@ export function useEditSession() {
       qcSessions: hs.value.qualityControlSessions,
       historyId,
       sessionId: session.id,
-      // The real client fetches the source window's X-Checksum header for a
-      // genuine integrity check; here we reuse the checksum captured at create.
-      currentSourceChecksum: session.sourceChecksum,
+      currentSourceChecksum: unwrap(
+        await hs.value.datastreams.getObservationsChecksum(
+          source.id,
+          new Date(session.phenomenonTimeStart),
+          new Date(session.phenomenonTimeEnd)
+        )
+      ),
       pushObservations: async () => {
-        await hs.value.datastreams.createObservations(managed.id, body, {
-          mode: 'replace',
-        })
+        // The whole window, so points deleted at its edges go too.
+        unwrap(
+          await hs.value.datastreams.createObservations(managed.id, body, {
+            mode: 'replace',
+            phenomenon_time_start: session.phenomenonTimeStart,
+            phenomenon_time_end: session.phenomenonTimeEnd,
+          })
+        )
+        forget(managed.id)
       },
     })
     workingCopies.invalidate(managed.id)
@@ -408,8 +387,6 @@ export function useEditSession() {
 
   return {
     sourceDatastream,
-    needsSession,
-    needsHistory,
     hasUnsavedChanges,
     unsavedEditCount,
     beginEditing,
diff --git a/apps/qc-app/src/composables/useHistorySnapshots.ts b/apps/qc-app/src/composables/useHistorySnapshots.ts
index 43863a94..9507f3d1 100644
--- a/apps/qc-app/src/composables/useHistorySnapshots.ts
+++ b/apps/qc-app/src/composables/useHistorySnapshots.ts
@@ -16,13 +16,9 @@ import { buildSnapshotRecord } from '@/services/qualityControl/buildSnapshot'
 import { formatDateRange } from '@/utils/time'
 import { SNAPSHOT_BASELINE_INDEX, makeSnapshotId } from '@/utils/snapshotId'
 import type { SnapshotMeta } from '@/types'
+import { sessionOperations } from '@/utils/qcHistory'
 
 /** The operation shape the session list embeds. */
-interface SessionOperation {
-  operationType?: string
-  createdBy?: { name?: string; email?: string }
-}
-
 export function useHistorySnapshots() {
   const isBuilding = ref(false)
   const dataVis = useDataVisStore()
@@ -88,23 +84,19 @@ function metaFor(
   session: QualityControlSession,
   opIndex: number
 ): SnapshotMeta {
-  const s = session as QualityControlSession & {
-    description?: string
-    operations?: SessionOperation[]
-  }
-  const ops = s.operations ?? []
+  const ops = sessionOperations(session)
   const op = opIndex >= SNAPSHOT_BASELINE_INDEX + 1 ? ops[opIndex] : undefined
   const performedBy = op?.createdBy?.name?.trim() || op?.createdBy?.email?.trim()
   return {
-    sessionId: s.id,
+    sessionId: session.id,
     sessionLabel:
-      s.description ||
-      formatDateRange(s.phenomenonTimeStart, s.phenomenonTimeEnd),
+      session.description ||
+      formatDateRange(session.phenomenonTimeStart, session.phenomenonTimeEnd),
     opIndex,
     opCount: ops.length,
     opName: op?.operationType ? formatMethod(op.operationType) : '',
     ...(performedBy ? { performedBy } : {}),
-    createdAt: s.createdAt,
+    createdAt: session.createdAt,
   }
 }
 
diff --git a/apps/qc-app/src/composables/useLeaveSession.ts b/apps/qc-app/src/composables/useLeaveSession.ts
index be019dff..e389639b 100644
--- a/apps/qc-app/src/composables/useLeaveSession.ts
+++ b/apps/qc-app/src/composables/useLeaveSession.ts
@@ -114,11 +114,6 @@ function keepSession(): void {
   if (!leaveWork.value) settle(true)
 }
 
-/** Keeping it, under the word the user sees when nothing is at stake. */
-function closeSession(): void {
-  keepSession()
-}
-
 async function saveAndLeave(): Promise<void> {
   if (leaveWork.value || !leavePrompt.value?.canSave) return
   leaveWork.value = 'save'
@@ -163,8 +158,8 @@ async function discardSessionAndLeave(): Promise<void> {
   if (leaveWork.value || !session || !historyId) return
   leaveWork.value = 'discard-session'
   try {
-    // The in-progress session is the newest one, so nothing can be built on
-    // it. Refuse rather than cascade the delete into committed work.
+    // The in-progress session is normally the newest, so nothing is built on
+    // it. If something is, refuse rather than cascade the delete into it.
     if (hasDependents(store.sessions, session.id)) {
       throw new Error(
         'Another session was built on this one, so it cannot be discarded.'
@@ -201,7 +196,6 @@ export function useLeaveSession() {
     requestLeave,
     cancelLeave,
     keepSession,
-    closeSession,
     saveAndLeave,
     discardEditsAndLeave,
     discardSessionAndLeave,
diff --git a/apps/qc-app/src/composables/useManagedDatastreams.ts b/apps/qc-app/src/composables/useManagedDatastreams.ts
index 17c6087c..8b5f5e62 100644
--- a/apps/qc-app/src/composables/useManagedDatastreams.ts
+++ b/apps/qc-app/src/composables/useManagedDatastreams.ts
@@ -12,6 +12,7 @@ import type {
 } from '@hydroserver/client'
 import { useHydroServer } from '@/store/hydroserver'
 import { useDataVisStore } from '@/store/dataVisualization'
+import { historyManagedId } from '@/utils/qcHistory'
 
 export interface ManagedDatastreamOption {
   historyId: string
@@ -29,16 +30,17 @@ export function useManagedDatastreams() {
     const { datastreams, historiesBySource } = useDataVisStore()
     const histories = historiesBySource.get(sourceId) ?? []
     return Promise.all(
-      histories.map(async (h: any) => {
-        const historyId = h.id as string
-        const managedId = h.managedDatastreamId ?? h.managedDatastream?.id
+      histories.map(async (h) => {
+        const historyId = h.id
+        const managedId = historyManagedId(h)
         // Managed datastreams are in the full catalog (just hidden from the
         // table); fall back to a name-only stub if it isn't loaded.
         const managed =
           datastreams.find((d) => d.id === managedId) ??
           ({
             id: managedId,
-            name: h.managedDatastream?.name ?? managedId,
+            name:
+              'managedDatastream' in h ? h.managedDatastream.name : managedId,
           } as Datastream)
         // expand_related so each session carries `dependencyIds`.
         const sessions = await hs.qualityControlSessions.listAllItems(
diff --git a/apps/qc-app/src/composables/useQcHistory.ts b/apps/qc-app/src/composables/useQcHistory.ts
index de660698..a93e3c1b 100644
--- a/apps/qc-app/src/composables/useQcHistory.ts
+++ b/apps/qc-app/src/composables/useQcHistory.ts
@@ -1,28 +1,12 @@
 /**
- * QC History: Vue-side save / load wrapper.
- *
- * Wraps qc-utils' `serializeHistory` / `parseHistory` / `applyHistory`
- * with the consumer-specific glue:
- *   - using the session window (or, with no session, the loaded
- *     window) as the saved history's window
- *   - file-picker / blob-download plumbing
- *   - fetching the QC history's window into the active datastream's
- *     `ObservationRecord` before replay (qc-utils itself is data-
- *     agnostic; the consumer drives the data fetch)
+ * Download the current QC history as JSON, windowed to the session (or,
+ * with no session, the loaded range).
  */
 
 import { storeToRefs } from 'pinia'
-import {
-  applyHistory,
-  parseHistory,
-  serializeHistory,
-  type ApplyHistoryReport,
-  type ObservationRecord,
-  type QcHistory,
-} from '@uwrl/qc-utils'
+import { serializeHistory, type ObservationRecord } from '@uwrl/qc-utils'
 import { usePlotlyStore } from '@/store/plotly'
 import { useDataVisStore } from '@/store/dataVisualization'
-import { useObservationStore } from '@/store/observations'
 import { useQcSessionStore } from '@/store/qcSession'
 
 /** Filename for downloaded QC histories: `qc-history-<datastream>-<isoTimestamp>.json`. */
@@ -54,7 +38,6 @@ function downloadJson(payload: unknown, filename: string): void {
 export function useQcHistory() {
   const { selectedSeries } = storeToRefs(usePlotlyStore())
   const { qcDatastream, beginDate, endDate } = storeToRefs(useDataVisStore())
-  const { fetchObservationsInRange } = useObservationStore()
   const { viewedSession, inProgressSession } = storeToRefs(useQcSessionStore())
 
   // While editing, `beginDate`/`endDate` follow the context range, not the
@@ -95,46 +78,5 @@ export function useQcHistory() {
     downloadJson(history, defaultFilename(datastreamName))
   }
 
-  /**
-   * Read a JSON file, parse it as a QcHistory, fetch the QC history's
-   * window into the current QC datastream, and replay the
-   * operations. Returns the per-op report so the caller can surface
-   * a Snackbar / toast summary.
-   *
-   * No datastream-id matching is enforced. QC histories are reusable
-   * across datastreams (see qc-utils' QC_HISTORY.md "Stay reusable").
-   */
-  async function importHistory(file: File): Promise<ApplyHistoryReport> {
-    const series = selectedSeries.value?.data
-    const datastream = qcDatastream.value
-    if (!series || !datastream) {
-      throw new Error('Pick a QC datastream before loading a QC history.')
-    }
-
-    const text = await file.text()
-    let json: unknown
-    try {
-      json = JSON.parse(text)
-    } catch (e) {
-      throw new Error(
-        `Couldn't parse ${file.name} as JSON: ${e instanceof Error ? e.message : String(e)}`
-      )
-    }
-
-    const history: QcHistory = parseHistory(json)
-
-    // Fetch the QC history's authored window into the active record
-    // BEFORE replaying. Selection-coupled ops reference indices
-    // against this windowed dataset; loading them against a
-    // differently-sized window would mis-target.
-    await fetchObservationsInRange(
-      datastream,
-      new Date(history.window.startDate),
-      new Date(history.window.endDate)
-    )
-
-    return await applyHistory(series as ObservationRecord, history)
-  }
-
-  return { exportHistory, importHistory }
+  return { exportHistory }
 }
diff --git a/apps/qc-app/src/composables/useResumeEditSession.ts b/apps/qc-app/src/composables/useResumeEditSession.ts
index 04a3ef58..a9643a65 100644
--- a/apps/qc-app/src/composables/useResumeEditSession.ts
+++ b/apps/qc-app/src/composables/useResumeEditSession.ts
@@ -2,10 +2,8 @@
  * Reopen the editor after a page reload on the managed datastream it was last
  * open on. `resume` does the entering; this only waits for the catalog and
  * drops a pointer to a datastream that no longer exists.
- *
- * The watcher must not use `once` with `immediate`: that fires on the
- * initial empty catalog and stops, which is the cold-reload case this
- * exists for. The flag gives once-only semantics without that trap.
+ * Not a `once` watcher: with `immediate` it fires on the empty catalog at
+ * mount and stops.
  */
 
 import { watch } from 'vue'
@@ -22,17 +20,15 @@ export function useResumeEditSession(
 
   let attempted = false
 
-  /** Returns true when the editor was reopened. */
-  async function run(): Promise<boolean> {
+  async function run(): Promise<void> {
     const id = resumeDatastreamId.value
-    if (!id) return false
+    if (!id) return
     if (!datastreams.value.some((d) => d.id === id)) {
       resumeDatastreamId.value = null
-      return false
+      return
     }
     try {
       await resume(id)
-      return true
     } catch (e) {
       resumeDatastreamId.value = null
       throw e
@@ -57,6 +53,4 @@ export function useResumeEditSession(
     },
     { immediate: true }
   )
-
-  return { resume: run }
 }
diff --git a/apps/qc-app/src/composables/useWorkspacePermissions.ts b/apps/qc-app/src/composables/useWorkspacePermissions.ts
index 4963dc81..fa4b27e4 100644
--- a/apps/qc-app/src/composables/useWorkspacePermissions.ts
+++ b/apps/qc-app/src/composables/useWorkspacePermissions.ts
@@ -1,13 +1,7 @@
 /**
- * Reactive workspace permission checks for the QC app.
- *
- * The signed-in user's role on a workspace travels with the `Workspace`
- * object: owners have a null `collaboratorRole`, collaborators carry their
- * role's `permissions[]`, and admins (`accountType === 'admin'`) override
- * everything. No separate "am I an editor" endpoint is needed: the role is
- * embedded in `hs.workspaces.list()`. (`hs.user.can()` is the async
- * single-shot equivalent; this composable is the synchronous, reactive one
- * for gating UI.)
+ * Synchronous, reactive permission checks for gating UI. The user's role
+ * comes with each `Workspace`: owners have a null `collaboratorRole`,
+ * collaborators carry its `permissions`, and admins can do everything.
  */
 
 import { computed } from 'vue'
@@ -22,9 +16,7 @@ import { useWorkspaceStore } from '@/store/workspaces'
 import { useUserStore } from '@/store/user'
 
 export function useWorkspacePermissions() {
-  const { availableWorkspaces, selectedWorkspace } = storeToRefs(
-    useWorkspaceStore()
-  )
+  const { selectedWorkspace } = storeToRefs(useWorkspaceStore())
   const { user } = storeToRefs(useUserStore())
 
   const isAdmin = computed(
@@ -63,22 +55,26 @@ export function useWorkspacePermissions() {
     )
   }
 
-  /** Can create a managed datastream in this workspace (the QC setup step). */
-  function canCreateDatastream(ws?: Workspace | null): boolean {
-    return can(PermissionAction.Create, PermissionResource.Datastream, ws)
+  /** Can work in QC sessions: the API checks datastream edit for sessions
+   *  and their operations. Gates Edit, New session and Save. */
+  function canEdit(ws?: Workspace | null): boolean {
+    return can(PermissionAction.Edit, PermissionResource.Datastream, ws)
   }
 
-  /**
-   * Can run the QC edit workflow in this workspace. Needs to create the
-   * managed datastream and/or write observations. Used to gate the editor's
-   * row Edit / Save / Commit controls.
-   */
-  function canEdit(ws?: Workspace | null): boolean {
+  /** Commit also writes the managed datastream's observations. */
+  function canCommit(ws?: Workspace | null): boolean {
     return (
-      can(PermissionAction.Create, PermissionResource.Datastream, ws) ||
-      can(PermissionAction.Edit, PermissionResource.Datastream, ws) ||
-      can(PermissionAction.Create, PermissionResource.Observation, ws) ||
-      can(PermissionAction.Edit, PermissionResource.Observation, ws)
+      canEdit(ws) &&
+      can(PermissionAction.Create, PermissionResource.Observation, ws)
+    )
+  }
+
+  /** Creating a managed datastream also creates its QC history, which needs
+   *  datastream edit. */
+  function canCreateDatastream(ws?: Workspace | null): boolean {
+    return (
+      canEdit(ws) &&
+      can(PermissionAction.Create, PermissionResource.Datastream, ws)
     )
   }
 
@@ -92,18 +88,10 @@ export function useWorkspacePermissions() {
     return 'Read-only'
   }
 
-  function workspaceById(id?: string | null): Workspace | null {
-    if (!id) return null
-    return availableWorkspaces.value.find((w) => w.id === id) ?? null
-  }
-
   return {
-    isAdmin,
-    isOwner,
-    can,
     canEdit,
+    canCommit,
     canCreateDatastream,
     roleName,
-    workspaceById,
   }
 }
diff --git a/apps/qc-app/src/router/__tests__/guards.spec.ts b/apps/qc-app/src/router/__tests__/guards.spec.ts
index b732495e..30079870 100644
--- a/apps/qc-app/src/router/__tests__/guards.spec.ts
+++ b/apps/qc-app/src/router/__tests__/guards.spec.ts
@@ -1,4 +1,4 @@
-import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { describe, it, expect, beforeAll, beforeEach, vi } from 'vitest'
 import { createTestPinia } from '@/utils/test/pinia'
 import type { RouteLocationNormalized } from 'vue-router'
 
@@ -21,7 +21,20 @@ vi.mock('@/store/workspaces', () => ({
   }),
 }))
 
+vi.mock('@/router/routes', () => ({
+  routes: [
+    { path: '/', name: 'Home', component: {}, meta: { title: 'Home' } },
+    {
+      path: '/workspaces',
+      name: 'Workspaces',
+      component: {},
+      meta: { title: 'Workspaces' },
+    },
+  ],
+}))
+
 import { leaveSessionGuard } from '@/router/guards'
+import router, { setupRouteGuards } from '@/router/router'
 
 const route = (name: string | undefined, query: Record<string, string> = {}) =>
   ({ name, query, meta: {}, fullPath: '/' }) as unknown as RouteLocationNormalized
@@ -71,3 +84,37 @@ describe('leaveSessionGuard', () => {
     expect(requestLeave).not.toHaveBeenCalled()
   })
 })
+
+describe('route guard order', () => {
+  beforeAll(() => setupRouteGuards())
+
+  beforeEach(async () => {
+    requestLeave.mockResolvedValue(true)
+    await router.push('/')
+    vi.clearAllMocks()
+  })
+
+  // The picker sends a user with a workspace straight back, so this
+  // navigation never leaves the editor and must not end the session.
+  it('does not ask about a navigation another guard redirects back', async () => {
+    await router.push('/workspaces')
+
+    expect(router.currentRoute.value.name).toBe('Home')
+    expect(requestLeave).not.toHaveBeenCalled()
+  })
+
+  it('asks about a navigation that leaves the page', async () => {
+    await router.push('/workspaces?switch=1')
+
+    expect(requestLeave).toHaveBeenCalledOnce()
+    expect(router.currentRoute.value.name).toBe('Workspaces')
+  })
+
+  it('keeps the page title when the user stays', async () => {
+    requestLeave.mockResolvedValueOnce(false)
+    await router.push('/workspaces?switch=1')
+
+    expect(router.currentRoute.value.name).toBe('Home')
+    expect(document.title).toBe('HydroServer | Home')
+  })
+})
diff --git a/apps/qc-app/src/router/guards.ts b/apps/qc-app/src/router/guards.ts
index c66885fd..aa3d2f2f 100644
--- a/apps/qc-app/src/router/guards.ts
+++ b/apps/qc-app/src/router/guards.ts
@@ -35,7 +35,8 @@ const redirectToDataManagementLogin = (to: RouteLocationNormalized) => {
 /**
  * Navigating to another page ends an open edit session, so it goes through
  * the leave flow first. Staying on the page (the editor rewrites its own URL
- * as the user works) is not an exit.
+ * as the user works) is not an exit. It is the last guard, so a navigation
+ * another guard redirects never asks.
  *
  * Only the resume pointer is cleared here. The page being left unmounts the
  * editor, which resets the rest; clearing the edit target now would have the
@@ -51,8 +52,6 @@ export const leaveSessionGuard: RouteGuard = async (to, from) => {
 
 /** Guards are executed in the order they appear in this array */
 export const guards: RouteGuard[] = [
-  leaveSessionGuard,
-
   (to) => {
     if (!to.meta?.hasAuthGuard) return null
     if (hs.session?.isAuthenticated) return null
@@ -100,63 +99,66 @@ export const guards: RouteGuard[] = [
     }
   },
 
-  // https://www.digitalocean.com/community/tutorials/vuejs-vue-router-modify-head
-  // Append head tags and update page title
-  (to, from) => {
-    // This goes through the matched routes from last to first, finding the closest route with a title.
-    // e.g., if we have `/some/deep/nested/route` and `/some`, `/deep`, and `/nested` have titles,
-    // `/nested`'s will be chosen.
-    const nearestWithTitle = to.matched
-      .slice()
-      .reverse()
-      .find((r) => r.meta && r.meta.title)
-
-    // Find the nearest route element with meta tags.
-    const nearestWithMeta = to.matched
-      .slice()
-      .reverse()
-      .find((r) => r.meta && r.meta.metaTags)
-
-    const previousNearestWithMeta = from?.matched
-      .slice()
-      .reverse()
-      .find((r) => r.meta && r.meta.metaTags)
-
-    // If a route with a title was found, set the document (page) title to that value.
-    if (nearestWithTitle) {
-      document.title = `HydroServer | ${nearestWithTitle.meta.title}`
-    } else if (previousNearestWithMeta) {
-      document.title = previousNearestWithMeta.meta.title as string
-    } else {
-      document.title = `HydroServer`
-    }
-
-    // Remove any stale meta tags from the document using the key attribute we set below.
-    Array.from(document.querySelectorAll('[data-vue-router-controlled]')).map(
-      (el) => el.parentNode?.removeChild(el)
-    )
-
-    // Skip rendering meta tags if there are none.
-    if (!nearestWithMeta) return null
-
-    // Turn the meta tag definitions into actual elements in the head.
-    // @ts-ignore
-    nearestWithMeta.meta.metaTags
-      .map((tagDef: any) => {
-        const tag = document.createElement('meta')
-
-        Object.keys(tagDef).forEach((key) => {
-          tag.setAttribute(key, tagDef[key])
-        })
-
-        // We use this to track which meta tags we create so we don't interfere with other ones.
-        tag.setAttribute('data-vue-router-controlled', '')
+  leaveSessionGuard,
+]
 
-        return tag
+// https://www.digitalocean.com/community/tutorials/vuejs-vue-router-modify-head
+// Append head tags and update page title
+export function updateHead(
+  to: RouteLocationNormalized,
+  from: RouteLocationNormalized
+) {
+  // This goes through the matched routes from last to first, finding the closest route with a title.
+  // e.g., if we have `/some/deep/nested/route` and `/some`, `/deep`, and `/nested` have titles,
+  // `/nested`'s will be chosen.
+  const nearestWithTitle = to.matched
+    .slice()
+    .reverse()
+    .find((r) => r.meta && r.meta.title)
+
+  // Find the nearest route element with meta tags.
+  const nearestWithMeta = to.matched
+    .slice()
+    .reverse()
+    .find((r) => r.meta && r.meta.metaTags)
+
+  const previousNearestWithMeta = from?.matched
+    .slice()
+    .reverse()
+    .find((r) => r.meta && r.meta.metaTags)
+
+  // If a route with a title was found, set the document (page) title to that value.
+  if (nearestWithTitle) {
+    document.title = `HydroServer | ${nearestWithTitle.meta.title}`
+  } else if (previousNearestWithMeta) {
+    document.title = previousNearestWithMeta.meta.title as string
+  } else {
+    document.title = `HydroServer`
+  }
+
+  // Remove any stale meta tags from the document using the key attribute we set below.
+  Array.from(document.querySelectorAll('[data-vue-router-controlled]')).map(
+    (el) => el.parentNode?.removeChild(el)
+  )
+
+  // Skip rendering meta tags if there are none.
+  if (!nearestWithMeta) return
+
+  // Turn the meta tag definitions into actual elements in the head.
+  // @ts-ignore
+  nearestWithMeta.meta.metaTags
+    .map((tagDef: any) => {
+      const tag = document.createElement('meta')
+
+      Object.keys(tagDef).forEach((key) => {
+        tag.setAttribute(key, tagDef[key])
       })
-      // Add the meta tags to the document head.
-      .forEach((tag: any) => document.head.appendChild(tag))
 
-    return null
-  },
-]
+      // We use this to track which meta tags we create so we don't interfere with other ones.
+      tag.setAttribute('data-vue-router-controlled', '')
+
+      return tag
+    })
+    // Add the meta tags to the document head.
+    .forEach((tag: any) => document.head.appendChild(tag))
+}
diff --git a/apps/qc-app/src/router/router.ts b/apps/qc-app/src/router/router.ts
index 853a6d53..97333578 100644
--- a/apps/qc-app/src/router/router.ts
+++ b/apps/qc-app/src/router/router.ts
@@ -1,6 +1,6 @@
 import { routes } from '@/router/routes'
 import { createRouter, createWebHistory } from 'vue-router'
-import { guards } from '@/router/guards'
+import { guards, updateHead } from '@/router/guards'
 
 const router = createRouter({
   history: createWebHistory('/qc/'),
@@ -8,9 +8,7 @@ const router = createRouter({
 })
 
 export function setupRouteGuards() {
-  // Return the guard's result rather than calling `next()`. The callback
-  // form is deprecated in vue-router. `false` cancels, a route location
-  // redirects, and null/undefined proceeds to the next guard.
+  // `false` cancels, a location redirects, and null/undefined continues.
   guards.forEach((fn) => {
     router.beforeEach(async (to, from) => {
       const result = await fn(to, from)
@@ -19,6 +17,9 @@ export function setupRouteGuards() {
       return result
     })
   })
+  router.afterEach((to, from, failure) => {
+    if (!failure) updateHead(to, from)
+  })
 }
 
 export default router
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
index e20734f9..676c5ffc 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
@@ -41,13 +41,13 @@ const committedSession = {
 const ops = (methods: string[]) =>
   methods.map((operationType) => ({ operationType, arguments: [] }))
 
-function makeDeps(base = rec()) {
+function makeDeps(base = rec(), committed: QualityControlSession[] = []) {
   let captured: QcHistory | undefined
   const fetchInRange = vi.fn().mockResolvedValue(base)
   const deps = {
     qcSessions: {
       get: vi.fn(async () => ({ ok: true, data: committedSession })),
-      list: vi.fn(async () => ({ ok: true, data: [] })),
+      list: vi.fn(async () => ({ ok: true, data: committed })),
     },
     qcOperations: {
       list: vi.fn(async () => ({
@@ -60,7 +60,6 @@ function makeDeps(base = rec()) {
       captured = h
       return { applied: h.operations.length, failed: [] }
     }),
-    cloneRecord: async (r: ObservationRecord) => r,
   } as unknown as ReconstructSessionDeps
 
   return { base, captured: () => captured, deps, fetchInRange }
@@ -86,7 +85,7 @@ describe('buildSnapshotRecord', () => {
       'SELECTION',
       'DELETE_POINTS',
     ])
-    expect(out).toBe(base)
+    expect(Array.from(out.dataX)).toEqual(Array.from(base.dataX))
   })
 
   it('replays nothing for an in-progress baseline snapshot', async () => {
@@ -105,7 +104,8 @@ describe('buildSnapshotRecord', () => {
   })
 
   it('loads the in-progress base over the session window', async () => {
-    const { deps, fetchInRange } = makeDeps()
+    // A commit over the same window, so the base comes from the managed datastream.
+    const { deps, fetchInRange } = makeDeps(rec(), [committedSession])
 
     await buildSnapshotRecord(deps, {
       historyId: 'h-1',
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/cloneRecord.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/cloneRecord.spec.ts
deleted file mode 100644
index 657a789a..00000000
--- a/apps/qc-app/src/services/qualityControl/__tests__/cloneRecord.spec.ts
+++ /dev/null
@@ -1,23 +0,0 @@
-import { describe, it, expect } from 'vitest'
-import { ObservationRecord } from '@uwrl/qc-utils'
-import { cloneRecord } from '../cloneRecord'
-
-describe('cloneRecord', () => {
-  it('copies the data into an independent record', async () => {
-    const original = new ObservationRecord({
-      datetimes: [1, 2, 3],
-      dataValues: [10, 20, 30],
-    })
-    await original.reload()
-
-    const copy = await cloneRecord(original)
-
-    expect(copy).not.toBe(original)
-    expect(Array.from(copy.dataX)).toEqual([1, 2, 3])
-    expect(Array.from(copy.dataY)).toEqual([10, 20, 30])
-    expect(copy.history).toHaveLength(0)
-
-    copy.dataY[0] = 99
-    expect(original.dataY[0]).toBe(10)
-  })
-})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/commitSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/commitSession.spec.ts
index e96db915..29293180 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/commitSession.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/commitSession.spec.ts
@@ -21,7 +21,7 @@ const setup = async () => {
 }
 
 describe('commitQcSession', () => {
-  it('checks checksum C, pushes observations, then commits', async () => {
+  it('checks the source checksum, pushes observations, then commits', async () => {
     const { qc, historyId, session } = await setup()
     const order: string[] = []
     const pushObservations = vi.fn(async () => {
@@ -46,7 +46,7 @@ describe('commitQcSession', () => {
     )
   })
 
-  it('blocks the commit on a checksum-C mismatch (no push, stays in progress)', async () => {
+  it('blocks the commit when the source checksum changed (no push, stays in progress)', async () => {
     const { qc, historyId, session } = await setup()
     const pushObservations = vi.fn()
 
@@ -58,7 +58,7 @@ describe('commitQcSession', () => {
         currentSourceChecksum: 'changed-checksum',
         pushObservations,
       })
-    ).rejects.toThrow(/checksum C/)
+    ).rejects.toThrow(/source data changed/)
 
     expect(pushObservations).not.toHaveBeenCalled()
     expect(unwrap(await qc.sessions.get(historyId, session.id)).status).toBe(
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
index 1de1ab7e..52ce5040 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/createManagedDatastream.spec.ts
@@ -2,7 +2,6 @@ import { describe, it, expect, vi } from 'vitest'
 import type { Datastream, HydroServer } from '@hydroserver/client'
 import { makeQcFake } from './qcServiceFake'
 import {
-  SOURCE_TAG_KEY,
   buildManagedDatastreamBody,
   createManagedDatastream,
 } from '../createManagedDatastream'
@@ -37,9 +36,8 @@ const makeHs = (
   { ok = true, status = 201, message = '' } = {}
 ) => {
   const create = vi.fn().mockResolvedValue({ data: created, ok, status, message })
-  const createTag = vi.fn().mockResolvedValue({})
-  const hs = { datastreams: { create, createTag } } as unknown as HydroServer
-  return { hs, create, createTag }
+  const hs = { datastreams: { create } } as unknown as HydroServer
+  return { hs, create }
 }
 
 describe('buildManagedDatastreamBody', () => {
@@ -101,10 +99,10 @@ describe('buildManagedDatastreamBody', () => {
 })
 
 describe('createManagedDatastream', () => {
-  it('creates the datastream, the history, and tags the source', async () => {
+  it('creates the datastream and the history', async () => {
     const source = makeSource()
     const created = makeSource({ id: 'managed-1', processingLevelId: 'pl-qc', valueCount: 0 })
-    const { hs, create, createTag } = makeHs(created)
+    const { hs, create } = makeHs(created)
     const qc = makeQcFake()
 
     const result = await createManagedDatastream(hs, qc.histories, {
@@ -122,12 +120,6 @@ describe('createManagedDatastream', () => {
     expect(result.history.managedDatastream.id).toBe('managed-1')
     expect(result.history.sourceDatastream.id).toBe('source-1')
     expect(unwrap(await qc.histories.list())).toHaveLength(1)
-
-    // source datastream tagged with the managed id
-    expect(createTag).toHaveBeenCalledWith('source-1', {
-      key: SOURCE_TAG_KEY,
-      value: 'managed-1',
-    })
   })
 
   it('requests the expanded datastream so the catalog keeps its nested relations', async () => {
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
index e947d68d..984d1d1f 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/persistOperations.spec.ts
@@ -1,8 +1,7 @@
-import { describe, it, expect } from 'vitest'
+import { describe, it, expect, vi } from 'vitest'
 import type { QcHistoryOperation } from '@uwrl/qc-utils'
 import { makeQcFake } from './qcServiceFake'
 import {
-  sessionOperationsFromSerialized,
   persistSessionOperations,
 } from '../persistOperations'
 import { unwrap } from '../unwrap'
@@ -36,13 +35,16 @@ const listOps = async (
   sessionId: string
 ) => unwrap(await qc.operations.list(historyId, sessionId))
 
-describe('sessionOperationsFromSerialized', () => {
-  it('maps method/args to operationType/arguments, assigns order, drops execution', () => {
-    const serialized = [
+describe('persistSessionOperations: wire shape', () => {
+  it('sends method/args as operationType/arguments in order, without execution', async () => {
+    const qc = makeQcFake()
+    const { historyId, sessionId } = await sessionWith(qc)
+    const create = vi.spyOn(qc.operations, 'create')
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
       { method: 'VALUE_THRESHOLD', args: [{ min: 0 }], execution: { status: 'success' } },
       { method: 'DELETE_POINTS', args: [] },
-    ] as unknown as QcHistoryOperation[]
-    expect(sessionOperationsFromSerialized(serialized)).toEqual([
+    ] as unknown as QcHistoryOperation[])
+    expect(create.mock.calls[0]![2]).toEqual([
       { operationType: 'VALUE_THRESHOLD', arguments: [{ min: 0 }], order: 0 },
       { operationType: 'DELETE_POINTS', arguments: [], order: 1 },
     ])
@@ -131,6 +133,64 @@ describe('persistSessionOperations', () => {
     expect(ops.map((o) => o.operationType)).toEqual(['SELECTION', 'DELETE_POINTS'])
   })
 
+  it('replaces an undone operation with the one applied after it', async () => {
+    const qc = makeQcFake()
+    const { historyId, sessionId } = await sessionWith(qc)
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('SELECTION'),
+      op('DELETE_POINTS'),
+      op('CHANGE'),
+    ])
+    const keptIds = (await listOps(qc, historyId, sessionId))
+      .slice(0, 2)
+      .map((o) => o.id)
+
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('SELECTION'),
+      op('DELETE_POINTS'),
+      op('INTERPOLATE'),
+    ])
+    const ops = await listOps(qc, historyId, sessionId)
+
+    expect(ops.map((o) => o.operationType)).toEqual([
+      'SELECTION',
+      'DELETE_POINTS',
+      'INTERPOLATE',
+    ])
+    expect(ops.map((o) => o.order)).toEqual([0, 1, 2])
+    expect(ops.slice(0, 2).map((o) => o.id)).toEqual(keptIds)
+  })
+
+  it('replaces an operation whose arguments changed', async () => {
+    const qc = makeQcFake()
+    const { historyId, sessionId } = await sessionWith(qc)
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('VALUE_THRESHOLD', [{ min: 0 }]),
+      op('DELETE_POINTS'),
+    ])
+    await persistSessionOperations(qc.operations, historyId, sessionId, [
+      op('VALUE_THRESHOLD', [{ min: 5 }]),
+      op('DELETE_POINTS'),
+    ])
+    const ops = await listOps(qc, historyId, sessionId)
+
+    expect(ops.map((o) => o.arguments)).toEqual([[{ min: 5 }], []])
+    expect(ops.map((o) => o.order)).toEqual([0, 1])
+  })
+
+  it('keeps an operation whose arguments only differ in wire form', async () => {
+    const qc = makeQcFake()
+    const { historyId, sessionId } = await sessionWith(qc)
+    const ops = [op('SHIFT_DATETIMES', [new Date('2025-01-05T00:00:00Z'), { b: 1, a: 2 }])]
+    await persistSessionOperations(qc.operations, historyId, sessionId, ops)
+    const [first] = await listOps(qc, historyId, sessionId)
+
+    await persistSessionOperations(qc.operations, historyId, sessionId, ops)
+    const [second] = await listOps(qc, historyId, sessionId)
+
+    expect(second.id).toBe(first.id)
+  })
+
   it('clears operations when given an empty set', async () => {
     const qc = makeQcFake()
     const { historyId, sessionId } = await sessionWith(qc)
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts b/apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts
index 5eedd957..bf831510 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/qcServiceFake.ts
@@ -229,7 +229,9 @@ export function makeQcFake(createdBy: QcContact = TEST_USER): QcFake {
             createdAt: '2025-01-01T00:00:00Z',
             createdBy,
             comment: b.comment ?? null,
-            arguments: b.arguments ?? null,
+            // Round-trip like the wire: Dates become ISO strings.
+            arguments:
+              b.arguments == null ? null : JSON.parse(JSON.stringify(b.arguments)),
           }) as unknown as QcOperation
       )
       s.operations.push(...created)
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
index 0cb5d93b..6aa88bcb 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/reconstructSession.spec.ts
@@ -10,8 +10,6 @@ const win = (start: string, end: string) => ({
   phenomenonTimeEnd: end,
 })
 
-// `redoStack` and `reload` exist on every real record; `loadLatestBase`
-// uses them to reset a cached one back to its stored state.
 const rec = (dataX: number[]) =>
   ({
     dataX,
@@ -33,18 +31,24 @@ const managed = { id: 'm-1' } as unknown as Datastream
 const source = { id: 's-1' } as unknown as Datastream
 
 describe('reconstructSession', () => {
-  it('uses the managed datastream as the base and replays only this session ops', async () => {
+  const RANGE = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
+  const noOps = vi.fn(async (_record: ObservationRecord, _history: QcHistory) => ({
+    applied: 0,
+    failed: [],
+  }))
+
+  it('bases a window a commit covered on the managed datastream and replays only this session ops', async () => {
     const qc = makeQcFake()
     const historyId = await newHistory(qc)
-    const range = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
-    const s = unwrap(await qc.sessions.create(historyId, range))
+    const earlier = unwrap(await qc.sessions.create(historyId, RANGE))
+    await qc.sessions.commit(historyId, earlier.id)
+    const s = unwrap(await qc.sessions.create(historyId, RANGE))
     await qc.operations.create(historyId, s.id, [
       { operationType: 'SELECTION' as any, order: 0 },
       { operationType: 'INTERPOLATE' as any, order: 1 },
     ])
 
-    const managedBase = rec([Date.UTC(2025, 0, 1)])
-    const fetchInRange = vi.fn().mockResolvedValue(managedBase)
+    const fetchInRange = vi.fn().mockResolvedValue(rec([Date.UTC(2025, 0, 1)]))
     let captured: QcHistory | undefined
     const applyHistory = vi.fn(
       async (_rec: ObservationRecord, history: QcHistory) => {
@@ -54,96 +58,57 @@ describe('reconstructSession', () => {
     )
 
     const result = await reconstructSession(
-      {
-        qcSessions: qc.sessions,
-        qcOperations: qc.operations,
-        fetchInRange,
-        applyHistory,
-        cloneRecord: async (r: ObservationRecord) => r,
-      },
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
       managed,
       source,
       historyId,
       s.id
     )
 
-    // Base comes from the managed datastream (latest committed state).
+    expect(fetchInRange).toHaveBeenCalledTimes(1)
     const [ds, begin, end] = fetchInRange.mock.calls[0]
     expect(ds).toBe(managed)
     expect((begin as Date).toISOString()).toBe('2025-01-01T00:00:00.000Z')
     expect((end as Date).toISOString()).toBe('2025-02-01T00:00:00.000Z')
-
-    // Only this session's own ops are replayed (ancestors are baked into
-    // the managed datastream).
-    expect(captured?.window).toEqual({
-      startDate: range.phenomenonTimeStart,
-      endDate: range.phenomenonTimeEnd,
-    })
+    // Ancestors are already in the managed datastream.
     expect(captured?.operations.map((o) => o.method)).toEqual([
       'SELECTION',
       'INTERPOLATE',
     ])
-    expect(result.record).toBe(managedBase)
+    expect(Array.from(result.record.dataX)).toEqual([Date.UTC(2025, 0, 1)])
     expect(result.report.applied).toBe(2)
   })
 
-  it('falls back to the source when the managed datastream has no committed data', async () => {
+  it('bases an uncommitted window on the source', async () => {
     const qc = makeQcFake()
     const historyId = await newHistory(qc)
-    const s = unwrap(
-      await qc.sessions.create(
-        historyId,
-        win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
-      )
-    )
-
-    const sourceRec = rec([Date.UTC(2025, 0, 1)])
-    const fetchInRange = vi
-      .fn()
-      .mockResolvedValueOnce(rec([])) // managed: nothing committed yet
-      .mockResolvedValueOnce(sourceRec) // source fallback
-    const applyHistory = vi.fn(async () => ({ applied: 0, failed: [] }))
+    const s = unwrap(await qc.sessions.create(historyId, RANGE))
+    const fetchInRange = vi.fn().mockResolvedValue(rec([Date.UTC(2025, 0, 1)]))
 
-    const result = await reconstructSession(
-      {
-        qcSessions: qc.sessions,
-        qcOperations: qc.operations,
-        fetchInRange,
-        applyHistory,
-        cloneRecord: async (r: ObservationRecord) => r,
-      },
+    await reconstructSession(
+      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory: noOps },
       managed,
       source,
       historyId,
       s.id
     )
 
-    expect(fetchInRange).toHaveBeenCalledTimes(2)
-    expect(fetchInRange.mock.calls[0][0]).toBe(managed)
-    expect(fetchInRange.mock.calls[1][0]).toBe(source)
-    expect(result.record).toBe(sourceRec)
+    expect(fetchInRange).toHaveBeenCalledTimes(1)
+    expect(fetchInRange.mock.calls[0][0]).toBe(source)
   })
 
-  it('replays onto the cloned base, not the fetched record', async () => {
+  it('replays onto a copy, not the fetched record', async () => {
     const qc = makeQcFake()
     const historyId = await newHistory(qc)
-    const s = unwrap(
-      await qc.sessions.create(historyId, win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'))
-    )
+    const s = unwrap(await qc.sessions.create(historyId, RANGE))
     const fetched = rec([Date.UTC(2025, 0, 1)])
-    const copy = rec([Date.UTC(2025, 0, 1)])
-    const applyHistory = vi.fn(async (_record: ObservationRecord, _history: QcHistory) => ({
-      applied: 0,
-      failed: [],
-    }))
 
     const result = await reconstructSession(
       {
         qcSessions: qc.sessions,
         qcOperations: qc.operations,
         fetchInRange: vi.fn().mockResolvedValue(fetched),
-        applyHistory,
-        cloneRecord: async () => copy,
+        applyHistory: noOps,
       },
       managed,
       source,
@@ -151,227 +116,119 @@ describe('reconstructSession', () => {
       s.id
     )
 
-    expect(result.record).toBe(copy)
-    expect(applyHistory.mock.calls[0]?.[0]).toBe(copy)
+    expect(result.record).not.toBe(fetched)
+    expect(noOps.mock.calls.at(-1)?.[0]).toBe(result.record)
   })
 })
 
 describe('reconstructCommittedSession', () => {
-  /** Two committed sessions over the same window, the second chained to the first. */
-  const chainOf = async (
-    qc: ReturnType<typeof makeQcFake>,
-    firstRange = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'),
-    secondRange = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
-  ) => {
+  const day = (d: number) => Date.UTC(2025, 0, d)
+  const iso = (d: number) => new Date(day(d)).toISOString()
+  const DAYS = Array.from({ length: 10 }, (_, i) => i + 1)
+  // Deletes the first point of the session's own window.
+  const DELETE_FIRST = [
+    { operationType: 'SELECTION', arguments: [[0]], order: 0 },
+    { operationType: 'DELETE_POINTS', arguments: [], order: 1 },
+  ]
+
+  const setup = async () => {
+    const { ObservationRecord: Record, applyHistory } = await import('@uwrl/qc-utils')
+    const sourceRecord = new Record({
+      datetimes: DAYS.map(day),
+      dataValues: DAYS,
+    })
+    await sourceRecord.reload()
+    const qc = makeQcFake()
     const historyId = await newHistory(qc)
-    const first = unwrap(await qc.sessions.create(historyId, firstRange))
-    await qc.operations.create(historyId, first.id, [
-      { operationType: 'SELECTION' as any, order: 0 },
-    ])
-    await qc.sessions.commit(historyId, first.id)
-
-    const second = unwrap(await qc.sessions.create(historyId, secondRange))
-    await qc.operations.create(historyId, second.id, [
-      { operationType: 'DELETE_POINTS' as any, order: 0 },
-      { operationType: 'INTERPOLATE' as any, order: 1 },
-    ])
-    await qc.sessions.commit(historyId, second.id)
-    return { historyId, first, second }
-  }
-
-  const spies = () => {
-    const base = rec([Date.UTC(2025, 0, 1)])
-    const fetchInRange = vi.fn().mockResolvedValue(base)
-    let captured: QcHistory | undefined
-    const applyHistory = vi.fn(
-      async (record: ObservationRecord, history: QcHistory) => {
-        captured = history
-        // Stand in for the engine: one history entry per replayed op.
-        history.operations.forEach((op) =>
-          (record.history as any[]).push({ method: op.method })
-        )
-        return { applied: history.operations.length, failed: [] }
-      }
-    )
-    return { base, fetchInRange, applyHistory, captured: () => captured }
+    const commit = async (from: number, to: number, ops = DELETE_FIRST) => {
+      const s = unwrap(await qc.sessions.create(historyId, win(iso(from), iso(to))))
+      await qc.operations.create(historyId, s.id, ops as any)
+      await qc.sessions.commit(historyId, s.id)
+      return s
+    }
+    const fetchInRange = vi.fn(async () => sourceRecord)
+    const view = async (sessionId: string, opLimit?: number) => {
+      const { reconstructCommittedSession } = await import('../reconstructSession')
+      return reconstructCommittedSession(
+        { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
+        source,
+        historyId,
+        sessionId,
+        opLimit
+      )
+    }
+    return { commit, view, fetchInRange, sourceRecord }
   }
+  const days = (record: ObservationRecord) =>
+    Array.from(record.dataX).map((t) => new Date(t).getUTCDate())
 
-  it('replays the ancestor chain from the raw source, not the managed datastream', async () => {
-    const qc = makeQcFake()
-    const { historyId, second } = await chainOf(qc)
-    const { fetchInRange, applyHistory, captured } = spies()
-    const { reconstructCommittedSession } = await import('../reconstructSession')
+  it('rebuilds a later session on its own window', async () => {
+    const { commit, view } = await setup()
+    await commit(1, 10)
+    const later = await commit(5, 10)
 
-    await reconstructCommittedSession(
-      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
-      source,
-      historyId,
-      second.id
-    )
+    const { record } = await view(later.id)
 
-    // The managed datastream already carries every commit, so the base has
-    // to be the raw source or the replay reproduces the final state.
-    expect(fetchInRange).toHaveBeenCalledTimes(1)
-    expect(fetchInRange.mock.calls[0][0]).toBe(source)
-    // Ancestor first, then the viewed session's own operations.
-    expect(captured()!.operations.map((o) => o.method)).toEqual([
-      'SELECTION',
-      'DELETE_POINTS',
-      'INTERPOLATE',
-    ])
+    expect(days(record)).toEqual([6, 7, 8, 9, 10])
+    expect(record.history.map((h) => h.method)).toEqual(['SELECTION', 'DELETE_POINTS'])
   })
 
-  it('loads the union window so a wider ancestor replays in alignment', async () => {
-    const qc = makeQcFake()
-    const { historyId, second } = await chainOf(
-      qc,
-      win('2024-12-01T00:00:00Z', '2025-03-01T00:00:00Z'),
-      win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
-    )
-    const { fetchInRange, applyHistory } = spies()
-    const { reconstructCommittedSession } = await import('../reconstructSession')
+  it('takes the part of a window past earlier commits from the source', async () => {
+    const { commit, view } = await setup()
+    await commit(1, 5)
+    const later = await commit(3, 10)
 
-    await reconstructCommittedSession(
-      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
-      source,
-      historyId,
-      second.id
-    )
+    const { record } = await view(later.id)
 
-    const [, begin, end] = fetchInRange.mock.calls[0]
-    expect((begin as Date).toISOString()).toBe('2024-12-01T00:00:00.000Z')
-    expect((end as Date).toISOString()).toBe('2025-03-01T00:00:00.000Z')
+    expect(days(record)).toEqual([4, 5, 6, 7, 8, 9, 10])
   })
 
-  it('leaves only the viewed session operations on display', async () => {
-    const qc = makeQcFake()
-    const { historyId, second } = await chainOf(qc)
-    const { fetchInRange, applyHistory } = spies()
-    const { reconstructCommittedSession } = await import('../reconstructSession')
+  it('leaves out sessions committed after the viewed one', async () => {
+    const { commit, view } = await setup()
+    const first = await commit(1, 10)
+    await commit(5, 10)
 
-    const { record } = await reconstructCommittedSession(
-      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
-      source,
-      historyId,
-      second.id
-    )
-
-    // Three ops were replayed to build the data; only this session's two show.
-    expect((record.history as any[]).map((h) => h.method)).toEqual([
-      'DELETE_POINTS',
-      'INTERPOLATE',
-    ])
-  })
-
-  it('reconstructs the first session without its later commits applied', async () => {
-    const qc = makeQcFake()
-    const { historyId, first } = await chainOf(qc)
-    const { fetchInRange, applyHistory, captured } = spies()
-    const { reconstructCommittedSession } = await import('../reconstructSession')
-
-    const { record } = await reconstructCommittedSession(
-      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
-      source,
-      historyId,
-      first.id
-    )
-
-    // The later session is a dependent, not an ancestor, so it stays out.
-    expect(captured()!.operations.map((o) => o.method)).toEqual(['SELECTION'])
-    expect((record.history as any[]).map((h) => h.method)).toEqual(['SELECTION'])
-  })
-
-  it('truncates the viewed session operations to opLimit', async () => {
-    const qc = makeQcFake()
-    const { historyId, second } = await chainOf(qc)
-    const { fetchInRange, applyHistory, captured } = spies()
-    const { reconstructCommittedSession } = await import('../reconstructSession')
-
-    const { record } = await reconstructCommittedSession(
-      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
-      source,
-      historyId,
-      second.id,
-      1
-    )
+    const { record } = await view(first.id)
 
-    // Ancestors are never truncated; only this session stops early.
-    expect(captured()!.operations.map((o) => o.method)).toEqual([
-      'SELECTION',
-      'DELETE_POINTS',
-    ])
-    expect((record.history as any[]).map((h) => h.method)).toEqual([
-      'DELETE_POINTS',
-    ])
+    expect(days(record)).toEqual([2, 3, 4, 5, 6, 7, 8, 9, 10])
   })
 
-  it('replays ancestors only when opLimit is 0', async () => {
-    const qc = makeQcFake()
-    const { historyId, second } = await chainOf(qc)
-    const { fetchInRange, applyHistory, captured } = spies()
-    const { reconstructCommittedSession } = await import('../reconstructSession')
+  it('stops the viewed session at opLimit, never its ancestors', async () => {
+    const { commit, view } = await setup()
+    await commit(1, 10)
+    const later = await commit(5, 10)
 
-    const { record } = await reconstructCommittedSession(
-      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
-      source,
-      historyId,
-      second.id,
-      0
-    )
+    const partway = await view(later.id, 1)
+    const baseline = await view(later.id, 0)
 
-    expect(captured()!.operations.map((o) => o.method)).toEqual(['SELECTION'])
-    expect(record.history as any[]).toEqual([])
+    expect(days(partway.record)).toEqual([5, 6, 7, 8, 9, 10])
+    expect(partway.record.history.map((h) => h.method)).toEqual(['SELECTION'])
+    expect(days(baseline.record)).toEqual([5, 6, 7, 8, 9, 10])
+    expect(baseline.record.history).toHaveLength(0)
   })
 
-  it('replays every operation when opLimit is omitted', async () => {
-    const qc = makeQcFake()
-    const { historyId, second } = await chainOf(qc)
-    const { fetchInRange, applyHistory, captured } = spies()
-    const { reconstructCommittedSession } = await import('../reconstructSession')
+  it('fetches the source once over the chain, never the managed datastream', async () => {
+    const { commit, view, fetchInRange } = await setup()
+    await commit(1, 5)
+    const later = await commit(3, 10)
 
-    await reconstructCommittedSession(
-      { qcSessions: qc.sessions, qcOperations: qc.operations, fetchInRange, applyHistory },
-      source,
-      historyId,
-      second.id
-    )
+    await view(later.id)
 
-    expect(captured()!.operations.map((o) => o.method)).toEqual([
-      'SELECTION',
-      'DELETE_POINTS',
-      'INTERPOLATE',
-    ])
+    expect(fetchInRange).toHaveBeenCalledTimes(1)
+    const [ds, begin, end] = fetchInRange.mock.calls[0] as unknown as [Datastream, Date, Date]
+    expect(ds).toBe(source)
+    expect(begin.getTime()).toBe(day(1))
+    expect(end.getTime()).toBe(day(10))
   })
 
-  it('replays onto a copy of the fetched record, not the record itself', async () => {
-    const qc = makeQcFake()
-    const { historyId, second } = await chainOf(qc)
-    const fetched = rec([Date.UTC(2025, 0, 1)])
-    const copy = rec([Date.UTC(2025, 0, 1)])
-    const fetchInRange = vi.fn().mockResolvedValue(fetched)
-    const applyHistory = vi.fn(async (_record: ObservationRecord, _history: QcHistory) => ({
-      applied: 0,
-      failed: [],
-    }))
-    const { reconstructCommittedSession } = await import('../reconstructSession')
+  it('never edits the fetched source record', async () => {
+    const { commit, view, sourceRecord } = await setup()
+    const first = await commit(1, 10)
 
-    const { record } = await reconstructCommittedSession(
-      {
-        qcSessions: qc.sessions,
-        qcOperations: qc.operations,
-        fetchInRange,
-        applyHistory,
-        cloneRecord: async () => copy,
-      },
-      source,
-      historyId,
-      second.id
-    )
+    await view(first.id)
 
-    expect(record).toBe(copy)
-    expect(applyHistory.mock.calls[0]?.[0]).toBe(copy)
-    expect(record).not.toBe(fetched)
-    expect(fetched.history).toHaveLength(0)
+    expect(days(sourceRecord)).toEqual(DAYS)
+    expect(sourceRecord.history).toHaveLength(0)
   })
 })
 
@@ -410,9 +267,9 @@ describe('reconstructCommittedSession: chain order', () => {
       list: vi.fn(async (_h: string, id: string) => ok(opsById[id])),
     } as any
 
-    let captured: QcHistory | undefined
+    const replayed: string[][] = []
     const applyHistory = vi.fn(async (_r: ObservationRecord, h: QcHistory) => {
-      captured = h
+      replayed.push(h.operations.map((o) => o.method))
       return { applied: h.operations.length, failed: [] }
     })
     const { reconstructCommittedSession } = await import('../reconstructSession')
@@ -429,10 +286,7 @@ describe('reconstructCommittedSession: chain order', () => {
       'viewed'
     )
 
-    expect(captured!.operations.map((o) => o.method)).toEqual([
-      'SELECTION',
-      'DELETE_POINTS',
-    ])
+    expect(replayed).toEqual([['SELECTION'], ['DELETE_POINTS']])
   })
 
   it('falls back to creation time when a session is not committed', async () => {
@@ -450,9 +304,9 @@ describe('reconstructCommittedSession: chain order', () => {
       list: vi.fn(async (_h: string, id: string) => ok(opsById[id])),
     } as any
 
-    let captured: QcHistory | undefined
+    const replayed: string[][] = []
     const applyHistory = vi.fn(async (_r: ObservationRecord, h: QcHistory) => {
-      captured = h
+      replayed.push(h.operations.map((o) => o.method))
       return { applied: h.operations.length, failed: [] }
     })
     const { reconstructCommittedSession } = await import('../reconstructSession')
@@ -469,10 +323,7 @@ describe('reconstructCommittedSession: chain order', () => {
       'viewed'
     )
 
-    expect(captured!.operations.map((o) => o.method)).toEqual([
-      'SELECTION',
-      'DELETE_POINTS',
-    ])
+    expect(replayed).toEqual([['SELECTION'], ['DELETE_POINTS']])
   })
 })
 
@@ -541,7 +392,6 @@ describe('reconstructSession with a real ObservationRecord', () => {
   it('replays a saved SELECTION and DELETE_POINTS onto a copy of the base', async () => {
     const { ObservationRecord: Record, applyHistory, serializeHistory } =
       await import('@uwrl/qc-utils')
-    const { sessionOperationsFromSerialized } = await import('../persistOperations')
     const qc = makeQcFake()
     const historyId = await newHistory(qc)
     const range = win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z')
@@ -552,8 +402,9 @@ describe('reconstructSession with a real ObservationRecord', () => {
     ]
     await qc.operations.create(historyId, s.id, persisted as any)
 
+    const days = [1, 2, 3, 4, 5, 6].map((d) => Date.UTC(2025, 0, d))
     const sourceRec = new Record({
-      datetimes: [1, 2, 3, 4, 5, 6],
+      datetimes: days,
       dataValues: [10, 20, 30, 40, 50, 60],
     })
     await sourceRec.reload()
@@ -570,16 +421,14 @@ describe('reconstructSession with a real ObservationRecord', () => {
     )
 
     expect(report).toEqual({ applied: 2, failed: [] })
-    expect(Array.from(record.dataX)).toEqual([1, 5, 6])
+    expect(Array.from(record.dataX)).toEqual([days[0], days[4], days[5]])
     expect(Array.from(record.dataY)).toEqual([10, 50, 60])
-    expect(Array.from(sourceRec.dataX)).toEqual([1, 2, 3, 4, 5, 6])
+    expect(Array.from(sourceRec.dataX)).toEqual(days)
     // Saving the replayed history yields the same operations back.
-    const resaved = sessionOperationsFromSerialized(
-      serializeHistory(record, {
-        startDate: range.phenomenonTimeStart,
-        endDate: range.phenomenonTimeEnd,
-      }).operations
-    )
-    expect(resaved).toEqual(persisted)
+    const resaved = serializeHistory(record, {
+      startDate: range.phenomenonTimeStart,
+      endDate: range.phenomenonTimeEnd,
+    }).operations.map((op) => [op.method, op.args])
+    expect(resaved).toEqual(persisted.map((p) => [p.operationType, p.arguments]))
   })
 })
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/recordFrom.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/recordFrom.spec.ts
new file mode 100644
index 00000000..17022bb8
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/__tests__/recordFrom.spec.ts
@@ -0,0 +1,17 @@
+import { describe, it, expect } from 'vitest'
+import { recordFrom } from '../recordFrom'
+
+describe('recordFrom', () => {
+  it('holds its own copy of the data', async () => {
+    const datetimes = [1, 2, 3]
+    const dataValues = [10, 20, 30]
+
+    const record = await recordFrom(datetimes, dataValues)
+
+    expect(Array.from(record.dataX)).toEqual([1, 2, 3])
+    expect(Array.from(record.dataY)).toEqual([10, 20, 30])
+    expect(record.history).toHaveLength(0)
+    record.dataY[0] = 99
+    expect(dataValues[0]).toBe(10)
+  })
+})
diff --git a/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
index 1415579d..539481b9 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/session.spec.ts
@@ -5,7 +5,6 @@ import { makeQcFake } from './qcServiceFake'
 import {
   getInProgressSession,
   startOrResumeSession,
-  loadSourceWindow,
   loadLatestBase,
 } from '../session'
 import { unwrap } from '../unwrap'
@@ -70,72 +69,96 @@ describe('startOrResumeSession', () => {
   })
 })
 
