Migrate a Windows project to ensure the window is shown

Flutter 3.13 fixed a bug that could result in the window not being shown. Windows projects created using Flutter 3.7 or Flutter 3.10 need to be migrated to fix this issue.

Verify you are on Flutter version 3.13 or newer using flutter --version. If needed, use flutter upgrade to update to the latest version of the Flutter SDK.

Projects that have not modified their windows/runner/flutter_window.cpp file will be migrated automatically by flutter run or flutter build windows.

Projects that have modified their windows/runner/flutter_window.cpp file might need to migrate manually.

Code before migration:

flutter_controller_->engine()->SetNextFrameCallback([&]() {
  this->Show();
});

Code after migration:

flutter_controller_->engine()->SetNextFrameCallback([&]() {
  this->Show();
});
​
// Flutter can complete the first frame before the "show window" callback is
// registered. The following call ensures a frame is pending to ensure the
// window is shown. It is a no-op if the first frame hasn't completed yet.
flutter_controller_->ForceRedraw();

PR 995 shows the migration work for the Flutter Gallery app.