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

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

Index: third_party/WebKit/Source/core/dom/Element.cpp
diff --git a/third_party/WebKit/Source/core/dom/Element.cpp b/third_party/WebKit/Source/core/dom/Element.cpp
index ba359cea2d719e2710bf57da93ab21c849aed03d..116b3efc50a51a949ba47e66ccfe63669e163a35 100644
--- a/third_party/WebKit/Source/core/dom/Element.cpp
+++ b/third_party/WebKit/Source/core/dom/Element.cpp
@@ -670,20 +670,22 @@ int Element::clientTop()
 
 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().frame()->isLocalRoot())
+                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;
@@ -691,8 +693,6 @@ int Element::clientWidth()
 
 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();
@@ -700,12 +700,16 @@ int Element::clientHeight()
     if ((!inQuirksMode && document().documentElement() == this)
         || (inQuirksMode && isHTMLElement() && document().body() == this)) {
         if (LayoutViewItem layoutView = LayoutViewItem(document().layoutView())) {
+            if (!RuntimeEnabledFeatures::overlayScrollbarsEnabled() || !document().frame()->isLocalRoot())
+                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;