[Windows] Ensure window is shown (flutter#127046)

## Background

The Windows runner has a race at startup:

1. **Platform thread**: creates a hidden window
2. **Platform thread**: launches the Flutter engine
3. **UI/Raster threads**: renders the first frame
4. **Platform thread**: Registers a callback to show the window once the next frame has been rendered.

Steps 3 and 4 happen in parallel and it is possible for step 3 to complete before step 4 starts. In this scenario, the next frame callback is never called and the window is never shown.

As a result the `windows_startup_test`'s test, which [verifies that the "show window" callback is called](https://github.com/flutter/flutter/blob/1f09a8662dad3bb1959b24e9124e05e2b9dbff1d/dev/integration_tests/windows_startup_test/windows/runner/flutter_window.cpp#L60-L64), can flake if the first frame is rendered before the show window callback has been registered.

## Solution

This change makes the runner schedule a frame after it registers the next frame callback. If step 3 hasn't completed yet, this no-ops as a frame is already scheduled. If step 3 has already completed, a new frame will be rendered, which will call the next frame callback and show the window.

Part of flutter#119415

See this thread for alternatives that were considered: flutter/engine#42061 (comment)

dev/integration_tests/flutter_gallery/windows/runner/flutter_window.cpp

@@ -35,6 +35,11 @@ bool FlutterWindow::OnCreate() {
     this->Show();
   });
 
+  // Flutter can complete the first frame before the "show window" callback is
+  // registered. Ensure a frame is pending to ensure the window is shown.
+  // This no-ops if the first frame hasn't completed yet.
+  flutter_controller_->ForceRedraw();
+
   return true;
 }
 

dev/integration_tests/ui/windows/runner/flutter_window.cpp

@@ -35,6 +35,11 @@ bool FlutterWindow::OnCreate() {
     this->Show();
   });
 
+  // Flutter can complete the first frame before the "show window" callback is
+  // registered. Ensure a frame is pending to ensure the window is shown.
+  // This no-ops if the first frame hasn't completed yet.
+  flutter_controller_->ForceRedraw();
+
   return true;
 }
 

dev/integration_tests/windows_startup_test/windows/runner/flutter_window.cpp

@@ -63,6 +63,11 @@ bool FlutterWindow::OnCreate() {
     visible = true;
   });
 
+  // Flutter can complete the first frame before the "show window" callback is
+  // registered. Ensure a frame is pending to ensure the window is shown.
+  // This no-ops if the first frame hasn't completed yet.
+  flutter_controller_->ForceRedraw();
+
   // Create a method channel to check the window's visibility.
   flutter::MethodChannel<> channel(
       flutter_controller_->engine()->messenger(), "tests.flutter.dev/windows_startup_test",

dev/manual_tests/windows/runner/flutter_window.cpp

@@ -35,6 +35,11 @@ bool FlutterWindow::OnCreate() {
     this->Show();
   });
 
+  // Flutter can complete the first frame before the "show window" callback is
+  // registered. Ensure a frame is pending to ensure the window is shown.
+  // This no-ops if the first frame hasn't completed yet.
+  flutter_controller_->ForceRedraw();
+
   return true;
 }
 

examples/flutter_view/windows/runner/flutter_window.cpp

@@ -35,6 +35,11 @@ bool FlutterWindow::OnCreate() {
     this->Show();
   });
 
+  // Flutter can complete the first frame before the "show window" callback is
+  // registered. Ensure a frame is pending to ensure the window is shown.
+  // This no-ops if the first frame hasn't completed yet.
+  flutter_controller_->ForceRedraw();
+
   return true;
 }
 

examples/hello_world/windows/runner/flutter_window.cpp

@@ -35,6 +35,11 @@ bool FlutterWindow::OnCreate() {
     this->Show();
   });
 
+  // Flutter can complete the first frame before the "show window" callback is
+  // registered. Ensure a frame is pending to ensure the window is shown.
+  // This no-ops if the first frame hasn't completed yet.
+  flutter_controller_->ForceRedraw();
+
   return true;
 }
 

examples/layers/windows/runner/flutter_window.cpp

@@ -35,6 +35,11 @@ bool FlutterWindow::OnCreate() {
     this->Show();
   });
 
