feat(FN-3884): anchor QuickChatFAB and chat messages to latest on layout sh

Merges FN-3884 chat scroll anchoring — ChatView now re-scrolls to the latest message when reopened or when activation transitions occur, with a bounded bottom-anchor helper preventing layout-shift overshoots. QuickChatFAB was updated to wire the new anchoring behavior, and both components gained com

Fusion-Task-Id: FN-3884

.changeset/fn-3884-chat-scroll-latest.md

@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix chat thread bottom anchoring when reopening sessions.
+
+Quick Chat and Chat now scroll to the latest message every time they are reopened, even when markdown/images/tool details render after the initial paint.

packages/dashboard/app/components/ChatView.tsx

@@ -785,7 +785,7 @@ export function ChatView({ projectId, addToast }: ChatViewProps) {
 
   const messagesEndRef = useRef<HTMLDivElement>(null);
   const isUserScrollingRef = useRef(false);
-  const lastScrolledSessionIdRef = useRef<string | null>(null);
+  const lastAnchoredSessionStateRef = useRef<{ sessionId: string; loaded: boolean; hasMessages: boolean } | null>(null);
   const hideSkillMenuTimeoutRef = useRef<number | null>(null);
   const messagesContainerRef = useRef<HTMLDivElement>(null);
   const inputRef = useRef<HTMLTextAreaElement>(null);
@@ -895,31 +895,76 @@ export function ChatView({ projectId, addToast }: ChatViewProps) {
     isUserScrollingRef.current = !atBottom;
   }, []);
 
+  const anchorToBottom = useCallback((container: HTMLElement) => {
+    if (!container.isConnected) return;
+
+    let frame = 0;
+    let stableFrames = 0;
+    let lastScrollHeight = -1;
+    const maxFrames = 6;
+
+    const writeBottom = () => {
+      if (!container.isConnected) return;
+
+      container.scrollTop = container.scrollHeight;
+      if (container.scrollHeight === lastScrollHeight) {
+        stableFrames += 1;
+      } else {
+        stableFrames = 0;
+        lastScrollHeight = container.scrollHeight;
+      }
+
+      frame += 1;
+      if (frame >= maxFrames || stableFrames >= 2) {
+        setIsUserScrolling(false);
+        isUserScrollingRef.current = false;
+        return;
+      }
+
+      window.requestAnimationFrame(writeBottom);
+    };
+
+    writeBottom();
+  }, []);
+
   const scrollToBottom = useCallback(() => {
     const messagesContainer = messagesContainerRef.current;
     if (!messagesContainer) return;
-    messagesContainer.scrollTop = messagesContainer.scrollHeight;
-    setIsUserScrolling(false);
-    isUserScrollingRef.current = false;
-  }, []);
+    anchorToBottom(messagesContainer);
+  }, [anchorToBottom]);
 
   useLayoutEffect(() => {
     const sessionId = activeSession?.id ?? null;
     if (!sessionId) {
-      lastScrolledSessionIdRef.current = null;
+      lastAnchoredSessionStateRef.current = null;
+      return;
+    }
+
+    const nextState = {
+      sessionId,
+      loaded: !messagesLoading,
+      hasMessages: messages.length > 0,
+    };
+    const previousState = lastAnchoredSessionStateRef.current;
+    const isSessionChanged = previousState?.sessionId !== sessionId;
+    const finishedLoading =
+      previousState?.sessionId === sessionId && !previousState.loaded && nextState.loaded;
+    const firstMessagesArrived =
+      previousState?.sessionId === sessionId && !previousState.hasMessages && nextState.hasMessages;
+
+    const shouldAnchor = previousState === null || isSessionChanged || finishedLoading || firstMessagesArrived;
+    if (!shouldAnchor) {
       return;
     }
-    if (messages.length === 0) return;
-    if (lastScrolledSessionIdRef.current === sessionId) return;
 
     const messagesContainer = messagesContainerRef.current;
-    if (!messagesContainer) return;
+    if (!messagesContainer) {
+      return;
+    }
 