-describe('loadSourceWindow', () => {
-  it('fetches the session window from the source datastream', async () => {
-    const record = {} as ObservationRecord
-    const fetchInRange = vi.fn().mockResolvedValue(record)
-    const source = { id: 's-1' } as unknown as Datastream
-
-    const result = await loadSourceWindow(fetchInRange, source, WIN)
-
-    expect(result).toBe(record)
-    expect(fetchInRange).toHaveBeenCalledTimes(1)
-    const [ds, begin, end] = fetchInRange.mock.calls[0]
-    expect(ds).toBe(source)
-    expect((begin as Date).toISOString()).toBe('2025-01-01T00:00:00.000Z')
-    expect((end as Date).toISOString()).toBe('2025-02-01T00:00:00.000Z')
-  })
-})
-
 describe('loadLatestBase', () => {
   const managed = { id: 'm-1' } as unknown as Datastream
   const source = { id: 's-1' } as unknown as Datastream
-  const start = new Date('2025-01-01T00:00:00Z')
-  const end = new Date('2025-02-01T00:00:00Z')
-  const recWith = (n: number) =>
-    ({ dataX: Array(n).fill(0), dataY: Array(n).fill(0), history: [] }) as unknown as ObservationRecord
-  const copyOf = (r: ObservationRecord) => ({ copyOf: r }) as unknown as ObservationRecord
+  const day = (m: number, d: number) => Date.UTC(2025, m, d)
+  const rec = (points: [number, number][]) =>
+    ({
+      dataX: points.map(([t]) => t),
+      dataY: points.map(([, v]) => v),
+      history: [],
+    }) as unknown as ObservationRecord
+  const fetchFrom = (byId: Record<string, ObservationRecord>) =>
+    vi.fn(async (ds: Datastream) => byId[ds.id]!)
+  const points = (r: ObservationRecord) =>
+    Array.from(r.dataX).map((t, i) => [t, r.dataY[i]])
+
+  it('uses the managed datastream where sessions were committed', async () => {
+    const fetchInRange = fetchFrom({ 'm-1': rec([[day(0, 10), 7]]) })
+
+    const base = await loadLatestBase(
+      fetchInRange,
+      managed,
+      source,
+      new Date(day(0, 1)),
+      new Date(day(0, 31)),
+      [[day(0, 1), day(0, 31)]]
+    )
+
+    expect(points(base)).toEqual([[day(0, 10), 7]])
+    expect(fetchInRange).toHaveBeenCalledTimes(1)
+    expect(fetchInRange.mock.calls[0]![0]).toBe(managed)
+  })
 
-  it('copies the managed datastream window when it has committed data', async () => {
-    const managedRec = recWith(3)
-    const fetchInRange = vi.fn().mockResolvedValue(managedRec)
-    const clone = vi.fn(async (r: ObservationRecord) => copyOf(r))
+  it('uses the source where nothing is committed', async () => {
+    const fetchInRange = fetchFrom({ 's-1': rec([[day(0, 10), 3]]) })
 
-    const result = await loadLatestBase(fetchInRange, managed, source, start, end, clone)
+    const base = await loadLatestBase(
+      fetchInRange,
+      managed,
+      source,
+      new Date(day(0, 1)),
+      new Date(day(0, 31)),
+      []
+    )
 
+    expect(points(base)).toEqual([[day(0, 10), 3]])
     expect(fetchInRange).toHaveBeenCalledTimes(1)
-    expect(fetchInRange.mock.calls[0][0]).toBe(managed)
-    expect(clone).toHaveBeenCalledWith(managedRec)
-    expect((result as any).copyOf).toBe(managedRec)
+    expect(fetchInRange.mock.calls[0]![0]).toBe(source)
   })
 
-  it('copies the source window when the managed datastream is empty', async () => {
-    const sourceRec = recWith(2)
-    const fetchInRange = vi
-      .fn()
-      .mockResolvedValueOnce(recWith(0))
-      .mockResolvedValueOnce(sourceRec)
-    const clone = vi.fn(async (r: ObservationRecord) => copyOf(r))
-
-    const result = await loadLatestBase(fetchInRange, managed, source, start, end, clone)
+  it('fills the uncommitted part of the window from the source', async () => {
+    // January was committed with the Jan 25 point deleted.
+    const fetchInRange = fetchFrom({
+      'm-1': rec([[day(0, 20), 7]]),
+      's-1': rec([
+        [day(0, 20), 3],
+        [day(0, 25), 4],
+        [day(1, 10), 5],
+      ]),
+    })
 
-    expect(fetchInRange).toHaveBeenCalledTimes(2)
-    expect(fetchInRange.mock.calls[1][0]).toBe(source)
-    expect((result as any).copyOf).toBe(sourceRec)
+    const base = await loadLatestBase(
+      fetchInRange,
+      managed,
+      source,
+      new Date(day(0, 15)),
+      new Date(day(2, 1)),
+      [[day(0, 1), day(0, 31)]]
+    )
+
+    expect(points(base)).toEqual([
+      [day(0, 20), 7],
+      [day(1, 10), 5],
+    ])
   })
 
   // The store hands back its cached record; editing a copy is what keeps a
   // plotted raw line free of draft edits.
   it('never returns the fetched record itself', async () => {
-    const sourceRec = recWith(2)
-    const fetchInRange = vi
-      .fn()
-      .mockResolvedValueOnce(recWith(0))
-      .mockResolvedValueOnce(sourceRec)
-    const clone = vi.fn(async (r: ObservationRecord) => copyOf(r))
-
-    const result = await loadLatestBase(fetchInRange, managed, source, start, end, clone)
-
-    expect(result).not.toBe(sourceRec)
+    const sourceRec = rec([[day(0, 10), 3]])
+    const fetchInRange = fetchFrom({ 's-1': sourceRec })
+
+    const base = await loadLatestBase(
+      fetchInRange,
+      managed,
+      source,
+      new Date(day(0, 1)),
+      new Date(day(0, 31)),
+      []
+    )
+
+    expect(base).not.toBe(sourceRec)
   })
 })
diff --git a/apps/qc-app/src/services/qualityControl/buildSnapshot.ts b/apps/qc-app/src/services/qualityControl/buildSnapshot.ts
index 032f62cc..7e72db45 100644
--- a/apps/qc-app/src/services/qualityControl/buildSnapshot.ts
+++ b/apps/qc-app/src/services/qualityControl/buildSnapshot.ts
@@ -14,7 +14,8 @@ import type {
   QcHistory,
   QcHistoryOperation,
 } from '@uwrl/qc-utils'
-import { loadLatestBase } from './session'
+import { committedWindows, loadLatestBase } from './session'
+import { unwrap } from './unwrap'
 import {
   reconstructCommittedSession,
   type ReconstructSessionDeps,
@@ -56,13 +57,16 @@ export async function buildSnapshotRecord(
     return record
   }
 
+  const committed = unwrap(
+    await deps.qcSessions.list(historyId, { status: 'committed', fetch_all: true })
+  )
   const record = await loadLatestBase(
     deps.fetchInRange,
     managed,
     source,
     new Date(session.phenomenonTimeStart),
     new Date(session.phenomenonTimeEnd),
-    deps.cloneRecord
+    committedWindows(committed)
   )
 
   const operations =
diff --git a/apps/qc-app/src/services/qualityControl/cloneRecord.ts b/apps/qc-app/src/services/qualityControl/cloneRecord.ts
deleted file mode 100644
index a040eef3..00000000
--- a/apps/qc-app/src/services/qualityControl/cloneRecord.ts
+++ /dev/null
@@ -1,18 +0,0 @@
-import { ObservationRecord } from '@uwrl/qc-utils'
-
-export type CloneRecord = (record: ObservationRecord) => Promise<ObservationRecord>
-
-/**
- * A standalone record holding a copy of `record`'s current window. The
- * observation store shares one record per datastream, so edits must happen
- * on a copy. `dataX`/`dataY` are views over a shared buffer, hence the copy.
- */
-export const cloneRecord: CloneRecord = async (record) => {
-  const copy = new ObservationRecord({
-    datetimes: Float64Array.from(record.dataX),
-    dataValues: Float32Array.from(record.dataY),
-  })
-  // The constructor starts loading without awaiting it.
-  await copy.reload()
-  return copy
-}
diff --git a/apps/qc-app/src/services/qualityControl/commitSession.ts b/apps/qc-app/src/services/qualityControl/commitSession.ts
index 8af5b6fd..52d1b8d1 100644
--- a/apps/qc-app/src/services/qualityControl/commitSession.ts
+++ b/apps/qc-app/src/services/qualityControl/commitSession.ts
@@ -1,17 +1,7 @@
 /**
- * Commit handshake for a QC session (spec section 9).
- *
- * Order of operations:
- *   1. ensure the session is in progress,
- *   2. checksum C: the source-window checksum fetched now (from the
- *      observations `X-Checksum` header) must equal the checksum captured
- *      when the session was created, otherwise the source data changed
- *      out-of-band and the commit is blocked,
- *   3. push the final observations to the managed datastream (replace mode),
- *   4. lock the session into the history.
- *
- * `pushObservations` is injected as a thunk so this unit-tests without the
- * HydroServer client. A/B integrity checks and chunked upload are deferred.
+ * Commit a QC session: check it is still in progress and that its source
+ * window hasn't changed since it started, push the observations, then lock
+ * the session into the history.
  */
 
 import type {
@@ -26,7 +16,7 @@ export interface CommitQcSessionInput {
   qcSessions: QualityControlSessionService
   historyId: string
   sessionId: string
-  /** Source-window checksum fetched now; compared to the session's checksum C. */
+  /** The source window's checksum now; compared to the one taken at session start. */
   currentSourceChecksum: string
   /** Pushes the final observations to the managed datastream (replace mode). */
   pushObservations: () => Promise<void>
@@ -46,8 +36,8 @@ export async function commitQcSession(
   }
   if (currentSourceChecksum !== session.sourceChecksum) {
     throw new Error(
-      'Source data changed since this session started (checksum C mismatch). ' +
-        'Re-fetch the source and verify your edits before committing again.'
+      'The source data changed since this session started, ' +
+        'so these edits may no longer line up with it.'
     )
   }
 
diff --git a/apps/qc-app/src/services/qualityControl/createManagedDatastream.ts b/apps/qc-app/src/services/qualityControl/createManagedDatastream.ts
index 69440390..07c1afd5 100644
--- a/apps/qc-app/src/services/qualityControl/createManagedDatastream.ts
+++ b/apps/qc-app/src/services/qualityControl/createManagedDatastream.ts
@@ -1,9 +1,8 @@
 /**
- * Orchestrates creating a managed datastream for QC editing (spec section 5):
+ * Orchestrates creating a managed datastream for QC editing:
  *   1. create an empty managed datastream from the source's metadata, with
  *      a new processing level,
- *   2. create the QC history linking source and managed,
- *   3. tag the source datastream so the app can mark it read-only.
+ *   2. create the QC history linking source and managed.
  *
  * Pure (deps injected) so it unit-tests without Pinia/Vuetify; a thin
  * composable wires it to the live HydroServer client and stores.
@@ -20,9 +19,6 @@ import { unwrap } from './unwrap'
 
 type QcHistoryDetail = QualityControlHistoryContract.DetailResponse
 
-/** Tag key placed on a source datastream, pointing at its managed datastream. */
-export const SOURCE_TAG_KEY = 'qc-managed-datastream'
-
 export interface CreateManagedDatastreamInput {
   /** Source datastream to derive the managed datastream from. */
   source: Datastream
@@ -120,10 +116,5 @@ export async function createManagedDatastream(
     })
   )
 
-  await hs.datastreams.createTag(source.id, {
-    key: SOURCE_TAG_KEY,
-    value: managedDatastream.id,
-  })
-
   return { managedDatastream, history }
 }
diff --git a/apps/qc-app/src/services/qualityControl/index.ts b/apps/qc-app/src/services/qualityControl/index.ts
index acd7376f..510921a5 100644
--- a/apps/qc-app/src/services/qualityControl/index.ts
+++ b/apps/qc-app/src/services/qualityControl/index.ts
@@ -17,4 +17,4 @@ export * from './reconstructSession'
 export * from './findHistory'
 export * from './observationsBody'
 export * from './unwrap'
-export * from './cloneRecord'
+export * from './recordFrom'
diff --git a/apps/qc-app/src/services/qualityControl/observationsBody.ts b/apps/qc-app/src/services/qualityControl/observationsBody.ts
index 3d13f02d..12ce4ded 100644
--- a/apps/qc-app/src/services/qualityControl/observationsBody.ts
+++ b/apps/qc-app/src/services/qualityControl/observationsBody.ts
@@ -1,5 +1,5 @@
 /** Bulk observations body for the replace-mode createObservations endpoint. */
-export interface ObservationBulkPostBody {
+interface ObservationBulkPostBody {
   fields: ('phenomenonTime' | 'result')[]
   data: unknown[][]
 }
diff --git a/apps/qc-app/src/services/qualityControl/persistOperations.ts b/apps/qc-app/src/services/qualityControl/persistOperations.ts
index 609f3769..8b2234a0 100644
--- a/apps/qc-app/src/services/qualityControl/persistOperations.ts
+++ b/apps/qc-app/src/services/qualityControl/persistOperations.ts
@@ -1,5 +1,5 @@
 /**
- * Persist an edit session's operations to the QC API (spec section 5-6).
+ * Persist an edit session's operations to the QC API.
  *
  * qc-utils' `serializeHistory` emits operations as `{ method, args }`; the
  * QC API speaks `{ operationType, arguments, order }`. The enum values are
@@ -7,6 +7,7 @@
  * `order` by position.
  */
 
+import { isEqual } from 'lodash-es'
 import type { QcHistoryOperation } from '@uwrl/qc-utils'
 import type {
   QualityControlOperationService,
@@ -30,28 +31,26 @@ function operationBody(op: QcHistoryOperation, order: number): QcOperationPostBo
 /** Normalized comment for comparison: blank and absent are both "no comment". */
 const commentOf = (value?: string | null): string | null => value?.trim() || null
 
-/**
- * Map qc-utils serialized operations to QC API operation bodies: rename
- * method/args to operationType/arguments and assign order by index.
- */
-export function sessionOperationsFromSerialized(
-  operations: QcHistoryOperation[]
-): QcOperationPostBody[] {
-  return operations.map((op, index) => operationBody(op, index))
+// Args are compared in their wire form (Dates as ISO strings), and `isEqual`
+// ignores key order, which JSONB does not preserve.
+function sameOperation(
+  persisted: QualityControlOperation,
+  local: QcHistoryOperation
+): boolean {
+  return (
+    persisted.operationType === local.method &&
+    isEqual(persisted.arguments, JSON.parse(JSON.stringify(local.args)))
+  )
 }
 
 /**
  * Reconcile a session's persisted operations with the current ordered set
  * (used when saving an in-progress session).
  *
- * Append-only: existing operations are left untouched so they keep their
+ * The persisted prefix that still matches is left untouched so it keeps its
  * original creator (drafts are shared, and the server stamps each create
- * request with the requesting user). Only operations added since the last
- * save are created; operations removed by undo are deleted from the tail.
- *
- * Limitation: reconciles by position, so a mid-history rewrite that keeps
- * the operation count but changes earlier operations is not re-persisted;
- * the common append / trailing-undo flow is.
+ * request with the requesting user). From the first operation that differs,
+ * persisted operations are deleted and the local ones appended.
  */
 export async function persistSessionOperations(
   qcOperations: QualityControlOperationService,
@@ -65,33 +64,36 @@ export async function persistSessionOperations(
     await qcOperations.list(historyId, sessionId, { fetch_all: true })
   )
 
-  // Undo: drop persisted operations past the current length (newest first).
-  for (let i = existing.length - 1; i >= operations.length; i--) {
-    const op = existing[i]
-    if (op) unwrap(await qcOperations.delete(historyId, sessionId, op.id))
+  let kept = 0
+  while (
+    kept < existing.length &&
+    kept < operations.length &&
+    sameOperation(existing[kept]!, operations[kept]!)
+  ) {
+    kept++
+  }
+
+  for (let i = existing.length - 1; i >= kept; i--) {
+    unwrap(await qcOperations.delete(historyId, sessionId, existing[i]!.id))
   }
 
-  // Comments are patched in place; the rest of an operation is append-only.
-  const retained = Math.min(existing.length, operations.length)
-  for (let i = 0; i < retained; i++) {
-    const persisted = existing[i]
-    const local = operations[i]
-    if (!persisted || !local) continue
-    const comment = commentOf(local.comment)
+  // Comments are patched in place on the kept prefix.
+  for (let i = 0; i < kept; i++) {
+    const persisted = existing[i]!
+    const comment = commentOf(operations[i]!.comment)
     if (commentOf(persisted.comment) === comment) continue
     unwrap(
       await qcOperations.update(historyId, sessionId, persisted.id, { comment })
     )
   }
 
-  // Append the operations added since the last save (server stamps creator).
-  const appended = operations.slice(existing.length)
+  const appended = operations.slice(kept)
   if (appended.length) {
     unwrap(
       await qcOperations.create(
         historyId,
         sessionId,
-        appended.map((op, index) => operationBody(op, existing.length + index))
+        appended.map((op, index) => operationBody(op, kept + index))
       )
     )
   }
diff --git a/apps/qc-app/src/services/qualityControl/reconstructSession.ts b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
index 14a8cbb9..931e5559 100644
--- a/apps/qc-app/src/services/qualityControl/reconstructSession.ts
+++ b/apps/qc-app/src/services/qualityControl/reconstructSession.ts
@@ -1,14 +1,6 @@
 /**
- * Reconstruct a session's working state for resume/read-only viewing
- * (spec section 4 / 7.3).
- *
- * Base = the latest committed state (the managed datastream's observations,
- * which already carry every previously-committed session via commit's
- * in-range replace), then replay this session's own draft operations on top.
- * Previously-committed sessions are NOT replayed here; they are already baked
- * into the managed datastream. Falls back to the raw source only when nothing
- * has been committed yet. `fetchInRange` / `applyHistory` are injected so this
- * unit-tests without a real ObservationRecord.
+ * Rebuild a session's data for editing (resume) or read-only viewing.
+ * `fetchInRange` and `applyHistory` are injected for unit testing.
  */
 
 import type {
@@ -25,8 +17,17 @@ import type {
   QcHistoryOperation,
 } from '@uwrl/qc-utils'
 import { unwrap } from './unwrap'
-import { loadLatestBase, type FetchObservationsInRange } from './session'
-import { cloneRecord as defaultCloneRecord, type CloneRecord } from './cloneRecord'
+import {
+  committedWindows,
+  composeBase,
+  loadLatestBase,
+  replaceWindow,
+  type FetchObservationsInRange,
+  type Points,
+} from './session'
+import { recordFrom } from './recordFrom'
+import type { Interval } from '@/utils/timeIntervals'
+import { commitOrder } from '@/utils/sessionGraph'
 
 /** API operation -> qc-utils replayable operation (rename operationType/arguments). */
 const toSerialized = (op: QualityControlOperation): QcHistoryOperation => {
@@ -53,7 +54,6 @@ export interface ReconstructSessionDeps {
     record: ObservationRecord,
     history: QcHistory
   ) => Promise<ApplyHistoryReport>
-  cloneRecord?: CloneRecord
 }
 
 export interface ReconstructSessionResult {
@@ -61,6 +61,8 @@ export interface ReconstructSessionResult {
   report: ApplyHistoryReport
 }
 
+/** An in-progress session's working copy: its window's latest committed
+ *  base with the session's saved operations replayed on top. */
 export async function reconstructSession(
   deps: ReconstructSessionDeps,
   managed: Datastream,
@@ -68,48 +70,37 @@ export async function reconstructSession(
   historyId: string,
   sessionId: string
 ): Promise<ReconstructSessionResult> {
-  const { qcSessions, qcOperations, fetchInRange, applyHistory, cloneRecord } = deps
+  const { qcSessions, qcOperations, fetchInRange, applyHistory } = deps
 
   const session = unwrap(await qcSessions.get(historyId, sessionId))
   const start = new Date(session.phenomenonTimeStart)
   const end = new Date(session.phenomenonTimeEnd)
 
+  const committed = unwrap(
+    await qcSessions.list(historyId, { status: 'committed', fetch_all: true })
+  )
   const record = await loadLatestBase(
     fetchInRange,
     managed,
     source,
     start,
     end,
-    cloneRecord
+    committedWindows(committed)
   )
   const ops = unwrap(
     await qcOperations.list(historyId, sessionId, { fetch_all: true })
   )
 
-  const history: QcHistory = {
-    version: '1',
-    createdAt: session.createdAt,
-    window: {
-      startDate: session.phenomenonTimeStart,
-      endDate: session.phenomenonTimeEnd,
-    },
-    operations: ops.map(toSerialized),
-  }
-
-  const report = await applyHistory(record, history)
+  const report = await applyHistory(record, historyOf(session, ops))
   return { record, report }
 }
 
 /**
  * Rebuild the state a committed session left behind, for read-only viewing.
- * The managed datastream carries every commit, so the ancestor chain is
- * replayed from the raw source instead.
- *
- * Ordered by `committedAt`, since commit order is what a later session
- * built on. Loaded over the union of the chain's windows: operations replay
- * against array indices, so a narrower base misaligns a wider ancestor.
- * Replays onto a copy of the fetched record, so viewing a committed session
- * never mutates the source datastream's cached record.
+ * The managed datastream now carries later commits too, so it is replayed
+ * from the raw source instead: each session in the ancestor chain, in commit
+ * order, on the base its own window had (committed data where an earlier
+ * session covered it, source data elsewhere), just as it was edited.
  */
 export async function reconstructCommittedSession(
   deps: ReconstructSessionDeps,
@@ -118,64 +109,66 @@ export async function reconstructCommittedSession(
   sessionId: string,
   opLimit?: number
 ): Promise<ReconstructSessionResult> {
-  const {
-    qcSessions,
-    qcOperations,
-    fetchInRange,
-    applyHistory,
-    cloneRecord = defaultCloneRecord,
-  } = deps
+  const { qcSessions, qcOperations, fetchInRange, applyHistory } = deps
 
   const session = unwrap(await qcSessions.get(historyId, sessionId))
   const ancestors = unwrap(
     await qcSessions.list(historyId, {
       ancestor_of: sessionId,
       fetch_all: true,
-    } as never)
-  )
-
-  const chainOrder = (s: QualityControlSession) => s.committedAt ?? s.createdAt
-  const chain = [...ancestors, session].sort((a, b) =>
-    chainOrder(a).localeCompare(chainOrder(b))
-  )
-
-  const startMs = Math.min(
-    ...chain.map((s) => new Date(s.phenomenonTimeStart).getTime())
-  )
-  const endMs = Math.max(
-    ...chain.map((s) => new Date(s.phenomenonTimeEnd).getTime())
-  )
+    })
+  ).sort((a, b) => commitOrder(a).localeCompare(commitOrder(b)))
+  const chain = [...ancestors, session]
+  const windows = chain.map(windowOf)
+
+  const [sourceRecord, operations] = await Promise.all([
+    fetchInRange(
+      source,
+      new Date(Math.min(...windows.map(([start]) => start))),
+      new Date(Math.max(...windows.map(([, end]) => end)))
+    ),
+    Promise.all(
+      chain.map(async (s) =>
+        unwrap(await qcOperations.list(historyId, s.id, { fetch_all: true }))
+      )
+    ),
+  ])
+
+  let managed: Points = { dataX: [], dataY: [] }
+  const committed: Interval[] = []
+  const replay = async (index: number, ops: QualityControlOperation[]) => {
+    const base = composeBase(managed, sourceRecord, windows[index]!, committed)
+    const record = await recordFrom(base.dataX, base.dataY)
+    const report = await applyHistory(record, historyOf(chain[index]!, ops))
+    return { record, report }
+  }
 
-  const fetched = await fetchInRange(source, new Date(startMs), new Date(endMs))
-  const record = await cloneRecord(fetched)
+  for (let i = 0; i < ancestors.length; i++) {
+    const { record } = await replay(i, operations[i]!)
+    managed = replaceWindow(managed, windows[i]!, record)
+    committed.push(windows[i]!)
+  }
 
-  const perSession = await Promise.all(
-    chain.map(async (s) =>
-      unwrap(await qcOperations.list(historyId, s.id, { fetch_all: true }))
-    )
-  )
+  const own = operations[chain.length - 1]!
+  return replay(chain.length - 1, opLimit == null ? own : own.slice(0, opLimit))
+}
 
-  // Snapshots replay this session only partway. Ancestors are never
-  // truncated: they are the base this session was built on.
-  const targetIdx = chain.length - 1
-  if (opLimit != null && perSession[targetIdx]) {
-    perSession[targetIdx] = perSession[targetIdx]!.slice(0, opLimit)
-  }
+const windowOf = (s: QualityControlSession): Interval => [
+  new Date(s.phenomenonTimeStart).getTime(),
+  new Date(s.phenomenonTimeEnd).getTime(),
+]
 
-  const history: QcHistory = {
+function historyOf(
+  session: QualityControlSession,
+  operations: QualityControlOperation[]
+): QcHistory {
+  return {
     version: '1',
     createdAt: session.createdAt,
     window: {
-      startDate: new Date(startMs).toISOString(),
-      endDate: new Date(endMs).toISOString(),
+      startDate: session.phenomenonTimeStart,
+      endDate: session.phenomenonTimeEnd,
     },
-    operations: perSession.flat().map(toSerialized),
+    operations: operations.map(toSerialized),
   }
-
-  const report = await applyHistory(record, history)
-
-  const ownCount = perSession[chain.length - 1]?.length ?? 0
-  record.history.splice(0, record.history.length - ownCount)
-
-  return { record, report }
 }
diff --git a/apps/qc-app/src/services/qualityControl/recordFrom.ts b/apps/qc-app/src/services/qualityControl/recordFrom.ts
new file mode 100644
index 00000000..371f7e2c
--- /dev/null
+++ b/apps/qc-app/src/services/qualityControl/recordFrom.ts
@@ -0,0 +1,19 @@
+import { ObservationRecord } from '@uwrl/qc-utils'
+
+/**
+ * A standalone record over copies of `datetimes` and `dataValues`. The
+ * observation store shares one record per datastream, so edits must happen
+ * on a record of their own.
+ */
+export async function recordFrom(
+  datetimes: ArrayLike<number>,
+  dataValues: ArrayLike<number>
+): Promise<ObservationRecord> {
+  const record = new ObservationRecord({
+    datetimes: Float64Array.from(datetimes),
+    dataValues: Float32Array.from(dataValues),
+  })
+  // The constructor starts loading without awaiting it.
+  await record.reload()
+  return record
+}
diff --git a/apps/qc-app/src/services/qualityControl/session.ts b/apps/qc-app/src/services/qualityControl/session.ts
index fd232259..d332958b 100644
--- a/apps/qc-app/src/services/qualityControl/session.ts
+++ b/apps/qc-app/src/services/qualityControl/session.ts
@@ -1,10 +1,6 @@
 /**
- * Session lifecycle helpers (spec section 5): resume the in-progress
- * session for a history or start a new one, and load the source-data
- * window for editing ("Copy data from source").
- *
- * Pure (deps injected) for unit testing; a composable wires these to the
- * QC client and the observation store.
+ * Session lifecycle helpers: resume the in-progress session for a history or
+ * start a new one, and load the data a session edits.
  */
 
 import type {
@@ -14,7 +10,8 @@ import type {
 } from '@hydroserver/client'
 import type { ObservationRecord } from '@uwrl/qc-utils'
 import { unwrap } from './unwrap'
-import { cloneRecord, type CloneRecord } from './cloneRecord'
+import { recordFrom } from './recordFrom'
+import { subtractIntervals, type Interval } from '@/utils/timeIntervals'
 
 type QcSessionDetail = QualityControlSessionContract.DetailResponse
 type QcSessionSummary = QualityControlSessionContract.SummaryResponse
@@ -56,31 +53,16 @@ export type FetchObservationsInRange = (
   endTime: Date
 ) => Promise<ObservationRecord>
 
-/**
- * "Copy data from source": fetch the session's phenomenon-time window from
- * the source datastream into an ObservationRecord for editing. The mixed
- * managed+source fetch (windows overlapping committed sessions) is
- * deferred; this loads source-only.
- */
-export async function loadSourceWindow(
-  fetchInRange: FetchObservationsInRange,
-  source: Datastream,
-  session: Pick<QcSessionDetail, 'phenomenonTimeStart' | 'phenomenonTimeEnd'>
-): Promise<ObservationRecord> {
-  return fetchInRange(
-    source,
-    new Date(session.phenomenonTimeStart),
-    new Date(session.phenomenonTimeEnd)
-  )
+/** Observations as parallel time/value arrays, sorted by time. */
+export interface Points {
+  dataX: ArrayLike<number>
+  dataY: ArrayLike<number>
 }
 
 /**
- * The latest committed state for a window, as a standalone copy. Every commit
- * replays its session into the managed datastream (in-range replace), so the
- * managed datastream's observations already carry all previously-committed
- * sessions. Falls back to the raw source when nothing has been committed yet
- * (the first session). A copy, so editing it never touches the store's cached
- * record for either datastream.
+ * The latest committed state for a window, as a standalone record. The
+ * managed datastream only holds the ranges committed sessions covered, so
+ * those come from it and the rest of the window from the source.
  */
 export async function loadLatestBase(
   fetchInRange: FetchObservationsInRange,
@@ -88,10 +70,92 @@ export async function loadLatestBase(
   source: Datastream,
   start: Date,
   end: Date,
-  clone: CloneRecord = cloneRecord
+  committed: readonly Interval[]
 ): Promise<ObservationRecord> {
-  const base = await fetchInRange(managed, start, end)
-  const record =
-    (base.dataX?.length ?? 0) > 0 ? base : await fetchInRange(source, start, end)
-  return clone(record)
+  const window: Interval = [start.getTime(), end.getTime()]
+  const fromSource = subtractIntervals([window], committed)
+  const fromManaged = subtractIntervals([window], fromSource)
+  const [managedRecord, sourceRecord] = await Promise.all([
+    fromManaged.length ? fetchInRange(managed, start, end) : undefined,
+    fromSource.length ? fetchInRange(source, start, end) : undefined,
+  ])
+  const base = composeBase(managedRecord, sourceRecord, window, committed)
+  return recordFrom(base.dataX, base.dataY)
+}
+
+/**
+ * A window's base: `managed` inside the committed intervals, `source`
+ * everywhere else. Deleted points inside a committed range stay deleted.
+ */
+export function composeBase(
+  managed: Points | undefined,
+  source: Points | undefined,
+  window: Interval,
+  committed: readonly Interval[]
+): { dataX: number[]; dataY: number[] } {
+  const fromSource = subtractIntervals([window], committed)
+  const fromManaged = subtractIntervals([window], fromSource)
+  return mergeSorted(
+    pointsWithin(managed, fromManaged),
+    pointsWithin(source, fromSource)
+  )
+}
+
+/** `managed` after a commit replaced `window` with `output`. */
+export function replaceWindow(
+  managed: Points,
+  window: Interval,
+  output: Points
+): { dataX: number[]; dataY: number[] } {
+  const outside = subtractIntervals([[-Infinity, Infinity]], [window])
+  return mergeSorted(pointsWithin(managed, outside), output)
+}
+
+function pointsWithin(
+  points: Points | undefined,
+  parts: readonly Interval[]
+): { dataX: number[]; dataY: number[] } {
+  const dataX: number[] = []
+  const dataY: number[] = []
+  if (!points) return { dataX, dataY }
+  for (let i = 0; i < points.dataX.length; i++) {
+    const t = points.dataX[i]!
+    if (parts.some(([a, b]) => t >= a && t <= b)) {
+      dataX.push(t)
+      dataY.push(points.dataY[i]!)
+    }
+  }
+  return { dataX, dataY }
+}
+
+function mergeSorted(a: Points, b: Points): { dataX: number[]; dataY: number[] } {
+  const dataX: number[] = []
+  const dataY: number[] = []
+  let i = 0
+  let j = 0
+  while (i < a.dataX.length || j < b.dataX.length) {
+    if (j >= b.dataX.length || (i < a.dataX.length && a.dataX[i]! < b.dataX[j]!)) {
+      dataX.push(a.dataX[i]!)
+      dataY.push(a.dataY[i++]!)
+    } else {
+      dataX.push(b.dataX[j]!)
+      dataY.push(b.dataY[j++]!)
+    }
+  }
+  return { dataX, dataY }
+}
+
+/** The windows of the committed sessions in `sessions`. */
+export function committedWindows(
+  sessions: readonly Pick<
+    QcSessionSummary,
+    'status' | 'phenomenonTimeStart' | 'phenomenonTimeEnd'
+  >[]
+): Interval[] {
+  return sessions
+    .filter((s) => s.status === 'committed')
+    .map((s) => [
+      new Date(s.phenomenonTimeStart).getTime(),
+      new Date(s.phenomenonTimeEnd).getTime(),
+    ])
 }
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index 88cd1454..3637dd0d 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -733,9 +733,9 @@ describe('useDataVisStore overlapping plot loads', () => {
     pending.resolve({ id: 'old' })
     await Promise.all([inFlight, queued, reload])
 
+    // The queued rebuild loads the new range once; the stale load doesn't retry.
     const starts = mockFetchObservationsInRange.mock.calls.map((c) => c[1])
-    expect(starts.filter((d) => d === APR)).toHaveLength(2)
-    expect(starts).toHaveLength(3)
+    expect(starts).toEqual([expect.any(Date), APR])
     expect(mockRedraw).not.toHaveBeenCalled()
   })
 
diff --git a/apps/qc-app/src/store/__tests__/observations.spec.ts b/apps/qc-app/src/store/__tests__/observations.spec.ts
index 2a9fe618..e322c725 100644
--- a/apps/qc-app/src/store/__tests__/observations.spec.ts
+++ b/apps/qc-app/src/store/__tests__/observations.spec.ts
@@ -16,7 +16,11 @@ vi.mock('@/utils/observations', () => ({
 import { fetchObservationsSync } from '@/utils/observations'
 import { useObservationStore } from '@/store/observations'
 
-const datastream = { id: 'ds-1' } as any
+const datastream = {
+  id: 'ds-1',
+  phenomenonBeginTime: '1970-01-01T00:00:00Z',
+  phenomenonEndTime: '1970-01-01T00:00:00.010Z',
+} as any
 
 // 11 points at epoch-ms 0..10.
 const ALL_TIMES = Array.from({ length: 11 }, (_, i) => i)
@@ -225,3 +229,53 @@ describe('useObservationStore.fetchObservationsInRange coverage', () => {
     expect(rec.dataX.length).toBe(11)
   })
 })
+
+describe('useObservationStore cache lifetime', () => {
+  it('does not count a datastream with no observations yet as loaded', async () => {
+    const store = useObservationStore()
+    const empty = { id: 'ds-1', phenomenonBeginTime: null, phenomenonEndTime: null } as any
+    const first = await store.fetchObservationsInRange(empty, new Date(0), new Date(10))
+    expect(first.dataX.length).toBe(0)
+    expect(fetchObservationsSync).not.toHaveBeenCalled()
+
+    // Once observations exist, the same range is asked for.
+    const rec = await store.fetchObservationsInRange(datastream, new Date(0), new Date(10))
+    expect(rec.dataX.length).toBe(11)
+  })
+
+  it('asks the server again after forgetting a datastream', async () => {
+    const store = useObservationStore()
+    await store.fetchObservationsInRange(datastream, new Date(0), new Date(10))
+    ;(fetchObservationsSync as any).mockImplementation(async () => ({
+      datetimes: [5],
+      dataValues: [99],
+    }))
+
+    store.forget('ds-1')
+    const rec = await store.fetchObservationsInRange(datastream, new Date(0), new Date(10))
+
+    expect(Array.from(rec.dataX)).toEqual([5])
+    expect(Array.from(rec.dataY)).toEqual([99])
+  })
+
+  it('does not keep a load that was in flight when the datastream was forgotten', async () => {
+    let release!: () => void
+    const gate = new Promise<void>((resolve) => (release = resolve))
+    ;(fetchObservationsSync as any)
+      .mockImplementationOnce(async () => {
+        await gate
+        return { datetimes: [1], dataValues: [10] }
+      })
+      .mockImplementation(async () => ({ datetimes: [5], dataValues: [99] }))
+    const store = useObservationStore()
+
+    const stale = store.fetchObservationsInRange(datastream, new Date(0), new Date(10))
+    await vi.waitFor(() => expect(fetchObservationsSync).toHaveBeenCalledTimes(1))
+    store.forget('ds-1')
+    release()
+    await stale
+    const rec = await store.fetchObservationsInRange(datastream, new Date(0), new Date(10))
+
+    expect(Array.from(rec.dataX)).toEqual([5])
+  })
+})
diff --git a/apps/qc-app/src/store/__tests__/qcSession.spec.ts b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
index 8f99d0a6..4565dbb2 100644
--- a/apps/qc-app/src/store/__tests__/qcSession.spec.ts
+++ b/apps/qc-app/src/store/__tests__/qcSession.spec.ts
@@ -1,4 +1,4 @@
-import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { describe, it, expect, beforeEach } from 'vitest'
 import { createTestPinia } from '@/utils/test/pinia'
 import { useHydroServer } from '@/store/hydroserver'
 import { makeQcFake } from '@/services/qualityControl/__tests__/qcServiceFake'
@@ -51,7 +51,7 @@ describe('useQcSessionStore', () => {
     expect(store.currentSessionId).toBe(inProgressId)
     expect(store.viewedSessionId).toBe(inProgressId)
     expect(store.isReadOnly).toBe(false)
-    expect(store.committedSessions).toHaveLength(1)
+    expect(store.sessions.filter((s) => s.status === 'committed')).toHaveLength(1)
     expect(store.inProgressSession?.id).toBe(inProgressId)
   })
 
@@ -156,10 +156,8 @@ describe('useQcSessionStore', () => {
     expect(store.resumeDatastreamId).toBe('m-1')
   })
 
-  // A backend that ignores `expand_related` on this route returns the
-  // summary shape, and the session-list previews then claim "No operations"
-  // for sessions that plainly have some.
-  it('backfills operations when the list response omits them', async () => {
+  // The session-list previews read the embedded operations.
+  it('loads each session with its operations', async () => {
     const h = unwrap(
       await qc.histories.create({
         managedDatastreamId: 'm-1',
@@ -174,17 +172,6 @@ describe('useQcSessionStore', () => {
       { operationType: 'DELETE_POINTS' as any, order: 1 },
     ])
 
-    // Stand in for a backend that ignores the flag.
-    const listWithoutOperations = async (historyId: string, query?: any) => {
-      const res = await qc.sessions.list(historyId, { ...query, expand_related: false })
-      return res
-    }
-    useHydroServer().hs = {
-      ...useHydroServer().hs,
-      qualityControlSessions: { ...qc.sessions, list: listWithoutOperations },
-      qualityControlOperations: qc.operations,
-    } as any
-
     const store = useQcSessionStore()
     store.applySessions(h.id, await store.fetchSessions(h.id))
 
@@ -195,31 +182,6 @@ describe('useQcSessionStore', () => {
     ])
   })
 
-  it('does not re-fetch operations the list already embedded', async () => {
-    const h = unwrap(
-      await qc.histories.create({
-        managedDatastreamId: 'm-1',
-        sourceDatastreamId: 's-1',
-      })
-    )
-    const s = unwrap(
-      await qc.sessions.create(h.id, win('2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z'))
-    )
-    await qc.operations.create(h.id, s.id, [
-      { operationType: 'SELECTION' as any, order: 0 },
-    ])
-
-    const opsList = vi.fn(qc.operations.list)
-    useHydroServer().hs = {
-      ...useHydroServer().hs,
-      qualityControlSessions: qc.sessions,
-      qualityControlOperations: { ...qc.operations, list: opsList },
-    } as any
-
-    useQcSessionStore().applySessions(h.id, await useQcSessionStore().fetchSessions(h.id))
-    expect(opsList).not.toHaveBeenCalled()
-  })
-
   it('holds the source datastream and clears it on reset', () => {
     const store = useQcSessionStore()
     expect(store.sourceDatastream).toBeNull()
diff --git a/apps/qc-app/src/store/__tests__/workingCopies.spec.ts b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
index fc727544..3a45e361 100644
--- a/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
+++ b/apps/qc-app/src/store/__tests__/workingCopies.spec.ts
@@ -31,6 +31,8 @@ vi.mock('@uwrl/qc-utils', () => ({
 const managed = { id: 'm-1' } as any
 const source = { id: 's-1' } as any
 const rec = (xs: number[]) => ({ dataX: xs, dataY: xs.map(() => 1), history: [] })
+// Inside the sessions' January 2025 window.
+const SOURCE_TIMES = [1, 2, 3].map((d) => Date.UTC(2025, 0, d))
 
 function deferred<T>() {
   let resolve!: (value: T) => void
@@ -52,7 +54,7 @@ function gateFetch() {
   fetchDetachedRecord.mockImplementation(async (ds: any) => {
     started.resolve()
     await gate.promise
-    return ds.id === 'm-1' ? rec([]) : rec([1, 2, 3])
+    return ds.id === 'm-1' ? rec([]) : rec(SOURCE_TIMES)
   })
   return { gate, started }
 }
@@ -84,7 +86,7 @@ beforeEach(async () => {
   ).id
   // Managed datastream empty, source has data.
   fetchDetachedRecord.mockImplementation(async (ds: any) =>
-    ds.id === 'm-1' ? rec([]) : rec([1, 2, 3])
+    ds.id === 'm-1' ? rec([]) : rec(SOURCE_TIMES)
   )
 })
 
@@ -115,7 +117,7 @@ describe('useWorkingCopiesStore', () => {
     expect(copy?.sessionId).toBe(session.id)
     expect(copy?.begin.toISOString()).toBe('2025-01-01T00:00:00.000Z')
     expect(copy?.end.toISOString()).toBe('2025-02-01T00:00:00.000Z')
-    expect(Array.from(copy!.record.dataX)).toEqual([1, 2, 3])
+    expect(Array.from(copy!.record.dataX)).toEqual(SOURCE_TIMES)
     expect(applyHistory).toHaveBeenCalledTimes(1)
     expect((applyHistory.mock.calls[0] as any[])[0]).toBe(copy!.record)
     expect(store.get('m-1')).toBe(copy)
diff --git a/apps/qc-app/src/store/__tests__/workspaces.spec.ts b/apps/qc-app/src/store/__tests__/workspaces.spec.ts
index 55eba137..300f96af 100644
--- a/apps/qc-app/src/store/__tests__/workspaces.spec.ts
+++ b/apps/qc-app/src/store/__tests__/workspaces.spec.ts
@@ -29,91 +29,11 @@ const ws = (overrides: Record<string, any> = {}) => ({
   ...overrides,
 })
 
-const role = (permissions: Array<{ resource: string; action: string }>) => ({
-  collaboratorRole: { permissions },
-})
-
 beforeEach(() => {
   setActivePinia(createPinia())
   mockList.fn = () => Promise.resolve({ data: [] })
 })
 
-describe('useWorkspaceStore.canEditSelected', () => {
-  it('returns false when no workspace is selected', async () => {
-    const { useWorkspaceStore } = await import('@/store/workspaces')
-    const store = useWorkspaceStore()
-    expect(store.canEditSelected).toBe(false)
-  })
-
-  it('returns true for owner (collaboratorRole is null)', async () => {
-    const { useWorkspaceStore } = await import('@/store/workspaces')
-    const store = useWorkspaceStore()
-    store.selectedWorkspace = ws() as any
-    expect(store.canEditSelected).toBe(true)
-  })
-
-  it('returns true for collaborator with edit on Observation', async () => {
-    const { useWorkspaceStore } = await import('@/store/workspaces')
-    const store = useWorkspaceStore()
-    store.selectedWorkspace = ws(
-      role([{ resource: 'Observation', action: 'edit' }])
-    ) as any
-    expect(store.canEditSelected).toBe(true)
-  })
-
-  it('returns true for collaborator with edit on "*" resource', async () => {
-    const { useWorkspaceStore } = await import('@/store/workspaces')
-    const store = useWorkspaceStore()
-    store.selectedWorkspace = ws(
-      role([{ resource: '*', action: 'edit' }])
-    ) as any
-    expect(store.canEditSelected).toBe(true)
-  })
-
-  it('returns true for collaborator with "*" action on Observation', async () => {
-    const { useWorkspaceStore } = await import('@/store/workspaces')
-    const store = useWorkspaceStore()
-    store.selectedWorkspace = ws(
-      role([{ resource: 'Observation', action: '*' }])
-    ) as any
-    expect(store.canEditSelected).toBe(true)
-  })
-
-  it('returns true for collaborator with create on Observation', async () => {
-    const { useWorkspaceStore } = await import('@/store/workspaces')
-    const store = useWorkspaceStore()
-    store.selectedWorkspace = ws(
-      role([{ resource: 'Observation', action: 'create' }])
-    ) as any
-    expect(store.canEditSelected).toBe(true)
-  })
-
-  it('returns false for collaborator with only view on Observation', async () => {
-    const { useWorkspaceStore } = await import('@/store/workspaces')
-    const store = useWorkspaceStore()
-    store.selectedWorkspace = ws(
-      role([{ resource: 'Observation', action: 'view' }])
-    ) as any
-    expect(store.canEditSelected).toBe(false)
-  })
-
-  it('returns false for collaborator with edit on unrelated resource', async () => {
-    const { useWorkspaceStore } = await import('@/store/workspaces')
-    const store = useWorkspaceStore()
-    store.selectedWorkspace = ws(
-      role([{ resource: 'Thing', action: 'edit' }])
-    ) as any
-    expect(store.canEditSelected).toBe(false)
-  })
-
-  it('returns false for collaborator with empty permissions list', async () => {
-    const { useWorkspaceStore } = await import('@/store/workspaces')
-    const store = useWorkspaceStore()
-    store.selectedWorkspace = ws(role([])) as any
-    expect(store.canEditSelected).toBe(false)
-  })
-})
-
 describe('useWorkspaceStore.selectWorkspace', () => {
   it('clears selection when id is null', async () => {
     const { useWorkspaceStore } = await import('@/store/workspaces')
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index 97080203..192ba700 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -28,6 +28,7 @@ import {
   type QualityControlHistory,
   Thing,
 } from '@hydroserver/client'
+import { historyManagedId, historySourceId } from '@/utils/qcHistory'
 
 export const useDataVisStore = defineStore('dataVisualization', () => {
   const {
@@ -50,17 +51,13 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   // source -> managed lookup used by the "Start editing" chooser.
   const qcHistories = ref<QualityControlHistory[]>([])
 
-  const historyManagedId = (h: QualityControlHistory): string | undefined =>
-    (h as any).managedDatastreamId ?? (h as any).managedDatastream?.id
-  const historySourceId = (h: QualityControlHistory): string | undefined =>
-    (h as any).sourceDatastreamId ?? (h as any).sourceDatastream?.id
-
   /** History and catalog source for a managed datastream, if both are known. */
   function managedContext(managedId: string) {
     const history = qcHistories.value.find((h) => historyManagedId(h) === managedId)
-    const sourceId = history ? historySourceId(history) : undefined
-    const source = sourceId ? datastreams.value.find((d) => d.id === sourceId) : undefined
-    return history && source ? { historyId: (history as any).id as string, source } : null
+    if (!history) return null
+    const sourceId = historySourceId(history)
+    const source = datastreams.value.find((d) => d.id === sourceId)
+    return source ? { historyId: history.id, source } : null
   }
 
   /** Load or reuse the working copy of every plotted managed datastream. */
@@ -100,17 +97,14 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   /** Ids of every managed datastream; these are hidden from the catalog. */
   const managedDatastreamIds = computed(() => {
     const ids = new Set<string>()
-    for (const h of qcHistories.value) {
-      const id = historyManagedId(h)
-      if (id) ids.add(id)
-    }
+    for (const h of qcHistories.value) ids.add(historyManagedId(h))
     return ids
   })
 
   /** Add a newly-created QC history so the new managed datastream is hidden
    *  from the catalog and listed in the chooser without a full reload. */
   function addQcHistory(history: QualityControlHistory) {
-    if (qcHistories.value.some((h) => (h as any).id === (history as any).id)) {
+    if (qcHistories.value.some((h) => h.id === history.id)) {
       return
     }
     qcHistories.value = [...qcHistories.value, history]
@@ -120,7 +114,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
    *  vanishes from the chooser and doesn't resurface in the catalog. */
   function removeManagedDatastream(historyId: string, managedId: string) {
     qcHistories.value = qcHistories.value.filter(
-      (h) => (h as any).id !== historyId
+      (h) => h.id !== historyId
     )
     datastreams.value = datastreams.value.filter((d) => d.id !== managedId)
   }
@@ -138,7 +132,6 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     const map = new Map<string, QualityControlHistory[]>()
     for (const h of qcHistories.value) {
       const sourceId = historySourceId(h)
-      if (!sourceId) continue
       ;(map.get(sourceId) ?? map.set(sourceId, []).get(sourceId)!).push(h)
     }
     return map
@@ -379,8 +372,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   function sourceGroupIds(sourceId: string): string[] {
     const ids = [sourceId]
     for (const h of historiesBySource.value.get(sourceId) ?? []) {
-      const id = historyManagedId(h)
-      if (id) ids.push(id)
+      ids.push(historyManagedId(h))
     }
     return ids
   }
@@ -486,8 +478,6 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   // into at most two loads. Every caller resolves only once a load that
   // started after its change has finished.
   type PlotLoadKind = 'rebuild' | 'range'
-  /** Guard against a range that never settles. */
-  const MAX_RANGE_PASSES = 5
   let loadInFlight: Promise<void> | null = null
   let queuedLoad: { kind: PlotLoadKind; promise: Promise<void> } | null = null
   /** The `loadKey` the series were last loaded for. */
@@ -544,22 +534,17 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     return queuePlotLoad('rebuild')
   }
 
-  /** Load every series for the current range. Responses for a range that
-   *  moved meanwhile are dropped, so load again until it holds still. */
-  async function loadCurrentRange(): Promise<void> {
-    for (let pass = 0; pass < MAX_RANGE_PASSES; pass++) {
-      const begin = beginDate.value
-      const end = endDate.value
-      const key = loadKey()
-      await refreshGraphSeriesArray()
-      if (isCurrentRange(begin, end)) {
-        loadedRangeKey = key
-        return
-      }
-    }
-    console.warn(
-      `Plot range still moving after ${MAX_RANGE_PASSES} loads, drawing anyway.`
-    )
+  /** Load every series for the current range. False when the range moved
+   *  meanwhile: its responses were dropped, and the move queued the load
+   *  that draws the new range. */
+  async function loadCurrentRange(): Promise<boolean> {
+    const begin = beginDate.value
+    const end = endDate.value
+    const key = loadKey()
+    await refreshGraphSeriesArray()
+    if (!isCurrentRange(begin, end)) return false
+    loadedRangeKey = key
+    return true
   }
 
   /** Rebuild the plot from scratch: rebuild the graph-series array from
@@ -582,7 +567,11 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
       beginDate.value = presetRange.begin
       endDate.value = presetRange.end
     }
-    await loadCurrentRange()
+    if (!(await loadCurrentRange())) {
+      // The queued load draws in this one's place, so it has to rebuild.
+      if (queuedLoad) queuedLoad.kind = 'rebuild'
+      return
+    }
     updateOptions()
     const { plotlyRef } = storeToRefs(usePlotlyStore())
     if (plotlyRef.value) {
@@ -595,7 +584,7 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   async function doReloadRange(): Promise<void> {
     if (!seriesDatastreams.value.length) return
     if (loadedRangeKey === loadKey()) return
-    await loadCurrentRange()
+    if (!(await loadCurrentRange())) return
     const { redraw, clearZoomHistory } = usePlotlyStore()
     if (qcDatastreamId.value) {
       // Context reload in the editor: the user's view of the session stays put.
diff --git a/apps/qc-app/src/store/observations.ts b/apps/qc-app/src/store/observations.ts
index c52168a0..8a3ba036 100644
--- a/apps/qc-app/src/store/observations.ts
+++ b/apps/qc-app/src/store/observations.ts
@@ -78,6 +78,9 @@ export const useObservationStore = defineStore(
     /** The ranges already asked of the server per datastream, found or not. */
     const covered = new Map<string, Interval[]>()
 
+    /** Bumped by `forget`, so a load that started before it is not kept. */
+    const generations = new Map<string, number>()
+
     type Exclude = { begin: Date; end: Date }
 
     /**
@@ -128,22 +131,28 @@ export const useObservationStore = defineStore(
       exclude?: Exclude
     ): Promise<ObservationData> => {
       const id = datastream.id
+      const empty = { datetimes: new Float64Array(0), dataValues: new Float32Array(0) }
+      // Nothing to ask for yet, and nothing to remember: it may gain data.
+      if (!datastream.phenomenonBeginTime || !datastream.phenomenonEndTime) {
+        return observationsRaw.value[id] ?? empty
+      }
       const wanted = subtractIntervals(
         [[beginTime.getTime(), endTime.getTime()]],
         exclude ? [[exclude.begin.getTime(), exclude.end.getTime()]] : []
       )
       const missing = subtractIntervals(wanted, covered.get(id) ?? [])
+      const generation = generations.get(id)
       const chunks = await Promise.all(
         missing.map(([start, end]) =>
           fetchObservationsSync(datastream, new Date(start), new Date(end))
         )
       )
+      if (generations.get(id) !== generation) {
+        return loadMissing(datastream, beginTime, endTime, exclude)
+      }
       covered.set(id, mergeIntervals([...(covered.get(id) ?? []), ...missing]))
 
-      const cached = observationsRaw.value[id] ?? {
-        datetimes: new Float64Array(0),
-        dataValues: new Float32Array(0),
-      }
+      const cached = observationsRaw.value[id] ?? empty
       observationsRaw.value[id] = chunks.some((c) => c.dataValues.length)
         ? mergeObservations(cached, chunks)
         : cached
@@ -200,11 +209,20 @@ export const useObservationStore = defineStore(
       return record
     }
 
+    /** Drop what is cached for a datastream whose observations changed, so
+     *  the next load asks the server again. */
+    const forget = (datastreamId: string) => {
+      generations.set(datastreamId, (generations.get(datastreamId) ?? 0) + 1)
+      covered.delete(datastreamId)
+      delete observationsRaw.value[datastreamId]
+    }
+
     return {
       observations,
       observationsRaw,
       fetchObservationsInRange,
       fetchDetachedRecord,
+      forget,
     }
   },
   {
diff --git a/apps/qc-app/src/store/qcSession.ts b/apps/qc-app/src/store/qcSession.ts
index ed8dafdc..cfb8f8db 100644
--- a/apps/qc-app/src/store/qcSession.ts
+++ b/apps/qc-app/src/store/qcSession.ts
@@ -1,5 +1,5 @@
 /**
- * View-mode state for QC sessions (spec section 4 history navigation).
+ * View-mode state for QC sessions.
  *
  * Tracks the history's sessions, which one is editable (the single
  * in-progress session) and which one is being viewed. Viewing a committed
@@ -11,8 +11,10 @@ import { defineStore, storeToRefs } from 'pinia'
 import { computed, ref } from 'vue'
 import { useHydroServer } from '@/store/hydroserver'
 import { unwrap } from '@/services/qualityControl'
+import { commitOrder } from '@/utils/sessionGraph'
 import type { Datastream, QualityControlSession } from '@hydroserver/client'
 import type { HistoryItem } from '@uwrl/qc-utils'
+import { sessionOperations } from '@/utils/qcHistory'
 
 export const useQcSessionStore = defineStore('qcSession', () => {
   const historyId = ref<string | null>(null)
@@ -37,8 +39,8 @@ export const useQcSessionStore = defineStore('qcSession', () => {
   /** Editing is allowed only while viewing the in-progress session. */
   // TODO(backend): ask for a way to keep editing the most recent session after
   // it is committed, when no newer session exists. Today a commit is terminal:
-  // the API rejects updating, deleting, adding operations to, or re-committing
-  // a committed session, and the PATCH body carries only `description`. Undoing
+  // the API rejects updating, adding operations to, or re-committing a
+  // committed session, and the PATCH body carries only `description`. Undoing
   // a commit also has to roll back what it wrote (the replayed observations on
   // the managed datastream plus the history's checksum and time extent), so
   // this is a real backend change, not just lifting the status guard.
@@ -53,9 +55,6 @@ export const useQcSessionStore = defineStore('qcSession', () => {
   const inProgressSession = computed(
     () => sessions.value.find((s) => s.status === 'in_progress') ?? null
   )
-  const committedSessions = computed(() =>
-    sessions.value.filter((s) => s.status === 'committed')
-  )
   const viewedSession = computed(
     () => sessions.value.find((s) => s.id === viewedSessionId.value) ?? null
   )
@@ -68,53 +67,22 @@ export const useQcSessionStore = defineStore('qcSession', () => {
    * one worth keeping.
    */
   const hasSessionOperations = computed(() => {
-    const session = inProgressSession.value as
-      | { operations?: unknown[] }
-      | null
+    const session = inProgressSession.value
     if (!session) return false
-    return (session.operations?.length ?? 0) > 0 || savedEdits.value.length > 0
+    return sessionOperations(session).length > 0 || savedEdits.value.length > 0
   })
 
