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

ui/events/event_processor.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/events/event_processor.h"
 
 #include "ui/events/event_target.h"
 #include "ui/events/event_targeter.h"
 
 namespace ui {
 
 EventDispatchDetails EventProcessor::OnEventFromSource(Event* event) {
   EventTarget* root = GetRootTarget();
   CHECK(root);
   EventTargeter* targeter = root->GetEventTargeter();
   CHECK(targeter);
 
   PrepareEventForDispatch(event);
   EventTarget* target = targeter->FindTargetForEvent(root, event);
 
   while (target) {
-    EventDispatchDetails details = DispatchEvent(target, event);
+    EventDispatchDetails details;
+    if (event->phase() != EP_PREDISPATCH) {
+      // |event| is currently being processed by another EventProcessor, which
+      // expects the event's state (e.g., target, phase, etc.) to remain
+      // unchanged once |this| has finished its nested processing (with the
+      // exception of whether or not the event was handled). Thus dispatch
+      // a copy of the event instead.
+      scoped_ptr<Event> event_copy(event->Clone());
+      details = DispatchEvent(target, event_copy.get());

[sadrul, 2014/08/29 21:12:17]

I think we want to clone the event before going into this loop?

[tdanderson, 2014/09/02 22:57:09]

Done.

+
+      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()) {
       return details;
     }
 
     target = targeter->FindNextBestTarget(target, event);
   }
 
   return EventDispatchDetails();
 }
 
 void EventProcessor::PrepareEventForDispatch(Event* event) {
 }
 
 }  // namespace ui