Don't force-update style and layout in Document::scrollingElement

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All
  * rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
@@ skipping 1327 matching lines @@
   PositionWithAffinity positionWithAffinity = result.position();
   if (positionWithAffinity.isNull())
     return nullptr;
 
   Position rangeCompliantPosition =
       positionWithAffinity.position().parentAnchoredEquivalent();
   return Range::createAdjustedToTreeScope(*this, rangeCompliantPosition);
 }
 
 Element* Document::scrollingElement() {
+  if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled() && inQuirksMode())
+    updateStyleAndLayoutTree();
+  return scrollingElementNoLayout();
+}
+
+Element* Document::scrollingElementNoLayout() {
   if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
     if (inQuirksMode()) {
-      updateStyleAndLayoutTree();
+      DCHECK(m_lifecycle.state() >= DocumentLifecycle::StyleClean);

[chrishtr, 2017/01/11 17:19:00]

StyleClean is the right one, since updateStyleAndLayoutTree()
does not layout, just updates style and creates the layout tree.

       HTMLBodyElement* body = firstBodyElement();
       if (body && body->layoutObject() &&
           body->layoutObject()->hasOverflowClip())
         return nullptr;
 
       return body;
     }
 
     return documentElement();
   }
@@ skipping 5222 matching lines @@
 }
 
 void showLiveDocumentInstances() {
   WeakDocumentSet& set = liveDocumentSet();
   fprintf(stderr, "There are %u documents currently alive:\n", set.size());
   for (Document* document : set)
     fprintf(stderr, "- Document %p URL: %s\n", document,
             document->url().getString().utf8().data());
 }
 #endif