Copy relations to thread root in the thread timeline (#2012)

src/client.ts

@@ -8569,6 +8569,10 @@ export class MatrixClient extends EventEmitter {
     }
 
     public partitionThreadedEvents(events: MatrixEvent[]): [MatrixEvent[], MatrixEvent[]] {
+        // Indices to the events array, for readibility
+        const ROOM = 0;
+        const THREAD = 1;
+        const threadRoots = new Set<string>();
         if (this.supportsExperimentalThreads) {
             return events.reduce((memo, event: MatrixEvent) => {
                 const room = this.getRoom(event.getRoomId());
@@ -8576,14 +8580,25 @@ export class MatrixClient extends EventEmitter {
                 // - It's a reply in thread event
                 // - It's related to a reply in thread event
                 let shouldLiveInThreadTimeline = event.isThreadRelation;
-                if (!shouldLiveInThreadTimeline) {
+                if (shouldLiveInThreadTimeline) {
+                    threadRoots.add(event.relationEventId);
+                } else {
                     const parentEventId = event.parentEventId;
                     const parentEvent = room?.findEventById(parentEventId) || events.find((mxEv: MatrixEvent) => {
                         return mxEv.getId() === parentEventId;
                     });
                     shouldLiveInThreadTimeline = parentEvent?.isThreadRelation;
+
+                    // Copy all the reactions and annotations to the root event
+                    // to the thread timeline. They will end up living in both
+                    // timelines at the same time
+                    const targetingThreadRoot = parentEvent?.isThreadRoot || threadRoots.has(event.relationEventId);
+                    if (targetingThreadRoot && !event.isThreadRelation && event.relationEventId) {
+                        memo[THREAD].push(event);
+                    }
                 }
-                memo[shouldLiveInThreadTimeline ? 1 : 0].push(event);
+                const targetTimeline = shouldLiveInThreadTimeline ? THREAD : ROOM;
+                memo[targetTimeline].push(event);
                 return memo;
             }, [[], []]);
         } else {

src/models/event.ts

@@ -438,14 +438,14 @@ export class MatrixEvent extends EventEmitter {
      * @experimental
      */
     public get isThreadRoot(): boolean {
+        // TODO, change the inner working of this getter for it to use the
+        // bundled relationship return on the event, view MSC3440
         const thread = this.getThread();
         return thread?.id === this.getId();
     }
 
     public get parentEventId(): string {
-        const relations = this.getWireContent()["m.relates_to"];
-        return relations?.["m.in_reply_to"]?.["event_id"]
-            || relations?.event_id;
+        return this.replyEventId || this.relationEventId;
     }
 
     public get replyEventId(): string {
@@ -453,6 +453,12 @@ export class MatrixEvent extends EventEmitter {
         return relations?.["m.in_reply_to"]?.["event_id"];
     }
 
+    public get relationEventId(): string {
+        return this.getWireContent()
+            ?.["m.relates_to"]
+            ?.event_id;
+    }
+
     /**
      * Get the previous event content JSON. This will only return something for
      * state events which exist in the timeline.

src/models/room.ts

@@ -1270,8 +1270,11 @@ export class Room extends EventEmitter {
         if (!event) {
             return null;
         }
+
         if (event.isThreadRelation) {
             return this.threads.get(event.threadRootId);
+        } else if (event.isThreadRoot) {
+            return this.threads.get(event.getId());
         } else {
             const parentEvent = this.findEventById(event.parentEventId);
             return this.findThreadForEvent(parentEvent);