Do not generate keydown event for empty modifier flags

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

@@ -39,6 +39,8 @@ - (nonnull instancetype)initWithChannel:(nonnull FlutterBasicMessageChannel*)cha
 }
 
 - (void)handleEvent:(NSEvent*)event callback:(FlutterAsyncKeyCallback)callback {
+  // Remove the modifier bits that Flutter is not interested in.
+  NSEventModifierFlags modifierFlags = event.modifierFlags & ~0x100;
   NSString* type;
   switch (event.type) {
     case NSEventTypeKeyDown:
@@ -48,9 +50,9 @@ - (void)handleEvent:(NSEvent*)event callback:(FlutterAsyncKeyCallback)callback {
       type = @"keyup";
       break;
     case NSEventTypeFlagsChanged:
-      if (event.modifierFlags < _previouslyPressedFlags) {
+      if (modifierFlags < _previouslyPressedFlags) {
         type = @"keyup";
-      } else if (event.modifierFlags > _previouslyPressedFlags) {
+      } else if (modifierFlags > _previouslyPressedFlags) {
         type = @"keydown";
       } else {
         // ignore duplicate modifiers; This can happen in situations like switching
@@ -61,7 +63,7 @@ - (void)handleEvent:(NSEvent*)event callback:(FlutterAsyncKeyCallback)callback {
     default:
       NSAssert(false, @"Unexpected key event type (got %lu).", event.type);
   }
-  _previouslyPressedFlags = event.modifierFlags;
+  _previouslyPressedFlags = modifierFlags;
   NSMutableDictionary* keyMessage = [@{
     @"keymap" : @"macos",
     @"type" : type,

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

@@ -52,9 +52,20 @@
         callback(keyMessage);
       }));
 
-  // Key down
   FlutterChannelKeyResponder* responder =
       [[FlutterChannelKeyResponder alloc] initWithChannel:mockKeyEventChannel];
+
+  // Initial empty modifiers. This can happen when user opens window while modifier key is pressed
+  // and then releases the modifier. Shouldn't result in an event being sent.
+  // Regression test for https://github.com/flutter/flutter/issues/87339.
+  [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)];