Return "auto" for computed z-index on non-stacking-context element

third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp

 /*
  * Copyright (C) 2004 Zack Rusin <zack@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2011 Sencha, Inc. All rights reserved.
  * Copyright (C) 2015 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 2815 matching lines @@
     case CSSPropertyWebkitFontSmoothing:
       return CSSIdentifierValue::create(
           style.getFontDescription().fontSmoothing());
     case CSSPropertyFontVariantLigatures:
       return valueForFontVariantLigatures(style);
     case CSSPropertyFontVariantCaps:
       return valueForFontVariantCaps(style);
     case CSSPropertyFontVariantNumeric:
       return valueForFontVariantNumeric(style);
     case CSSPropertyZIndex:
-      if (style.hasAutoZIndex())
+      if (style.hasAutoZIndex() || !style.isStackingContext())
         return CSSIdentifierValue::create(CSSValueAuto);
       return CSSPrimitiveValue::create(style.zIndex(),
                                        CSSPrimitiveValue::UnitType::Integer);
     case CSSPropertyZoom:
       return CSSPrimitiveValue::create(style.zoom(),
                                        CSSPrimitiveValue::UnitType::Number);
     case CSSPropertyBoxSizing:
       if (style.boxSizing() == BoxSizingContentBox)
         return CSSIdentifierValue::create(CSSValueContentBox);
       return CSSIdentifierValue::create(CSSValueBorderBox);
@@ skipping 766 matching lines @@
     case CSSPropertyAll:
       return nullptr;
     default:
       break;
   }
   ASSERT_NOT_REACHED();
   return nullptr;
 }
 
 }  // namespace blink