fix(webui): keep streaming tail visible (#5140)

webui/src/components/thread/ThreadViewport.tsx

@@ -275,7 +275,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
     if (el) {
       const top = Math.max(0, el.scrollHeight - el.clientHeight);
       if (smooth) {
-        threadMotionRef.current?.animateTo(top);
+        threadMotionRef.current?.navigateLatestTo(top);
       } else {
         threadMotionRef.current?.jumpTo(top);
       }
@@ -289,7 +289,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
     (smooth = false, options?: { force?: boolean }) => {
       const force = options?.force ?? false;
       if (!force && threadMotionRef.current?.isAutoFollowPaused()) return;
-      threadMotionRef.current?.resumeAutoFollow();
+      if (!smooth) threadMotionRef.current?.resumeAutoFollow();
       scrollToBottomNow(smooth);
     },
     [scrollToBottomNow],

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

@@ -4,7 +4,7 @@ interface ThreadCameraMotionProfile {
    * camera closes roughly 95% of an uncapped distance in three time constants.
    */
   responseTimeMs: number;
-  /** Prevents a large completion batch from turning into a one-frame jump. */
+  /** Prevents a long explicit navigation from turning into a one-frame jump. */
   maxSpeedPxPerSecond: number;
   /** Avoids spending frames chasing sub-pixel layout noise. */
   settleDistancePx: number;
@@ -12,13 +12,6 @@ interface ThreadCameraMotionProfile {
   maxFrameDeltaMs: number;
 }
 
-const THREAD_CAMERA_FOLLOW_MOTION: Readonly<ThreadCameraMotionProfile> = {
-  responseTimeMs: 90,
-  maxSpeedPxPerSecond: 1_200,
-  settleDistancePx: 0.5,
-  maxFrameDeltaMs: 50,
-};
-
 const THREAD_CAMERA_NAVIGATION_MOTION: Readonly<ThreadCameraMotionProfile> = {
   responseTimeMs: 110,
   maxSpeedPxPerSecond: 12_000,
@@ -26,18 +19,6 @@ const THREAD_CAMERA_NAVIGATION_MOTION: Readonly<ThreadCameraMotionProfile> = {
   maxFrameDeltaMs: 50,
 };
 
-/**
- * Reduced motion still preserves spatial continuity. Snapping a long thread
- * to its destination removes the very context that helps users understand
- * where the viewport moved; this profile shortens that motion instead.
- */
-const THREAD_CAMERA_REDUCED_MOTION: Readonly<ThreadCameraMotionProfile> = {
-  responseTimeMs: 55,
-  maxSpeedPxPerSecond: 2_400,
-  settleDistancePx: 0.5,
-  maxFrameDeltaMs: 50,
-};
-
 const THREAD_CAMERA_REDUCED_NAVIGATION_MOTION: Readonly<ThreadCameraMotionProfile> = {
   responseTimeMs: 45,
   maxSpeedPxPerSecond: 24_000,
@@ -63,12 +44,10 @@ interface ThreadCameraOptions {
   prefersReducedMotion?: () => boolean;
 }
 
-type ThreadCameraMotionKind = "follow" | "navigation";
-
 /**
  * A time-based ease-out chase rather than a start/end tween. The target can
- * move on every streamed line without restarting a duration or adding another
+ * move during explicit history navigation without restarting a duration or
- * frame loop.
+ * adding another frame loop.
  */
 function easeOutChase(
   current: number,
@@ -108,7 +87,6 @@ export class ThreadCameraController {
   private phase: "idle" | "following" = "idle";
   private target = 0;
   private lastTimestamp: number | null = null;
-  private motionKind: ThreadCameraMotionKind = "follow";
 
   constructor(
     getViewport: () => ThreadCameraViewport | null,
@@ -131,25 +109,31 @@ export class ThreadCameraController {
     this.write(viewport, this.target);
   }
 
+  /**
+   * Automatic follow is a layout constraint, not navigation. Resolve it in
+   * the geometry frame so streamed content and viewport resizing cannot build
+   * up hidden travel below the visible tail.
+   */
   followTo(top: number): ThreadCameraFollowResult | null {
-    return this.moveTo(top, "follow");
+    const viewport = this.getViewport();
+    if (!viewport) return null;
+    this.cancel();
+    this.target = Math.max(0, top);
+    this.write(viewport, this.target);
+    return "settled";
   }
 
   navigateTo(top: number): ThreadCameraFollowResult | null {
-    return this.moveTo(top, "navigation");
+    return this.moveTo(top);
   }
 
