Do not generate keydown event for empty modifier flags

Author: knopp

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

@@ -50,7 +50,8 @@ - (void)handleEvent:(NSEvent*)event callback:(FlutterAsyncKeyCallback)callback {
     case NSEventTypeFlagsChanged:
       if (event.modifierFlags < _previouslyPressedFlags) {
         type = @"keyup";
-      } else if (event.modifierFlags > _previouslyPressedFlags) {
+      } else if (event.modifierFlags > _previouslyPressedFlags &&
+                 event.modifierFlags > 0x100) {  // 0x100 is empty modifierFlags
         type = @"keydown";
       } else {
         // ignore duplicate modifiers; This can happen in situations like switching

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

@@ -52,9 +52,18 @@
         callback(keyMessage);
       }));
 
-  // Key down
   FlutterChannelKeyResponder* responder =
       [[FlutterChannelKeyResponder alloc] initWithChannel:mockKeyEventChannel];
+
+  // Empty modifiers. Shouldn't result in any event
+  [responder handleEvent:keyEvent(NSEventTypeFlagsChanged, 0x100, @"", @"", FALSE, 60)
+                callback:^(BOOL handled) {
+                  [responses addObject:@(handled)];
+                }];
+
+  EXPECT_EQ([messages count], 0u);
+
+  // Key down
   [responder handleEvent:keyEvent(NSEventTypeKeyDown, 0x100, @"a", @"a", FALSE, 0)
                 callback:^(BOOL handled) {
                   [responses addObject:@(handled)];