[ash-md] Prevents a crash when entering overview mode while dragging a window

ash/common/wm/overview/window_selector_item.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 "ash/common/wm/overview/window_selector_item.h"
 
 #include <algorithm>
 #include <vector>
 
 #include "ash/common/material_design/material_design_controller.h"
@@ skipping 409 matching lines @@
 }
 
 void WindowSelectorItem::Shutdown() {
   if (transform_window_.GetTopInset()) {
     // Activating a window (even when it is the window that was active before
     // overview) results in stacking it at the top. Maintain the label window
     // stacking position above the item to make the header transformation more
     // gradual upon exiting the overview mode.
     WmWindow* label_window =
         WmLookup::Get()->GetWindowForWidget(window_label_.get());
-    label_window->GetParent()->StackChildAbove(label_window,
-                                               transform_window_.window());
+
+    // |label_window| was originally created in the same container as the
+    // |transform_window_| but when closing overview the |transform_window_|
+    // could have been reparented if a drag was active. Only change stacking
+    // if the windows still belong to the same container.
+    if (label_window->GetParent() == transform_window_.window()->GetParent()) {

[tdanderson, 2016/10/20 21:09:33]

For my own learning, in general must two windows in the same container have the
same parent (i.e., the container itself) or is that just true of this specific
situation?

[varkha, 2016/10/20 23:54:40]

Yes the two windows in the same container have the same parent (that container).
The problem here is that one of those two windows in this case (the real window
rather than the label that shows the title in overview) is reparented when
ending a drag. Ending the drag also ends overview mode because the window
hierarchy changes. So we get into shutdown but still assume that the title
window and the real window belong to the same container when they are not.

Note: while windows are dragged they get reparented to one of the top-level
containers (like a docked container) to allow them to be on top of any other
windows including docked or panels. At the end of the drag this is reverted.

+      label_window->GetParent()->StackChildAbove(label_window,
+                                                 transform_window_.window());
+    }
   }
   if (background_view_) {
     background_view_->OnItemRestored();
     background_view_ = nullptr;
   }
   FadeOut(std::move(window_label_));
 }
 
 void WindowSelectorItem::ShowWindowOnExit() {
   transform_window_.ShowWindowOnExit();
@@ skipping 339 matching lines @@
   window_selector_->delegate()->AddDelayedAnimationObserver(
       std::move(observer));
   widget_ptr->SetOpacity(0.f);
 }
 
 gfx::SlideAnimation* WindowSelectorItem::GetBackgroundViewAnimation() {
   return background_view_ ? background_view_->animation() : nullptr;
 }
 
 }  // namespace ash