fix(tree-compare): default Base to the older revision

dashboard/src/locales/messages/index.ts

@@ -410,6 +410,7 @@ export const messages = {
     'tree.path': 'Trees',
     'tree.searchPlaceholder': 'Search by tree, branch or tag with a regex',
     'treeCompare.backToDetails': 'Back to tree details',
+    'treeCompare.base': 'Base',
     'treeCompare.breadcrumb': 'Compare',
     'treeCompare.breakdownTitle': 'Changed results',
     'treeCompare.change.appeared': 'Appeared',
@@ -425,10 +426,10 @@ export const messages = {
     'treeCompare.change.unchanged': 'Unchanged',
     'treeCompare.changed': 'Changed',
     'treeCompare.description':
-      'Compare pass/fail counts between two revisions on the same tree and branch.',
-    'treeCompare.detail.missingSide': 'No result on this side',
+      'Compare pass/fail counts from a base revision to a target revision on the same tree and branch.',
+    'treeCompare.detail.missingSide': 'No result on this revision',
     'treeCompare.drilldownHint':
-      'Individual builds, boots, and tests that changed between Side A and Side B.',
+      'Individual builds, boots, and tests that changed from the base revision to the target revision.',
     'treeCompare.failures.change': 'Change',
     'treeCompare.failures.configArch': 'Config / Arch',
     'treeCompare.failures.pathArch': 'Path / Arch',
@@ -447,24 +448,23 @@ export const messages = {
       'Select two different revisions to compare. This tree needs at least two commits with results.',
     'treeCompare.openCompare': 'Compare revisions',
     'treeCompare.selectRevision': 'Select a revision',
-    'treeCompare.sideA': 'Side A',
-    'treeCompare.sideB': 'Side B',
     'treeCompare.statusPairFilter.active': 'Active status pair filters',
     'treeCompare.statusPairFilter.add': 'Add',
     'treeCompare.statusPairFilter.from': 'From',
     'treeCompare.statusPairFilter.glossary':
       'PASS — completed successfully{br}' +
       'FAIL — completed with a failure{br}' +
       'INCONCLUSIVE — no definitive pass or fail result{br}' +
-      '— — absent on this side',
+      '— — absent on this revision',
     'treeCompare.statusPairFilter.remove': 'Remove {from} to {to} filter',
     'treeCompare.statusPairFilter.select': 'Select status',
     'treeCompare.statusPairFilter.to': 'To',
     'treeCompare.suggestion.branchHead': 'Branch head',
     'treeCompare.suggestion.previous': 'Previous commit',
-    'treeCompare.suggestion.swap': 'Swap sides',
+    'treeCompare.suggestion.swap': 'Swap base and target',
     'treeCompare.suggestions': 'Suggestions',
     'treeCompare.summaryTitle': 'Tree summary',
+    'treeCompare.target': 'Target',
     'treeDetails.bootsHistory': 'Boots History',
     'treeDetails.branch': 'Branch',
     'treeDetails.buildsHistory': 'Builds History',

dashboard/src/pages/TreeCompare/TreeCompareLink.tsx

@@ -24,7 +24,7 @@ export function TreeCompareLink({
       <Link
         to="/tree/$treeName/$branch/compare"
         params={{ treeName, branch }}
-        search={{ hashA: hash, hashB: '', origin }}
+        search={{ hashA: '', hashB: hash, origin }}
         state={s => s}
       >
         <GitCompareArrows className="mr-2 h-4 w-4" />

dashboard/src/pages/TreeCompare/TreeComparePage.tsx

@@ -43,6 +43,7 @@ import {
   mapBootOrTestDiffRows,
   mapBuildDiffRows,
   readStoredStatusPairs,
+  resolveCompareHashes,
   resolveStatusPairs,
   serializeStatusPairs,
   writeStoredStatusPairs,
@@ -86,11 +87,10 @@ const TreeComparePage = (): JSX.Element => {
     [commitsQuery.data],
   );
 
-  const resolvedHashA = hashA || revisions[0]?.hash || '';
-  const resolvedHashB =
-    hashB ||
-    revisions.find(revision => revision.hash !== resolvedHashA)?.hash ||
-    '';
+  const { hashA: resolvedHashA, hashB: resolvedHashB } = useMemo(
+    () => resolveCompareHashes(revisions, hashA, hashB),
+    [revisions, hashA, hashB],
+  );
 
   const canCompare = Boolean(resolvedHashA && resolvedHashB);
 