+  // Flutter can complete the first frame before the "show window" callback is
+  // registered. Ensure a frame is pending to ensure the window is shown.
+  // This no-ops if the first frame hasn't completed yet.
+  flutter_controller_->ForceRedraw();
+
   return true;
 }
 

examples/platform_channel/windows/runner/flutter_window.cpp

@@ -98,6 +98,11 @@ bool FlutterWindow::OnCreate() {
     this->Show();
   });
 
+  // Flutter can complete the first frame before the "show window" callback is
+  // registered. Ensure a frame is pending to ensure the window is shown.
+  // This no-ops if the first frame hasn't completed yet.
+  flutter_controller_->ForceRedraw();
+
   return true;
 }
 

packages/flutter_tools/lib/src/cmake_project.dart

@@ -64,6 +64,9 @@ class WindowsProject extends FlutterProjectPlatform implements CmakeBasedProject
   /// The native entrypoint's CMake specification.
   File get runnerCmakeFile => runnerDirectory.childFile('CMakeLists.txt');
 
+  /// The native entrypoint's file that adds Flutter to the window.
+  File get runnerFlutterWindowFile => runnerDirectory.childFile('flutter_window.cpp');
+
   /// The native entrypoint's resource file. Used to configure things
   /// like the application icon, name, and version.
   File get runnerResourceFile => runnerDirectory.childFile('Runner.rc');

packages/flutter_tools/lib/src/windows/build_windows.dart

@@ -18,6 +18,7 @@ import '../convert.dart';
 import '../flutter_plugins.dart';
 import '../globals.dart' as globals;
 import '../migrations/cmake_custom_command_migration.dart';
+import 'migrations/show_window_migration.dart';
 import 'migrations/version_migration.dart';
 import 'visual_studio.dart';
 
@@ -54,6 +55,7 @@ Future<void> buildWindows(WindowsProject windowsProject, BuildInfo buildInfo, {
   final List<ProjectMigrator> migrators = <ProjectMigrator>[
     CmakeCustomCommandMigration(windowsProject, globals.logger),
     VersionMigration(windowsProject, globals.logger),
+    ShowWindowMigration(windowsProject, globals.logger),
   ];
 
   final ProjectMigration migration = ProjectMigration(migrators);

packages/flutter_tools/lib/src/windows/migrations/show_window_migration.dart

@@ -0,0 +1,72 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import '../../base/file_system.dart';
+import '../../base/project_migrator.dart';
+import '../../cmake_project.dart';
+import 'utils.dart';
+
+const String _before = r'''
+  flutter_controller_->engine()->SetNextFrameCallback([&]() {
+    this->Show();
+  });
+
+  return true;
+''';
+const String _after = r'''
+  flutter_controller_->engine()->SetNextFrameCallback([&]() {
+    this->Show();
+  });
+
+  // Flutter can complete the first frame before the "show window" callback is
+  // registered. Ensure a frame is pending to ensure the window is shown.
+  // This no-ops if the first frame hasn't completed yet.
+  flutter_controller_->ForceRedraw();
+
+  return true;
+''';
+
+/// Migrates Windows apps to ensure the window is shown.
+///
+/// This prevents a race condition between Flutter rendering the first frame
+/// and the app registering the callback to show the window on the first frame.
+/// See https://github.com/flutter/flutter/issues/119415.
+class ShowWindowMigration extends ProjectMigrator {
+  ShowWindowMigration(WindowsProject project, super.logger)
+    : _file = project.runnerFlutterWindowFile;
+
+  final File _file;
+
+  @override
+  void migrate() {
+    // Skip this migration if the affected file does not exist. This indicates
+    // the app has done non-trivial changes to its runner and this migration
+    // might not work as expected if applied.
+    if (!_file.existsSync()) {
+      logger.printTrace('''
+windows/runner/flutter_window.cpp file not found, skipping show window migration.
+
+This indicates non-trivial changes have been made to the Windows runner in the
+"windows" folder. If needed, you can reset the Windows runner by deleting the
+"windows" folder and then using the "flutter create --platforms=windows ." command.
+''');
+      return;
+    }
+
+    // Migrate the windows/runner/flutter_window.cpp file.
+    final String originalContents = _file.readAsStringSync();
+    final String newContents = replaceFirst(
+      originalContents,
+      _before,
+      _after,
+    );
+    if (originalContents != newContents) {
+      logger.printStatus(
+        'windows/runner/flutter_window.cpp does not ensure the show window '
+        'callback is called, updating.'
+      );
+      _file.writeAsStringSync(newContents);
+    }
+  }
+}

