Merge main (anchored-turn chat scrolling) into perf/switch-and-scroll

Main replaced the chat timeline scroll engine while this branch was in
flight, which obsoletes two of its subareas and reshapes a third:

- Chat timeline: main's LegendList-based MessageList/ChatContainer win;
  the activation-overscan staircase targeted the removed tanstack path
  (LegendList provides adaptive rendering natively) and is dropped along
  with its test.
- Scroll shadows: main's hook-based masks stay (the virtualized list owns
  its scroll element — there is no wrapper to hand the styling to); the
  viewport-wrapper ScrollShadow rewrite, its index.css replacement, its
  test, and the call-site viewportClassName adaptations are reverted to
  main. The chat OverlayScrollbar keeps this branch's disableHorizontal.
- OverlayScrollbar: the direct-DOM rewrite lands, but binding now follows
  the live container node instead of binding once per ref object — the
  chat scroller remounts on every session switch, and a bind-once
  contract left the scrollbar attached to a dead element.
- Markdown renderer: the detached-DOM cache and warm-block fast path
  merge with main's block-commit reveal (enter cascade), streaming code
  highlighting, and gutter reservation; the per-block reconcile keeps
  both the decoration-refresh path and the reveal cascade.

packages/ui/src/stores/useUIStore.ts

@@ -674,6 +674,7 @@ interface UIStore {
   eventStreamStatus: EventStreamStatus;
   eventStreamHint: string | null;
   showReasoningTraces: boolean;
+  streamingAutoFollowEnabled: boolean;
   sessionRecapEnabled: boolean;
   sessionSuggestionEnabled: boolean;
   sessionGoalEnabled: boolean;
@@ -859,6 +860,7 @@ interface UIStore {
   setSettingsRemoteInstancesSelectedId: (instanceId: string | null) => void;
   setEventStreamStatus: (status: EventStreamStatus, hint?: string | null) => void;
   setShowReasoningTraces: (value: boolean) => void;
+  setStreamingAutoFollowEnabled: (value: boolean) => void;
   setSessionRecapEnabled: (value: boolean) => void;
   setSessionSuggestionEnabled: (value: boolean) => void;
   setSessionGoalEnabled: (value: boolean) => void;
@@ -1027,6 +1029,7 @@ export const useUIStore = create<UIStore>()(
         eventStreamStatus: 'idle',
         eventStreamHint: null,
         showReasoningTraces: true,
+        streamingAutoFollowEnabled: true,
         sessionRecapEnabled: true,
         sessionSuggestionEnabled: true,
         sessionGoalEnabled: true,
@@ -1759,6 +1762,10 @@ export const useUIStore = create<UIStore>()(
           set({ showReasoningTraces: value });
         },
 
+        setStreamingAutoFollowEnabled: (value) => {
+          set({ streamingAutoFollowEnabled: value });
+        },
+
         setSessionRecapEnabled: (value) => {
           set({ sessionRecapEnabled: value });
         },
@@ -2639,6 +2646,7 @@ export const useUIStore = create<UIStore>()(
           isSessionCreateDialogOpen: state.isSessionCreateDialogOpen,
           // Note: isSettingsDialogOpen intentionally NOT persisted
           showReasoningTraces: state.showReasoningTraces,
+          streamingAutoFollowEnabled: state.streamingAutoFollowEnabled,
           sessionRecapEnabled: state.sessionRecapEnabled,
           sessionSuggestionEnabled: state.sessionSuggestionEnabled,
           sessionGoalEnabled: state.sessionGoalEnabled,