-    messagesContainer.scrollTop = messagesContainer.scrollHeight;
-    setIsUserScrolling(false);
-    isUserScrollingRef.current = false;
-    lastScrolledSessionIdRef.current = sessionId;
-  }, [activeSession?.id, messages.length]);
+    anchorToBottom(messagesContainer);
+    lastAnchoredSessionStateRef.current = nextState;
+  }, [activeSession?.id, messages.length, messagesLoading, anchorToBottom]);
 
   // Scroll thread container to bottom on new messages or streaming when user is near live tail.
   // Avoid Element.scrollIntoView() here because on mobile Safari it can

packages/dashboard/app/components/QuickChatFAB.tsx

@@ -981,7 +981,7 @@ export function QuickChatFAB({
   // slides down on top of it.
   const suppressVvShrinkRef = useRef(false);
   const isUserScrollingRef = useRef(false);
-  const lastScrolledOpenSessionKeyRef = useRef<string | null>(null);
+  const previousOpenStateRef = useRef<{ isOpen: boolean; sessionId: string | null }>({ isOpen: false, sessionId: null });
 
   // Pin the document at the top while the panel is open on mobile.
   // Otherwise iOS can leave window.scrollY > 0 (e.g. after the keyboard
@@ -1417,33 +1417,64 @@ export function QuickChatFAB({
     isUserScrollingRef.current = !atBottom;
   }, []);
 
+  const anchorToBottom = useCallback((container: HTMLElement) => {
+    if (!container.isConnected) return;
+
+    let frame = 0;
+    let stableFrames = 0;
+    let lastScrollHeight = -1;
+    const maxFrames = 6;
+
+    const writeBottom = () => {
+      if (!container.isConnected) return;
+
+      container.scrollTop = container.scrollHeight;
+      if (container.scrollHeight === lastScrollHeight) {
+        stableFrames += 1;
+      } else {
+        stableFrames = 0;
+        lastScrollHeight = container.scrollHeight;
+      }
+
+      frame += 1;
+      if (frame >= maxFrames || stableFrames >= 2) {
+        setIsUserScrolling(false);
+        isUserScrollingRef.current = false;
+        return;
+      }
+
+      window.requestAnimationFrame(writeBottom);
+    };
+
+    writeBottom();
+  }, []);
+
   const scrollToBottom = useCallback(() => {
     const messagesEl = messagesRef.current;
     if (!messagesEl) return;
-    messagesEl.scrollTop = messagesEl.scrollHeight;
-    setIsUserScrolling(false);
-    isUserScrollingRef.current = false;
-  }, []);
+    anchorToBottom(messagesEl);
+  }, [anchorToBottom]);
 
   useLayoutEffect(() => {
     const sessionId = activeSession?.id ?? null;
+    const previousState = previousOpenStateRef.current;
+    previousOpenStateRef.current = { isOpen, sessionId };
+
     if (!isOpen || !sessionId) {
-      lastScrolledOpenSessionKeyRef.current = null;
       return;
     }
-    if (messages.length === 0) return;
 
-    const openSessionKey = `${isOpen}:${sessionId}`;
-    if (lastScrolledOpenSessionKeyRef.current === openSessionKey) return;
+    const openingNow = !previousState.isOpen && isOpen;
+    const sessionChangedWhileOpen = previousState.isOpen && previousState.sessionId !== sessionId;
+    if (!openingNow && !sessionChangedWhileOpen) {
+      return;
+    }
 
     const messagesEl = messagesRef.current;
     if (!messagesEl) return;
 
-    messagesEl.scrollTop = messagesEl.scrollHeight;
-    setIsUserScrolling(false);
-    isUserScrollingRef.current = false;
-    lastScrolledOpenSessionKeyRef.current = openSessionKey;
-  }, [isOpen, activeSession?.id, messages.length]);
+    anchorToBottom(messagesEl);
+  }, [isOpen, activeSession?.id, anchorToBottom]);
 
   // Auto-scroll messages when user is near the live tail.
   useEffect(() => {

packages/dashboard/app/components/__tests__/ChatView.test.tsx

@@ -3212,7 +3212,7 @@ describe("ChatView mobile behavior", () => {
     }
   });
 
