Avoid touching z-index in StyleAdjuster by using an isStackingContext flag instead

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 101 matching lines @@
 static bool isInTopLayer(const Element* element, const ComputedStyle& style)
 {
     return (element && element->isInTopLayer()) || style.styleType() == PseudoIdBackdrop;
 }
 
 static bool parentStyleForcesZIndexToCreateStackingContext(const ComputedStyle& parentStyle)
 {
     return parentStyle.isDisplayFlexibleOrGridBox();
 }
 
-static bool hasWillChangeThatCreatesStackingContext(const ComputedStyle& style)
-{
-    for (size_t i = 0; i < style.willChangeProperties().size(); ++i) {
-        switch (style.willChangeProperties()[i]) {
-        case CSSPropertyOpacity:
-        case CSSPropertyTransform:
-        case CSSPropertyAliasWebkitTransform:
-        case CSSPropertyTransformStyle:
-        case CSSPropertyAliasWebkitTransformStyle:
-        case CSSPropertyPerspective:
-        case CSSPropertyAliasWebkitPerspective:
-        case CSSPropertyWebkitMask:
-        case CSSPropertyWebkitMaskBoxImage:
-        case CSSPropertyWebkitClipPath:
-        case CSSPropertyWebkitBoxReflect:
-        case CSSPropertyWebkitFilter:
-        case CSSPropertyBackdropFilter:
-        case CSSPropertyZIndex:
-        case CSSPropertyPosition:
-        case CSSPropertyMixBlendMode:
-        case CSSPropertyIsolation:
-            return true;
-        default:
-            break;
-        }
-    }
-    return false;
-}
-
 void StyleAdjuster::adjustComputedStyle(ComputedStyle& style, const ComputedStyle& parentStyle, Element* element)
 {
     if (style.display() != NONE) {
         if (element && element->isHTMLElement())
             adjustStyleForHTMLElement(style, parentStyle, toHTMLElement(*element));
 
         // Per the spec, position 'static' and 'relative' in the top layer compute to 'absolute'.
         if (isInTopLayer(element, style) && (style.position() == StaticPosition || style.position() == RelativePosition))
             style.setPosition(AbsolutePosition);
 
@@ skipping 12 matching lines @@
         if (style.containsPaint() && style.display() == INLINE)
             style.setDisplay(BLOCK);
     } else {
         adjustStyleForFirstLetter(style);
     }
 
     if (element && element->hasCompositorProxy())
         style.setHasCompositorProxy(true);
 
     // Make sure our z-index value is only applied if the object is positioned.
-    if (style.position() == StaticPosition && !parentStyleForcesZIndexToCreateStackingContext(parentStyle))
-        style.setHasAutoZIndex();
-
-    // Auto z-index becomes 0 for the root element and transparent objects. This prevents
-    // cases where objects that should be blended as a single unit end up with a non-transparent
-    // object wedged in between them. Auto z-index also becomes 0 for objects that specify transforms/masks/reflections.
-    if (style.hasAutoZIndex() && ((element && element->document().documentElement() == element)
-        || style.hasOpacity()
-        || style.hasTransformRelatedProperty()
-        || style.hasMask()
-        || style.clipPath()
-        || style.boxReflect()
-        || style.hasFilterInducingProperty()
-        || style.hasBlendMode()
-        || style.hasIsolation()
-        || style.hasViewportConstrainedPosition()
-        || isInTopLayer(element, style)
-        || hasWillChangeThatCreatesStackingContext(style)
-        || style.containsPaint()))
-        style.setZIndex(0);
+    if (style.position() == StaticPosition && !parentStyleForcesZIndexToCreateStackingContext(parentStyle)) {
+        style.setIsStackingContext(false);
+        // TODO(alancutter): Avoid altering z-index here.

[rune, 2016/06/10 12:25:52]

Could you add the crbug issue here as well? I've attached another case to the
issue which shows the problem with this adjustment.

+        if (!style.hasAutoZIndex())
+            style.setZIndex(0);
+    } else if (!style.hasAutoZIndex()) {
+        style.setIsStackingContext(true);
+    }
 
     if (doesNotInheritTextDecoration(style, element))
         style.clearAppliedTextDecorations();
 
     style.applyTextDecorations();
 
     if (style.overflowX() != OverflowVisible || style.overflowY() != OverflowVisible)
         adjustOverflow(style);
 
     // Cull out any useless layers and also repeat patterns into additional layers.
@@ skipping 244 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