Fix macOS crash when modifier keys pressed.

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

@@ -504,7 +504,8 @@ - (void)propagateKeyEvent:(NSEvent*)event ofType:(NSString*)type {
         return;
       }
     }
-    if ([self.nextResponder respondsToSelector:@selector(keyDown:)]) {
+    if ([self.nextResponder respondsToSelector:@selector(keyDown:)] &&
+        event.type == NSEventTypeKeyDown) {
       [self.nextResponder keyDown:event];
     }
   } else if ([type isEqual:@"keyup"]) {
@@ -513,13 +514,20 @@ - (void)propagateKeyEvent:(NSEvent*)event ofType:(NSString*)type {
         return;
       }
     }
-    if ([self.nextResponder respondsToSelector:@selector(keyUp:)]) {
+    if ([self.nextResponder respondsToSelector:@selector(keyUp:)] &&
+        event.type == NSEventTypeKeyUp) {
       [self.nextResponder keyUp:event];
     }
   }
+  if ([self.nextResponder respondsToSelector:@selector(flagsChanged:)] &&
+      event.type == NSEventTypeFlagsChanged) {
+    [self.nextResponder flagsChanged:event];
+  }
 }
 
 - (void)dispatchKeyEvent:(NSEvent*)event ofType:(NSString*)type {
+  // Be sure to add a handler in propagateKeyEvent if you allow more event
+  // types here.
   if (event.type != NSEventTypeKeyDown && event.type != NSEventTypeKeyUp &&
       event.type != NSEventTypeFlagsChanged) {
     return;

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

@@ -18,6 +18,7 @@ @interface FlutterViewControllerTestObjC : NSObject
 - (bool)testKeyEventsAreSentToFramework;
 - (bool)testKeyEventsArePropagatedIfNotHandled;
 - (bool)testKeyEventsAreNotPropagatedIfHandled;
+- (bool)testFlagsChangedEventsArePropagatedIfNotHandled;
 @end
 
 namespace flutter::testing {
@@ -98,6 +99,11 @@ id mockViewController(NSString* pasteboardString) {
   ASSERT_TRUE([[FlutterViewControllerTestObjC alloc] testKeyEventsAreNotPropagatedIfHandled]);
 }
 
+TEST(FlutterViewControllerTest, TestFlagsChangedEventsArePropagatedIfNotHandled) {
+  ASSERT_TRUE(
+      [[FlutterViewControllerTestObjC alloc] testFlagsChangedEventsArePropagatedIfNotHandled]);
+}
+
 }  // namespace flutter::testing
 
 @implementation FlutterViewControllerTestObjC
@@ -185,6 +191,53 @@ - (bool)testKeyEventsArePropagatedIfNotHandled {
   return true;
 }
 
+- (bool)testFlagsChangedEventsArePropagatedIfNotHandled {
+  id engineMock = OCMClassMock([FlutterEngine class]);
+  id binaryMessengerMock = OCMProtocolMock(@protocol(FlutterBinaryMessenger));
+  OCMStub(  // NOLINT(google-objc-avoid-throwing-exception)
+      [engineMock binaryMessenger])
+      .andReturn(binaryMessengerMock);
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:engineMock
+                                                                                nibName:@""
+                                                                                 bundle:nil];
+  id responderMock = OCMClassMock([FlutterIntermediateKeyResponder class]);
+  [viewController addKeyResponder:responderMock];
+  NSDictionary* expectedEvent = @{
+    @"keymap" : @"macos",
+    @"type" : @"keydown",
+    @"keyCode" : @(56),  // SHIFT key
+    @"modifiers" : @(537001986),
+  };
+  NSData* encodedKeyEvent = [[FlutterJSONMessageCodec sharedInstance] encode:expectedEvent];
+  CGEventRef cgEvent = CGEventCreateKeyboardEvent(NULL, 56, TRUE);  // SHIFT key
+  CGEventSetType(cgEvent, kCGEventFlagsChanged);
+  NSEvent* event = [NSEvent eventWithCGEvent:cgEvent];
+  OCMExpect(  // NOLINT(google-objc-avoid-throwing-exception)
+      [binaryMessengerMock sendOnChannel:@"flutter/keyevent"
+                                 message:encodedKeyEvent
+                             binaryReply:[OCMArg any]])
+      .andDo((^(NSInvocation* invocation) {
+        FlutterBinaryReply handler;
+        [invocation getArgument:&handler atIndex:4];
+        NSDictionary* reply = @{
+          @"handled" : @(false),
+        };
+        NSData* encodedReply = [[FlutterJSONMessageCodec sharedInstance] encode:reply];
+        handler(encodedReply);
+      }));
+  [viewController viewWillAppear];  // Initializes the event channel.
+  [viewController flagsChanged:event];
+  @try {
+    OCMVerify(  // NOLINT(google-objc-avoid-throwing-exception)
+        [binaryMessengerMock sendOnChannel:@"flutter/keyevent"
+                                   message:encodedKeyEvent
+                               binaryReply:[OCMArg any]]);
+  } @catch (...) {
+    return false;
+  }
+  return true;
+}
+
 - (bool)testKeyEventsAreNotPropagatedIfHandled {
   id engineMock = OCMClassMock([FlutterEngine class]);
   id binaryMessengerMock = OCMProtocolMock(@protocol(FlutterBinaryMessenger));