@@ -315,7 +315,7 @@ const TreeComparePage = (): JSX.Element => {
                 params={{
                   treeName,
                   branch,
-                  hash: resolvedHashA,
+                  hash: resolvedHashB,
                 }}
                 state={s => s}
               >
@@ -347,7 +347,7 @@ const TreeComparePage = (): JSX.Element => {
             </div>
             <Link
               to="/tree/$treeName/$branch/$hash"
-              params={{ treeName, branch, hash: resolvedHashA }}
+              params={{ treeName, branch, hash: resolvedHashB }}
               className="text-blue text-sm font-medium hover:underline"
               state={s => s}
             >

dashboard/src/pages/TreeCompare/components/CompareDetailSheet.tsx

@@ -64,7 +64,7 @@ function SideColumn({
   logData,
   isLoading,
 }: {
-  labelId: 'treeCompare.sideA' | 'treeCompare.sideB';
+  labelId: 'treeCompare.base' | 'treeCompare.target';
   status: CompareItemStatus;
   id: string | null;
   logType: LogType;
@@ -154,15 +154,15 @@ export function CompareDetailSheet({
             </div>
             <div className="grid gap-6 lg:grid-cols-2">
               <SideColumn
-                labelId="treeCompare.sideA"
+                labelId="treeCompare.base"
                 status={item.sideA}
                 id={item.idA}
                 logType={logType}
                 logData={logA.data}
                 isLoading={logA.isLoading}
               />
               <SideColumn
-                labelId="treeCompare.sideB"
+                labelId="treeCompare.target"
                 status={item.sideB}
                 id={item.idB}
                 logType={logType}

dashboard/src/pages/TreeCompare/components/CompareFailuresTables.tsx

@@ -374,15 +374,15 @@ export function CompareBuildsFailuresTable({
             />
             <SortableHead
               className="text-center"
-              intlKey="treeCompare.sideA"
+              intlKey="treeCompare.base"
               sortKey="sideA"
               sort={sort}
               onSort={key => setSort(current => cycleSort(current, key))}
             />
             <TableHead className="bg-light-gray w-8" />
             <SortableHead
               className="text-center"
-              intlKey="treeCompare.sideB"
+              intlKey="treeCompare.target"
               sortKey="sideB"
               sort={sort}
               onSort={key => setSort(current => cycleSort(current, key))}
@@ -507,15 +507,15 @@ function PathHardwareTable({
             />
             <SortableHead
               className="text-center"
-              intlKey="treeCompare.sideA"
+              intlKey="treeCompare.base"
               sortKey="sideA"
               sort={sort}
               onSort={key => setSort(current => cycleSort(current, key))}
             />
             <TableHead className="bg-light-gray w-8" />
             <SortableHead
               className="text-center"
-              intlKey="treeCompare.sideB"
+              intlKey="treeCompare.target"
               sortKey="sideB"
               sort={sort}
               onSort={key => setSort(current => cycleSort(current, key))}

dashboard/src/pages/TreeCompare/components/RevisionSelector.tsx

@@ -68,11 +68,11 @@ function RevisionCard({
             side === 'A' ? 'bg-blue' : 'bg-dim-gray',
           )}
         >
-          {side}
+          {side === 'A' ? 'B' : 'T'}
         </span>
         <span className="text-dim-black text-sm font-semibold">
           <FormattedMessage
-            id={side === 'A' ? 'treeCompare.sideA' : 'treeCompare.sideB'}
+            id={side === 'A' ? 'treeCompare.base' : 'treeCompare.target'}
           />
         </span>
       </div>
@@ -180,7 +180,7 @@ export function RevisionSelectorBar({
             type="button"
             onClick={onSwap}
             className="bg-medium-gray flex h-10 w-10 items-center justify-center rounded-full"
-            aria-label="Swap sides"
+            aria-label="Swap base and target"
           >
             <ArrowLeftRight className="text-dim-gray h-5 w-5" />
             <span className="sr-only">

dashboard/src/utils/treeCompareDiff.test.ts

@@ -7,6 +7,7 @@ import {
   mapBuildDiffRows,
   compareRowNav,
   parseStatusPairs,
+  resolveCompareHashes,
   resolveStatusPairs,
   serializeStatusPairs,
   toggleChangeTypePairs,
@@ -44,6 +45,43 @@ describe('deriveCompareChange', () => {
   });
 });
 
+describe('resolveCompareHashes', () => {
+  const revisions = [{ hash: 'new' }, { hash: 'mid' }, { hash: 'old' }];
+
+  it('defaults to the head on B and the revision before it on A', () => {
+    expect(resolveCompareHashes(revisions, '', '')).toEqual({
+      hashA: 'mid',
+      hashB: 'new',
+    });
+  });
+
+  it('keeps A one revision older than an explicit B', () => {
+    expect(resolveCompareHashes(revisions, '', 'mid')).toEqual({
+      hashA: 'old',
+      hashB: 'mid',
+    });
+  });
+
+  it('never defaults B to the same revision as A', () => {
+    expect(resolveCompareHashes(revisions, 'new', '')).toEqual({
+      hashA: 'new',
+      hashB: 'mid',
+    });
+  });
+
+  it('leaves explicit hashes alone and copes with too few revisions', () => {
+    expect(resolveCompareHashes(revisions, 'old', 'new')).toEqual({
+      hashA: 'old',
+      hashB: 'new',
+    });
+    expect(resolveCompareHashes([{ hash: 'only' }], '', '')).toEqual({
+      hashA: '',
+      hashB: 'only',
+    });
+    expect(resolveCompareHashes([], '', '')).toEqual({ hashA: '', hashB: '' });
+  });
+});
+
 describe('applyStatusPairFilter', () => {
   const rows = [
     { id: '1', sideA: 'PASS' as const, sideB: 'FAIL' as const },

