Cache a screen compatible DC to avoid re-creating it excessively.

ui/gfx/screen.h

 // Copyright (c) 2011 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 UI_GFX_SCREEN_H_
 #define UI_GFX_SCREEN_H_
 #pragma once
 
 #include "ui/gfx/native_widget_types.h"
 #include "ui/gfx/point.h"
@@ skipping 47 matching lines @@
   static gfx::NativeWindow GetWindowAtCursorScreenPoint();
 
   // Returns the dimensions of the primary monitor in pixels.
   static gfx::Size GetPrimaryMonitorSize();
 
   // Returns the number of monitors.
   // Mirrored displays are excluded; this method is intended to return the
   // number of distinct, usable displays.
   static int GetNumMonitors();
 
+#if defined(OS_WIN)

[sky, 2012/02/03 18:34:06]

I don't like defining this here. Move it into it's own class/namespace.

+  // Returns a device context compatible with the screen. The context can be
+  // used as a temporary context on the UI thread.
+  static HDC GetCompatibleDC();
+
+  // Should be called on WM_DISPLAYCHANGE to release the cached compatible
+  // device context, if any.
+  static void ReleaseCompatibleDC();
+#endif
+
  protected:
   virtual gfx::Point GetCursorScreenPointImpl() = 0;
   virtual gfx::Rect GetMonitorWorkAreaNearestWindowImpl(
       gfx::NativeView view) = 0;
   virtual gfx::Rect GetMonitorAreaNearestWindowImpl(
       gfx::NativeView view) = 0;
   virtual gfx::Rect GetMonitorWorkAreaNearestPointImpl(
       const gfx::Point& point) = 0;
   virtual gfx::Rect GetMonitorAreaNearestPointImpl(const gfx::Point& point) = 0;
   virtual gfx::NativeWindow GetWindowAtCursorScreenPointImpl() = 0;
   virtual gfx::Size GetPrimaryMonitorSizeImpl() = 0;
   virtual int GetNumMonitorsImpl() = 0;
 
 private:
 #if defined(USE_AURA)
   // The singleton screen instance. Only used on aura.
   static Screen* instance_;
 #endif
+
+#if defined(OS_WIN)
+  // Cached device context compatible with the screen.
+  static HDC compatible_dc_;
+#endif
 };
 
 }  // namespace gfx
 
 #endif  // VIEWS_SCREEN_H_