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

Source/core/events/MouseEvent.cpp

Index: Source/core/events/MouseEvent.cpp
diff --git a/Source/core/events/MouseEvent.cpp b/Source/core/events/MouseEvent.cpp
index eb4c1bf47978b85189ae7a7957406e0a674f8c1f..3cefc999f74c1d101e48018fa306e4081049bca6 100644
--- a/Source/core/events/MouseEvent.cpp
+++ b/Source/core/events/MouseEvent.cpp
@@ -260,6 +260,7 @@ bool MouseEventDispatchMediator::dispatchEvent(EventDispatcher& dispatcher) cons
 {
     if (isSyntheticMouseEvent()) {
         event().eventPath().adjustForRelatedTarget(dispatcher.node(), event().relatedTarget());
+        event().markAsSynthetic();
         return dispatcher.dispatch();
     }