[Mac] Implement highlight-on-hover for the Wrench menu buttons.

chrome/browser/cocoa/menu_tracked_button.h

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_COCOA_MENU_TRACKED_BUTTON_H_
 #define CHROME_BROWSER_COCOA_MENU_TRACKED_BUTTON_H_
 #pragma once
 
 #import <Cocoa/Cocoa.h>
 
 // A MenuTrackedButton is meant to be used whenever a button is placed inside
 // the custom view of an NSMenuItem. If the user opens the menu in a non-sticky
 // fashion (i.e. clicks, holds, and drags) and then releases the mouse over
 // a MenuTrackedButton, it will |-performClick:| itself.
+//
+// To create the hover state effects, there are two code paths.  When the menu
+// is opened sticky, a tracking rect produces mouse entered/exit events that
+// allow for setting the cell's highlight property.  When in a drag cycle,
+// however, the only event received is |-mouseDragged:|.  Therefore, a
+// delayed selector is scheduled to poll the mouse location after each drag
+// event.  This checks if the user is still over the button after the drag
+// events stop being sent, indicating either the user is hovering without
+// movement or that the mouse is no longer over the receiver.
 @interface MenuTrackedButton : NSButton {
  @private
   // If the button received a |-mouseEntered:| event. This short-circuits the
   // custom drag tracking logic.
   BOOL didEnter_;
 
   // Whether or not the user is in a click-drag-release event sequence. If so
   // and this receives a |-mouseUp:|, then this will click itself.
   BOOL tracking_;
+
+  // In order to get hover effects when the menu is sticky-opened, a tracking
+  // rect needs to be installed on the button.
+  NSTrackingRectTag trackingTag_;
 }
 
 @end
 
 #endif  // CHROME_BROWSER_COCOA_MENU_TRACKED_BUTTON_H_