fix(webui): prevent composer resize scroll jitter (#5121)

webui/src/components/thread/ThreadViewport.tsx

@@ -664,6 +664,11 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
           <div
             ref={composerDockRef}
             data-testid="thread-composer-dock"
+            onInputCapture={(event) => {
+              if (event.target instanceof HTMLTextAreaElement) {
+                threadMotionRef.current?.handleComposerInput();
+              }
+            }}
             className={cn(
               "row-start-2 z-10 w-full",
               hasMessages ? "sticky bottom-0 bg-background" : "relative self-center",

webui/src/components/thread/thread-motion.ts

@@ -21,6 +21,7 @@ type ThreadMotionEvent =
   | "navigation-settled"
   | "user-scroll"
   | "boundary-scroll"
+  | "composer-input"
   | "turn-completed"
   | "resume-follow";
 
@@ -49,6 +50,7 @@ const THREAD_MOTION_TRANSITIONS: Readonly<
   "follow-completion": {
     "navigate-history": "navigating-history",
     "user-scroll": "browsing-history",
+    "composer-input": "idle",
   },
   "navigating-history": {
     "navigate-history": "navigating-history",
@@ -141,6 +143,7 @@ export class ThreadMotionCoordinator {
   private promptPositioned = false;
   private measurementFrameId: number | null = null;
   private geometryDirty = false;
+  private composerInputDuringTurn = false;
 
   constructor(options: ThreadMotionCoordinatorOptions) {
     this.camera = options.camera;
@@ -170,6 +173,7 @@ export class ThreadMotionCoordinator {
     this.turn = turn;
     if (isNewTurn) {
       this.camera.cancel();
+      this.composerInputDuringTurn = false;
       this.promptPositioned = turn.entry === "restored";
       this.mode = this.promptPositioned && turn.hasOutput
         ? "follow-output"
@@ -187,10 +191,17 @@ export class ThreadMotionCoordinator {
     }
     // Protocol completion can share a React commit with the last large text
     // batch and begins the run-drawer exit. Keep camera ownership through
-    // those final layout changes; only a new turn or explicit user navigation
-    // may end completion follow.
+    // those final layout changes; only a new turn, explicit user navigation,
+    // or input for the next prompt may end completion follow.
     this.transition("turn-completed");
+    if (
+      this.composerInputDuringTurn
+      && this.transition("composer-input")
+    ) {
+      this.camera.cancel();
+    }
     this.turn = { id: null, promptId: null, hasOutput: false };
+    this.composerInputDuringTurn = false;
     this.promptPositioned = false;
     this.invalidateGeometry();
   }
@@ -201,6 +212,15 @@ export class ThreadMotionCoordinator {
     this.measurementFrameId = this.scheduler.request(this.flushGeometry);
   }
 
+  handleComposerInput(): void {
+    // Input and protocol completion can arrive in either order. Remember
+    // editing that starts just before turn_end so the completion drawer
+    // cannot reacquire the camera a few milliseconds later.
+    if (this.turn.id) this.composerInputDuringTurn = true;
+    if (!this.transition("composer-input")) return;
+    this.camera.cancel();
+  }
+
   takeUserControl(): void {
     this.handleUserScrollIntent(true);
   }
@@ -275,6 +295,7 @@ export class ThreadMotionCoordinator {
     this.geometryDirty = false;
     this.camera.cancel();
     this.turn = { id: null, promptId: null, hasOutput: false };
+    this.composerInputDuringTurn = false;
     this.mode = "idle";
     this.promptPositioned = false;
   }

webui/src/tests/thread-motion.test.ts

@@ -192,6 +192,71 @@ describe("ThreadMotionCoordinator", () => {
     expect(camera.followTo).toHaveBeenLastCalledWith(1_950);
   });
 