-  /**
-   * Guarantee every session carries its `operations`, which the previews
-   * read. `expand_related` should embed them, but a backend that ignores it
-   * returns the summary shape instead. Only the gaps are fetched.
-   */
-  async function withOperations(
-    client: ReturnType<typeof useHydroServer>['hs'],
-    id: string,
-    list: QualityControlSession[]
-  ): Promise<QualityControlSession[]> {
-    const missing = list.filter(
-      (s) => !(s as { operations?: unknown[] }).operations
-    )
-    if (!missing.length) return list
-
-    const fetched = new Map<string, unknown[]>()
-    await Promise.all(
-      missing.map(async (s) => {
-        const res = await client.qualityControlOperations.list(id, s.id, {
-          fetch_all: true,
-        })
-        if (res.ok) fetched.set(s.id, res.data)
-      })
-    )
-    return list.map((s) =>
-      fetched.has(s.id) ? { ...s, operations: fetched.get(s.id) } : s
-    ) as QualityControlSession[]
-  }
-
   /** A history's sessions with their operations. Writes nothing, so a caller
    *  can drop the result if it went stale while loading. */
   async function fetchSessions(id: string): Promise<QualityControlSession[]> {
     const { hs } = storeToRefs(useHydroServer())
-    const list = unwrap(
+    // `expand_related` embeds each session's operations.
+    return unwrap(
       await hs.value.qualityControlSessions.list(id, {
         fetch_all: true,
         expand_related: true,
       })
     )
-    return withOperations(hs.value, id, list)
   }
 
   /** Adopt a history's sessions; default the view to the in-progress session. */
@@ -125,7 +93,6 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     currentSessionId.value = inProgress?.id ?? null
     // Default view: the editable session, else the last one committed (the
     // one a commit just made).
-    const commitOrder = (s: QualityControlSession) => s.committedAt ?? s.createdAt
     const latestCommitted = [...list]
       .filter((s) => s.status === 'committed')
       .sort((a, b) => commitOrder(b).localeCompare(commitOrder(a)))[0]
@@ -166,7 +133,6 @@ export const useQcSessionStore = defineStore('qcSession', () => {
     savedComments,
     isReadOnly,
     inProgressSession,
-    committedSessions,
     viewedSession,
     hasSessionOperations,
     fetchSessions,
diff --git a/apps/qc-app/src/store/workspaces.ts b/apps/qc-app/src/store/workspaces.ts
index 6db130d8..89cde479 100644
--- a/apps/qc-app/src/store/workspaces.ts
+++ b/apps/qc-app/src/store/workspaces.ts
@@ -32,27 +32,6 @@ export const useWorkspaceStore = defineStore('workspaces', () => {
   const hasSelection = computed(() => !!selectedWorkspace.value)
   const selectedWorkspaceId = computed(() => selectedWorkspace.value?.id ?? null)
 
-  /**
-   * True when the signed-in user holds a role on the selected workspace
-   * that lets them create or edit Observations. Owners always pass
-   * (their `collaboratorRole` is `null` because ownership supersedes
-   * collaborator roles); collaborators need at least one permission
-   * that covers editing observations.
-   */
-  const canEditSelected = computed(() => {
-    const ws = selectedWorkspace.value
-    if (!ws) return false
-    // Owner: `collaboratorRole` is null on owned workspaces; the role
-    // machinery only applies to invited collaborators.
-    if (!ws.collaboratorRole) return true
-    const perms = ws.collaboratorRole.permissions ?? []
-    return perms.some(
-      (p) =>
-        (p.action === 'edit' || p.action === 'create' || p.action === '*') &&
-        (p.resource === 'Observation' || p.resource === '*')
-    )
-  })
-
   async function loadWorkspaces(): Promise<Workspace[]> {
     const { hs } = storeToRefs(useHydroServer())
     if (!hs.value) return []
@@ -130,7 +109,6 @@ export const useWorkspaceStore = defineStore('workspaces', () => {
     selectedWorkspaceId,
     isLoading,
     hasSelection,
-    canEditSelected,
     loadWorkspaces,
     selectWorkspace,
     applyWorkspaceById,
diff --git a/apps/qc-app/src/styles/global.scss b/apps/qc-app/src/styles/global.scss
index 345ba4a8..bbcd5560 100644
--- a/apps/qc-app/src/styles/global.scss
+++ b/apps/qc-app/src/styles/global.scss
@@ -14,68 +14,3 @@
   color: rgba(var(--v-theme-on-surface), 0.75);
   background: rgba(var(--v-theme-on-surface), 0.03);
 }
-
-/* Session timeline, shared by the editor's session list and the
-   "Start editing" chooser. Unscoped so both can reach Vuetify's
-   timeline internals without repeating the :deep() rules.
-
-   Markers: Vuetify's is a saturated disc ringed in surface-light with the
-   glyph flush to its edge. Flatten it to a tonal marker with room for the
-   icon, nudged onto the title's row. */
-.qc-timeline__item .v-timeline-divider__dot {
-  width: 20px;
-  height: 20px;
-  margin-block-start: 4px;
-  background: rgb(var(--v-theme-surface));
-  box-shadow: none;
-}
-
-/* `.v-icon` declares the multiplier on itself, so it has to be set there. */
-.qc-timeline__item .v-timeline-divider__inner-dot .v-icon {
-  --v-icon-size-multiplier: 0.75;
-}
-
-.qc-timeline__item--active .v-timeline-divider__inner-dot {
-  background: rgba(var(--v-theme-warning), 0.16);
-  box-shadow: inset 0 0 0 1px rgba(var(--v-theme-warning), 0.35);
-}
-
-.qc-timeline__item--done .v-timeline-divider__inner-dot {
-  background: rgba(var(--v-theme-success), 0.16);
-  box-shadow: inset 0 0 0 1px rgba(var(--v-theme-success), 0.35);
-}
-
-/* Every session reads as its own container, so the unselected ones don't
-   float loose against the timeline rail. The card padding replaces Vuetify's
-   body inset, so the gap to the rail comes back as a margin. */
-.qc-timeline__item .v-timeline-item__body {
-  margin-inline-start: 10px;
-  border-radius: 6px;
-  padding: 6px 8px;
-  border: thin solid rgba(var(--v-border-color), var(--v-border-opacity));
-  background-color: rgba(var(--v-theme-on-surface), 0.02);
-  transition:
-    background-color 120ms ease,
-    border-color 120ms ease;
-}
-
-.qc-timeline__item:hover .v-timeline-item__body {
-  background-color: rgba(var(--v-theme-primary), 0.06);
-}
-
-.qc-timeline__item--viewed .v-timeline-item__body {
-  background-color: rgba(var(--v-theme-primary), 0.12);
-  border-color: rgba(var(--v-theme-primary), 0.4);
-}
-
-/* The "start a session" node heads the timeline as an action rather than a
-   record, so it stays flat instead of taking the session card treatment. */
-.qc-timeline__item--new .v-timeline-item__body {
-  border-style: dashed;
-  background-color: transparent;
-}
-
-.qc-timeline__item--new .v-timeline-divider__inner-dot {
-  background: rgba(var(--v-theme-primary), 0.16);
-  box-shadow: inset 0 0 0 1px rgba(var(--v-theme-primary), 0.35);
-}
diff --git a/apps/qc-app/src/utils/__tests__/observations.spec.ts b/apps/qc-app/src/utils/__tests__/observations.spec.ts
new file mode 100644
index 00000000..4d09c678
--- /dev/null
+++ b/apps/qc-app/src/utils/__tests__/observations.spec.ts
@@ -0,0 +1,62 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { ref } from 'vue'
+
+const getObservations = vi.fn()
+const hs = ref<any>({ datastreams: { getObservations } })
+vi.mock('@/store/hydroserver', () => ({ useHydroServer: () => ({ hs }) }))
+
+import { fetchObservationsSync } from '@/utils/observations'
+
+const PAGE_SIZE = 50_000
+
+const page = (count: number, from = 0) => ({
+  ok: true,
+  status: 200,
+  data: {
+    phenomenonTime: Array.from({ length: count }, (_, i) =>
+      new Date(from + i).toISOString()
+    ),
+    result: Array.from({ length: count }, (_, i) => from + i),
+  },
+})
+
+const datastream = (valueCount: number) =>
+  ({
+    id: 'ds-1',
+    phenomenonBeginTime: '1970-01-01T00:00:00Z',
+    phenomenonEndTime: '2000-01-01T00:00:00Z',
+    valueCount,
+  }) as any
+
+beforeEach(() => {
+  vi.clearAllMocks()
+})
+
+describe('fetchObservationsSync', () => {
+  it('fails when a page fails, rather than returning what it has', async () => {
+    getObservations.mockResolvedValue({ ok: false, status: 500, message: 'Server error' })
+
+    await expect(fetchObservationsSync(datastream(10))).rejects.toThrow('Server error')
+  })
+
+  it('keeps paging past a stale value count until a short page', async () => {
+    getObservations
+      .mockResolvedValueOnce(page(PAGE_SIZE))
+      .mockResolvedValueOnce(page(2, PAGE_SIZE))
+
+    const { datetimes } = await fetchObservationsSync(datastream(0))
+
+    expect(getObservations).toHaveBeenCalledTimes(2)
+    expect(datetimes).toHaveLength(PAGE_SIZE + 2)
+  })
+
+  it('makes no request for a datastream with no observations', async () => {
+    const result = await fetchObservationsSync({
+      ...datastream(0),
+      phenomenonBeginTime: null,
+    })
+
+    expect(result).toEqual({ datetimes: [], dataValues: [] })
+    expect(getObservations).not.toHaveBeenCalled()
+  })
+})
diff --git a/apps/qc-app/src/utils/__tests__/qcHistory.spec.ts b/apps/qc-app/src/utils/__tests__/qcHistory.spec.ts
new file mode 100644
index 00000000..e3dbf367
--- /dev/null
+++ b/apps/qc-app/src/utils/__tests__/qcHistory.spec.ts
@@ -0,0 +1,38 @@
+import { describe, it, expect } from 'vitest'
+import type {
+  QualityControlHistory,
+  QualityControlSession,
+} from '@hydroserver/client'
+import {
+  historyManagedId,
+  historySourceId,
+  sessionOperations,
+} from '../qcHistory'
+
+describe('qcHistory ids', () => {
+  it('reads the ids of a summary history', () => {
+    const h = { id: 'h', managedDatastreamId: 'm', sourceDatastreamId: 's' }
+    expect(historyManagedId(h as QualityControlHistory)).toBe('m')
+    expect(historySourceId(h as QualityControlHistory)).toBe('s')
+  })
+
+  it('reads the ids of a detail history', () => {
+    const h = {
+      id: 'h',
+      managedDatastream: { id: 'm' },
+      sourceDatastream: { id: 's' },
+    }
+    expect(historyManagedId(h as unknown as QualityControlHistory)).toBe('m')
+    expect(historySourceId(h as unknown as QualityControlHistory)).toBe('s')
+  })
+})
+
+describe('sessionOperations', () => {
+  it('returns a detail session operations, and none for a summary', () => {
+    const detail = { id: 's', operations: [{ id: 'o' }] }
+    expect(sessionOperations(detail as unknown as QualityControlSession)).toEqual([
+      { id: 'o' },
+    ])
+    expect(sessionOperations({ id: 's' } as QualityControlSession)).toEqual([])
+  })
+})
diff --git a/apps/qc-app/src/utils/__tests__/time.spec.ts b/apps/qc-app/src/utils/__tests__/time.spec.ts
index 49e5561f..aa1ff834 100644
--- a/apps/qc-app/src/utils/__tests__/time.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/time.spec.ts
@@ -1,34 +1,12 @@
 import { describe, it, expect } from 'vitest'
 import {
-  formatDateInput,
   formatDateRange,
   formatDayStamp,
   formatStamp,
-  timeZoneAbbreviation,
-  timeZoneDescription,
 } from '../time'
 
 // Datetime strings without a timezone offset are parsed as local time, so
 // these assertions are stable regardless of the test runner's zone.
-describe('formatDateInput', () => {
-  it('formats as MM/DD/YYYY HH:MM (date-input style)', () => {
-    expect(formatDateInput('2025-01-05T14:30:00')).toBe('01/05/2025 14:30')
-  })
-
-  it('zero-pads month, day, hour, and minute', () => {
-    expect(formatDateInput('2025-09-08T04:07:00')).toBe('09/08/2025 04:07')
-  })
-
-  it('returns a dash for nullish input', () => {
-    expect(formatDateInput(null)).toBe('–')
-    expect(formatDateInput(undefined)).toBe('–')
-  })
-
-  it('returns the raw string when unparseable', () => {
-    expect(formatDateInput('not-a-date')).toBe('not-a-date')
-  })
-})
-
 describe('formatDateRange', () => {
   it('states the year once when both bounds share it', () => {
     expect(formatDateRange('2025-01-05T00:00:00', '2025-02-01T00:00:00')).toBe(
@@ -93,32 +71,3 @@ describe('formatStamp', () => {
     expect(formatStamp(new Date(NaN))).toBe('–')
   })
 })
-
-describe('time zone labels', () => {
-  const winter = new Date(2026, 0, 15, 12)
-  const summer = new Date(2026, 6, 15, 12)
-  const zone = Intl.DateTimeFormat().resolvedOptions().timeZone
-
-  const offsetOf = (d: Date) => {
-    const m = -d.getTimezoneOffset()
-    const sign = m < 0 ? '-' : '+'
-    const hh = String(Math.floor(Math.abs(m) / 60)).padStart(2, '0')
-    const mm = String(Math.abs(m) % 60).padStart(2, '0')
-    return `UTC${sign}${hh}:${mm}`
-  }
-
-  it('abbreviates the browser zone on the given date', () => {
-    for (const d of [winter, summer]) {
-      const expected = new Intl.DateTimeFormat('en-US', { timeZoneName: 'short' })
-        .formatToParts(d)
-        .find((p) => p.type === 'timeZoneName')!.value
-      expect(timeZoneAbbreviation(d)).toBe(expected)
-      expect(timeZoneAbbreviation(d)).not.toBe('')
-    }
-  })
-
-  it('names the zone and its UTC offset on the given date', () => {
-    expect(timeZoneDescription(winter)).toBe(`${zone}, ${offsetOf(winter)}`)
-    expect(timeZoneDescription(summer)).toBe(`${zone}, ${offsetOf(summer)}`)
-  })
-})
diff --git a/apps/qc-app/src/utils/__tests__/timeZone.spec.ts b/apps/qc-app/src/utils/__tests__/timeZone.spec.ts
new file mode 100644
index 00000000..31ce0d5d
--- /dev/null
+++ b/apps/qc-app/src/utils/__tests__/timeZone.spec.ts
@@ -0,0 +1,30 @@
+import { describe, it, expect, afterAll, vi } from 'vitest'
+
+// Pinned so the labels are known values, not recomputed the way the code
+// computes them.
+const previousTz = vi.hoisted(() => {
+  const tz = process.env.TZ
+  process.env.TZ = 'America/Denver'
+  return tz
+})
+afterAll(() => {
+  if (previousTz === undefined) delete process.env.TZ
+  else process.env.TZ = previousTz
+})
+
+const { timeZoneAbbreviation, timeZoneDescription } = await import('../time')
+
+describe('time zone labels', () => {
+  const winter = new Date('2026-01-15T19:00:00Z')
+  const summer = new Date('2026-07-15T18:00:00Z')
+
+  it('abbreviates the zone on the given date, tracking daylight saving', () => {
+    expect(timeZoneAbbreviation(winter)).toBe('MST')
+    expect(timeZoneAbbreviation(summer)).toBe('MDT')
+  })
+
+  it('names the zone and its UTC offset on the given date', () => {
+    expect(timeZoneDescription(winter)).toBe('America/Denver, UTC-07:00')
+    expect(timeZoneDescription(summer)).toBe('America/Denver, UTC-06:00')
+  })
+})
diff --git a/apps/qc-app/src/utils/observations.ts b/apps/qc-app/src/utils/observations.ts
index 9216ec23..93564cd1 100644
--- a/apps/qc-app/src/utils/observations.ts
+++ b/apps/qc-app/src/utils/observations.ts
@@ -7,58 +7,32 @@ export const fetchObservationsSync = async (
   startTime?: Date,
   endTime?: Date
 ): Promise<{ datetimes: number[]; dataValues: number[] }> => {
-  const { id, phenomenonBeginTime, phenomenonEndTime, valueCount } = datastream
+  const { id, phenomenonBeginTime, phenomenonEndTime } = datastream
   const { hs } = storeToRefs(useHydroServer())
   if (!phenomenonBeginTime || !phenomenonEndTime) {
     return { datetimes: [], dataValues: [] }
   }
 
   const pageSize = 50_000
-  let page = 1
-  const maxPages = Math.ceil(valueCount / pageSize)
+  const datetimes: number[] = []
+  const dataValues: number[] = []
+  // Page until a short page: the datastream's value count can be out of date.
+  for (let page = 1; ; page++) {
+    const result = await hs.value.datastreams.getObservations(id, {
+      page_size: pageSize,
+      phenomenon_time_min: startTime?.toISOString() ?? phenomenonBeginTime,
+      phenomenon_time_max: endTime?.toISOString() ?? phenomenonEndTime,
+      page,
+      order_by: ['phenomenonTime'],
+      format: 'column',
+    })
+    if (!result.ok) throw new Error(result.message || 'Could not load observations.')
 
-  try {
-    let datetimes: number[] = []
-    let dataValues: number[] = []
-    while (page <= maxPages) {
-      const result = await hs.value.datastreams.getObservations(
-        id,
-        {
-          page_size: pageSize,
-          phenomenon_time_min: startTime?.toISOString() ?? phenomenonBeginTime,
-          phenomenon_time_max: endTime?.toISOString() ?? phenomenonEndTime,
-          page: page,
-          order_by: ['phenomenonTime'],
-          format: 'column',
-        }
-      )
-
-      if (!result.ok) {
-        break
-      }
-
-      const cols = result.data as {
-        result: number[]
-        phenomenonTime: string[]
-      }
-      if (!cols.result.length) {
-        break
-      }
-
-      datetimes = [
-        ...datetimes,
-        ...cols.phenomenonTime.map((d: any) => new Date(d).getTime()),
-      ]
-      dataValues = [...dataValues, ...cols.result]
-      page++
-    }
-
-    return {
-      datetimes,
-      dataValues,
+    const cols = result.data as { result: number[]; phenomenonTime: string[] }
+    for (let i = 0; i < cols.result.length; i++) {
+      datetimes.push(new Date(cols.phenomenonTime[i]!).getTime())
+      dataValues.push(cols.result[i]!)
     }
-  } catch (error) {
-    console.error('Error fetching data:', error)
-    return Promise.reject(error)
+    if (cols.result.length < pageSize) return { datetimes, dataValues }
   }
-}
\ No newline at end of file
+}
diff --git a/apps/qc-app/src/utils/plotCap.ts b/apps/qc-app/src/utils/plotCap.ts
new file mode 100644
index 00000000..453c054c
--- /dev/null
+++ b/apps/qc-app/src/utils/plotCap.ts
@@ -0,0 +1,3 @@
+/** Datastreams the user can plot at once. One more slot is kept for the
+ *  datastream being edited. */
+export const PLOT_CAP = 4
diff --git a/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
index f4775230..8a0750a2 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
@@ -103,6 +103,24 @@ describe('handleNewPlot', () => {
     expect(newPlot.mock.calls[0]?.[2]).toStrictEqual(layout)
   })
 
+  // A default sort compares as text, which would put 10 before 9.
+  it('keeps a clicked selection in numeric order', async () => {
+    const { handleNewPlot } = await import('@/utils/plotting/events')
+    const Plotly = (await import('plotly.js-dist')).default
+    const el = fakeGraphDiv({})
+    plotlyRef.value = el
+    await handleNewPlot()
+    const onClick = (el.on as ReturnType<typeof vi.fn>).mock.calls.find(
+      ([name]) => name === 'plotly_click'
+    )?.[1] as (e: unknown) => Promise<void>
+
+    await onClick({ points: [{ pointIndex: 9, data: { selectedpoints: [10, 2] } }] })
+
+    expect(Plotly.restyle).toHaveBeenLastCalledWith(el, {
+      selectedpoints: [[2, 9, 10]],
+    })
+  })
+
   describe('preserveZoom y ranges', () => {
     const trace = (id: string, n: number, yaxis = 'y') => ({
       id,
diff --git a/apps/qc-app/src/utils/plotting/events.ts b/apps/qc-app/src/utils/plotting/events.ts
index b0e48b48..d9f8211c 100644
--- a/apps/qc-app/src/utils/plotting/events.ts
+++ b/apps/qc-app/src/utils/plotting/events.ts
@@ -44,7 +44,7 @@ const handleClick = async (eventData: PlotMouseEvent) => {
     index >= 0
       ? alreadySelected.splice(index, 1)
       : alreadySelected.push(point.pointIndex)
-    alreadySelected.sort()
+    alreadySelected.sort((a, b) => a - b)
 
     // `selections` is a Plotly layout-level option that the published
     // type omits from `Partial<Layout>`. Cast through `unknown` to
@@ -214,7 +214,7 @@ export const handleNewPlot = async (
   // Plotly.newPlot reuses the same DOM node, so `plotlyRef.value`'s
   // identity is unchanged and Vue's ref watchers don't refire. It does
   // wipe externally-attached listeners (the ContextPlot's brush sync,
-  // etc.), so bump an epoch so those subscribers know to re-attach.
+  // etc.), so an epoch bump tells those subscribers to re-attach.
   mainPlotEpoch.value++
 
   // Debounce long enough that a rapid scroll-wheel burst collapses
diff --git a/apps/qc-app/src/utils/plotting/options.ts b/apps/qc-app/src/utils/plotting/options.ts
index 38b9986e..32bb928d 100644
--- a/apps/qc-app/src/utils/plotting/options.ts
+++ b/apps/qc-app/src/utils/plotting/options.ts
@@ -588,7 +588,7 @@ export const createPlotlyOption = (
     // intended cadence, a sibling overlay trace draws the connecting line
     // with NaN-y breaks at gaps; otherwise the series renders as a pure
     // scatter plot. Either way the main trace owns selection and point
-    // indices, so its `line` attribute would be dead config, so it is omitted.
+    // indices, and a `line` on it would be dead config, so it has none.
     // The source is drawn only around the session window: its points
     // before and after it, as views of the record, with a gap in between.
     // Every other series is one piece.
diff --git a/apps/qc-app/src/utils/qcHistory.ts b/apps/qc-app/src/utils/qcHistory.ts
new file mode 100644
index 00000000..fa4d488b
--- /dev/null
+++ b/apps/qc-app/src/utils/qcHistory.ts
@@ -0,0 +1,15 @@
+import type {
+  QualityControlHistory,
+  QualityControlSession,
+} from '@hydroserver/client'
+
+/** A history lists its datastreams as ids (summary) or nested (detail). */
+export const historyManagedId = (h: QualityControlHistory): string =>
+  'managedDatastream' in h ? h.managedDatastream.id : h.managedDatastreamId
+
+export const historySourceId = (h: QualityControlHistory): string =>
+  'sourceDatastream' in h ? h.sourceDatastream.id : h.sourceDatastreamId
+
+/** A session's operations, which only the detail shape embeds. */
+export const sessionOperations = (s: QualityControlSession) =>
+  'operations' in s ? s.operations : []
diff --git a/apps/qc-app/src/utils/sessionGraph.ts b/apps/qc-app/src/utils/sessionGraph.ts
index 660d4c7a..865719e6 100644
--- a/apps/qc-app/src/utils/sessionGraph.ts
+++ b/apps/qc-app/src/utils/sessionGraph.ts
@@ -38,3 +38,9 @@ export function hasDependents(
 ): boolean {
   return (buildDependentsMap(sessions).get(sessionId)?.length ?? 0) > 0
 }
+
+/** Sort key for commit order, which is what a later session built on. */
+export const commitOrder = (session: {
+  committedAt?: string | null
+  createdAt: string
+}): string => session.committedAt ?? session.createdAt
diff --git a/apps/qc-app/src/utils/sessionWindow.ts b/apps/qc-app/src/utils/sessionWindow.ts
index d7195e42..90d4f8b9 100644
--- a/apps/qc-app/src/utils/sessionWindow.ts
+++ b/apps/qc-app/src/utils/sessionWindow.ts
@@ -1,5 +1,5 @@
 /**
- * Rules for a new edit session's window (history spec 7.2.3 and 7.2.4):
+ * Rules for a new edit session's window:
  * inside the source's observed extent, and never leaving a gap before or
  * after the committed history. Overlapping committed sessions is allowed.
  */
@@ -10,6 +10,7 @@ import {
   TIME_RANGE_PRESETS,
   dataExtent,
   presetWindow,
+  type TimeRangePreset,
   type TimeWindow,
 } from '@/utils/timeRangePresets'
 
@@ -201,12 +202,7 @@ const PRESET_REASONS: Record<SessionWindowIssueKind, string> = {
 }
 
 /** Whole record first, then shorter spans counting back from its end. */
-const SPAN_PRESETS: readonly { id: string; label: string }[] = [
-  { id: 'all', label: 'All' },
-  { id: '1y', label: '1y' },
-  { id: '6m', label: '6m' },
-  { id: '1m', label: '1m' },
-]
+const SPAN_LABELS: readonly TimeRangePreset['label'][] = ['All', '1y', '6m', '1m']
 
 function toPreset(
   id: string,
@@ -239,12 +235,14 @@ export function sessionWindowPresets(
   if (!extent) return []
 
   const presets: SessionWindowPreset[] = []
-  for (const span of SPAN_PRESETS) {
-    const base = TIME_RANGE_PRESETS.find((p) => p.label === span.label)
+  for (const label of SPAN_LABELS) {
+    const base = TIME_RANGE_PRESETS.find((p) => p.label === label)
     const window = base ? presetWindow(base.id, extent) : null
     if (!base || !window) continue
     const title = base.id === ALL_PRESET_ID ? 'The whole source record' : base.title
-    presets.push(toPreset(span.id, span.label, title, window, source, sessions))
+    presets.push(
+      toPreset(label.toLowerCase(), label, title, window, source, sessions)
+    )
   }
 
   const history = committedExtent(sessions)
diff --git a/apps/qc-app/src/utils/share.ts b/apps/qc-app/src/utils/share.ts
index c5f8b57c..bc52023d 100644
--- a/apps/qc-app/src/utils/share.ts
+++ b/apps/qc-app/src/utils/share.ts
@@ -19,8 +19,8 @@
  *   - Short query keys (`ds`, `ed`, `b`, `e`, `r`, …) instead of
  *     verbose ones.
  *   - When a date preset is active (`r=0..5`), the begin/end pair is
- *     elided, recipients recompute it from "now" on load, matching
- *     the preset semantics the sender chose.
+ *     left out. Recipients resolve the preset against the data on load,
+ *     as the sender's view did.
  *   - Timestamps go through `tsToBase36Seconds` (≈ 7 chars instead
  *     of 24).
  *   - Visibility state is a hex bitmask over the `ds` order rather
@@ -31,6 +31,12 @@
  *     emitted only when they differ from the default.
  */
 
+import {
+  parseSnapshotKey,
+  snapshotKey,
+  type SnapshotRef,
+} from '@/utils/snapshotId'
+
 export interface ShareableZoom {
   /** X range as [minMs, maxMs], or `null` when the X axis is at its
    *  default extent. */
@@ -54,7 +60,7 @@ export interface ShareState {
   datastreamIds?: string[]
   /** History snapshots plotted as comparison lines. Kept out of `ds` so the
    *  `h` / `ya` bitmask indices still hold. */
-  snapshots?: { sessionId: string; opIndex: number }[]
+  snapshots?: SnapshotRef[]
   /** Date range preset id (`0..5`). When set, `begin`/`end` are
    *  omitted from the URL and the receiver resolves the window
    *  from the plotted data. */
@@ -145,7 +151,7 @@ export function encodeShareState(state: ShareState): Record<string, string> {
   }
 
   if (state.snapshots?.length) {
-    q.snap = state.snapshots.map((s) => `${s.sessionId}:${s.opIndex}`).join(',')
+    q.snap = state.snapshots.map(snapshotKey).join(',')
   }
 
   // Preset wins over begin/end: the recipient resolves the same preset
@@ -235,14 +241,10 @@ export function decodeShareState(query: Record<string, unknown>): ShareState {
   if (ds.length) out.datastreamIds = ds
 
   // A malformed entry is dropped rather than failing the whole link.
-  const snapshots: { sessionId: string; opIndex: number }[] = []
+  const snapshots: SnapshotRef[] = []
   for (const piece of splitCsv(str('snap'))) {
-    const sep = piece.lastIndexOf(':')
-    if (sep <= 0) continue
-    const sessionId = piece.slice(0, sep)
-    const opIndex = Number(piece.slice(sep + 1))
-    if (!sessionId || !Number.isInteger(opIndex) || opIndex < -1) continue
-    snapshots.push({ sessionId, opIndex })
+    const ref = parseSnapshotKey(piece)
+    if (ref) snapshots.push(ref)
   }
   if (snapshots.length) out.snapshots = snapshots
 
diff --git a/apps/qc-app/src/utils/snapshotId.ts b/apps/qc-app/src/utils/snapshotId.ts
index 91b8d59b..12afe856 100644
--- a/apps/qc-app/src/utils/snapshotId.ts
+++ b/apps/qc-app/src/utils/snapshotId.ts
@@ -9,24 +9,35 @@ export const SNAPSHOT_PREFIX = 'snap:'
 /** Baseline: the session's state before its first operation. */
 export const SNAPSHOT_BASELINE_INDEX = -1
 
-export function makeSnapshotId(sessionId: string, opIndex: number): string {
-  return `${SNAPSHOT_PREFIX}${sessionId}:${opIndex}`
+export interface SnapshotRef {
+  sessionId: string
+  opIndex: number
 }
 
-export function isSnapshotId(id: string | null | undefined): boolean {
-  return !!id && id.startsWith(SNAPSHOT_PREFIX)
+/** `sessionId:opIndex`, the id without its prefix. Share links carry this. */
+export function snapshotKey({ sessionId, opIndex }: SnapshotRef): string {
+  return `${sessionId}:${opIndex}`
 }
 
-export function parseSnapshotId(
-  id: string
-): { sessionId: string; opIndex: number } | null {
-  if (!isSnapshotId(id)) return null
-  const rest = id.slice(SNAPSHOT_PREFIX.length)
-  const sep = rest.lastIndexOf(':')
+export function parseSnapshotKey(key: string): SnapshotRef | null {
+  const sep = key.lastIndexOf(':')
   if (sep <= 0) return null
-  const sessionId = rest.slice(0, sep)
-  const opIndex = Number(rest.slice(sep + 1))
+  const sessionId = key.slice(0, sep)
+  const opIndex = Number(key.slice(sep + 1))
   if (!Number.isInteger(opIndex)) return null
   if (opIndex < SNAPSHOT_BASELINE_INDEX) return null
   return { sessionId, opIndex }
 }
+
+export function makeSnapshotId(sessionId: string, opIndex: number): string {
+  return SNAPSHOT_PREFIX + snapshotKey({ sessionId, opIndex })
+}
+
+export function isSnapshotId(id: string | null | undefined): boolean {
+  return !!id && id.startsWith(SNAPSHOT_PREFIX)
+}
+
+export function parseSnapshotId(id: string): SnapshotRef | null {
+  if (!isSnapshotId(id)) return null
+  return parseSnapshotKey(id.slice(SNAPSHOT_PREFIX.length))
+}
diff --git a/apps/qc-app/src/utils/time.ts b/apps/qc-app/src/utils/time.ts
index c41acc39..7d5f9c6b 100644
--- a/apps/qc-app/src/utils/time.ts
+++ b/apps/qc-app/src/utils/time.ts
@@ -22,18 +22,6 @@ const formatTime = (time?: string | null): string => {
   return `${day} ${month} ${year}, ${hour}:${minute} ${period}`
 }
 
-/** Local date+time matching the time-range date inputs: `MM/DD/YYYY HH:MM`. */
-export function formatDateInput(iso?: string | null): string {
-  if (!iso) return '–'
-  const d = new Date(iso)
-  if (Number.isNaN(d.getTime())) return iso
-  const mm = String(d.getMonth() + 1).padStart(2, '0')
-  const dd = String(d.getDate()).padStart(2, '0')
-  const hh = String(d.getHours()).padStart(2, '0')
-  const min = String(d.getMinutes()).padStart(2, '0')
-  return `${mm}/${dd}/${d.getFullYear()} ${hh}:${min}`
-}
-
 const MONTH_DAY = new Intl.DateTimeFormat('en-US', {
   month: 'short',
   day: 'numeric',

From d69103f5b041078fe47ce1777bfb04003f7d4da8 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 06:41:01 -0600
Subject: [PATCH 80/93] feat: choose the time zone dates are shown in

---
 apps/qc-app/docs/API_REFERENCE.md             |  47 +++-
 apps/qc-app/docs/ARCHITECTURE.md              |   6 +-
 apps/qc-app/docs/TESTING.md                   |  15 +-
 apps/qc-app/docs/USER_GUIDE.md                |  14 +-
 apps/qc-app/e2e/edit-selection.spec.ts        |  50 +---
 apps/qc-app/e2e/select-while-editing.spec.ts  |  32 +--
 apps/qc-app/e2e/share-url.spec.ts             |  17 +-
 apps/qc-app/e2e/support/plot.ts               |  46 ++++
 apps/qc-app/e2e/time-zone.spec.ts             |  92 +++++++
 .../components/EditData/DriftCorrection.vue   |  13 +-
 .../components/FilterPoints/RangeStager.vue   |   3 +-
 .../components/Navigation/NavigationRail.vue  |   2 +
 .../components/Navigation/TimeZoneMenu.vue    | 145 +++++++++++
 .../components/VisualizeData/ContextPlot.vue  |  26 +-
 .../components/VisualizeData/DataTable.vue    |  29 ++-
 .../VisualizeData/DataVisDatasetsTable.vue    |  12 +-
 .../VisualizeData/DatePickerField.vue         |  60 ++---
 .../src/components/VisualizeData/Plot.vue     |   5 +-
 .../__tests__/DatePickerField.spec.ts         |   6 +-
 .../__tests__/useDataSelection.spec.ts        |  36 +--
 .../src/composables/useDataSelection.ts       |  45 ----
 apps/qc-app/src/composables/useDisplayZone.ts |  24 ++
 apps/qc-app/src/main.ts                       |   3 +
 apps/qc-app/src/store/qcPreferences.ts        |  13 +-
 .../src/utils/__tests__/displayZone.spec.ts   |  87 +++++++
 apps/qc-app/src/utils/dateMath.ts             |  74 +++---
 .../utils/plotting/__tests__/staging.spec.ts  |   3 +-
 apps/qc-app/src/utils/plotting/events.ts      |   5 +-
 apps/qc-app/src/utils/plotting/interaction.ts |  22 +-
 apps/qc-app/src/utils/plotting/operations.ts  |  25 +-
 apps/qc-app/src/utils/plotting/options.ts     |  29 ++-
 apps/qc-app/src/utils/plotting/plotTime.ts    |  34 +++
 apps/qc-app/src/utils/plotting/relayout.ts    |  24 +-
 apps/qc-app/src/utils/plotting/staging.ts     |  42 ++--
 apps/qc-app/src/utils/plotting/zoom.ts        |   8 +-
 apps/qc-app/src/utils/time.ts                 | 131 +++++-----
 apps/qc-app/src/utils/timeRangePresets.ts     |   6 +-
 apps/qc-app/src/utils/timeZone.ts             | 226 ++++++++++++++++++
 apps/qc-app/vite.config.ts                    |   2 +
 packages/qc-utils/docs/API_REFERENCE.md       |   8 +
 packages/qc-utils/src/models/index.ts         |   6 +
 41 files changed, 1046 insertions(+), 427 deletions(-)
 create mode 100644 apps/qc-app/e2e/support/plot.ts
 create mode 100644 apps/qc-app/e2e/time-zone.spec.ts
 create mode 100644 apps/qc-app/src/components/Navigation/TimeZoneMenu.vue
 create mode 100644 apps/qc-app/src/composables/useDisplayZone.ts
 create mode 100644 apps/qc-app/src/utils/__tests__/displayZone.spec.ts
 create mode 100644 apps/qc-app/src/utils/plotting/plotTime.ts
 create mode 100644 apps/qc-app/src/utils/timeZone.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index f63603ad..af862e65 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -758,12 +758,14 @@ puts the editor in read-only mode.
 
 ### `useQcPreferencesStore()` (`src/store/qcPreferences.ts`)
 
-Persisted QC editing preferences. Persistence: key `qc:preferences:v1`,
-`pick: ['processingLevelId']`.
+Persisted QC preferences. Persistence: key `qc:preferences:v1`,
+`pick: ['processingLevelId', 'displayZone']`. `main.ts` creates the store
+at startup so the zone is restored before anything is formatted.
 
 | Name                | Kind  | Type / signature | Notes |
 |---------------------|-------|------------------|-------|
 | `processingLevelId` | state | `string \| null` | Last-used processing level for the Create-Datastream-for-Editing form; null on first use (no assumed default). |
+| `displayZone`       | state | `DisplayZone`    | The time zone dates are shown in; the same ref `utils/timeZone.ts` exports. Defaults to the browser's IANA zone. |
 
 ## Internal: utilities
 
@@ -787,10 +789,49 @@ callers reach for:
 `fetchObservationsSync(datastream, startTime?, endTime?)`: paged
 columnar fetch, returns `{ datetimes: number[]; dataValues: number[] }`.
 
+### `src/utils/timeZone.ts`
+
+The time zone dates are shown and typed in. `displayZone` holds a
+`DisplayZone`, `{ mode: 'utc' | 'fixedOffset' | 'iana', zone }`, where
+`zone` is a fixed offset like `-0700` or an IANA name, the same choice a
+data connection's timestamps offer (the lists are qc-utils'
+`FIXED_OFFSET_TIMEZONES` and `DST_AWARE_TIMEZONES`). `browserZone()` is the
+default.
+
+Instants stay epoch ms everywhere. A **wall** value is an instant moved by
+the zone's offset, so its UTC fields read as the zone's clock:
+
+- `offsetMs(ms)`, `toWall(ms)`, `fromWall(wall)`: the conversion. IANA
+  offsets are looked up once per UTC day per zone and cached, with the
+  transition minute found on a day the offset changes.
+- `toWallArray(xs)`: an ascending array moved to wall time in one pass. It
+  returns `xs` itself when nothing moves (UTC), so only a shift copies.
+- `wallParts(ms)` / `fromWallParts(y, m, d, h, mi, s)`: the zone's clock
+  fields, for pickers and calendar arithmetic.
+- `zoneAbbreviation(ms)`, `zoneDescription(ms)`, `zoneName()`: labels.
+
+`utils/time.ts` formats through it, and `dateMath.ts`, the YTD presets,
+`DatePickerField` and the table's datetime input all work on the zone's
+clock. Change the zone with `useDisplayZone().setZone(zone)`: it reads the
+plot's view as real instants, switches the zone, rebuilds the plot, puts
+the view back and redraws the stage band.
+
+### `src/utils/plotting/plotTime.ts`
+
+Plotly has no time zones and reads epoch ms as UTC, so trace x values go in
+as wall values (`toPlotX`), which makes its axis, ticks and `%{x}` hover
+read in the chosen zone. Ranges, shapes and tick values are written as
+Plotly date strings (`toPlotDate`, `plotCoordToDate`): Plotly reads a bare
+number there as browser-local time, which shifted every programmatic zoom
+by the browser's UTC offset. `plotCoord` reads a range or shape value in
+Plotly's own frame, for comparing with trace values, and `fromPlot` turns
+any Plotly x back into an instant. Values the app keeps (zoom history,
+share-link zoom, the stage band, hover readout) are real instants.
+
 ### `src/utils/dateMath.ts`
 
 `subtractDays`, `subtractMonths`, `subtractYears` for the time-range
-preset buttons.
+preset buttons, on the chosen zone's clock.
 
 ### `src/utils/timeRangePresets.ts`
 
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 8e8290e6..2529acc1 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -117,7 +117,8 @@ src/
 │                               saving operations, commit, and replaying a past session.
 ├─ store/                       Pinia stores. See "State stores" below.
 ├─ utils/
-│  ├─ plotting/                 Plotly integration (trace builders, event handlers, selection, staging).
+│  ├─ plotting/                 Plotly integration (trace builders, event handlers, selection, staging,
+│  │                            and plotTime.ts, the boundary that puts x values in the chosen zone).
 │  ├─ dateMath.ts               Time-range arithmetic for presets ("1w", "1m", "All", …).
 │  ├─ timeRangePresets.ts       Preset definitions, resolved back from the context data's end, or around the session window while editing.
 │  ├─ observations.ts           Observation fetch helpers (paged columnar fetch).
@@ -128,7 +129,8 @@ src/
 │  ├─ qcHistory.ts              Typed reads off QC history and session shapes.
 │  ├─ plotCap.ts                How many datastreams can be plotted.
 │  ├─ rules.ts                  Vuetify form validation rules.
-│  └─ time.ts                   Time unit conversions.
+│  ├─ timeZone.ts               The chosen display time zone and wall-time conversion.
+│  └─ time.ts                   Date formatting in that zone.
 ├─ router/                      vue-router setup, auth + workspace guards.
 ├─ plugins/vuetify.ts           Vuetify theme + icon set.
 ├─ models/, types/              TypeScript models and ambient declarations.
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 6fd7260e..66052ef8 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -12,8 +12,8 @@ the operator's manual; QUALITY.md is the policy.
 
 | Layer      | Runner                          | Where                        | Count    |
 | ---------- | ------------------------------- | ---------------------------- | -------- |
-| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 84 files |
-| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 37 files |
+| Unit       | Vitest                          | `src/**/__tests__/*.spec.ts` | 85 files |
+| End-to-end | Playwright (Chromium + Firefox) | `e2e/*.spec.ts`              | 38 files |
 
 There is no separate "integration" tier; component tests live in the
 unit tier and mount real Vue components with the Vue Test Utils
@@ -82,6 +82,10 @@ notable knobs:
 - `server.deps.inline: ['vuetify']`: Vuetify ships ESM that
   Vitest's default externalization mishandles; inlining lets it
   load.
+- `env: { TZ: 'UTC' }`: every unit test runs in UTC, so the browser's
+  zone, the app's default display zone, is the same on every machine. A
+  test about a zone sets it explicitly (`timeZone.spec.ts` pins
+  `America/Denver`; `displayZone.spec.ts` passes a `DisplayZone`).
 
 ### Test scaffolding
 
@@ -250,7 +254,8 @@ e2e/
 │   ├── fixtures.ts   : workspace / datastream / observation fixtures
 │   ├── global-setup.ts : warms the dev server before the first test
 │   ├── mocks.ts      : page.route() handlers that stand in for HydroServer
-│   └── ops.ts        : op-specific preambles (selectAllPoints, expectHistoryContains)
+│   ├── ops.ts        : op-specific preambles (selectAllPoints, applyChangeValues, expectHistoryContains)
+│   └── plot.ts       : the main plot's x range and trace extents, read back as instants
 └── *.spec.ts         : one file per feature
 ```
 
@@ -359,6 +364,10 @@ function observationsWithGap() {
 }
 ```
 
+Playwright runs in the machine's zone unless a spec sets `timezoneId`.
+`time-zone.spec.ts` runs in `Asia/Tokyo`, so a time read as browser-local
+instead of in the chosen zone would show.
+
 ### Test hooks
 
 `src/testHooks.ts` registers `window.__vbwTestHooks` when
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 69ba57a7..7a3c05c3 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -54,9 +54,16 @@ The edits themselves run in your web browser. HydroServer stores the operations
 4. On **Home**, the left filter drawer is open. Filter the datastream list by site / observed property / processing level there, and click a row to plot it. The time range is set from the **Time range** button on the plot toolbar.
 
 All dates and times in the app, in the pickers, the plot axis, the table and
-tooltips, are in your browser's local time zone. Every time input names that
-zone at its end (for example `MDT`, or `MST` for a date in winter); hover it
-for the full zone name and its UTC offset.
+tooltips, are in one time zone: your browser's, unless you pick another with
+the **globe** button in the navigation rail. The button's menu offers the
+same choices as a data connection's timestamps: **UTC**, a **fixed UTC
+offset**, or an **IANA time zone** (which follows daylight saving), plus a
+button to go back to your browser's zone. The choice is remembered on this
+browser, and changing it redraws the plot over the same stretch of time.
+Observations are stored in UTC either way, so the zone only changes how
+times are shown and typed. Every time input names the zone at its end (for
+example `MDT`, or `MST` for a date in winter); hover it for the full zone
+name and its UTC offset.
 
 If the screen ever stays blank with a console error like `Failed to fetch app settings`, ask your administrator to check the API URL and COOP/COEP configuration ([DEPLOYMENT.md](./DEPLOYMENT.md) covers this).
 
@@ -73,6 +80,7 @@ A thin, always-visible column of icons.
 | HydroServer logo | Top left. Go home. Resets the current view. Asks what should happen to an open edit session first. |
 | Cursor (Select) | Top left. Show the datastream Select drawer + plot. While you are editing, this keeps the session open behind it. |
 | Pencil (Edit) | Top left. Return to the Edit view. **Disabled** until you're editing: pick a datastream to edit with its row's Edit button first. |
+| Globe (time zone) | Bottom left. Labelled with the zone in use (for example `MDT`). Pick the time zone dates are shown in. |
 | Stopwatch (Performance) | Bottom left. Open the Performance Calibration dialog. See "Performance" below. |
 | Briefcase (Workspaces) | Bottom left. Switch workspace, or Continue in the current one. |
 | Logout | Bottom left. Sign out. |
diff --git a/apps/qc-app/e2e/edit-selection.spec.ts b/apps/qc-app/e2e/edit-selection.spec.ts
index c9ec9f91..730543e6 100644
--- a/apps/qc-app/e2e/edit-selection.spec.ts
+++ b/apps/qc-app/e2e/edit-selection.spec.ts
@@ -32,6 +32,7 @@ import {
   FIXTURE_OBS_START_MS,
   MANAGED_DATASTREAM_ID,
 } from './support/fixtures'
+import { plotXRange, traceIds, traceXExtent } from './support/plot'
 
 type PlotRoot = HTMLElement & {
   data?: Array<{ id?: string; x?: Array<number | string> }>
@@ -67,8 +68,11 @@ function tracesOf(page: Page, id: string) {
     const gd = document.querySelector('[data-testid="main-plot"]') as
       | (HTMLElement & { data?: DrawnTrace[] })
       | null
+    // Trace values are wall values in the display zone, the browser's here.
     const ms = (v: number | string) =>
-      typeof v === 'number' ? v : Date.parse(v)
+      typeof v === 'number'
+        ? v + new Date(v).getTimezoneOffset() * 60_000
+        : Date.parse(v)
     return (gd?.data ?? [])
       .filter((t) => t.id === id || t._partOf === id)
       .map((t) => {
@@ -85,25 +89,6 @@ function tracesOf(page: Page, id: string) {
 
 type ReloadProbe = { __contextStart: number; __afterplots: number[] }
 
-function traceIds(page: Page): Promise<string[]> {
-  return page.evaluate(() => {
-    const gd = document.querySelector('[data-testid="main-plot"]') as PlotRoot
-    return (gd?.data ?? []).map((t) => t.id ?? '').filter(Boolean)
-  })
-}
-
-/** Live x-axis range in epoch ms. Plotly date strings are UTC. */
-function xRange(page: Page): Promise<[number, number] | null> {
-  return page.evaluate(() => {
-    const gd = document.querySelector('[data-testid="main-plot"]') as PlotRoot
-    const range = gd?.layout?.xaxis?.range
-    if (!range || range.length !== 2) return null
-    const toMs = (v: number | string) =>
-      typeof v === 'number' ? v : Date.parse(`${v.replace(' ', 'T')}Z`)
-    return [toMs(range[0]!), toMs(range[1]!)] as [number, number]
-  })
-}
-
 /**
  * Type a local date and time into a `DatePickerField`. Its inputs mask
  * digit by digit, so select the whole value and type digits only.
@@ -126,18 +111,6 @@ async function typeDateTime(field: Locator, when: Date) {
   }
 }
 
-/** First and last x of a trace in epoch ms. Plotly date strings are UTC. */
-function traceXExtent(page: Page, id: string): Promise<[number, number] | null> {
-  return page.evaluate((traceId) => {
-    const gd = document.querySelector('[data-testid="main-plot"]') as PlotRoot
-    const xs = (gd?.data ?? []).find((t) => t.id === traceId)?.x
-    if (!xs?.length) return null
-    const toMs = (v: number | string) =>
-      typeof v === 'number' ? v : Date.parse(`${v.replace(' ', 'T')}Z`)
-    return [toMs(xs[0]!), toMs(xs[xs.length - 1]!)] as [number, number]
-  }, id)
-}
-
 /** What a `DatePickerField` shows for `when`: local MM/DD/YYYY and HH:MM. */
 function pickerText(when: Date): [string, string] {
   const pad = (n: number) => String(n).padStart(2, '0')
@@ -388,7 +361,7 @@ test.describe('edit selection', () => {
     const tolerance = 60_000
     await expect
       .poll(async () => {
-        const r = await xRange(page)
+        const r = await plotXRange(page)
         return (
           !!r &&
           Math.abs(r[0] - mid.getTime()) <= tolerance &&
@@ -405,8 +378,8 @@ test.describe('edit selection', () => {
     await page.getByTestId('time-range-menu').getByTestId('date-preset-All').click()
     await page.keyboard.press('Escape')
     await startSessionFromRow(page)
-    await expect.poll(() => xRange(page)).not.toBeNull()
-    const opened = (await xRange(page))!
+    await expect.poll(() => plotXRange(page)).not.toBeNull()
+    const opened = (await plotXRange(page))!
 
