Mac: Don't repaint scrollbars every frame

third_party/WebKit/Source/platform/scroll/Scrollbar.cpp

 /*
  * Copyright (C) 2004, 2006, 2008 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 43 matching lines @@
     , m_pressedPart(NoPart)
     , m_pressedPos(0)
     , m_scrollPos(0)
     , m_draggingDocument(false)
     , m_documentDragPos(0)
     , m_enabled(true)
     , m_scrollTimer(this, &Scrollbar::autoscrollTimerFired)
     , m_overlapsResizer(false)
     , m_isAlphaLocked(false)
     , m_elasticOverscroll(0)
+    , m_trackNeedsRepaint(true)
+    , m_thumbNeedsRepaint(true)
 {
     if (!m_theme)
         m_theme = ScrollbarTheme::theme();
 
     m_theme->registerScrollbar(this);
 
     // FIXME: This is ugly and would not be necessary if we fix cross-platform code to actually query for
     // scrollbar thickness and use it when sizing scrollbars (rather than leaving one dimension of the scrollbar
     // alone when sizing).
     int thickness = m_theme->scrollbarThickness(controlSize);
@@ skipping 460 matching lines @@
         return 0;
 
     if (m_orientation == HorizontalScrollbar)
         return m_scrollableArea->scrollPosition().x() - m_scrollableArea->minimumScrollPosition().x();
 
     return m_scrollableArea->scrollPosition().y() - m_scrollableArea->minimumScrollPosition().y();
 }
 
 void Scrollbar::setNeedsPaintInvalidation()
 {
+    if (m_theme->shouldRepaintAllPartsOnInvalidation()) {
+        m_trackNeedsRepaint = true;
+        m_thumbNeedsRepaint = true;
+    }
     if (m_scrollableArea)
         m_scrollableArea->setScrollbarNeedsPaintInvalidation(this);
 }
 
 } // namespace blink