-  private moveTo(
+  private moveTo(top: number): ThreadCameraFollowResult | null {
-    top: number,
-    motionKind: ThreadCameraMotionKind,
-  ): ThreadCameraFollowResult | null {
     const viewport = this.getViewport();
     if (!viewport) return null;
     const current = viewport.scrollTop;
     this.target = Math.max(0, top);
-    this.motionKind = motionKind;
 
-    const motion = this.currentMotion(motionKind);
+    const motion = this.currentMotion();
     if (this.phase === "following") {
       return "retargeted";
     }
@@ -171,7 +155,6 @@ export class ThreadCameraController {
     }
     this.phase = "idle";
     this.lastTimestamp = null;
-    this.motionKind = "follow";
   }
 
   dispose(): void {
@@ -186,7 +169,7 @@ export class ThreadCameraController {
       return;
     }
 
-    const motion = this.currentMotion(this.motionKind);
+    const motion = this.currentMotion();
     const previousTimestamp = this.lastTimestamp ?? timestamp - (1000 / 60);
     const deltaMs = Math.min(
       motion.maxFrameDeltaMs,
@@ -220,15 +203,10 @@ export class ThreadCameraController {
     this.frameId = this.scheduler.request(this.advance);
   };
 
-  private currentMotion(kind: ThreadCameraMotionKind): ThreadCameraMotionProfile {
+  private currentMotion(): ThreadCameraMotionProfile {
-    if (kind === "navigation") {
-      return this.prefersReducedMotion()
-        ? THREAD_CAMERA_REDUCED_NAVIGATION_MOTION
-        : THREAD_CAMERA_NAVIGATION_MOTION;
-    }
     return this.prefersReducedMotion()
-      ? THREAD_CAMERA_REDUCED_MOTION
+      ? THREAD_CAMERA_REDUCED_NAVIGATION_MOTION
-      : THREAD_CAMERA_FOLLOW_MOTION;
+      : THREAD_CAMERA_NAVIGATION_MOTION;
   }
 
   private write(viewport: ThreadCameraViewport, top: number): void {

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

@@ -8,6 +8,7 @@ type ThreadMotionMode =
   | "anchor-prompt"
   | "follow-output"
   | "follow-completion"
+  | "navigating-latest"
   | "navigating-history"
   | "browsing-history";
 
@@ -17,6 +18,7 @@ type AutomaticThreadMotionMode =
   | "follow-output";
 
 type ThreadMotionEvent =
+  | "navigate-latest"
   | "navigate-history"
   | "navigation-settled"
   | "user-scroll"
@@ -34,25 +36,38 @@ const THREAD_MOTION_TRANSITIONS: Readonly<
   >
 > = {
   idle: {
+    "navigate-latest": "navigating-latest",
     "navigate-history": "navigating-history",
     "user-scroll": "browsing-history",
   },
   "anchor-prompt": {
+    "navigate-latest": "navigating-latest",
     "navigate-history": "navigating-history",
     "user-scroll": "browsing-history",
     "turn-completed": "follow-completion",
   },
   "follow-output": {
+    "navigate-latest": "navigating-latest",
     "navigate-history": "navigating-history",
     "user-scroll": "browsing-history",
     "turn-completed": "follow-completion",
   },
   "follow-completion": {
+    "navigate-latest": "navigating-latest",
     "navigate-history": "navigating-history",
     "user-scroll": "browsing-history",
     "composer-input": "idle",
   },
+  "navigating-latest": {
+    "navigate-latest": "navigating-latest",
+    "navigate-history": "navigating-history",
+    "navigation-settled": "current-automatic-mode",
+    "user-scroll": "browsing-history",
+    "boundary-scroll": "browsing-history",
+    "resume-follow": "current-automatic-mode",
+  },
   "navigating-history": {
+    "navigate-latest": "navigating-latest",
     "navigate-history": "navigating-history",
     "navigation-settled": "browsing-history",
     "user-scroll": "browsing-history",
@@ -60,6 +75,7 @@ const THREAD_MOTION_TRANSITIONS: Readonly<
     "resume-follow": "current-automatic-mode",
   },
   "browsing-history": {
+    "navigate-latest": "navigating-latest",
     "navigate-history": "navigating-history",
     "resume-follow": "current-automatic-mode",
   },
@@ -124,9 +140,10 @@ function defaultScheduler(): ThreadMotionScheduler {
 }
 
