fix: Correctly store lastEvent in database after decryption

lib/encryption/key_manager.dart

@@ -193,9 +193,17 @@ class KeyManager {
           event.content['session_id'] == sessionId) {
         final decrypted = encryption.decryptRoomEventSync(roomId, event);
         if (decrypted.type != EventTypes.Encrypted) {
-          // No need to persist it as the lastEvent is persisted in the sync
-          // right after processing to-device messages:
+          // Update the last event in memory first
           room.lastEvent = decrypted;
+
+          // To persist it in database and trigger UI updates:
+          await client.database?.transaction(() async {
+            await client.handleSync(
+              SyncUpdate(
+                  nextBatch: '',
+                  rooms: RoomsUpdate(join: {room.id: JoinedRoomUpdate()})),
+            );
+          });
         }
       }
       // and finally broadcast the new session

test/encryption/key_manager_test.dart

@@ -318,6 +318,9 @@ void main() {
       final roomId = '!someroom:example.org';
       final sessionId = inbound.session_id();
       final room = Room(id: roomId, client: client);
+      final nextSyncUpdateFuture = client.onSync.stream
+          .firstWhere((update) => update.rooms != null)
+          .timeout(const Duration(seconds: 5));
       client.rooms.add(room);
       // we build up an encrypted message so that we can test if it successfully decrypted afterwards
       room.setState(
@@ -475,6 +478,11 @@ void main() {
       expect(room.lastEvent?.type, 'm.room.message');
       expect(room.lastEvent?.content['body'], 'foxies');
 
+      // test if a fake sync has been performed to update the GUI and store the
+      // decrypted last event
+      final syncUpdate = await nextSyncUpdateFuture;
+      expect(syncUpdate.rooms?.join?.containsKey(room.id), true);
+
       inbound.free();
       session.free();
     });