MouseEvent.offsetX/Y should just return 0,0 for simulated clicks.

Source/core/events/MouseRelatedEvent.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 166 matching lines @@
 
 int MouseRelatedEvent::layerY()
 {
     if (!m_hasCachedRelativePosition)
         computeRelativePosition();
     return m_layerLocation.y();
 }
 
 int MouseRelatedEvent::offsetX()
 {
+    if (isSimulated())
+        return 0;
     if (!m_hasCachedRelativePosition)
         computeRelativePosition();
     return roundToInt(m_offsetLocation.x());
 }
 
 int MouseRelatedEvent::offsetY()
 {
+    if (isSimulated())
+        return 0;
     if (!m_hasCachedRelativePosition)
         computeRelativePosition();
     return roundToInt(m_offsetLocation.y());
 }
 
 int MouseRelatedEvent::pageX() const
 {
     return m_pageLocation.x();
 }
 
@@ skipping 15 matching lines @@
     // See Microsoft documentation and <http://www.quirksmode.org/dom/w3c_events.html>.
     return m_clientLocation.y();
 }
 
 void MouseRelatedEvent::trace(Visitor* visitor)
 {
     UIEventWithKeyState::trace(visitor);
 }
 
 } // namespace WebCore