 /**
- * Owns the policy that turns discrete layout events into continuous camera
+ * Owns the policy that turns discrete layout events into automatic tail
- * motion. Callers only invalidate geometry; one display frame coalesces those
+ * pinning or explicit camera navigation. Callers only invalidate geometry;
- * notifications, reads the authoritative layout, and retargets the camera.
+ * one display frame coalesces those notifications and reads the authoritative
+ * layout before applying either policy.
  */
 export class ThreadMotionCoordinator {
   private readonly camera: ThreadMotionCamera;
@@ -241,8 +258,18 @@ export class ThreadMotionCoordinator {
     this.camera.jumpTo(top);
   }
 
-  animateTo(top: number): ThreadCameraFollowResult | null {
+  /**
-    return this.camera.navigateTo(top);
+   * Explicitly navigate to the live tail while allowing authoritative layout
+   * frames to retarget that destination as streamed output continues to grow.
+   */
+  navigateLatestTo(top: number): ThreadCameraFollowResult | null {
+    this.camera.cancel();
+    this.transition("navigate-latest");
+    const result = this.camera.navigateTo(top);
+    if (!result || result === "settled") {
+      this.settleLatestNavigation();
+    }
+    return result;
   }
 
   navigateHistoryTo(top: number): ThreadCameraFollowResult | null {
@@ -271,6 +298,15 @@ export class ThreadMotionCoordinator {
    */
   observeScroll(nearBottom: boolean): ThreadScrollOwner {
     switch (this.mode) {
+      case "navigating-latest":
+        if (!this.camera.isFollowing()) {
+          if (nearBottom) {
+            this.settleLatestNavigation();
+          } else {
+            this.invalidateGeometry();
+          }
+        }
+        return "navigation";
       case "navigating-history":
         if (!this.camera.isFollowing()) {
           this.transition("navigation-settled");
@@ -307,7 +343,8 @@ export class ThreadMotionCoordinator {
 
   private isHistoryMode(): boolean {
     return (
-      this.mode === "navigating-history"
+      this.mode === "navigating-latest"
+      || this.mode === "navigating-history"
       || this.mode === "browsing-history"
     );
   }
@@ -336,15 +373,17 @@ export class ThreadMotionCoordinator {
     const result = this.camera.followTo(target);
     if (
       result
-      && (
+      && Math.abs(target - geometry.scrollTop) > GEOMETRY_EPSILON_PX
-        Math.abs(target - geometry.scrollTop) > GEOMETRY_EPSILON_PX
-        || result === "retargeted"
-      )
     ) {
       this.onAutoFollow?.();
     }
   }
 
+  private settleLatestNavigation(): void {
+    if (!this.transition("navigation-settled")) return;
+    this.invalidateGeometry();
+  }
+
   private readonly flushGeometry = (): void => {
     this.measurementFrameId = null;
     if (!this.geometryDirty) return;
@@ -358,6 +397,13 @@ export class ThreadMotionCoordinator {
     if (!geometry) return;
     this.onGeometry?.(geometry);
 
+    if (this.mode === "navigating-latest") {
+      const result = this.camera.navigateTo(geometry.maxScrollTop);
+      if (!result || result === "settled") {
+        this.settleLatestNavigation();
+      }
+      return;
+    }
     if (this.mode === "follow-completion") {
       this.followGeometry(geometry);
       return;
@@ -374,8 +420,9 @@ export class ThreadMotionCoordinator {
         return;
       }
       // Before output exists, the real lower scroll boundary is the only
-      // position with zero hidden downward travel. Once output exists, start
+      // position with zero hidden downward travel. Once output exists, first
-      // from the prompt origin and let the follow camera reveal its growth.
+      // establish the prompt origin; automatic follow below then resolves the
+      // current tail in this same authoritative geometry frame.
       this.camera.jumpTo(
         this.turn.hasOutput ? geometry.promptTop : geometry.maxScrollTop,
       );

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

@@ -39,74 +39,57 @@ function cameraHarness(prefersReducedMotion = false) {
 }
 
 describe("ThreadCameraController", () => {
-  it("responds immediately, then eases out as a static target gets closer", () => {
+  it("pins automatic follow in the geometry frame without camera debt", () => {
-    const { camera, viewport, advance } = cameraHarness();
+    const { camera, viewport, frames } = cameraHarness();
 
-    camera.followTo(60);
+    expect(camera.followTo(60)).toBe("settled");
-    advance(16);
-    const firstStep = viewport.scrollTop;
-    advance(16);
-    const secondStep = viewport.scrollTop - firstStep;
-    advance(16);
-    const thirdStep = viewport.scrollTop - firstStep - secondStep;
 
