Set overflow: auto in StyleResolver::styleForDocument.

third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
@@ skipping 604 matching lines @@
   documentStyle->setZIndex(0);
   documentStyle->setIsStackingContext(true);
   documentStyle->setUserModify(document.inDesignMode() ? READ_WRITE
                                                        : READ_ONLY);
   // These are designed to match the user-agent stylesheet values for the
   // document element so that the common case doesn't need to create a new
   // ComputedStyle in Document::inheritHtmlAndBodyElementStyles.
   documentStyle->setDisplay(EDisplay::Block);
   documentStyle->setPosition(AbsolutePosition);
 
+  // Document::inheritHtmlAndBodyElementStyles will set the final overflow
+  // style values, but they should initially be auto to avoid premature
+  // scrollbar removal in PaintLayerScrollableArea::updateAfterStyleChange.
+  documentStyle->setOverflowX(EOverflow::Auto);
+  documentStyle->setOverflowY(EOverflow::Auto);
+
   document.setupFontBuilder(*documentStyle);
 
   return documentStyle.release();
 }
 
 void StyleResolver::adjustComputedStyle(StyleResolverState& state,
                                         Element* element) {
   StyleAdjuster::adjustComputedStyle(state.mutableStyleRef(),
                                      *state.parentStyle(), element);
 }
@@ skipping 1341 matching lines @@
 DEFINE_TRACE(StyleResolver) {
   visitor->trace(m_matchedPropertiesCache);
   visitor->trace(m_selectorFilter);
   visitor->trace(m_styleSharingLists);
   visitor->trace(m_pendingStyleSheets);
   visitor->trace(m_document);
   visitor->trace(m_tracker);
 }
 
 }  // namespace blink