-  it("snaps to bottom when opening a session with loaded messages", async () => {
+  it("FN-3884: snaps to bottom when opening a session with loaded messages", async () => {
     const restoreMatchMedia = mockDesktopViewport();
     try {
       setupMockChat({ activeSession: activeSessionFixture, messages: [] });
@@ -3243,7 +3243,81 @@ describe("ChatView mobile behavior", () => {
     }
   });
 
-  it("snaps to bottom when switching active session id", async () => {
+  it("FN-3884: re-anchors when messagesLoading transitions to loaded with messages", async () => {
+    const restoreMatchMedia = mockDesktopViewport();
+    try {
+      setupMockChat({ activeSession: activeSessionFixture, messages: [], messagesLoading: true });
+      const { rerender } = render(<ChatView projectId="proj-123" addToast={vi.fn()} />);
+
+      const messagesContainer = document.querySelector(".chat-messages") as HTMLDivElement;
+      let scrollTopValue = 0;
+      Object.defineProperty(messagesContainer, "scrollHeight", { configurable: true, get: () => 980 });
+      Object.defineProperty(messagesContainer, "scrollTop", {
+        configurable: true,
+        get: () => scrollTopValue,
+        set: (value: number) => {
+          scrollTopValue = value;
+        },
+      });
+
+      setupMockChat({
+        activeSession: activeSessionFixture,
+        messagesLoading: false,
+        messages: [{ id: "msg-001", sessionId: "session-001", role: "assistant", content: "Loaded", createdAt: "2026-04-08T00:00:00.000Z" }],
+      });
+      rerender(<ChatView projectId="proj-123" addToast={vi.fn()} />);
+
+      await waitFor(() => {
+        expect(scrollTopValue).toBe(980);
+      });
+    } finally {
+      restoreMatchMedia.mockRestore();
+    }
+  });
+
+  it("FN-3884: retries bottom anchor while container height keeps growing", async () => {
+    const restoreMatchMedia = mockDesktopViewport();
+    const originalRaf = window.requestAnimationFrame;
+    const rafQueue: FrameRequestCallback[] = [];
+    window.requestAnimationFrame = vi.fn((cb: FrameRequestCallback) => {
+      rafQueue.push(cb);
+      return rafQueue.length;
+    });
+
+    try {
+      setupMockChat({
+        activeSession: activeSessionFixture,
+        messages: [{ id: "msg-001", sessionId: "session-001", role: "assistant", content: "One", createdAt: "2026-04-08T00:00:00.000Z" }],
+      });
+
+      render(<ChatView projectId="proj-123" addToast={vi.fn()} />);
+
+      const messagesContainer = document.querySelector(".chat-messages") as HTMLDivElement;
+      let scrollTopValue = 0;
+      let scrollHeightValue = 600;
+      Object.defineProperty(messagesContainer, "scrollHeight", { configurable: true, get: () => scrollHeightValue });
+      Object.defineProperty(messagesContainer, "scrollTop", {
+        configurable: true,
+        get: () => scrollTopValue,
+        set: (value: number) => {
+          scrollTopValue = value;
+        },
+      });
+
+      scrollHeightValue = 900;
+      while (rafQueue.length > 0) {
+        const cb = rafQueue.shift();
+        cb?.(performance.now());
+      }
+
+      expect(scrollTopValue).toBe(900);
+    } finally {
+      window.requestAnimationFrame = originalRaf;
+      restoreMatchMedia.mockRestore();
+    }
+  });
+
+  it("FN-3884: snaps to bottom when switching active session id", async () => {
     const restoreMatchMedia = mockDesktopViewport();
     try {
       setupMockChat({
@@ -3279,7 +3353,7 @@ describe("ChatView mobile behavior", () => {
     }
   });
 
-  it("does not clobber scroll position on same-session history pagination", async () => {
+  it("FN-3884: does not yank when user scrolled up on same-session updates", async () => {
     const restoreMatchMedia = mockDesktopViewport();
     try {
       setupMockChat({

packages/dashboard/app/components/__tests__/QuickChatFAB.test.tsx

@@ -470,10 +470,12 @@ describe("QuickChatFAB session-first UX", () => {
 
     fireEvent.click(screen.getByTestId("quick-chat-jump-to-latest"));
     expect(scrollTopValue).toBe(1200);
-    expect(screen.queryByTestId("quick-chat-jump-to-latest")).toBeNull();
+    await waitFor(() => {
+      expect(screen.queryByTestId("quick-chat-jump-to-latest")).toBeNull();
+    });
   });
 
