Merge pull request #40433 from xxxigm/fix/desktop-chat-autoscroll

fix(desktop): stop chat transcript from jumping/flickering while reading (#37549)

apps/desktop/src/app/session/hooks/use-session-state-cache.ts

@@ -9,6 +9,28 @@ import { $busy, $messages, noteSessionActivity, setSessionAttention, setSessionW
 
 import type { ClientSessionState } from '../../types'
 
+// Shallow per-message identity check. When a flush carries no transcript
+// changes, `preserveLocalAssistantErrors` returns the same message objects in
+// the same order, so reference equality per slot is enough to detect "nothing
+// to publish" and avoid a needless `$messages` churn.
+function sameMessageList(a: ChatMessage[], b: ChatMessage[]): boolean {
+  if (a === b) {
+    return true
+  }
+
+  if (a.length !== b.length) {
+    return false
+  }
+
+  for (let index = 0; index < a.length; index += 1) {
+    if (a[index] !== b[index]) {
+      return false
+    }
+  }
+
+  return true
+}
+
 interface SessionStateCacheOptions {
   activeSessionId: string | null
   busyRef: MutableRefObject<boolean>
@@ -88,7 +110,20 @@ export function useSessionStateCache({
       return
     }
 
-    setMessages(preserveLocalAssistantErrors(pending.state.messages, $messages.get()))
+    // `preserveLocalAssistantErrors` always returns a fresh array, so publishing
+    // it unconditionally puts a new `$messages` reference on the store every
+    // flush — including the periodic `session.info` heartbeats that don't touch
+    // the transcript. That churns ChatView → runtimeMessageRepository → the
+    // assistant-ui runtime → the virtualizer, which re-measures and visibly
+    // jerks the scroll position while the user is reading. Skip the publish when
+    // the merged result is content-identical to what's already on screen.
+    const currentMessages = $messages.get()
+    const nextMessages = preserveLocalAssistantErrors(pending.state.messages, currentMessages)
+
+    if (!sameMessageList(nextMessages, currentMessages)) {
+      setMessages(nextMessages)
+    }
+
     setBusy(pending.state.busy)
     setMutableRef(busyRef, pending.state.busy)
     setAwaitingResponse(pending.state.awaitingResponse)

apps/desktop/src/components/assistant-ui/thread-virtualizer.tsx

@@ -264,8 +264,27 @@ function useThreadScrollAnchor({
       return
     }
 
+    // Already parked at the bottom: writing `scrollTop` is a no-op and the
+    // browser fires NO scroll event, so arming the programmatic gate here would
+    // leave it permanently set. Repeated pins (streaming heartbeats, the
+    // post-run lock loop) then accumulate the gate, and the next genuine user
+    // scroll-up is misread as one of our programmatic scrolls — re-arming
+    // sticky-bottom and yanking the viewport back down. Refresh trackers, bail.
+    const distFromBottom = el.scrollHeight - (el.scrollTop + el.clientHeight)
+
+    if (distFromBottom <= AT_BOTTOM_THRESHOLD) {
+      lastTopRef.current = el.scrollTop
+      lastHeightRef.current = el.scrollHeight
+      lastClientHeightRef.current = el.clientHeight
+
+      return
+    }
+
     // Hold the disarm gate across the scroll event the next line will fire.
-    programmaticScrollPendingRef.current += 1
+    // Set to 1 rather than incrementing: coalesced writes within a frame fire a
+    // single scroll event, so a counter > 1 can never drain and would swallow a
+    // later real user scroll.
+    programmaticScrollPendingRef.current = 1
     scrollElementToBottom(el)
     lastTopRef.current = el.scrollTop
     lastHeightRef.current = el.scrollHeight