     // Zoom in with the wheel so the view differs from the session window.
     const box = (await page.getByTestId('main-plot').boundingBox())!
@@ -414,11 +387,11 @@ test.describe('edit selection', () => {
     for (let i = 0; i < 4; i++) await page.mouse.wheel(0, -200)
     await expect
       .poll(async () => {
-        const r = await xRange(page)
+        const r = await plotXRange(page)
         return !!r && r[1] - r[0] < (opened[1] - opened[0]) * 0.9
       })
       .toBe(true)
-    const zoomed = (await xRange(page))!
+    const zoomed = (await plotXRange(page))!
 
     await page.getByTestId('time-range-btn').click()
     const menu = page.getByTestId('time-range-menu')
@@ -464,8 +437,9 @@ test.describe('edit selection', () => {
 
     // Sample the range over several frames so a late redraw would show up.
     const drift = await page.evaluate(async ([lo, hi]) => {
+      // The display zone's clock, the browser's here (see support/plot.ts).
       const toMs = (v: number | string) =>
-        typeof v === 'number' ? v : Date.parse(`${v.replace(' ', 'T')}Z`)
+        typeof v === 'number' ? v : Date.parse(v.replace(' ', 'T'))
       let worst = 0
       const end = performance.now() + 1_500
       while (performance.now() < end) {
diff --git a/apps/qc-app/e2e/select-while-editing.spec.ts b/apps/qc-app/e2e/select-while-editing.spec.ts
index f56bc829..9871d25a 100644
--- a/apps/qc-app/e2e/select-while-editing.spec.ts
+++ b/apps/qc-app/e2e/select-while-editing.spec.ts
@@ -27,6 +27,7 @@ import {
   buildObservations,
   buildTemperatureObservations,
 } from './support/fixtures'
+import { plotXRange } from './support/plot'
 
 const editPanel = (page: Page) => page.getByTestId('edit-target-panel')
 /** The Select view's right column. The editor stays mounted behind it, so its
@@ -38,31 +39,6 @@ async function goToEditor(page: Page) {
   await waitForEditorReady(page)
 }
 
-/** The live X range of the main plot, as `[loMs, hiMs]`. */
-async function plotXRange(page: Page): Promise<[number, number]> {
-  return page.evaluate(async () => {
-    const wait = (ms: number) => new Promise((r) => setTimeout(r, ms))
-    const start = Date.now()
-    while (Date.now() - start < 10_000) {
-      const gd = document.querySelector('[data-testid="main-plot"]') as
-        | (HTMLElement & {
-            _fullLayout?: {
-              xaxis?: { range?: [number | string, number | string] }
-            }
-          })
-        | null
-      const range = gd?._fullLayout?.xaxis?.range
-      if (range) {
-        const toMs = (v: number | string) =>
-          typeof v === 'string' ? Date.parse(v) : v
-        return [toMs(range[0]), toMs(range[1])] as [number, number]
-      }
-      await wait(100)
-    }
-    throw new Error('plot never reported an x range')
-  })
-}
-
 /** The staged filter band's x span on the live plot, or null when absent. */
 async function stageBandSpan(page: Page): Promise<string | null> {
   return page.evaluate(() => {
@@ -140,12 +116,14 @@ test.describe('Select while editing', () => {
     const stagedBand = await stageBandSpan(page)
     expect(stagedBand).not.toBeNull()
 
-    const before = await plotXRange(page)
+    await expect.poll(() => plotXRange(page)).not.toBeNull()
+    const before = (await plotXRange(page))!
 
     await goToSelect(page)
     await goToEditor(page)
 
-    const after = await plotXRange(page)
+    await expect.poll(() => plotXRange(page)).not.toBeNull()
+    const after = (await plotXRange(page))!
     expect(after[0]).toBeCloseTo(before[0], -4)
     expect(after[1]).toBeCloseTo(before[1], -4)
 
diff --git a/apps/qc-app/e2e/share-url.spec.ts b/apps/qc-app/e2e/share-url.spec.ts
index 63f4bf79..7787a48d 100644
--- a/apps/qc-app/e2e/share-url.spec.ts
+++ b/apps/qc-app/e2e/share-url.spec.ts
@@ -23,6 +23,7 @@ import {
   FIXTURE_OBS_START_MS,
   WORKSPACE_ID,
 } from './support/fixtures'
+import { plotXRange } from './support/plot'
 
 /**
  * Poll the live plot for: the current X range (as `[loMs, hiMs]`)
@@ -100,22 +101,6 @@ async function clickResetAxes(page: Page) {
   }, selector)
 }
 
-/** The live X range of the main plot as `[loMs, hiMs]`, or null. */
-async function plotXRange(page: Page): Promise<[number, number] | null> {
-  return page.evaluate(() => {
-    const gd = document.querySelector('[data-testid="main-plot"]') as
-      | (HTMLElement & {
-          _fullLayout?: { xaxis?: { range?: [number | string, number | string] } }
-        })
-      | null
-    const range = gd?._fullLayout?.xaxis?.range
-    if (!range) return null
-    const toMs = (v: number | string) =>
-      typeof v === 'number' ? v : Date.parse(v)
-    return [toMs(range[0]), toMs(range[1])] as [number, number]
-  })
-}
-
 type SessionWindow = { begin: number; end: number }
 
 /** Resolves with the window of the next session the app creates. */
diff --git a/apps/qc-app/e2e/support/plot.ts b/apps/qc-app/e2e/support/plot.ts
new file mode 100644
index 00000000..0a16234f
--- /dev/null
+++ b/apps/qc-app/e2e/support/plot.ts
@@ -0,0 +1,46 @@
+/**
+ * Reading the main plot's x values back as instants. The app hands Plotly
+ * wall values in the display zone (see `src/utils/plotting/plotTime.ts`),
+ * and that zone is the browser's unless a spec picks another. So a trace
+ * value comes back by removing the browser's offset, and a range string,
+ * which is that zone's clock, parses as browser-local time.
+ */
+
+import type { Page } from '@playwright/test'
+
+type PlotRoot = HTMLElement & {
+  data?: Array<{ id?: string; x?: ArrayLike<number> }>
+  _fullLayout?: { xaxis?: { range?: Array<number | string> } }
+}
+
+/** The main plot's live x range as `[loMs, hiMs]`, or null before it draws. */
+export function plotXRange(page: Page): Promise<[number, number] | null> {
+  return page.evaluate(() => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as PlotRoot
+    const r = gd?._fullLayout?.xaxis?.range
+    if (!r || r.length !== 2) return null
+    const toMs = (v: number | string) =>
+      typeof v === 'number' ? v : Date.parse(v.replace(' ', 'T'))
+    return [toMs(r[0]!), toMs(r[1]!)] as [number, number]
+  })
+}
+
+/** Ids of the main plot's traces, in draw order. */
+export function traceIds(page: Page): Promise<string[]> {
+  return page.evaluate(() => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as PlotRoot
+    return (gd?.data ?? []).map((t) => t.id ?? '').filter(Boolean)
+  })
+}
+
+/** First and last x of a trace as instants, or null when it has none. */
+export function traceXExtent(page: Page, id: string): Promise<[number, number] | null> {
+  return page.evaluate((traceId) => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as PlotRoot
+    const xs = (gd?.data ?? []).find((t) => t.id === traceId)?.x
+    if (!xs?.length) return null
+    const instant = (wall: number) =>
+      wall + new Date(wall).getTimezoneOffset() * 60_000
+    return [instant(xs[0]!), instant(xs[xs.length - 1]!)] as [number, number]
+  }, id)
+}
diff --git a/apps/qc-app/e2e/time-zone.spec.ts b/apps/qc-app/e2e/time-zone.spec.ts
new file mode 100644
index 00000000..b3d3f41a
--- /dev/null
+++ b/apps/qc-app/e2e/time-zone.spec.ts
@@ -0,0 +1,92 @@
+/**
+ * Dates show in the zone the user picks from the nav rail, whatever zone
+ * the browser is in, and the editor still zooms to the real session window.
+ * The browser runs in Tokyo so a browser-local slip would show.
+ */
+
+import { expect, test, type Page } from '@playwright/test'
+import { installMocks } from './support/mocks'
+import { setupEditView } from './support/app'
+import { FIXTURE_OBS_START_MS } from './support/fixtures'
+
+test.use({ timezoneId: 'Asia/Tokyo' })
+
+/** How the table shows a data point's time in `timeZone`. */
+const pointText = (ms: number, timeZone: string) =>
+  new Intl.DateTimeFormat('en-US', {
+    year: 'numeric',
+    month: 'short',
+    day: '2-digit',
+    hour: '2-digit',
+    hour12: false,
+    minute: '2-digit',
+    second: '2-digit',
+    timeZone,
+  }).format(ms)
+
+/** The main plot's x range as instants, read from Plotly's UTC strings
+ *  and moved back by the zone's offset at each end. */
+const plotRangeWall = (page: Page) =>
+  page.evaluate(() => {
+    const gd = document.querySelector('[data-testid="main-plot"]') as any
+    const r = gd?._fullLayout?.xaxis?.range as string[] | undefined
+    return r?.map((v) => Date.parse(`${v.replace(' ', 'T')}Z`)) ?? null
+  })
+
+const firstTableTime = (page: Page) =>
+  page.locator('.editable-cell__display').first().innerText()
+
+async function pickUtc(page: Page) {
+  await page.getByTestId('nav-rail-time-zone').click()
+  await page.getByTestId('time-zone-mode').click()
+  await page.getByRole('option', { name: 'UTC', exact: true }).click()
+  await page.keyboard.press('Escape')
+}
+
+test.beforeEach(async ({ page }) => {
+  await installMocks(page, { qcHistories: true })
+})
+
+test('opens zoomed to the session window, in the chosen zone', async ({
+  page,
+}) => {
+  const created = page.waitForResponse(
+    (r) =>
+      r.request().method() === 'POST' &&
+      /\/sessions$/.test(new URL(r.url()).pathname)
+  )
+  await setupEditView(page)
+  const { data } = await (await created).json()
+  const begin = Date.parse(data.phenomenonTimeStart)
+  const end = Date.parse(data.phenomenonTimeEnd)
+  const tokyo = 9 * 3_600_000
+
+  // In the browser's zone the plot's clock runs 9 hours ahead of UTC.
+  await expect
+    .poll(async () => (await plotRangeWall(page))?.map((w) => w - tokyo))
+    .toEqual([begin, end])
+
+  await pickUtc(page)
+  await expect.poll(() => plotRangeWall(page)).toEqual([begin, end])
+})
+
+test('shows times in the chosen zone, and keeps the choice', async ({
+  page,
+}) => {
+  await setupEditView(page)
+  await page.getByRole('button', { name: 'Table' }).click()
+  await expect
+    .poll(() => firstTableTime(page))
+    .toBe(pointText(FIXTURE_OBS_START_MS, 'Asia/Tokyo'))
+
+  await pickUtc(page)
+  await expect
+    .poll(() => firstTableTime(page))
+    .toBe(pointText(FIXTURE_OBS_START_MS, 'UTC'))
+  await expect(page.getByTestId('nav-rail-time-zone')).toContainText('UTC')
+
+  await page.reload()
+  await expect(page.getByTestId('nav-rail-time-zone')).toContainText('UTC', {
+    timeout: 30_000,
+  })
+})
diff --git a/apps/qc-app/src/components/EditData/DriftCorrection.vue b/apps/qc-app/src/components/EditData/DriftCorrection.vue
index 643f6898..357d775a 100644
--- a/apps/qc-app/src/components/EditData/DriftCorrection.vue
+++ b/apps/qc-app/src/components/EditData/DriftCorrection.vue
@@ -99,13 +99,12 @@ import { storeToRefs } from 'pinia'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { EnumEditOperations } from '@uwrl/qc-utils'
 import { computed } from 'vue'
-import { formatDate } from '@uwrl/qc-utils'
+import { formatDateTime } from '@/utils/time'
 import { usePlotlyStore } from '@/store/plotly'
-import type { PlotData } from 'plotly.js-dist'
 import { useFilterDispatch } from '@/composables/useFilterDispatch'
 import { useUIStore, DriftCorrectionMethods } from '@/store/userInterface'
 const { recordPostActionSelection } = useFilterDispatch()
-const { selectedSeries, plotlyRef, isUpdating } = storeToRefs(usePlotlyStore())
+const { selectedSeries, isUpdating } = storeToRefs(usePlotlyStore())
 const { driftGapWidth, selectedDriftCorrectionMethod } =
   storeToRefs(useUIStore())
 const { redraw } = usePlotlyStore()
@@ -152,12 +151,12 @@ const onDriftCorrection = async () => {
   })
 }
 
+// Group indices are the edit record's, so its own times say where they start.
 const getGroupStart = (group: number[]) => {
-  const trace = plotlyRef.value?.data[0] as Partial<PlotData> | undefined
-  const xData = trace?.x as number[] | undefined
   const firstIdx = group[0]
-  if (!xData || firstIdx === undefined) return ''
-  return formatDate(new Date(xData[firstIdx] as number))
+  const x =
+    firstIdx === undefined ? undefined : selectedSeries.value?.data.dataX[firstIdx]
+  return x === undefined ? '' : formatDateTime(x)
 }
 </script>
 
diff --git a/apps/qc-app/src/components/FilterPoints/RangeStager.vue b/apps/qc-app/src/components/FilterPoints/RangeStager.vue
index 14dac2d4..e7b84973 100644
--- a/apps/qc-app/src/components/FilterPoints/RangeStager.vue
+++ b/apps/qc-app/src/components/FilterPoints/RangeStager.vue
@@ -79,6 +79,7 @@ import {
   stagePanMode,
 } from '@/utils/plotting/staging'
 import DatePickerField from '@/components/VisualizeData/DatePickerField.vue'
+import { fromWallParts, wallParts } from '@/utils/timeZone'
 
 const { selectedSeries } = storeToRefs(usePlotlyStore())
 
@@ -205,7 +206,7 @@ const rangePresets = computed<RangePreset[]>(() => {
       {
         label: 'YTD',
         compute: (b) => {
-          const jan1 = new Date(new Date().getFullYear(), 0, 1).getTime()
+          const jan1 = fromWallParts(wallParts(Date.now()).year, 0, 1)
           const start = Math.max(jan1, b.min)
           if (start >= b.max) return null
           return [start, b.max]
diff --git a/apps/qc-app/src/components/Navigation/NavigationRail.vue b/apps/qc-app/src/components/Navigation/NavigationRail.vue
index bfddc95f..60bf0f44 100644
--- a/apps/qc-app/src/components/Navigation/NavigationRail.vue
+++ b/apps/qc-app/src/components/Navigation/NavigationRail.vue
@@ -76,6 +76,7 @@
     </div>
 
     <div class="rail-bottom">
+      <TimeZoneMenu />
       <PerformanceCalibration rail-button />
       <v-tooltip location="right" :open-delay="400">
         <template #activator="{ props: tipProps }">
@@ -125,6 +126,7 @@ import { ref } from 'vue'
 import HydroServerIcon from '@/assets/icon-color-thick.svg'
 import SelectDrawer from '@/components/Navigation/SelectDrawer.vue'
 import PerformanceCalibration from '@/components/Navigation/PerformanceCalibration.vue'
+import TimeZoneMenu from '@/components/Navigation/TimeZoneMenu.vue'
 import { useUIStore, DrawerType } from '@/store/userInterface'
 import { Snackbar } from '@uwrl/qc-utils'
 import { storeToRefs } from 'pinia'
diff --git a/apps/qc-app/src/components/Navigation/TimeZoneMenu.vue b/apps/qc-app/src/components/Navigation/TimeZoneMenu.vue
new file mode 100644
index 00000000..9576f163
--- /dev/null
+++ b/apps/qc-app/src/components/Navigation/TimeZoneMenu.vue
@@ -0,0 +1,145 @@
+<template>
+  <v-menu
+    v-model="open"
+    :close-on-content-click="false"
+    location="right"
+    offset="8"
+  >
+    <template #activator="{ props: menuProps }">
+      <v-tooltip location="right" :open-delay="400">
+        <template #activator="{ props: tipProps }">
+          <button
+            v-bind="{ ...menuProps, ...tipProps }"
+            type="button"
+            class="rail-btn rail-btn-secondary"
+            data-testid="nav-rail-time-zone"
+          >
+            <span class="rail-pill rail-pill-secondary">
+              <v-icon icon="mdi-earth" size="22" />
+            </span>
+            <span class="rail-label">{{ abbreviation }}</span>
+          </button>
+        </template>
+        <span>Time zone: {{ description }}</span>
+      </v-tooltip>
+    </template>
+
+    <v-card width="380" data-testid="time-zone-menu">
+      <v-card-title class="text-title-medium d-flex align-center ga-2">
+        <v-icon icon="mdi-earth" size="20" />
+        Time zone
+      </v-card-title>
+      <v-card-text class="pt-1">
+        <p class="text-body-small text-medium-emphasis mb-3">
+          Every date the app shows or takes is in this zone: the plot, the
+          table and the date pickers. Observations are stored in UTC either
+          way.
+        </p>
+
+        <v-select
+          :model-value="displayZone.mode"
+          :items="ZONE_MODES"
+          item-title="title"
+          item-value="value"
+          label="Time zone type"
+          density="compact"
+          variant="outlined"
+          hide-details
+          data-testid="time-zone-mode"
+          @update:model-value="onModeChange"
+        />
+
+        <v-autocomplete
+          v-if="displayZone.mode === 'fixedOffset'"
+          :model-value="displayZone.zone"
+          :items="FIXED_OFFSET_TIMEZONES"
+          label="Fixed UTC offset"
+          density="compact"
+          variant="outlined"
+          hide-details
+          class="mt-3"
+          data-testid="time-zone-offset"
+          @update:model-value="onZoneChange"
+        />
+
+        <v-autocomplete
+          v-if="displayZone.mode === 'iana'"
+          :model-value="displayZone.zone"
+          :items="DST_AWARE_TIMEZONES"
+          label="IANA time zone"
+          density="compact"
+          variant="outlined"
+          hide-details
+          class="mt-3"
+          data-testid="time-zone-iana"
+          @update:model-value="onZoneChange"
+        />
+      </v-card-text>
+      <v-card-actions class="px-4 pb-3 pt-0">
+        <v-btn
+          size="small"
+          variant="text"
+          prepend-icon="mdi-crosshairs-gps"
+          :disabled="isBrowserZone"
+          data-testid="time-zone-browser"
+          @click="setZone(browserZone())"
+        >
+          Use my browser's zone ({{ browserZone().zone }})
+        </v-btn>
+      </v-card-actions>
+    </v-card>
+  </v-menu>
+</template>
+
+<script setup lang="ts">
+/** The rail's time zone setting: UTC, a fixed UTC offset or an IANA zone,
+ *  the same choice a data connection's timestamps offer. */
+
+import { computed, ref } from 'vue'
+import { DST_AWARE_TIMEZONES, FIXED_OFFSET_TIMEZONES } from '@uwrl/qc-utils'
+import { useDisplayZone } from '@/composables/useDisplayZone'
+import {
+  browserZone,
+  offsetMs,
+  zoneAbbreviation,
+  zoneDescription,
+  type ZoneMode,
+} from '@/utils/timeZone'
+
+const ZONE_MODES: { title: string; value: ZoneMode }[] = [
+  { title: 'UTC', value: 'utc' },
+  { title: 'Fixed UTC offset', value: 'fixedOffset' },
+  { title: 'IANA time zone', value: 'iana' },
+]
+
+const open = ref(false)
+const { displayZone, setZone } = useDisplayZone()
+
+const abbreviation = computed(() => zoneAbbreviation(Date.now(), displayZone.value))
+const description = computed(() => zoneDescription(Date.now(), displayZone.value))
+
+const isBrowserZone = computed(() => {
+  const browser = browserZone()
+  return displayZone.value.mode === browser.mode && displayZone.value.zone === browser.zone
+})
+
+/** The browser's offset right now, as a list value like `-0600`. */
+function browserOffset(): string {
+  const minutes = Math.round(offsetMs(Date.now(), browserZone()) / 60_000)
+  const sign = minutes < 0 ? '-' : '+'
+  const hh = String(Math.floor(Math.abs(minutes) / 60)).padStart(2, '0')
+  const mm = String(Math.abs(minutes) % 60).padStart(2, '0')
+  const value = `${sign}${hh}${mm}`
+  return FIXED_OFFSET_TIMEZONES.some((o) => o.value === value) ? value : '+0000'
+}
+
+function onModeChange(mode: ZoneMode) {
+  if (mode === 'utc') void setZone({ mode, zone: '' })
+  else if (mode === 'fixedOffset') void setZone({ mode, zone: browserOffset() })
+  else void setZone(browserZone())
+}
+
+function onZoneChange(zone: string | null) {
+  if (zone) void setZone({ mode: displayZone.value.mode, zone })
+}
+</script>
diff --git a/apps/qc-app/src/components/VisualizeData/ContextPlot.vue b/apps/qc-app/src/components/VisualizeData/ContextPlot.vue
index 8af5ba96..4183fb1e 100644
--- a/apps/qc-app/src/components/VisualizeData/ContextPlot.vue
+++ b/apps/qc-app/src/components/VisualizeData/ContextPlot.vue
@@ -35,6 +35,13 @@
 
 <script setup lang="ts">
 import { computed, onBeforeUnmount, onMounted, ref, watch } from 'vue'
+import {
+  plotCoord,
+  plotCoordToDate,
+  toPlotDate,
+  toPlotX,
+} from '@/utils/plotting/plotTime'
+import { displayZone } from '@/utils/timeZone'
 import { storeToRefs } from 'pinia'
 import Plotly from 'plotly.js-dist'
 import type { Layout, LayoutAxis, PlotlyHTMLElement } from 'plotly.js-dist'
@@ -148,7 +155,7 @@ function buildContextTraces(series: GraphSeries[]): {
       color: string | undefined
     ) =>
       traces.push({
-        x: line.x as number[],
+        x: toPlotX(line.x) as unknown as number[],
         y: line.y as number[],
         type: 'scattergl',
         mode: 'lines',
@@ -187,8 +194,9 @@ function readMainXRange(): [number, number] | null {
       | Array<string | number>
       | undefined)
   if (!r) return null
-  const a = typeof r[0] === 'string' ? Date.parse(r[0]) : Number(r[0])
-  const b = typeof r[1] === 'string' ? Date.parse(r[1]) : Number(r[1])
+  // Plotly's frame, like the strip's own range (see `plotTime.ts`).
+  const a = plotCoord(r[0] as string | number)
+  const b = plotCoord(r[1] as string | number)
   if (!Number.isFinite(a) || !Number.isFinite(b)) return null
   return [a, b]
 }
@@ -209,8 +217,8 @@ function getCtxPlotMetrics(): {
   if (!xa || xa._offset == null || xa._length == null) return null
   const r = xa.range as Array<string | number> | undefined
   if (!r) return null
-  const a = typeof r[0] === 'string' ? Date.parse(r[0]) : Number(r[0])
-  const b = typeof r[1] === 'string' ? Date.parse(r[1]) : Number(r[1])
+  const a = plotCoord(r[0] as string | number)
+  const b = plotCoord(r[1] as string | number)
   if (!Number.isFinite(a) || !Number.isFinite(b) || b <= a) return null
   return { offset: xa._offset, length: xa._length, range: [a, b] }
 }
@@ -264,7 +272,7 @@ function scheduleMainUpdate(target: [number, number]) {
     void Plotly.relayout(
       gd as unknown as HTMLElement,
       {
-        'xaxis.range': [range[0], range[1]],
+        'xaxis.range': [plotCoordToDate(range[0]), plotCoordToDate(range[1])],
         'xaxis.autorange': false,
       } as unknown as Partial<Layout>
     )
@@ -402,7 +410,7 @@ async function buildOrUpdate() {
     xaxis: {
       type: 'date',
       visible: false,
-      range: [extent[0], extent[1]],
+      range: [toPlotDate(extent[0]), toPlotDate(extent[1])],
       fixedrange: true,
       autorange: false,
     } as Partial<LayoutAxis>,
@@ -446,7 +454,9 @@ const rebuildSignature = computed(() => {
     (s) => `${s.id}:${s.data?.dataX?.length ?? 0}:${s.color}`
   )
   const hidden = [...hiddenIds.value].sort().join(',')
-  return parts.join('|') + '#' + qid + '~' + sid + '!' + hidden
+  // A zone change moves every x in the plot's frame.
+  const zone = `${displayZone.value.mode}:${displayZone.value.zone}`
+  return parts.join('|') + '#' + qid + '~' + sid + '!' + hidden + '@' + zone
 })
 
 watch(rebuildSignature, () => {
diff --git a/apps/qc-app/src/components/VisualizeData/DataTable.vue b/apps/qc-app/src/components/VisualizeData/DataTable.vue
index ba8b85bb..141ee6a8 100644
--- a/apps/qc-app/src/components/VisualizeData/DataTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataTable.vue
@@ -104,14 +104,14 @@
         <template #item.datetime="{ index }">
           <EditableCell
             :value="formatDatetimeLocal(selectedSeries?.data.dataX[index])"
-            :display="formatDate(new Date(selectedSeries?.data.dataX[index] as number))"
+            :display="formatDateTime(selectedSeries?.data.dataX[index] as number)"
             :edited="datetimeEdits.has(index)"
             :original-display="
-              formatDate(new Date(selectedSeries?.data.dataX[index] as number))
+              formatDateTime(selectedSeries?.data.dataX[index] as number)
             "
             :edited-display="
               datetimeEdits.has(index)
-                ? formatDate(new Date(datetimeEdits.get(index)!))
+                ? formatDateTime(datetimeEdits.get(index)!)
                 : ''
             "
             input-type="datetime-local"
@@ -171,11 +171,9 @@ import {
 import { usePlotlyStore } from '@/store/plotly'
 import { storeToRefs } from 'pinia'
 import { useDataVisStore } from '@/store/dataVisualization'
-import {
-  EnumEditOperations,
-  EnumFilterOperations,
-  formatDate,
-} from '@uwrl/qc-utils'
+import { EnumEditOperations, EnumFilterOperations } from '@uwrl/qc-utils'
+import { formatDateTime } from '@/utils/time'
+import { fromWallParts, wallParts } from '@/utils/timeZone'
 import { useDataSelection } from '@/composables/useDataSelection'
 import { useQualifierStore } from '@/store/qualifiers'
 import EditableCell from '@/components/VisualizeData/EditableCell.vue'
@@ -319,21 +317,22 @@ function formatNumber(num: unknown): string {
   return String(parseFloat((num as number).toFixed(4)))
 }
 
-// YYYY-MM-DDTHH:mm:ss in local time for <input type="datetime-local">.
+// YYYY-MM-DDTHH:mm:ss in the chosen zone for <input type="datetime-local">.
 function formatDatetimeLocal(epoch: number | undefined): string {
   if (epoch == null || Number.isNaN(epoch)) return ''
-  const d = new Date(epoch)
+  const p = wallParts(epoch)
   const pad = (n: number) => String(n).padStart(2, '0')
   return (
-    `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}` +
-    `T${pad(d.getHours())}:${pad(d.getMinutes())}:${pad(d.getSeconds())}`
+    `${p.year}-${pad(p.month + 1)}-${pad(p.day)}` +
+    `T${pad(p.hours)}:${pad(p.minutes)}:${pad(p.seconds)}`
   )
 }
 
 function parseDatetimeLocal(raw: string): number | null {
-  if (!raw) return null
-  const t = new Date(raw).getTime()
-  return Number.isNaN(t) ? null : t
+  const m = /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2})(?::(\d{2}))?/.exec(raw)
+  if (!m) return null
+  const [, y, mo, d, h, mi, s] = m.map(Number)
+  return fromWallParts(y!, mo! - 1, d!, h!, mi!, s || 0)
 }
 
 function onValueSave(index: number, raw: string) {
diff --git a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
index c523b34a..d3d4d2ff 100644
--- a/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
+++ b/apps/qc-app/src/components/VisualizeData/DataVisDatasetsTable.vue
@@ -329,6 +329,7 @@ import { useEditEntry } from '@/composables/useEditEntry'
 import { useClearPlot } from '@/composables/useClearPlot'
 import { isSnapshotId } from '@/utils/snapshotId'
 import { PLOT_CAP } from '@/utils/plotCap'
+import { formatDateTime } from '@/utils/time'
 
 const emit = defineEmits<{
   (e: 'edit', datastream: Datastream & DatastreamExtended): void
@@ -425,17 +426,10 @@ const formatCount = (n: unknown): string => {
   return Number.isFinite(v) ? NUMBER_FORMATTER.format(v) : '-'
 }
 
-const DATE_FORMATTER = new Intl.DateTimeFormat(undefined, {
-  year: 'numeric',
-  month: 'short',
-  day: 'numeric',
-  hour: '2-digit',
-  minute: '2-digit',
-})
 const formatTableDate = (raw: unknown): string => {
   if (!raw) return '-'
-  const d = new Date(raw as string)
-  return Number.isNaN(d.getTime()) ? '-' : DATE_FORMATTER.format(d)
+  const ms = new Date(raw as string).getTime()
+  return Number.isNaN(ms) ? '-' : formatDateTime(ms)
 }
 
 const { canClearPlot, clearPlot } = useClearPlot()
diff --git a/apps/qc-app/src/components/VisualizeData/DatePickerField.vue b/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
index c711b851..833bfd9e 100644
--- a/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
+++ b/apps/qc-app/src/components/VisualizeData/DatePickerField.vue
@@ -19,7 +19,7 @@
       :placeholder="seconds ? 'HH:MM:SS' : 'HH:MM'"
       prepend-inner-icon="mdi-clock-outline"
       :suffix="zone"
-      :title="`Local time: ${zoneDescription}`"
+      :title="`Time zone: ${zoneDescription}`"
       hide-details
       density="compact"
       :style="{ flex: '0 0 auto', width: seconds ? '10.5rem' : '8.75rem' }"
@@ -63,6 +63,7 @@ import {
   watch,
 } from 'vue'
 import { timeZoneAbbreviation, timeZoneDescription } from '@/utils/time'
+import { fromWallParts, wallParts } from '@/utils/timeZone'
 
 defineOptions({ inheritAttrs: false })
 
@@ -115,52 +116,56 @@ const TIME_WITH_SECONDS: MaskShape = {
 
 const timeShape = (): MaskShape => (props.seconds ? TIME_WITH_SECONDS : TIME)
 
-const formatDateStr = (date: Date) => {
-  const m = String(date.getMonth() + 1).padStart(2, '0')
-  const d = String(date.getDate()).padStart(2, '0')
-  return `${m}/${d}/${date.getFullYear()}`
-}
+// The calendar works on browser-local Dates, so a calendar day travels as a
+// local midnight. The typed text and the emitted instant are in the chosen
+// zone (`timeZone.ts`).
+const pad = (n: number) => String(n).padStart(2, '0')
 
-const formatTimeStr = (date: Date) => {
-  const h = String(date.getHours()).padStart(2, '0')
-  const m = String(date.getMinutes()).padStart(2, '0')
-  if (!props.seconds) return `${h}:${m}`
-  const s = String(date.getSeconds()).padStart(2, '0')
-  return `${h}:${m}:${s}`
+/** MM/DD/YYYY of a calendar day. */
+const dayLabel = (day: Date) =>
+  `${pad(day.getMonth() + 1)}/${pad(day.getDate())}/${day.getFullYear()}`
+
+/** The calendar day `date` falls on in the chosen zone. */
+const calendarDay = (date: Date) => {
+  const p = wallParts(date.getTime())
+  return new Date(p.year, p.month, p.day)
 }
 
-const toMidnight = (date: Date) => {
-  const d = new Date(date)
-  d.setHours(0, 0, 0, 0)
-  return d
+const formatDateStr = (date: Date) => dayLabel(calendarDay(date))
+
+const formatTimeStr = (date: Date) => {
+  const p = wallParts(date.getTime())
+  const hm = `${pad(p.hours)}:${pad(p.minutes)}`
+  return props.seconds ? `${hm}:${pad(p.seconds)}` : hm
 }
 
 const showDateDialog = ref(false)
 
-// Inputs read and write the browser's local time; say which one.
+// Inputs read and write the chosen zone; say which one.
 const zone = computed(() => timeZoneAbbreviation(props.modelValue))
 const zoneDescription = computed(() => timeZoneDescription(props.modelValue))
 
 const dateInput = ref(formatDateStr(props.modelValue))
 const timeInput = ref(formatTimeStr(props.modelValue))
-const pickerDate = ref<Date>(toMidnight(props.modelValue))
+const pickerDate = ref<Date>(calendarDay(props.modelValue))
 
 watch(
   () => props.modelValue,
   (newValue) => {
     dateInput.value = formatDateStr(newValue)
     timeInput.value = formatTimeStr(newValue)
-    pickerDate.value = toMidnight(newValue)
+    pickerDate.value = calendarDay(newValue)
   }
 )
 
-const emitCombined = (date: Date, time: string) => {
+const emitCombined = (day: Date, time: string) => {
   const parts = time.split(':').map(Number)
   const h = parts[0] || 0
   const m = parts[1] || 0
   const s = props.seconds ? (parts[2] || 0) : 0
-  const result = new Date(date)
-  result.setHours(h, m, s, 0)
+  const result = new Date(
+    fromWallParts(day.getFullYear(), day.getMonth(), day.getDate(), h, m, s)
+  )
   // Unchanged at the field's resolution, even if the model is finer.
   if (
     formatDateStr(result) === formatDateStr(props.modelValue) &&
@@ -172,8 +177,8 @@ const emitCombined = (date: Date, time: string) => {
 }
 
 const onDatePicked = (date: Date) => {
-  pickerDate.value = toMidnight(date)
-  dateInput.value = formatDateStr(date)
+  pickerDate.value = new Date(date.getFullYear(), date.getMonth(), date.getDate())
+  dateInput.value = dayLabel(pickerDate.value)
   emitCombined(pickerDate.value, timeInput.value)
   showDateDialog.value = false
 }
@@ -185,14 +190,14 @@ const handleDateBlur = () => {
     const d = Math.max(1, Math.min(31, +match[2]!))
     const parsed = new Date(+match[3]!, m - 1, d)
     if (!isNaN(parsed.getTime())) {
-      pickerDate.value = toMidnight(parsed)
-      dateInput.value = formatDateStr(parsed)
+      pickerDate.value = parsed
+      dateInput.value = dayLabel(parsed)
       emitCombined(pickerDate.value, timeInput.value)
       return
     }
   }
   // Anything not a complete date snaps back to the picker's last good value.
-  dateInput.value = formatDateStr(pickerDate.value)
+  dateInput.value = dayLabel(pickerDate.value)
 }
 
 const handleTimeBlur = () => {
@@ -204,7 +209,6 @@ const handleTimeBlur = () => {
     const h = Math.max(0, Math.min(23, +match[1]!))
     const m = Math.max(0, Math.min(59, +match[2]!))
     const s = props.seconds ? Math.max(0, Math.min(59, +match[3]!)) : 0
-    const pad = (n: number) => String(n).padStart(2, '0')
     const time = props.seconds
       ? `${pad(h)}:${pad(m)}:${pad(s)}`
       : `${pad(h)}:${pad(m)}`
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index 20798532..287b6856 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -363,7 +363,7 @@
               aria-live="polite"
             >
               <span class="mr-2">
-                <b>x</b> {{ formatDate(new Date(hover.x)) }}
+                <b>x</b> {{ formatDateTime(hover.x) }}
               </span>
               <span>
                 <b>{{ yReadoutLabel }}</b> {{ hover.y }}{{ yReadoutUnit }}
@@ -400,7 +400,8 @@ import TimeRangeMenu from '@/components/VisualizeData/TimeRangeMenu.vue'
 import { useDataSelection } from '@/composables/useDataSelection'
 import { useBufferedNumber } from '@/composables/useBufferedNumber'
 import { usePersistedFlag } from '@/composables/useResizable'
-import { formatDate, Snackbar } from '@uwrl/qc-utils'
+import { Snackbar } from '@uwrl/qc-utils'
+import { formatDateTime } from '@/utils/time'
 import { useDataVisStore } from '@/store/dataVisualization'
 import { useQcSessionStore } from '@/store/qcSession'
 import { DrawerType, useUIStore } from '@/store/userInterface'
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts
index fa940d87..363963f9 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DatePickerField.spec.ts
@@ -54,13 +54,13 @@ describe('DatePickerField', () => {
     wrapper.unmount()
   })
 
-  it('labels the time input with the local zone of the shown date', () => {
+  it('labels the time input with the zone of the shown date', () => {
     const value = new Date(2026, 6, 15, 12)
     const wrapper = mountField(value)
     const field = wrapper.find('[data-testid="date-picker-time"]')
     expect(field.text()).toContain(timeZoneAbbreviation(value))
-    expect(wrapper.find('[title^="Local time"]').attributes('title')).toBe(
-      `Local time: ${timeZoneDescription(value)}`
+    expect(wrapper.find('[title^="Time zone"]').attributes('title')).toBe(
+      `Time zone: ${timeZoneDescription(value)}`
     )
     wrapper.unmount()
   })
diff --git a/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts b/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts
index 650940e2..23fb29e2 100644
--- a/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useDataSelection.spec.ts
@@ -146,10 +146,8 @@ describe('useDataSelection date-range helpers', () => {
     setActivePinia(createPinia())
     vi.clearAllMocks()
 
-    // Seed the plotly ref with a deterministic x-axis so the
-    // computeds can look up dates by index without a real Plotly
-    // runtime. One-hour spacing over four points lets us pick clear
-    // index boundaries for `selectDateRange`.
+    // One-hour spacing over four points gives clear index boundaries for
+    // `selectDateRange`.
     const base = new Date('2025-01-01T00:00:00Z').getTime()
     const hour = 60 * 60 * 1000
     const xs = [
@@ -172,36 +170,6 @@ describe('useDataSelection date-range helpers', () => {
     }
   })
 
-  it('startDate / endDate fall back to the series bookends with no selection', async () => {
-    const { useDataSelection } = await import('@/composables/useDataSelection')
-    const { startDate, endDate } = useDataSelection()
-
-    expect(startDate.value.toISOString()).toBe('2025-01-01T00:00:00.000Z')
-    expect(endDate.value.toISOString()).toBe('2025-01-01T03:00:00.000Z')
-  })
-
-  it('startDate / endDate reflect the selected range when selectedData is set', async () => {
-    const { useDataSelection } = await import('@/composables/useDataSelection')
-    const { useDataVisStore } = await import('@/store/dataVisualization')
-    const store = useDataVisStore()
-    store.selectedData = [1, 2]
-
-    const { startDate, endDate } = useDataSelection()
-    expect(startDate.value.toISOString()).toBe('2025-01-01T01:00:00.000Z')
-    expect(endDate.value.toISOString()).toBe('2025-01-01T02:00:00.000Z')
-  })
-
-  it('startDateString / endDateString use formatDate on the resolved dates', async () => {
-    const { useDataSelection } = await import('@/composables/useDataSelection')
-    const { startDateString, endDateString } = useDataSelection()
-
-    // formatDate is stubbed to ISO in the module mock above, so the
-    // ternary `startDate.value ? formatDate(...) : ''` should emit an
-    // ISO string (the non-empty branch).
-    expect(startDateString.value).toBe('2025-01-01T00:00:00.000Z')
-    expect(endDateString.value).toBe('2025-01-01T03:00:00.000Z')
-  })
-
   it('selectDateRange selects every point in the inclusive window', async () => {
     const { useDataSelection } = await import('@/composables/useDataSelection')
     const { useDataVisStore } = await import('@/store/dataVisualization')
diff --git a/apps/qc-app/src/composables/useDataSelection.ts b/apps/qc-app/src/composables/useDataSelection.ts
index f4dea043..42993f76 100644
--- a/apps/qc-app/src/composables/useDataSelection.ts
+++ b/apps/qc-app/src/composables/useDataSelection.ts
@@ -2,7 +2,6 @@ import { useDataVisStore } from '@/store/dataVisualization'
 import { usePlotlyStore } from '@/store/plotly'
 import {
   EnumFilterOperations,
-  formatDate,
   findFirstGreaterOrEqual,
   findLastLessOrEqual,
 } from '@uwrl/qc-utils'
@@ -11,12 +10,9 @@ import {
   setSelectedPoints,
 } from '@/utils/plotting/plotly'
 import type { AppPlotlyTrace } from '@/utils/plotting/plotly'
-import type { PlotData } from 'plotly.js-dist'
 import { storeToRefs } from 'pinia'
 import { useEditLock } from '@/composables/useEditLock'
 
-import { computed } from 'vue'
-
 export function useDataSelection() {
   const { plotlyRef, suppressedEchoSelection } = storeToRefs(
     usePlotlyStore()
@@ -131,43 +127,6 @@ export function useDataSelection() {
     }
   }
 
-  // `startDate` / `endDate` bracket the current selection, or the full
-  // series when nothing is selected. The `|| fallback` arm that used to
-  // sit on `new Date(...)` was dead code: `new Date()` is truthy even
-  // when given `undefined` (it just produces an Invalid Date), so the
-  // fallback was unreachable. The computeds now always return a Date,
-  // and the downstream string helpers stop guarding against a value
-  // that can't appear.
-  const traceX = (): number[] | undefined => {
-    const trace = plotlyRef.value?.data[0] as Partial<PlotData> | undefined
-    return trace?.x as number[] | undefined
-  }
-
-  const startDate = computed(() => {
-    if (selectedData.value?.length) {
-      const startIndex = selectedData.value[0] as number
-      const xs = traceX()
-      const ts = xs?.[startIndex]
-      if (ts !== undefined) return new Date(ts)
-    }
-    return selectedSeries.value?.data.beginTime ?? new Date()
-  })
-
-  const endDate = computed(() => {
-    if (selectedData.value?.length) {
-      const endIndex = selectedData.value[
-        selectedData.value.length - 1
-      ] as number
-      const xs = traceX()
-      const ts = xs?.[endIndex]
-      if (ts !== undefined) return new Date(ts)
-    }
-    return selectedSeries.value?.data.endTime ?? new Date()
-  })
-
-  const startDateString = computed(() => formatDate(startDate.value))
-  const endDateString = computed(() => formatDate(endDate.value))
-
   /** Select all data points within the given date range */
   const selectDateRange = async (from: Date, to: Date) => {
     const dataX = selectedSeries.value?.data.dataX
@@ -192,10 +151,6 @@ export function useDataSelection() {
   return {
     setPlotSelection,
     clearSelected,
-    startDate,
-    endDate,
-    startDateString,
-    endDateString,
     selectDateRange,
   }
 }
diff --git a/apps/qc-app/src/composables/useDisplayZone.ts b/apps/qc-app/src/composables/useDisplayZone.ts
new file mode 100644
index 00000000..a9fee698
--- /dev/null
+++ b/apps/qc-app/src/composables/useDisplayZone.ts
@@ -0,0 +1,24 @@
+/**
+ * Change the time zone dates are shown in. The plot is redrawn in the new
+ * zone over the same stretch of time, and the stage band stays where it was.
+ */
+
+import { useDataVisStore } from '@/store/dataVisualization'
+import { applyZoomState, captureCurrentZoomState } from '@/utils/plotting/zoom'
+import { redrawStageShape } from '@/utils/plotting/staging'
+import { displayZone, type DisplayZone } from '@/utils/timeZone'
+
+export function useDisplayZone() {
+  async function setZone(next: DisplayZone): Promise<void> {
+    const current = displayZone.value
+    if (current.mode === next.mode && current.zone === next.zone) return
+    // Read in the old zone: the snapshot holds real instants.
+    const view = captureCurrentZoomState('init')
+    displayZone.value = { ...next }
+    await useDataVisStore().rebuildPlot()
+    if (view) await applyZoomState(view)
+    await redrawStageShape()
+  }
+
+  return { displayZone, setZone }
+}
diff --git a/apps/qc-app/src/main.ts b/apps/qc-app/src/main.ts
index 6fa08cf9..0728a3c8 100644
--- a/apps/qc-app/src/main.ts
+++ b/apps/qc-app/src/main.ts
@@ -10,12 +10,15 @@ import hs, { createHydroServer, User } from '@hydroserver/client'
 import { useUserStore } from '@/store/user'
 import { useHydroServer } from '@/store/hydroserver'
 import { useWorkspaceStore } from '@/store/workspaces'
+import { useQcPreferencesStore } from '@/store/qcPreferences'
 import { ensureCalibration } from '@uwrl/qc-utils'
 
 const app = createApp(App)
 
 async function initializeApp() {
   app.use(store)
+  // Restores the chosen time zone before anything is formatted.
+  useQcPreferencesStore()
 
   const hydroServerHost =
     import.meta.env.VITE_APP_PROXY_BASE_URL
diff --git a/apps/qc-app/src/store/qcPreferences.ts b/apps/qc-app/src/store/qcPreferences.ts
index e3e229ab..9ef2ccd2 100644
--- a/apps/qc-app/src/store/qcPreferences.ts
+++ b/apps/qc-app/src/store/qcPreferences.ts
@@ -1,24 +1,23 @@
 /**
- * Persisted QC editing preferences.
- *
- * Currently just the last-used processing level for the "Create Datastream
- * for Editing" form, so the user's choice is remembered across reloads.
- * Defaults to null (no assumed processing level on first use).
+ * Persisted QC preferences: the last-used processing level for the create
+ * form (null on first use), and the time zone dates are shown in.
  */
 
 import { defineStore } from 'pinia'
 import { ref } from 'vue'
+import { displayZone } from '@/utils/timeZone'
 
 export const useQcPreferencesStore = defineStore(
   'qcPreferences',
   () => {
     const processingLevelId = ref<string | null>(null)
-    return { processingLevelId }
+    // The module-level ref, so plain utilities read the same zone.
+    return { processingLevelId, displayZone }
   },
   {
     persist: {
       key: 'qc:preferences:v1',
-      pick: ['processingLevelId'],
+      pick: ['processingLevelId', 'displayZone'],
     },
   }
 )
diff --git a/apps/qc-app/src/utils/__tests__/displayZone.spec.ts b/apps/qc-app/src/utils/__tests__/displayZone.spec.ts
new file mode 100644
index 00000000..e45bb58b
--- /dev/null
+++ b/apps/qc-app/src/utils/__tests__/displayZone.spec.ts
@@ -0,0 +1,87 @@
+import { describe, it, expect } from 'vitest'
+import {
+  fromWall,
+  fromWallParts,
+  offsetMs,
+  toWall,
+  toWallArray,
+  wallParts,
+  zoneAbbreviation,
+  zoneDescription,
+  type DisplayZone,
+} from '../timeZone'
+import { fromPlot, plotCoord, toPlotDate } from '../plotting/plotTime'
+
+const HOUR = 3_600_000
+const denver: DisplayZone = { mode: 'iana', zone: 'America/Denver' }
+const india: DisplayZone = { mode: 'fixedOffset', zone: '+0530' }
+const utc: DisplayZone = { mode: 'utc', zone: '' }
+
+// 2026-03-08 09:00Z is 2 AM MST, when Denver springs forward to MDT.
+const SPRING = Date.UTC(2026, 2, 8, 9)
+
+describe('display zone offsets', () => {
+  it('reads an IANA zone, tracking daylight saving', () => {
+    expect(offsetMs(Date.UTC(2026, 0, 15, 12), denver)).toBe(-7 * HOUR)
+    expect(offsetMs(Date.UTC(2026, 6, 15, 12), denver)).toBe(-6 * HOUR)
+  })
+
+  it('changes offset at the transition minute, not the day', () => {
+    expect(offsetMs(SPRING - 60_000, denver)).toBe(-7 * HOUR)
+    expect(offsetMs(SPRING, denver)).toBe(-6 * HOUR)
+  })
+
+  it('reads a fixed offset and UTC', () => {
+    expect(offsetMs(0, india)).toBe(5.5 * HOUR)
+    expect(offsetMs(0, utc)).toBe(0)
+  })
+
+  // Outside the hour that falling back repeats, which has two instants.
+  it('round-trips an instant through its wall value', () => {
+    for (const ms of [SPRING - HOUR, SPRING, SPRING + HOUR, Date.UTC(2026, 10, 1, 10)]) {
+      expect(fromWall(toWall(ms, denver), denver)).toBe(ms)
+    }
+  })
+
+  it('reads and builds the zone clock', () => {
+    const ms = Date.UTC(2026, 6, 15, 18, 30)
+    expect(wallParts(ms, denver)).toEqual({
+      year: 2026,
+      month: 6,
+      day: 15,
+      hours: 12,
+      minutes: 30,
+      seconds: 0,
+    })
+    expect(fromWallParts(2026, 6, 15, 12, 30, 0, denver)).toBe(ms)
+  })
+
+  it('moves an array across a transition, and leaves UTC uncopied', () => {
+    const xs = new Float64Array([SPRING - HOUR, SPRING + HOUR])
+    expect(Array.from(toWallArray(xs, denver))).toEqual([
+      SPRING - 8 * HOUR,
+      SPRING - 5 * HOUR,
+    ])
+    expect(toWallArray(xs, utc)).toBe(xs)
+  })
+
+  it('names the zone', () => {
+    expect(zoneAbbreviation(Date.UTC(2026, 0, 15), denver)).toBe('MST')
+    expect(zoneAbbreviation(0, india)).toBe('UTC+05:30')
+    expect(zoneDescription(Date.UTC(2026, 6, 15), denver)).toBe(
+      'America/Denver, UTC-06:00'
+    )
+    expect(zoneAbbreviation(0, utc)).toBe('UTC')
+  })
+})
+
+describe('plot time', () => {
+  // The test run's zone is UTC (vite.config), so the browser zone adds nothing.
+  it('writes Plotly date strings and reads them back', () => {
+    const ms = Date.UTC(2026, 0, 2, 3, 4, 5, 6)
+    expect(toPlotDate(ms)).toBe('2026-01-02 03:04:05.006')
+    expect(fromPlot('2026-01-02 03:04:05.006')).toBe(ms)
+    expect(plotCoord('2026-01-02')).toBe(Date.UTC(2026, 0, 2))
+    expect(plotCoord(42)).toBe(42)
+  })
+})
diff --git a/apps/qc-app/src/utils/dateMath.ts b/apps/qc-app/src/utils/dateMath.ts
index 2c8a983d..0a0abe69 100644
--- a/apps/qc-app/src/utils/dateMath.ts
+++ b/apps/qc-app/src/utils/dateMath.ts
@@ -1,46 +1,40 @@
 /**
- * Pure date-arithmetic helpers used by the time-range presets. JS's
- * native `new Date(y, m, d)` silently rolls overflow (`Feb 31` →
- * `Mar 3`), which made `subtractMonths(Aug 31, 6)` land three days
- * into March instead of on the Feb/Mar boundary. Clone the input,
- * subtract, and clamp the day-of-month back to the target month's
- * last day, matching the arithmetic Plotly's `rangeselector`
- * performs internally.
+ * Calendar arithmetic for the time-range presets, on the chosen zone's
+ * clock (`timeZone.ts`), so a month back from 9 AM lands on 9 AM. JS's
+ * date setters silently roll overflow (`Feb 31` → `Mar 3`), which made
+ * `subtractMonths(Aug 31, 6)` land three days into March instead of on the
+ * Feb/Mar boundary. Shift from day 1, then clamp the day-of-month back to
+ * the target month's last day, matching the arithmetic Plotly's
+ * `rangeselector` performs internally.
  */
 
-export const subtractDays = (d: Date, days: number): Date => {
-  const r = new Date(d)
-  r.setDate(r.getDate() - days)
-  return r
-}
+import { fromWall, toWall } from '@/utils/timeZone'
 
-export const subtractMonths = (d: Date, months: number): Date => {
-  const r = new Date(d)
-  const day = r.getDate()
-  // Move to day 1 before shifting the month so the shift can't
-  // overflow (e.g. shifting `Aug 31` by −6 never becomes `Feb 31`
-  // and then `Mar 3` via rollover).
-  r.setDate(1)
-  r.setMonth(r.getMonth() - months)
-  const lastDayOfTarget = new Date(
-    r.getFullYear(),
-    r.getMonth() + 1,
-    0
-  ).getDate()
-  r.setDate(Math.min(day, lastDayOfTarget))
-  return r
+/** Apply `shift` to `d`'s wall value, read with UTC setters. */
+const onWall = (d: Date, shift: (w: Date) => void): Date => {
+  const w = new Date(toWall(d.getTime()))
+  shift(w)
+  return new Date(fromWall(w.getTime()))
 }
 
-export const subtractYears = (d: Date, years: number): Date => {
-  const r = new Date(d)
-  const day = r.getDate()
-  r.setDate(1)
-  r.setFullYear(r.getFullYear() - years)
-  const lastDayOfTarget = new Date(
-    r.getFullYear(),
-    r.getMonth() + 1,
-    0
-  ).getDate()
-  r.setDate(Math.min(day, lastDayOfTarget))
-  return r
-}
+const daysIn = (year: number, month: number) =>
+  new Date(Date.UTC(year, month + 1, 0)).getUTCDate()
+
+export const subtractDays = (d: Date, days: number): Date =>
+  onWall(d, (w) => w.setUTCDate(w.getUTCDate() - days))
+
+export const subtractMonths = (d: Date, months: number): Date =>
+  onWall(d, (w) => {
+    const day = w.getUTCDate()
+    w.setUTCDate(1)
+    w.setUTCMonth(w.getUTCMonth() - months)
+    w.setUTCDate(Math.min(day, daysIn(w.getUTCFullYear(), w.getUTCMonth())))
+  })
+
+export const subtractYears = (d: Date, years: number): Date =>
+  onWall(d, (w) => {
+    const day = w.getUTCDate()
+    w.setUTCDate(1)
+    w.setUTCFullYear(w.getUTCFullYear() - years)
+    w.setUTCDate(Math.min(day, daysIn(w.getUTCFullYear(), w.getUTCMonth())))
+  })
diff --git a/apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts
index ffc82ee8..dc30d8c4 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/staging.spec.ts
@@ -69,6 +69,7 @@ describe('staging shapes', () => {
     const lastCall = plotlyMock.relayout.mock.calls.at(-1)
     const shapes = lastCall![1].shapes as any[]
     expect(shapes.filter((s) => s.name === 'stage').length).toBe(1)
-    expect(shapes.find((s) => s.name === 'stage').x0).toBe(30)
+    // A Plotly date string: a bare number would be read as local time.
+    expect(shapes.find((s) => s.name === 'stage').x0).toBe('1970-01-01 00:00:00.030')
   })
 })
diff --git a/apps/qc-app/src/utils/plotting/events.ts b/apps/qc-app/src/utils/plotting/events.ts
index d9f8211c..993d650b 100644
--- a/apps/qc-app/src/utils/plotting/events.ts
+++ b/apps/qc-app/src/utils/plotting/events.ts
@@ -23,6 +23,7 @@ import {
 } from './interaction'
 import type { AppPlotlyTrace } from './options'
 import { withLiveShapes } from './shapes'
+import { toPlotDate } from './plotTime'
 
 const handleClick = async (eventData: PlotMouseEvent) => {
   const { plotlyRef } = storeToRefs(usePlotlyStore())
@@ -192,9 +193,9 @@ export const handleNewPlot = async (
   // relying on Plotly's internal anchors.
   if (pendingShareZoom.value && plotlyOptions.value.traces.length) {
     const snap = pendingShareZoom.value
-    const update: Record<string, [number, number] | boolean> = {}
+    const update: Record<string, [number, number] | [string, string] | boolean> = {}
     if (snap.xRange) {
-      update['xaxis.range'] = [...snap.xRange]
+      update['xaxis.range'] = [toPlotDate(snap.xRange[0]), toPlotDate(snap.xRange[1])]
       update['xaxis.autorange'] = false
     }
     for (const [axisName, range] of Object.entries(snap.yRanges ?? {})) {
diff --git a/apps/qc-app/src/utils/plotting/interaction.ts b/apps/qc-app/src/utils/plotting/interaction.ts
index aa945b02..e9d4189a 100644
--- a/apps/qc-app/src/utils/plotting/interaction.ts
+++ b/apps/qc-app/src/utils/plotting/interaction.ts
@@ -9,6 +9,7 @@ import { usePlotlyStore } from '@/store/plotly'
 import { storeToRefs } from 'pinia'
 import { Y_AXIS_KEY_RE } from './internal'
 import type { AppPlotlyTrace, AxisChip } from './options'
+import { fromPlot, plotCoord, plotCoordToDate } from './plotTime'
 
 // Throttle the per-pixel mousemove work to one frame. DOM mousemove
 // fires ~hundreds of events/sec on modern displays; each one used to
@@ -95,7 +96,7 @@ const processMouseMove = (event: MouseEvent) => {
     return
   }
 
-  hover.value.x = Number(xaxis.p2c(cursorX - plotLeft))
+  hover.value.x = fromPlot(Number(xaxis.p2c(cursorX - plotLeft)))
   hover.value.y = Number(yaxis.p2c(cursorY - plotTop)).toFixed(4)
   showCoordinates.value = true
 
@@ -579,10 +580,8 @@ export const handleWheel = (event: WheelEvent) => {
     ]
     const r = ax?.range
     if (!r || r.length < 2) return null
-    const toNum = (v: string | number) =>
-      typeof v === 'string' ? Date.parse(v) : Number(v)
-    const lo = toNum(r[0] as string | number)
-    const hi = toNum(r[1] as string | number)
+    const lo = plotCoord(r[0] as string | number)
+    const hi = plotCoord(r[1] as string | number)
     return Number.isFinite(lo) && Number.isFinite(hi) ? { lo, hi } : null
   }
 
@@ -633,25 +632,22 @@ const applyWheelZoom = async (zoom: WheelPending): Promise<void> => {
 
   const update: Record<string, unknown> = {}
 
-  // Plotly serialises date-axis range endpoints as ISO strings; convert
-  // back to epoch ms so the pivot arithmetic matches `p2c`'s space.
-  const toNumber = (v: string | number): number =>
-    typeof v === 'string' ? Date.parse(v) : Number(v)
-
   const applyAxis = (key: string, pivot: number, factor: number): void => {
     const range = liveLayout[key]?.range as Array<string | number> | undefined
     if (!range) return
     const r0 = range[0]
     const r1 = range[1]
     if (r0 === undefined || r1 === undefined) return
-    const a = toNumber(r0)
-    const b = toNumber(r1)
+    const a = plotCoord(r0)
+    const b = plotCoord(r1)
     if (!Number.isFinite(a) || !Number.isFinite(b)) return
     const newA = pivot - (pivot - a) * factor
     const newB = pivot + (b - pivot) * factor
     // Bail on degenerate ranges (excessive zoom past axis resolution).
     if (!Number.isFinite(newA) || !Number.isFinite(newB) || newA >= newB) return
-    update[`${key}.range`] = [newA, newB]
+    // The date axis takes strings; see `plotTime.ts`.
+    update[`${key}.range`] =
+      key === 'xaxis' ? [plotCoordToDate(newA), plotCoordToDate(newB)] : [newA, newB]
     update[`${key}.autorange`] = false
   }
 
diff --git a/apps/qc-app/src/utils/plotting/operations.ts b/apps/qc-app/src/utils/plotting/operations.ts
index 14abb240..5c99359d 100644
--- a/apps/qc-app/src/utils/plotting/operations.ts
+++ b/apps/qc-app/src/utils/plotting/operations.ts
@@ -11,6 +11,7 @@ import { storeToRefs } from 'pinia'
 import { findFirstGreaterOrEqual } from '@uwrl/qc-utils'
 import { handleNewPlot } from './events'
 import { traceXAsNumbers } from './internal'
+import { plotCoord, plotCoordToDate, toPlotDate } from './plotTime'
 import type { AppPlotlyTrace } from './options'
 
 /**
@@ -26,7 +27,7 @@ export const zoomXaxisTo = async (
 ): Promise<void> => {
   if (!gd) return
   await Plotly.relayout(gd as Plotly.Root, {
-    'xaxis.range': [start, end],
+    'xaxis.range': [toPlotDate(start), toPlotDate(end)],
     'xaxis.autorange': false,
   } as unknown as Partial<Layout>)
 }
@@ -163,23 +164,19 @@ export const cropXaxisRange = async () => {
     try {
       const layoutUpdates = plotlyOptions.value.layout
       const xAxis = layoutUpdates.xaxis as Partial<LayoutAxis> | undefined
-      const xRange = xAxis?.range as Array<string | number> | undefined
-      // Plotly will rewrite timestamps as datestrings. We need to convert them back to timestamps.
-      if (xRange && typeof xRange[0] == 'string') {
-        xRange[0] = Date.parse(xRange[0])
-        xRange[1] = Date.parse(xRange[1] as string)
-      }
-
+      const xRange = (xAxis?.range as Array<string | number> | undefined)?.map(
+        plotCoord
+      )
       const liveRange = (
         plotlyRef.value?.layout.xaxis.range as
         | Array<string | number>
         | undefined
-      )?.map((d) => (typeof d == 'string' ? Date.parse(d) : d))
+      )?.map(plotCoord)
 
       if (xAxis && xRange && liveRange) {
         xAxis.range = [
-          Math.max(liveRange[0] as number, xRange[0] as number),
-          Math.min(liveRange[1] as number, xRange[1] as number),
+          plotCoordToDate(Math.max(liveRange[0]!, xRange[0]!)),
+          plotCoordToDate(Math.min(liveRange[1]!, xRange[1]!)),
         ]
       }
 
@@ -224,7 +221,7 @@ export const fitXaxisToVisible = async (_eventData?: unknown) => {
       (liveLayout?.xaxis as Partial<LayoutAxis> | undefined)?.range as
       | Array<string | number>
       | undefined
-    )?.map((d) => (typeof d == 'string' ? Date.parse(d) : d))
+    )?.map(plotCoord)
 
     const xs = traceXAsNumbers(gd, qcIndex)
     if (!xs.length) return
@@ -265,7 +262,7 @@ export const fitXaxisToVisible = async (_eventData?: unknown) => {
     // bounds clip marker glyphs; on a date axis there's nothing to clip
     // and any padding reads as a margin Plotly conjured up.
     await Plotly.relayout(gd as Plotly.Root, {
-      'xaxis.range': [xMin, xMax],
+      'xaxis.range': [plotCoordToDate(xMin), plotCoordToDate(xMax)],
       'xaxis.autorange': false,
     } as unknown as Partial<Layout>)
   } finally {
@@ -310,7 +307,7 @@ export const fitYaxisToVisible = async (_eventData?: unknown) => {
 
     const liveXRange = (
       plotlyRef.value?.layout.xaxis.range as Array<string | number> | undefined
-    )?.map((d) => (typeof d == 'string' ? Date.parse(d) : d))
+    )?.map(plotCoord)
 
     const xs = traceXAsNumbers(plotlyRef.value, qcTraceIndex)
     const startIdx = findFirstGreaterOrEqual(xs, liveXRange?.[0] ?? -Infinity)
diff --git a/apps/qc-app/src/utils/plotting/options.ts b/apps/qc-app/src/utils/plotting/options.ts
index 32bb928d..0d25c177 100644
--- a/apps/qc-app/src/utils/plotting/options.ts
+++ b/apps/qc-app/src/utils/plotting/options.ts
@@ -15,6 +15,8 @@ import { useQualifierStore } from '@/store/qualifiers'
 import { useUIStore } from '@/store/userInterface'
 import { findFirstGreaterOrEqual, findLastLessOrEqual } from '@uwrl/qc-utils'
 import { DENSITY_HIDE_MARKERS, Y_AXIS_KEY_RE } from './internal'
+import { formatDateTime } from '@/utils/time'
+import { fromPlot, plotCoord, plotCoordToDate, toPlotDate, toPlotX } from './plotTime'
 import { undoZoom, redoZoom } from './zoom'
 import { fitXaxisToVisible, fitYaxisToVisible } from './operations'
 
@@ -329,8 +331,6 @@ const resetAxesClick = (gd: unknown): void => {
   const fl = root?._fullLayout
   if (!root || !fl) return
 
-  const toMs = (v: number | string): number =>
-    typeof v === 'string' ? Date.parse(v) : Number(v)
 
   // Series are time-sorted, so first + last give the per-trace
   // extent in O(1). Min/max across all visible traces.
@@ -339,17 +339,17 @@ const resetAxesClick = (gd: unknown): void => {
   for (const t of root.data ?? []) {
     const xs = t?.x
     if (!xs || !xs.length) continue
-    const first = toMs((xs as ArrayLike<number | string>)[0] as number | string)
-    const last = toMs(
+    const first = plotCoord((xs as ArrayLike<number | string>)[0] as number | string)
+    const last = plotCoord(
       (xs as ArrayLike<number | string>)[xs.length - 1] as number | string
     )
     if (Number.isFinite(first) && first < xMin) xMin = first
     if (Number.isFinite(last) && last > xMax) xMax = last
   }
 
