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..bcf98912982df0c3f5df8bb0e5662ded8dbeccca 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)
+    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());