-  it("snaps to bottom when first opened", async () => {
+  it("FN-3884: snaps to bottom when first opened", async () => {
     const deferredMessages = createDeferredPromise<{
       messages: Array<{ id: string; sessionId: string; role: "assistant"; content: string; createdAt: string }>;
     }>();
@@ -512,7 +514,82 @@ describe("QuickChatFAB session-first UX", () => {
     });
   });
 
-  it("snaps to bottom when switching sessions while open", async () => {
+  it("FN-3884: reopens same session and scrolls to latest again", async () => {
+    mockFetchChatMessages.mockResolvedValue({
+      messages: [{ id: "msg-1", sessionId: "session-model", role: "assistant", content: "hello", createdAt: new Date().toISOString() }],
+    });
+
+    render(<QuickChatFAB addToast={vi.fn()} projectId="proj-1" />);
+    const fab = screen.getByTestId("quick-chat-fab");
+    fireEvent.click(fab);
+
+    let messages = await screen.findByTestId("quick-chat-messages");
+    let scrollTopValue = 0;
+    const installScrollDescriptors = (target: HTMLElement) => {
+      Object.defineProperty(target, "scrollHeight", { configurable: true, get: () => 1000 });
+      Object.defineProperty(target, "scrollTop", {
+        configurable: true,
+        get: () => scrollTopValue,
+        set: (value: number) => {
+          scrollTopValue = value;
+        },
+      });
+    };
+    installScrollDescriptors(messages);
+
+    fireEvent.click(screen.getByTestId("quick-chat-close"));
+    scrollTopValue = 0;
+    fireEvent.click(fab);
+
+    messages = await screen.findByTestId("quick-chat-messages");
+    installScrollDescriptors(messages);
+
+    await waitFor(() => {
+      expect(scrollTopValue).toBe(1000);
+    });
+  });
+
+  it("FN-3884: retries anchor when quick chat thread height grows after open", async () => {
+    const originalRaf = window.requestAnimationFrame;
+    const rafQueue: FrameRequestCallback[] = [];
+    window.requestAnimationFrame = vi.fn((cb: FrameRequestCallback) => {
+      rafQueue.push(cb);
+      return rafQueue.length;
+    });
+
+    mockFetchChatMessages.mockResolvedValue({
+      messages: [{ id: "msg-1", sessionId: "session-model", role: "assistant", content: "hello", createdAt: new Date().toISOString() }],
+    });
+
+    try {
+      render(<QuickChatFAB addToast={vi.fn()} projectId="proj-1" />);
+      fireEvent.click(screen.getByTestId("quick-chat-fab"));
+
+      const messages = await screen.findByTestId("quick-chat-messages");
+      let scrollTopValue = 0;
+      let scrollHeightValue = 500;
+      Object.defineProperty(messages, "scrollHeight", { configurable: true, get: () => scrollHeightValue });
+      Object.defineProperty(messages, "scrollTop", {
+        configurable: true,
+        get: () => scrollTopValue,
+        set: (value: number) => {
+          scrollTopValue = value;
+        },
+      });
+
+      scrollHeightValue = 900;
+      while (rafQueue.length > 0) {
+        const cb = rafQueue.shift();
+        cb?.(performance.now());
+      }
+
+      expect(scrollTopValue).toBe(900);
+    } finally {
+      window.requestAnimationFrame = originalRaf;
+    }
+  });
+
+  it("FN-3884: snaps to bottom when switching sessions while open", async () => {
     mockFetchChatMessages
       .mockResolvedValueOnce({ messages: [{ id: "msg-1", sessionId: "session-model", role: "assistant", content: "A", createdAt: new Date().toISOString() }] })
       .mockResolvedValueOnce({ messages: [{ id: "msg-2", sessionId: "session-agent", role: "assistant", content: "B", createdAt: new Date().toISOString() }] });