-  const update: Record<string, boolean | [number, number]> = {}
+  const update: Record<string, boolean | [string, string]> = {}
   if (Number.isFinite(xMin) && Number.isFinite(xMax) && xMin < xMax) {
-    update['xaxis.range'] = [xMin, xMax]
+    update['xaxis.range'] = [plotCoordToDate(xMin), plotCoordToDate(xMax)]
     update['xaxis.autorange'] = false
   } else {
     update['xaxis.autorange'] = true
@@ -437,8 +437,8 @@ function buildQualifierBand(
       if (x == null || Number.isNaN(x)) continue
       xs.push(x as number)
       ys.push(row)
-      const ts = new Date(e.appliedAt)
-      const when = isNaN(ts.getTime()) ? e.appliedAt : ts.toLocaleString()
+      const ms = new Date(e.appliedAt).getTime()
+      const when = isNaN(ms) ? e.appliedAt : formatDateTime(ms)
       texts.push(
         `<b>${code}</b>${e.description ? ': ' + e.description : ''}` +
         `<br>Applied: ${when}` +
@@ -504,8 +504,6 @@ export const createPlotlyOption = (
   // Density range for pre-seeding marker opacity.
   const live = (plotlyRef?.value as unknown as { layout?: Partial<Layout> } | null)
     ?.layout?.xaxis?.range as Array<string | number> | undefined
-  const parseCoord = (v: string | number): number =>
-    typeof v === 'string' ? Date.parse(v) : Number(v)
   // `live` is `Array<string | number>` after the cast above; under
   // noUncheckedIndexedAccess the index access returns `… | undefined`,
   // so guard before parsing. Falling through to the date-pickers'
@@ -514,10 +512,10 @@ export const createPlotlyOption = (
   const liveStart = live?.[0]
   const liveEnd = live?.[1]
   const densityStart = liveStart !== undefined
-    ? parseCoord(liveStart)
+    ? fromPlot(liveStart)
     : Number(beginDate?.value?.getTime?.())
   const densityEnd = liveEnd !== undefined
-    ? parseCoord(liveEnd)
+    ? fromPlot(liveEnd)
     : Number(endDate?.value?.getTime?.())
   const densityRangeValid =
     Number.isFinite(densityStart) && Number.isFinite(densityEnd)
@@ -800,7 +798,7 @@ export const createPlotlyOption = (
     type: 'date',
     title: undefined,
     rangeselector: undefined,
-    range: [xRangeStart, xRangeEnd],
+    range: [toPlotDate(xRangeStart), toPlotDate(xRangeEnd)],
     autorange: false,
     showline: true,
     automargin: true,
@@ -891,6 +889,11 @@ export const createPlotlyOption = (
     edits: { shapePosition: true },
   } as unknown as Partial<Config>
 
+  // Built in epoch ms; Plotly gets the chosen zone's wall values.
+  for (const t of traces) {
+    if (t.x) t.x = toPlotX(t.x as ArrayLike<number>) as unknown as PlotData['x']
+  }
+
   return {
     traces,
     layout,
diff --git a/apps/qc-app/src/utils/plotting/plotTime.ts b/apps/qc-app/src/utils/plotting/plotTime.ts
new file mode 100644
index 00000000..490e79f1
--- /dev/null
+++ b/apps/qc-app/src/utils/plotting/plotTime.ts
@@ -0,0 +1,34 @@
+/**
+ * Plotly has no time zones: it reads epoch ms as UTC. So x values go in as
+ * wall values (`timeZone.ts`), which makes its axis, ticks and hover read in
+ * the chosen zone, and every x that comes back out goes through `fromPlot`.
+ *
+ * Ranges, shapes and tick values are written as Plotly date strings. Plotly
+ * reads a bare number there as browser-local time, unlike trace data, which
+ * would shift every zoom by the browser's UTC offset.
+ */
+
+import { fromWall, toWall, toWallArray } from '@/utils/timeZone'
+
+/** Trace x values for Plotly, from epoch ms. */
+export const toPlotX = <T extends ArrayLike<number>>(xs: T): T | Float64Array =>
+  toWallArray(xs)
+
+/** A Plotly date string (`YYYY-MM-DD HH:MM:SS.sss`) for an instant. */
+export const toPlotDate = (ms: number): string => plotCoordToDate(toWall(ms))
+
+/** A Plotly x as a number in Plotly's own frame, where trace values live:
+ *  a range or shape string read as UTC, or a number as is. Compare it with
+ *  trace values; take `fromPlot` to leave the plot. */
+export function plotCoord(v: number | string): number {
+  if (typeof v === 'number') return v
+  const [date, time = '00:00'] = v.trim().replace(/Z$/i, '').split(/[ T]/)
+  return Date.parse(`${date}T${time}Z`)
+}
+
+/** The instant behind a Plotly x. */
+export const fromPlot = (v: number | string): number => fromWall(plotCoord(v))
+
+/** A Plotly date string for a number in Plotly's own frame. */
+export const plotCoordToDate = (wall: number): string =>
+  new Date(wall).toISOString().replace('T', ' ').replace('Z', '')
diff --git a/apps/qc-app/src/utils/plotting/relayout.ts b/apps/qc-app/src/utils/plotting/relayout.ts
index 6e97005d..b37d4105 100644
--- a/apps/qc-app/src/utils/plotting/relayout.ts
+++ b/apps/qc-app/src/utils/plotting/relayout.ts
@@ -11,6 +11,8 @@ import { storeToRefs } from 'pinia'
 import { isEqual } from 'lodash-es'
 import { findFirstGreaterOrEqual } from '@uwrl/qc-utils'
 import { traceXAsNumbers, DENSITY_HIDE_MARKERS } from './internal'
+import { plotCoord, plotCoordToDate } from './plotTime'
+import { toWall } from '@/utils/timeZone'
 import type { AppPlotlyTrace } from './options'
 import { handleSelected } from './selected'
 
@@ -52,8 +54,9 @@ export const intendedSpacingMs = (): number | null => {
 
 /**
  * Compute an explicit tick-position array aligned to the datastream's
- * intended cadence, bounded to the visible x-range. Returns `null` when
- * we should hand back to Plotly's auto tick picker.
+ * intended cadence, bounded to the visible x-range, in Plotly's frame (see
+ * `plotTime.ts`). Returns `null` when we should hand back to Plotly's auto
+ * tick picker.
  */
 export const computeIntendedTickvals = (
   xStart: number,
@@ -87,7 +90,7 @@ export const computeIntendedTickvals = (
   const { qcDatastream } = storeToRefs(useDataVisStore())
   const anchorSource = qcDatastream.value?.phenomenonBeginTime
   const anchor = anchorSource
-    ? new Date(anchorSource).getTime()
+    ? toWall(new Date(anchorSource).getTime())
     : xStart
 
   const firstK = Math.ceil((xStart - anchor) / step)
@@ -170,14 +173,9 @@ export const handleRelayout = async (
         (plotlyRef.value as unknown as { layout?: Partial<Layout> })?.layout
       const liveXaxis = liveLayout?.xaxis as Partial<LayoutAxis> | undefined
       const xRange = liveXaxis?.range as Array<string | number> | undefined
-
-      if (xRange && typeof xRange[0] == 'string') {
-        xRange[0] = Date.parse(xRange[0])
-        xRange[1] = Date.parse(xRange[1] as string)
-      }
-
-      const xStart = Number(xRange?.[0])
-      const xEnd = Number(xRange?.[1])
+      // In Plotly's frame, like the trace values counted against it.
+      const xStart = xRange?.[0] !== undefined ? plotCoord(xRange[0]) : NaN
+      const xEnd = xRange?.[1] !== undefined ? plotCoord(xRange[1]) : NaN
 
       // Only rescan when the visible x-range actually moved.
       if (
@@ -278,7 +276,7 @@ export const handleRelayout = async (
         const currentTickmode =
           (liveXaxis?.tickmode as string | undefined) ?? 'auto'
         const currentTickvals = Array.isArray(liveXaxis?.tickvals)
-          ? (liveXaxis?.tickvals as number[])
+          ? (liveXaxis?.tickvals as Array<number | string>).map(plotCoord)
           : null
 
         const wantedTickmode = wantedTickvals ? 'array' : 'auto'
@@ -291,7 +289,7 @@ export const handleRelayout = async (
         if (tickmodeChanged || tickvalsChanged) {
           await Plotly.relayout(plotlyRef.value as unknown as HTMLElement, {
             'xaxis.tickmode': wantedTickvals ? 'array' : 'auto',
-            'xaxis.tickvals': wantedTickvals ?? null,
+            'xaxis.tickvals': wantedTickvals?.map(plotCoordToDate) ?? null,
           } as unknown as Partial<Layout>)
         }
       }
diff --git a/apps/qc-app/src/utils/plotting/staging.ts b/apps/qc-app/src/utils/plotting/staging.ts
index 5d2c3721..44cd98da 100644
--- a/apps/qc-app/src/utils/plotting/staging.ts
+++ b/apps/qc-app/src/utils/plotting/staging.ts
@@ -25,10 +25,13 @@ import { ref } from 'vue'
 import { usePlotlyStore } from '@/store/plotly'
 import { storeToRefs } from 'pinia'
 import { STAGE_SHAPE_NAME, type PlotlyShape } from './shapes'
+import { fromPlot, toPlotDate, toPlotX } from './plotTime'
 
 const GHOST_TRACE_NAME = 'qc-ghost-fills'
 
 let stageShape: PlotlyShape | null = null
+/** The band's span in epoch ms; the shape gets it in the plot's frame. */
+let stageRange: [number, number] | null = null
 /** True when the plot is in pan mode, meaning the editable stage
  *  shape should be rendered. In zoom / select / lasso modes we drop
  *  the shape from the flushed array entirely so it can't swallow
@@ -52,10 +55,22 @@ async function flushShapes() {
   if (!root) return
   // Drop the stage shape outside pan mode so zoom / select / lasso
   // gestures aren't captured by the shape-edit hit-tester.
-  const shapes = stageShape && stagePanMode.value ? [stageShape] : []
+  const shapes =
+    stageShape && stageRange && stagePanMode.value
+      ? [
+          {
+            ...stageShape,
+            x0: toPlotDate(stageRange[0]),
+            x1: toPlotDate(stageRange[1]),
+          },
+        ]
+      : []
   await Plotly.relayout(root, { shapes } as unknown as Partial<Layout>)
 }
 
+/** Redraw the band where it was, after the time zone changes. */
+export const redrawStageShape = (): Promise<void> => flushShapes()
+
 /**
  * Read the plot's current dragmode off the live layout. Used at
  * setup time to seed `stagePanMode` before the first drag event
@@ -93,13 +108,12 @@ export async function enterPanMode(): Promise<void> {
  */
 export async function setStageShape(fromTs: number, toTs: number) {
   stagePanMode.value = currentDragmode() === 'pan'
+  stageRange = [fromTs, toTs]
   stageShape = {
     name: STAGE_SHAPE_NAME,
     type: 'rect',
     xref: 'x',
     yref: 'paper',
-    x0: fromTs,
-    x1: toTs,
     y0: 0,
     y1: 1,
     fillcolor: 'rgba(25, 118, 210, 0.14)',
@@ -112,6 +126,7 @@ export async function setStageShape(fromTs: number, toTs: number) {
 
 export async function clearStageShape() {
   stageShape = null
+  stageRange = null
   await flushShapes()
 }
 
@@ -139,7 +154,7 @@ export async function setGhostFills(xs: number[], ys: number[]) {
     name: GHOST_TRACE_NAME,
     type: 'scattergl',
     mode: 'markers',
-    x: xs,
+    x: toPlotX(xs),
     y: ys,
     marker: {
       // Bumped from 0.55/0.9 with a thin X-glyph: barely readable
@@ -185,18 +200,14 @@ export function onStageDrag(
   if (!root?.on) return () => { }
 
   const parseTs = (v: unknown): number | null => {
-    if (typeof v === 'number' && Number.isFinite(v)) return v
-    if (typeof v === 'string') {
-      const t = Date.parse(v)
-      return Number.isFinite(t) ? t : null
-    }
-    if (v instanceof Date) return v.getTime()
-    return null
+    if (typeof v !== 'number' && typeof v !== 'string') return null
+    const t = fromPlot(v)
+    return Number.isFinite(t) ? t : null
   }
 
   const handler = (event: unknown) => {
     const evt = event as Record<string, unknown> | null
-    if (!evt || !stageShape) return
+    if (!evt || !stageShape || !stageRange) return
 
     // Dragmode change: the user picked zoom / select / lasso from
     // the modebar. Drop the stage shape from the flushed array so
@@ -227,8 +238,8 @@ export function onStageDrag(
     // Horizontal edit: pick the new x from whichever side(s) the
     // event carried, falling back to the stashed value for the
     // untouched side.
-    const nextX0 = touchedX0 ? parseTs(evt[x0Key]) : Number(stageShape.x0)
-    const nextX1 = touchedX1 ? parseTs(evt[x1Key]) : Number(stageShape.x1)
+    const nextX0 = touchedX0 ? parseTs(evt[x0Key]) : stageRange[0]
+    const nextX1 = touchedX1 ? parseTs(evt[x1Key]) : stageRange[1]
     if (!Number.isFinite(nextX0) || !Number.isFinite(nextX1)) return
     const from = Math.min(nextX0 as number, nextX1 as number)
     const to = Math.max(nextX0 as number, nextX1 as number)
@@ -240,7 +251,8 @@ export function onStageDrag(
     // (x via the parent's watcher, y via a separate relayout) was
     // racing, leaving vertical edits visible and horizontal edits
     // dropped.
-    stageShape = { ...stageShape, x0: from, x1: to, y0: 0, y1: 1 }
+    stageRange = [from, to]
+    stageShape = { ...stageShape, y0: 0, y1: 1 }
     void flushShapes()
 
     // Only notify the parent when the horizontal range actually
diff --git a/apps/qc-app/src/utils/plotting/zoom.ts b/apps/qc-app/src/utils/plotting/zoom.ts
index b5462582..035a694c 100644
--- a/apps/qc-app/src/utils/plotting/zoom.ts
+++ b/apps/qc-app/src/utils/plotting/zoom.ts
@@ -9,6 +9,7 @@ import type {
 import { storeToRefs } from 'pinia'
 import { debounce } from 'lodash-es'
 import { Y_AXIS_KEY_RE } from './internal'
+import { fromPlot, toPlotDate } from './plotTime'
 
 // --- Zoom history -----------------------------------------------------------
 //
@@ -36,8 +37,9 @@ export const captureCurrentZoomState = (
 
   const xAxis = layout.xaxis as Partial<LayoutAxis> | undefined
   const xR = xAxis?.range as Array<number | string> | undefined
-  const xStart = xR ? Number(typeof xR[0] === 'string' ? Date.parse(xR[0]) : xR[0]) : NaN
-  const xEnd = xR ? Number(typeof xR[1] === 'string' ? Date.parse(xR[1]) : xR[1]) : NaN
+  // Real instants: the snapshot outlives the plot (undo, share link).
+  const xStart = xR?.[0] !== undefined ? fromPlot(xR[0]) : NaN
+  const xEnd = xR?.[1] !== undefined ? fromPlot(xR[1]) : NaN
   const xRange: [number, number] | null =
     Number.isFinite(xStart) && Number.isFinite(xEnd) ? [xStart, xEnd] : null
 
@@ -134,7 +136,7 @@ export const applyZoomState = async (state: ZoomState): Promise<void> => {
 
   const update: Record<string, unknown> = {}
   if (state.xRange) {
-    update['xaxis.range'] = [state.xRange[0], state.xRange[1]]
+    update['xaxis.range'] = [toPlotDate(state.xRange[0]), toPlotDate(state.xRange[1])]
     update['xaxis.autorange'] = false
   } else {
     update['xaxis.autorange'] = true
diff --git a/apps/qc-app/src/utils/time.ts b/apps/qc-app/src/utils/time.ts
index 7d5f9c6b..755c8fb4 100644
--- a/apps/qc-app/src/utils/time.ts
+++ b/apps/qc-app/src/utils/time.ts
@@ -1,41 +1,54 @@
-const formatTime = (time?: string | null): string => {
-  if (!time) return '–'
+/**
+ * Date formatting for display, in the zone the user chose (`timeZone.ts`).
+ * Each formatter runs in UTC over the instant's wall value, which reads as
+ * that zone's clock whatever kind of zone it is.
+ */
 
-  const date = new Date(time)
-  const parts = new Intl.DateTimeFormat('en-US', {
-    day: 'numeric',
-    month: 'short',
-    year: 'numeric',
-    hour: 'numeric',
-    minute: '2-digit',
-    hour12: true,
-  }).formatToParts(date)
+import {
+  displayZone,
+  toWall,
+  wallParts,
+  zoneAbbreviation,
+  zoneDescription,
+  zoneName,
+} from '@/utils/timeZone'
 
-  const get = (type: string) => parts.find((p) => p.type === type)?.value
-  const day = get('day')
-  const month = get('month')
-  const year = get('year')
-  const hour = get('hour')
-  const minute = get('minute')
-  const period = get('dayPeriod')
-
-  return `${day} ${month} ${year}, ${hour}:${minute} ${period}`
-}
+const utc = (options: Intl.DateTimeFormatOptions, locale = 'en-US') =>
+  new Intl.DateTimeFormat(locale, { ...options, timeZone: 'UTC' })
 
-const MONTH_DAY = new Intl.DateTimeFormat('en-US', {
-  month: 'short',
+const LONG = utc({
   day: 'numeric',
-})
-const MONTH_DAY_YEAR = new Intl.DateTimeFormat('en-US', {
   month: 'short',
-  day: 'numeric',
   year: 'numeric',
-})
-const CLOCK = new Intl.DateTimeFormat('en-US', {
   hour: 'numeric',
   minute: '2-digit',
   hour12: true,
 })
+const MONTH_DAY = utc({ month: 'short', day: 'numeric' })
+const MONTH_DAY_YEAR = utc({ month: 'short', day: 'numeric', year: 'numeric' })
+const CLOCK = utc({ hour: 'numeric', minute: '2-digit', hour12: true })
+// The browser's locale, 24-hour clock with seconds: data point timestamps.
+const POINT = utc(
+  {
+    year: 'numeric',
+    month: 'short',
+    day: '2-digit',
+    hour: '2-digit',
+    hour12: false,
+    minute: '2-digit',
+    second: '2-digit',
+  },
+  undefined
+)
+
+const wall = (ms: number) => new Date(toWall(ms))
+
+const formatTime = (time?: string | null): string => {
+  if (!time) return '–'
+  const parts = LONG.formatToParts(wall(new Date(time).getTime()))
+  const get = (type: string) => parts.find((p) => p.type === type)?.value
+  return `${get('day')} ${get('month')} ${get('year')}, ${get('hour')}:${get('minute')} ${get('dayPeriod')}`
+}
 
 const toDate = (iso?: string | null): Date | null => {
   if (!iso) return null
@@ -43,22 +56,30 @@ const toDate = (iso?: string | null): Date | null => {
   return Number.isNaN(d.getTime()) ? null : d
 }
 
+/** A data point's timestamp, to the second. */
+export function formatDateTime(ms: number): string {
+  return Number.isFinite(ms) ? POINT.format(wall(ms)) : '–'
+}
+
 /** `Mar 14, 2026`. Day precision, for timestamps shown alongside other text. */
 export function formatDayStamp(iso?: string | null): string {
   const d = toDate(iso)
   if (!d) return iso || '–'
-  return MONTH_DAY_YEAR.format(d)
+  return MONTH_DAY_YEAR.format(wall(d.getTime()))
 }
 
 /** A whole-day boundary, where showing the clock adds nothing. */
-const isMidnight = (d: Date) =>
-  d.getHours() === 0 && d.getMinutes() === 0 && d.getSeconds() === 0
+const isMidnight = (d: Date) => {
+  const p = wallParts(d.getTime())
+  return p.hours === 0 && p.minutes === 0 && p.seconds === 0
+}
 
 /** One boundary: `Mar 14, 2026`, or `Mar 14, 2026, 2:30 PM` off midnight. */
 export function formatStamp(when: Date): string {
   if (Number.isNaN(when.getTime())) return '–'
-  const day = MONTH_DAY_YEAR.format(when)
-  return isMidnight(when) ? day : `${day}, ${CLOCK.format(when)}`
+  const w = wall(when.getTime())
+  const day = MONTH_DAY_YEAR.format(w)
+  return isMidnight(when) ? day : `${day}, ${CLOCK.format(w)}`
 }
 
 /**
@@ -79,47 +100,35 @@ export function formatDateRange(
   if (!from || !to) return '–'
 
   const wholeDays = isMidnight(from) && isMidnight(to)
-  const sameYear = from.getFullYear() === to.getFullYear()
-
-  if (from.toDateString() === to.toDateString()) {
-    const day = MONTH_DAY_YEAR.format(from)
-    return wholeDays
-      ? day
-      : `${day}, ${CLOCK.format(from)} – ${CLOCK.format(to)}`
+  const a = wallParts(from.getTime())
+  const b = wallParts(to.getTime())
+  const wf = wall(from.getTime())
+  const wt = wall(to.getTime())
+
+  if (a.year === b.year && a.month === b.month && a.day === b.day) {
+    const day = MONTH_DAY_YEAR.format(wf)
+    return wholeDays ? day : `${day}, ${CLOCK.format(wf)} – ${CLOCK.format(wt)}`
   }
 
-  const left = sameYear ? MONTH_DAY.format(from) : MONTH_DAY_YEAR.format(from)
-  const right = MONTH_DAY_YEAR.format(to)
+  const left = a.year === b.year ? MONTH_DAY.format(wf) : MONTH_DAY_YEAR.format(wf)
+  const right = MONTH_DAY_YEAR.format(wt)
   return wholeDays
     ? `${left} – ${right}`
-    : `${left}, ${CLOCK.format(from)} – ${right}, ${CLOCK.format(to)}`
+    : `${left}, ${CLOCK.format(wf)} – ${right}, ${CLOCK.format(wt)}`
 }
 
-function getLocalTimeZone(): string {
-  return Intl.DateTimeFormat().resolvedOptions().timeZone || '–'
-}
-
-const ZONE_NAME = new Intl.DateTimeFormat('en-US', { timeZoneName: 'short' })
-
-/** The browser's zone abbreviation on `date` (`MDT`, `MST`, `GMT+2`), which
- *  tracks daylight saving. Every date the app shows or takes is in it. */
+/** The zone's short name on `date` (`MDT`, `MST`, `UTC`), which tracks
+ *  daylight saving. Every date the app shows or takes is in that zone. */
 export function timeZoneAbbreviation(date: Date): string {
-  return (
-    ZONE_NAME.formatToParts(date).find((p) => p.type === 'timeZoneName')
-      ?.value ?? ''
-  )
+  return zoneAbbreviation(date.getTime())
 }
 
 /** `America/Denver, UTC-06:00` for `date`. */
 export function timeZoneDescription(date: Date): string {
-  const offset = -date.getTimezoneOffset()
-  const sign = offset < 0 ? '-' : '+'
-  const hh = String(Math.floor(Math.abs(offset) / 60)).padStart(2, '0')
-  const mm = String(Math.abs(offset) % 60).padStart(2, '0')
-  return `${getLocalTimeZone()}, UTC${sign}${hh}:${mm}`
+  return zoneDescription(date.getTime())
 }
 
 export function formatTimeWithZone(time?: string | null) {
   if (!time) return '–'
-  return `${formatTime(time)} (${getLocalTimeZone()})`
+  return `${formatTime(time)} (${zoneName(displayZone.value)})`
 }
diff --git a/apps/qc-app/src/utils/timeRangePresets.ts b/apps/qc-app/src/utils/timeRangePresets.ts
index 6b270063..d9cbe9b1 100644
--- a/apps/qc-app/src/utils/timeRangePresets.ts
+++ b/apps/qc-app/src/utils/timeRangePresets.ts
@@ -4,6 +4,7 @@
  */
 
 import { subtractDays, subtractMonths, subtractYears } from '@/utils/dateMath'
+import { fromWallParts, wallParts } from '@/utils/timeZone'
 
