Add UseCounter for weird body+viewport scrolling

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.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 1619 matching lines @@
     }
     EImageRendering imageRendering = backgroundStyle->imageRendering();
 
     const ComputedStyle* overflowStyle = nullptr;
     if (Element* element = viewportDefiningElement(documentElementStyle.get())) {
         if (element == body) {
             overflowStyle = bodyStyle.get();
         } else {
             ASSERT(element == documentElement());
             overflowStyle = documentElementStyle.get();
+
+            // The body element has its own scrolling box, independent from the viewport.
+            // This is a bit of a weird edge case in the CSS spec that we might want to try to
+            // eliminate some day (eg. for ScrollTopLeftInterop - see http://crbug.com/157855).
+            if (bodyStyle && !bodyStyle->isOverflowVisible())
+                UseCounter::count(*this, UseCounter::BodyScrollsInAdditionToViewport);
         }
     }
 
     // Resolved rem units are stored in the matched properties cache so we need to make sure to
     // invalidate the cache if the documentElement needed to reattach or the font size changed
     // and then trigger a full document recalc. We also need to clear it here since the
     // call to styleForElement on the body above can cache bad values for rem units if the
     // documentElement's style was dirty. We could keep track of which elements depend on
     // rem units like we do for viewport styles, but we assume root font size changes are
     // rare and just invalidate the cache for now.
@@ skipping 4266 matching lines @@
 #ifndef NDEBUG
 using namespace blink;
 void showLiveDocumentInstances()
 {
     Document::WeakDocumentSet& set = Document::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().string().utf8().data());
 }
 #endif