LegacyJavaScriptObject crash on Flutter web DDC after hot restart

[thang72]

Bug Description

After a hot restart on Flutter web (DDC debug mode), SqliteAsyncDriftConnection's BroadcastChannel listener crashes with a TypeError. This kills all Drift watch query update notifications, resulting in stale data. The error does not occur on:

• Fresh app launch
• Full page reload (Ctrl+Shift+R)
• Hot reload

Error

DartError: TypeError: Instance of 'LegacyJavaScriptObject': type 'LegacyJavaScriptObject' is not a subtype of type 'UpdateNotification?'
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 274:3       throw_
dart-sdk/lib/_internal/js_shared/lib/rti.dart 1516:3                              _generalNullableAsCheckImplementation
dart-sdk/lib/_internal/js_shared/lib/rti.dart 89:34                               _installSpecializedAsCheck
dart-sdk/lib/async/stream_pipe.dart 152:15                                        [_add]
dart-sdk/lib/async/stream_pipe.dart 252:9                                         [_handleData]
dart-sdk/lib/async/stream_pipe.dart 184:5                                         [_handleData]
package:web/src/helpers/events/streams.dart 165:63                                <fn>
dart-sdk/lib/_internal/js_dev_runtime/patch/js_allow_interop_patch.dart 224:27    _callDartFunctionFast1

Root Cause

Hot restart recompiles Dart code and regenerates DDC runtime type information. However, the JavaScript BroadcastChannel.onmessage callback (created by SqliteAsyncDriftConnection via sqlite_async's BroadcastUpdates) retains references to the old DDC type system. When BroadcastChannel fires after hot restart, the old callback casts the message using stale type references → TypeError → listener dies → all future Drift watch query updates lost.

The crash occurs in sqlite_async/lib/src/web/database/broadcast_updates.dart where event.data is cast to _BroadcastMessage. After hot restart, the DDC runtime cannot resolve the type, and it arrives as a raw LegacyJavaScriptObject.

Impact

• Dev workflow: Hot restart is unusable on web — developers must use full page refresh
• Production: Not affected (release builds don't hot restart)

Environment

• powersync: 1.18.0
• powersync_core: 1.8.0
• drift_sqlite_async: 0.2.6
• sqlite_async: 0.13.1
• drift: 2.31.0
• Dart SDK: 3.9.0
• Flutter web, DDC (debug mode)
• Chrome 130+
• macOS

Steps to Reproduce

1. Create a Flutter web app with PowerSync + Drift using SqliteAsyncDriftConnection
2. Run on Chrome in debug mode (DDC): flutter run -d chrome
3. Verify data loads and watch queries work (Drift streams emit)
4. Trigger a hot restart (Shift+R in terminal, or restart from IDE)
5. Perform any database write that triggers a BroadcastChannel notification
6. Observe the LegacyJavaScriptObject error in console
7. Drift watch queries stop updating — data is stale

Workaround

Use full page refresh (Ctrl+Shift+R) instead of hot restart during web development.

Suggested Fix

The BroadcastUpdates class in sqlite_async could wrap the BroadcastChannel.onmessage handler with error handling, or re-register the listener on hot restart. Alternatively, the _BroadcastMessage JS interop type could use a more resilient deserialization path that doesn't rely on DDC runtime type identity.