Fix the issue on macOS where, after a hot restart with multiple windows, unresponsive windows are left behind. (flutter#180287)

There is currently an issue with multiple windows on macOS: after a hot
restart, new windows are created, but the original windows are not
closed. You can reproduce this by running examples/multiple_windows and
then performing a hot restart.

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Author: yiiim

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine.mm

@@ -152,14 +152,6 @@ - (void)sendUserLocales;
  */
 - (void)engineCallbackOnPlatformMessage:(const FlutterPlatformMessage*)message;
 
-/**
- * Invoked right before the engine is restarted.
- *
- * This should reset states to as if the application has just started.  It
- * usually indicates a hot restart (Shift-R in Flutter CLI.)
- */
-- (void)engineCallbackOnPreEngineRestart;
-
 /**
  * Requests that the task be posted back the to the Flutter engine at the target time. The target
  * time is in the clock used by the Flutter engine.
@@ -1289,6 +1281,7 @@ - (void)engineCallbackOnPreEngineRestart {
   while ((nextViewController = [viewControllerEnumerator nextObject])) {
     [nextViewController onPreEngineRestart];
   }
+  [_windowController closeAllWindows];
   [_platformViewController reset];
   _keyboardManager = [[FlutterKeyboardManager alloc] initWithDelegate:self];
 }

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine_Internal.h

@@ -268,6 +268,13 @@ typedef NS_ENUM(NSInteger, FlutterAppExitResponse) {
  */
 + (nullable FlutterEngine*)engineForIdentifier:(int64_t)identifier;
 
+/**
+ * Invoked right before the engine is restarted.
+ *
+ * This should reset states to as if the application has just started.  It
+ * usually indicates a hot restart (Shift-R in Flutter CLI.)
+ */
+- (void)engineCallbackOnPreEngineRestart;
 @end
 
 NS_ASSUME_NONNULL_END

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterWindowController.h

@@ -16,10 +16,6 @@
 
 @property(nonatomic, weak) FlutterEngine* engine;
 
-@end
-
-@interface FlutterWindowController (Testing)
-
 - (void)closeAllWindows;
 
 @end

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterWindowControllerTest.mm

@@ -202,4 +202,43 @@ void TearDown() {
   CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0.5, false);
   EXPECT_EQ(window.miniaturized, YES);
 }
+
+TEST_F(FlutterWindowControllerTest, ClosesAllWindowsOnEngineRestart) {
+  FlutterWindowCreationRequest request{
+      .has_size = true,
+      .size = {.width = 800, .height = 600},
+      .on_should_close = [] {},
+      .on_will_close = [] {},
+      .notify_listeners = [] {},
+  };
+
+  FlutterEngine* engine = GetFlutterEngine();
+  int64_t engine_id = reinterpret_cast<int64_t>(engine);
+
+  IsolateScope isolate_scope(isolate());
+
+  // Create multiple windows
+  int64_t handle1 = InternalFlutter_WindowController_CreateRegularWindow(engine_id, &request);
+  int64_t handle2 = InternalFlutter_WindowController_CreateRegularWindow(engine_id, &request);
+  int64_t handle3 = InternalFlutter_WindowController_CreateRegularWindow(engine_id, &request);
+
+  // Verify windows are created
+  FlutterViewController* viewController1 = [engine viewControllerForIdentifier:handle1];
+  FlutterViewController* viewController2 = [engine viewControllerForIdentifier:handle2];
+  FlutterViewController* viewController3 = [engine viewControllerForIdentifier:handle3];
+  EXPECT_NE(viewController1, nil);
+  EXPECT_NE(viewController2, nil);
+  EXPECT_NE(viewController3, nil);
+
+  // Close all windows on engine restart
+  [engine engineCallbackOnPreEngineRestart];
+
+  // Verify all windows are closed and view controllers are disposed
+  viewController1 = [engine viewControllerForIdentifier:handle1];
+  viewController2 = [engine viewControllerForIdentifier:handle2];
+  viewController3 = [engine viewControllerForIdentifier:handle3];
+  EXPECT_EQ(viewController1, nil);
+  EXPECT_EQ(viewController2, nil);
+  EXPECT_EQ(viewController3, nil);
+}
 }  // namespace flutter::testing