Don't re-fetch thread root if we already have it (#4088)

The root event of a thread used to arrive with the pagination request, but this was unspecced and so got changed to simply fetch the root event. In many (almost all) cases this shouldn't be necessary because the thread should already have its root event: re-use it if it's already there. This is only in pagination, so there's no reason to believe that the root event would have changed and needs to be re-fetched.

This removes a number of duplicate calls to the /event/ endpoint from the tests.

spec/integ/matrix-client-event-timeline.spec.ts

@@ -607,11 +607,6 @@ describe("MatrixClient event timelines", function () {
             await client.stopClient(); // we don't need the client to be syncing at this time
             const room = client.getRoom(roomId)!;
 
-            httpBackend
-                .when("GET", "/rooms/!foo%3Abar/event/" + encodeURIComponent(THREAD_ROOT.event_id!))
-                .respond(200, function () {
-                    return THREAD_ROOT;
-                });
             httpBackend
                 .when("GET", "/rooms/!foo%3Abar/event/" + encodeURIComponent(THREAD_ROOT.event_id!))
                 .respond(200, function () {
@@ -1545,9 +1540,7 @@ describe("MatrixClient event timelines", function () {
                 expect(timelineSets).not.toBeNull();
                 respondToThreads(threadsResponse);
                 respondToThreads(threadsResponse);
-                respondToEvent(THREAD_ROOT);
                 respondToEvent(THREAD2_ROOT);
-                respondToThread(THREAD_ROOT, [THREAD_REPLY]);
                 respondToThread(THREAD2_ROOT, [THREAD2_REPLY]);
                 await flushHttp(room.fetchRoomThreads());
                 const threadIds = room.getThreads().map((thread) => thread.id);
@@ -1566,7 +1559,6 @@ describe("MatrixClient event timelines", function () {
                 thread.initialEventsFetched = true;
                 const prom = emitPromise(room, ThreadEvent.NewReply);
                 respondToEvent(THREAD_ROOT_UPDATED);
-                respondToEvent(THREAD2_ROOT);
                 await room.addLiveEvents([THREAD_REPLY2]);
                 await httpBackend.flushAllExpected();
                 await prom;
@@ -1643,7 +1635,7 @@ describe("MatrixClient event timelines", function () {
                             ...THREAD_ROOT.unsigned!["m.relations"],
                             "io.element.thread": {
                                 ...THREAD_ROOT.unsigned!["m.relations"]!["io.element.thread"],
-                                count: 2,
+                                count: 1,
                                 latest_event: THREAD_REPLY,
                             },
                         },
@@ -1692,7 +1684,6 @@ describe("MatrixClient event timelines", function () {
                 thread.initialEventsFetched = true;
                 const prom = emitPromise(room, ThreadEvent.Update);
                 respondToEvent(THREAD_ROOT_UPDATED);
-                respondToEvent(THREAD2_ROOT);
                 await room.addLiveEvents([THREAD_REPLY_REACTION]);
                 await httpBackend.flushAllExpected();
                 await prom;
@@ -2007,11 +1998,6 @@ describe("MatrixClient event timelines", function () {
                     },
                 },
             });
-            httpBackend
-                .when("GET", "/rooms/!foo%3Abar/event/" + encodeURIComponent(THREAD_ROOT.event_id!))
-                .respond(200, function () {
-                    return THREAD_ROOT;
-                });
             httpBackend
                 .when("GET", "/rooms/!foo%3Abar/event/" + encodeURIComponent(THREAD_ROOT.event_id!))
                 .respond(200, function () {

src/client.ts

@@ -6565,8 +6565,10 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
                     const timelineSet = eventTimeline.getTimelineSet();
                     timelineSet.addEventsToTimeline(matrixEvents, backwards, eventTimeline, newToken ?? null);
                     if (!newToken && backwards) {
-                        const originalEvent = await this.fetchRoomEvent(eventTimeline.getRoomId() ?? "", thread.id);
-                        timelineSet.addEventsToTimeline([mapper(originalEvent)], true, eventTimeline, null);
+                        const originalEvent =
+                            thread.rootEvent ??
+                            mapper(await this.fetchRoomEvent(eventTimeline.getRoomId() ?? "", thread.id));
+                        timelineSet.addEventsToTimeline([originalEvent], true, eventTimeline, null);
                     }
                     this.processAggregatedTimelineEvents(timelineSet.room, matrixEvents);