feat(ui): unify list virtualization on @tanstack/react-virtual and polish scroll behavior

- Migrate sidebar session groups, git changes panel, virtualized code
  blocks, and JSON tree viewer from virtua to @tanstack/react-virtual;
  virtua remains only inside the Pierre diff viewer integration
- Sidebar: preserve scroll position when virtualization enables
  mid-session (enable only once the ancestor scroll element is resolved,
  seed initial offset from its live scrollTop, render plain rows for the
  single pre-paint frame); disable native scroll anchoring on the
  sessions scroller; keep row spacing identical between plain and
  virtualized modes; absolute row positioning so variable-height rows
  cannot drift past the container
- Chat: expand tool/thinking blocks downward by only adjusting scroll
  for rows growing above the viewport; raise the desktop history-load
  lead to 1.5 viewports so prepends land above the visible area
- Git changes: compute the prefetch window from the first visible row,
  skipping overscan rows above the viewport
- Sidebar rows: make the whole highlighted row area clickable, guarded
  against double-firing from interactive children

packages/ui/src/components/chat/hooks/useChatTimelineController.ts

@@ -59,7 +59,17 @@ export interface UseChatTimelineControllerResult {
 }
 
 const TURN_MODEL_CACHE_MAX = 30
-const HISTORY_SCROLL_THRESHOLD = 200
+// Desktop load-older lead distance. Trigger well before the top: the fetch
+// then completes and the prepend lands ABOVE the viewport, where key-anchored
+// compensation is exact and invisible. A short lead (the old 200px) let the
+// user reach the estimated-height region near the absolute top mid-fetch,
+// where the post-insert restore is least precise and reads as a small jump.
+const HISTORY_SCROLL_THRESHOLD_MIN_PX = 1200
+const HISTORY_SCROLL_VIEWPORT_FACTOR = 1.5
+const resolveHistoryScrollThreshold = (clientHeight: number): number => Math.max(
+    HISTORY_SCROLL_THRESHOLD_MIN_PX,
+    clientHeight * HISTORY_SCROLL_VIEWPORT_FACTOR,
+)
 const VSCODE_TURN_MODEL_CACHE_MAX = 4
 const VSCODE_TURN_MODEL_CACHE_MAX_MESSAGES = 30
 const MOBILE_TURN_MODEL_CACHE_MAX = 4
@@ -692,7 +702,7 @@ export const useChatTimelineController = ({
         const container = scrollRef.current;
         if (!container) return;
         if (isPinnedRef.current) return;
-        if (container.scrollTop >= HISTORY_SCROLL_THRESHOLD) return;
+        if (container.scrollTop >= resolveHistoryScrollThreshold(container.clientHeight)) return;
         if (!historySignalsRef.current.canLoadEarlier) return;
         if (isLoadingOlderRef.current || pendingRevealWorkRef.current) return;