+  it("releases completion follow when composer editing begins", () => {
+    const {
+      camera,
+      coordinator,
+      advanceFrame,
+      setGeometry,
+    } = motionHarness();
+    coordinator.updateTurn({
+      id: "turn-1",
+      promptId: "prompt-1",
+      hasOutput: true,
+    });
+    advanceFrame();
+    coordinator.completeTurn();
+    camera.jumpTo.mockClear();
+    camera.followTo.mockClear();
+    camera.cancel.mockClear();
+
+    coordinator.handleComposerInput();
+    setGeometry({
+      scrollTop: 1_400,
+      scrollHeight: 1_940,
+      composerHeight: 160,
+    });
+    coordinator.invalidateGeometry();
+    advanceFrame();
+
+    expect(camera.cancel).toHaveBeenCalledTimes(1);
+    expect(camera.jumpTo).not.toHaveBeenCalled();
+    expect(camera.followTo).not.toHaveBeenCalled();
+    expect(coordinator.snapshot().mode).toBe("idle");
+  });
+
+  it("remembers composer editing that begins before turn completion", () => {
+    const {
+      camera,
+      coordinator,
+      advanceFrame,
+      setGeometry,
+    } = motionHarness();
+    coordinator.updateTurn({
+      id: "turn-1",
+      promptId: "prompt-1",
+      hasOutput: true,
+    });
+    advanceFrame();
+    camera.followTo.mockClear();
+    camera.cancel.mockClear();
+
+    coordinator.handleComposerInput();
+    expect(coordinator.snapshot().mode).toBe("follow-output");
+
+    coordinator.completeTurn();
+    setGeometry({
+      scrollTop: 1_400,
+      scrollHeight: 1_940,
+      composerHeight: 160,
+    });
+    advanceFrame();
+
+    expect(camera.cancel).toHaveBeenCalledTimes(1);
+    expect(camera.followTo).not.toHaveBeenCalled();
+    expect(coordinator.snapshot().mode).toBe("idle");
+  });
+
   it("keeps turn identity stable when canonical replay replaces DOM ids", () => {
     const {
       camera,

webui/src/tests/thread-viewport.test.tsx

@@ -867,6 +867,85 @@ describe("ThreadViewport", () => {
     }
   });
 
+  it("releases completion follow when single-line input starts before completion", async () => {
+    const resizeObserver = stubResizeObserver();
+    const jumpTo = vi.spyOn(ThreadCameraController.prototype, "jumpTo");
+    const followTo = vi.spyOn(ThreadCameraController.prototype, "followTo");
+
+    try {
+      const threaded: UIMessage[] = [
+        { id: "u1", role: "user", content: "question", turnId: "turn-1", createdAt: 1 },
+        { id: "a1", role: "assistant", content: "answer", turnId: "turn-1", createdAt: 2 },
+      ];
+      const viewport = (isStreaming: boolean, activeTurnId?: string) => (
+        <ThreadViewport
+          messages={threaded}
+          isStreaming={isStreaming}
+          composer={<textarea aria-label="Message input" />}
+          activeTurnId={activeTurnId}
+        />
+      );
+      const { container, rerender } = render(viewport(true));
+      const scroller = container.firstElementChild?.firstElementChild as HTMLElement;
+      Object.defineProperties(scroller, {
+        scrollHeight: { configurable: true, value: 1_200 },
+        clientHeight: { configurable: true, value: 500 },
+        scrollTop: { configurable: true, writable: true, value: 700 },
+      });
+
+      rerender(viewport(true, "turn-1"));
+      await flushAnimationFrame();
+      jumpTo.mockClear();
+      followTo.mockClear();
+
+      const composerDock = screen.getByTestId("thread-composer-dock");
+      const composerObserver = resizeObserver.observers.find(
+        (observer) => observer.elements.includes(composerDock),
+      );
+      expect(composerObserver).toBeDefined();
+      Object.defineProperty(scroller, "scrollHeight", {
+        configurable: true,
+        value: 1_240,
+      });
+
+      act(() => {
+        composerObserver!.callback(
+          [{ target: composerDock }] as ResizeObserverEntry[],
+          composerObserver as unknown as ResizeObserver,
+        );
+      });
+      await flushAnimationFrame();
+
+      expect(followTo).toHaveBeenCalled();
+
+      followTo.mockClear();
+      fireEvent.input(screen.getByLabelText("Message input"), {
+        target: { value: "x" },
+      });
+      const scrollTopAfterInput = scroller.scrollTop;
+      rerender(viewport(false));
+      await flushAnimationFrame();
+      expect(followTo).not.toHaveBeenCalled();
+      Object.defineProperty(scroller, "scrollHeight", {
+        configurable: true,
+        value: 1_280,
+      });
+      act(() => {
+        composerObserver!.callback(
+          [{ target: composerDock }] as ResizeObserverEntry[],
+          composerObserver as unknown as ResizeObserver,
+        );
+      });
+      await flushAnimationFrame();
+
+      expect(jumpTo).not.toHaveBeenCalled();
+      expect(followTo).not.toHaveBeenCalled();
+      expect(scroller.scrollTop).toBe(scrollTopAfterInput);
+    } finally {
+      resizeObserver.restore();
+    }
+  });
+
   it("keeps the thread scrollport above a mobile soft keyboard", async () => {
     const visualViewport = stubVisualViewport({ innerHeight: 800, height: 480 });
     try {