diff --git a/apps/web/src/components/chat/AssistantCitationChip.browser.test.tsx b/apps/web/src/components/chat/AssistantCitationChip.browser.test.tsx
index 667c19fe4718c..5540f8eafa1fd 100644
--- a/apps/web/src/components/chat/AssistantCitationChip.browser.test.tsx
+++ b/apps/web/src/components/chat/AssistantCitationChip.browser.test.tsx
@@ -55,6 +55,7 @@ function mount(
   sourceAnchor: AssistantCitationSourceAnchor | undefined,
   onSave: () => boolean,
   chipCitation: AssistantCitation = citation,
+  container?: HTMLElement,
 ) {
   function Composer() {
     const [open, setOpen] = useState(true);
@@ -65,7 +66,7 @@ function mount(
       />
     );
   }
-  return render(<Composer />);
+  return render(<Composer />, container ? { container } : {});
 }
 
 const textarea = () =>
@@ -217,4 +218,67 @@ describe("citation comment popup position", () => {
     expect(rect.left).toBeGreaterThanOrEqual(0);
     expect(rect.right).toBeLessThanOrEqual(window.innerWidth);
   });
+
+  it("pins to the timeline edge the quote scrolled past, and typing scrolls nothing", async () => {
+    using source = setup();
+    const spacer = () => {
+      const element = document.createElement("div");
+      element.style.height = `${window.innerHeight * 2}px`;
+      return element;
+    };
+    const timeline = document.createElement("div");
+    timeline.style.cssText = "position: fixed; inset: 0; overflow-y: auto;";
+    timeline.append(spacer(), source.sourceAnchor.source, spacer());
+    document.body.append(timeline);
+    using _timeline = { [Symbol.dispose]: () => timeline.remove() };
+    // Moving the source collapsed the range.
+    source.sourceAnchor.range.selectNodeContents(source.sourceAnchor.source);
+    const quoteAt = (top: number) => {
+      timeline.scrollTop += source.sourceAnchor.source.getBoundingClientRect().top - top;
+    };
+    quoteAt(100);
+    // The composer floats over the bottom of the timeline.
+    const composer = document.createElement("div");
+    composer.dataset.chatComposerStack = "true";
+    composer.style.cssText = "position: fixed; inset-inline: 0; bottom: 0; height: 120px;";
+    const composerTop = window.innerHeight - 120;
+
+    await using _screen = await mount(
+      { ...source.sourceAnchor, viewport: timeline },
+      vi.fn(() => true),
+      citation,
+      composer,
+    );
+    const popup = () => document.querySelector<HTMLElement>('[data-slot="popover-popup"]')!;
+    await vi.waitFor(() => {
+      const quote = source.sourceAnchor.source.getBoundingClientRect();
+      expect(popup().getBoundingClientRect().top).toBeGreaterThanOrEqual(quote.bottom);
+    });
+
+    // Scrolled up, the quote goes under the composer and the popup stops just above it.
+    quoteAt(window.innerHeight);
+    const scrolledUp = timeline.scrollTop;
+    await vi.waitFor(() => {
+      const rect = popup().getBoundingClientRect();
+      expect(rect.bottom).toBeLessThanOrEqual(composerTop);
+      expect(rect.bottom).toBeGreaterThan(composerTop - 20);
+    });
+    await userEvent.keyboard("note");
+    expect(textarea()?.value).toBe("note");
+    expect(document.scrollingElement?.scrollTop).toBe(0);
+    expect(timeline.scrollTop).toBe(scrolledUp);
+
+    // Scrolled down past the quote, the popup stops at the top of the timeline.
+    quoteAt(-200);
+    const scrolledDown = timeline.scrollTop;
+    await vi.waitFor(() => {
+      const rect = popup().getBoundingClientRect();
+      expect(rect.top).toBeGreaterThanOrEqual(0);
+      expect(rect.top).toBeLessThan(20);
+    });
+    await userEvent.keyboard("!");
+    expect(textarea()?.value).toBe("note!");
+    expect(document.scrollingElement?.scrollTop).toBe(0);
+    expect(timeline.scrollTop).toBe(scrolledDown);
+  });
 });
diff --git a/apps/web/src/components/chat/AssistantCitationChip.tsx b/apps/web/src/components/chat/AssistantCitationChip.tsx
index 5a86f65399fcd..3acb9d712ef25 100644
--- a/apps/web/src/components/chat/AssistantCitationChip.tsx
+++ b/apps/web/src/components/chat/AssistantCitationChip.tsx
@@ -50,6 +50,32 @@ function useCitationSourceLinkProps(citation: AssistantCitation) {
   };
 }
 
