Fix read receipt sending behaviour around thread roots (#3600)

* Fix read receipt sending behaviour around thread roots

* Update src/client.ts

Co-authored-by: Eric Eastwood <erice@element.io>

---------

Co-authored-by: Eric Eastwood <erice@element.io>
(cherry picked from commit b05f933)

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,
             };
         }