Prevent default actions for JS-generated mouse events other than click

Source/core/events/EventDispatcher.cpp

Index: Source/core/events/EventDispatcher.cpp
diff --git a/Source/core/events/EventDispatcher.cpp b/Source/core/events/EventDispatcher.cpp
index 4608101c264a159845dfa964eb9e09c77dbb7310..6e904cdf397f7fc9c14a51a547e8efd7bdbe1fc0 100644
--- a/Source/core/events/EventDispatcher.cpp
+++ b/Source/core/events/EventDispatcher.cpp
@@ -206,7 +206,11 @@ inline void EventDispatcher::dispatchEventPostProcess(void* preDispatchEventHand
     // Call default event handlers. While the DOM does have a concept of preventing
     // default handling, the detail of which handlers are called is an internal
     // implementation detail and not part of the DOM.
-    if (!m_event->defaultPrevented() && !m_event->defaultHandled()) {
+
+    // Prevent calling default handlers for synthetic mouse events (issue #423975)

[Rick Byers, 2015/03/05 16:34:18]

Rather than say what the code is doing, this comment should attempt to explain
why.  Eg. "The DOM Events spec says that events dispatched by JS (other than
"click") should not have their default handlers invoked.  To mitigate
compatibility risk, we are trying to apply this rule for only mouse events at
first."

+    bool isSyntheticMouseEventOtherThanClick = m_event->isMouseEvent() && toMouseEvent(*m_event).isSynthetic() && toMouseEvent(*m_event).type() != EventTypeNames::click;
+
+    if (!m_event->defaultPrevented() && !m_event->defaultHandled() && !isSyntheticMouseEventOtherThanClick) {
         // Non-bubbling events call only one default event handler, the one for the target.
         m_node->willCallDefaultEventHandler(*m_event);
         m_node->defaultEventHandler(m_event.get());