Stop explicit inheritance if LayoutStyle doesn't change.

Source/core/layout/style/LayoutStyle.cpp

 /*
  * Copyright (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 170 matching lines @@
 
     if (oldStyle->display() != newStyle->display()
         || oldStyle->hasPseudoStyle(FIRST_LETTER) != newStyle->hasPseudoStyle(FIRST_LETTER)
         || oldStyle->columnSpan() != newStyle->columnSpan()
         || !oldStyle->contentDataEquivalent(newStyle)
         || oldStyle->hasTextCombine() != newStyle->hasTextCombine()
         || oldStyle->justifyItems() != newStyle->justifyItems()
         || oldStyle->alignItems() != newStyle->alignItems())
         return Reattach;
 
-    if (oldStyle->inheritedNotEqual(*newStyle)
-        || oldStyle->hasExplicitlyInheritedProperties()
-        || newStyle->hasExplicitlyInheritedProperties())
+    if (oldStyle->inheritedNotEqual(*newStyle))
         return Inherit;
 
     if (*oldStyle == *newStyle)
         return diffPseudoStyles(*oldStyle, *newStyle);
 
+    if (oldStyle->hasExplicitlyInheritedProperties())
+        return Inherit;
+
     return NoInherit;
 }
 
 ItemPosition LayoutStyle::resolveAlignment(const LayoutStyle& parentStyle, const LayoutStyle& childStyle, ItemPosition resolvedAutoPositionForRenderer)
 {
     // The auto keyword computes to the parent's align-items computed value, or to "stretch", if not set or "auto".
     if (childStyle.alignSelf() == ItemPositionAuto)
         return (parentStyle.alignItems() == ItemPositionAuto) ? resolvedAutoPositionForRenderer : parentStyle.alignItems();
     return childStyle.alignSelf();
 }
@@ skipping 14 matching lines @@
         setUserModify(currentUserModify);
     } else {
         rareInheritedData = inheritParent.rareInheritedData;
     }
     inherited = inheritParent.inherited;
     inherited_flags = inheritParent.inherited_flags;
     if (m_svgStyle != inheritParent.m_svgStyle)
         m_svgStyle.access()->inheritFrom(inheritParent.m_svgStyle.get());
 }
 
-void LayoutStyle::copyNonInheritedFrom(const LayoutStyle& other)
+void LayoutStyle::copyNonInheritedFromCached(const LayoutStyle& other)
 {
     m_box = other.m_box;
     visual = other.visual;
     m_background = other.m_background;
     surround = other.surround;
     rareNonInheritedData = other.rareNonInheritedData;
+
     // The flags are copied one-by-one because noninherited_flags contains a bunch of stuff other than real style data.
+    // See comments for each skipped flag below.
     noninherited_flags.effectiveDisplay = other.noninherited_flags.effectiveDisplay;
     noninherited_flags.originalDisplay = other.noninherited_flags.originalDisplay;
     noninherited_flags.overflowX = other.noninherited_flags.overflowX;
     noninherited_flags.overflowY = other.noninherited_flags.overflowY;
     noninherited_flags.verticalAlign = other.noninherited_flags.verticalAlign;
     noninherited_flags.clear = other.noninherited_flags.clear;
     noninherited_flags.position = other.noninherited_flags.position;
     noninherited_flags.floating = other.noninherited_flags.floating;
     noninherited_flags.tableLayout = other.noninherited_flags.tableLayout;
     noninherited_flags.unicodeBidi = other.noninherited_flags.unicodeBidi;
+    noninherited_flags.hasViewportUnits = other.noninherited_flags.hasViewportUnits;
     noninherited_flags.pageBreakBefore = other.noninherited_flags.pageBreakBefore;
     noninherited_flags.pageBreakAfter = other.noninherited_flags.pageBreakAfter;
     noninherited_flags.pageBreakInside = other.noninherited_flags.pageBreakInside;
-    noninherited_flags.explicitInheritance = other.noninherited_flags.explicitInheritance;
-    noninherited_flags.hasViewportUnits = other.noninherited_flags.hasViewportUnits;
+
+    // Correctly set during selector matching:
+    // noninherited_flags.styleType
+    // noninherited_flags.pseudoBits
+
+    // Set correctly while computing style for children:
+    // noninherited_flags.explicitInheritance
+
+    // unique() styles are not cacheable.
+    ASSERT(!other.noninherited_flags.unique);
+
+    // The following flags are set during matching before we decide that we get a
+    // match in the MatchedPropertiesCache which in turn calls this method. The
+    // reason why we don't copy these flags is that they're already correctly set
+    // and that they may differ between elements which have the same set of matched
+    // properties. For instance, given the rule:
+    //
+    // :-webkit-any(:hover, :focus) { background-color: green }"
+    //
+    // A hovered element, and a focused element may use the same cached matched
+    // properties here, but the affectedBy flags will be set differently based on
+    // the matching order of the :-webkit-any components.
+    //
+    // noninherited_flags.emptyState
+    // noninherited_flags.affectedByFocus
+    // noninherited_flags.affectedByHover
+    // noninherited_flags.affectedByActive
+    // noninherited_flags.affectedByDrag
+    // noninherited_flags.isLink
+
     if (m_svgStyle != other.m_svgStyle)
-        m_svgStyle.access()->copyNonInheritedFrom(other.m_svgStyle.get());
+        m_svgStyle.access()->copyNonInheritedFromCached(other.m_svgStyle.get());
     ASSERT(zoom() == initialZoom());
 }
 
 bool LayoutStyle::operator==(const LayoutStyle& o) const
 {
     // compare everything except the pseudoStyle pointer
     return inherited_flags == o.inherited_flags
         && noninherited_flags == o.noninherited_flags
         && m_box == o.m_box
         && visual == o.visual
@@ skipping 1398 matching lines @@
         horizontal || includeLogicalRightEdge);
 
     edges[BSLeft] = BorderEdge(borderLeftWidth(),
         visitedDependentColor(CSSPropertyBorderLeftColor),
         borderLeftStyle(),
         borderLeftIsTransparent(),
         !horizontal || includeLogicalLeftEdge);
 }
 
 } // namespace blink