Mac: Don't repaint scrollbars every frame

third_party/WebKit/public/platform/WebScrollbarThemePainter.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 46 matching lines @@
     BLINK_PLATFORM_EXPORT void paintTrackBackground(WebCanvas*, const WebRect&);
     BLINK_PLATFORM_EXPORT void paintBackTrackPart(WebCanvas*, const WebRect&);
     BLINK_PLATFORM_EXPORT void paintForwardTrackPart(WebCanvas*, const WebRect&);
     BLINK_PLATFORM_EXPORT void paintBackButtonStart(WebCanvas*, const WebRect&);
     BLINK_PLATFORM_EXPORT void paintBackButtonEnd(WebCanvas*, const WebRect&);
     BLINK_PLATFORM_EXPORT void paintForwardButtonStart(WebCanvas*, const WebRect&);
     BLINK_PLATFORM_EXPORT void paintForwardButtonEnd(WebCanvas*, const WebRect&);
     BLINK_PLATFORM_EXPORT void paintTickmarks(WebCanvas*, const WebRect&);
     BLINK_PLATFORM_EXPORT void paintThumb(WebCanvas*, const WebRect&);
 
+    BLINK_PLATFORM_EXPORT float trackOpacity() const;
+    BLINK_PLATFORM_EXPORT float thumbOpacity() const;
+
+    BLINK_PLATFORM_EXPORT bool needsPaintTrack() const;

[chrishtr, 2015/11/23 18:59:42]

trackBackgroudNeedsRepaint()
thumbNeedsRepaint()

[ccameron, 2015/11/24 01:12:05]

Done.

+    BLINK_PLATFORM_EXPORT bool needsPaintThumb() const;
+
 #if INSIDE_BLINK
     BLINK_PLATFORM_EXPORT WebScrollbarThemePainter(ScrollbarTheme*, Scrollbar*, float deviceScaleFactor);
 #endif
 
 private:
     // The theme is not owned by this class. It is assumed that the theme is a
     // static pointer and its lifetime is essentially infinite. The functions
     // called from the painter may not be thread-safe, so all calls must be made
     // from the same thread that it is created on.
     ScrollbarTheme* m_theme;
 
     // It is assumed that the constructor of this paint object is responsible
     // for the lifetime of this scrollbar. The painter has to use the real
     // scrollbar (and not a WebScrollbar wrapper) due to static_casts for
     // LayoutScrollbar and pointer-based HashMap lookups for Lion scrollbars.
     WebPrivatePtr<Scrollbar> m_scrollbar;
 
     float m_deviceScaleFactor;
 };
 
 } // namespace blink
 
 #endif