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

Source/core/events/MouseRelatedEvent.cpp

Index: Source/core/events/MouseRelatedEvent.cpp
diff --git a/Source/core/events/MouseRelatedEvent.cpp b/Source/core/events/MouseRelatedEvent.cpp
index 2da77c38f307d290c4c363c7c88e30b40c964d4c..c5be7a6a08a679f0014c9f16f8e23cf5261a8753 100644
--- a/Source/core/events/MouseRelatedEvent.cpp
+++ b/Source/core/events/MouseRelatedEvent.cpp
@@ -184,6 +184,8 @@ int MouseRelatedEvent::layerY()
 
 int MouseRelatedEvent::offsetX()
 {
+    if (isSimulated())
+        return 0;
     if (!m_hasCachedRelativePosition)
         computeRelativePosition();
     return roundToInt(m_offsetLocation.x());
@@ -191,6 +193,8 @@ int MouseRelatedEvent::offsetX()
 
 int MouseRelatedEvent::offsetY()
 {
+    if (isSimulated())
+        return 0;
     if (!m_hasCachedRelativePosition)
         computeRelativePosition();
     return roundToInt(m_offsetLocation.y());