fix: stop restored chats snapping to latest

Restored transcripts can still present stale running state while their final markdown and tool output continue to reflow. Resize-driven auto-follow now stays enabled only for live resizable output, so readers can scroll up after completion while active streams still remain pinned.

Constraint: Old H5 sessions can reconnect with stale non-idle UI state even when the transcript has an end_turn.
Rejected: Disable resize following entirely | active H5 streaming would regress when rendered content grows after tokens arrive.
Confidence: high
Scope-risk: narrow
Directive: Do not broaden ResizeObserver auto-follow without a restored completed-session regression test.
Tested: cd desktop && bun run test -- MessageList.test.tsx
Tested: cd desktop && bun run lint
Tested: cd desktop && bun run build
Tested: Browser smoke on session 85862799-ec6b-43d0-b3b8-6e9fd9130954 kept scrollTop at 320 after content resize.

desktop/src/components/chat/MessageList.test.tsx

@@ -1076,6 +1076,167 @@ describe('MessageList nested tool calls', () => {
     expect(screen.queryByRole('button', { name: 'Latest' })).toBeNull()
   })
 
+  it('does not pull a completed session back to the bottom when content resizes', async () => {
+    const scrollIntoView = vi.fn()
+    Object.defineProperty(HTMLElement.prototype, 'scrollIntoView', {
+      configurable: true,
+      value: scrollIntoView,
+    })
+
+    let resizeCallback: ResizeObserverCallback | null = null
+    class TestResizeObserver {
+      observe = vi.fn()
+      unobserve = vi.fn()
+      disconnect = vi.fn()
+
+      constructor(callback: ResizeObserverCallback) {
+        resizeCallback = callback
+      }
+    }
+    vi.stubGlobal('ResizeObserver', TestResizeObserver)
+
+    useChatStore.setState({
+      sessions: {
+        [ACTIVE_TAB]: makeSessionState({
+          chatState: 'idle',
+          messages: [
+            {
+              id: 'user-1',
+              type: 'user_text',
+              content: '生成一个 todo app',
+              timestamp: 1,
+            },
+            {
+              id: 'assistant-1',
+              type: 'assistant_text',
+              content: [
+                '已完成。',
+                '',
+                '```bash',
+                'cd /private/tmp/todo-app',
+                'npm run dev',
+                '```',
+              ].join('\n'),
+              timestamp: 2,
+            },
+          ],
+        }),
+      },
+    })
+
+    const { container } = render(<MessageList />)
+    const scroller = container.querySelector('.overflow-y-auto') as HTMLDivElement
+    let scrollTop = 180
+    let scrollHeight = 1400
+    Object.defineProperty(scroller, 'scrollHeight', {
+      configurable: true,
+      get: () => scrollHeight,
+    })
+    Object.defineProperty(scroller, 'clientHeight', { configurable: true, value: 400 })
+    Object.defineProperty(scroller, 'scrollTop', {
+      configurable: true,
+      get: () => scrollTop,
+      set: (value) => {
+        scrollTop = value
+      },
+    })
+
+    await waitFor(() => {
+      expect(resizeCallback).not.toBeNull()
+    })
+
+    scrollIntoView.mockClear()
+    scrollHeight = 1600
+    act(() => {
+      resizeCallback?.([], {} as ResizeObserver)
+    })
+
+    expect(scrollIntoView).not.toHaveBeenCalled()
+    expect(scrollTop).toBe(180)
+  })
+
+  it('does not pull a restored completed session back to the bottom from stale running state', async () => {
+    const scrollIntoView = vi.fn()
+    Object.defineProperty(HTMLElement.prototype, 'scrollIntoView', {
+      configurable: true,
+      value: scrollIntoView,
+    })
+
+    let resizeCallback: ResizeObserverCallback | null = null
+    class TestResizeObserver {
+      observe = vi.fn()
+      unobserve = vi.fn()
+      disconnect = vi.fn()
+
+      constructor(callback: ResizeObserverCallback) {
+        resizeCallback = callback
+      }
+    }
+    vi.stubGlobal('ResizeObserver', TestResizeObserver)
+
+    useChatStore.setState({
+      sessions: {
+        [ACTIVE_TAB]: makeSessionState({
+          chatState: 'thinking',
+          activeThinkingId: null,
+          messages: [
+            {
+              id: 'user-1',
+              type: 'user_text',
+              content: '复盘这个已完成会话',
+              timestamp: 1,
+            },
+            {
+              id: 'assistant-1',
+              type: 'assistant_text',
+              content: [
+                '这个会话已经完成。',
+                '',
+                '```tsx',
+                'export function TodoListView() {',
+                '  return <section>Done</section>',
+                '}',
+                '```',
+              ].join('\n'),
+              timestamp: 2,
+            },
+          ],
+          streamingText: '',
+        }),
+      },
+    })
+
+    const { container } = render(<MessageList />)
+    const scroller = container.querySelector('.overflow-y-auto') as HTMLDivElement
+    let scrollTop = 260
+    let scrollHeight = 1800
+    Object.defineProperty(scroller, 'scrollHeight', {
+      configurable: true,
+      get: () => scrollHeight,
+    })
+    Object.defineProperty(scroller, 'clientHeight', { configurable: true, value: 400 })
+    Object.defineProperty(scroller, 'scrollTop', {
+      configurable: true,
+      get: () => scrollTop,
+      set: (value) => {
+        scrollTop = value
+      },
+    })
+
+    await waitFor(() => {
+      expect(resizeCallback).not.toBeNull()
+    })
+
+    scrollIntoView.mockClear()
+    scrollHeight = 2100
+    act(() => {
+      resizeCallback?.([], {} as ResizeObserver)
+    })
+
+    expect(scrollIntoView).not.toHaveBeenCalled()
+    expect(scrollTop).toBe(260)
+  })
+
   it('restores a session scroll position when switching back to a tab', async () => {
     const scrollIntoView = vi.fn()
     Object.defineProperty(HTMLElement.prototype, 'scrollIntoView', {

desktop/src/components/chat/MessageList.tsx

@@ -540,6 +540,11 @@ export function MessageList({ sessionId, compact = false }: MessageListProps = {
   const streamingText = sessionState?.streamingText ?? ''
   const activeThinkingId = sessionState?.activeThinkingId ?? null
   const agentTaskNotifications = sessionState?.agentTaskNotifications ?? {}
+  const shouldFollowContentResize =
+    streamingText.trim().length > 0 ||
+    chatState === 'streaming' ||
+    chatState === 'tool_executing' ||
+    (chatState === 'thinking' && Boolean(activeThinkingId))
   const scrollContainerRef = useRef<HTMLDivElement>(null)
   const scrollContentRef = useRef<HTMLDivElement>(null)
   const shouldAutoScrollRef = useRef(true)
@@ -669,13 +674,14 @@ export function MessageList({ sessionId, compact = false }: MessageListProps = {
     if (!content || typeof ResizeObserver === 'undefined') return
 
     const observer = new ResizeObserver(() => {
+      if (!shouldFollowContentResize) return
       if (!shouldAutoScrollRef.current) return
       scrollToBottom('auto')
     })
     observer.observe(content)
 
     return () => observer.disconnect()
-  }, [scrollToBottom])
+  }, [scrollToBottom, shouldFollowContentResize])
 
   const { toolResultMap, childToolCallsByParent, renderItems } = useMemo(
     () => buildRenderModel(messages),