packages/flutter_tools/lib/src/windows/migrations/utils.dart

@@ -0,0 +1,27 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// Creates a new string with the first occurrence of [before] replaced by
+/// [after].
+///
+/// If the [originalContents] uses CRLF line endings, the [before] and [after]
+/// will be converted to CRLF line endings before the replacement is made.
+/// This is necessary for users that have git autocrlf enabled.
+///
+/// Example:
+/// ```dart
+/// 'a\n'.replaceFirst('a\n', 'b\n'); // 'b\n'
+/// 'a\r\n'.replaceFirst('a\n', 'b\n'); // 'b\r\n'
+/// ```
+String replaceFirst(String originalContents, String before, String after) {
+  final String result = originalContents.replaceFirst(before, after);
+  if (result != originalContents) {
+    return result;
+  }
+
+  final String beforeCrlf = before.replaceAll('\n', '\r\n');
+  final String afterCrlf = after.replaceAll('\n', '\r\n');
+
+  return originalContents.replaceFirst(beforeCrlf, afterCrlf);
+}

packages/flutter_tools/lib/src/windows/migrations/version_migration.dart

@@ -5,6 +5,7 @@
 import '../../base/file_system.dart';
 import '../../base/project_migrator.dart';
 import '../../cmake_project.dart';
+import 'utils.dart';
 
 const String _cmakeFileBefore = r'''
 # Apply the standard set of build settings. This can be removed for applications
@@ -96,7 +97,7 @@ This indicates non-trivial changes have been made to the Windows runner in the
 
     // Migrate the windows/runner/CMakeLists.txt file.
     final String originalCmakeContents = _cmakeFile.readAsStringSync();
-    final String newCmakeContents = _replaceFirst(
+    final String newCmakeContents = replaceFirst(
       originalCmakeContents,
       _cmakeFileBefore,
       _cmakeFileAfter,
@@ -108,7 +109,7 @@ This indicates non-trivial changes have been made to the Windows runner in the
 
     // Migrate the windows/runner/Runner.rc file.
     final String originalResourceFileContents = _resourceFile.readAsStringSync();
-    final String newResourceFileContents = _replaceFirst(
+    final String newResourceFileContents = replaceFirst(
       originalResourceFileContents,
       _resourceFileBefore,
       _resourceFileAfter,
@@ -121,27 +122,3 @@ This indicates non-trivial changes have been made to the Windows runner in the
     }
   }
 }
-
-  /// Creates a new string with the first occurrence of [before] replaced by
-  /// [after].
-  ///
-  /// If the [originalContents] uses CRLF line endings, the [before] and [after]
-  /// will be converted to CRLF line endings before the replacement is made.
-  /// This is necessary for users that have git autocrlf enabled.
-  ///
-  /// Example:
-  /// ```dart
-  /// 'a\n'.replaceFirst('a\n', 'b\n'); // 'b\n'
-  /// 'a\r\n'.replaceFirst('a\n', 'b\n'); // 'b\r\n'
-  /// ```
-String _replaceFirst(String originalContents, String before, String after) {
-  final String result = originalContents.replaceFirst(before, after);
-  if (result != originalContents) {
-    return result;
-  }
-
-  final String beforeCrlf = before.replaceAll('\n', '\r\n');
-  final String afterCrlf = after.replaceAll('\n', '\r\n');
-
-  return originalContents.replaceFirst(beforeCrlf, afterCrlf);
-}

packages/flutter_tools/templates/app_shared/windows.tmpl/runner/flutter_window.cpp

@@ -31,6 +31,11 @@ bool FlutterWindow::OnCreate() {
     this->Show();
   });
 
+  // Flutter can complete the first frame before the "show window" callback is
+  // registered. Ensure a frame is pending to ensure the window is shown.
+  // This no-ops if the first frame hasn't completed yet.
+  flutter_controller_->ForceRedraw();
+
   return true;
 }