Determine if event belongs to thread on jumping to event

This allows us to jump to any event in any thread, even if neither the thread nor the event are part of any timeline yet

src/utils/EventUtils.ts

@@ -238,8 +238,11 @@ export async function fetchInitialEvent(
     ) {
         const threadId = initialEvent.threadRootId;
         const room = client.getRoom(roomId);
+        const mapper = client.getEventMapper();
+        const rootEvent = room.findEventById(threadId)
+            ?? mapper(await client.fetchRoomEvent(roomId, threadId));
         try {
-            room.createThread(threadId, room.findEventById(threadId), [initialEvent], true);
+            room.createThread(threadId, rootEvent, [initialEvent], true);
         } catch (e) {
             logger.warn("Could not find root event: " + threadId);
         }