Match MSC behaviour for threads when disabled (thread-aware mode) (#8476)

src/utils/Reply.ts

@@ -152,8 +152,17 @@ export function makeReplyMixIn(ev?: MatrixEvent): IEventRelation {
         },
     };
 
-    if (SettingsStore.getValue("feature_thread") && ev.threadRootId) {
+    if (ev.threadRootId) {
+        if (SettingsStore.getValue("feature_thread")) {
             mixin.is_falling_back = false;
+        } else {
+            // Clients that do not offer a threading UI should behave as follows when replying, for best interaction
+            // with those that do. They should set the m.in_reply_to part as usual, and then add on
+            // "rel_type": "m.thread" and "event_id": "$thread_root", copying $thread_root from the replied-to event.
+            const relation = ev.getRelation();
+            mixin.rel_type = relation.rel_type;
+            mixin.event_id = relation.event_id;
+        }
     }
 
     return mixin;