Fix edge cases around non-thread relations to thread roots and read receipts (#3607)

* Ensure non-thread relations to a thread root are actually in both timelines

* Make thread in sendReceipt & sendReadReceipt explicit rather than guessing it

* Apply suggestions from code review

* Fix Room::eventShouldLiveIn to better match Synapse to diverging ideas of notifications

* Update read receipt sending behaviour to align with Synapse

* Fix tests

* Fix thread rel type

src/client.ts

@@ -5000,8 +5000,15 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         });
 
         if (!unthreaded) {
-            // 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;
+            // XXX: the spec currently says a threaded read receipt can be sent for the root of a thread,
+            // but in practice this isn't possible and the spec needs updating.
+            const isThread =
+                !!event.threadRootId &&
+                // A thread cannot be just a thread root and a thread root can only be read in the main timeline
+                !event.isThreadRoot &&
+                // Similarly non-thread relations upon the thread root (reactions, edits) should also be for the main timeline.
+                event.isRelation() &&
+                (event.isRelation(THREAD_RELATION_TYPE.name) || event.relationEventId !== event.threadRootId);
             body = {
                 ...body,
                 // Only thread replies should define a specific thread. Thread roots can only be read in the main timeline.