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)
+    bool isSyntheticMouseEvent = m_event->isMouseEvent() && toMouseEvent(*m_event).isSynthetic();
+
+    if (!m_event->defaultPrevented() && !m_event->defaultHandled() && !isSyntheticMouseEvent) {
         // 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;
             }
         }
     }
 }
 
 }