fix first responder update after accessibility is turned off

Author: chunhtai

shell/platform/darwin/macos/framework/Source/FlutterViewController.mm

@@ -546,7 +546,20 @@ - (void)onAccessibilityStatusChanged:(NSNotification*)notification {
   if (!_engine) {
     return;
   }
-  _engine.semanticsEnabled = !!notification.userInfo[EnhancedUserInterfaceKey];
+  BOOL enabled = [notification.userInfo[EnhancedUserInterfaceKey] boolValue];
+  if (!enabled && self.viewLoaded && [_textInputPlugin isFirstResponder]) {
+    // The client (i.e. the FlutterTextField) of the textInputPlugin is a sibling
+    // of the FlutterView. The macOS will pick the ancestor to be the next responder
+    // when the client is removed from the view hierarchy, which is the result of
+    // turning off semantics. This will cause the keyboard focus to stuck at the
+    // NSWindow.
+    //
+    // Since we create an illustion that the FlutterTextField is below the FlutterView
+    // in accessibility (See FlutterViewWrapper), we have to manually pick our next
+    // responder.
+    [self.view.window makeFirstResponder:_flutterView];
+  }
+  _engine.semanticsEnabled = [notification.userInfo[EnhancedUserInterfaceKey] boolValue];
 }
 
 - (void)onSettingsChanged:(NSNotification*)notification {

shell/platform/darwin/macos/framework/Source/FlutterViewControllerTest.mm

@@ -29,6 +29,15 @@ + (void)respondFalseForSendEvent:(const FlutterKeyEvent&)event
 
 namespace {
 
+// Returns an engine configured for the test fixture resource configuration.
+FlutterEngine* CreateTestEngine() {
+  NSString* fixtures = @(testing::GetFixturesPath());
+  FlutterDartProject* project = [[FlutterDartProject alloc]
+      initWithAssetsPath:fixtures
+             ICUDataPath:[fixtures stringByAppendingString:@"/icudtl.dat"]];
+  return [[FlutterEngine alloc] initWithName:@"test" project:project allowHeadlessExecution:true];
+}
+
 NSResponder* mockResponder() {
   NSResponder* mock = OCMStrictClassMock([NSResponder class]);
   OCMStub([mock keyDown:[OCMArg any]]).andDo(nil);
@@ -98,6 +107,39 @@ + (void)respondFalseForSendEvent:(const FlutterKeyEvent&)event
   EXPECT_EQ(accessibilityChildren[0], viewControllerMock.flutterView);
 }
 
+TEST(FlutterViewController,
+     MakesFlutterViewFirstResponderWhenAccessibilityIsTurnedOffAndTextInputPluginIsFirstResponder) {
+  FlutterEngine* engine = CreateTestEngine();
+  NSString* fixtures = @(testing::GetFixturesPath());
+  FlutterDartProject* project = [[FlutterDartProject alloc]
+      initWithAssetsPath:fixtures
+             ICUDataPath:[fixtures stringByAppendingString:@"/icudtl.dat"]];
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithProject:project];
+  [viewController loadView];
+  [engine setViewController:viewController];
+  // Creates a NSWindow so that sub view can be first responder.
+  NSWindow* window = [[NSWindow alloc] initWithContentRect:NSMakeRect(0, 0, 800, 600)
+                                                 styleMask:NSBorderlessWindowMask
+                                                   backing:NSBackingStoreBuffered
+                                                     defer:NO];
+  window.contentView = viewController.view;
+  // Attaches FlutterTextInputPlugin to the view;
+  [viewController.view addSubview:viewController.textInputPlugin];
+  // Makes sure the textInputPlugin can be the first responder.
+  EXPECT_TRUE([window makeFirstResponder:viewController.textInputPlugin]);
+  EXPECT_EQ([window firstResponder], viewController.textInputPlugin);
+  // Sends a notification to turn off the accessibility.
+  NSDictionary* userInfo = @{
+    @"AXEnhancedUserInterface" : @(NO),
+  };
+  NSNotification* accessibilityOff = [NSNotification notificationWithName:@""
+                                                                   object:nil
+                                                                 userInfo:userInfo];
+  [viewController onAccessibilityStatusChanged:accessibilityOff];
+  // FlutterView becomes the first responder.
+  EXPECT_EQ([window firstResponder], viewController.flutterView);
+}
+
 TEST(FlutterViewControllerTest, TestKeyEventsAreSentToFramework) {
   ASSERT_TRUE([[FlutterViewControllerTestObjC alloc] testKeyEventsAreSentToFramework]);
 }

shell/platform/darwin/macos/framework/Source/FlutterViewController_Internal.h

@@ -37,3 +37,8 @@
                                 bundle:(nullable NSBundle*)nibBundle NS_DESIGNATED_INITIALIZER;
 
 @end
+
+// Private methods made visible for testing
+@interface FlutterViewController (TestMethods)
+- (void)onAccessibilityStatusChanged:(nonnull NSNotification*)notification;
+@end