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

ash/wm/panels/panel_layout_manager.cc

Index: ash/wm/panels/panel_layout_manager.cc
diff --git a/ash/wm/panels/panel_layout_manager.cc b/ash/wm/panels/panel_layout_manager.cc
index 3bd70369c6c621d34451a5dc70f64c8801bc11dd..3d7eee43b0d3c0eddd09eb18afc72622e9ded175 100644
--- a/ash/wm/panels/panel_layout_manager.cc
+++ b/ash/wm/panels/panel_layout_manager.cc
@@ -238,14 +238,11 @@ void PanelLayoutManager::Shutdown() {
 
 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();
 }
@@ -279,7 +276,10 @@ void PanelLayoutManager::OnWindowAddedToLayout(aura::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) {