GTK Panels - fix crash when clicking on titlebar to restore a panel.

chrome/browser/ui/panels/panel_drag_gtk.h

 // 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.
 
 #ifndef CHROME_BROWSER_UI_PANELS_PANEL_DRAG_GTK_H_
 #define CHROME_BROWSER_UI_PANELS_PANEL_DRAG_GTK_H_
 #pragma once
 
 #include <gtk/gtk.h>
 
@@ skipping 30 matching lines @@
   // |titlebar_widget| should handle the mouse release event if the mouse
   // is released without exceeding the drag threshold (a mouse click).
   void InitialTitlebarMousePress(GdkEventButton* event,
                                  GtkWidget* titlebar_widget);
 
  private:
   friend class NativePanelTestingGtk;
 
   enum DragState {
     NOT_DRAGGING,
-    DRAG_IN_PROGRESS,
+    DRAG_CAN_START,  // mouse pressed
+    DRAG_IN_PROGRESS,  // mouse moved beyond drag threshold
     DRAG_ENDED_WAITING_FOR_MOUSE_RELEASE
   };
 
   // Callbacks for GTK mouse and key events.
   CHROMEGTK_CALLBACK_1(PanelDragGtk, gboolean, OnMouseMoveEvent,
                        GdkEventMotion*);
   CHROMEGTK_CALLBACK_1(PanelDragGtk, gboolean, OnButtonPressEvent,
                        GdkEventButton*);
   CHROMEGTK_CALLBACK_1(PanelDragGtk, gboolean, OnButtonReleaseEvent,
                        GdkEventButton*);
@@ skipping 35 matching lines @@
   // handling is necessary.
   GtkWidget* click_handler_;
 
   // Delegate for processing drag depends on actual type of drag.
   PanelDragDelegate* drag_delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(PanelDragGtk);
 };
 
 #endif  // CHROME_BROWSER_UI_PANELS_PANEL_DRAG_GTK_H_