Caches the scrollbar's thickness for the given theme and control size

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 51 matching lines @@
       m_elasticOverscroll(0),
       m_trackNeedsRepaint(true),
       m_thumbNeedsRepaint(true) {
   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);
+  m_themeScrollbarThickness = thickness;

[oshima, 2016/10/14 23:17:30]

don't you have to cache for each controlSize?

[malaykeshav, 2016/10/14 23:18:54]

For a given scrollbar instance the ControlSize does not change. It is
initialized in the constructor.

   if (m_hostWindow)
     thickness = m_hostWindow->windowToViewportScalar(thickness);
   Widget::setFrameRect(IntRect(0, 0, thickness, thickness));
 
   m_currentPos = scrollableAreaCurrentPos();
 }
 
 Scrollbar::~Scrollbar() {
   m_theme.unregisterScrollbar(*this);
 }
@@ skipping 437 matching lines @@
     return;
   m_enabled = e;
   theme().updateEnabledState(*this);
   setNeedsPaintInvalidation(AllParts);
 }
 
 int Scrollbar::scrollbarThickness() const {
   int thickness = orientation() == HorizontalScrollbar ? height() : width();
   if (!thickness || !m_hostWindow)
     return thickness;
-  return m_hostWindow->windowToViewportScalar(
-      m_theme.scrollbarThickness(controlSize()));
+  return m_hostWindow->windowToViewportScalar(m_themeScrollbarThickness);
 }
 
 bool Scrollbar::isOverlayScrollbar() const {
   return m_theme.usesOverlayScrollbars();
 }
 
 bool Scrollbar::shouldParticipateInHitTesting() {
   // Non-overlay scrollbars should always participate in hit testing.
   if (!isOverlayScrollbar())
     return true;
@@ skipping 76 matching lines @@
     invalidParts = AllParts;
   if (invalidParts & ~ThumbPart)
     m_trackNeedsRepaint = true;
   if (invalidParts & ThumbPart)
     m_thumbNeedsRepaint = true;
   if (m_scrollableArea)
     m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }
 
 }  // namespace blink