Do not mutate ui::Event properties during nested event processing

ui/events/event_processor.cc

Index: ui/events/event_processor.cc
diff --git a/ui/events/event_processor.cc b/ui/events/event_processor.cc
index 3077cc85b168aea483a9e506021af80b5e64375e..437a08e8223ce247b87e5b72a919f283fa67885e 100644
--- a/ui/events/event_processor.cc
+++ b/ui/events/event_processor.cc
@@ -18,8 +18,31 @@ EventDispatchDetails EventProcessor::OnEventFromSource(Event* event) {
   PrepareEventForDispatch(event);
   EventTarget* target = targeter->FindTargetForEvent(root, event);
 
+  // Create a copy of |event| if it is currently being processed by another
+  // EventProcessor.

[sadrul, 2014/09/04 15:45:06]

If the event is in the process of being dispatched, or has already been
dispatched, then dispatch a copy of the event instead.

[tdanderson, 2014/09/04 17:19:03]

Done.

+  scoped_ptr<Event> event_copy;
+  if (event->phase() != EP_PREDISPATCH)
+    event_copy = Event::Clone(*event);
+
   while (target) {
-    EventDispatchDetails details = DispatchEvent(target, event);
+    EventDispatchDetails details;
+    if (event_copy) {
+      // The state of |event| (such as its target and phase) should remain
+      // unchanged once we have finished our nested processing, with the
+      // exception of whether or not the event was handled. Thus dispatch
+      // |event_copy| instead of |event|.

[sadrul, 2014/09/04 15:45:06]

You don't need this comment. The comment above in lines 21:22 is sufficient.

[tdanderson, 2014/09/04 17:19:03]

Done.

+      details = DispatchEvent(target, event_copy.get());
+
+      if (event_copy->stopped_propagation())
+        event->StopPropagation();
+      else if (event_copy->handled())
+        event->SetHandled();
+    } else {
+      // |this| is the only EventProcessor currently processing |event|, so
+      // copying the event prior to dispatch is unnecessary.
+      details = DispatchEvent(target, event);
+    }
+
     if (details.dispatcher_destroyed ||
         details.target_destroyed ||
         event->handled()) {