Spurious scrollbars should not be drawn when overflow is applied to a frame.

third_party/WebKit/Source/core/css/resolver/StyleAdjuster.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/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 331 matching lines @@
     }
 
     if (isHTMLFrameElement(element) || isHTMLFrameSetElement(element)) {
         // Frames and framesets never honor position:relative or position:absolute. This is necessary to
         // fix a crash where a site tries to position these objects. They also never honor display.
         style.setPosition(StaticPosition);
         style.setDisplay(BLOCK);
         return;
     }
 
+    if (isHTMLFrameElementBase(element)) {
+        // Frames cannot overflow (they are always the size we ask them to be).
+        // Some compositing code paths may try to draw scrollbars anyhow.
+        style.setOverflowX(OverflowVisible);
+        style.setOverflowY(OverflowVisible);

[leviw_travelin_and_unemployed, 2016/03/29 18:40:42]

Early return?

+    }
+
     if (isHTMLRTElement(element)) {
         // Ruby text does not support float or position. This might change with evolution of the specification.
         style.setPosition(StaticPosition);
         style.setFloating(NoFloat);
         return;
     }
 
     if (isHTMLMarqueeElement(element)) {
         // For now, <marquee> requires an overflow clip to work properly.
         style.setOverflowX(OverflowHidden);
@@ skipping 89 matching lines @@
         // We want to count vertical percentage paddings/margins on flex items because our current
         // behavior is different from the spec and we want to gather compatibility data.
         if (style.paddingBefore().hasPercent() || style.paddingAfter().hasPercent())
             UseCounter::count(document, UseCounter::FlexboxPercentagePaddingVertical);
         if (style.marginBefore().hasPercent() || style.marginAfter().hasPercent())
             UseCounter::count(document, UseCounter::FlexboxPercentageMarginVertical);
     }
 }
 
 } // namespace blink