[GTK] correct tabstrip drag point calculation.

chrome/browser/gtk/tabs/dragged_tab_controller_gtk.cc

 // Copyright (c) 2010 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 "chrome/browser/gtk/tabs/dragged_tab_controller_gtk.h"
 
 #include <algorithm>
 
 #include "base/callback.h"
 #include "chrome/browser/browser.h"
@@ skipping 420 matching lines @@
   }
 
   // Detaching resets the delegate, but we still want to be the delegate.
   dragged_contents_->set_delegate(this);
 
   attached_tabstrip_ = NULL;
 }
 
 gfx::Point DraggedTabControllerGtk::ConvertScreenPointToTabStripPoint(
     TabStripGtk* tabstrip, const gfx::Point& screen_point) {
-  gint x, y;
-  gdk_window_get_origin(tabstrip->tabstrip_->window, &x, &y);
-  return gfx::Point(screen_point.x() - x, screen_point.y() - y);
+  gfx::Point tabstrip_screen_point =
+      gtk_util::GetWidgetScreenPosition(tabstrip->tabstrip_.get());
+  return screen_point.Subtract(tabstrip_screen_point);
 }
 
 gfx::Rect DraggedTabControllerGtk::GetDraggedTabTabStripBounds(
     const gfx::Point& screen_point) {
   gfx::Point client_point =
       ConvertScreenPointToTabStripPoint(attached_tabstrip_, screen_point);
   gfx::Size tab_size = dragged_tab_->attached_tab_size();
   return gfx::Rect(client_point.x(), client_point.y(),
                    tab_size.width(), tab_size.height());
 }
@@ skipping 318 matching lines @@
     gfx::NativeView dragged_tab = dragged_tab_->widget();
     dock_windows_.insert(dragged_tab);
     window = DockInfo::GetLocalProcessWindowAtPoint(GetCursorScreenPoint(),
                                                     dock_windows_);
     dock_windows_.erase(dragged_tab);
   }
 
   if (window)
     gtk_window_present(GTK_WINDOW(window));
 }