fix(threads): review-wave fixes — decryption re-render, receipt dedupe, chip perf

Two-reviewer audit of the thread stack; confirmed findings fixed:
- ThreadTimeline: wrap encrypted events in EncryptedContent so a live-arriving
  E2EE reply re-renders when its key decrypts (decryption emits neither
  RoomEvent.Timeline nor ThreadEvent.Update — previously stuck at "Unable to
  decrypt").
- ThreadPanel: mark-read deduped on the latest event id (RoomEvent.Timeline
  re-emits per backfilled event/edit/reaction; previously up to N receipt POSTs
  per panel open) + rejection handled with retry.
- RoomTimeline: ThreadSummary chips now mount only for events carrying thread
  data (each chip holds a room-level listener; one per rendered message would
  blow the SDK's 100-listener emitter cap) with a single room-level
  ThreadEvent.New tick for new-thread liveness.
- useThreadPendingEvents: keep a sent reply visible through the /send-response→
  /sync window (was flashing out of the pending strip before landing).
- ThreadTimeline: reseed the window on RoomEvent.TimelineReset (gappy sync left
  a detached timeline).

Documented-acceptable (reviewer-noted): thread typing shows as room typing (no
per-thread typing in the spec; Element matches), thread panel + members drawer
can be open together, scheduled-send is thread-unaware but unreachable there.

Gates: tsc clean, eslint 0 errors, build OK, 616/617 tests (1 IDB skip).

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

src/app/features/room/RoomTimeline.tsx

@@ -18,9 +18,11 @@ import {
   IContent,
   MatrixClient,
   MatrixEvent,
+  RelationType,
   Room,
   RoomEvent,
   RoomEventHandlerMap,
+  ThreadEvent,
 } from 'matrix-js-sdk';
 import { HTMLReactParserOptions } from 'html-react-parser';
 import classNames from 'classnames';
@@ -477,6 +479,19 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
 
   const setReplyDraft = useSetAtom(roomIdToReplyDraftAtomFamily(room.roomId));
   const setActiveThreadId = useSetAtom(roomIdToActiveThreadIdAtomFamily(room.roomId));
+  // Thread summary chips only mount for events that already carry thread data
+  // (perf: a chip subscribes room-level listeners, so mounting one per rendered
+  // message would exceed the SDK's emitter cap). This single room-level
+  // ThreadEvent.New subscription re-renders the timeline once when a brand-new
+  // thread appears, so the root's chip shows up without unrelated activity.
+  const [, setThreadNewTick] = useState(0);
+  useEffect(() => {
+    const handleThreadNew = () => setThreadNewTick((c) => c + 1);
+    room.on(ThreadEvent.New, handleThreadNew);
+    return () => {
+      room.removeListener(ThreadEvent.New, handleThreadNew);
+    };
+  }, [room]);
   const powerLevels = usePowerLevelsContext();
   const creators = useRoomCreators(room);
 
@@ -1136,9 +1151,11 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
                     onReactionToggle={handleReactionToggle}
                   />
                 )}
-                {(!threadRootId || threadRootId === mEventId) && (
-                  <ThreadSummary rootEvent={mEvent} room={room} onOpen={setActiveThreadId} />
-                )}
+                {(!threadRootId || threadRootId === mEventId) &&
+                  (mEvent.getThread() !== undefined ||
+                    mEvent.getServerAggregatedRelation(RelationType.Thread) !== undefined) && (
+                    <ThreadSummary rootEvent={mEvent} room={room} onOpen={setActiveThreadId} />
+                  )}
               </>
             }
             hideReadReceipts={hideActivity}
@@ -1227,9 +1244,11 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
                     onReactionToggle={handleReactionToggle}
                   />
                 )}
-                {(!threadRootId || threadRootId === mEventId) && (
-                  <ThreadSummary rootEvent={mEvent} room={room} onOpen={setActiveThreadId} />
-                )}
+                {(!threadRootId || threadRootId === mEventId) &&
+                  (mEvent.getThread() !== undefined ||
+                    mEvent.getServerAggregatedRelation(RelationType.Thread) !== undefined) && (
+                    <ThreadSummary rootEvent={mEvent} room={room} onOpen={setActiveThreadId} />
+                  )}
               </>
             }
             hideReadReceipts={hideActivity}