Emit relations created when target event added later

This changes the "relations created" event to ensure it is properly emitted even
if the target event is added to the timeline after the relation event. There was
perhaps always a risk of this happening in the past, but is seems more likely to
bite now with delayed decryption.

Part of https://github.com/vector-im/element-web/issues/17461

spec/unit/relations.spec.ts

@@ -14,6 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import { EventTimelineSet } from "../../src/models/event-timeline-set";
 import { MatrixEvent } from "../../src/models/event";
 import { Relations } from "../../src/models/relations";
 
@@ -70,4 +71,63 @@ describe("Relations", function() {
             expect(events.size).toEqual(1);
         }
     });
+
+    it("should emit created regardless of ordering", async function () {
+        const targetEvent = new MatrixEvent({
+            "sender": "@bob:example.com",
+            "type": "m.room.message",
+            "event_id": "$2s4yYpEkVQrPglSCSqB_m6E8vDhWsg0yFNyOJdVIb_o",
+            "room_id": "!pzVjCQSoQPpXQeHpmK:example.com",
+            "content": {},
+        });
+        const relationEvent = new MatrixEvent({
+            "sender": "@bob:example.com",
+            "type": "m.reaction",
+            "event_id": "$cZ1biX33ENJqIm00ks0W_hgiO_6CHrsAc3ZQrnLeNTw",
+            "room_id": "!pzVjCQSoQPpXQeHpmK:example.com",
+            "content": {
+                "m.relates_to": {
+                    "event_id": "$2s4yYpEkVQrPglSCSqB_m6E8vDhWsg0yFNyOJdVIb_o",
+                    "key": "👍️",
+                    "rel_type": "m.annotation",
+                },
+            },
+        });
+
+        // Stub the room
+        const room = {
+            getPendingEvent() { return null; },
+            getUnfilteredTimelineSet() { return null; },
+        };
+
+        // Add the target event first, then the relation event
+        {
+            const relationsCreated = new Promise(resolve => {
+                targetEvent.once("Event.relationsCreated", resolve);
+            })
+
+            const timelineSet = new EventTimelineSet(room, {
+                unstableClientRelationAggregation: true,
+            });
+            timelineSet.addLiveEvent(targetEvent);
+            timelineSet.addLiveEvent(relationEvent);
+
+            await relationsCreated;
+        }
+
+        // Add the relation event first, then the target event
+        {
+            const relationsCreated = new Promise(resolve => {
+                targetEvent.once("Event.relationsCreated", resolve);
+            })
+
+            const timelineSet = new EventTimelineSet(room, {
+                unstableClientRelationAggregation: true,
+            });
+            timelineSet.addLiveEvent(relationEvent);
+            timelineSet.addLiveEvent(targetEvent);
+
+            await relationsCreated;
+        }
+    });
 });