dashboard/src/utils/treeCompareDiff.ts

@@ -28,7 +28,7 @@ export function apiStatusToItemStatus(
   return status;
 }
 
-/** Mirror backend _CHANGE_COUNT_SELECT categories for A→B transitions. */
+/** Mirror backend _CHANGE_COUNT_SELECT categories for base→target transitions. */
 export function deriveCompareChange(
   statusA: CompareItemStatus,
   statusB: CompareItemStatus,
@@ -300,6 +300,25 @@ export function applyStatusPairFilter<
   );
 }
 
+/**
+ * Fill in missing sides so the default comparison runs oldest → newest:
+ * B defaults to the branch head, A to the revision right before B.
+ * `revisions` is newest-first.
+ */
+export function resolveCompareHashes(
+  revisions: readonly { hash: string }[],
+  hashA: string,
+  hashB: string,
+): { hashA: string; hashB: string } {
+  const resolvedB =
+    hashB || revisions.find(revision => revision.hash !== hashA)?.hash || '';
+  const indexB = revisions.findIndex(revision => revision.hash === resolvedB);
+  return {
+    hashA: hashA || revisions[indexB + 1]?.hash || '',
+    hashB: resolvedB,
+  };
+}
+
 /** Next/prev over the currently visible (searched/sorted) table rows. */
 export function compareRowNav<T extends { id: string }>(
   rows: T[],

docs/tree-compare.md

@@ -4,9 +4,9 @@ High-level overview of the Tree Compare feature: side-by-side comparison of buil
 
 ## Purpose
 
-Given a tree name and branch, pick two commit hashes (side A and side B) and answer:
+Given a tree name and branch, pick two commit hashes (base and target) and answer:
 
-- How did overall pass/fail/inconclusive counts move from A → B?
+- How did overall pass/fail/inconclusive counts move from base → target?
 - Which individual builds / boots / tests changed category (regressions, fixes, new failures, etc.)?
 
 Entry point: **Compare revisions** on Tree Details (`TreeCompareLink`), which opens:
@@ -15,14 +15,14 @@ Entry point: **Compare revisions** on Tree Details (`TreeCompareLink`), which op
 
 ## User flow
 
-1. Open compare from Tree Details (current revision pre-fills as side A).
-2. Choose / swap revisions via the revision selector (commit history + shortcuts: previous commit, branch head, swap sides).
+1. Open compare from Tree Details (current revision pre-fills as **Target**, **Base** defaults to the revision before it, so base → target reads oldest → newest). URL params stay `hashA` (base) and `hashB` (target).
+2. Choose / swap revisions via the revision selector (commit history + shortcuts: previous commit, branch head, swap base and target).
 3. Read the summary matrix (fixes, regressions, pass/fail/other counts per builds / boots / tests).
 4. Drill into **Changed results** tabs (Builds / Boots / Tests). Quick change-type chips add or remove their status pairs; custom From/To pairs can be added too (default: `PASS → FAIL` and `FAIL → PASS`). Last edited pairs are remembered in localStorage when the URL omits `statusPair`.
 
 URL search state owns: `hashA`, `hashB`, `origin`, `currentPageTab`, and optional `statusPair`.
 
-## Change categories (A → B)
+## Change categories (base → target)
 
 Statuses are grouped into **PASS**, **FAIL**, and **INCONCLUSIVE** (everything else, including null). Absent on one side is treated as missing (`null` / `—`).
 
@@ -34,7 +34,7 @@ Statuses are grouped into **PASS**, **FAIL**, and **INCONCLUSIVE** (everything e
 | `stillFailing` | FAIL → FAIL |
 | `newPass` | missing/INCONCLUSIVE → PASS |
 | `appeared` | missing → INCONCLUSIVE |
-| `disappeared` | present on A, missing on B |
+| `disappeared` | present on base, missing on target |
 | `unchanged` | same status on both sides (except FAIL → FAIL); frontend-only, has no backend count and no filter chip |
 
 Backend SQL aggregates (`_CHANGE_COUNT_SELECT` in `queries/tree.py`) and the frontend `deriveCompareChange` helper must stay in sync.