fix: preserve per-session scroll position on session switch (#1083)

* fix: restore scroll position when switching chat sessions

When switching between chat sessions, scroll position now restores to
where the user left off instead of always jumping to the bottom.

- Save pixel-level scrollPosition (scrollTop/scrollHeight/clientHeight)
  in viewport store on every scroll event
- Add restoreSavedScrollPosition to timeline controller for ratio-based
  restoration (handles content size changes between visits)
- Suppress intermediate scroll events during session transition with an
  explicit flag, cleared deterministically after restore completes
- Cancel in-flight animations/follow-loops on session switch
- Preserve scrollPosition when session-ui-store rebuilds SessionMemoryState

* fix: keep streaming sessions pinned on restore

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/sync/session-ui-store.ts

@@ -819,10 +819,11 @@ export const useSessionUIStore = create<SessionUIState>()((set, get) => ({
       if (!memState || !memState.lastUserMessageAt) {
         const newMemState = new Map(viewportState.sessionMemoryState)
         newMemState.set(currentSessionId, {
-          viewportAnchor: memState?.viewportAnchor ?? 0,
-          isStreaming: memState?.isStreaming ?? false,
+          viewportAnchor: 0,
+          isStreaming: false,
           lastAccessedAt: Date.now(),
-          backgroundMessageCount: memState?.backgroundMessageCount ?? 0,
+          backgroundMessageCount: 0,
+          ...memState,
           lastUserMessageAt: Date.now(),
         })
         useViewportStore.setState({ sessionMemoryState: newMemState })

packages/ui/src/sync/viewport-store.ts

@@ -7,6 +7,12 @@ import { create } from "zustand"
 
 export type SessionMemoryState = {
   viewportAnchor: number
+  /** Last known scrollbar pixel state — saved on every scroll event. */
+  scrollPosition?: {
+    scrollTop: number
+    scrollHeight: number
+    clientHeight: number
+  }
   isStreaming: boolean
   streamStartTime?: number
   lastAccessedAt: number
@@ -27,14 +33,14 @@ export type ViewportState = {
   sessionMemoryState: Map<string, SessionMemoryState>
   isSyncing: boolean
 
-  updateViewportAnchor: (sessionId: string, anchor: number) => void
+  updateViewportAnchor: (sessionId: string, anchor: number, scrollPosition?: SessionMemoryState['scrollPosition']) => void
 }
 
 export const useViewportStore = create<ViewportState>()((set) => ({
   sessionMemoryState: new Map(),
   isSyncing: false,
 
-  updateViewportAnchor: (sessionId, anchor) =>
+  updateViewportAnchor: (sessionId, anchor, scrollPosition) =>
     set((s) => {
       const map = new Map(s.sessionMemoryState)
       const existing = map.get(sessionId) ?? {
@@ -43,7 +49,12 @@ export const useViewportStore = create<ViewportState>()((set) => ({
         lastAccessedAt: Date.now(),
         backgroundMessageCount: 0,
       }
-      map.set(sessionId, { ...existing, viewportAnchor: anchor, lastAccessedAt: Date.now() })
+      map.set(sessionId, {
+        ...existing,
+        viewportAnchor: anchor,
+        ...(scrollPosition ? { scrollPosition } : {}),
+        lastAccessedAt: Date.now(),
+      })
       return { sessionMemoryState: map }
     }),
 }))