[macOS] performKeyEquivalent cleanup (#45946)

#40706 added a duplicate `NSEvent (KeyEquivalentMarker)` category. This PR removes it. It also removes the call to `markAsKeyEquivalent` from `FlutterViewController`. That call is internal to `FlutterTextInputPlugin` and classes outside should not be calling it.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

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

@@ -164,8 +164,6 @@ @interface FlutterViewWrapper : NSView
 
 - (void)setBackgroundColor:(NSColor*)color;
 
-- (BOOL)performKeyEquivalent:(NSEvent*)event;
-
 @end
 
 /**
@@ -242,37 +240,6 @@ - (void)onKeyboardLayoutChanged;
 
 @end
 
-#pragma mark - NSEvent (KeyEquivalentMarker) protocol
-
-@interface NSEvent (KeyEquivalentMarker)
-
-// Internally marks that the event was received through performKeyEquivalent:.
-// When text editing is active, keyboard events that have modifier keys pressed
-// are received through performKeyEquivalent: instead of keyDown:. If such event
-// is passed to TextInputContext but doesn't result in a text editing action it
-// needs to be forwarded by FlutterKeyboardManager to the next responder.
-- (void)markAsKeyEquivalent;
-
-// Returns YES if the event is marked as a key equivalent.
-- (BOOL)isKeyEquivalent;
-
-@end
-
-@implementation NSEvent (KeyEquivalentMarker)
-
-// This field doesn't need a value because only its address is used as a unique identifier.
-static char markerKey;
-
-- (void)markAsKeyEquivalent {
-  objc_setAssociatedObject(self, &markerKey, @true, OBJC_ASSOCIATION_RETAIN);
-}
-
-- (BOOL)isKeyEquivalent {
-  return [objc_getAssociatedObject(self, &markerKey) boolValue] == YES;
-}
-
-@end
-
 #pragma mark - Private dependant functions
 
 namespace {
@@ -312,19 +279,15 @@ - (void)setBackgroundColor:(NSColor*)color {
 }
 
 - (BOOL)performKeyEquivalent:(NSEvent*)event {
-  if ([_controller isDispatchingKeyEvent:event]) {
-    // When NSWindow is nextResponder, keyboard manager will send to it
-    // unhandled events (through [NSWindow keyDown:]). If event has both
-    // control and cmd modifiers set (i.e. cmd+control+space - emoji picker)
-    // NSWindow will then send this event as performKeyEquivalent: to first
-    // responder, which might be FlutterTextInputPlugin. If that's the case, the
-    // plugin must not handle the event, otherwise the emoji picker would not
-    // work (due to first responder returning YES from performKeyEquivalent:)
-    // and there would be an infinite loop, because FlutterViewController will
-    // send the event back to [keyboardManager handleEvent:].
+  // Do not intercept the event if flutterView is not first responder, otherwise this would
+  // interfere with TextInputPlugin, which also handles key equivalents.
+  //
+  // Also do not intercept the event if key equivalent is a product of an event being
+  // redispatched by the TextInputPlugin, in which case it needs to bubble up so that menus
+  // can handle key equivalents.
+  if (self.window.firstResponder != _flutterView || [_controller isDispatchingKeyEvent:event]) {
     return NO;
   }
-  [event markAsKeyEquivalent];
   [_flutterView keyDown:event];
   return YES;
 }

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

@@ -331,6 +331,13 @@ - (bool)testCtrlTabKeyEventIsPropagated {
   const uint64_t kPhysicalKeyTab = 0x7002b;
 
   [viewController viewWillAppear];  // Initializes the event channel.
+  // Creates a NSWindow so that FlutterView 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;
+  [window makeFirstResponder:viewController.flutterView];
   [viewController.view performKeyEquivalent:event];
 
   EXPECT_TRUE(called);