Avoid style recalc and layout when not necessary for client size.

third_party/WebKit/Source/core/dom/Element.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012, 2013 Apple Inc. All rights reserved.
  *           (C) 2007 Eric Seidel (eric@webkit.org)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 652 matching lines @@
 {
     document().updateLayoutIgnorePendingStylesheetsForNode(this);
 
     if (LayoutBox* layoutObject = layoutBox())
         return adjustLayoutUnitForAbsoluteZoom(layoutObject->clientTop(), layoutObject->styleRef()).round();
     return 0;
 }
 
 int Element::clientWidth()
 {
-    document().updateLayoutIgnorePendingStylesheetsForNode(this);
-
     // When in strict mode, clientWidth for the document element should return the width of the containing frame.
     // When in quirks mode, clientWidth for the body element should return the width of the containing frame.
     bool inQuirksMode = document().inQuirksMode();
     if ((!inQuirksMode && document().documentElement() == this)
         || (inQuirksMode && isHTMLElement() && document().body() == this)) {
         if (LayoutViewItem layoutView = LayoutViewItem(document().layoutView())) {
+            if (!RuntimeEnabledFeatures::overlayScrollbarsEnabled())
+                document().updateLayoutIgnorePendingStylesheetsForNode(this);
             if (document().page()->settings().forceZeroLayoutHeight())
                 return adjustLayoutUnitForAbsoluteZoom(layoutView.overflowClipRect(LayoutPoint()).width(), layoutView.styleRef()).round();
             return adjustLayoutUnitForAbsoluteZoom(LayoutUnit(layoutView.layoutSize().width()), layoutView.styleRef()).round();
         }
     }
 
+    document().updateLayoutIgnorePendingStylesheetsForNode(this);
+
     if (LayoutBox* layoutObject = layoutBox())
         return adjustLayoutUnitForAbsoluteZoom(LayoutUnit(layoutObject->pixelSnappedClientWidth()), layoutObject->styleRef()).round();
     return 0;
 }
 
 int Element::clientHeight()
 {
-    document().updateLayoutIgnorePendingStylesheetsForNode(this);
-
     // When in strict mode, clientHeight for the document element should return the height of the containing frame.
     // When in quirks mode, clientHeight for the body element should return the height of the containing frame.
     bool inQuirksMode = document().inQuirksMode();
 
     if ((!inQuirksMode && document().documentElement() == this)
         || (inQuirksMode && isHTMLElement() && document().body() == this)) {
         if (LayoutViewItem layoutView = LayoutViewItem(document().layoutView())) {
+            if (!RuntimeEnabledFeatures::overlayScrollbarsEnabled())
+                document().updateLayoutIgnorePendingStylesheetsForNode(this);
             if (document().page()->settings().forceZeroLayoutHeight())
                 return adjustLayoutUnitForAbsoluteZoom(layoutView.overflowClipRect(LayoutPoint()).height(), layoutView.styleRef()).round();
             return adjustLayoutUnitForAbsoluteZoom(LayoutUnit(layoutView.layoutSize().height()), layoutView.styleRef()).round();
         }
     }
 
+    document().updateLayoutIgnorePendingStylesheetsForNode(this);
+
     if (LayoutBox* layoutObject = layoutBox())
         return adjustLayoutUnitForAbsoluteZoom(LayoutUnit(layoutObject->pixelSnappedClientHeight()), layoutObject->styleRef()).round();
     return 0;
 }
 
 double Element::scrollLeft()
 {
     document().updateLayoutIgnorePendingStylesheetsForNode(this);
 
     if (document().scrollingElement() == this) {
@@ skipping 2976 matching lines @@
 
 DEFINE_TRACE(Element)
 {
     if (hasRareData())
         visitor->trace(elementRareData());
     visitor->trace(m_elementData);
     ContainerNode::trace(visitor);
 }
 
 } // namespace blink