Fix button event on mouse events.

third_party/WebKit/Source/core/events/MouseEvent.cpp

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2005, 2006, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 366 matching lines @@
 }
 
 const AtomicString& MouseEvent::InterfaceName() const {
   return EventNames::MouseEvent;
 }
 
 bool MouseEvent::IsMouseEvent() const {
   return true;
 }
 
+short MouseEvent::button() const {
+  const AtomicString& event_name = type();
+  if (button_ == -1 || event_name == EventTypeNames::mousemove ||
+      event_name == EventTypeNames::mouseleave ||
+      event_name == EventTypeNames::mouseenter ||
+      event_name == EventTypeNames::mouseover ||
+      event_name == EventTypeNames::mouseout) {
+    return 0;
+  }
+  return button_;
+}
+
 int MouseEvent::which() const {
   // For the DOM, the return values for left, middle and right mouse buttons are
   // 0, 1, 2, respectively.
   // For the Netscape "which" property, the return values for left, middle and
   // right mouse buttons are 1, 2, 3, respectively.
   // So we must add 1.
   return button_ + 1;
 }
 
 Node* MouseEvent::toElement() const {
@@ skipping 189 matching lines @@
 
 int MouseEvent::offsetY() {
   if (!HasPosition())
     return 0;
   if (!has_cached_relative_position_)
     ComputeRelativePosition();
   return std::round(offset_location_.Y());
 }
 
 }  // namespace blink