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If the embedder API's `FlutterEngineAddView` and `FlutterEngineRemoveView` don't return `kSuccess`, their callbacks won't be invoked. See: [flutter.dev/go/multi-view-embedder-apis](https://flutter.dev/go/multi-view-embedder-apis). Previously, the Windows embedder would hang in this scenario as it blocks until the callbacks are invoked. Now, the Windows embedder only blocks if these embedder APIs return `kSuccess`. Kudos to @dkwingsmt for catching this! Part of https://github.com/flutter/flutter/issues/144810 Part of https://github.com/flutter/flutter/issues/142845 [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style