feat: Support fallback for threads in Event.getReplyEvent()

lib/src/event.dart

@@ -481,13 +481,35 @@ class Event extends MatrixEvent {
   Future<String?> redactEvent({String? reason, String? txid}) async =>
       await room.redactEvent(eventId, reason: reason, txid: txid);
 
-  /// Searches for the reply event in the given timeline.
+  /// Searches for the reply event in the given timeline. Also returns the
+  /// event fallback if the relationship type is `m.thread`.
+  /// https://spec.matrix.org/v1.14/client-server-api/#fallback-for-unthreaded-clients
   Future<Event?> getReplyEvent(Timeline timeline) async {
-    if (relationshipType != RelationshipTypes.reply) return null;
-    final relationshipEventId = this.relationshipEventId;
-    return relationshipEventId == null
-        ? null
-        : await timeline.getEventById(relationshipEventId);
+    switch (relationshipType) {
+      case RelationshipTypes.reply:
+        final relationshipEventId = this.relationshipEventId;
+        return relationshipEventId == null
+            ? null
+            : await timeline.getEventById(relationshipEventId);
+
+      case RelationshipTypes.thread:
+        final relationshipContent =
+            content.tryGetMap<String, Object?>('m.relates_to');
+        if (relationshipContent == null) return null;
+        final String? relationshipEventId;
+        if (relationshipContent.tryGet<bool>('is_falling_back') == true) {
+          relationshipEventId = relationshipContent
+              .tryGetMap<String, Object?>('m.in_reply_to')
+              ?.tryGet<String>('event_id');
+        } else {
+          relationshipEventId = this.relationshipEventId;
+        }
+        return relationshipEventId == null
+            ? null
+            : await timeline.getEventById(relationshipEventId);
+      default:
+        return null;
+    }
   }
 
   /// If this event is encrypted and the decryption was not successful because

test/event_test.dart

@@ -2862,5 +2862,49 @@ void main() {
         matrixEvent.toJson(),
       );
     });
+
+    test('getReplyEvent fallback', () async {
+      final event = Event.fromJson(
+        {
+          'content': {
+            'msgtype': 'text',
+            'body': 'Hello world',
+            'm.relates_to': {
+              'rel_type': 'm.thread',
+              'event_id': '\$root',
+              'm.in_reply_to': {'event_id': '\$target'},
+              'is_falling_back': true,
+            },
+          },
+          'event_id': '\$143273582443PhrSn:example.org',
+          'origin_server_ts': 1432735824653,
+          'room_id': room.id,
+          'sender': '@example:example.org',
+          'type': 'm.room.message',
+          'unsigned': {'age': 1234},
+          'redacts': 'abcd',
+          'prev_content': <String, Object?>{
+            'foo': 'bar',
+          },
+        },
+        room,
+      );
+      expect(event.relationshipType, RelationshipTypes.thread);
+      expect(event.relationshipEventId, '\$root');
+      final targetEvent = Event(
+        eventId: '\$target',
+        senderId: '@example:example.org',
+        type: 'm.room.message',
+        content: {
+          'msgtype': 'text',
+          'body': 'Hello world',
+        },
+        originServerTs: DateTime.now(),
+        room: room,
+      );
+      final timeline =
+          Timeline(room: room, chunk: TimelineChunk(events: [targetEvent]));
+      expect(await event.getReplyEvent(timeline), targetEvent);
+    });
   });
 }