EventDispatcher shouldn't always cancel on a hierarchy change

components/mus/ws/event_dispatcher.cc

 // Copyright 2015 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 "components/mus/ws/event_dispatcher.h"
 
 #include <algorithm>
 
 #include "base/time/time.h"
 #include "components/mus/ws/accelerator.h"
@@ skipping 492 matching lines @@
     if (pair.second->MatchesEvent(event, phase)) {
       return pair.second.get();
     }
   }
   return nullptr;
 }
 
 void EventDispatcher::OnWillChangeWindowHierarchy(ServerWindow* window,
                                                   ServerWindow* new_parent,
                                                   ServerWindow* old_parent) {
-  CancelPointerEventsToTarget(window);
+  // TODO(sky): moving to a different root likely needs to transfer capture.
+  // TODO(sky): this isn't quite right, I think the logic should be (assuming
+  // moving in same root and still drawn):
+  // . if there is capture and window is still in the same root, continue
+  //   sending to it.
+  // . if there isn't capture, then reevaluate each of the pointer targets
+  //   sending exit as necessary.
+  // http://crbug.com/613646 .
+  if (!new_parent || !new_parent->IsDrawn() ||
+      new_parent->GetRoot() != old_parent->GetRoot()) {
+    CancelPointerEventsToTarget(window);
+  }
 }
 
 void EventDispatcher::OnWindowVisibilityChanged(ServerWindow* window) {
   CancelPointerEventsToTarget(window);
 }
 
 void EventDispatcher::OnWindowDestroyed(ServerWindow* window) {
   CancelPointerEventsToTarget(window);
 
   if (mouse_cursor_source_window_ == window)
     mouse_cursor_source_window_ = nullptr;
 }
 
 }  // namespace ws
 }  // namespace mus