fix: invalidate lazy-loaded older history when the live window reshapes

Address review (PR pingdotgg#3510, Cursor Bugbot):

- Reconnect / checkpoint revert can re-snapshot or filter the live activity
  window, but the prepended `olderActivities` weren't invalidated — leaving gaps
  or showing reverted history. Reset the lazy-load state when the live window's
  oldest activity id changes (it's stable while activities only append, so this
  doesn't fire during a normal turn), in addition to on thread switch. Web + mobile.
- Web only deduped a new older page against already-loaded older pages, not the
  live window (mobile already did both). Dedup against both so a boundary overlap
  can't produce duplicate ids / React keys.

Not changed — the "unsequenced cursor hides sequenced history" finding is a false
positive: NULL-sequence (legacy) rows always sort oldest in the window/cursor
ordering, so the oldest-loaded row is only unsequenced once every sequenced row
is already loaded; the unsequenced cursor can never strand sequenced rows.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: olafura

apps/mobile/src/state/use-thread-composer-state.ts

@@ -113,14 +113,19 @@ export function useThreadComposerState() {
     : null;
   const activityRequestKeyRef = useRef(activityRequestKey);
   activityRequestKeyRef.current = activityRequestKey;
+
+  const liveActivities = selectedThreadDetail?.activities ?? EMPTY_ACTIVITIES;
+  // The live window's oldest activity is stable while new activities only
+  // append; it changes when the window is re-snapshotted (reconnect) or rows are
+  // removed (checkpoint revert), either of which can make prepended older pages
+  // stale or gappy — so reset the lazy-load state when it (or the thread) changes.
+  const liveOldestActivityId = liveActivities[0]?.id ?? null;
   useEffect(() => {
     setOlderActivities([]);
     setOlderLoaded(false);
     setOlderHasMore(false);
     setLoadingOlderActivities(false);
-  }, [activityRequestKey]);
-
-  const liveActivities = selectedThreadDetail?.activities ?? EMPTY_ACTIVITIES;
+  }, [activityRequestKey, liveOldestActivityId]);
   const mergedActivities = useMemo(
     () =>
       olderActivities.length > 0 ? [...olderActivities, ...liveActivities] : liveActivities,

apps/web/src/components/ChatView.tsx

@@ -1997,14 +1997,19 @@ function ChatViewContent(props: ChatViewProps) {
     : null;
   const activeThreadActivityRequestKeyRef = useRef(activeThreadActivityRequestKey);
   activeThreadActivityRequestKeyRef.current = activeThreadActivityRequestKey;
+  const liveThreadActivities = activeThread?.activities ?? EMPTY_ACTIVITIES;
+  // The live window's oldest activity is stable while new activities only
+  // append; it changes when the window is re-snapshotted (reconnect) or rows are
+  // removed (checkpoint revert), either of which can make prepended older pages
+  // stale or gappy — so reset the lazy-load state when it (or the thread) changes.
+  const liveOldestActivityId = liveThreadActivities[0]?.id ?? null;
   useEffect(() => {
     setOlderActivities([]);
     setOlderLoaded(false);
     setOlderHasMore(false);
     setLoadingOlderActivities(false);
-  }, [activeThreadActivityRequestKey]);
+  }, [activeThreadActivityRequestKey, liveOldestActivityId]);
 
-  const liveThreadActivities = activeThread?.activities ?? EMPTY_ACTIVITIES;
   const threadActivities = useMemo(
     () =>
       olderActivities.length > 0
@@ -2053,7 +2058,13 @@ function ChatViewContent(props: ChatViewProps) {
         }
         const page = result.value;
         setOlderActivities((prev) => {
+          // Dedup against both already-loaded older pages and the live window so
+          // an overlap at the window boundary can't leave duplicate ids (which
+          // would break timeline keys and work-log derivation).
           const seen = new Set(prev.map((activity) => activity.id));
+          for (const activity of liveThreadActivities) {
+            seen.add(activity.id);
+          }
           const fresh = page.activities.filter((activity) => !seen.has(activity.id));
           return [...fresh, ...prev];
         });
@@ -2073,6 +2084,7 @@ function ChatViewContent(props: ChatViewProps) {
     activeThreadActivityRequestKey,
     hasMoreOlderActivities,
     threadActivities,
+    liveThreadActivities,
     loadThreadActivities,
   ]);