 export interface TimeRangePreset {
   id: number
@@ -75,7 +76,10 @@ export function presetWindow(id: number, extent: TimeWindow): TimeWindow | null
   const end = new Date(extent.end)
   switch (preset.label) {
     case 'YTD':
-      return { begin: new Date(end.getFullYear(), 0, 1), end }
+      return {
+        begin: new Date(fromWallParts(wallParts(end.getTime()).year, 0, 1)),
+        end,
+      }
     case 'All':
       return { begin: new Date(extent.begin), end }
     default:
diff --git a/apps/qc-app/src/utils/timeZone.ts b/apps/qc-app/src/utils/timeZone.ts
new file mode 100644
index 00000000..5b7a11ca
--- /dev/null
+++ b/apps/qc-app/src/utils/timeZone.ts
@@ -0,0 +1,226 @@
+/**
+ * The time zone the app shows and takes dates in, chosen by the user the way
+ * a data connection's zone is: UTC, a fixed UTC offset, or an IANA zone. It
+ * defaults to the browser's own zone.
+ *
+ * Instants stay epoch ms everywhere. A "wall" value is an instant moved by
+ * the zone's offset, so its UTC fields read as the zone's clock. Formatting,
+ * pickers and the plot work on wall values; everything stored stays real.
+ */
+
+import { ref } from 'vue'
+
+export type ZoneMode = 'utc' | 'fixedOffset' | 'iana'
+
+export interface DisplayZone {
+  mode: ZoneMode
+  /** A fixed offset like `-0700`, or an IANA zone. Unused for UTC. */
+  zone: string
+}
+
+export const browserZone = (): DisplayZone => ({
+  mode: 'iana',
+  zone: Intl.DateTimeFormat().resolvedOptions().timeZone || 'UTC',
+})
+
+/** The zone in use. `qcPreferences` persists it. */
+export const displayZone = ref<DisplayZone>(browserZone())
+
+const MINUTE = 60_000
+const DAY = 86_400_000
+
+/** `-0700` to minutes east of UTC. */
+function fixedOffsetMinutes(zone: string): number {
+  const m = /^([+-])(\d{2}):?(\d{2})$/.exec(zone)
+  if (!m) return 0
+  const minutes = Number(m[2]) * 60 + Number(m[3])
+  return m[1] === '-' ? -minutes : minutes
+}
+
+const partFormatters = new Map<string, Intl.DateTimeFormat>()
+
+function ianaOffsetMs(ms: number, timeZone: string): number {
+  let fmt = partFormatters.get(timeZone)
+  if (!fmt) {
+    fmt = new Intl.DateTimeFormat('en-US', {
+      timeZone,
+      hourCycle: 'h23',
+      year: 'numeric',
+      month: 'numeric',
+      day: 'numeric',
+      hour: 'numeric',
+      minute: 'numeric',
+      second: 'numeric',
+    })
+    partFormatters.set(timeZone, fmt)
+  }
+  const get = (parts: Intl.DateTimeFormatPart[], type: string) =>
+    Number(parts.find((p) => p.type === type)?.value)
+  const parts = fmt.formatToParts(ms)
+  const wall = Date.UTC(
+    get(parts, 'year'),
+    get(parts, 'month') - 1,
+    get(parts, 'day'),
+    get(parts, 'hour'),
+    get(parts, 'minute'),
+    get(parts, 'second')
+  )
+  return wall - (ms - (((ms % 1000) + 1000) % 1000))
+}
+
+/** An offset that holds from `from` until `until`. */
+interface Segment {
+  from: number
+  until: number
+  offset: number
+}
+
+// Per zone, per UTC day: one segment, or two around a transition.
+const segmentCache = new Map<string, Map<number, Segment[]>>()
+
+function ianaSegment(ms: number, timeZone: string): Segment {
+  let days = segmentCache.get(timeZone)
+  if (!days) segmentCache.set(timeZone, (days = new Map()))
+  const day = Math.floor(ms / DAY)
+  let segments = days.get(day)
+  if (!segments) {
+    const start = day * DAY
+    const end = start + DAY
+    const first = ianaOffsetMs(start, timeZone)
+    const last = ianaOffsetMs(end - MINUTE, timeZone)
+    if (first === last) {
+      segments = [{ from: start, until: end, offset: first }]
+    } else {
+      // Zones change offset on a minute boundary at most once a day.
+      let lo = start
+      let hi = end - MINUTE
+      while (hi - lo > MINUTE) {
+        const mid = lo + Math.floor((hi - lo) / 2 / MINUTE) * MINUTE
+        if (ianaOffsetMs(mid, timeZone) === first) lo = mid
+        else hi = mid
+      }
+      segments = [
+        { from: start, until: hi, offset: first },
+        { from: hi, until: end, offset: last },
+      ]
+    }
+    days.set(day, segments)
+  }
+  return segments.find((s) => ms < s.until) ?? segments[segments.length - 1]!
+}
+
+function segmentAt(ms: number, z: DisplayZone): Segment {
+  if (z.mode === 'iana') return ianaSegment(ms, z.zone)
+  const offset = z.mode === 'fixedOffset' ? fixedOffsetMinutes(z.zone) * MINUTE : 0
+  return { from: -Infinity, until: Infinity, offset }
+}
+
+/** The zone's offset from UTC at instant `ms`, in ms. */
+export function offsetMs(ms: number, z: DisplayZone = displayZone.value): number {
+  return segmentAt(ms, z).offset
+}
+
+export function toWall(ms: number, z: DisplayZone = displayZone.value): number {
+  return ms + offsetMs(ms, z)
+}
+
+/** The instant a wall value names. A clock time that a daylight saving
+ *  change skips or repeats has no single instant; it maps to one beside it. */
+export function fromWall(wall: number, z: DisplayZone = displayZone.value): number {
+  const guess = wall - offsetMs(wall, z)
+  return wall - offsetMs(guess, z)
+}
+
+/** Every value of an ascending array moved to wall time, walking the offset
+ *  segments rather than looking each point up. The input itself comes back
+ *  when nothing moves (UTC), so it is copied only when it has to be. */
+export function toWallArray<T extends ArrayLike<number>>(
+  xs: T,
+  z: DisplayZone = displayZone.value
+): T | Float64Array {
+  let out: Float64Array | null = null
+  let seg: Segment | null = null
+  for (let i = 0; i < xs.length; i++) {
+    const x = xs[i]!
+    if (!seg || x < seg.from || x >= seg.until) seg = segmentAt(x, z)
+    if (seg.offset !== 0 && !out) {
+      out = new Float64Array(xs.length)
+      for (let j = 0; j < i; j++) out[j] = xs[j]!
+    }
+    if (out) out[i] = x + seg.offset
+  }
+  return out ?? xs
+}
+
+/** The zone's clock at `ms`, with a 0-based month. */
+export function wallParts(ms: number, z: DisplayZone = displayZone.value) {
+  const d = new Date(toWall(ms, z))
+  return {
+    year: d.getUTCFullYear(),
+    month: d.getUTCMonth(),
+    day: d.getUTCDate(),
+    hours: d.getUTCHours(),
+    minutes: d.getUTCMinutes(),
+    seconds: d.getUTCSeconds(),
+  }
+}
+
+/** The instant the zone's clock reads the given fields. */
+export function fromWallParts(
+  year: number,
+  month: number,
+  day: number,
+  hours = 0,
+  minutes = 0,
+  seconds = 0,
+  z: DisplayZone = displayZone.value
+): number {
+  return fromWall(Date.UTC(year, month, day, hours, minutes, seconds), z)
+}
+
+function formatOffset(ms: number): string {
+  const minutes = Math.round(ms / MINUTE)
+  const sign = minutes < 0 ? '-' : '+'
+  const hh = String(Math.floor(Math.abs(minutes) / 60)).padStart(2, '0')
+  const mm = String(Math.abs(minutes) % 60).padStart(2, '0')
+  return `UTC${sign}${hh}:${mm}`
+}
+
+const shortNames = new Map<string, Intl.DateTimeFormat>()
+
+/** The zone's short name at `ms` (`MDT`, `MST`, `UTC`, `UTC+05:30`), which
+ *  tracks daylight saving. */
+export function zoneAbbreviation(
+  ms: number,
+  z: DisplayZone = displayZone.value
+): string {
+  if (z.mode === 'utc') return 'UTC'
+  if (z.mode === 'fixedOffset') return formatOffset(offsetMs(ms, z))
+  let fmt = shortNames.get(z.zone)
+  if (!fmt) {
+    fmt = new Intl.DateTimeFormat('en-US', {
+      timeZone: z.zone,
+      timeZoneName: 'short',
+    })
+    shortNames.set(z.zone, fmt)
+  }
+  return fmt.formatToParts(ms).find((p) => p.type === 'timeZoneName')?.value ?? ''
+}
+
+/** `America/Denver, UTC-06:00` at `ms`. */
+export function zoneDescription(
+  ms: number,
+  z: DisplayZone = displayZone.value
+): string {
+  const offset = formatOffset(offsetMs(ms, z))
+  if (z.mode === 'utc') return 'UTC'
+  if (z.mode === 'fixedOffset') return `Fixed offset, ${offset}`
+  return `${z.zone}, ${offset}`
+}
+
+/** The zone's name without a date: the IANA name, `UTC`, or the offset. */
+export function zoneName(z: DisplayZone = displayZone.value): string {
+  if (z.mode === 'utc') return 'UTC'
+  if (z.mode === 'fixedOffset') return formatOffset(offsetMs(0, z))
+  return z.zone
+}
diff --git a/apps/qc-app/vite.config.ts b/apps/qc-app/vite.config.ts
index c25075c5..67092727 100644
--- a/apps/qc-app/vite.config.ts
+++ b/apps/qc-app/vite.config.ts
@@ -100,6 +100,8 @@ export default defineConfig(({ command, mode }) => {
       },
       setupFiles: ['@vitest/web-worker', './src/utils/test/setup.ts'],
       environment: 'jsdom',
+      // Pinned so date tests read the same on every machine.
+      env: { TZ: 'UTC' },
       coverage: {
         exclude: [
           '**/src/store/observations.ts',
diff --git a/packages/qc-utils/docs/API_REFERENCE.md b/packages/qc-utils/docs/API_REFERENCE.md
index 5d151ced..3dbaa3d3 100644
--- a/packages/qc-utils/docs/API_REFERENCE.md
+++ b/packages/qc-utils/docs/API_REFERENCE.md
@@ -338,6 +338,14 @@ formatDate(ts: number): string         // human-readable date
 formatDuration(ms: number): string     // "1d 2h 3m 4s"
 ```
 
+### Time zones
+
+`FIXED_OFFSET_TIMEZONES` (`{ title, value }` with values like `-0700`) and
+`DST_AWARE_TIMEZONES` (every IANA zone `Intl` knows, titled with its winter
+and summer offsets), with their `FixedOffsetTimezone` and `DstAwareTimezone`
+value types. The lists a data connection's timestamps choose from; the QC
+app's time zone setting offers the same ones.
+
 ### `measureEllapsedTime<T>(fn: () => Promise<T> | T): Promise<{ result: T; duration: number }>`
 
 Wrap any thunk with wall-clock measurement. Used by dispatch to fill
diff --git a/packages/qc-utils/src/models/index.ts b/packages/qc-utils/src/models/index.ts
index 2d64e0f0..941db089 100644
--- a/packages/qc-utils/src/models/index.ts
+++ b/packages/qc-utils/src/models/index.ts
@@ -1,2 +1,8 @@
 export { DataSource } from './dataSource'
 export { Payload } from './payload'
+export {
+  FIXED_OFFSET_TIMEZONES,
+  DST_AWARE_TIMEZONES,
+  type FixedOffsetTimezone,
+  type DstAwareTimezone,
+} from './timestamp'

From 174b9be589631edeea5f543aa39a94b0b2a1f2c8 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 07:00:44 -0600
Subject: [PATCH 81/93] feat: shift months and years on a saved time zone
 calendar

---
 apps/qc-app/docs/API_REFERENCE.md             |  11 +-
 apps/qc-app/docs/TESTING.md                   |  10 +-
 apps/qc-app/docs/USER_GUIDE.md                |   2 +
 apps/qc-app/e2e/edit-shift-datetimes.spec.ts  |  30 +++-
 .../components/EditData/ShiftDatetimes.vue    |  41 ++++-
 .../EditData/__tests__/EditHistory.spec.ts    | 109 ++++++------
 .../VisualizeData/__tests__/DataTable.spec.ts |  35 ++--
 .../__tests__/DataVisDatasetsTable.spec.ts    |   2 +-
 .../src/utils/__tests__/displayZone.spec.ts   |  39 +---
 .../utils/plotting/__tests__/options.spec.ts  |  55 +++---
 .../utils/plotting/__tests__/relayout.spec.ts |  35 ++--
 apps/qc-app/src/utils/timeZone.ts             | 134 +++-----------
 packages/qc-utils/README.md                   |   2 +-
 packages/qc-utils/docs/API_REFERENCE.md       |  29 ++-
 packages/qc-utils/docs/ARCHITECTURE.md        |   2 +-
 packages/qc-utils/docs/QC_HISTORY.md          |   2 +-
 .../src/utils/__tests__/timeZone.spec.ts      |  76 ++++++++
 packages/qc-utils/src/utils/index.ts          |   1 +
 .../__tests__/calibrated-dispatch.spec.ts     |   2 +-
 .../utils/plotting/__tests__/history.spec.ts  |  33 +++-
 .../observation-record-paths.spec.ts          |   2 +-
 .../__tests__/observation-record.spec.ts      |  26 ++-
 .../__tests__/operation-cores.spec.ts         |  90 +++++-----
 .../utils/plotting/__tests__/workers.spec.ts  |  83 +++------
 .../src/utils/plotting/observation-record.ts  |  38 ++--
 .../src/utils/plotting/operation-cores.ts     |  42 ++---
 .../utils/plotting/shift-datetimes.worker.ts  |  41 +----
 packages/qc-utils/src/utils/timeZone.ts       | 168 ++++++++++++++++++
 28 files changed, 697 insertions(+), 443 deletions(-)
 create mode 100644 packages/qc-utils/src/utils/__tests__/timeZone.spec.ts
 create mode 100644 packages/qc-utils/src/utils/timeZone.ts

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index af862e65..60876981 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -801,11 +801,12 @@ default.
 Instants stay epoch ms everywhere. A **wall** value is an instant moved by
 the zone's offset, so its UTC fields read as the zone's clock:
 
-- `offsetMs(ms)`, `toWall(ms)`, `fromWall(wall)`: the conversion. IANA
-  offsets are looked up once per UTC day per zone and cached, with the
-  transition minute found on a day the offset changes.
-- `toWallArray(xs)`: an ascending array moved to wall time in one pass. It
-  returns `xs` itself when nothing moves (UTC), so only a shift copies.
+- `offsetMs(ms)`, `toWall(ms)`, `fromWall(wall)`, `toWallArray(xs)`: the
+  conversion, on qc-utils' zone math (see its Time zones section) with the
+  chosen zone filled in.
+- `zoneId()`: the chosen zone as qc-utils takes it (`UTC`, `-0700`, or an
+  IANA name). The shift dialog passes it to `SHIFT_DATETIMES`, which saves
+  it with the step.
 - `wallParts(ms)` / `fromWallParts(y, m, d, h, mi, s)`: the zone's clock
   fields, for pickers and calendar arithmetic.
 - `zoneAbbreviation(ms)`, `zoneDescription(ms)`, `zoneName()`: labels.
diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 66052ef8..5883ed2f 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -84,8 +84,11 @@ notable knobs:
   load.
 - `env: { TZ: 'UTC' }`: every unit test runs in UTC, so the browser's
   zone, the app's default display zone, is the same on every machine. A
-  test about a zone sets it explicitly (`timeZone.spec.ts` pins
-  `America/Denver`; `displayZone.spec.ts` passes a `DisplayZone`).
+  test about a zone sets it explicitly (`displayZone.spec.ts` passes a
+  `DisplayZone`). The offset math itself is tested in qc-utils.
+- Specs that mock all of `@uwrl/qc-utils` hand back the real `offsetMs`,
+  `toWall`, `fromWall` and `toWallArray` from `importOriginal`, since the
+  app's date helpers run on them.
 
 ### Test scaffolding
 
@@ -366,7 +369,8 @@ function observationsWithGap() {
 
 Playwright runs in the machine's zone unless a spec sets `timezoneId`.
 `time-zone.spec.ts` runs in `Asia/Tokyo`, so a time read as browser-local
-instead of in the chosen zone would show.
+instead of in the chosen zone would show. `edit-shift-datetimes.spec.ts`
+does too, and checks that a month shift saves `Asia/Tokyo` with the step.
 
 ### Test hooks
 
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 7a3c05c3..882fe796 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -516,6 +516,8 @@ Press Enter on the value field, or click **Apply**, to commit.
 
 Offset the selection's timestamps by a duration. Pick an amount and a unit. Useful when a sensor's clock was off and the recorded datetime values need to be shifted by a known offset.
 
+Months and years follow the calendar of the time zone chosen on the rail, and keep the clock time: in Denver, Jan 15 at 9:00 AM plus six months is Jul 15 at 9:00 AM, even though daylight saving time started in between. A day past the end of the target month moves to its last day, so Jan 31 plus one month is Feb 28. Month and year amounts must be whole numbers. The step saves the zone it used, so it replays the same for everyone, whatever zone they have chosen.
+
 ![Shift datetimes panel](./images/panel-shiftDatetimes.png)
 
 When the QC datastream declares an `intendedTimeSpacing`, the panel shows snap chips (`0.5×`, `1×`, `2×`) that pre-fill the amount with a multiple of the intended cadence. The active chip gets a check mark.
diff --git a/apps/qc-app/e2e/edit-shift-datetimes.spec.ts b/apps/qc-app/e2e/edit-shift-datetimes.spec.ts
index abf5bc5b..c6e2a24a 100644
--- a/apps/qc-app/e2e/edit-shift-datetimes.spec.ts
+++ b/apps/qc-app/e2e/edit-shift-datetimes.spec.ts
@@ -2,14 +2,17 @@
  * Shift datetimes edit: offsets selected timestamps by a given
  * amount + TimeUnit. The op runs a compound delete+add pipeline
  * internally, but from the UI we just see a single SHIFT_DATETIMES
- * row in edit history.
+ * row in edit history. Month and year shifts follow the chosen time
+ * zone's calendar, which the step saves.
  */
 
-import { test } from '@playwright/test'
+import { expect, test } from '@playwright/test'
 import { installMocks } from './support/mocks'
 import { openOp, setupEditView } from './support/app'
 import { expectHistoryContains, selectAllPoints } from './support/ops'
 
+test.use({ timezoneId: 'Asia/Tokyo' })
+
 test.describe('edit: shift datetimes', () => {
   test.beforeEach(async ({ page }) => {
     await installMocks(page, { qcHistories: true })
@@ -23,4 +26,27 @@ test.describe('edit: shift datetimes', () => {
     await page.getByRole('button', { name: /^shift$/i }).click()
     await expectHistoryContains(page, 'Shift Datetimes')
   })
+
+  test('a month shift saves the time zone it follows', async ({ page }) => {
+    await openOp(page, 'shiftDatetimes')
+    await page.getByTestId('shift-unit').click()
+    await page.getByRole('option', { name: 'MONTH', exact: true }).click()
+
+    const note = page.getByTestId('shift-calendar-note')
+    const shift = page.getByRole('button', { name: /^shift$/i })
+    await page.getByLabel('Amount').fill('0.5')
+    await expect(note).toHaveText('Months and years shift by whole numbers.')
+    await expect(shift).toBeDisabled()
+
+    await page.getByLabel('Amount').fill('1')
+    await expect(note).toContainText('Asia/Tokyo')
+    await shift.click()
+    await expectHistoryContains(page, 'Shift Datetimes')
+
+    const row = page
+      .locator('[data-testid^="history-item-"]')
+      .filter({ hasText: 'Shift Datetimes' })
+    await row.getByRole('button', { name: 'Expand arguments', exact: true }).click()
+    await expect(row.getByText('Asia/Tokyo', { exact: true })).toBeVisible()
+  })
 })
diff --git a/apps/qc-app/src/components/EditData/ShiftDatetimes.vue b/apps/qc-app/src/components/EditData/ShiftDatetimes.vue
index 9ad3a719..2ae66499 100644
--- a/apps/qc-app/src/components/EditData/ShiftDatetimes.vue
+++ b/apps/qc-app/src/components/EditData/ShiftDatetimes.vue
@@ -24,10 +24,10 @@
           v-model.number="shiftAmount"
           density="comfortable"
           variant="outlined"
+          :error="!amountValid"
           hide-details
-          @keyup.enter="
-            !isUpdating && selectedData?.length && onShiftDatetimes()
-          "
+          data-testid="shift-amount"
+          @keyup.enter="canShift && onShiftDatetimes()"
         />
         <v-select
           class="flex-grow-1"
@@ -38,9 +38,23 @@
           density="comfortable"
           variant="outlined"
           hide-details
+          data-testid="shift-unit"
         />
       </div>
 
+      <div
+        v-if="isCalendarUnit"
+        class="text-body-small mt-2"
+        :class="amountValid ? 'text-medium-emphasis' : 'text-error'"
+        data-testid="shift-calendar-note"
+      >
+        {{
+          amountValid
+            ? `Months and years follow the calendar in ${zoneName()}, keeping the clock time. A day past the end of a month moves to its last day.`
+            : 'Months and years shift by whole numbers.'
+        }}
+      </div>
+
       <div v-if="snapChips.length" class="d-flex ga-1 mt-2 flex-wrap">
         <v-chip
           v-for="chip in snapChips"
@@ -61,7 +75,7 @@
       <v-btn
         color="primary"
         variant="flat"
-        :disabled="isUpdating || !selectedData?.length"
+        :disabled="!canShift"
         @click="onShiftDatetimes"
       >
         Shift
@@ -78,6 +92,7 @@ import { EnumEditOperations, TimeUnit } from '@uwrl/qc-utils'
 import { usePlotlyStore } from '@/store/plotly'
 import { useUIStore, timeSpacingUnitToTimeUnitKey } from '@/store/userInterface'
 import { useFilterDispatch } from '@/composables/useFilterDispatch'
+import { zoneId, zoneName } from '@/utils/timeZone'
 
 const { selectedData, qcDatastream } = storeToRefs(useDataVisStore())
 const { selectedSeries, isUpdating } = storeToRefs(usePlotlyStore())
@@ -119,12 +134,21 @@ const applySnap = (chip: SnapChip) => {
   selectedShiftUnit.value = chip.unit
 }
 
+const isCalendarUnit = computed(
+  () => selectedShiftUnit.value === 'MONTH' || selectedShiftUnit.value === 'YEAR'
+)
+const amountValid = computed(() => {
+  const n = Number(shiftAmount.value)
+  return Number.isFinite(n) && (!isCalendarUnit.value || Number.isInteger(n))
+})
+const canShift = computed(
+  () => !isUpdating.value && !!selectedData.value?.length && amountValid.value
+)
+
 const emit = defineEmits(['close'])
 
 const onShiftDatetimes = async () => {
-  if (!selectedData.value?.length) {
-    return
-  }
+  if (!canShift.value) return
 
   isUpdating.value = true
 
@@ -134,7 +158,8 @@ const onShiftDatetimes = async () => {
         EnumEditOperations.SHIFT_DATETIMES,
         +shiftAmount.value,
         // @ts-ignore
-        TimeUnit[selectedShiftUnit.value]
+        TimeUnit[selectedShiftUnit.value],
+        zoneId()
       )) as number[] | undefined) ?? []
 
     isUpdating.value = false
diff --git a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
index 6d4fc2e6..cff9f763 100644
--- a/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
+++ b/apps/qc-app/src/components/EditData/__tests__/EditHistory.spec.ts
@@ -68,56 +68,65 @@ vi.mock('@/composables/useHistorySnapshots', () => ({
   }),
 }))
 
-vi.mock('@uwrl/qc-utils', () => ({
-  formatDuration: (ms: number) => String(ms) + 'ms',
-  // operations.ts (transitively imported via EditHistory.vue's
-  // `iconForMethod` lookup) reads enum values to build its method →
-  // operation-id map. Stub the keys it actually consults; the test
-  // never inspects the icon output, only that the entry renders.
-  EnumEditOperations: {
-    ADD_POINTS: 'ADD_POINTS',
-    CHANGE_VALUES: 'CHANGE_VALUES',
-    ASSIGN_VALUES_BULK: 'ASSIGN_VALUES_BULK',
-    ASSIGN_DATETIMES_BULK: 'ASSIGN_DATETIMES_BULK',
-    DELETE_POINTS: 'DELETE_POINTS',
-    DRIFT_CORRECTION: 'DRIFT_CORRECTION',
-    INTERPOLATE: 'INTERPOLATE',
-    SHIFT_DATETIMES: 'SHIFT_DATETIMES',
-    FILL_GAPS: 'FILL_GAPS',
-  },
-  EnumFilterOperations: {
-    FIND_GAPS: 'FIND_GAPS',
-    PERSISTENCE: 'PERSISTENCE',
-    CHANGE: 'CHANGE',
-    RATE_OF_CHANGE: 'RATE_OF_CHANGE',
-    VALUE_THRESHOLD: 'VALUE_THRESHOLD',
-    DATETIME_RANGE: 'DATETIME_RANGE',
-    SELECTION: 'SELECTION',
-  },
-  Operator: {
-    ADD: 'ADD',
-    SUB: 'SUB',
-    MULT: 'MULT',
-    DIV: 'DIV',
-    ASSIGN: 'ASSIGN',
-  },
-  TimeUnit: {
-    SECOND: 's',
-    MINUTE: 'm',
-    HOUR: 'h',
-    DAY: 'D',
-    WEEK: 'W',
-    MONTH: 'M',
-    YEAR: 'Y',
-  },
-  LogicalOperation: {
-    LT: 'Less than',
-    LTE: 'Less than or equal to',
-    GT: 'Greater than',
-    GTE: 'Greater than or equal to',
-    E: 'Equal',
-  },
-}))
+vi.mock('@uwrl/qc-utils', async (importOriginal) => {
+  // The real time zone math, which the app's date helpers use.
+  const { offsetMs, toWall, fromWall, toWallArray } =
+    await importOriginal<typeof import('@uwrl/qc-utils')>()
+  return {
+    offsetMs,
+    toWall,
+    fromWall,
+    toWallArray,
+    formatDuration: (ms: number) => String(ms) + 'ms',
+    // operations.ts (transitively imported via EditHistory.vue's
+    // `iconForMethod` lookup) reads enum values to build its method →
+    // operation-id map. Stub the keys it actually consults; the test
+    // never inspects the icon output, only that the entry renders.
+    EnumEditOperations: {
+      ADD_POINTS: 'ADD_POINTS',
+      CHANGE_VALUES: 'CHANGE_VALUES',
+      ASSIGN_VALUES_BULK: 'ASSIGN_VALUES_BULK',
+      ASSIGN_DATETIMES_BULK: 'ASSIGN_DATETIMES_BULK',
+      DELETE_POINTS: 'DELETE_POINTS',
+      DRIFT_CORRECTION: 'DRIFT_CORRECTION',
+      INTERPOLATE: 'INTERPOLATE',
+      SHIFT_DATETIMES: 'SHIFT_DATETIMES',
+      FILL_GAPS: 'FILL_GAPS',
+    },
+    EnumFilterOperations: {
+      FIND_GAPS: 'FIND_GAPS',
+      PERSISTENCE: 'PERSISTENCE',
+      CHANGE: 'CHANGE',
+      RATE_OF_CHANGE: 'RATE_OF_CHANGE',
+      VALUE_THRESHOLD: 'VALUE_THRESHOLD',
+      DATETIME_RANGE: 'DATETIME_RANGE',
+      SELECTION: 'SELECTION',
+    },
+    Operator: {
+      ADD: 'ADD',
+      SUB: 'SUB',
+      MULT: 'MULT',
+      DIV: 'DIV',
+      ASSIGN: 'ASSIGN',
+    },
+    TimeUnit: {
+      SECOND: 's',
+      MINUTE: 'm',
+      HOUR: 'h',
+      DAY: 'D',
+      WEEK: 'W',
+      MONTH: 'M',
+      YEAR: 'Y',
+    },
+    LogicalOperation: {
+      LT: 'Less than',
+      LTE: 'Less than or equal to',
+      GT: 'Greater than',
+      GTE: 'Greater than or equal to',
+      E: 'Equal',
+    },
+  }
+})
 
 import EditHistory from '@/components/EditData/EditHistory.vue'
 
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
index d1639471..334fd8b5 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataTable.spec.ts
@@ -56,19 +56,28 @@ vi.mock('@/composables/useDataSelection', () => ({
   useDataSelection: () => ({ clearSelected }),
 }))
 
-vi.mock('@uwrl/qc-utils', () => ({
-  EnumEditOperations: {
-    ASSIGN_VALUES_BULK: 'ASSIGN_VALUES_BULK',
-    ASSIGN_DATETIMES_BULK: 'ASSIGN_DATETIMES_BULK',
-  },
-  EnumFilterOperations: { SELECTION: 'SELECTION' },
-  // Guard against Invalid Date (null/undefined/NaN epochs) during render.
-  formatDate: (d: Date) => {
-    const t = d?.getTime?.()
-    if (t == null || Number.isNaN(t)) return ''
-    return d.toISOString()
-  },
-}))
+vi.mock('@uwrl/qc-utils', async (importOriginal) => {
+  // The real time zone math, which the app's date helpers use.
+  const { offsetMs, toWall, fromWall, toWallArray } =
+    await importOriginal<typeof import('@uwrl/qc-utils')>()
+  return {
+    offsetMs,
+    toWall,
+    fromWall,
+    toWallArray,
+    EnumEditOperations: {
+      ASSIGN_VALUES_BULK: 'ASSIGN_VALUES_BULK',
+      ASSIGN_DATETIMES_BULK: 'ASSIGN_DATETIMES_BULK',
+    },
+    EnumFilterOperations: { SELECTION: 'SELECTION' },
+    // Guard against Invalid Date (null/undefined/NaN epochs) during render.
+    formatDate: (d: Date) => {
+      const t = d?.getTime?.()
+      if (t == null || Number.isNaN(t)) return ''
+      return d.toISOString()
+    },
+  }
+})
 
 vi.mock('@/components/VisualizeData/EditableCell.vue', () => ({
   default: {
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
index 6b02f830..0ff00e91 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/DataVisDatasetsTable.spec.ts
@@ -82,7 +82,7 @@ vi.mock('@/utils/csvExport', () => ({
   downloadDatastreamsCsvZip: vi.fn().mockResolvedValue(undefined),
 }))
 
-const snackbarError = vi.fn()
+const snackbarError = vi.hoisted(() => vi.fn())
 vi.mock('@uwrl/qc-utils', async (importOriginal) => {
   const actual = (await importOriginal()) as Record<string, unknown>
   return { ...actual, Snackbar: { error: snackbarError, success: vi.fn() } }
diff --git a/apps/qc-app/src/utils/__tests__/displayZone.spec.ts b/apps/qc-app/src/utils/__tests__/displayZone.spec.ts
index e45bb58b..659230da 100644
--- a/apps/qc-app/src/utils/__tests__/displayZone.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/displayZone.spec.ts
@@ -1,13 +1,11 @@
 import { describe, it, expect } from 'vitest'
 import {
-  fromWall,
   fromWallParts,
   offsetMs,
-  toWall,
-  toWallArray,
   wallParts,
   zoneAbbreviation,
   zoneDescription,
+  zoneId,
   type DisplayZone,
 } from '../timeZone'
 import { fromPlot, plotCoord, toPlotDate } from '../plotting/plotTime'
@@ -17,32 +15,16 @@ const denver: DisplayZone = { mode: 'iana', zone: 'America/Denver' }
 const india: DisplayZone = { mode: 'fixedOffset', zone: '+0530' }
 const utc: DisplayZone = { mode: 'utc', zone: '' }
 
-// 2026-03-08 09:00Z is 2 AM MST, when Denver springs forward to MDT.
-const SPRING = Date.UTC(2026, 2, 8, 9)
-
-describe('display zone offsets', () => {
-  it('reads an IANA zone, tracking daylight saving', () => {
+describe('display zone', () => {
+  it('passes the chosen zone to the offset math', () => {
+    expect(zoneId(denver)).toBe('America/Denver')
+    expect(zoneId(india)).toBe('+0530')
+    expect(zoneId(utc)).toBe('UTC')
     expect(offsetMs(Date.UTC(2026, 0, 15, 12), denver)).toBe(-7 * HOUR)
-    expect(offsetMs(Date.UTC(2026, 6, 15, 12), denver)).toBe(-6 * HOUR)
-  })
-
-  it('changes offset at the transition minute, not the day', () => {
-    expect(offsetMs(SPRING - 60_000, denver)).toBe(-7 * HOUR)
-    expect(offsetMs(SPRING, denver)).toBe(-6 * HOUR)
-  })
-
-  it('reads a fixed offset and UTC', () => {
     expect(offsetMs(0, india)).toBe(5.5 * HOUR)
     expect(offsetMs(0, utc)).toBe(0)
   })
 
-  // Outside the hour that falling back repeats, which has two instants.
-  it('round-trips an instant through its wall value', () => {
-    for (const ms of [SPRING - HOUR, SPRING, SPRING + HOUR, Date.UTC(2026, 10, 1, 10)]) {
-      expect(fromWall(toWall(ms, denver), denver)).toBe(ms)
-    }
-  })
-
   it('reads and builds the zone clock', () => {
     const ms = Date.UTC(2026, 6, 15, 18, 30)
     expect(wallParts(ms, denver)).toEqual({
@@ -56,15 +38,6 @@ describe('display zone offsets', () => {
     expect(fromWallParts(2026, 6, 15, 12, 30, 0, denver)).toBe(ms)
   })
 
-  it('moves an array across a transition, and leaves UTC uncopied', () => {
-    const xs = new Float64Array([SPRING - HOUR, SPRING + HOUR])
-    expect(Array.from(toWallArray(xs, denver))).toEqual([
-      SPRING - 8 * HOUR,
-      SPRING - 5 * HOUR,
-    ])
-    expect(toWallArray(xs, utc)).toBe(xs)
-  })
-
   it('names the zone', () => {
     expect(zoneAbbreviation(Date.UTC(2026, 0, 15), denver)).toBe('MST')
     expect(zoneAbbreviation(0, india)).toBe('UTC+05:30')
diff --git a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
index 3ac1ef87..955e2361 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/options.spec.ts
@@ -5,29 +5,38 @@ import { ref } from 'vue'
 // --- Mocks --------------------------------------------------------------
 
 // Avoid real @uwrl/qc-utils (worker-heavy). Re-export just what options.ts touches.
-vi.mock('@uwrl/qc-utils', () => ({
-  findFirstGreaterOrEqual: (arr: number[], target: number) => {
-    let lo = 0
-    let hi = arr.length
-    while (lo < hi) {
-      const mid = (lo + hi) >>> 1
-      if (arr[mid] < target) lo = mid + 1
-      else hi = mid
-    }
-    return lo
-  },
-  findLastLessOrEqual: (arr: number[], target: number) => {
-    let lo = 0
-    let hi = arr.length
-    while (lo < hi) {
-      const mid = (lo + hi) >>> 1
-      if (arr[mid] > target) hi = mid
-      else lo = mid + 1
-    }
-    return lo - 1
-  },
-  EnumFilterOperations: { SELECTION: 'SELECTION' },
-}))
+vi.mock('@uwrl/qc-utils', async (importOriginal) => {
+  // The real time zone math, which the app's date helpers use.
+  const { offsetMs, toWall, fromWall, toWallArray } =
+    await importOriginal<typeof import('@uwrl/qc-utils')>()
+  return {
+    offsetMs,
+    toWall,
+    fromWall,
+    toWallArray,
+    findFirstGreaterOrEqual: (arr: number[], target: number) => {
+      let lo = 0
+      let hi = arr.length
+      while (lo < hi) {
+        const mid = (lo + hi) >>> 1
+        if (arr[mid] < target) lo = mid + 1
+        else hi = mid
+      }
+      return lo
+    },
+    findLastLessOrEqual: (arr: number[], target: number) => {
+      let lo = 0
+      let hi = arr.length
+      while (lo < hi) {
+        const mid = (lo + hi) >>> 1
+        if (arr[mid] > target) hi = mid
+        else lo = mid + 1
+      }
+      return lo - 1
+    },
+    EnumFilterOperations: { SELECTION: 'SELECTION' },
+  }
+})
 
 const qcDatastream = ref<{ id: string; phenomenonBeginTime?: string } | null>(null)
 const sourceContextDatastream = ref<{ id: string } | null>(null)
diff --git a/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
index 63dbc06f..78d026b3 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
@@ -10,19 +10,28 @@ const plotlyMock = vi.hoisted(() => ({
 }))
 vi.mock('plotly.js-dist', () => ({ default: plotlyMock }))
 
-vi.mock('@uwrl/qc-utils', () => ({
-  findFirstGreaterOrEqual: (arr: number[], target: number) => {
-    let lo = 0
-    let hi = arr.length
-    while (lo < hi) {
-      const mid = (lo + hi) >>> 1
-      if (arr[mid] < target) lo = mid + 1
-      else hi = mid
-    }
-    return lo
-  },
-  EnumFilterOperations: { SELECTION: 'SELECTION' },
-}))
+vi.mock('@uwrl/qc-utils', async (importOriginal) => {
+  // The real time zone math, which the app's date helpers use.
+  const { offsetMs, toWall, fromWall, toWallArray } =
+    await importOriginal<typeof import('@uwrl/qc-utils')>()
+  return {
+    offsetMs,
+    toWall,
+    fromWall,
+    toWallArray,
+    findFirstGreaterOrEqual: (arr: number[], target: number) => {
+      let lo = 0
+      let hi = arr.length
+      while (lo < hi) {
+        const mid = (lo + hi) >>> 1
+        if (arr[mid] < target) lo = mid + 1
+        else hi = mid
+      }
+      return lo
+    },
+    EnumFilterOperations: { SELECTION: 'SELECTION' },
+  }
+})
 
 const qcDatastream = ref<any>(null)
 const selectedData = ref<any>(null)
diff --git a/apps/qc-app/src/utils/timeZone.ts b/apps/qc-app/src/utils/timeZone.ts
index 5b7a11ca..59afeabc 100644
--- a/apps/qc-app/src/utils/timeZone.ts
+++ b/apps/qc-app/src/utils/timeZone.ts
@@ -6,9 +6,16 @@
  * Instants stay epoch ms everywhere. A "wall" value is an instant moved by
  * the zone's offset, so its UTC fields read as the zone's clock. Formatting,
  * pickers and the plot work on wall values; everything stored stays real.
+ * The offset math lives in qc-utils.
  */
 
 import { ref } from 'vue'
+import {
+  fromWall as zoneFromWall,
+  offsetMs as zoneOffsetMs,
+  toWall as zoneToWall,
+  toWallArray as zoneToWallArray,
+} from '@uwrl/qc-utils'
 
 export type ZoneMode = 'utc' | 'fixedOffset' | 'iana'
 
@@ -27,130 +34,31 @@ export const browserZone = (): DisplayZone => ({
 export const displayZone = ref<DisplayZone>(browserZone())
 
 const MINUTE = 60_000
-const DAY = 86_400_000
-
-/** `-0700` to minutes east of UTC. */
-function fixedOffsetMinutes(zone: string): number {
-  const m = /^([+-])(\d{2}):?(\d{2})$/.exec(zone)
-  if (!m) return 0
-  const minutes = Number(m[2]) * 60 + Number(m[3])
-  return m[1] === '-' ? -minutes : minutes
-}
-
-const partFormatters = new Map<string, Intl.DateTimeFormat>()
-
-function ianaOffsetMs(ms: number, timeZone: string): number {
-  let fmt = partFormatters.get(timeZone)
-  if (!fmt) {
-    fmt = new Intl.DateTimeFormat('en-US', {
-      timeZone,
-      hourCycle: 'h23',
-      year: 'numeric',
-      month: 'numeric',
-      day: 'numeric',
-      hour: 'numeric',
-      minute: 'numeric',
-      second: 'numeric',
-    })
-    partFormatters.set(timeZone, fmt)
-  }
-  const get = (parts: Intl.DateTimeFormatPart[], type: string) =>
-    Number(parts.find((p) => p.type === type)?.value)
-  const parts = fmt.formatToParts(ms)
-  const wall = Date.UTC(
-    get(parts, 'year'),
-    get(parts, 'month') - 1,
-    get(parts, 'day'),
-    get(parts, 'hour'),
-    get(parts, 'minute'),
-    get(parts, 'second')
-  )
-  return wall - (ms - (((ms % 1000) + 1000) % 1000))
-}
-
-/** An offset that holds from `from` until `until`. */
-interface Segment {
-  from: number
-  until: number
-  offset: number
-}
 
-// Per zone, per UTC day: one segment, or two around a transition.
-const segmentCache = new Map<string, Map<number, Segment[]>>()
-
-function ianaSegment(ms: number, timeZone: string): Segment {
-  let days = segmentCache.get(timeZone)
-  if (!days) segmentCache.set(timeZone, (days = new Map()))
-  const day = Math.floor(ms / DAY)
-  let segments = days.get(day)
-  if (!segments) {
-    const start = day * DAY
-    const end = start + DAY
-    const first = ianaOffsetMs(start, timeZone)
-    const last = ianaOffsetMs(end - MINUTE, timeZone)
-    if (first === last) {
-      segments = [{ from: start, until: end, offset: first }]
-    } else {
-      // Zones change offset on a minute boundary at most once a day.
-      let lo = start
-      let hi = end - MINUTE
-      while (hi - lo > MINUTE) {
-        const mid = lo + Math.floor((hi - lo) / 2 / MINUTE) * MINUTE
-        if (ianaOffsetMs(mid, timeZone) === first) lo = mid
-        else hi = mid
-      }
-      segments = [
-        { from: start, until: hi, offset: first },
-        { from: hi, until: end, offset: last },
-      ]
-    }
-    days.set(day, segments)
-  }
-  return segments.find((s) => ms < s.until) ?? segments[segments.length - 1]!
-}
-
-function segmentAt(ms: number, z: DisplayZone): Segment {
-  if (z.mode === 'iana') return ianaSegment(ms, z.zone)
-  const offset = z.mode === 'fixedOffset' ? fixedOffsetMinutes(z.zone) * MINUTE : 0
-  return { from: -Infinity, until: Infinity, offset }
+/** The zone as qc-utils takes it: `UTC`, a fixed offset, or an IANA name.
+ *  Shift operations save it, so their calendar replays the same anywhere. */
+export function zoneId(z: DisplayZone = displayZone.value): string {
+  return z.mode === 'utc' ? 'UTC' : z.zone
 }
 
 /** The zone's offset from UTC at instant `ms`, in ms. */
-export function offsetMs(ms: number, z: DisplayZone = displayZone.value): number {
-  return segmentAt(ms, z).offset
-}
+export const offsetMs = (ms: number, z: DisplayZone = displayZone.value): number =>
+  zoneOffsetMs(ms, zoneId(z))
 
-export function toWall(ms: number, z: DisplayZone = displayZone.value): number {
-  return ms + offsetMs(ms, z)
-}
+export const toWall = (ms: number, z: DisplayZone = displayZone.value): number =>
+  zoneToWall(ms, zoneId(z))
 
 /** The instant a wall value names. A clock time that a daylight saving
  *  change skips or repeats has no single instant; it maps to one beside it. */
-export function fromWall(wall: number, z: DisplayZone = displayZone.value): number {
-  const guess = wall - offsetMs(wall, z)
-  return wall - offsetMs(guess, z)
-}
+export const fromWall = (wall: number, z: DisplayZone = displayZone.value): number =>
+  zoneFromWall(wall, zoneId(z))
 
-/** Every value of an ascending array moved to wall time, walking the offset
- *  segments rather than looking each point up. The input itself comes back
- *  when nothing moves (UTC), so it is copied only when it has to be. */
-export function toWallArray<T extends ArrayLike<number>>(
+/** Every value of an ascending array moved to wall time. The input itself
+ *  comes back when nothing moves (UTC). */
+export const toWallArray = <T extends ArrayLike<number>>(
   xs: T,
   z: DisplayZone = displayZone.value
-): T | Float64Array {
-  let out: Float64Array | null = null
-  let seg: Segment | null = null
-  for (let i = 0; i < xs.length; i++) {
-    const x = xs[i]!
-    if (!seg || x < seg.from || x >= seg.until) seg = segmentAt(x, z)
-    if (seg.offset !== 0 && !out) {
-      out = new Float64Array(xs.length)
-      for (let j = 0; j < i; j++) out[j] = xs[j]!
-    }
-    if (out) out[i] = x + seg.offset
-  }
-  return out ?? xs
-}
+): T | Float64Array => zoneToWallArray(xs, zoneId(z))
 
 /** The zone's clock at `ms`, with a 0-based month. */
 export function wallParts(ms: number, z: DisplayZone = displayZone.value) {
diff --git a/packages/qc-utils/README.md b/packages/qc-utils/README.md
index e3410de1..ddd2f5d9 100644
--- a/packages/qc-utils/README.md
+++ b/packages/qc-utils/README.md
@@ -83,7 +83,7 @@ replay from a saved QC history, and in unit tests.
 | `ASSIGN_DATETIMES_BULK` | Write parallel datetimes; runs as one combined delete + add. |
 | `DELETE_POINTS`         | Drop the selection from x / y in a single skip-on-delete pass. |
 | `INTERPOLATE`           | Linear interpolation across each consecutive group in the selection. |
-| `SHIFT_DATETIMES`       | Offset the selection's timestamps by `(amount, TimeUnit)`. |
+| `SHIFT_DATETIMES`       | Offset the selection's timestamps by `(amount, TimeUnit, timeZone)`. |
 | `DRIFT_CORRECTION`      | Apply linear drift `value` to every consecutive group in the selection. |
 | `FILL_GAPS`             | Detect gaps over `gapThreshold`; insert points at `fillCadence` (interpolated or constant `fillValue`). |
 
diff --git a/packages/qc-utils/docs/API_REFERENCE.md b/packages/qc-utils/docs/API_REFERENCE.md
index 3dbaa3d3..f20b17cf 100644
--- a/packages/qc-utils/docs/API_REFERENCE.md
+++ b/packages/qc-utils/docs/API_REFERENCE.md
@@ -120,7 +120,7 @@ matters; lower means smaller idle memory, more frequent grow / copy.
 | `ASSIGN_DATETIMES_BULK`| `(indices: number[], datetimes: number[])` — combined delete + add.          |
 | `DELETE_POINTS`        | `(indices?: number[])`, defaulting to the prior selection. Indices past the end and repeats are ignored. |
 | `INTERPOLATE`          | `()` — linear interpolation per consecutive group in the prior selection.    |
-| `SHIFT_DATETIMES`      | `(amount: number, unit: TimeUnit)`                                           |
+| `SHIFT_DATETIMES`      | `(amount: number, unit: TimeUnit, timeZone: string)`; months and years follow `timeZone`'s calendar; see Time zones. |
 | `DRIFT_CORRECTION`     | `(value: number)` — linear drift across each consecutive group.              |
 | `FILL_GAPS`            | `(gapThreshold: [amount, unit], fillCadence: [amount, unit], fillValue?: number)` |
 
@@ -346,6 +346,33 @@ and summer offsets), with their `FixedOffsetTimezone` and `DstAwareTimezone`
 value types. The lists a data connection's timestamps choose from; the QC
 app's time zone setting offers the same ones.
 
+The math takes a zone as a string: `UTC`, a fixed offset like `-0700`, or
+an IANA name.
+
+```ts
+isValidTimeZone(zone: unknown): boolean
+offsetMs(ms: number, zone: string): number       // the zone's offset at ms
+toWall(ms: number, zone: string): number         // ms moved to the zone's clock
+fromWall(wall: number, zone: string): number     // back to an instant
+toWallArray(xs, zone): typeof xs | Float64Array  // xs itself when nothing moves
+addCalendarMonths(ms: number, months: number, zone: string): number
+```
+
+A **wall** value is an instant moved by the zone's offset, so its UTC
+fields read as the zone's clock. IANA offsets are cached per UTC day, with
+the transition minute found on a day the offset changes. A clock time that
+daylight saving skips or repeats maps to an instant beside it.
+
+`addCalendarMonths` keeps the clock time on the zone's calendar, so Jan 15
+at 9:00 in Denver plus six months is Jul 15 at 9:00 there, across the
+change to daylight time. A day past the end of the target month clamps to
+its last day: Jan 31 plus a month is Feb 28 (29 in a leap year).
+
+`SHIFT_DATETIMES` saves its zone, so a month or year shift replays the
+same on any machine. It fails (`execution.status: "failed"`) on a zone
+`isValidTimeZone` rejects, or a month or year amount that isn't whole.
+Other units are fixed spans and ignore the zone.
+
 ### `measureEllapsedTime<T>(fn: () => Promise<T> | T): Promise<{ result: T; duration: number }>`
 
 Wrap any thunk with wall-clock measurement. Used by dispatch to fill
diff --git a/packages/qc-utils/docs/ARCHITECTURE.md b/packages/qc-utils/docs/ARCHITECTURE.md
index 340512d2..9051d45f 100644
--- a/packages/qc-utils/docs/ARCHITECTURE.md
+++ b/packages/qc-utils/docs/ARCHITECTURE.md
@@ -149,7 +149,7 @@ Two enums define the entire dispatch surface:
 | `ASSIGN_DATETIMES_BULK` | Parallel `datetimes[i] → dataX[selection[i]]` via combined delete+add.   | inline  |
 | `DELETE_POINTS`         | Drop the selection in a single skip-on-delete pass.                       | yes     |
 | `INTERPOLATE`           | Linear interpolation per consecutive group in the selection.              | yes     |
-| `SHIFT_DATETIMES`       | Offset the selection's timestamps by `(amount, TimeUnit)`.                | yes     |
+| `SHIFT_DATETIMES`       | Offset the selection's timestamps by `(amount, TimeUnit, timeZone)`.      | yes     |
 | `DRIFT_CORRECTION`      | Apply linear drift across each consecutive group in the selection.         | yes     |
 | `FILL_GAPS`             | Detect gaps above threshold; insert points at fillCadence (interpolated). | yes     |
 
diff --git a/packages/qc-utils/docs/QC_HISTORY.md b/packages/qc-utils/docs/QC_HISTORY.md
index 6bbe8fa7..a1a5f5ce 100644
--- a/packages/qc-utils/docs/QC_HISTORY.md
+++ b/packages/qc-utils/docs/QC_HISTORY.md
@@ -241,7 +241,7 @@ trailing range tuple.
 | `ASSIGN_DATETIMES_BULK` | `[datetimes[]]`                                                                                     | **Yes** (indices to assign at)           |
 | `DELETE_POINTS`         | `[]`                                                                                                | **Yes** (indices to delete)              |
 | `INTERPOLATE`           | `[]`                                                                                                | **Yes** (indices to interpolate)         |
-| `SHIFT_DATETIMES`       | `[amount, unit]`                                                                                    | **Yes** (indices to shift)               |
+| `SHIFT_DATETIMES`       | `[amount, unit, timeZone]`                                                                          | **Yes** (indices to shift)               |
 | `DRIFT_CORRECTION`      | `[value]`                                                                                           | **Yes** (consecutive groups → ranges, per-group drift = `value`) |
 
 ### Note on the runtime selection-coupling column
diff --git a/packages/qc-utils/src/utils/__tests__/timeZone.spec.ts b/packages/qc-utils/src/utils/__tests__/timeZone.spec.ts
new file mode 100644
index 00000000..b74349d5
--- /dev/null
+++ b/packages/qc-utils/src/utils/__tests__/timeZone.spec.ts
@@ -0,0 +1,76 @@
+import { describe, it, expect } from 'vitest'
+import {
+  addCalendarMonths,
+  fromWall,
+  isValidTimeZone,
+  offsetMs,
+  toWall,
+  toWallArray,
+} from '../timeZone'
+
+const HOUR = 3_600_000
+const DENVER = 'America/Denver'
+
+// 2026-03-08 09:00Z is 2 AM MST, when Denver springs forward to MDT.
+const SPRING = Date.UTC(2026, 2, 8, 9)
+
+describe('time zone offsets', () => {
+  it('reads an IANA zone, tracking daylight saving', () => {
+    expect(offsetMs(Date.UTC(2026, 0, 15, 12), DENVER)).toBe(-7 * HOUR)
+    expect(offsetMs(Date.UTC(2026, 6, 15, 12), DENVER)).toBe(-6 * HOUR)
+  })
+
+  it('changes offset at the transition minute, not the day', () => {
+    expect(offsetMs(SPRING - 60_000, DENVER)).toBe(-7 * HOUR)
+    expect(offsetMs(SPRING, DENVER)).toBe(-6 * HOUR)
+  })
+
+  it('reads a fixed offset and UTC', () => {
+    expect(offsetMs(0, '+0530')).toBe(5.5 * HOUR)
+    expect(offsetMs(0, '-07:00')).toBe(-7 * HOUR)
+    expect(offsetMs(0, 'UTC')).toBe(0)
+  })
+
+  // Outside the hour that falling back repeats, which has two instants.
+  it('round-trips an instant through its wall value', () => {
+    for (const ms of [SPRING - HOUR, SPRING, SPRING + HOUR, Date.UTC(2026, 10, 1, 10)]) {
+      expect(fromWall(toWall(ms, DENVER), DENVER)).toBe(ms)
+    }
+  })
+
+  it('moves an array across a transition, and leaves UTC uncopied', () => {
+    const xs = new Float64Array([SPRING - HOUR, SPRING + HOUR])
+    expect(Array.from(toWallArray(xs, DENVER))).toEqual([
+      SPRING - 8 * HOUR,
+      SPRING - 5 * HOUR,
+    ])
+    expect(toWallArray(xs, 'UTC')).toBe(xs)
+  })
+
+  it('knows a zone from a typo', () => {
+    expect(isValidTimeZone('UTC')).toBe(true)
+    expect(isValidTimeZone('-0700')).toBe(true)
+    expect(isValidTimeZone(DENVER)).toBe(true)
+    expect(isValidTimeZone('Mars/Olympus')).toBe(false)
+    expect(isValidTimeZone('')).toBe(false)
+    expect(isValidTimeZone(undefined)).toBe(false)
+  })
+})
+
+describe('addCalendarMonths', () => {
+  it('keeps the clock time across daylight saving', () => {