-    expect(firstStep).toBeGreaterThan(0);
+    expect(viewport.scrollTop).toBe(60);
-    expect(secondStep).toBeGreaterThan(0);
+    expect(frames).toHaveLength(0);
-    expect(thirdStep).toBeGreaterThan(0);
+    expect(camera.isFollowing()).toBe(false);
-    expect(secondStep).toBeLessThan(firstStep);
-    expect(thirdStep).toBeLessThan(secondStep);
   });
 
-  it("retargets an active follow without adding another loop", () => {
+  it("retargets active history navigation without adding another loop", () => {
     const { camera, viewport, frames, advance } = cameraHarness();
 
-    expect(camera.followTo(100)).toBe("started");
+    expect(camera.navigateTo(100)).toBe("started");
     expect(frames).toHaveLength(1);
     advance(16);
     expect(frames).toHaveLength(1);
 
-    expect(camera.followTo(180)).toBe("retargeted");
+    expect(camera.navigateTo(180)).toBe("retargeted");
     expect(frames).toHaveLength(1);
     for (let frame = 0; frame < 120; frame += 1) advance(16);
     expect(viewport.scrollTop).toBe(180);
   });
 
-  it("tracks repeated target growth as one monotonic camera movement", () => {
+  it("pins repeated automatic targets without accumulating lag", () => {
-    const { camera, viewport, advance } = cameraHarness();
+    const { camera, viewport, frames } = cameraHarness();
 
     camera.followTo(80);
-    advance(16);
+    expect(viewport.scrollTop).toBe(80);
-    const first = viewport.scrollTop;
     camera.followTo(140);
-    advance(16);
+    expect(viewport.scrollTop).toBe(140);
-    const second = viewport.scrollTop;
     camera.followTo(220);
-    advance(16);
+    expect(viewport.scrollTop).toBe(220);
-    const third = viewport.scrollTop;
+    expect(frames).toHaveLength(0);
-
+    expect(camera.isFollowing()).toBe(false);
-    expect(first).toBeGreaterThan(0);
-    expect(second).toBeGreaterThan(first);
-    expect(third).toBeGreaterThan(second);
-    expect(camera.isFollowing()).toBe(true);
   });
 
-  it("uses a faster motion profile for explicit long-distance navigation", () => {
+  it("eases explicit long-distance navigation across frames", () => {
-    const follow = cameraHarness();
+    const { camera, viewport, advance } = cameraHarness();
-    const navigation = cameraHarness();
 
-    follow.camera.followTo(1_000);
+    camera.navigateTo(1_000);
-    navigation.camera.navigateTo(1_000);
+    advance(16);
-    follow.advance(16);
-    navigation.advance(16);
 
-    expect(navigation.viewport.scrollTop).toBeGreaterThan(follow.viewport.scrollTop);
+    expect(viewport.scrollTop).toBeGreaterThan(0);
-    expect(navigation.viewport.scrollTop).toBeLessThan(1_000);
+    expect(viewport.scrollTop).toBeLessThan(1_000);
   });
 
   it("gives an immediate jump command priority over an active follow", () => {
     const { camera, viewport, scheduler, frames } = cameraHarness();
 
-    camera.followTo(240);
+    camera.navigateTo(240);
     expect(frames).toHaveLength(1);
     camera.jumpTo(40);
 
@@ -116,12 +99,12 @@ describe("ThreadCameraController", () => {
     expect(frames).toHaveLength(0);
   });
 
-  it("preserves spatial continuity with a shorter reduced-motion chase", () => {
+  it("preserves spatial continuity with shorter reduced-motion navigation", () => {
     const regular = cameraHarness();
     const reduced = cameraHarness(true);
 
-    expect(regular.camera.followTo(240)).toBe("started");
+    expect(regular.camera.navigateTo(240)).toBe("started");
-    expect(reduced.camera.followTo(240)).toBe("started");
+    expect(reduced.camera.navigateTo(240)).toBe("started");
 
     regular.advance(16);
     reduced.advance(16);

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

@@ -36,7 +36,7 @@ function motionHarness(initial?: Partial<ThreadMotionGeometry>) {
     }),
     dispose: vi.fn(),
     jumpTo: vi.fn(),
-    followTo: vi.fn(() => "started" as const),
+    followTo: vi.fn(() => "settled" as const),
     isFollowing: vi.fn(() => cameraFollowing),
     navigateTo: vi.fn(() => {
       cameraFollowing = true;
@@ -120,7 +120,7 @@ describe("ThreadMotionCoordinator", () => {
     });
   });
 
