Merge pull request #3603 from matrix-org/backport-3600-to-staging

[Backport staging] Fix read receipt sending behaviour around thread roots

src/client.ts

@@ -4992,9 +4992,11 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         });
 
         if (!unthreaded) {
-            const isThread = !!event.threadRootId;
+            // A thread cannot be just a thread root and a thread root can only be read in the main timeline
+            const isThread = !!event.threadRootId && !event.isThreadRoot;
             body = {
                 ...body,
+                // Only thread replies should define a specific thread. Thread roots can only be read in the main timeline.
                 thread_id: isThread ? event.threadRootId : MAIN_ROOM_TIMELINE,
             };
         }