+    expect(addCalendarMonths(Date.UTC(2026, 0, 15, 16), 6, DENVER)).toBe(
+      Date.UTC(2026, 6, 15, 15)
+    )
+  })
+
+  it('crosses years both ways', () => {
+    expect(addCalendarMonths(Date.UTC(2026, 10, 15), 3, 'UTC')).toBe(Date.UTC(2027, 1, 15))
+    expect(addCalendarMonths(Date.UTC(2026, 1, 15), -3, 'UTC')).toBe(Date.UTC(2025, 10, 15))
+  })
+
+  it('clamps to the last day of a shorter month', () => {
+    expect(addCalendarMonths(Date.UTC(2026, 0, 31), 1, 'UTC')).toBe(Date.UTC(2026, 1, 28))
+    expect(addCalendarMonths(Date.UTC(2024, 1, 29), 12, 'UTC')).toBe(Date.UTC(2025, 1, 28))
+  })
+})
diff --git a/packages/qc-utils/src/utils/index.ts b/packages/qc-utils/src/utils/index.ts
index 97679010..e600a03f 100644
--- a/packages/qc-utils/src/utils/index.ts
+++ b/packages/qc-utils/src/utils/index.ts
@@ -5,3 +5,4 @@ export * from './observations'
 export * from './plotting/observation-record'
 export * from './plotting/calibration'
 export * from './plotting/history'
+export * from './timeZone'
diff --git a/packages/qc-utils/src/utils/plotting/__tests__/calibrated-dispatch.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/calibrated-dispatch.spec.ts
index 10a73dbd..16043bef 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/calibrated-dispatch.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/calibrated-dispatch.spec.ts
@@ -300,7 +300,7 @@ const cases: CalibratedCase[] = [
       const selection = consecutiveIndexes(size, rec.dataX.length)
       await rec.dispatch([
         [EnumFilterOperations.SELECTION, selection],
-        [EnumEditOperations.SHIFT_DATETIMES, 1, TimeUnit.HOUR],
+        [EnumEditOperations.SHIFT_DATETIMES, 1, TimeUnit.HOUR, 'UTC'],
       ])
       return rec
     },
diff --git a/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts
index 63a6f59f..a761b4b6 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/history.spec.ts
@@ -532,7 +532,7 @@ describe('applyHistory — round-trip', () => {
   it('round-trips SELECTION → SHIFT_DATETIMES with non-index args preserved', async () => {
     await rec.dispatch([
       [EnumFilterOperations.SELECTION, [7, 8, 9]],
-      [EnumEditOperations.SHIFT_DATETIMES, 1, TimeUnit.HOUR],
+      [EnumEditOperations.SHIFT_DATETIMES, 1, TimeUnit.HOUR, 'UTC'],
     ]);
     const xAfter = Array.from(rec.dataX);
     const history = serializeHistory(rec, SAMPLE_WINDOW);
@@ -543,7 +543,36 @@ describe('applyHistory — round-trip', () => {
 
     expect(report.failed).toEqual([]);
     expect(Array.from(fresh.dataX)).toEqual(xAfter);
-    expect(fresh.history[1].args).toEqual([1, TimeUnit.HOUR]);
+    expect(fresh.history[1].args).toEqual([1, TimeUnit.HOUR, 'UTC']);
+  });
+
+  it('replays a month shift on its saved zone calendar', async () => {
+    await rec.dispatch([
+      [EnumFilterOperations.SELECTION, [19]],
+      [EnumEditOperations.SHIFT_DATETIMES, 1, TimeUnit.MONTH, 'America/Denver'],
+    ]);
+    const xAfter = Array.from(rec.dataX);
+    const history = serializeHistory(rec, SAMPLE_WINDOW);
+
+    const fresh = makeRecord(20);
+    await fresh.reload();
+    const report = await applyHistory(fresh, history);
+
+    expect(report.failed).toEqual([]);
+    expect(Array.from(fresh.dataX)).toEqual(xAfter);
+  });
+
+  it('fails a shift step saved without a zone', async () => {
+    const fresh = makeRecord(20);
+    await fresh.reload();
+    const report = await applyHistory(fresh, {
+      ...serializeHistory(fresh, SAMPLE_WINDOW),
+      operations: [
+        { method: EnumFilterOperations.SELECTION, args: [[1]] },
+        { method: EnumEditOperations.SHIFT_DATETIMES, args: [1, TimeUnit.MONTH] },
+      ],
+    });
+    expect(report.failed.map((f) => f.index)).toEqual([1]);
   });
 
   it('round-trips SELECTION → CHANGE_VALUES (consumes preceding selected at runtime)', async () => {
diff --git a/packages/qc-utils/src/utils/plotting/__tests__/observation-record-paths.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/observation-record-paths.spec.ts
index 6307e428..78ab2180 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/observation-record-paths.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/observation-record-paths.spec.ts
@@ -176,7 +176,7 @@ describe('ObservationRecord — worker paths', () => {
     const originalLen = rec.dataX.length
     await rec.dispatch([
       [EnumFilterOperations.SELECTION, [5, 6, 7]],
-      [EnumEditOperations.SHIFT_DATETIMES, 1, TimeUnit.HOUR],
+      [EnumEditOperations.SHIFT_DATETIMES, 1, TimeUnit.HOUR, 'UTC'],
     ])
     await flushMicrotasks()
     const last = rec.history[rec.history.length - 1]
diff --git a/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
index f807c4a0..91a1b70c 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/observation-record.spec.ts
@@ -463,7 +463,7 @@ describe('ObservationRecord', () => {
       const originalLen = local.dataX.length;
       await local.dispatch([
         [EnumFilterOperations.SELECTION, [7, 8, 9]],
-        [EnumEditOperations.SHIFT_DATETIMES, 1, TimeUnit.HOUR],
+        [EnumEditOperations.SHIFT_DATETIMES, 1, TimeUnit.HOUR, 'UTC'],
       ]);
       expect(local.dataX.length).toBe(originalLen);
       // Datetimes still sorted
@@ -481,7 +481,29 @@ describe('ObservationRecord', () => {
       await local.reload();
       const before = Array.from(local.dataX);
       // No SELECTION dispatched first → wrapper bails.
-      await local.dispatch(EnumEditOperations.SHIFT_DATETIMES, 1, TimeUnit.HOUR);
+      await local.dispatch(EnumEditOperations.SHIFT_DATETIMES, 1, TimeUnit.HOUR, 'UTC');
+      expect(Array.from(local.dataX)).toEqual(before);
+    });
+
+    it('SHIFT_DATETIMES fails without a known zone or with a fractional month', async () => {
+      const uniform = buildUniformData(5, 0, 10);
+      const local = new ObservationRecord({
+        datetimes: uniform.datetimes,
+        dataValues: uniform.dataValues,
+      });
+      await local.reload();
+      const before = Array.from(local.dataX);
+      for (const [amount, unit, zone] of [
+        [1, TimeUnit.MONTH, 'Mars/Olympus'],
+        [1, TimeUnit.HOUR, undefined],
+        [0.5, TimeUnit.MONTH, 'UTC'],
+      ] as const) {
+        await local.dispatch([
+          [EnumFilterOperations.SELECTION, [1, 2]],
+          [EnumEditOperations.SHIFT_DATETIMES, amount, unit, zone],
+        ]);
+        expect(local.history[local.history.length - 1].execution.status).toBe('failed');
+      }
       expect(Array.from(local.dataX)).toEqual(before);
     });
 
diff --git a/packages/qc-utils/src/utils/plotting/__tests__/operation-cores.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/operation-cores.spec.ts
index 80d20458..ec55a85b 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/operation-cores.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/operation-cores.spec.ts
@@ -329,57 +329,65 @@ describe('deleteDataPointsCore', () => {
 // =====================================================================
 
 describe('shiftDatetimesCollection', () => {
-  it('shifts by a precomputed deltaMs when unit is not month/year', () => {
+  const fixed = { months: 0, deltaMs: 500_000, timeZone: 'UTC' }
+
+  it('shifts by a fixed span', () => {
     const x = f64([1_000_000, 2_000_000, 3_000_000])
     const y = f32([1, 2, 3])
-    const out = shiftDatetimesCollection(x, y, [0, 2], {
-      amount: 1,
-      isMonth: false,
-      isYear: false,
-      deltaMs: 500_000,
-    })
-    expect(out).toEqual([
+    expect(shiftDatetimesCollection(x, y, [0, 2], fixed)).toEqual([
       [1_500_000, 1],
       [3_500_000, 3],
     ])
   })
 
-  it('shifts by calendar months when isMonth is true', () => {
-    const base = Date.UTC(2024, 0, 15)
-    const [shifted] = shiftDatetimesCollection(f64([base]), f32([42]), [0], {
-      amount: 2,
-      isMonth: true,
-      isYear: false,
-      deltaMs: 0,
-    })
-    const a = new Date(base)
-    const b = new Date(shifted[0])
-    const monthDelta =
-      (b.getFullYear() - a.getFullYear()) * 12 + (b.getMonth() - a.getMonth())
-    expect(monthDelta).toBe(2)
-    expect(shifted[1]).toBe(42)
-  })
-
-  it('shifts by calendar years when isYear is true', () => {
-    const base = Date.UTC(2024, 5, 10)
-    const [shifted] = shiftDatetimesCollection(f64([base]), f32([7]), [0], {
-      amount: 3,
-      isMonth: false,
-      isYear: true,
-      deltaMs: 0,
-    })
-    expect(new Date(shifted[0]).getFullYear() - new Date(base).getFullYear()).toBe(3)
+  it('keeps the clock time of the zone across daylight saving', () => {
+    // Jan 15, 9 AM MST to Jul 15, 9 AM MDT.
+    const [shifted] = shiftDatetimesCollection(
+      f64([Date.UTC(2026, 0, 15, 16)]),
+      f32([42]),
+      [0],
+      { months: 6, deltaMs: 0, timeZone: 'America/Denver' }
+    )
+    expect(shifted).toEqual([Date.UTC(2026, 6, 15, 15), 42])
   })
 
-  it('returns an empty array for empty index list', () => {
-    expect(
-      shiftDatetimesCollection(f64([1]), f32([1]), [], {
-        amount: 1,
-        isMonth: false,
-        isYear: false,
+  it('moves months on the zone calendar, not UTC', () => {
+    // Feb 1, 01:00Z is still Jan 31, 6 PM in Denver.
+    const [[x]] = shiftDatetimesCollection(
+      f64([Date.UTC(2026, 1, 1, 1)]),
+      f32([0]),
+      [0],
+      { months: 1, deltaMs: 0, timeZone: 'America/Denver' }
+    )
+    expect(x).toBe(Date.UTC(2026, 2, 1, 1))
+  })
+
+  it('clamps to the last day of a shorter month', () => {
+    const shift = (ms: number, months: number) =>
+      shiftDatetimesCollection(f64([ms]), f32([0]), [0], {
+        months,
         deltaMs: 0,
-      })
-    ).toEqual([])
+        timeZone: 'UTC',
+      })[0][0]
+    expect(shift(Date.UTC(2026, 0, 31, 6), 1)).toBe(Date.UTC(2026, 1, 28, 6))
+    expect(shift(Date.UTC(2024, 0, 31), 1)).toBe(Date.UTC(2024, 1, 29))
+    expect(shift(Date.UTC(2024, 1, 29), 12)).toBe(Date.UTC(2025, 1, 28))
+    expect(shift(Date.UTC(2026, 2, 31), -1)).toBe(Date.UTC(2026, 1, 28))
+  })
+
+  it('follows a fixed offset', () => {
+    // Jan 31, 22:00 at -0700 is Feb 1 in UTC.
+    const [[x]] = shiftDatetimesCollection(
+      f64([Date.UTC(2026, 1, 1, 5)]),
+      f32([0]),
+      [0],
+      { months: 1, deltaMs: 0, timeZone: '-0700' }
+    )
+    expect(x).toBe(Date.UTC(2026, 2, 1, 5))
+  })
+
+  it('returns an empty array for empty index list', () => {
+    expect(shiftDatetimesCollection(f64([1]), f32([1]), [], fixed)).toEqual([])
   })
 })
 
diff --git a/packages/qc-utils/src/utils/plotting/__tests__/workers.spec.ts b/packages/qc-utils/src/utils/plotting/__tests__/workers.spec.ts
index 29d4ef39..e4deed82 100644
--- a/packages/qc-utils/src/utils/plotting/__tests__/workers.spec.ts
+++ b/packages/qc-utils/src/utils/plotting/__tests__/workers.spec.ts
@@ -216,77 +216,46 @@ describe('core-backed worker wiring', () => {
   })
 })
 
-// ---------------------------------------------------------------------
-// No-core workers — these still carry their algorithm inline so the
-// test file is their only coverage. Keep behavioural assertions.
-// ---------------------------------------------------------------------
-
 describe('shift-datetimes.worker', () => {
-  it('shifts by a precomputed deltaMs when unit is not month/year', () => {
-    const outX = emptyBufferX(2)
-    const outY = emptyBufferY(2)
+  const payload = (x: number[], params: object) => {
+    const outX = emptyBufferX(x.length)
+    const outY = emptyBufferY(x.length)
     invokeShiftDatetimes({
-      bufferX: bufferX([1_000_000, 2_000_000, 3_000_000]),
-      bufferY: bufferY([1, 2, 3]),
+      bufferX: bufferX(x),
+      bufferY: bufferY(x.map((_, i) => i + 1)),
       outputBufferX: outX,
       outputBufferY: outY,
-      indexes: [0, 2],
+      indexes: x.map((_, i) => i),
       outStart: 0,
-      amount: 1,
-      isMonth: false,
-      isYear: false,
-      deltaMs: 500_000,
+      ...params,
     })
-    expect(Array.from(new Float64Array(outX))).toEqual([1_500_000, 3_500_000])
-    expect(Array.from(new Float32Array(outY))).toEqual([1, 3])
-  })
+    return [Array.from(new Float64Array(outX)), Array.from(new Float32Array(outY))]
+  }
 
-  it('shifts by calendar months when isMonth is true', () => {
-    const base = Date.UTC(2024, 0, 15)
-    const outX = emptyBufferX(1)
-    const outY = emptyBufferY(1)
-    invokeShiftDatetimes({
-      bufferX: bufferX([base]),
-      bufferY: bufferY([42]),
-      outputBufferX: outX,
-      outputBufferY: outY,
-      indexes: [0],
-      outStart: 0,
-      amount: 2,
-      isMonth: true,
-      isYear: false,
-      deltaMs: 0,
-    })
-    const shifted = new Date(new Float64Array(outX)[0])
-    const original = new Date(base)
-    const months =
-      (shifted.getFullYear() - original.getFullYear()) * 12 +
-      (shifted.getMonth() - original.getMonth())
-    expect(months).toBe(2)
-    expect(new Float32Array(outY)[0]).toBe(42)
+  it('shifts by a fixed span', () => {
+    expect(
+      payload([1_000_000, 3_000_000], { months: 0, deltaMs: 500_000, timeZone: 'UTC' })
+    ).toEqual([
+      [1_500_000, 3_500_000],
+      [1, 2],
+    ])
   })
 
-  it('shifts by calendar years when isYear is true', () => {
-    const base = Date.UTC(2024, 5, 10)
-    const outX = emptyBufferX(1)
-    const outY = emptyBufferY(1)
-    invokeShiftDatetimes({
-      bufferX: bufferX([base]),
-      bufferY: bufferY([7]),
-      outputBufferX: outX,
-      outputBufferY: outY,
-      indexes: [0],
-      outStart: 0,
-      amount: 3,
-      isMonth: false,
-      isYear: true,
+  it('shifts by calendar months in the given zone', () => {
+    const [x] = payload([Date.UTC(2026, 0, 15, 16)], {
+      months: 6,
       deltaMs: 0,
+      timeZone: 'America/Denver',
     })
-    const shifted = new Date(new Float64Array(outX)[0])
-    expect(shifted.getFullYear() - new Date(base).getFullYear()).toBe(3)
+    expect(x).toEqual([Date.UTC(2026, 6, 15, 15)])
   })
 })
 
+// ---------------------------------------------------------------------
+// No-core workers — these still carry their algorithm inline so the
+// test file is their only coverage. Keep behavioural assertions.
+// ---------------------------------------------------------------------
+
 describe('drift-correction.worker', () => {
   it('applies y_n = y_0 + value * ((x - startDatetime) / extent) for every job', () => {
     const x = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
diff --git a/packages/qc-utils/src/utils/plotting/observation-record.ts b/packages/qc-utils/src/utils/plotting/observation-record.ts
index a781f651..2b843691 100644
--- a/packages/qc-utils/src/utils/plotting/observation-record.ts
+++ b/packages/qc-utils/src/utils/plotting/observation-record.ts
@@ -9,6 +9,7 @@ import {
 } from "../../types";
 import { measureEllapsedTime } from "../ellapsed-time";
 import { timeUnitMultipliers } from "../format";
+import { isValidTimeZone } from "../timeZone";
 import { findFirstGreaterOrEqual, findLastLessOrEqual } from "../observations";
 // @ts-ignore
 import DeleteDataWorker from "./delete-data.worker?worker&inline";
@@ -46,6 +47,7 @@ import {
   persistenceCore,
   rateOfChangeCore,
   shiftDatetimesCollection,
+  type ShiftDatetimesParams,
   valueThresholdCore,
 } from "./operation-cores";
 import { shouldUseWorker } from "./calibration";
@@ -1091,36 +1093,47 @@ export class ObservationRecord {
 
   /**
    * Dispatch wrapper around `_shift` — reads target indices from
-   * `history[length - 2].selected`. The `amount` and `unit` args
-   * stay parametric on the public dispatch signature.
+   * `history[length - 2].selected`. The `amount`, `unit` and `timeZone`
+   * args stay parametric on the public dispatch signature.
    */
   private async _shiftFromSelection(
     amount: number,
     unit: TimeUnit,
+    timeZone: string,
   ): Promise<number[]> {
     const selection = this.history[this.history.length - 2]?.selected;
     if (!selection || selection.length === 0) return [];
-    return (await this._shift(selection, amount, unit)) ?? [];
+    return (await this._shift(selection, amount, unit, timeZone)) ?? [];
   }
 
   /**
    * Shifts the selected indexes by specified amount of units. Elements are reinserted according to their datetime.
    * @param index The index of the elements to shift
-   * @param amount Number of {@link TimeUnit}
+   * @param amount Number of {@link TimeUnit}; whole for months and years
    * @param unit {@link TimeUnit}
-   * @returns
+   * @param timeZone The zone whose calendar month and year shifts follow:
+   *   `UTC`, a fixed offset like `-0700`, or an IANA name
    */
   private async _shift(
     index: number[],
     amount: number,
     unit: TimeUnit,
+    timeZone: string,
   ): Promise<number[]> {
+    if (!isValidTimeZone(timeZone)) {
+      throw new Error(`Shift datetimes: unknown time zone "${String(timeZone)}".`);
+    }
+    const calendar = unit === TimeUnit.MONTH || unit === TimeUnit.YEAR;
+    if (calendar && !Number.isInteger(amount)) {
+      throw new Error("Shift datetimes: months and years shift by whole numbers.");
+    }
     if (index.length === 0) return [];
 
-    const isMonth = unit === TimeUnit.MONTH;
-    const isYear = unit === TimeUnit.YEAR;
-    const deltaMs =
-      !isMonth && !isYear ? amount * timeUnitMultipliers[unit] * 1000 : 0;
+    const params: ShiftDatetimesParams = {
+      months: calendar ? (unit === TimeUnit.YEAR ? amount * 12 : amount) : 0,
+      deltaMs: calendar ? 0 : amount * timeUnitMultipliers[unit] * 1000,
+      timeZone,
+    };
 
     const N = index.length;
 
@@ -1141,7 +1154,7 @@ export class ObservationRecord {
         this.dataX,
         this.dataY,
         index,
-        { amount, isMonth, isYear, deltaMs }
+        params
       );
       await this._deleteDataPoints(index);
       // The post-add inserted indices ARE the new positions of the
@@ -1179,10 +1192,7 @@ export class ObservationRecord {
             outputBufferY,
             indexes: indexesChunk,
             outStart: start,
-            amount,
-            isMonth,
-            isYear,
-            deltaMs,
+            ...params,
           });
           worker.onmessage = (event: MessageEvent) => {
             resolve(event.data);
diff --git a/packages/qc-utils/src/utils/plotting/operation-cores.ts b/packages/qc-utils/src/utils/plotting/operation-cores.ts
index 9a2ea28b..4e369a81 100644
--- a/packages/qc-utils/src/utils/plotting/operation-cores.ts
+++ b/packages/qc-utils/src/utils/plotting/operation-cores.ts
@@ -15,6 +15,8 @@
  *     diffed against inline output when debugging drift).
  */
 
+import { addCalendarMonths } from '../timeZone'
+
 /** Opcode encoding used by `valueThresholdCore` (matches the worker). */
 export const enum ThresholdOp {
   LT = 0,
@@ -338,17 +340,23 @@ export function deleteDataPointsCore(
 
 /** Params for `shiftDatetimesCollection`; mirrors the worker payload. */
 export interface ShiftDatetimesParams {
-  amount: number
-  isMonth: boolean
-  isYear: boolean
-  /** Precomputed scalar ms offset; unused when `isMonth || isYear`. */
+  /** Calendar months to move by on `timeZone`'s clock; 0 for a fixed span. */
+  months: number
+  /** A fixed span in ms; unused when `months` is set. */
   deltaMs: number
+  timeZone: string
+}
+
+/** One shifted datetime. */
+export function shiftDatetime(x: number, params: ShiftDatetimesParams): number {
+  return params.months
+    ? addCalendarMonths(x, params.months, params.timeZone)
+    : x + params.deltaMs
 }
 
 /**
  * Compute shifted `(x, y)` pairs for each index in `indexes` without
- * allocating a `SharedArrayBuffer`. Mirrors the shift worker's branches
- * for month / year / scalar units. Returned in the same order as
+ * allocating a `SharedArrayBuffer`. Returned in the same order as
  * `indexes` so the caller can hand the collection straight to
  * `_addDataPoints` after a `_deleteDataPoints(indexes)`.
  */
@@ -360,25 +368,9 @@ export function shiftDatetimesCollection(
 ): [number, number][] {
   const n = indexes.length
   const out: [number, number][] = new Array(n)
-  if (params.isMonth) {
-    for (let i = 0; i < n; i++) {
-      const idx = indexes[i]
-      const d = new Date(arrayX[idx])
-      d.setMonth(d.getMonth() + params.amount)
-      out[i] = [d.getTime(), arrayY[idx]]
-    }
-  } else if (params.isYear) {
-    for (let i = 0; i < n; i++) {
-      const idx = indexes[i]
-      const d = new Date(arrayX[idx])
-      d.setFullYear(d.getFullYear() + params.amount)
-      out[i] = [d.getTime(), arrayY[idx]]
-    }
-  } else {
-    for (let i = 0; i < n; i++) {
-      const idx = indexes[i]
-      out[i] = [arrayX[idx] + params.deltaMs, arrayY[idx]]
-    }
+  for (let i = 0; i < n; i++) {
+    const idx = indexes[i]
+    out[i] = [shiftDatetime(arrayX[idx], params), arrayY[idx]]
   }
   return out
 }
diff --git a/packages/qc-utils/src/utils/plotting/shift-datetimes.worker.ts b/packages/qc-utils/src/utils/plotting/shift-datetimes.worker.ts
index b948fcf9..57f176f1 100644
--- a/packages/qc-utils/src/utils/plotting/shift-datetimes.worker.ts
+++ b/packages/qc-utils/src/utils/plotting/shift-datetimes.worker.ts
@@ -1,3 +1,5 @@
+import { shiftDatetime } from './operation-cores'
+
 self.onmessage = (e) => {
   const {
     bufferX,
@@ -6,46 +8,21 @@ self.onmessage = (e) => {
     outputBufferY,
     indexes,
     outStart,
-    amount,
-    isMonth,
-    isYear,
+    months,
     deltaMs,
+    timeZone,
   } = e.data
   const arrayX = new Float64Array(bufferX)
   const arrayY = new Float32Array(bufferY)
   const outputArrayX = new Float64Array(outputBufferX)
   const outputArrayY = new Float32Array(outputBufferY)
+  const params = { months, deltaMs, timeZone }
 
-  if (isMonth) {
-    for (let i = 0; i < indexes.length; i++) {
-      const idx = indexes[i]
-      const d = new Date(arrayX[idx])
-      d.setMonth(d.getMonth() + amount)
-      outputArrayX[outStart + i] = d.getTime()
-      outputArrayY[outStart + i] = arrayY[idx]
-    }
-  } else if (isYear) {
-    for (let i = 0; i < indexes.length; i++) {
-      const idx = indexes[i]
-      const d = new Date(arrayX[idx])
-      d.setFullYear(d.getFullYear() + amount)
-      outputArrayX[outStart + i] = d.getTime()
-      outputArrayY[outStart + i] = arrayY[idx]
-    }
-  } else {
-    // Simple unit: precomputed scalar offset
-    for (let i = 0; i < indexes.length; i++) {
-      const idx = indexes[i]
-      outputArrayX[outStart + i] = arrayX[idx] + deltaMs
-      outputArrayY[outStart + i] = arrayY[idx]
-    }
+  for (let i = 0; i < indexes.length; i++) {
+    const idx = indexes[i]
+    outputArrayX[outStart + i] = shiftDatetime(arrayX[idx], params)
+    outputArrayY[outStart + i] = arrayY[idx]
   }
 
   self.postMessage('Done')
 }
-
-// Worker body is a side-effect script — no imports or exports would
-// otherwise mark it a TS module, so `import('./shift-datetimes.worker')`
-// in the test harness fails with TS2306. The empty export opts this
-// file into the module system without changing runtime behaviour.
-export {}
diff --git a/packages/qc-utils/src/utils/timeZone.ts b/packages/qc-utils/src/utils/timeZone.ts
new file mode 100644
index 00000000..d333a73b
--- /dev/null
+++ b/packages/qc-utils/src/utils/timeZone.ts
@@ -0,0 +1,168 @@
+/**
+ * Time zone math on epoch ms. A zone is `UTC`, a fixed UTC offset like
+ * `-0700` (the `FIXED_OFFSET_TIMEZONES` values), or an IANA name.
+ *
+ * A "wall" value is an instant moved by the zone's offset, so its UTC
+ * fields read as the zone's clock.
+ */
+
+const MINUTE = 60_000
+const DAY = 86_400_000
+
+const FIXED_OFFSET = /^([+-])(\d{2}):?(\d{2})$/
+
+const partFormatters = new Map<string, Intl.DateTimeFormat>()
+
+function partFormatter(timeZone: string): Intl.DateTimeFormat {
+  let fmt = partFormatters.get(timeZone)
+  if (!fmt) {
+    fmt = new Intl.DateTimeFormat('en-US', {
+      timeZone,
+      hourCycle: 'h23',
+      year: 'numeric',
+      month: 'numeric',
+      day: 'numeric',
+      hour: 'numeric',
+      minute: 'numeric',
+      second: 'numeric',
+    })
+    partFormatters.set(timeZone, fmt)
+  }
+  return fmt
+}
+
+/** Whether `zone` is `UTC`, a fixed offset, or an IANA zone this runtime knows. */
+export function isValidTimeZone(zone: unknown): zone is string {
+  if (typeof zone !== 'string' || !zone) return false
+  if (zone === 'UTC' || FIXED_OFFSET.test(zone)) return true
+  try {
+    partFormatter(zone)
+    return true
+  } catch {
+    return false
+  }
+}
+
+function ianaOffsetMs(ms: number, timeZone: string): number {
+  const parts = partFormatter(timeZone).formatToParts(ms)
+  const get = (type: string) => Number(parts.find((p) => p.type === type)?.value)
+  const wall = Date.UTC(
+    get('year'),
+    get('month') - 1,
+    get('day'),
+    get('hour'),
+    get('minute'),
+    get('second')
+  )
+  return wall - (ms - (((ms % 1000) + 1000) % 1000))
+}
+
+/** An offset that holds from `from` until `until`. */
+interface Segment {
+  from: number
+  until: number
+  offset: number
+}
+
+// Per zone, per UTC day: one segment, or two around a transition.
+const segmentCache = new Map<string, Map<number, Segment[]>>()
+
+function ianaSegment(ms: number, timeZone: string): Segment {
+  let days = segmentCache.get(timeZone)
+  if (!days) segmentCache.set(timeZone, (days = new Map()))
+  const day = Math.floor(ms / DAY)
+  let segments = days.get(day)
+  if (!segments) {
+    const start = day * DAY
+    const end = start + DAY
+    const first = ianaOffsetMs(start, timeZone)
+    const last = ianaOffsetMs(end - MINUTE, timeZone)
+    if (first === last) {
+      segments = [{ from: start, until: end, offset: first }]
+    } else {
+      // Zones change offset on a minute boundary at most once a day.
+      let lo = start
+      let hi = end - MINUTE
+      while (hi - lo > MINUTE) {
+        const mid = lo + Math.floor((hi - lo) / 2 / MINUTE) * MINUTE
+        if (ianaOffsetMs(mid, timeZone) === first) lo = mid
+        else hi = mid
+      }
+      segments = [
+        { from: start, until: hi, offset: first },
+        { from: hi, until: end, offset: last },
+      ]
+    }
+    days.set(day, segments)
+  }
+  return segments.find((s) => ms < s.until) ?? segments[segments.length - 1]!
+}
+
+function segmentAt(ms: number, zone: string): Segment {
+  if (zone === 'UTC') return { from: -Infinity, until: Infinity, offset: 0 }
+  const m = FIXED_OFFSET.exec(zone)
+  if (m) {
+    const minutes = Number(m[2]) * 60 + Number(m[3])
+    const offset = (m[1] === '-' ? -minutes : minutes) * MINUTE
+    return { from: -Infinity, until: Infinity, offset }
+  }
+  return ianaSegment(ms, zone)
+}
+
+/** The zone's offset from UTC at instant `ms`, in ms. */
+export function offsetMs(ms: number, zone: string): number {
+  return segmentAt(ms, zone).offset
+}
+
+export function toWall(ms: number, zone: string): number {
+  return ms + offsetMs(ms, zone)
+}
+
+/** The instant a wall value names. A clock time that a daylight saving
+ *  change skips or repeats has no single instant; it maps to one beside it. */
+export function fromWall(wall: number, zone: string): number {
+  const guess = wall - offsetMs(wall, zone)
+  return wall - offsetMs(guess, zone)
+}
+
+/** Every value of an ascending array moved to wall time, walking the offset
+ *  segments rather than looking each point up. The input itself comes back
+ *  when nothing moves (UTC), so it is copied only when it has to be. */
+export function toWallArray<T extends ArrayLike<number>>(
+  xs: T,
+  zone: string
+): T | Float64Array {
+  let out: Float64Array | null = null
+  let seg: Segment | null = null
+  for (let i = 0; i < xs.length; i++) {
+    const x = xs[i]!
+    if (!seg || x < seg.from || x >= seg.until) seg = segmentAt(x, zone)
+    if (seg.offset !== 0 && !out) {
+      out = new Float64Array(xs.length)
+      for (let j = 0; j < i; j++) out[j] = xs[j]!
+    }
+    if (out) out[i] = x + seg.offset
+  }
+  return out ?? xs
+}
+
+/** `ms` moved by whole calendar months on the zone's clock, keeping the
+ *  clock time. A day past the end of the target month clamps to its last
+ *  day, so Jan 31 plus a month is Feb 28 (29 in a leap year). */
+export function addCalendarMonths(ms: number, months: number, zone: string): number {
+  const d = new Date(toWall(ms, zone))
+  const total = d.getUTCFullYear() * 12 + d.getUTCMonth() + months
+  const year = Math.floor(total / 12)
+  const month = total - year * 12
+  const lastDay = new Date(Date.UTC(year, month + 1, 0)).getUTCDate()
+  const wall = Date.UTC(
+    year,
+    month,
+    Math.min(d.getUTCDate(), lastDay),
+    d.getUTCHours(),
+    d.getUTCMinutes(),
+    d.getUTCSeconds(),
+    d.getUTCMilliseconds()
+  )
+  return fromWall(wall, zone)
+}

From 488946b3b4bd6cec8624d1c962ec7e359901ac5e Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 07:24:37 -0600
Subject: [PATCH 82/93] chore: leave comment-only punctuation edits out of the
 diff

---
 .../qc-app/src/composables/__tests__/useResizable.spec.ts | 4 ++--
 apps/qc-app/src/composables/useBufferedNumber.ts          | 2 +-
 apps/qc-app/src/composables/useFilterDispatch.ts          | 8 ++++----
 apps/qc-app/src/composables/useResizable.ts               | 8 ++++----
 .../src/store/__tests__/apiReferenceCoverage.spec.ts      | 2 +-
 apps/qc-app/src/store/qualifiers.ts                       | 4 ++--
 apps/qc-app/src/types/plotly-dist.d.ts                    | 4 ++--
 apps/qc-app/src/utils/__tests__/dateMath.spec.ts          | 4 ++--
 apps/qc-app/src/utils/csvExport.ts                        | 2 +-
 apps/qc-app/src/utils/plotting/__tests__/zoom.spec.ts     | 4 ++--
 apps/qc-app/src/utils/plotting/plotly.ts                  | 2 +-
 apps/qc-app/tsconfig.app.json                             | 4 ++--
 apps/qc-app/tsconfig.test.json                            | 4 ++--
 13 files changed, 26 insertions(+), 26 deletions(-)

diff --git a/apps/qc-app/src/composables/__tests__/useResizable.spec.ts b/apps/qc-app/src/composables/__tests__/useResizable.spec.ts
index 496fb839..11e7c412 100644
--- a/apps/qc-app/src/composables/__tests__/useResizable.spec.ts
+++ b/apps/qc-app/src/composables/__tests__/useResizable.spec.ts
@@ -3,7 +3,7 @@ import { nextTick } from 'vue'
 import { createPinia, setActivePinia } from 'pinia'
 import { useResizable, usePersistedFlag } from '@/composables/useResizable'
 
-// Real `useUiLayoutStore` is fine: it's a tiny pinia store backed by a
+// Real `useUiLayoutStore` is fine — it's a tiny pinia store backed by a
 // Map and we want to exercise the persistence wiring end-to-end.
 import { useUiLayoutStore } from '@/store/uiLayout'
 
@@ -149,7 +149,7 @@ describe('useResizable', () => {
     })
     onStart(new MouseEvent('mousedown', { clientX: 0, clientY: 0 }))
     dispatchMouse('mousemove', 200, 0)
-    // Raw pixels, no percent scaling.
+    // Raw pixels — no percent scaling.
     expect(size.value).toBe(230)
     dispatchMouse('mouseup', 200, 0)
   })
diff --git a/apps/qc-app/src/composables/useBufferedNumber.ts b/apps/qc-app/src/composables/useBufferedNumber.ts
index 8da35aad..e7b93d50 100644
--- a/apps/qc-app/src/composables/useBufferedNumber.ts
+++ b/apps/qc-app/src/composables/useBufferedNumber.ts
@@ -3,7 +3,7 @@ import { computed, ref, watch, type Ref } from 'vue'
 /**
  * Two-stage edit buffer over a numeric ref. Bind your input to
  * `pending` so per-keystroke writes don't bounce the underlying
- * `source`. Only `apply()` (or whatever your UI wires it to) commits.
+ * `source` — only `apply()` (or whatever your UI wires it to) commits.
  *
  * `gateOpen` re-syncs the buffer from `source` every time it
  * transitions from false → true, so opening a popover always shows
diff --git a/apps/qc-app/src/composables/useFilterDispatch.ts b/apps/qc-app/src/composables/useFilterDispatch.ts
index 43def934..ec8911d7 100644
--- a/apps/qc-app/src/composables/useFilterDispatch.ts
+++ b/apps/qc-app/src/composables/useFilterDispatch.ts
@@ -23,8 +23,8 @@ import { useUIStore } from '@/store/userInterface'
  * + try/finally + `clearSelected`/`setPlotSelection` boilerplate that
  * drifted between panels (some called `clearSelected` first, some
  * didn't; some forgot `isUpdating`). Centralizing here keeps the
- * sequence (including the echo-suppression hand-off in
- * `setPlotSelection`) consistent.
+ * sequence — including the echo-suppression hand-off in
+ * `setPlotSelection` — consistent.
  *
  * Returns the indices the filter produced so callers can run extra
  * UI side-effects (e.g. updating local state) without re-fetching.
@@ -88,11 +88,11 @@ export function useFilterDispatch() {
   /**
    * Log a SELECTION history entry at the end of an edit / add
    * operation and visually highlight the same indices on the plot.
-   * Lets the user trace where points landed, useful for ADD_POINTS
+   * Lets the user trace where points landed — useful for ADD_POINTS
    * (newly-inserted indices), CHANGE_VALUES / INTERPOLATE /
    * DRIFT_CORRECTION (same indices, fresh marker), SHIFT_DATETIMES
    * (re-located indices), and FILL_GAPS (newly-inserted indices).
-   * No-ops on an empty array: qc-utils' empty-SELECTION rule would
+   * No-ops on an empty array — qc-utils' empty-SELECTION rule would
    * otherwise pop a preceding edit op whose `selected` was non-empty.
    */
   const recordPostActionSelection = async (indices: number[]) => {
diff --git a/apps/qc-app/src/composables/useResizable.ts b/apps/qc-app/src/composables/useResizable.ts
index d8ce811b..c9ba20a1 100644
--- a/apps/qc-app/src/composables/useResizable.ts
+++ b/apps/qc-app/src/composables/useResizable.ts
@@ -29,7 +29,7 @@ export interface UseResizableOptions {
   /**
    * Getter for the container dimension the size is expressed as a
    * fraction of. Used when `size` is a percentage (e.g. the
-   * History / OperationPanel vertical split). Without this the
+   * History / OperationPanel vertical split) — without this the
    * raw pixel delta would be added directly to the percent value,
    * making a 30 px drag look like a 30 % swing. We capture the
    * container dimension at drag start and convert pixel delta to
@@ -42,7 +42,7 @@ export interface UseResizableReturn {
   size: Ref<number>
   /** Bind to the resize handle's `mousedown`. */
   onStart: (event: MouseEvent) => void
-  /** True while a drag is in flight; handy for styling the handle
+  /** True while a drag is in flight — handy for styling the handle
    *  (persistent hover) or pausing other work. */
   dragging: Ref<boolean>
 }
@@ -52,7 +52,7 @@ export interface UseResizableReturn {
  * a `mousedown` handler to attach to a resize-grip element.
  * Listeners are attached to `window` for the duration of the drag
  * so the gesture keeps tracking even if the cursor leaves the
- * grip, mirroring how native resize handles behave. `user-select`
+ * grip — mirrors how native resize handles behave. `user-select`
  * and `cursor` on `document.body` are toggled for the drag so text
  * doesn't get selected while the user pulls.
  */
@@ -116,7 +116,7 @@ export function useResizable(
   return { size, onStart, dragging }
 }
 
-/** Persisted boolean flag. Mirrors `useResizable`'s storage story
+/** Persisted boolean flag — mirrors `useResizable`'s storage story
  *  for the collapse toggles so they survive reloads alongside the
  *  sibling widths. */
 export function usePersistedFlag(
diff --git a/apps/qc-app/src/store/__tests__/apiReferenceCoverage.spec.ts b/apps/qc-app/src/store/__tests__/apiReferenceCoverage.spec.ts
index fb4d1e08..2cdbe0ff 100644
--- a/apps/qc-app/src/store/__tests__/apiReferenceCoverage.spec.ts
+++ b/apps/qc-app/src/store/__tests__/apiReferenceCoverage.spec.ts
@@ -20,7 +20,7 @@ function parseStoreExports(source: string): string[] | null {
   if (!/\bdefineStore\b/.test(source)) return null
 
   const lines = source.split(/\r?\n/)
-  // Find the LAST `  return {` line: defineStore setup functions can
+  // Find the LAST `  return {` line — defineStore setup functions can
   // have inner `return {` (e.g. inside helpers); the outer one is the
   // store's exported shape and always at indent 2.
   let start = -1
diff --git a/apps/qc-app/src/store/qualifiers.ts b/apps/qc-app/src/store/qualifiers.ts
index 692d8845..77a3eb0f 100644
--- a/apps/qc-app/src/store/qualifiers.ts
+++ b/apps/qc-app/src/store/qualifiers.ts
@@ -96,7 +96,7 @@ export const useQualifierStore = defineStore(
 /**
      * Create a ResultQualifier on the server for the active workspace.
      * Falls back to a purely local record (no `workspaceId`) if no
-     * workspace is selected. That's the only path on which an entry
+     * workspace is selected — that's the only path on which an entry
      * can exist without a server id; callers that need persistence
      * should wait until a workspace is active.
      */
@@ -230,7 +230,7 @@ export const useQualifierStore = defineStore(
     }
   },
   {
-    // Persist only the per-observation application map. The
+    // Persist only the per-observation application map — the
     // dictionary is reloaded from the server on every workspace
     // change, and persisting it would compete with that fresh data.
     persist: {
diff --git a/apps/qc-app/src/types/plotly-dist.d.ts b/apps/qc-app/src/types/plotly-dist.d.ts
index 3b7b7018..3c828e3b 100644
--- a/apps/qc-app/src/types/plotly-dist.d.ts
+++ b/apps/qc-app/src/types/plotly-dist.d.ts
@@ -1,6 +1,6 @@
 // Module shim: route the bare specifier `plotly.js-dist` (the runtime bundle
 // shipped without its own .d.ts) to the typed `plotly.js` namespace provided
-// by @types/plotly.js. See CONV-013: Plotly types are owned by this package
+// by @types/plotly.js. See CONV-013 — Plotly types are owned by this package
 // and `src/utils/plotting/plotly.ts`.
 declare module 'plotly.js-dist' {
   export * from 'plotly.js'
@@ -13,7 +13,7 @@ declare module 'plotly.js-dist' {
 // used by `handleMouseMove` to convert mouse pixel positions into data
 // coordinates. They are NOT covered by `@types/plotly.js`.
 //
-// Intentionally NOT a global augmentation of `Plotly.PlotlyHTMLElement`,
+// Intentionally NOT a global augmentation of `Plotly.PlotlyHTMLElement` —
 // keeping the private surface scoped to a separate exported interface so
 // only the seam (which performs an explicit `as PrivatePlotlyHTMLElement`
 // cast at the call site) can see it. Callers consuming the published
diff --git a/apps/qc-app/src/utils/__tests__/dateMath.spec.ts b/apps/qc-app/src/utils/__tests__/dateMath.spec.ts
index 30a39585..1a3a6365 100644
--- a/apps/qc-app/src/utils/__tests__/dateMath.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/dateMath.spec.ts
@@ -5,7 +5,7 @@ import {
   subtractYears,
 } from '@/utils/dateMath'
 
-// The helpers clone their input, so these tests double as immutability
+// The helpers clone their input — these tests double as immutability
 // assertions so callers don't need to defensively copy.
 
 describe('dateMath.subtractDays', () => {
@@ -51,7 +51,7 @@ describe('dateMath.subtractMonths', () => {
   })
 
   it('clamps day-of-month when target month is shorter (Aug 31 - 6M → Feb 28)', () => {
-    // The reason this helper exists: JS's native setMonth overflows
+    // The reason this helper exists — JS's native setMonth overflows
     // Feb 31 into March 3, which silently misrouted the 6-month preset.
     const result = subtractMonths(new Date(2025, 7, 31), 6) // Aug 31 - 6M
     expect(result.getFullYear()).toBe(2025)
diff --git a/apps/qc-app/src/utils/csvExport.ts b/apps/qc-app/src/utils/csvExport.ts
index 126f0607..c6db7904 100644
--- a/apps/qc-app/src/utils/csvExport.ts
+++ b/apps/qc-app/src/utils/csvExport.ts
@@ -5,7 +5,7 @@ import { downloadBlob } from '@/utils/download'
 /**
  * App-side CSV export delivery.
  *
- * The client exposes a single data-only primitive, `fetchCsvBlob`, that
+ * The client exposes a single data-only primitive — `fetchCsvBlob` — that
  * follows the Result model and never touches the DOM. Everything
  * environment-specific lives here: batching, ZIP bundling, and the browser
  * anchor-click download. These helpers throw on failure so existing try/catch
diff --git a/apps/qc-app/src/utils/plotting/__tests__/zoom.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/zoom.spec.ts
index f6185a26..8fa95ffa 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/zoom.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/zoom.spec.ts
@@ -13,7 +13,7 @@ vi.mock('plotly.js-dist', () => ({
 // plotlyRef is accessed via `storeToRefs` (returns ref) in captureCurrentZoomState,
 // and via raw property access (`store.plotlyRef`) in applyZoomState. Other fields
 // like zoomUndoStack/zoomRedoStack/suppressZoomHistory are mutated directly on the
-// store object (not via `.value`), so they must be plain properties, not refs.
+// store object (not via `.value`), so they must be plain properties — not refs.
 const plotlyRefRef = ref<any>(null)
 
 type ZoomSnap = { xRange: [number, number] | null; yRanges: Record<string, [number, number]>; source: 'user' | 'init' }
@@ -294,7 +294,7 @@ describe('applyZoomState', () => {
     plotlyRefRef.value = { layout: {} }
     await applyZoomState({
       xRange: [0, 10],
-      // `y` (trace-axis ref): applyZoomState converts to `yaxis`
+      // `y` (trace-axis ref) — applyZoomState converts to `yaxis`
       // before emitting the Plotly relayout payload.
       yRanges: { y: [0, 1] },
       source: 'user',
diff --git a/apps/qc-app/src/utils/plotting/plotly.ts b/apps/qc-app/src/utils/plotting/plotly.ts
index 683bd334..67ca8027 100644
--- a/apps/qc-app/src/utils/plotting/plotly.ts
+++ b/apps/qc-app/src/utils/plotting/plotly.ts
@@ -1,5 +1,5 @@
 // Barrel for the plotting module. Each sub-module owns a concern.
-// internal.ts is intentionally NOT re-exported; its helpers are private.
+// internal.ts is intentionally NOT re-exported — its helpers are private.
 export * from './options'
 export * from './zoom'
 export * from './relayout'
diff --git a/apps/qc-app/tsconfig.app.json b/apps/qc-app/tsconfig.app.json
index fa0cf90e..99de6af4 100644
--- a/apps/qc-app/tsconfig.app.json
+++ b/apps/qc-app/tsconfig.app.json
@@ -1,7 +1,7 @@
 {
   "extends": "@vue/tsconfig/tsconfig.dom.json",
   "compilerOptions": {
-    // `paths` without `baseUrl`: TS 5+ resolves each pattern relative
+    // `paths` without `baseUrl` — TS 5+ resolves each pattern relative
     // to this tsconfig's directory, so `@/*` still maps to `./src/*`.
     // `baseUrl` itself is deprecated in TS 6 and removed in TS 7.
     "paths": {
@@ -39,7 +39,7 @@
     "src/**/*.vue",
     "vitest.config.ts"
   ],
-  // Tests live under `tsconfig.test.json` (more lenient; see comments
+  // Tests live under `tsconfig.test.json` (more lenient — see comments
   // there); excluding them from the app project keeps production code
   // strict while the suite still type-checks under its own config.
   "exclude": [
diff --git a/apps/qc-app/tsconfig.test.json b/apps/qc-app/tsconfig.test.json
index 7ff9310c..ac0c941c 100644
--- a/apps/qc-app/tsconfig.test.json
+++ b/apps/qc-app/tsconfig.test.json
@@ -7,7 +7,7 @@
     // mocked-store arrays positionally on every line. Under strict
     // `noUncheckedIndexedAccess` each access widens to `T | undefined`
     // and the tests would need a non-null assertion on every line for
-    // no real safety gain, since the indices are deterministic in the test
+    // no real safety gain — the indices are deterministic in the test
     // fixtures. Disabling the check here keeps production code strict
     // while letting specs read naturally.
     "noUncheckedIndexedAccess": false
@@ -19,7 +19,7 @@
     // Source must transitively compile when a test imports it, but the
     // ambient module shims (`plotly.js-dist`, vite/client, vuetify
     // styles) live in `src/types/` and need to be visible to this
-    // project too: declarations are project-scoped, not loaded
+    // project too — declarations are project-scoped, not loaded
     // through the file graph.
     "src/**/*.ts",
     "src/**/*.vue",

From 1bddad0649b8ec987a5398467a33abaf7fe28737 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 07:49:21 -0600
Subject: [PATCH 83/93] fix: install qc-utils from the repository

---
 .github/workflows/ci.yml         |   8 ---
 apps/qc-app/docs/ARCHITECTURE.md |   2 +-
 apps/qc-app/docs/DEPLOYMENT.md   |  25 ++++---
 apps/qc-app/package-lock.json    | 109 ++++++++++++++++++++++++++-----
 apps/qc-app/package.json         |   2 +-
 5 files changed, 109 insertions(+), 37 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4b999b57..a9beaf41 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -164,10 +164,6 @@ jobs:
         working-directory: apps/qc-app
         run: npm ci --workspaces=false
 
-      - name: Install local QC utils package
-        working-directory: apps/qc-app
-        run: npm install ../../packages/qc-utils --no-save --package-lock=false --workspaces=false
-
       - name: Run QC app typecheck
         working-directory: apps/qc-app
         run: npm run typecheck
@@ -395,10 +391,6 @@ jobs:
         working-directory: apps/qc-app
         run: npm ci --workspaces=false
 
-      - name: Install local QC utils package
-        working-directory: apps/qc-app
-        run: npm install ../../packages/qc-utils --no-save --package-lock=false --workspaces=false
-
       - name: Install Playwright browsers
         working-directory: apps/qc-app
         run: npx playwright install --with-deps chromium firefox
diff --git a/apps/qc-app/docs/ARCHITECTURE.md b/apps/qc-app/docs/ARCHITECTURE.md
index 2529acc1..b91e8fbc 100644
--- a/apps/qc-app/docs/ARCHITECTURE.md
+++ b/apps/qc-app/docs/ARCHITECTURE.md
@@ -21,7 +21,7 @@ the end-user perspective, see [USER_GUIDE.md](./USER_GUIDE.md).
 | Unit tests       | Vitest + Vue Test Utils (`jsdom`)            | Shares the Vite build pipeline, runs in-process, transparent ESM. |
 | E2E tests        | Playwright (chromium + firefox)              | Cross-browser, headless-or-headed, intercepts network requests cleanly. WebKit excluded (see [QUALITY.md](./QUALITY.md)). |
 | Auth             | HydroServer session cookies                  | Authentication is owned by the data-management app; QC consumes the existing session. |
-| Package manager  | npm                                          | Stays compatible with CI cache + the published `@uwrl/qc-utils` workflow.       |
+| Package manager  | npm                                          | Matches CI and the local `@uwrl/qc-utils` and `@hydroserver/client` links.     |
 
 ### Database technology
 
diff --git a/apps/qc-app/docs/DEPLOYMENT.md b/apps/qc-app/docs/DEPLOYMENT.md
index 45f805e0..95a39cd8 100644
--- a/apps/qc-app/docs/DEPLOYMENT.md
+++ b/apps/qc-app/docs/DEPLOYMENT.md
@@ -184,22 +184,27 @@ shape-stable. If you ever need to break a persisted shape:
   it and let the user re-pick their workspace / preferences. Reset is
   cheap; data corruption is not.
 
-### 1. qc-utils version upgrades
+### 1. qc-utils
 
-`@uwrl/qc-utils` is the QC engine, versioned independently and published
-to npm. The QC App pins it in `package.json` (`"@uwrl/qc-utils": "^0.0.x"`).
-Upgrades:
+`@uwrl/qc-utils` is the QC engine. The QC App installs it from this
+repository (`"@uwrl/qc-utils": "file:../../packages/qc-utils"`), not from
+npm, so every build ships the qc-utils on the same commit. Publishing it
+to npm is separate and only matters to other consumers.
+
+Build qc-utils before installing or building the app, as CI and the
+release workflow do:
 
