fix: prevent scroll position jump when AI completes response while user reading

web-app/src/routes/threads/$threadId.tsx

@@ -37,6 +37,8 @@ function ThreadDetail() {
   const [isAtBottom, setIsAtBottom] = useState(true)
   const [hasScrollbar, setHasScrollbar] = useState(false)
   const lastScrollTopRef = useRef(0)
+  const userIntendedPositionRef = useRef<number | null>(null)
+  const wasStreamingRef = useRef(false)
   const { currentThreadId, setCurrentThreadId } = useThreads()
   const { setCurrentAssistant, assistants } = useAssistant()
   const { setMessages, deleteMessage } = useMessages()
@@ -112,6 +114,8 @@ function ThreadDetail() {
       scrollToBottom()
       setIsAtBottom(true)
       setIsUserScrolling(false)
+      userIntendedPositionRef.current = null
+      wasStreamingRef.current = false
       checkScrollState()
       return
     }
@@ -123,11 +127,39 @@ function ThreadDetail() {
     scrollToBottom()
     setIsAtBottom(true)
     setIsUserScrolling(false)
+    userIntendedPositionRef.current = null
+    wasStreamingRef.current = false
     checkScrollState()
   }, [threadId])
 
   // Single useEffect for all auto-scrolling logic
   useEffect(() => {
+    // Track streaming state changes
+    const isCurrentlyStreaming = !!streamingContent
+    const justFinishedStreaming = wasStreamingRef.current && !isCurrentlyStreaming
+    wasStreamingRef.current = isCurrentlyStreaming
+
+    // If streaming just finished and user had an intended position, restore it
+    if (justFinishedStreaming && userIntendedPositionRef.current !== null) {
+      // Small delay to ensure DOM has updated
+      setTimeout(() => {
+        if (scrollContainerRef.current && userIntendedPositionRef.current !== null) {
+          scrollContainerRef.current.scrollTo({
+            top: userIntendedPositionRef.current,
+            behavior: 'smooth'
+          })
+          userIntendedPositionRef.current = null
+          setIsUserScrolling(false)
+        }
+      }, 100)
+      return
+    }
+
+    // Clear intended position when streaming starts fresh
+    if (isCurrentlyStreaming && !wasStreamingRef.current) {
+      userIntendedPositionRef.current = null
+    }
+
     // Only auto-scroll when the user is not actively scrolling
     // AND either at the bottom OR there's streaming content
     if (!isUserScrolling && (streamingContent || isAtBottom) && messagesCount) {
@@ -163,6 +195,11 @@ function ThreadDetail() {
     // Detect if this is a user-initiated scroll
     if (Math.abs(scrollTop - lastScrollTopRef.current) > 10) {
       setIsUserScrolling(!isBottom)
+
+      // If user scrolls during streaming and moves away from bottom, record their intended position
+      if (streamingContent && !isBottom) {
+        userIntendedPositionRef.current = scrollTop
+      }
     }
     setIsAtBottom(isBottom)
     setHasScrollbar(hasScroll)
@@ -180,6 +217,11 @@ function ThreadDetail() {
     // Detect if this is a user-initiated scroll
     if (Math.abs(scrollTop - lastScrollTopRef.current) > 10) {
       setIsUserScrolling(!isBottom)
+
+      // If user scrolls during streaming and moves away from bottom, record their intended position
+      if (streamingContent && !isBottom) {
+        userIntendedPositionRef.current = scrollTop
+      }
     }
     setIsAtBottom(isBottom)
     setHasScrollbar(hasScroll)