-  it("retargets repeated output growth without restarting camera ownership", () => {
+  it("pins repeated output growth on each authoritative geometry frame", () => {
     const {
       camera,
       coordinator,
@@ -465,6 +465,83 @@ describe("ThreadMotionCoordinator", () => {
     expect(coordinator.snapshot().mode).toBe("browsing-history");
   });
 
+  it("retargets smooth latest navigation before resuming automatic follow", () => {
+    const {
+      camera,
+      coordinator,
+      advanceFrame,
+      setCameraFollowing,
+      setGeometry,
+    } = motionHarness();
+    coordinator.updateTurn({
+      id: "turn-1",
+      promptId: "prompt-1",
+      hasOutput: true,
+    });
+    advanceFrame();
+    coordinator.takeUserControl();
+    camera.followTo.mockClear();
+    camera.navigateTo.mockClear();
+
+    expect(coordinator.navigateLatestTo(1_400)).toBe("started");
+    expect(coordinator.snapshot().mode).toBe("navigating-latest");
+
+    setGeometry({
+      scrollTop: 900,
+      scrollHeight: 2_100,
+    });
+    coordinator.invalidateGeometry();
+    advanceFrame();
+
+    expect(camera.navigateTo.mock.calls).toEqual([[1_400], [1_600]]);
+    expect(camera.followTo).not.toHaveBeenCalled();
+
+    setGeometry({ scrollTop: 1_600 });
+    setCameraFollowing(false);
+    expect(coordinator.observeScroll(true)).toBe("navigation");
+    expect(coordinator.snapshot().mode).toBe("follow-output");
+
+    advanceFrame();
+    expect(camera.followTo).toHaveBeenCalledWith(1_600);
+  });
+
+  it("keeps latest navigation through completion and final layout growth", () => {
+    const {
+      camera,
+      coordinator,
+      advanceFrame,
+      setCameraFollowing,
+      setGeometry,
+    } = motionHarness();
+    coordinator.updateTurn({
+      id: "turn-1",
+      promptId: "prompt-1",
+      hasOutput: true,
+    });
+    advanceFrame();
+    coordinator.takeUserControl();
+    coordinator.navigateLatestTo(1_400);
+    camera.followTo.mockClear();
+    camera.navigateTo.mockClear();
+
+    coordinator.completeTurn();
+    setGeometry({
+      scrollTop: 1_000,
+      scrollHeight: 2_300,
+    });
+    coordinator.invalidateGeometry();
+    advanceFrame();
+
+    expect(camera.navigateTo).toHaveBeenCalledWith(1_800);
+    expect(camera.followTo).not.toHaveBeenCalled();
+    expect(coordinator.snapshot().mode).toBe("navigating-latest");
+
+    setGeometry({ scrollTop: 1_800 });
+    setCameraFollowing(false);
+    expect(coordinator.observeScroll(true)).toBe("navigation");
+    expect(coordinator.snapshot().mode).toBe("idle");
+  });
+
   it("pins a waiting prompt to the exact lower boundary across all layout changes", () => {
     const {
       camera,

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

@@ -11,6 +11,7 @@ import {
   windowMessages,
 } from "@/components/thread/ThreadViewport";
 import { ThreadCameraController } from "@/components/thread/thread-camera";
+import { ThreadMotionCoordinator } from "@/components/thread/thread-motion";
 import type { UIMessage } from "@/lib/types";
 
 const messages: UIMessage[] = [
@@ -819,6 +820,33 @@ describe("ThreadViewport", () => {
     }
   });
 
+  it("gives smooth scroll-to-bottom navigation ownership of the latest target", () => {
+    const navigateLatestTo = vi.spyOn(
+      ThreadMotionCoordinator.prototype,
+      "navigateLatestTo",
+    ).mockReturnValue("started");
+    const { container } = render(
+      <ThreadViewport
+        messages={messages}
+        isStreaming
+        composer={<div>composer</div>}
+      />,
+    );
+    const scroller = getScroller(container);
+    Object.defineProperties(scroller, {
+      scrollHeight: { configurable: true, value: 2_400 },
+      clientHeight: { configurable: true, value: 600 },
+      scrollTop: { configurable: true, writable: true, value: 0 },
+    });
+
+    act(() => {
+      dispatchUserScroll(scroller);
+    });
+    fireEvent.click(screen.getByRole("button", { name: "Scroll to bottom" }));
+
+    expect(navigateLatestTo).toHaveBeenCalledWith(1_800);
+  });
+
   it("pins the waiting boundary across composer and grid-track growth", async () => {
     const resizeObserver = stubResizeObserver();
     const jumpTo = vi.spyOn(ThreadCameraController.prototype, "jumpTo");