+/** The part of the timeline left visible by the composer, which floats over its bottom. */
+function visibleTimelineRect(viewport: HTMLElement, composer: Element | null | undefined) {
+  const bounds = viewport.getBoundingClientRect();
+  const composerTop = composer?.getBoundingClientRect().top ?? bounds.bottom;
+  const bottom = Math.max(bounds.top, Math.min(bounds.bottom, composerTop));
+  return new DOMRect(bounds.left, bounds.top, bounds.width, bottom - bounds.top);
+}
+
+/** Floating UI reads a rect boundary on every position update, so getters keep it current. */
+function liveBoundary(read: () => DOMRect) {
+  return {
+    get x() {
+      return read().x;
+    },
+    get y() {
+      return read().y;
+    },
+    get width() {
+      return read().width;
+    },
+    get height() {
+      return read().height;
+    },
+  };
+}
+
 const flat = (text: string) => text.replace(/\s+/g, " ");
 
 /**
@@ -195,6 +221,7 @@ export function AssistantCitationChip({
   const sourceLinkProps = useCitationSourceLinkProps(citation);
   const commentInputRef = useRef<HTMLTextAreaElement>(null);
   const commentPopupRef = useRef<HTMLDivElement>(null);
+  const [commentTrigger, setCommentTrigger] = useState<HTMLButtonElement | null>(null);
   const draftCommentRef = useRef<string | null>(null);
   const [voiceSetupOpen, setVoiceSetupOpen] = useState(false);
   const [unavailableSourceAnchor, setUnavailableSourceAnchor] =
@@ -234,6 +261,11 @@ export function AssistantCitationChip({
       onUnavailable: onSourceUnavailable,
     });
   }, [citation, commentOpen, sourceAnchor, unavailableSourceAnchor]);
+  const visibleTimeline = (anchor: AssistantCitationSourceAnchor) =>
+    visibleTimelineRect(
+      anchor.viewport,
+      commentTrigger?.closest('[data-chat-composer-stack="true"]'),
+    );
   const popupAnchor = activeSourceAnchor
     ? {
         contextElement: activeSourceAnchor.source,
@@ -247,10 +279,17 @@ export function AssistantCitationChip({
           const bounds = activeSourceAnchor.source.getBoundingClientRect();
           const left = Math.min(Math.max(rect.left, bounds.left), bounds.right);
           const right = Math.max(Math.min(rect.right, bounds.right), left);
-          return new DOMRect(left, rect.top, right - left, rect.height);
+          // A quote scrolled out of view pins the popup to the timeline edge it left by.
+          const timeline = visibleTimeline(activeSourceAnchor);
+          const top = Math.min(Math.max(rect.top, timeline.top), timeline.bottom);
+          const bottom = Math.min(Math.max(rect.bottom, top), timeline.bottom);
+          return new DOMRect(left, top, right - left, bottom - top);
         },
       }
     : undefined;
+  const popupBoundary = activeSourceAnchor
+    ? liveBoundary(() => visibleTimeline(activeSourceAnchor))
+    : undefined;
   const label = assistantCitationLabel(citation);
   const composerSourceLink = (
     <Link
@@ -284,6 +323,7 @@ export function AssistantCitationChip({
           }}
         >
           <PopoverTrigger
+            ref={setCommentTrigger}
             aria-label={citation.comment ? "Edit citation comment" : "Add comment to citation"}
             data-citation-comment-trigger="true"
             render={<ContextChipAction />}
@@ -298,6 +338,7 @@ export function AssistantCitationChip({
               side={activeSourceAnchor ? "bottom" : "top"}
               align="end"
               anchor={popupAnchor}
+              collisionBoundary={popupBoundary}
               initialFocus={() => {
                 const input = commentInputRef.current;
                 if (input) {
diff --git a/apps/web/src/components/ui/popover.tsx b/apps/web/src/components/ui/popover.tsx
index ecbf7a6667062..3f87af7fff635 100644
--- a/apps/web/src/components/ui/popover.tsx
+++ b/apps/web/src/components/ui/popover.tsx
@@ -40,6 +40,7 @@ function PopoverPopup({
   sideOffset = 4,
   alignOffset = 0,
   collisionAvoidance,
+  collisionBoundary,
   tooltipStyle = false,
   keepMounted = false,
   anchor,
@@ -53,6 +54,7 @@ function PopoverPopup({
   sideOffset?: PopoverPrimitive.Positioner.Props["sideOffset"];
   alignOffset?: PopoverPrimitive.Positioner.Props["alignOffset"];
   collisionAvoidance?: PopoverPrimitive.Positioner.Props["collisionAvoidance"];
+  collisionBoundary?: PopoverPrimitive.Positioner.Props["collisionBoundary"];
   tooltipStyle?: boolean;
   keepMounted?: PopoverPrimitive.Portal.Props["keepMounted"];
   anchor?: PopoverPrimitive.Positioner.Props["anchor"];
@@ -76,6 +78,7 @@ function PopoverPopup({
         alignOffset={alignOffset}
         anchor={anchor}
         collisionAvoidance={collisionAvoidance}
+        collisionBoundary={collisionBoundary}
         collisionPadding={variant === "panel" ? 0 : undefined}
         className={cn(
           "h-(--positioner-height) max-h-(--available-height) w-(--positioner-width) max-w-(--available-width)",