Attach panel while dragging to bring it in front of other panels.

ash/wm/panels/panel_layout_manager.cc

 // Copyright (c) 2012 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/wm/panels/panel_layout_manager.h"
 
 #include <algorithm>
 #include <map>
 
 #include "ash/launcher/launcher.h"
@@ skipping 220 matching lines @@
     delete iter->callout_widget;
   }
   panel_windows_.clear();
   if (launcher_)
     launcher_->RemoveIconObserver(this);
   launcher_ = NULL;
 }
 
 void PanelLayoutManager::StartDragging(aura::Window* panel) {
   DCHECK(!dragged_panel_);
-  DCHECK(panel->parent() == panel_container_);
   dragged_panel_ = panel;
   Relayout();
 }
 
 void PanelLayoutManager::FinishDragging() {
-  // Note, dragged panel may be null if the panel was just attached to the
-  // panel layout.
   dragged_panel_ = NULL;
   Relayout();
 }
 
 void PanelLayoutManager::SetLauncher(ash::Launcher* launcher) {
   launcher_ = launcher;
   launcher_->AddIconObserver(this);
 }
 
 void PanelLayoutManager::ToggleMinimize(aura::Window* panel) {
@@ skipping 13 matching lines @@
 }
 
 void PanelLayoutManager::OnWindowAddedToLayout(aura::Window* child) {
   if (child->type() == aura::client::WINDOW_TYPE_POPUP)
     return;
   PanelInfo panel_info;
   panel_info.window = child;
   panel_info.callout_widget = new PanelCalloutWidget(panel_container_);
   panel_windows_.push_back(panel_info);
   child->AddObserver(this);
-  Relayout();
+  // When dragging a panel on the default container it is temporarily added to
+  // the panel container and doesn't require a relayout.
+  if (child != dragged_panel_)
+    Relayout();
 }
 
 void PanelLayoutManager::OnWillRemoveWindowFromLayout(aura::Window* child) {
 }
 
 void PanelLayoutManager::OnWindowRemovedFromLayout(aura::Window* child) {
   if (child->type() == aura::client::WINDOW_TYPE_POPUP)
     return;
   PanelList::iterator found =
       std::find(panel_windows_.begin(), panel_windows_.end(), child);
@@ skipping 326 matching lines @@
 
     SetChildBoundsDirect(callout_widget->GetNativeWindow(), callout_bounds);
     panel_container_->StackChildAbove(callout_widget->GetNativeWindow(),
                                       panel);
     callout_widget->Show();
   }
 }
 
 }  // namespace internal
 }  // namespace ash