perf(chat): throttle scroll geometry sampling

packages/client/ui-chat/src/client/chat/ChatView.tsx

@@ -17,6 +17,7 @@ import { formatRunDuration } from './message-chrome.ts'
 import css from './ChatView.module.css'
 
 const FOLLOW_THRESHOLD = 24
+const SCROLL_SAMPLE_INTERVAL_MS = 500
 
 /** Active column host when present; otherwise the view-local scroller. */
 function scrollerOf(from: HTMLElement): HTMLElement {
@@ -307,6 +308,8 @@ export function ChatView({
   // restores it and normalizes a floor-clamped position back to following.
   const [atBottom, setAtBottom] = useState(() => chatScroll.read() === null)
   const atBottomRef = useRef(atBottom)
+  const scrollSamplePendingRef = useRef(false)
+  const [, setScrollSampleTick] = useState(0)
   const [activeTurn, setActiveTurn] = useState<number | null>(
     () => turnNavigationItems.at(-1)?.turn ?? null,
   )
@@ -343,6 +346,7 @@ export function ChatView({
   const followSig = `${openState}:${firstSeq}:${lastKey}:${order.length}:${running ? 1 : 0}:${lastSteeringId ?? ''}:${lastSubmissionId ?? ''}`
 
   const syncActiveTurn = useCallback((): void => {
+    if (scrollSamplePendingRef.current) return
     const local = listRef.current
     const first = turnNavigationItems[0]
     if (local === null || first === undefined) {
@@ -461,6 +465,7 @@ export function ChatView({
   }
 
   useLayoutEffect(() => {
+    if (scrollSamplePendingRef.current) return
     const local = listRef.current
     /* v8 ignore next -- ref-null guard: React attaches the ref before layout effects run. */
     if (local === null) return
@@ -574,18 +579,33 @@ export function ChatView({
     scheduleActiveTurn()
   }
 
-  // Bind the scroll listener on the resolved scrollport once per mount;
-  // reader-input attribution rides the observed-top ledger, not per-device
-  // input listeners.
+  // Raw scroll events only schedule work. Geometry is sampled at most once
+  // per interval, with scrollend providing the final sample for a short burst.
   useEffect(() => {
     const local = listRef.current
     /* v8 ignore next -- ref-null guard: effect runs after the list node commits. */
     if (local === null) return
     const el = scrollerOf(local)
-    const onScroll = (): void => { onScrollRef.current() }
+    let sampleTimer: number | undefined
+    const sample = (): void => {
+      if (!scrollSamplePendingRef.current) return
+      scrollSamplePendingRef.current = false
+      if (sampleTimer !== undefined) window.clearTimeout(sampleTimer)
+      sampleTimer = undefined
+      onScrollRef.current()
+      setScrollSampleTick(tick => tick + 1)
+    }
+    const onScroll = (): void => {
+      scrollSamplePendingRef.current = true
+      sampleTimer ??= window.setTimeout(sample, SCROLL_SAMPLE_INTERVAL_MS)
+    }
     el.addEventListener('scroll', onScroll, { passive: true })
+    el.addEventListener('scrollend', sample, { passive: true })
     return () => {
       el.removeEventListener('scroll', onScroll)
+      el.removeEventListener('scrollend', sample)
+      if (sampleTimer !== undefined) window.clearTimeout(sampleTimer)
+      scrollSamplePendingRef.current = false
     }
   }, [])
 
@@ -593,6 +613,7 @@ export function ChatView({
   // initializer a function initial value would need never exists.
   const followRef = useRef<(() => void) | null>(null)
   followRef.current = () => {
+    if (scrollSamplePendingRef.current) return
     const local = listRef.current
     if (local !== null && atBottomRef.current) {
       const el = scrollerOf(local)

packages/client/ui-chat/tests/chat-view.client.spec.tsx

@@ -439,6 +439,7 @@ function makeHarness(
 function readerScroll(element: HTMLElement, top: number): void {
   element.scrollTop = top
   fireEvent.scroll(element)
+  fireEvent(element, new Event('scrollend'))
 }
 
 function turnProcessControl(container: HTMLElement): HTMLButtonElement | null {
@@ -2315,6 +2316,7 @@ describe('ChatView', () => {
     // lands exactly on the ledger's floor min, so it is not reader input.
     metrics.setLayout(800, 700)
     fireEvent.scroll(scroller)
+    fireEvent(scroller, new Event('scrollend'))
     expect(scroller.scrollTop).toBe(500)
     expect(view.queryByLabelText('回到底部')).toBeNull()
     expect(h.chatScroll.read()).toBeNull()
@@ -2337,6 +2339,7 @@ describe('ChatView', () => {
     // baseline sampled from already-moved raw geometry.
     scroller.scrollTop = 500
     fireEvent.scroll(scroller)
+    fireEvent(scroller, new Event('scrollend'))
     expect(view.getByLabelText('回到底部')).toBeTruthy()
   })
 
@@ -2359,6 +2362,7 @@ describe('ChatView', () => {
     Object.defineProperty(scroller, 'clientHeight', { value: 300, writable: true })
     scroller.scrollTop = 700
     fireEvent.scroll(scroller)
+    fireEvent(scroller, new Event('scrollend'))
     Object.defineProperty(scroller, 'scrollHeight', { value: 1_200, writable: true })
     act(() => { notify?.() })
     expect(scroller.scrollTop).toBe(1_200)