Extra insurance that we don't mix events in the wrong timelines

spec/unit/event-timeline-set.spec.ts

@@ -55,6 +55,23 @@ describe('EventTimelineSet', () => {
         });
     };
 
+    const mkThreadResponse = (root: MatrixEvent) => utils.mkEvent({
+        event: true,
+        type: EventType.RoomMessage,
+        user: userA,
+        room: roomId,
+        content: {
+            "body": "Thread response :: " + Math.random(),
+            "m.relates_to": {
+                "event_id": root.getId(),
+                "m.in_reply_to": {
+                    "event_id": root.getId(),
+                },
+                "rel_type": "m.thread",
+            },
+        },
+    }, room.client);
+
     beforeEach(() => {
         client = utils.mock(MatrixClient, 'MatrixClient');
         client.reEmitter = utils.mock(ReEmitter, 'ReEmitter');
@@ -117,6 +134,13 @@ describe('EventTimelineSet', () => {
     });
 
     describe('addEventToTimeline', () => {
+        let thread: Thread;
+
+        beforeEach(() => {
+            (client.supportsExperimentalThreads as jest.Mock).mockReturnValue(true);
+            thread = new Thread("!thread_id:server", messageEvent, { room, client });
+        });
+
         it("Adds event to timeline", () => {
             const liveTimeline = eventTimelineSet.getLiveTimeline();
             expect(liveTimeline.getEvents().length).toStrictEqual(0);
@@ -144,6 +168,41 @@ describe('EventTimelineSet', () => {
                 );
             }).not.toThrow();
         });
+
+        it("should not add an event to a timeline that does not belong to the timelineSet", () => {
+            const eventTimelineSet2 = new EventTimelineSet(room);
+            const liveTimeline2 = eventTimelineSet2.getLiveTimeline();
+            expect(liveTimeline2.getEvents().length).toStrictEqual(0);
+
+            expect(() => {
+                eventTimelineSet.addEventToTimeline(messageEvent, liveTimeline2, {
+                    toStartOfTimeline: true,
+                });
+            }).toThrowError();
+        });
+
+        it("should not add a threaded reply to the main room timeline", () => {
+            const liveTimeline = eventTimelineSet.getLiveTimeline();
+            expect(liveTimeline.getEvents().length).toStrictEqual(0);
+
+            const threadedReplyEvent = mkThreadResponse(messageEvent);
+
+            eventTimelineSet.addEventToTimeline(threadedReplyEvent, liveTimeline, {
+                toStartOfTimeline: true,
+            });
+            expect(liveTimeline.getEvents().length).toStrictEqual(0);
+        });
+
+        it("should not add a normal message to the timelineSet representing a thread", () => {
+            const eventTimelineSetForThread = new EventTimelineSet(room, {}, client, thread);
+            const liveTimeline = eventTimelineSetForThread.getLiveTimeline();
+            expect(liveTimeline.getEvents().length).toStrictEqual(0);
+
+            eventTimelineSetForThread.addEventToTimeline(messageEvent, liveTimeline, {
+                toStartOfTimeline: true,
+            });
+            expect(liveTimeline.getEvents().length).toStrictEqual(0);
+        });
     });
 
     describe('aggregateRelations', () => {

src/models/event-timeline-set.ts

@@ -702,6 +702,19 @@ export class EventTimelineSet extends TypedEventEmitter<EmittedEvents, EventTime
             );
         }
 
+        if (timeline.getTimelineSet() !== this) {
+            throw new Error(`EventTimelineSet.addEventToTimeline: Timeline=${timeline.toString()} does not belong " + 
+                "in timelineSet(threadId=${this.thread?.id})`);
+        }
+
+        // Make sure events don't get mixed in timelines they shouldn't be in
+        // (e.g. a threaded message should not be in the main timeline).
+        if (!this.canContain(event)) {
+            logger.warn(`EventTimelineSet.addEventToTimeline: Ignoring event=${event.getId()} that does not belong " + 
+                "in timeline=${timeline.toString()} timelineSet(threadId=${this.thread?.id})`);
+            return;
+        }
+
         const eventId = event.getId()!;
         timeline.addEvent(event, {
             toStartOfTimeline,

src/models/event.ts

@@ -107,7 +107,7 @@ export interface IEventRelation {
     event_id?: string;
     is_falling_back?: boolean;
     "m.in_reply_to"?: {
-        event_id: string;
+        event_id?: string;
     };
     key?: string;
 }

src/models/room.ts

@@ -1862,7 +1862,7 @@ export class Room extends ReadReceipt<RoomEmittedEvents, RoomEventHandlerMap> {
         shouldLiveInThread: boolean;
         threadId?: string;
     } {
-        if (!this.client.supportsExperimentalThreads()) {
+        if (!this.client?.supportsExperimentalThreads()) {
             return {
                 shouldLiveInRoom: true,
                 shouldLiveInThread: false,