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

Source/core/events/EventDispatcher.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 188 matching lines @@
     m_event->setTarget(EventPath::eventTargetRespectingTargetRules(*m_node));
     m_event->setCurrentTarget(0);
     m_event->setEventPhase(0);
 
     // Pass the data from the preDispatchEventHandler to the postDispatchEventHandler.
     m_node->postDispatchEventHandler(m_event.get(), preDispatchEventHandlerResult);
 
     // 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());
         ASSERT(!m_event->defaultPrevented());
         if (m_event->defaultHandled())
             return;
         // For bubbling events, call default event handlers on the same targets in the
         // same order as the bubbling phase.
         if (m_event->bubbles()) {
             size_t size = m_event->eventPath().size();
             for (size_t i = 1; i < size; ++i) {
                 m_event->eventPath()[i].node()->willCallDefaultEventHandler(*m_event);
                 m_event->eventPath()[i].node()->defaultEventHandler(m_event.get());
                 ASSERT(!m_event->defaultPrevented());
                 if (m_event->defaultHandled())
                     return;
             }
         }
     }
 }
 
 }