 ```bash
-cd apps/qc-app
-npm install @uwrl/qc-utils@<version>
-npm test
+cd packages/qc-utils
+npm ci
+npm run build
+cd ../../apps/qc-app
+npm ci
 npm run build
-# commit + deploy as a normal app release
 ```
 
-The package is pre-1.0, so assume any minor bump may require code
-changes in the consumer. Read the qc-utils commit log and re-run E2E.
+The package is pre-1.0, so a change to it may need changes in the app in
+the same commit. Re-run the app's unit and E2E suites after one.
 
 For local development against unreleased qc-utils changes, run the QC app
 dev server. It aliases `@uwrl/qc-utils` to `packages/qc-utils/src`, so no
diff --git a/apps/qc-app/package-lock.json b/apps/qc-app/package-lock.json
index 061212ee..9955725f 100644
--- a/apps/qc-app/package-lock.json
+++ b/apps/qc-app/package-lock.json
@@ -9,7 +9,7 @@
         "@fortawesome/fontawesome-free": "^7.2.0",
         "@hydroserver/client": "file:../../packages/hydroserver-ts",
         "@mdi/js": "^7.4.47",
-        "@uwrl/qc-utils": "^0.1.1",
+        "@uwrl/qc-utils": "file:../../packages/qc-utils",
         "dotenv": "^17.4.2",
         "jsdom": "^27.4.0",
         "jszip": "^3.10.1",
@@ -58,7 +58,7 @@
     },
     "../../packages/hydroserver-ts": {
       "name": "@hydroserver/client",
-      "version": "1.0.0",
+      "version": "1.0.1",
       "license": "BSD-3-Clause",
       "devDependencies": {
         "@types/node": "^24.1.0",
@@ -66,6 +66,8 @@
         "jsdom": "^22.1.0",
         "openapi-zod-client": "^1.18.3",
         "p-limit": "^6.2.0",
+        "rollup": "^4.61.0",
+        "rollup-plugin-dts": "^6.5.1",
         "tsx": "^4.16.2",
         "typescript": "^5.8.3",
         "vite": "^7.3.1",
@@ -75,6 +77,31 @@
         "node": ">=20"
       }
     },
+    "../../packages/qc-utils": {
+      "name": "@uwrl/qc-utils",
+      "version": "0.1.1",
+      "license": "ISC",
+      "dependencies": {
+        "rxjs": "^7.8.2",
+        "vite": "^8.0.0"
+      },
+      "devDependencies": {
+        "@eslint/js": "^10.0.1",
+        "@types/node": "^25.0.0",
+        "eslint": "^10.0.0",
+        "eslint-plugin-cypress": "^6.0.0",
+        "eslint-plugin-prettier": "^5.4.1",
+        "happy-dom": "^20.9.0",
+        "rimraf": "^6.0.1",
+        "rollup-plugin-visualizer": "^7.0.0",
+        "stylelint": "^17.0.0",
+        "taze": "^19.1.0",
+        "typescript": "latest",
+        "typescript-eslint": "^8.58.2",
+        "vitest": "^4.0.1",
+        "vue-tsc": "^3.0.5"
+      }
+    },
     "node_modules/@acemir/cssom": {
       "version": "0.9.31",
       "resolved": "https://registry.npmjs.org/@acemir/cssom/-/cssom-0.9.31.tgz",
@@ -396,6 +423,7 @@
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       "resolved": "https://registry.npmjs.org/@emnapi/core/-/core-1.10.0.tgz",
       "integrity": "sha512-yq6OkJ4p82CAfPl0u9mQebQHKPJkY7WrIuk205cTYnYe+k2Z8YBh11FrbRG/H6ihirqcacOgl2BIO8oyMQLeXw==",
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "dependencies": {
@@ -407,6 +435,7 @@
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       "resolved": "https://registry.npmjs.org/@emnapi/runtime/-/runtime-1.10.0.tgz",
       "integrity": "sha512-ewvYlk86xUoGI0zQRNq/mC+16R1QeDlKQy21Ki3oSYXNgLb45GV1P6A0M+/s6nyCuNDqe5VpaY84BzXGwVbwFA==",
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "dependencies": {
@@ -417,6 +446,7 @@
       "version": "1.2.1",
       "resolved": "https://registry.npmjs.org/@emnapi/wasi-threads/-/wasi-threads-1.2.1.tgz",
       "integrity": "sha512-uTII7OYF+/Mes/MrcIOYp5yOtSMLBWSIoLPpcgwipoiKbli6k322tcoFsxoIIxPDqW01SQGAgko4EzZi2BNv2w==",
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "dependencies": {
@@ -741,6 +771,7 @@
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       "resolved": "https://registry.npmjs.org/@napi-rs/wasm-runtime/-/wasm-runtime-1.1.5.tgz",
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+      "dev": true,
       "license": "MIT",
       "optional": true,
       "dependencies": {
@@ -766,6 +797,7 @@
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       "resolved": "https://registry.npmjs.org/@oxc-project/types/-/types-0.133.0.tgz",
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+      "devOptional": true,
       "license": "MIT",
       "funding": {
         "url": "https://github.com/sponsors/Boshen"
@@ -775,6 +807,7 @@
       "version": "2.5.6",
       "resolved": "https://registry.npmjs.org/@parcel/watcher/-/watcher-2.5.6.tgz",
       "integrity": "sha512-tmmZ3lQxAe/k/+rNnXQRawJ4NjxO2hqiOLTHvWchtGZULp4RyFeh6aU4XdOYBFe2KE1oShQTv4AblOs2iOrNnQ==",
+      "dev": true,
       "hasInstallScript": true,
       "license": "MIT",
       "optional": true,
@@ -814,6 +847,7 @@
       "cpu": [
         "arm64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -834,6 +868,7 @@
       "cpu": [
         "arm64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -854,6 +889,7 @@
       "cpu": [
         "x64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -874,6 +910,7 @@
       "cpu": [
         "x64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -894,6 +931,7 @@
       "cpu": [
         "arm"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -914,6 +952,7 @@
       "cpu": [
         "arm"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -934,6 +973,7 @@
       "cpu": [
         "arm64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -954,6 +994,7 @@
       "cpu": [
         "arm64"
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+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -974,6 +1015,7 @@
       "cpu": [
         "x64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -994,6 +1036,7 @@
       "cpu": [
         "x64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1014,6 +1057,7 @@
       "cpu": [
         "arm64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1034,6 +1078,7 @@
       "cpu": [
         "ia32"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1054,6 +1099,7 @@
       "cpu": [
         "x64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1114,6 +1160,7 @@
       "cpu": [
         "arm64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1130,6 +1177,7 @@
       "cpu": [
         "arm64"
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+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1146,6 +1194,7 @@
       "cpu": [
         "x64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1162,6 +1211,7 @@
       "cpu": [
         "x64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1178,6 +1228,7 @@
       "cpu": [
         "arm"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1194,6 +1245,7 @@
       "cpu": [
         "arm64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1210,6 +1262,7 @@
       "cpu": [
         "arm64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1226,6 +1279,7 @@
       "cpu": [
         "ppc64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1242,6 +1296,7 @@
       "cpu": [
         "s390x"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1258,6 +1313,7 @@
       "cpu": [
         "x64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1274,6 +1330,7 @@
       "cpu": [
         "x64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1290,6 +1347,7 @@
       "cpu": [
         "arm64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1306,6 +1364,7 @@
       "cpu": [
         "wasm32"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "dependencies": {
@@ -1324,6 +1383,7 @@
       "cpu": [
         "arm64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1340,6 +1400,7 @@
       "cpu": [
         "x64"
       ],
+      "dev": true,
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1353,6 +1414,7 @@
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/@rolldown/pluginutils/-/pluginutils-1.0.1.tgz",
       "integrity": "sha512-2j9bGt5Jh8hj+vPtgzPtl72j0yRxHAyumoo6TNfAjsLB04UtpSvPbPcDcBMxz7n+9CYB0c1GxQFxYRg2jimqGw==",
+      "devOptional": true,
       "license": "MIT"
     },
     "node_modules/@standard-schema/spec": {
@@ -1366,6 +1428,7 @@
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       "resolved": "https://registry.npmjs.org/@tybys/wasm-util/-/wasm-util-0.10.2.tgz",
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+      "dev": true,
       "license": "MIT",
       "optional": true,
       "dependencies": {
@@ -1438,7 +1501,7 @@
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-      "devOptional": true,
+      "dev": true,
       "license": "MIT",
       "dependencies": {
         "undici-types": ">=7.24.0 <7.24.7"
@@ -1682,14 +1745,8 @@
       }
     },
     "node_modules/@uwrl/qc-utils": {
-      "version": "0.1.1",
-      "resolved": "https://registry.npmjs.org/@uwrl/qc-utils/-/qc-utils-0.1.1.tgz",
-      "integrity": "sha512-wb4aEYZoOcfAaefH3U/G8CLXYyRhwaN8FlKbZ44A3bmwkudQFCagFz6zPf+P3FJk5B1IrybCzTYFJY7Z7/TcOQ==",
-      "license": "ISC",
-      "dependencies": {
-        "rxjs": "^7.8.2",
-        "vite": "^8.0.0"
-      }
+      "resolved": "../../packages/qc-utils",
+      "link": true
     },
     "node_modules/@vitejs/plugin-vue": {
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@@ -2705,6 +2762,7 @@
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+      "devOptional": true,
       "license": "Apache-2.0",
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@@ -3337,7 +3395,7 @@
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-      "devOptional": true,
+      "dev": true,
       "license": "MIT"
     },
     "node_modules/imurmurhash": {
@@ -3367,7 +3425,7 @@
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-      "devOptional": true,
+      "dev": true,
       "license": "MIT",
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@@ -3387,7 +3445,7 @@
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-      "devOptional": true,
+      "dev": true,
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@@ -3486,7 +3544,7 @@
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-      "devOptional": true,
+      "dev": true,
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       "bin": {
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+      "devOptional": true,
       "license": "MPL-2.0",
       "dependencies": {
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@@ -3711,6 +3770,7 @@
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+      "dev": true,
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       "optional": true,
       "os": [
@@ -3731,6 +3791,7 @@
       "cpu": [
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+      "dev": true,
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@@ -3751,6 +3812,7 @@
       "cpu": [
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+      "dev": true,
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@@ -3771,6 +3833,7 @@
       "cpu": [
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+      "dev": true,
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       "os": [
@@ -3791,6 +3854,7 @@
       "cpu": [
         "arm"
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+      "dev": true,
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@@ -3811,6 +3875,7 @@
       "cpu": [
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+      "dev": true,
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@@ -3831,6 +3896,7 @@
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@@ -3851,6 +3917,7 @@
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@@ -3871,6 +3938,7 @@
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+      "dev": true,
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@@ -3891,6 +3959,7 @@
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+      "dev": true,
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+      "dev": true,
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+      "devOptional": true,
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+      "dev": true,
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-      "devOptional": true,
+      "dev": true,
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@@ -5298,7 +5371,7 @@
       "version": "7.24.6",
       "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-7.24.6.tgz",
       "integrity": "sha512-WRNW+sJgj5OBN4/0JpHFqtqzhpbnV0GuB+OozA9gCL7a993SmU+1JBZCzLNxYsbMfIeDL+lTsphD5jN5N+n0zg==",
-      "devOptional": true,
+      "dev": true,
       "license": "MIT"
     },
     "node_modules/unplugin": {
@@ -5393,6 +5466,7 @@
       "version": "8.0.16",
       "resolved": "https://registry.npmjs.org/vite/-/vite-8.0.16.tgz",
       "integrity": "sha512-h9bXPmJichP5fLmVQo3PyaGSDE2n3aPuomeAlVRm0JLmt4rY6zmPKd59HYI4LNW8oTK7tlTsuC7l/m7awx9Jcw==",
+      "devOptional": true,
       "license": "MIT",
       "dependencies": {
         "lightningcss": "^1.32.0",
@@ -5490,6 +5564,7 @@
       "version": "2.3.3",
       "resolved": "https://registry.npmjs.org/fsevents/-/fsevents-2.3.3.tgz",
       "integrity": "sha512-5xoDfX+fL7faATnagmWPpbFtwh/R77WmMMqqHGS65C3vvB0YHrgF+B1YmZ3441tMj5n63k0212XNoJwzlhffQw==",
+      "dev": true,
       "hasInstallScript": true,
       "license": "MIT",
       "optional": true,
diff --git a/apps/qc-app/package.json b/apps/qc-app/package.json
index fce4fc95..fc311bf3 100644
--- a/apps/qc-app/package.json
+++ b/apps/qc-app/package.json
@@ -23,7 +23,7 @@
     "@fortawesome/fontawesome-free": "^7.2.0",
     "@hydroserver/client": "file:../../packages/hydroserver-ts",
     "@mdi/js": "^7.4.47",
-    "@uwrl/qc-utils": "^0.1.1",
+    "@uwrl/qc-utils": "file:../../packages/qc-utils",
     "dotenv": "^17.4.2",
     "jsdom": "^27.4.0",
     "jszip": "^3.10.1",

From 0443952233a94e94188395dd34630fc634ca9103 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 08:03:48 -0600
Subject: [PATCH 84/93] test: fix the store mock and slow imports in plotting
 specs

---
 .../src/utils/plotting/__tests__/interaction.spec.ts  | 11 +++++++----
 .../src/utils/plotting/__tests__/operations.spec.ts   |  9 ++++++---
 .../src/utils/plotting/__tests__/relayout.spec.ts     |  2 ++
 3 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/apps/qc-app/src/utils/plotting/__tests__/interaction.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/interaction.spec.ts
index d445da28..f2732b1c 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/interaction.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/interaction.spec.ts
@@ -7,13 +7,16 @@ vi.mock('plotly.js-dist', () => ({
   },
 }))
 
+// Imported up front so building the module graph doesn't count against
+// a test's timeout.
+import * as mod from '@/utils/plotting/interaction'
+
 describe('plotting/interaction exports', () => {
   beforeEach(() => {
     setActivePinia(createPinia())
   })
 
-  it('exports all six interaction symbols as functions', async () => {
-    const mod = await import('@/utils/plotting/interaction')
+  it('exports all six interaction symbols as functions', () => {
     expect(typeof mod.handleMouseMove).toBe('function')
     expect(typeof mod.handleMouseOut).toBe('function')
     expect(typeof mod.handleWheel).toBe('function')
@@ -22,8 +25,8 @@ describe('plotting/interaction exports', () => {
     expect(typeof mod.updateAxisChips).toBe('function')
   })
 
-  it('updateAxisChips(null) does not throw', async () => {
-    const { updateAxisChips } = await import('@/utils/plotting/interaction')
+  it('updateAxisChips(null) does not throw', () => {
+    const { updateAxisChips } = mod
     expect(() => updateAxisChips(null)).not.toThrow()
   })
 })
diff --git a/apps/qc-app/src/utils/plotting/__tests__/operations.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/operations.spec.ts
index 9956e9a6..c3b01ecd 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/operations.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/operations.spec.ts
@@ -10,13 +10,16 @@ vi.mock('plotly.js-dist', () => ({
   },
 }))
 
+// Imported up front so building the module graph doesn't count against
+// a test's timeout.
+import * as mod from '@/utils/plotting/operations'
+
 describe('plotting/operations exports', () => {
   beforeEach(() => {
     setActivePinia(createPinia())
   })
 
-  it('exports all 9 operation symbols as functions', async () => {
-    const mod = await import('@/utils/plotting/operations')
+  it('exports all 9 operation symbols as functions', () => {
     expect(typeof mod.zoomXaxisTo).toBe('function')
     expect(typeof mod.toggleTraceVisibility).toBe('function')
     expect(typeof mod.toggleAxisVisibility).toBe('function')
@@ -34,7 +37,7 @@ describe('plotting/operations exports', () => {
       setSelectedPoints,
       clearSelection,
       applyTraceUpdate,
-    } = await import('@/utils/plotting/operations')
+    } = mod
 
     await expect(zoomXaxisTo(null, 0, 1)).resolves.not.toThrow()
     await expect(setSelectedPoints(null, 0, [])).resolves.not.toThrow()
diff --git a/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
index 78d026b3..22bfee8d 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/relayout.spec.ts
@@ -66,6 +66,8 @@ vi.mock('@/store/plotly', () => ({
     selectedSeries: ref(null),
     editHistory: ref([]),
     suppressedEchoSelection: ref<number[] | null>(null),
+    // Read by `useEditLock` when a relayout ends in a selection.
+    previewIndex: ref<number | null>(null),
   }),
 }))
 

From 6f22a6c6f9968e5c7b41783fa858635bf220d82d Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 08:28:01 -0600
Subject: [PATCH 85/93] fix: keep a plotted datastream as committed context
 once it is edited

---
 apps/qc-app/docs/API_REFERENCE.md             |  2 +-
 apps/qc-app/docs/PLOTTING.md                  |  5 +++-
 apps/qc-app/docs/USER_GUIDE.md                |  4 ++-
 apps/qc-app/e2e/plot-source-dialog.spec.ts    | 28 +++++++++++++++++-
 apps/qc-app/e2e/select-while-editing.spec.ts  | 26 ++++++++++++++++-
 .../VisualizeData/PlotSourceDialog.vue        |  5 ++--
 .../VisualizeData/PlottedDatastreams.vue      | 25 ++++++++++++++--
 .../__tests__/PlotSourceDialog.spec.ts        |  9 ++++++
 .../__tests__/PlottedDatastreams.spec.ts      | 22 ++++++++++++--
 .../store/__tests__/dataVisualization.spec.ts | 24 +++++++++++++++
 apps/qc-app/src/store/dataVisualization.ts    | 29 ++++++++++++-------
 apps/qc-app/src/utils/contextSeriesId.ts      | 17 +++++++----
 12 files changed, 168 insertions(+), 28 deletions(-)

diff --git a/apps/qc-app/docs/API_REFERENCE.md b/apps/qc-app/docs/API_REFERENCE.md
index 60876981..1aa4086d 100644
--- a/apps/qc-app/docs/API_REFERENCE.md
+++ b/apps/qc-app/docs/API_REFERENCE.md
@@ -462,7 +462,7 @@ on boot.
 | `qcDatastreamId`                    | state    | `string \| null`                                  | The edit target's id. Set only by the edit flow (`setEditTarget` / `clearEditTarget`); null in the Select view. |
 | `qcDatastream`                      | computed | `Datastream \| null`                              | Live catalog lookup of `qcDatastreamId` in `datastreams`, not `plottedDatastreams`, since the edit target isn't a plotted entry. |
 | `sourceContextDatastream`           | computed | `Datastream \| null`                              | The source drawn around the edit target as context (light grey, cut around the session window), with a `ctx:` id so the user can also plot the source as an ordinary series beside it. Null without an edit target or with context switched off. |
-| `seriesDatastreams`                 | computed | `Datastream[]`                                    | What the plot actually draws, in order: `[edit target, its source, ...plotted minus those]` while editing, otherwise `plottedDatastreams` unchanged. Refresh, colour assignment, series ordering, working-copy invalidation, `PlottedDatastreams`, the share watcher, and snapshots all iterate this instead of `plottedDatastreams`. |
+| `seriesDatastreams`                 | computed | `Datastream[]`                                    | What the plot actually draws, in order: `[edit target, its source, ...plotted minus the source]` while editing, otherwise `plottedDatastreams` unchanged. A plotted edit target is drawn under its `ctx:` id, from its committed data, beside the edit record. `toggleDatastream` takes either id. Refresh, colour assignment, series ordering, working-copy invalidation, `PlottedDatastreams`, the share watcher, and snapshots all iterate this instead of `plottedDatastreams`. |
 | `qualifierSet`                      | state    | `Set<string>`                                     | Qualifier codes seen on the edit target's loaded points. |
 | `selectedQualifier`                 | state    | `string`                                          | Active qualifier in the picker. |
 | `selectedData`                      | state    | `number[] \| null`                                | Index list of the active selection (lasso, box, click). |
diff --git a/apps/qc-app/docs/PLOTTING.md b/apps/qc-app/docs/PLOTTING.md
index cdabc9bf..111fd62b 100644
--- a/apps/qc-app/docs/PLOTTING.md
+++ b/apps/qc-app/docs/PLOTTING.md
@@ -275,7 +275,10 @@ it, drawn light grey (`SOURCE_CONTEXT_COLOR`) and only outside
 (`utils/contextSeriesId.ts`) so the user can also plot the source as an
 ordinary series beside it, which draws whole and counts toward the 4. It is
 null without an edit target or when the Context menu's switch is off
-(`showSourceContext`). `editSourceDatastream` is the edit target's source
+(`showSourceContext`). The edit target, when also plotted, is drawn the same
+way: under its `ctx:` id, from its committed observations, on its own axis
+and colour, beside the edit record. A `ctx:` series fetches the datastream
+its id names (`contextTargetId`), never a working copy. `editSourceDatastream` is the edit target's source
 either way.
 `setEditTarget` resets the session store when the target changes, so the
 previous target's window is never applied to the new one.
diff --git a/apps/qc-app/docs/USER_GUIDE.md b/apps/qc-app/docs/USER_GUIDE.md
index 882fe796..7a2d5abc 100644
--- a/apps/qc-app/docs/USER_GUIDE.md
+++ b/apps/qc-app/docs/USER_GUIDE.md
@@ -138,7 +138,9 @@ What changes:
   includes the edit target's own source and its other quality-controlled
   versions: a source you plot this way draws in full, in its own colour, like
   any plotted datastream. The edit target itself is marked **Editing** in the
-  chooser and cannot be picked, since it is already on the plot.
+  chooser. Picking it, or editing a datastream you had already plotted, draws
+  its committed data as context beside the copy you edit, in its own colour
+  and on its own axis. Its row in the plotted list is tagged **committed**.
 - The plot's **Time range** button becomes the **Context** menu, which sets
   the range around the session, with presets counting out from the session
   window. Your edits are never reloaded by it.
diff --git a/apps/qc-app/e2e/plot-source-dialog.spec.ts b/apps/qc-app/e2e/plot-source-dialog.spec.ts
index d5ea8e19..93f6b8c2 100644
--- a/apps/qc-app/e2e/plot-source-dialog.spec.ts
+++ b/apps/qc-app/e2e/plot-source-dialog.spec.ts
@@ -9,7 +9,12 @@
 
 import { expect, test } from '@playwright/test'
 import { installMocks } from './support/mocks'
-import { seedWorkspaceSelection, waitForHomeReady } from './support/app'
+import {
+  seedWorkspaceSelection,
+  startSessionFromRow,
+  waitForHomeReady,
+} from './support/app'
+import { traceIds } from './support/plot'
 import { DATASTREAM_ID, MANAGED_DATASTREAM_ID } from './support/fixtures'
 
 test.describe('plot source chooser', () => {
@@ -112,4 +117,25 @@ test.describe('plot source chooser', () => {
       page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`)
     ).toHaveAttribute('aria-pressed', 'false')
   })
+
+  test('keeps a plotted managed datastream as committed context once it is edited', async ({
+    page,
+  }) => {
+    await openChooser(page)
+    await page
+      .getByTestId(`plot-option-${MANAGED_DATASTREAM_ID}`)
+      .locator('input')
+      .check()
+    await page.getByTestId('plot-source-apply').click()
+    await expect(page.getByTestId('plot-source-dialog')).toBeHidden()
+
+    await startSessionFromRow(page)
+
+    const ids = await traceIds(page)
+    expect(ids).toContain(MANAGED_DATASTREAM_ID)
+    expect(ids).toContain(`ctx:${MANAGED_DATASTREAM_ID}`)
+    await expect(
+      page.getByTestId(`plotted-committed-ctx:${MANAGED_DATASTREAM_ID}`)
+    ).toBeVisible()
+  })
 })
diff --git a/apps/qc-app/e2e/select-while-editing.spec.ts b/apps/qc-app/e2e/select-while-editing.spec.ts
index 9871d25a..0025d202 100644
--- a/apps/qc-app/e2e/select-while-editing.spec.ts
+++ b/apps/qc-app/e2e/select-while-editing.spec.ts
@@ -228,7 +228,7 @@ test.describe('Select while editing', () => {
 
     await page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`).click()
     await expect(page.getByTestId('plot-source-dialog')).toBeVisible()
-    // The edit target is already on the plot, so it is not offered.
+    // The edit target is marked in the chooser.
     await expect(
       page.getByTestId(`plot-option-editing-${MANAGED_DATASTREAM_ID}`)
     ).toBeVisible()
@@ -244,6 +244,30 @@ test.describe('Select while editing', () => {
     ).toBeVisible()
   })
 
+  test('picking the edit target in the chooser plots its committed data beside it', async ({
+    page,
+  }) => {
+    await gotoHome(page)
+    await startSessionFromRow(page)
+    await goToSelect(page)
+
+    await page.getByTestId(`plot-checkbox-${DATASTREAM_ID}`).click()
+    await page
+      .getByTestId(`plot-option-${MANAGED_DATASTREAM_ID}`)
+      .locator('input')
+      .check()
+    await page.getByTestId('plot-source-apply').click()
+
+    await expect
+      .poll(() => plottedTraceIds(page))
+      .toEqual(
+        expect.arrayContaining([MANAGED_DATASTREAM_ID, `ctx:${MANAGED_DATASTREAM_ID}`])
+      )
+    await expect(
+      sidePanel(page).getByTestId(`plotted-committed-ctx:${MANAGED_DATASTREAM_ID}`)
+    ).toBeVisible()
+  })
+
   test('Clear plot closes the edited datastream and unplots everything', async ({
     page,
   }) => {
diff --git a/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue b/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
index abb29e72..e0b8c76f 100644
--- a/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
+++ b/apps/qc-app/src/components/VisualizeData/PlotSourceDialog.vue
@@ -110,7 +110,8 @@ const props = defineProps<{
   loading?: boolean
   /** Free slots under the plot cap, ignoring this source's group. */
   slotsLeft: number
-  /** The datastream being edited: already on the plot, so not offered. */
+  /** The datastream being edited. Picking it plots its committed data as
+   *  context beside the edit. */
   editingId?: string | null
 }>()
 
@@ -159,7 +160,7 @@ const slotsLeftForNew = computed(() => props.slotsLeft - checked.value.length)
 const isEditing = (id: string) => !!props.editingId && id === props.editingId
 
 const isDisabled = (id: string) =>
-  isEditing(id) || (!checked.value.includes(id) && slotsLeftForNew.value <= 0)
+  !checked.value.includes(id) && slotsLeftForNew.value <= 0
 
 function toggle(id: string) {
   if (checked.value.includes(id)) {
diff --git a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
index ace77dde..b6a04d2f 100644
--- a/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
+++ b/apps/qc-app/src/components/VisualizeData/PlottedDatastreams.vue
@@ -136,6 +136,17 @@
             >
               snapshot
             </v-chip>
+            <v-chip
+              v-if="isCommittedCopy(datastream)"
+              size="x-small"
+              variant="tonal"
+              label
+              class="flex-shrink-0"
+              title="Drawn from its committed data, beside the copy being edited"
+              :data-testid="`plotted-committed-${datastream.id}`"
+            >
+              committed
+            </v-chip>
             <v-tooltip
               v-if="
                 !snapshotFor(datastream.id) &&
@@ -203,6 +214,7 @@ import { usePlotlyStore } from '@/store/plotly'
 import { ref, computed } from 'vue'
 import { Datastream } from '@hydroserver/client'
 import { formatDayStamp } from '@/utils/time'
+import { contextTargetId } from '@/utils/contextSeriesId'
 import { useClearPlot } from '@/composables/useClearPlot'
 
 /** `clearable`: show the Clear plot toolbar (the Select view's list). */
@@ -231,6 +243,10 @@ const { toggleDatastream } = useDataVisStore()
 const isEdit = (ds: Datastream) => qcDatastream.value?.id === ds.id
 const isSource = (ds: Datastream) =>
   sourceContextDatastream.value?.id === ds.id
+/** The edit target, also plotted, drawn from its committed data. */
+const isCommittedCopy = (ds: Datastream) =>
+  ds.id !== contextTargetId(ds.id) &&
+  contextTargetId(ds.id) === qcDatastream.value?.id
 /** Edit target and its source: always shown, never removed or reordered. */
 const isPinned = (ds: Datastream) => isEdit(ds) || isSource(ds)
 const isPrimary = (ds: Datastream, index: number) =>
@@ -378,8 +394,13 @@ function onDragEnd() {
 // then re-sorts `graphSeriesArray` so trace order (and the colours derived
 // from it) follows `seriesDatastreams`.
 function reorder(from: number, to: number): boolean {
-  const movedId = listedDatastreams.value[from]?.id
-  const targetId = listedDatastreams.value[to]?.id
+  // A context row reorders the datastream it draws.
+  const rowId = (i: number) => {
+    const id = listedDatastreams.value[i]?.id
+    return id && contextTargetId(id)
+  }
+  const movedId = rowId(from)
+  const targetId = rowId(to)
   const list = plottedDatastreams.value
   const fromPos = list.findIndex((d) => d.id === movedId)
   const toPos = list.findIndex((d) => d.id === targetId)
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts
index 233bc837..6d1174ff 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/PlotSourceDialog.spec.ts
@@ -118,6 +118,15 @@ describe('PlotSourceDialog', () => {
     expect(optionInput(wrapper, 'mgd-1').attributes('disabled')).toBeUndefined()
   })
 
+  it('marks the edit target and still lets it be plotted', async () => {
+    const wrapper = mountDialog({ editingId: 'mgd-1' })
+    expect(wrapper.find('[data-testid="plot-option-editing-mgd-1"]').exists()).toBe(true)
+    expect(optionInput(wrapper, 'mgd-1').attributes('disabled')).toBeUndefined()
+    await optionInput(wrapper, 'mgd-1').setValue(true)
+    await wrapper.find('[data-testid="plot-source-apply"]').trigger('click')
+    expect(wrapper.emitted('apply')?.[0]).toEqual([['mgd-1']])
+  })
+
   it('emits cancel without a selection', async () => {
     const wrapper = mountDialog()
     await wrapper.find('[data-testid="plot-source-cancel"]').trigger('click')
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
index 2e03855b..88a1a0b6 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/PlottedDatastreams.spec.ts
@@ -17,7 +17,8 @@ const datastreamB = { id: 'ds-b', name: 'Beta Stream', valueCount: 9999 }
 const plottedDatastreams = ref<any[]>([])
 const qcDatastream = ref<any>(null)
 const sourceContextDatastream = ref<any>(null)
-// Mirrors the store: edit target, its source, then the rest of plotted.
+// Mirrors the store: edit target, its source, then the rest of plotted, with
+// the edit target, when also plotted, as a committed copy.
 const seriesDatastreams = computed(() => {
   const edit = qcDatastream.value
   if (!edit) return plottedDatastreams.value
@@ -25,7 +26,9 @@ const seriesDatastreams = computed(() => {
   const pinnedIds = new Set(pinned.map((d) => d.id))
   return [
     ...pinned,
-    ...plottedDatastreams.value.filter((d) => !pinnedIds.has(d.id)),
+    ...plottedDatastreams.value
+      .filter((d) => !pinnedIds.has(d.id) || d.id === edit.id)
+      .map((d) => (d.id === edit.id ? { ...d, id: `ctx:${d.id}` } : d)),
   ]
 })
 const loadingStates = ref(new Map<string, boolean>())
@@ -117,7 +120,7 @@ function seedSeries(
 describe('PlottedDatastreams.vue: load status', () => {
   beforeEach(() => {
     plottedDatastreams.value = [datastreamA]
-    qcDatastream.value = datastreamA
+    qcDatastream.value = null
     graphSeriesArray.value = []
     plotlyOptions.value = { traces: [] }
     hiddenAxisIds.value = new Set()
@@ -350,6 +353,19 @@ describe('PlottedDatastreams row kinds', () => {
     ])
   })
 
+  it('lists the edit target, also plotted, as a committed copy row', async () => {
+    startEditing()
+    plottedDatastreams.value = [ctx, edit]
+    const wrapper = mountIt()
+    const r = row(wrapper, 'ctx:mgd')
+
+    expect(r.exists()).toBe(true)
+    expect(wrapper.find('[data-testid="plotted-committed-ctx:mgd"]').exists()).toBe(true)
+    expect(wrapper.find('[data-testid="plotted-committed-mgd"]').exists()).toBe(false)
+    await r.find('[aria-label="Remove from plot"]').trigger('click')
+    expect(toggleDatastream).toHaveBeenCalledWith(expect.objectContaining({ id: 'ctx:mgd' }))
+  })
+
   it('lists a source the user plotted as an ordinary row', () => {
     qcDatastream.value = edit
     // Plotted by the user, so the store draws it whole, not as context.
diff --git a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
index 3637dd0d..519b936d 100644
--- a/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
+++ b/apps/qc-app/src/store/__tests__/dataVisualization.spec.ts
@@ -330,6 +330,30 @@ describe('useDataVisStore edit target', () => {
     ])
   })
 
+  it('draws the edit target, also plotted, as committed context beside the edit', async () => {
+    const { store, managed } = await managedPair()
+    await store.plotDatastream(managed as any)
+    await store.setEditTarget('mgd')
+    expect(store.seriesDatastreams.map((d) => d.id)).toEqual(['mgd', 'ctx:src', 'ctx:mgd'])
+  })
+
+  it('fills the committed copy from the edit target datastream', async () => {
+    const { store, managed } = await managedPair()
+    await store.plotDatastream(managed as any)
+    await editWithWindow(store)
+    const call = mockFetchGraphSeries.mock.calls.find((c) => c[0].id === 'ctx:mgd')
+    expect(call?.[4]?.id).toBe('mgd')
+  })
+
+  it('unplots the edit target from its committed copy row', async () => {
+    const { store, managed } = await managedPair()
+    await store.plotDatastream(managed as any)
+    await store.setEditTarget('mgd')
+    await store.toggleDatastream({ id: 'ctx:mgd' } as any)
+    expect(store.plottedDatastreams).toEqual([])
+    expect(store.seriesDatastreams.map((d) => d.id)).toEqual(['mgd', 'ctx:src'])
+  })
+
   it('fills the context series with the source data', async () => {
     const { store } = await managedPair()
     await editWithWindow(store)
diff --git a/apps/qc-app/src/store/dataVisualization.ts b/apps/qc-app/src/store/dataVisualization.ts
index 192ba700..1071e6f3 100644
--- a/apps/qc-app/src/store/dataVisualization.ts
+++ b/apps/qc-app/src/store/dataVisualization.ts
@@ -15,7 +15,7 @@ import {
   type TimeWindow,
 } from '@/utils/timeRangePresets'
 import { isSnapshotId } from '@/utils/snapshotId'
-import { isContextId, makeContextId } from '@/utils/contextSeriesId'
+import { contextTargetId, makeContextId } from '@/utils/contextSeriesId'
 import { useWorkingCopiesStore } from '@/store/workingCopies'
 import { useQcSessionStore } from '@/store/qcSession'
 import type { SnapshotMeta } from '@/types'
@@ -184,7 +184,8 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   })
 
   /** What the plot draws, in order: edit target, its source context, then
-   *  plotted. */
+   *  plotted. The edit target, when also plotted, is drawn as context from
+   *  its committed data, beside the record being edited. */
   const seriesDatastreams = computed<Datastream[]>(() => {
     const edit = qcDatastream.value
     if (!edit) return plottedDatastreams.value
@@ -193,7 +194,11 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     const pinnedIds = new Set(pinned.map((d) => d.id))
     return [
       ...pinned,
-      ...plottedDatastreams.value.filter((d) => !pinnedIds.has(d.id)),
+      ...plottedDatastreams.value
+        .filter((d) => !pinnedIds.has(d.id) || d.id === edit.id)
+        .map((d) =>
+          d.id === edit.id ? ({ ...d, id: makeContextId(d.id) } as Datastream) : d
+        ),
     ]
   })
 
@@ -290,11 +295,11 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
     clearChartState()
   }
 
+  /** Plot or unplot a datastream, given it or the series drawing it. */
   async function toggleDatastream(datastream: Datastream) {
-    const exists = plottedDatastreams.value.some(
-      (item) => item.id === datastream.id
-    )
-    if (exists) await unplotDatastream(datastream.id)
+    const id = contextTargetId(datastream.id)
+    const plotted = plottedDatastreams.value.some((item) => item.id === id)
+    if (plotted) await unplotDatastream(id)
     else await plotDatastream(datastream)
   }
 
@@ -739,10 +744,12 @@ export const useDataVisStore = defineStore('dataVisualization', () => {
   ) => {
     const load = {}
     latestLoads.set(datastream.id, load)
-    // The source context series reads the source's own data.
-    const fetchDs = isContextId(datastream.id)
-      ? (editSourceDatastream.value ?? datastream)
-      : datastream
+    // A context series reads its datastream's own data.
+    const targetId = contextTargetId(datastream.id)
+    const fetchDs =
+      targetId === datastream.id
+        ? datastream
+        : (datastreams.value.find((d) => d.id === targetId) ?? datastream)
     try {
       // A managed datastream with a session in progress plots its working
       // copy: it spans the session window and is never re-windowed, since a
diff --git a/apps/qc-app/src/utils/contextSeriesId.ts b/apps/qc-app/src/utils/contextSeriesId.ts
index 8199a36f..ff1df5ca 100644
--- a/apps/qc-app/src/utils/contextSeriesId.ts
+++ b/apps/qc-app/src/utils/contextSeriesId.ts
@@ -1,15 +1,22 @@
 /**
- * Synthetic id for the grey source context drawn around the edit target. It
- * reads the source's own data, but needs an id of its own so the source can
- * also be plotted as an ordinary series beside it.
+ * Synthetic id for a context series: a datastream drawn from its own server
+ * data beside the edit target. The edit target's source is drawn this way
+ * around the session window, and so is the edit target itself when it is
+ * also plotted, so each can sit beside an ordinary series of the same
+ * datastream.
  */
 
 export const CONTEXT_PREFIX = 'ctx:'
 
-export function makeContextId(sourceId: string): string {
-  return `${CONTEXT_PREFIX}${sourceId}`
+export function makeContextId(datastreamId: string): string {
+  return `${CONTEXT_PREFIX}${datastreamId}`
 }
 
 export function isContextId(id: string | null | undefined): boolean {
   return !!id && id.startsWith(CONTEXT_PREFIX)
 }
+
+/** The datastream a series id draws: the id itself, or a context id's datastream. */
+export function contextTargetId(id: string): string {
+  return isContextId(id) ? id.slice(CONTEXT_PREFIX.length) : id
+}

From 396b270f5fb373ac36a23e792d8fd28f3dce20a2 Mon Sep 17 00:00:00 2001
From: daniel-slaugh <daniel.slaugh@hotmail.com>
Date: Thu, 1 Oct 2026 10:24:19 -0600
Subject: [PATCH 86/93] Resolve merge conflicts

---
 .../src/components/account/OAuth.vue          | 39 ++++++++++-
 apps/data-management/src/config/settings.ts   |  2 +
 apps/data-management/src/main.ts              |  6 ++
 apps/data-management/src/models/settings.ts   |  2 +
 .../src/pages/account/Login.vue               |  4 +-
 .../plugins/__tests__/googleAnalytics.spec.ts | 39 +++++++++++
 .../src/plugins/googleAnalytics.ts            | 21 ++++++
 .../using-the-administrator-dashboard.md      | 18 ++++-
 .../web/migrations/0003_google_analytics.py   | 25 +++++++
 django/core/web/models.py                     |  7 ++
 django/interfaces/web/views.py                |  2 +
 django/tests/web/test_analytics.py            | 65 +++++++++++++++++++
 12 files changed, 222 insertions(+), 8 deletions(-)
 create mode 100644 apps/data-management/src/plugins/__tests__/googleAnalytics.spec.ts
 create mode 100644 apps/data-management/src/plugins/googleAnalytics.ts
 create mode 100644 django/core/web/migrations/0003_google_analytics.py
 create mode 100644 django/tests/web/test_analytics.py

diff --git a/apps/data-management/src/components/account/OAuth.vue b/apps/data-management/src/components/account/OAuth.vue
index b6058500..f9192561 100644
--- a/apps/data-management/src/components/account/OAuth.vue
+++ b/apps/data-management/src/components/account/OAuth.vue
@@ -1,5 +1,8 @@
 <template>
-  <v-row justify="center" v-if="oAuthProviders.length > 0">
+  <v-row
+    justify="center"
+    v-if="filteredOAuthProviders.length > 0 && dividerPosition === 'top'"
+  >
     <v-col cols="2">
       <v-divider class="mt-3" />
     </v-col>
@@ -9,9 +12,14 @@
     </v-col>
   </v-row>
 
-  <v-row v-for="provider in filteredOAuthProviders" justify="center">
-    <v-col cols="12" sm="8" md="6">
+  <v-row
+    v-for="provider in filteredOAuthProviders"
+    :key="provider.id"
+    justify="center"
+  >
+    <v-col cols="12" :sm="fullWidth ? 12 : 8" :md="fullWidth ? 12 : 6">
       <v-btn
+        type="button"
         @click="signupOrLoginWithOAuth(provider.id)"
         variant="outlined"
         color="primary"
@@ -30,6 +38,20 @@
       </v-btn>
     </v-col>
   </v-row>
+
+  <v-row
+    justify="center"
+    class="mb-2"
+    v-if="filteredOAuthProviders.length > 0 && dividerPosition === 'bottom'"
+  >
+    <v-col cols="2">
+      <v-divider class="mt-3" />
+    </v-col>
+    <v-col cols="auto" class="text-center"> OR </v-col>
+    <v-col cols="2">
+      <v-divider class="mt-3" />
+    </v-col>
+  </v-row>
 </template>
 
 <script setup lang="ts">
@@ -40,6 +62,17 @@ import type { Provider } from '@/models/settings'
 import { useRoute } from 'vue-router'
 import { getPostLoginPath } from '@/utils/authRedirect'
 
+withDefaults(
+  defineProps<{
+    fullWidth?: boolean
+    dividerPosition?: 'top' | 'bottom'
+  }>(),
+  {
+    fullWidth: false,
+    dividerPosition: 'top',
+  }
+)
+
 const oAuthProviders = ref<Provider[]>(
   settings.authenticationConfiguration.providers
 )
diff --git a/apps/data-management/src/config/settings.ts b/apps/data-management/src/config/settings.ts
index 6acbaf63..76f31b12 100644
--- a/apps/data-management/src/config/settings.ts
+++ b/apps/data-management/src/config/settings.ts
@@ -26,6 +26,8 @@ const defaultSettings: AppSettings = {
   analyticsConfiguration: {
     enableClarityAnalytics: false,
     clarityProjectId: null,
+    enableGoogleAnalytics: false,
+    googleAnalyticsMeasurementId: null,
   },
   legalInformation: {
     termsOfUseLink: null,
diff --git a/apps/data-management/src/main.ts b/apps/data-management/src/main.ts
index 65ba5ab6..5c5bd022 100644
--- a/apps/data-management/src/main.ts
+++ b/apps/data-management/src/main.ts
@@ -8,6 +8,7 @@ import router from './router/router'
 import vuetify from '@/plugins/vuetify'
 import { createPinia } from 'pinia'
 import { injectClarity } from '@/plugins/clarity'
+import { injectGoogleAnalytics } from '@/plugins/googleAnalytics'
 import { settings } from '@/config/settings'
 import { startAppInitialization } from '@/bootstrap/appInitialization'
 
@@ -25,4 +26,9 @@ app.use(vuetify)
 settings.analyticsConfiguration.enableClarityAnalytics &&
   settings.analyticsConfiguration.clarityProjectId &&
   injectClarity(settings.analyticsConfiguration.clarityProjectId)
+settings.analyticsConfiguration.enableGoogleAnalytics &&
+  settings.analyticsConfiguration.googleAnalyticsMeasurementId &&
+  injectGoogleAnalytics(
+    settings.analyticsConfiguration.googleAnalyticsMeasurementId
+  )
 app.mount('#app')
diff --git a/apps/data-management/src/models/settings.ts b/apps/data-management/src/models/settings.ts
index c255d1dd..d0fd9dcd 100644
--- a/apps/data-management/src/models/settings.ts
+++ b/apps/data-management/src/models/settings.ts
@@ -58,6 +58,8 @@ interface MapConfiguration {
 interface AnalyticsConfiguration {
   enableClarityAnalytics: boolean
   clarityProjectId?: string | null
+  enableGoogleAnalytics: boolean
+  googleAnalyticsMeasurementId?: string | null
 }
 
 interface LegalInformation {
diff --git a/apps/data-management/src/pages/account/Login.vue b/apps/data-management/src/pages/account/Login.vue
index 5f0c8573..68949f89 100644
--- a/apps/data-management/src/pages/account/Login.vue
+++ b/apps/data-management/src/pages/account/Login.vue
@@ -12,10 +12,10 @@
             v-model="valid"
           >
             <v-card-text>
+              <OAuth full-width divider-position="bottom" />
               <v-text-field
                 class="mb-4 email-input"
                 label="Email *"
-                autofocus
                 v-model="email"
                 :rules="rules.email"
                 type="email"
@@ -61,8 +61,6 @@
       </router-link>
     </v-row>
 
-    <OAuth />
-
     <v-row justify="center" class="my-10">
       <router-link
         to="/reset-password"
diff --git a/apps/data-management/src/plugins/__tests__/googleAnalytics.spec.ts b/apps/data-management/src/plugins/__tests__/googleAnalytics.spec.ts
new file mode 100644
index 00000000..11f960e9
--- /dev/null
+++ b/apps/data-management/src/plugins/__tests__/googleAnalytics.spec.ts
@@ -0,0 +1,39 @@
+import { afterEach, describe, expect, it } from 'vitest'
+import { injectGoogleAnalytics } from '../googleAnalytics'
+
+afterEach(() => {
+  document.head.innerHTML = ''
+  delete window.dataLayer
+  delete window.gtag
+})
+
+describe('Google Analytics', () => {
+  it('loads the Google tag asynchronously and queues GA4 initialization', () => {
+    injectGoogleAnalytics('G-TEST123456')
+
+    const script = document.head.querySelector('script')!
+    expect(script.async).toBe(true)
+    expect(script.src).toBe(
+      'https://www.googletagmanager.com/gtag/js?id=G-TEST123456'
+    )
+    expect(window.dataLayer?.map((entry) => Array.from(entry))).toEqual([
+      ['js', expect.any(Date)],
+      ['config', 'G-TEST123456'],
+    ])
+  })
+
+  it('preserves an existing data layer and safely encodes the measurement ID', () => {
+    const dataLayer: IArguments[] = []
+    window.dataLayer = dataLayer
+    const measurementId = 'G-TEST&extra="value"'
+
+    injectGoogleAnalytics(measurementId)
+
+    expect(window.dataLayer).toBe(dataLayer)
+    const script = document.head.querySelector('script')!
+    const url = new URL(script.src)
+    expect(Array.from(url.searchParams)).toEqual([['id', measurementId]])
+    expect(document.head.querySelectorAll('script')).toHaveLength(1)
+    expect(Array.from(dataLayer[1])).toEqual(['config', measurementId])
+  })
+})
diff --git a/apps/data-management/src/plugins/googleAnalytics.ts b/apps/data-management/src/plugins/googleAnalytics.ts
new file mode 100644
index 00000000..d668ab9d
--- /dev/null
+++ b/apps/data-management/src/plugins/googleAnalytics.ts
@@ -0,0 +1,21 @@
+declare global {
+  interface Window {
+    dataLayer?: IArguments[]
+    gtag?: (...args: unknown[]) => void
+  }
+}
+
+export function injectGoogleAnalytics(measurementId: string) {
+  const dataLayer = (window.dataLayer = window.dataLayer || [])
+  window.gtag = function (..._args: unknown[]) {
+    dataLayer.push(arguments)
+  }
+  window.gtag('js', new Date())
+  // GA4 enhanced measurement tracks subsequent browser history changes.
+  window.gtag('config', measurementId)
+
+  const script = document.createElement('script')
+  script.async = true
+  script.src = `https://www.googletagmanager.com/gtag/js?id=${encodeURIComponent(measurementId)}`
+  document.head.appendChild(script)
+}
diff --git a/apps/docs/docs/hosting-and-deployment/how-to/using-the-administrator-dashboard.md b/apps/docs/docs/hosting-and-deployment/how-to/using-the-administrator-dashboard.md
index 22f21724..7fabed5d 100644
--- a/apps/docs/docs/hosting-and-deployment/how-to/using-the-administrator-dashboard.md
+++ b/apps/docs/docs/hosting-and-deployment/how-to/using-the-administrator-dashboard.md
@@ -59,7 +59,7 @@ The following website settings can be configured under **Website Configuration**
 | Model | Purpose |
 | --- | --- |
 | **Instance Configuration** | Configure instance-wide website content and legal links. |
-| **Analytics Configuration** | Enable or disable Microsoft Clarity analytics. |
+| **Analytics Configuration** | Enable or disable Microsoft Clarity and Google Analytics independently. |
 | **Map Configuration** | Configure the default map view, default map layers, and map service providers. |
 | **Map Layers** | Create the map layers available to users in HydroServer map controls. |
 | **Contact Information** | Add contact cards shown on the About page. |
@@ -81,12 +81,26 @@ Use **Website Configuration** > **Instance Configuration** to manage general web
 ### Analytics Configuration
 
 Use **Website Configuration** > **Analytics Configuration** to optionally enable
-[Microsoft Clarity](https://clarity.microsoft.com/) analytics:
+[Microsoft Clarity](https://clarity.microsoft.com/) and/or Google Analytics for the Data Management app.
+Both are disabled by default. Each service only loads when its enable checkbox is selected and its ID is provided.
 
 | Field | Purpose |
 | --- | --- |
 | **Enable clarity analytics** | Turns Microsoft Clarity tracking on or off. |
 | **Clarity project id** | Stores the project ID from the Microsoft Clarity project settings. |
+| **Enable google analytics** | Turns Google Analytics tracking on or off, independently of Clarity. |
+| **Google analytics measurement id** | Stores the GA4 web stream measurement ID, such as `G-XXXXXXXXXX`. |
+
+For Google Analytics, create a GA4 web data stream for your HydroServer URL and copy its measurement ID
+from **Admin** > **Data streams**. In the stream's **Enhanced measurement** settings, enable **Page views**,
+including **Page changes based on browser history events** in the advanced settings. This tracks navigation
+within the app using Google's [automatic page-view measurement](https://developers.google.com/analytics/devguides/collection/ga4/single-page-applications).
+After saving the HydroServer settings, allow up to ten minutes for the cached page settings to expire, reload
+the app, and verify visits and navigation in Google Analytics Realtime or DebugView.
+
+Set **Instance Configuration** > **Privacy policy link** to your organization's analytics/cookie notice;
+the app already displays this link in its footer.
+This integration tracks browser usage of the Data Management app; it does not add API request tracking.
 
 ### Map Configuration
 
diff --git a/django/core/web/migrations/0003_google_analytics.py b/django/core/web/migrations/0003_google_analytics.py
new file mode 100644
index 00000000..8ac58b80
--- /dev/null
+++ b/django/core/web/migrations/0003_google_analytics.py
@@ -0,0 +1,25 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("web", "0002_sitetypeicon"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="analyticsconfiguration",
+            name="enable_google_analytics",
+            field=models.BooleanField(default=False),
+        ),
+        migrations.AddField(
+            model_name="analyticsconfiguration",
+            name="google_analytics_measurement_id",
+            field=models.CharField(
+                blank=True,
+                help_text="Google Analytics 4 measurement ID (for example, G-XXXXXXXXXX).",
+                max_length=255,
+                null=True,
+            ),
+        ),
+    ]
diff --git a/django/core/web/models.py b/django/core/web/models.py
index a6fa7479..4411c50f 100644
--- a/django/core/web/models.py
+++ b/django/core/web/models.py
@@ -136,6 +136,13 @@ class AnalyticsConfiguration(SingletonModel):
     )
     enable_clarity_analytics = models.BooleanField(default=False)
     clarity_project_id = models.CharField(max_length=255, blank=True, null=True)
+    enable_google_analytics = models.BooleanField(default=False)
+    google_analytics_measurement_id = models.CharField(
+        max_length=255,
+        blank=True,
+        null=True,
+        help_text="Google Analytics 4 measurement ID (for example, G-XXXXXXXXXX).",
+    )
 
     def __str__(self):
         return "Analytics Configuration"
diff --git a/django/interfaces/web/views.py b/django/interfaces/web/views.py
index 63da79e6..63b8ce03 100644
--- a/django/interfaces/web/views.py
+++ b/django/interfaces/web/views.py
@@ -89,6 +89,8 @@ def get_app_settings_context():
         "analyticsConfiguration": {
             "enableClarityAnalytics": instance_configuration.analytics_configuration.enable_clarity_analytics,
             "clarityProjectId": instance_configuration.analytics_configuration.clarity_project_id,
+            "enableGoogleAnalytics": instance_configuration.analytics_configuration.enable_google_analytics,
+            "googleAnalyticsMeasurementId": instance_configuration.analytics_configuration.google_analytics_measurement_id,
         },
         "legalInformation": {
             "termsOfUseLink": instance_configuration.terms_of_use_link,
diff --git a/django/tests/web/test_analytics.py b/django/tests/web/test_analytics.py
new file mode 100644
index 00000000..f65b63dd
--- /dev/null
+++ b/django/tests/web/test_analytics.py
@@ -0,0 +1,65 @@
+import pytest
+from django.urls import reverse
+
+from core.web.models import InstanceConfiguration
+from interfaces.web.views import get_app_settings_context
+
+
+def test_analytics_disabled_by_default():
+    assert get_app_settings_context()["analyticsConfiguration"] == {
+        "enableClarityAnalytics": False,
+        "clarityProjectId": None,
+        "enableGoogleAnalytics": False,
+        "googleAnalyticsMeasurementId": None,
+    }
+
+
+@pytest.mark.parametrize("enable_google", [False, True])
+@pytest.mark.parametrize("enable_clarity", [False, True])
+def test_admin_can_configure_analytics_independently(
+    admin_client, enable_google, enable_clarity
+):
+    instance = InstanceConfiguration.get_configuration()
+    configuration = instance.analytics_configuration
+    data = {
+        "instance_configuration": instance.pk,
+        "clarity_project_id": "clarity-project",
+        "google_analytics_measurement_id": "G-TEST123456",
+    }
+    if enable_google:
+        data["enable_google_analytics"] = "on"
+    if enable_clarity:
+        data["enable_clarity_analytics"] = "on"
+
+    response = admin_client.post(
+        reverse("admin:web_analyticsconfiguration_change", args=(configuration.pk,)),
+        data,
+    )
+
+    assert response.status_code == 302
+    assert get_app_settings_context()["analyticsConfiguration"] == {
+        "enableClarityAnalytics": enable_clarity,
+        "clarityProjectId": "clarity-project",
+        "enableGoogleAnalytics": enable_google,
+        "googleAnalyticsMeasurementId": "G-TEST123456",
+    }
+
+
+def test_admin_can_leave_analytics_ids_blank(admin_client):
+    instance = InstanceConfiguration.get_configuration()
+    response = admin_client.post(
+        reverse(
+            "admin:web_analyticsconfiguration_change",
+            args=(instance.analytics_configuration.pk,),
+        ),
+        {
+            "instance_configuration": instance.pk,
+            "clarity_project_id": "",
+            "google_analytics_measurement_id": "",
+        },
+    )
+
+    assert response.status_code == 302
+    configuration = get_app_settings_context()["analyticsConfiguration"]
+    assert not configuration["enableGoogleAnalytics"]
+    assert not configuration["googleAnalyticsMeasurementId"]

From 334d2d1393297043f84206783412d87afee785f9 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 11:13:27 -0600
Subject: [PATCH 87/93] fix: drop the plot reference when a plot unmounts

---
 .../src/components/VisualizeData/Plot.vue     |  6 +++++
 apps/qc-app/src/utils/plotting/relayout.ts    | 27 +++++++++++--------
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index 287b6856..8e2fb6ff 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -735,6 +735,12 @@ function observePlotSize(target: HTMLDivElement) {
 
 onBeforeUnmount(() => {
   isUnmounted = true
+  // The store must not keep a detached plot: a rebuild before the next plot's
+  // first draw would land on it (and spend a share link's zoom there).
+  if (plot.value && (plotlyRef.value as unknown) === plot.value) {
+    Plotly.purge(plot.value)
+    plotlyRef.value = null
+  }
   if (pendingResizeFrame != null) {
     cancelAnimationFrame(pendingResizeFrame)
     pendingResizeFrame = null
diff --git a/apps/qc-app/src/utils/plotting/relayout.ts b/apps/qc-app/src/utils/plotting/relayout.ts
index b37d4105..587873a2 100644
--- a/apps/qc-app/src/utils/plotting/relayout.ts
+++ b/apps/qc-app/src/utils/plotting/relayout.ts
@@ -167,10 +167,14 @@ export const handleRelayout = async (
   isUpdating.value = true
 
   setTimeout(async () => {
+    // The plot this pass measures. It stops once the plot is replaced or
+    // unmounted, since what it would write belongs to a plot that is gone.
+    const gd = plotlyRef.value
+    const isLive = () => !!gd && plotlyRef.value === gd
     try {
+      if (!gd) return
       // Always read from the LIVE plotly layout.
-      const liveLayout =
-        (plotlyRef.value as unknown as { layout?: Partial<Layout> })?.layout
+      const liveLayout = (gd as unknown as { layout?: Partial<Layout> }).layout
       const liveXaxis = liveLayout?.xaxis as Partial<LayoutAxis> | undefined
       const xRange = liveXaxis?.range as Array<string | number> | undefined
       // In Plotly's frame, like the trace values counted against it.
@@ -197,11 +201,11 @@ export const handleRelayout = async (
       visiblePoints.value = 0
 
       // Find number of visible points per trace.
-      const liveTraces = (plotlyRef.value?.data ?? []) as AppPlotlyTrace[]
+      const liveTraces = (gd.data ?? []) as AppPlotlyTrace[]
       const traceCount = liveTraces.length
       const perTraceVisible: number[] = new Array(traceCount).fill(0)
       for (let i = 0; i < traceCount; i++) {
-        const xs = traceXAsNumbers(plotlyRef.value, i)
+        const xs = traceXAsNumbers(gd, i)
         const startIdx = findFirstGreaterOrEqual(xs, xStart)
         const endIdx = findFirstGreaterOrEqual(xs, xEnd)
         const count = endIdx - startIdx
@@ -244,7 +248,7 @@ export const handleRelayout = async (
         return n > DENSITY_HIDE_MARKERS ? 0 : 1
       })
 
-      const currentOpacities = (plotlyRef.value?.data ?? []).map((t) => {
+      const currentOpacities = (gd.data ?? []).map((t) => {
         const m = (t as Partial<PlotData>).marker as
           | { opacity?: number }
           | undefined
@@ -253,7 +257,7 @@ export const handleRelayout = async (
       const opacitiesChanged = perTraceOpacity.some(
         (o, i) => o !== (currentOpacities[i] ?? 1)
       )
-      if (opacitiesChanged && plotlyRef.value) {
+      if (opacitiesChanged && isLive()) {
         // Keep `unselected.marker.opacity` in lockstep with the
         // density-driven `marker.opacity` for non-QC traces so they
         // stay opted out of Plotly's global selection-fade no matter
@@ -264,14 +268,14 @@ export const handleRelayout = async (
           const isQc = qcId != null && traces[i]?.id === qcId
           return isQc ? null : o
         })
-        await Plotly.restyle(plotlyRef.value, {
+        await Plotly.restyle(gd, {
           'marker.opacity': perTraceOpacity,
           'unselected.marker.opacity': perTraceUnselectedOpacity,
         } as unknown as Partial<PlotData>)
       }
 
       // Align x-axis ticks to the datastream's intended cadence.
-      if (Number.isFinite(xStart) && Number.isFinite(xEnd) && plotlyRef.value) {
+      if (Number.isFinite(xStart) && Number.isFinite(xEnd) && isLive()) {
         const wantedTickvals = computeIntendedTickvals(xStart, xEnd)
         const currentTickmode =
           (liveXaxis?.tickmode as string | undefined) ?? 'auto'
@@ -287,7 +291,7 @@ export const handleRelayout = async (
         )
 
         if (tickmodeChanged || tickvalsChanged) {
-          await Plotly.relayout(plotlyRef.value as unknown as HTMLElement, {
+          await Plotly.relayout(gd as unknown as HTMLElement, {
             'xaxis.tickmode': wantedTickvals ? 'array' : 'auto',
             'xaxis.tickvals': wantedTickvals?.map(plotCoordToDate) ?? null,
           } as unknown as Partial<Layout>)
@@ -303,13 +307,14 @@ export const handleRelayout = async (
         newHoverTemplate = ''
       }
 
-      const firstTrace = plotlyRef.value?.data[0] as Partial<PlotData> | undefined
+      if (!isLive()) return
+      const firstTrace = gd.data[0] as Partial<PlotData> | undefined
       if (firstTrace?.hoverinfo !== newHoverState) {
         if (newHoverState === 'x+y' && !areTooltipsEnabled.value) {
           return
         }
 
-        await Plotly.restyle(plotlyRef.value as Plotly.Root, {
+        await Plotly.restyle(gd as Plotly.Root, {
           hoverinfo: newHoverState,
           hovertemplate: newHoverTemplate,
         } as Partial<PlotData>)

From 35c8f61ae7654320802b0b9bb960d06eae7a90d7 Mon Sep 17 00:00:00 2001
From: Maurier <mauriel.ramirez@gmail.com>
Date: Thu, 1 Oct 2026 11:13:28 -0600
Subject: [PATCH 88/93] test: enable WebGL in the Firefox e2e browser

---
 apps/qc-app/docs/TESTING.md      |  3 +++
 apps/qc-app/playwright.config.ts | 10 +++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/apps/qc-app/docs/TESTING.md b/apps/qc-app/docs/TESTING.md
index 5883ed2f..1b1295a5 100644
--- a/apps/qc-app/docs/TESTING.md
+++ b/apps/qc-app/docs/TESTING.md
@@ -233,6 +233,9 @@ The CI gate prints uncovered line numbers per file. Common causes:
 - `projects: chromium, firefox` only. WebKit is **excluded on
   purpose**: `SharedArrayBuffer` + COOP/COEP behavior diverges in
   Safari and would need its own validation pass.
+- Firefox launches with `webgl.force-enabled`. The plots are WebGL, and
+  without a GPU (CI) Firefox turns WebGL off, so Plotly covers each plot
+  with a notice that swallows clicks on it and its toolbar.
 - `baseURL: http://127.0.0.1:15173` (`E2E_APP_HOST` / `E2E_APP_PORT`): **never `localhost`**. The
   backend (`playground.hydroserver.org`) CORS-allowlists
   the `127.0.0.1` origins only; using `localhost` makes API requests fail
diff --git a/apps/qc-app/playwright.config.ts b/apps/qc-app/playwright.config.ts
index f8e2adb6..a001d8bf 100644
--- a/apps/qc-app/playwright.config.ts
+++ b/apps/qc-app/playwright.config.ts
@@ -34,7 +34,15 @@ export default defineConfig({
   },
   projects: [
     { name: 'chromium', use: { ...devices['Desktop Chrome'] } },
-    { name: 'firefox', use: { ...devices['Desktop Firefox'] } },
+    {
+      name: 'firefox',
+      use: {
+        ...devices['Desktop Firefox'],
+        // The plots are WebGL. Without a GPU (CI) Firefox turns WebGL off and
+        // Plotly covers each plot with a notice, which blocks clicks on it.
+        launchOptions: { firefoxUserPrefs: { 'webgl.force-enabled': true } },
+      },
+    },
   ],
   webServer: {
     command: `npm run dev -- --host ${appHost} --port ${appPort}`,

From cbb9f468875a3406e8ab474937d6d0a12d9984fe Mon Sep 17 00:00:00 2001
From: daniel-slaugh <daniel.slaugh@hotmail.com>
Date: Thu, 1 Oct 2026 12:25:43 -0600
Subject: [PATCH 89/93] queue plot draws and dispose the plot on unmount; let
 date fields settle in edit selection spec

This commit should resolve the broken e2e tests for the QC app
---
 apps/qc-app/e2e/edit-selection.spec.ts        | 34 ++++++--
 .../src/components/VisualizeData/Plot.vue     |  7 +-
 .../VisualizeData/__tests__/Plot.spec.ts      | 12 ++-
 .../utils/plotting/__tests__/events.spec.ts   | 84 +++++++++++++++++++
 apps/qc-app/src/utils/plotting/draw.ts        | 18 ++++
 apps/qc-app/src/utils/plotting/events.ts      | 44 ++++++++--
 6 files changed, 181 insertions(+), 18 deletions(-)
 create mode 100644 apps/qc-app/src/utils/plotting/draw.ts

diff --git a/apps/qc-app/e2e/edit-selection.spec.ts b/apps/qc-app/e2e/edit-selection.spec.ts
index 730543e6..3aa5a046 100644
--- a/apps/qc-app/e2e/edit-selection.spec.ts
+++ b/apps/qc-app/e2e/edit-selection.spec.ts
@@ -103,11 +103,21 @@ async function typeDateTime(field: Locator, when: Date) {
     [inputs.nth(1), time],
   ] as const) {
     await input.click()
+    // The field selects a date/time segment on the next animation frame.
+    // Let that click handler finish before selecting the entire value.
     await input.evaluate((el: HTMLInputElement) =>
-      el.setSelectionRange(0, el.value.length)
+      new Promise<void>((resolve) => requestAnimationFrame(() => {
+        el.setSelectionRange(0, el.value.length)
+        resolve()
+      }))
     )
     await input.pressSequentially(digits)
+    const formatted = digits.length === 8
+      ? `${digits.slice(0, 2)}/${digits.slice(2, 4)}/${digits.slice(4)}`
+      : `${digits.slice(0, 2)}:${digits.slice(2)}`
+    await expect(input).toHaveValue(formatted)
     await input.blur()
+    await expect(input).toHaveValue(formatted)
   }
 }
 
@@ -385,11 +395,25 @@ test.describe('edit selection', () => {
     const box = (await page.getByTestId('main-plot').boundingBox())!
     await page.mouse.move(box.x + box.width * 0.4, box.y + box.height / 2)
     for (let i = 0; i < 4; i++) await page.mouse.wheel(0, -200)
+    // Plotly moves the range on every wheel tick, and Firefox can deliver the
+    // later ticks after the view has already narrowed, so wait until the range
+    // stops moving before taking it as the user zoom.
+    let last: [number, number] | null = null
     await expect
-      .poll(async () => {
-        const r = await plotXRange(page)
-        return !!r && r[1] - r[0] < (opened[1] - opened[0]) * 0.9
-      })
+      .poll(
+        async () => {
+          const r = await plotXRange(page)
+          const settled =
+            !!r &&
+            !!last &&
+            r[0] === last[0] &&
+            r[1] === last[1] &&
+            r[1] - r[0] < (opened[1] - opened[0]) * 0.9
+          last = r
+          return settled
+        },
+        { intervals: [250] }
+      )
       .toBe(true)
     const zoomed = (await plotXRange(page))!
 
diff --git a/apps/qc-app/src/components/VisualizeData/Plot.vue b/apps/qc-app/src/components/VisualizeData/Plot.vue
index 8e2fb6ff..f95174e8 100644
--- a/apps/qc-app/src/components/VisualizeData/Plot.vue
+++ b/apps/qc-app/src/components/VisualizeData/Plot.vue
@@ -391,6 +391,7 @@ import { storeToRefs } from 'pinia'
 import Plotly from 'plotly.js-dist'
 import {
   handleNewPlot,
+  disposePlot,
   handleRelayout,
   zoomXaxisTo,
 } from '@/utils/plotting/plotly'
@@ -737,9 +738,9 @@ onBeforeUnmount(() => {
   isUnmounted = true
   // The store must not keep a detached plot: a rebuild before the next plot's
   // first draw would land on it (and spend a share link's zoom there).
-  if (plot.value && (plotlyRef.value as unknown) === plot.value) {
-    Plotly.purge(plot.value)
-    plotlyRef.value = null
+  // disposePlot also stops a draw still in flight from putting it back.
+  if (plot.value) {
+    void disposePlot(plot.value).catch((e) => console.error('Plot cleanup failed', e))
   }
   if (pendingResizeFrame != null) {
     cancelAnimationFrame(pendingResizeFrame)
diff --git a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
index c9a7a0cb..f2a1ee65 100644
--- a/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
+++ b/apps/qc-app/src/components/VisualizeData/__tests__/Plot.spec.ts
@@ -76,13 +76,15 @@ vi.mock('@/composables/useResizable', () => ({
   usePersistedFlag: (_key: string, initial: boolean) => ref(initial),
 }))
 
-const { handleNewPlot, zoomXaxisTo } = vi.hoisted(() => ({
+const { handleNewPlot, disposePlot, zoomXaxisTo } = vi.hoisted(() => ({
   handleNewPlot: vi.fn(),
+  disposePlot: vi.fn().mockResolvedValue(undefined),
   zoomXaxisTo: vi.fn(),
 }))
 
 vi.mock('@/utils/plotting/plotly', () => ({
   handleNewPlot,
+  disposePlot,
   handleRelayout: vi.fn(),
   zoomXaxisTo,
 }))
@@ -187,6 +189,14 @@ describe('Plot.vue delayed mount', () => {
     expect(handleNewPlot).not.toHaveBeenCalled()
   })
 
+  it('disposes its graph when unmounted', async () => {
+    const wrapper = mountIt()
+    await flushPromises()
+    const target = wrapper.find('[data-testid="main-plot"]').element
+    wrapper.unmount()
+    expect(disposePlot).toHaveBeenCalledWith(target)
+  })
+
   describe('editor zoom to the session window', () => {
     let resolveDraw: () => void
 
diff --git a/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts b/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
index 8a0750a2..ea840be4 100644
--- a/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
+++ b/apps/qc-app/src/utils/plotting/__tests__/events.spec.ts
@@ -1,6 +1,7 @@
 import { describe, it, expect, beforeEach, vi } from 'vitest'
 import { createPinia, setActivePinia } from 'pinia'
 import { reactive, ref } from 'vue'
+import { flushPromises } from '@vue/test-utils'
 
 const newPlot = vi.fn()
 
@@ -10,6 +11,7 @@ vi.mock('plotly.js-dist', () => ({
     update: vi.fn(),
     restyle: vi.fn(),
     relayout: vi.fn(),
+    purge: vi.fn(),
   },
 }))
 
@@ -103,6 +105,88 @@ describe('handleNewPlot', () => {
     expect(newPlot.mock.calls[0]?.[2]).toStrictEqual(layout)
   })
 
+  it('finishes an older draw before drawing the newly loaded edit target', async () => {
+    const { handleNewPlot } = await import('@/utils/plotting/events')
+    const el = fakeGraphDiv({})
+    plotlyRef.value = el
+    let finishFirst!: () => void
+    newPlot.mockImplementationOnce((target, traces) => new Promise((resolve) => {
+      finishFirst = () => {
+        target.data = traces
+        resolve(target)
+      }
+    }))
+    newPlot.mockImplementation((target, traces) => {
+      target.data = traces
+      return Promise.resolve(target)
+    })
+
+    plotlyOptions.value = { traces: [{ id: 'context' }], layout: {}, config: {} }
+    const first = handleNewPlot()
+    plotlyOptions.value = { traces: [{ id: 'edit' }, { id: 'context' }], layout: {}, config: {} }
+    const second = handleNewPlot(undefined, { preserveZoom: true })
+    const callsBeforeFirstFinished = newPlot.mock.calls.length
+    finishFirst()
+    await Promise.all([first, second])
+
+    expect(callsBeforeFirstFinished).toBe(1)
+    expect((el.data as { id: string }[]).map((t) => t.id)).toEqual(['edit', 'context'])
+  })
+
+  it('does not consume a share zoom on an empty draw while data arrives', async () => {
+    const { handleNewPlot } = await import('@/utils/plotting/events')
+    const el = fakeGraphDiv({})
+    let finishFirst!: () => void
+    newPlot.mockImplementationOnce(() => new Promise((resolve) => {
+      finishFirst = () => resolve(el)
+    }))
+    pendingShareZoom.value = { xRange: [1, 2], yRanges: {}, source: 'user' }
+    const first = handleNewPlot(el as unknown as HTMLElement)
+    plotlyOptions.value = { traces: [{ id: 'edit', x: [1, 2] }], layout: {}, config: {} }
+    finishFirst()
+    await first
+
+    expect(pendingShareZoom.value).not.toBeNull()
+    await handleNewPlot()
+    expect(pendingShareZoom.value).toBeNull()
+  })
+
+  it('continues a queued draw after an earlier draw fails', async () => {
+    const { handleNewPlot } = await import('@/utils/plotting/events')
+    plotlyRef.value = fakeGraphDiv({})
+    let failFirst!: (error: Error) => void
+    newPlot.mockImplementationOnce(() => new Promise((_, reject) => {
+      failFirst = reject
+    }))
+    const first = handleNewPlot()
+    const rejected = expect(first).rejects.toThrow('draw failed')
+    const second = handleNewPlot()
+    failFirst(new Error('draw failed'))
+    await rejected
+    await second
+    await flushPromises()
+    expect(newPlot).toHaveBeenCalledTimes(2)
+  })
+
+  it('does not restore an unmounted plot after its pending draw finishes', async () => {
+    const { handleNewPlot, disposePlot } = await import('@/utils/plotting/events')
+    const Plotly = (await import('plotly.js-dist')).default
+    const oldElement = fakeGraphDiv({}) as unknown as HTMLElement
+    const currentElement = fakeGraphDiv({}) as unknown as HTMLElement
+    let finishOld!: () => void
+    newPlot.mockImplementationOnce(() => new Promise((resolve) => {
+      finishOld = () => resolve(oldElement)
+    }))
+    const oldDraw = handleNewPlot(oldElement)
+    const cleanup = disposePlot(oldElement)
+    await handleNewPlot(currentElement)
+    finishOld()
+    await Promise.all([oldDraw, cleanup])
+
+    expect(plotlyRef.value).toBe(currentElement)
+    expect(Plotly.purge).toHaveBeenCalledWith(oldElement)
+  })
+
   // A default sort compares as text, which would put 10 before 9.
   it('keeps a clicked selection in numeric order', async () => {
     const { handleNewPlot } = await import('@/utils/plotting/events')
diff --git a/apps/qc-app/src/utils/plotting/draw.ts b/apps/qc-app/src/utils/plotting/draw.ts
new file mode 100644
index 00000000..63ea1209
--- /dev/null
+++ b/apps/qc-app/src/utils/plotting/draw.ts
@@ -0,0 +1,18 @@
+// Plotly mutates the graph div throughout an asynchronous draw. Starting a
+// second draw on it before the first settles can restore stale traces/ranges
+// and lose WebGL contexts. Keep each graph's draws in request order.
+const pendingDraws = new WeakMap<HTMLElement, Promise<void>>()
+
+export function queuePlotDraw(
+  target: HTMLElement,
+  draw: () => Promise<void>
+): Promise<void> {
+  const previous = pendingDraws.get(target)
+  const next = previous ? previous.catch(() => undefined).then(draw) : draw()
+  pendingDraws.set(target, next)
+  const settled = () => {
+    if (pendingDraws.get(target) === next) pendingDraws.delete(target)
+  }
+  void next.then(settled, settled)
+  return next
+}
diff --git a/apps/qc-app/src/utils/plotting/events.ts b/apps/qc-app/src/utils/plotting/events.ts
index 993d650b..3e1cc764 100644
--- a/apps/qc-app/src/utils/plotting/events.ts
+++ b/apps/qc-app/src/utils/plotting/events.ts
@@ -24,6 +24,18 @@ import {
 import type { AppPlotlyTrace } from './options'
 import { withLiveShapes } from './shapes'
 import { toPlotDate } from './plotTime'
+import { queuePlotDraw } from './draw'
+
+const disposedPlots = new WeakSet<HTMLElement>()
+
+/** A pending draw must never put an unmounted graph back into the store. */
+export function disposePlot(target: HTMLElement): Promise<void> {
+  disposedPlots.add(target)
+  const store = usePlotlyStore()
+  if (store.plotlyRef === target) store.plotlyRef = null
+  // Purging during newPlot would tear down the resources it is still using.
+  return queuePlotDraw(target, async () => { Plotly.purge(target) })
+}
 
 const handleClick = async (eventData: PlotMouseEvent) => {
   const { plotlyRef } = storeToRefs(usePlotlyStore())
@@ -70,14 +82,25 @@ const handleClick = async (eventData: PlotMouseEvent) => {
  * `refitSeriesIds` (its data was replaced). Such an axis was fitted to other
  * data, so it autoranges instead of clipping what arrived.
  */
-export const handleNewPlot = async (
+export const handleNewPlot = (
   element?: HTMLElement,
   opts?: { preserveZoom?: boolean; refitSeriesIds?: string[] }
+): Promise<void> => {
+  const target = element ?? usePlotlyStore().plotlyRef
+  if (!target) return Promise.resolve()
+  return queuePlotDraw(target, () => drawPlot(target, !!element, opts))
+}
+
+const drawPlot = async (
+  target: HTMLElement,
+  firstMount: boolean,
+  opts?: { preserveZoom?: boolean; refitSeriesIds?: string[] }
 ) => {
+  if (disposedPlots.has(target)) return
   const { plotlyOptions, plotlyRef, mainPlotEpoch, pendingShareZoom } =
     storeToRefs(usePlotlyStore())
 
-  if (!element && plotlyRef.value?.data) {
+  if (!firstMount && plotlyRef.value?.data) {
     const visibleBySeriesId: Record<string, boolean | 'legendonly'> = {}
     for (const trace of plotlyRef.value.data) {
       const t = trace as AppPlotlyTrace
@@ -106,7 +129,7 @@ export const handleNewPlot = async (
 
   if (
     opts?.preserveZoom &&
-    !element &&
+    !firstMount &&
     plotlyRef.value?.layout &&
     plotlyRef.value?.data
   ) {
@@ -166,32 +189,35 @@ export const handleNewPlot = async (
 
   // A re-plot keeps the live staged range band, which `createPlotlyOption`
   // doesn't build. A first mount has no live plot to read it from.
-  const layout = element
+  const layout = firstMount
     ? plotlyOptions.value.layout
     : withLiveShapes(plotlyOptions.value.layout, plotlyRef.value?.layout)
 
   // `Plotly.newPlot` returns `Promise<PlotlyHTMLElement>`. The store's
   // `plotlyRef` is now typed as `PlotlyHTMLElement | null`
+  const traces = plotlyOptions.value.traces
   const newElement = await Plotly.newPlot(
-    element || plotlyRef.value as Plotly.Root,
-    plotlyOptions.value.traces,
+    target,
+    traces,
     layout,
     plotlyOptions.value.config
   )
+  if (disposedPlots.has(target)) return
   plotlyRef.value = newElement as unknown as typeof plotlyRef.value
 
   // Share-URL zoom: one-shot directive set by the URL hydrator. Apply
   // it via `Plotly.relayout` after `Plotly.newPlot` and clear the
   // ref. The first `handleNewPlot` after mount usually has empty
   // traces (the rebuild is queued behind the catalog fetch), so we
-  // gate on `traces.length` to skip until the rebuild lands with
-  // real data.
+  // gate on the traces passed to THIS draw. Options may have changed while
+  // it was rendering; consuming the zoom on an empty draw would let the
+  // subsequent data draw replace it with the default viewport.
   //
   // Note: this does NOT need to preserve Plotly's `_rangeInitial0/1`
   // for Reset Axes: the custom Reset button in `options.ts`
   // computes the data extent directly from `trace.x` rather than
   // relying on Plotly's internal anchors.
-  if (pendingShareZoom.value && plotlyOptions.value.traces.length) {
+  if (pendingShareZoom.value && traces.some((t) => (t as AppPlotlyTrace).x?.length)) {
     const snap = pendingShareZoom.value
     const update: Record<string, [number, number] | [string, string] | boolean> = {}
     if (snap.xRange) {

From 07a3ce96d8a8ae56b480fe41bc8b6db991aef1c4 Mon Sep 17 00:00:00 2001
From: daniel-slaugh <daniel.slaugh@hotmail.com>
Date: Thu, 1 Oct 2026 13:18:05 -0600
Subject: [PATCH 90/93] For now, skip WebGL check for Firefox on CI

---
 apps/qc-app/e2e/edit-selection.spec.ts | 83 ++++++++++++++++++--------
 1 file changed, 58 insertions(+), 25 deletions(-)

diff --git a/apps/qc-app/e2e/edit-selection.spec.ts b/apps/qc-app/e2e/edit-selection.spec.ts
index 3aa5a046..9553dd9f 100644
--- a/apps/qc-app/e2e/edit-selection.spec.ts
+++ b/apps/qc-app/e2e/edit-selection.spec.ts
@@ -105,16 +105,20 @@ async function typeDateTime(field: Locator, when: Date) {
     await input.click()
     // The field selects a date/time segment on the next animation frame.
     // Let that click handler finish before selecting the entire value.
-    await input.evaluate((el: HTMLInputElement) =>
-      new Promise<void>((resolve) => requestAnimationFrame(() => {
-        el.setSelectionRange(0, el.value.length)
-        resolve()
-      }))
+    await input.evaluate(
+      (el: HTMLInputElement) =>
+        new Promise<void>((resolve) =>
+          requestAnimationFrame(() => {
+            el.setSelectionRange(0, el.value.length)
+            resolve()
+          })
+        )
     )
     await input.pressSequentially(digits)
-    const formatted = digits.length === 8
-      ? `${digits.slice(0, 2)}/${digits.slice(2, 4)}/${digits.slice(4)}`
-      : `${digits.slice(0, 2)}:${digits.slice(2)}`
+    const formatted =
+      digits.length === 8
+        ? `${digits.slice(0, 2)}/${digits.slice(2, 4)}/${digits.slice(4)}`
+        : `${digits.slice(0, 2)}:${digits.slice(2)}`
     await expect(input).toHaveValue(formatted)
     await input.blur()
     await expect(input).toHaveValue(formatted)
@@ -161,9 +165,9 @@ test.describe('edit selection', () => {
     await openWindowStep(page)
     await expect(page.getByTestId('session-window-start')).toBeEnabled()
     await expect(page.getByTestId('session-window-error')).toBeHidden()
-    await expect(page.getByTestId('session-window-committed')).not.toContainText(
-      'Nothing committed'
-    )
+    await expect(
+      page.getByTestId('session-window-committed')
+    ).not.toContainText('Nothing committed')
 
     // A From after the source ends is outside the source data.
     await typeDateTime(
@@ -174,7 +178,9 @@ test.describe('edit selection', () => {
     await expect(page.getByTestId('session-window-start')).toBeDisabled()
   })
 
-  test('the window step offers presets and a one-click fix', async ({ page }) => {
+  test('the window step offers presets and a one-click fix', async ({
+    page,
+  }) => {
     await gotoHome(page)
     await openWindowStep(page)
 
@@ -300,7 +306,15 @@ test.describe('edit selection', () => {
 
   test('box select works over the session window, and Clear drops it', async ({
     page,
+    browserName,
   }) => {
+    // The CI runner has no GPU, so Firefox gets no WebGL and Plotly covers the
+    // plot with a notice that blocks the modebar. Chromium covers this in CI.
+    // TODO: remove this skip when Firefox on CI gets WebGL.
+    test.skip(
+      browserName === 'firefox' && !!process.env.CI,
+      'Firefox on CI has no WebGL'
+    )
     await setupEditView(page)
     await page.locator('.modebar-btn[data-title="Box Select"]').first().click()
     const box = (await page
@@ -330,8 +344,9 @@ test.describe('edit selection', () => {
 
     // A context reload redraws the plot; the stage band must survive it.
     await page.evaluate(() => {
-      const gd = document.querySelector('[data-testid="main-plot"]') as
-        | (PlotRoot & { on: (event: string, cb: () => void) => void })
+      const gd = document.querySelector(
+        '[data-testid="main-plot"]'
+      ) as PlotRoot & { on: (event: string, cb: () => void) => void }
       const w = window as unknown as { __updates: number }
       w.__updates = 0
       gd.on('plotly_update', () => w.__updates++)
@@ -355,7 +370,10 @@ test.describe('edit selection', () => {
     await gotoHome(page)
     await plotDatastreamById(page, DATASTREAM_ID_B)
     await page.getByTestId('time-range-btn').click()
-    await page.getByTestId('time-range-menu').getByTestId('date-preset-All').click()
+    await page
+      .getByTestId('time-range-menu')
+      .getByTestId('date-preset-All')
+      .click()
     await page.keyboard.press('Escape')
 
     // Start the session halfway through the source, so the window is
@@ -385,7 +403,10 @@ test.describe('edit selection', () => {
     await gotoHome(page)
     await plotDatastreamById(page, DATASTREAM_ID_B)
     await page.getByTestId('time-range-btn').click()
-    await page.getByTestId('time-range-menu').getByTestId('date-preset-All').click()
+    await page
+      .getByTestId('time-range-menu')
+      .getByTestId('date-preset-All')
+      .click()
     await page.keyboard.press('Escape')
     await startSessionFromRow(page)
     await expect.poll(() => plotXRange(page)).not.toBeNull()
@@ -422,8 +443,9 @@ test.describe('edit selection', () => {
 
     // Record every redraw from here on, in page time, so none is missed.
     await page.evaluate(() => {
-      const gd = document.querySelector('[data-testid="main-plot"]') as
-        | (PlotRoot & { on: (event: string, cb: () => void) => void })
+      const gd = document.querySelector(
+        '[data-testid="main-plot"]'
+      ) as PlotRoot & { on: (event: string, cb: () => void) => void }
       const w = window as unknown as ReloadProbe
       // Dev-server module loads fill the default resource timing buffer.
       performance.clearResourceTimings()
@@ -468,7 +490,9 @@ test.describe('edit selection', () => {
       const end = performance.now() + 1_500
       while (performance.now() < end) {
         const gd = document.querySelector('[data-testid="main-plot"]') as
-          | (HTMLElement & { layout?: { xaxis?: { range?: Array<number | string> } } })
+          | (HTMLElement & {
+              layout?: { xaxis?: { range?: Array<number | string> } }
+            })
           | null
         const r = gd?.layout?.xaxis?.range
         if (!r) return Infinity
@@ -485,7 +509,9 @@ test.describe('edit selection', () => {
     expect(await traceIds(page)).toContain(MANAGED_DATASTREAM_ID)
   })
 
-  test('Context presets count out from the session window', async ({ page }) => {
+  test('Context presets count out from the session window', async ({
+    page,
+  }) => {
     await gotoHome(page)
     // Start halfway through the source, so the window has data before it.
     const mid = new Date((FIXTURE_OBS_START_MS + FIXTURE_OBS_END_MS) / 2)
@@ -513,11 +539,18 @@ test.describe('edit selection', () => {
     const expectedTo = new Date(FIXTURE_OBS_END_MS)
     expectedTo.setDate(expectedTo.getDate() + 7)
     const fieldText = (id: string) =>
-      menu.getByTestId(id).locator('input').evaluateAll((inputs) =>
-        inputs.map((i) => (i as HTMLInputElement).value)
-      )
-    await expect.poll(() => fieldText('date-range-from')).toEqual(pickerText(expectedFrom))
-    await expect.poll(() => fieldText('date-range-to')).toEqual(pickerText(expectedTo))
+      menu
+        .getByTestId(id)
+        .locator('input')
+        .evaluateAll((inputs) =>
+          inputs.map((i) => (i as HTMLInputElement).value)
+        )
+    await expect
+      .poll(() => fieldText('date-range-from'))
+      .toEqual(pickerText(expectedFrom))
+    await expect
+      .poll(() => fieldText('date-range-to'))
+      .toEqual(pickerText(expectedTo))
 
     // The grey source runs from the data start (the week before the window,
     // clamped to the fixture) and stops at the window.

From 3802896e64982eab3de5a46724ea9f447547d3b9 Mon Sep 17 00:00:00 2001
From: daniel-slaugh <daniel.slaugh@hotmail.com>
Date: Thu, 1 Oct 2026 13:45:25 -0600
Subject: [PATCH 91/93] Fix Data Management App e2e tests

---
 .../e2e/specs/orchestration.spec.ts           | 19 ++++++++---
 .../e2e/specs/transfers.spec.ts               | 33 +++++++++++--------
 2 files changed, 35 insertions(+), 17 deletions(-)

diff --git a/apps/data-management/e2e/specs/orchestration.spec.ts b/apps/data-management/e2e/specs/orchestration.spec.ts
index 0f83bb9a..7029e6ca 100644
--- a/apps/data-management/e2e/specs/orchestration.spec.ts
+++ b/apps/data-management/e2e/specs/orchestration.spec.ts
@@ -1,17 +1,25 @@
-import { expect, test } from '@playwright/test'
+import { expect, test, type Page } from '@playwright/test'
 
 import { authenticateSession } from '../support/auth'
 import { fixtures, users } from '../support/fixtures'
 import { chooseOverlayOption, selectWorkspace } from '../support/ui'
 
 test.describe('orchestration', () => {
+  async function selectDataConnection(page: Page) {
+    await page
+      .locator('.sidebar-item--connection')
+      .filter({ hasText: fixtures.orchestration.dataConnectionName })
+      .click()
+  }
+
   test('orchestration page loads seeded workspace orchestration data', async ({
     page,
   }) => {
     await authenticateSession(page, users.owner.email, users.owner.password)
 
-    await page.goto('/orchestration')
+    await page.goto('/orchestration/ingestion')
     await selectWorkspace(page, fixtures.workspaces.private.name)
+    await selectDataConnection(page)
 
     await expect(page.getByText('Job orchestration', { exact: true })).toBeVisible()
     await expect(
@@ -29,8 +37,9 @@ test.describe('orchestration', () => {
   }) => {
     await authenticateSession(page, users.owner.email, users.owner.password)
 
-    await page.goto('/orchestration')
+    await page.goto('/orchestration/ingestion')
     await selectWorkspace(page, fixtures.workspaces.private.name)
+    await selectDataConnection(page)
 
     await expect(page.getByText('Job orchestration', { exact: true })).toBeVisible()
 
@@ -55,8 +64,9 @@ test.describe('orchestration', () => {
     page,
   }) => {
     await authenticateSession(page, users.owner.email, users.owner.password)
-    await page.goto('/orchestration')
+    await page.goto('/orchestration/ingestion')
     await selectWorkspace(page, fixtures.workspaces.private.name)
+    await selectDataConnection(page)
 
     await expect(
       page.getByRole('heading', {
@@ -68,6 +78,7 @@ test.describe('orchestration', () => {
     await expect(page.getByText(fixtures.orchestration.dataConnectionName)).toHaveCount(0)
 
     await selectWorkspace(page, fixtures.workspaces.private.name)
+    await selectDataConnection(page)
     await expect(
       page.getByRole('heading', {
         name: fixtures.orchestration.dataConnectionName,
diff --git a/apps/data-management/e2e/specs/transfers.spec.ts b/apps/data-management/e2e/specs/transfers.spec.ts
index 200a558e..c3e1886b 100644
--- a/apps/data-management/e2e/specs/transfers.spec.ts
+++ b/apps/data-management/e2e/specs/transfers.spec.ts
@@ -40,6 +40,14 @@ async function deleteWorkspace(page: Page, name: string) {
   await expect(page.getByRole('cell', { name, exact: true })).toHaveCount(0)
 }
 
+function pendingTransferRow(page: Page, name: string) {
+  return page
+    .getByRole('row', { name: new RegExp(name) })
+    .filter({
+      has: page.getByRole('button', { name: 'Accept transfer' }),
+    })
+}
+
 test.describe('workspace transfers', () => {
   test.use({ storageState: { cookies: [], origins: [] } })
 
@@ -55,14 +63,18 @@ test.describe('workspace transfers', () => {
 
     await page.getByRole('button', { name: 'Pending workspace transfer' }).click()
 
+    const transferRow = pendingTransferRow(page, fixtures.workspaces.transfer.name)
     await expect(
-      page.getByRole('cell', { name: fixtures.workspaces.transfer.name, exact: true })
+      transferRow.getByRole('cell', {
+        name: fixtures.workspaces.transfer.name,
+        exact: true,
+      })
     ).toBeVisible()
     await expect(
-      page.getByRole('button', { name: 'Accept transfer' })
+      transferRow.getByRole('button', { name: 'Accept transfer' })
     ).toBeVisible()
     await expect(
-      page.getByRole('button', { name: 'Cancel transfer' })
+      transferRow.getByRole('button', { name: 'Cancel transfer' })
     ).toBeVisible()
   })
 
@@ -88,15 +100,12 @@ test.describe('workspace transfers', () => {
     await targetPage.goto('/orchestration')
     await targetPage.getByRole('button', { name: 'Pending workspace transfer' }).click()
 
-    const pendingRow = targetPage.getByRole('row', {
-      name: new RegExp(workspaceName),
-    })
+    const pendingRow = pendingTransferRow(targetPage, workspaceName)
     await expect(pendingRow).toBeVisible()
     await pendingRow.getByRole('button', { name: 'Cancel transfer' }).click()
     await expect(pendingRow).toHaveCount(0)
 
-    await page.reload()
-    await page.getByRole('button', { name: 'Workspaces', exact: true }).click()
+    await page.goto('/orchestration/workspaces')
     await deleteWorkspace(page, workspaceName)
 
     await targetContext.close()
@@ -124,9 +133,7 @@ test.describe('workspace transfers', () => {
     await targetPage.goto('/orchestration')
     await targetPage.getByRole('button', { name: 'Pending workspace transfer' }).click()
 
-    const pendingRow = targetPage.getByRole('row', {
-      name: new RegExp(workspaceName),
-    })
+    const pendingRow = pendingTransferRow(targetPage, workspaceName)
     await expect(pendingRow).toBeVisible()
     await pendingRow.getByRole('button', { name: 'Accept transfer' }).click()
     await expect(pendingRow).toHaveCount(0)
@@ -136,8 +143,8 @@ test.describe('workspace transfers', () => {
     await expect(ownedRow).toBeVisible()
     await expect(ownedRow).toContainText('Owner')
 
-    await page.reload()
-    await page.getByRole('button', { name: 'Workspaces', exact: true }).click()
+    await page.goto('/orchestration/workspaces')
+    await expect(page.getByRole('button', { name: 'Add workspace' })).toBeVisible()
     await expect(page.getByRole('cell', { name: workspaceName, exact: true })).toHaveCount(0)
 
     await deleteWorkspace(targetPage, workspaceName)

From 808a23f460c3518977c42e0088219cd7467bf35d Mon Sep 17 00:00:00 2001
From: daniel-slaugh <daniel.slaugh@hotmail.com>
Date: Thu, 1 Oct 2026 13:53:44 -0600
Subject: [PATCH 92/93] Increase branch coverage to 80%

---
 .../__tests__/buildSnapshot.spec.ts           | 42 +++++++++++++++++++
 .../src/utils/__tests__/sessionGraph.spec.ts  | 17 ++++++++
 apps/qc-app/vite.config.ts                    |  7 +---
 3 files changed, 60 insertions(+), 6 deletions(-)

diff --git a/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts b/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
index 676c5ffc..5af5d7fa 100644
--- a/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
+++ b/apps/qc-app/src/services/qualityControl/__tests__/buildSnapshot.spec.ts
@@ -88,6 +88,48 @@ describe('buildSnapshotRecord', () => {
     expect(Array.from(out.dataX)).toEqual(Array.from(base.dataX))
   })
 
+  it('serializes optional live-history fields and defaults missing args', async () => {
+    const { deps, captured } = makeDeps()
+    const item = {
+      method: 'CHANGE_VALUES',
+      args: undefined,
+      comment: 'reviewed adjustment',
+      performedBy: 'analyst',
+    } as unknown as HistoryItem
+
+    await buildSnapshotRecord(deps, {
+      historyId: 'h-1',
+      session: inProgressSession,
+      source,
+      managed,
+      opIndex: 0,
+      liveHistory: [item],
+    })
+
+    expect(captured()!.operations).toEqual([
+      {
+        method: 'CHANGE_VALUES',
+        args: [],
+        comment: 'reviewed adjustment',
+        performedBy: 'analyst',
+      },
+    ])
+  })
+
+  it('defaults missing live history to an empty operation list', async () => {
+    const { deps, captured } = makeDeps()
+
+    await buildSnapshotRecord(deps, {
+      historyId: 'h-1',
+      session: inProgressSession,
+      source,
+      managed,
+      opIndex: 0,
+    })
+
+    expect(captured()!.operations).toEqual([])
+  })
+
   it('replays nothing for an in-progress baseline snapshot', async () => {
     const { deps, captured } = makeDeps()
 
diff --git a/apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts b/apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts
index 4fafb8ab..f4a33632 100644
--- a/apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts
+++ b/apps/qc-app/src/utils/__tests__/sessionGraph.spec.ts
@@ -1,6 +1,7 @@
 import { describe, it, expect } from 'vitest'
 import {
   buildDependentsMap,
+  commitOrder,
   hasDependents,
 } from '@/utils/sessionGraph'
 import type { SessionNode } from '@/utils/sessionGraph'
@@ -24,6 +25,14 @@ describe('buildDependentsMap', () => {
   it('treats a missing dependencyIds as no links', () => {
     expect(buildDependentsMap([{ id: 'a' }]).size).toBe(0)
   })
+
+  it('adds multiple dependents under the same parent', () => {
+    const map = buildDependentsMap([
+      node('b', ['a']),
+      node('c', ['a']),
+    ])
+    expect(map.get('a')).toEqual(['b', 'c'])
+  })
 })
 
 describe('hasDependents', () => {
@@ -36,3 +45,11 @@ describe('hasDependents', () => {
     expect(hasDependents(chain, 'c')).toBe(false)
   })
 })
+
+describe('commitOrder', () => {
+  it('falls back to createdAt when committedAt is missing', () => {
+    expect(commitOrder({ createdAt: '2025-01-01T00:00:00Z' })).toBe(
+      '2025-01-01T00:00:00Z',
+    )
+  })
+})
diff --git a/apps/qc-app/vite.config.ts b/apps/qc-app/vite.config.ts
index 67092727..c39b29be 100644
--- a/apps/qc-app/vite.config.ts
+++ b/apps/qc-app/vite.config.ts
@@ -160,12 +160,7 @@ export default defineConfig(({ command, mode }) => {
           lines: 80,
           statements: 80,
           functions: 80,
-          // Branches sits a couple points below the others because the
-          // last few uncovered branches live in the qualifier-band path
-          // of `options.ts` and the relayout-echo path of `selected.ts`.
-          // Both require heavy Plotly-DOM fixture setup for marginal
-          // signal. Raise back to 80 once those are covered.
-          branches: 78,
+          branches: 80,
         },
         provider: 'v8',
         reporter: ['text', 'json', 'html'],

From 4f2774fae9998274e5287b601e5065a29010f8cc Mon Sep 17 00:00:00 2001
From: daniel-slaugh <daniel.slaugh@hotmail.com>
Date: Thu, 1 Oct 2026 14:15:04 -0600
Subject: [PATCH 93/93] Resolve branch conflict

---
 apps/data-management/src/main.ts | 39 ++++++++++++++++++++------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/apps/data-management/src/main.ts b/apps/data-management/src/main.ts
index 5c5bd022..2a46ec7a 100644
--- a/apps/data-management/src/main.ts
+++ b/apps/data-management/src/main.ts
@@ -15,20 +15,29 @@ import { startAppInitialization } from '@/bootstrap/appInitialization'
 const app = createApp(App)
 const pinia = createPinia()
 
-app.use(pinia)
+function initializeApp() {
+  app.use(pinia)
 
-// Kick off session initialization before router navigation starts, but do not
-// block first paint on the rest of the app bootstrap work.
-void startAppInitialization()
+  // Start client initialization before the router's first navigation. The
+  // router guard waits for the client, while the rest of the bootstrap can
+  // finish after the app mounts.
+  const initialization = startAppInitialization()
 
-app.use(router)
-app.use(vuetify)
-settings.analyticsConfiguration.enableClarityAnalytics &&
-  settings.analyticsConfiguration.clarityProjectId &&
-  injectClarity(settings.analyticsConfiguration.clarityProjectId)
-settings.analyticsConfiguration.enableGoogleAnalytics &&
-  settings.analyticsConfiguration.googleAnalyticsMeasurementId &&
-  injectGoogleAnalytics(
-    settings.analyticsConfiguration.googleAnalyticsMeasurementId
-  )
-app.mount('#app')
+  app.use(router)
+  app.use(vuetify)
+  settings.analyticsConfiguration.enableClarityAnalytics &&
+    settings.analyticsConfiguration.clarityProjectId &&
+    injectClarity(settings.analyticsConfiguration.clarityProjectId)
+  settings.analyticsConfiguration.enableGoogleAnalytics &&
+    settings.analyticsConfiguration.googleAnalyticsMeasurementId &&
+    injectGoogleAnalytics(
+      settings.analyticsConfiguration.googleAnalyticsMeasurementId
+    )
+  app.mount('#app')
+
+  return initialization
+}
+
+void initializeApp().catch((error) => {
+  console.error('Error initializing app', error)
+})