Attribute reader scroll input through the observed-top ledger

ChatView's bottom-follow recognized only wheel gestures as reader input,
so touch panning, scrollbar dragging, and keyboard paging could not leave
the bottom of a streaming transcript. Replace the wheel listener with
device-agnostic attribution: a scroll position deviating from the
observed-top ledger of the last delivered or written scrollTop is reader
input. Adds keyboard-paging and touch-style fling e2e scenarios (red under
the old implementation) and the bilingual Agent Note triplet.

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

@@ -372,9 +372,6 @@ export function ChatView({
   const [atBottom, setAtBottom] = useState(true)
   /** Last position delivered or written on the main thread. */
   const observedTopRef = useRef(0)
-  /** Pre-input position for the current wheel gesture. */
-  const wheelStartRef = useRef<number | null>(null)
-  const wheelEpochRef = useRef(0)
   /** Paging anchor: semantic row/position at click, updated by reader scrolls
    * while the request is pending and restored after the prepend lands. */
   const anchorRef = useRef<PagingAnchor | null>(null)
@@ -394,8 +391,6 @@ export function ChatView({
   const followSig = `${openState}:${firstSeq}:${lastKey}:${nodes.length}:${running ? 1 : 0}:${runningCalls.length}:${lastSteeringId ?? ''}`
 
   const toBottom = (el: HTMLElement): void => {
-    wheelStartRef.current = null
-    wheelEpochRef.current += 1
     anchorRef.current = null
     el.scrollTop = el.scrollHeight
     observedTopRef.current = el.scrollTop
@@ -472,17 +467,19 @@ export function ChatView({
     /* v8 ignore next -- ref-null guard: the handler only fires while mounted. */
     if (local === null) return
     const el = scrollerOf(local)
-    // Only wheel input may make raw scroll geometry change follow ownership.
-    // Browser clamping and delayed programmatic scroll events otherwise have
-    // the same event shape and must preserve the current ownership state.
+    // Only reader input may make raw scroll geometry change follow ownership:
+    // a delivered position that deviates from the observed-top ledger (every
+    // programmatic write records itself there synchronously). This covers
+    // wheel, touch, scrollbar, and keyboard alike without naming devices.
+    // Browser shrink-clamps land exactly on the floor min and delayed
+    // programmatic deliveries land on the ledger itself, so both preserve
+    // the current ownership state.
     const floor = Math.max(0, el.scrollHeight - el.clientHeight)
-    const wheelStart = wheelStartRef.current
-    const movedByWheel = wheelStart !== null
-      && Math.abs(el.scrollTop - Math.min(wheelStart, floor)) > 0.5
-    const isAtBottom = movedByWheel
+    const movedByReader = Math.abs(el.scrollTop - Math.min(observedTopRef.current, floor)) > 0.5
+    const isAtBottom = movedByReader
       ? floor - el.scrollTop <= FOLLOW_THRESHOLD + 1
       : atBottomRef.current
-    if (!movedByWheel && isAtBottom) {
+    if (!movedByReader && isAtBottom) {
       toBottom(el)
       return
     }
@@ -501,34 +498,18 @@ export function ChatView({
     observedTopRef.current = el.scrollTop
   }
 
-  // Bind scroll and the wheel provenance needed to distinguish reader input
-  // from layout-driven scrolls on the resolved scrollport once per mount.
+  // Bind the scroll listener on the resolved scrollport once per mount;
+  // reader-input attribution rides the observed-top ledger, not per-device
+  // input listeners.
   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() }
-    const onWheel = (event: WheelEvent): void => {
-      if (event.ctrlKey || event.deltaY === 0) return
-      const startTop = observedTopRef.current
-      const floor = Math.max(0, el.scrollHeight - el.clientHeight)
-      const canMove = event.deltaY < 0 ? startTop > 1 : startTop < floor - 1
-      if (!canMove) return
-      wheelStartRef.current = startTop
-      const epoch = ++wheelEpochRef.current
-      requestAnimationFrame(() => {
-        requestAnimationFrame(() => {
-          if (wheelEpochRef.current === epoch) wheelStartRef.current = null
-        })
-      })
-    }
     el.addEventListener('scroll', onScroll, { passive: true })
-    el.addEventListener('wheel', onWheel, { capture: true, passive: true })
     return () => {
-      wheelStartRef.current = null
       el.removeEventListener('scroll', onScroll)
-      el.removeEventListener('wheel', onWheel, true)
     }
   }, [])
 

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

@@ -158,9 +158,9 @@ function makeHarness(init?: Partial<ConversationSnapshot>) {
   return { set, ChatView, props, openDetails, openFile, loadOlder, inspectCall, chatScroll, forkAt, setSelection }
 }
 
-/** Simulate reader input before the browser delivers the host scroll event. */
+/** Simulate reader input (any device): a delivered position that deviates
+ * from the observed-top ledger of programmatic writes. */
 function readerScroll(element: HTMLElement, top: number): void {
-  fireEvent.wheel(element, { deltaY: top < element.scrollTop ? -120 : 120 })
   element.scrollTop = top
   fireEvent.scroll(element)
 }
@@ -939,7 +939,7 @@ describe('ChatView', () => {
     expect(view.queryByLabelText('回到底部')).toBeNull()
   })
 
-  it('keeps following when a delayed clamp scroll arrives after layout regrows', () => {
+  it('keeps following when a stream-finalization shrink clamp delivers its scroll', () => {
     const h = makeHarness({ nodes: [user(1, 'q'), assistant(2, 'a')] })
     const view = render(<h.ChatView {...h.props} />)
     const scroller = view.container.querySelector('[class*="scroll"]') as HTMLDivElement
@@ -947,12 +947,12 @@ describe('ChatView', () => {
     scroller.scrollTop = 700
     fireEvent.scroll(scroller)
 
-    // The wheel cannot move farther down. A stream-finalization shrink clamps
-    // the old position, then reflow grows the layout before scroll delivery.
-    fireEvent.wheel(scroller, { deltaY: 120 })
-    metrics.setLayout(1_040, 500)
+    // Stream finalization shrinks the column: the browser clamps the pinned
+    // position onto the new floor and delivers a scroll event. The clamp
+    // lands exactly on the ledger's floor min, so it is not reader input.
+    metrics.setLayout(800, 700)
     fireEvent.scroll(scroller)
-    expect(scroller.scrollTop).toBe(740)
+    expect(scroller.scrollTop).toBe(500)
     expect(view.queryByLabelText('回到底部')).toBeNull()
     expect(h.chatScroll.read()).toBeNull()
 
@@ -961,7 +961,7 @@ describe('ChatView', () => {
     expect(scroller.scrollTop).toBe(900)
   })
 
-  it('uses the last delivered top when compositor scrolling precedes passive wheel delivery', () => {
+  it('uses the last delivered top when compositor scrolling precedes scroll delivery', () => {
     const h = makeHarness({ nodes: [user(1, 'q'), assistant(2, 'a')] })
     const view = render(<h.ChatView {...h.props} />)
     const scroller = view.container.querySelector('[class*="scroll"]') as HTMLDivElement
@@ -969,8 +969,10 @@ describe('ChatView', () => {
     scroller.scrollTop = 700
     fireEvent.scroll(scroller)
 
+    // Chromium advances compositor geometry before delivering the event:
+    // attribution must compare against the observed-top ledger, never a
+    // baseline sampled from already-moved raw geometry.
     scroller.scrollTop = 500
-    fireEvent.wheel(scroller, { deltaY: -200 })
     fireEvent.scroll(scroller)
     expect(view.getByLabelText('回到底部')).toBeTruthy()
   })