fix: prevent Windows chat content jitter (#603)

Windows WebView2 can report one-pixel content resize oscillations while a thinking or tool response is pinned to the bottom. That fed repeated bottom-scroll corrections back into the message list and made otherwise static chat content visibly move.

Ignore sub-2px content resize deltas before following live content growth, so real message growth still stays pinned without turning tiny layout jitter into scroll movement.

Constraint: Issue #603 reproduces on some Windows WebView2/display paths but not consistently across macOS or all Windows machines
Rejected: Disable bottom-following during thinking | would regress active response visibility when content actually grows
Confidence: medium
Scope-risk: narrow
Directive: Keep ResizeObserver follow behavior tolerant of sub-pixel or one-pixel WebView2 height jitter
Tested: cd desktop && bun run test -- --run src/components/chat/MessageList.test.tsx src/components/chat/virtualHeightCache.test.ts
Tested: cd desktop && bun run lint
Tested: bun run check:desktop
Tested: Browser smoke at http://127.0.0.1:17999 loaded Claude Code Companion with no frontend error logs
Not-tested: Live Windows WebView2 runtime retest on reporter machine
Related: #603

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

@@ -2053,6 +2053,94 @@ describe('MessageList nested tool calls', () => {
     expect(screen.queryByRole('button', { name: 'Latest' })).toBeNull()
   })
 
+  it('ignores one-pixel content resize jitter while pinned to active thinking output', async () => {
+    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: 'thinking-1',
+          messages: [
+            {
+              id: 'user-1',
+              type: 'user_text',
+              content: '触发 Windows WebView2 细微重排',
+              timestamp: 1,
+            },
+            {
+              id: 'thinking-1',
+              type: 'thinking',
+              content: '正在分析一个静态问题',
+              timestamp: 2,
+            },
+          ],
+        }),
+      },
+    })
+
+    const { container } = render(<MessageList />)
+    const scroller = container.querySelector('.overflow-y-auto') as HTMLDivElement
+    let scrollTop = 600
+    let scrollTopWriteCount = 0
+    let scrollHeight = 1000
+    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) => {
+        scrollTopWriteCount += 1
+        scrollTop = value
+      },
+    })
+
+    await waitFor(() => {
+      expect(resizeCallback).not.toBeNull()
+    })
+    await waitForProgrammaticScrollReset()
+
+    const makeResizeEntry = (height: number) => ([{
+      contentRect: { height },
+    } as ResizeObserverEntry])
+
+    act(() => {
+      resizeCallback?.(makeResizeEntry(400), {} as ResizeObserver)
+    })
+    expect(scrollTop).toBe(600)
+
+    scrollTopWriteCount = 0
+    act(() => {
+      resizeCallback?.(makeResizeEntry(401), {} as ResizeObserver)
+    })
+    act(() => {
+      resizeCallback?.(makeResizeEntry(400), {} as ResizeObserver)
+    })
+
+    expect(scrollTopWriteCount).toBe(0)
+    expect(scrollTop).toBe(600)
+
+    scrollHeight = 1040
+    act(() => {
+      resizeCallback?.(makeResizeEntry(420), {} as ResizeObserver)
+    })
+
+    expect(scrollTop).toBe(640)
+  })
+
   it('does not pull a completed session back to the bottom when content resizes', async () => {
     const scrollIntoView = vi.fn()
     Object.defineProperty(HTMLElement.prototype, 'scrollIntoView', {

desktop/src/components/chat/MessageList.tsx

@@ -795,6 +795,9 @@ const VIRTUAL_OVERSCAN_PX = 1200
 const VIRTUAL_DEFAULT_VIEWPORT_HEIGHT = 720
 const VIRTUAL_MIN_ITEM_HEIGHT = 48
 const VIRTUAL_MAX_ITEM_HEIGHT = 24_000
+// Windows WebView2 can report 1px oscillations for live chat content; don't
+// convert those into bottom-scroll corrections.
+const CONTENT_RESIZE_FOLLOW_MIN_DELTA_PX = 2
 const EMPTY_MESSAGES: UIMessage[] = []
 const CHAT_SCROLL_AREA_CLASS = [
   'chat-scroll-area',
@@ -1241,6 +1244,7 @@ export function MessageList({ sessionId, compact = false }: MessageListProps = {
   const pendingMeasuredHeightsRef = useRef(false)
   const measureFlushFrameRef = useRef<number | null>(null)
   const lastAutoScrollAtRef = useRef(0)
+  const lastContentResizeFollowHeightRef = useRef<number | null>(null)
   const shouldAutoScrollRef = useRef(true)
   const isProgrammaticScrollingRef = useRef(false)
   const ignoreProgrammaticScrollUntilRef = useRef(0)
@@ -1397,6 +1401,7 @@ export function MessageList({ sessionId, compact = false }: MessageListProps = {
         ? getMetricsForSession(resolvedSessionId)
         : new Map<string, VirtualRenderItemMetric>()
       pendingMeasuredHeightsRef.current = false
+      lastContentResizeFollowHeightRef.current = null
       if (measureFlushFrameRef.current !== null) {
         cancelAnimationFrame(measureFlushFrameRef.current)
         measureFlushFrameRef.current = null
@@ -1474,7 +1479,18 @@ export function MessageList({ sessionId, compact = false }: MessageListProps = {
     const content = scrollContentRef.current
     if (!content || typeof ResizeObserver === 'undefined') return
 
-    const observer = new ResizeObserver(() => {
+    const observer = new ResizeObserver((entries) => {
+      const nextHeight = entries[0]?.contentRect.height
+      if (typeof nextHeight === 'number' && Number.isFinite(nextHeight)) {
+        const previousFollowHeight = lastContentResizeFollowHeightRef.current
+        if (
+          previousFollowHeight !== null &&
+          Math.abs(nextHeight - previousFollowHeight) < CONTENT_RESIZE_FOLLOW_MIN_DELTA_PX
+        ) {
+          return
+        }
+        lastContentResizeFollowHeightRef.current = nextHeight
+      }
       if (!shouldFollowContentResize) return
       if (!shouldAutoScrollRef.current) return
       scrollToBottom('auto')