Implement the ::backdrop pseudo-element for modal <dialog>.

Source/core/dom/Element.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012, 2013 Apple Inc. All rights reserved.
  *           (C) 2007 Eric Seidel (eric@webkit.org)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 187 matching lines @@
     }
 #endif
 
     if (PropertySetCSSStyleDeclaration* cssomWrapper = inlineStyleCSSOMWrapper())
         cssomWrapper->clearParentElement();
 
     if (hasRareData()) {
         ElementRareData* data = elementRareData();
         data->setPseudoElement(BEFORE, 0);
         data->setPseudoElement(AFTER, 0);
+        data->setPseudoElement(BACKDROP, 0);
         data->clearShadow();
     }
 
     if (isCustomElement() && document() && document()->registry()) {
         document()->registry()->customElementWasDestroyed(this);
     }
 
     if (hasSyntheticAttrChildNodes())
         detachAllAttrNodesFromElement();
 
@@ skipping 1118 matching lines @@
     // When a shadow root exists, it does the work of attaching the children.
     if (ElementShadow* shadow = this->shadow()) {
         parentPusher.push();
         shadow->attach(context);
     } else if (firstChild())
         parentPusher.push();
 
     ContainerNode::attach(context);
 
     createPseudoElementIfNeeded(AFTER);
+    createPseudoElementIfNeeded(BACKDROP);
 
     if (hasRareData()) {
         ElementRareData* data = elementRareData();
         if (data->needsFocusAppearanceUpdateSoonAfterAttach()) {
             if (isFocusable() && document()->focusedNode() == this)
                 document()->updateFocusAppearanceSoon(false /* don't restore selection */);
             data->setNeedsFocusAppearanceUpdateSoonAfterAttach(false);
         }
     }
 
@@ skipping 1039 matching lines @@
         // when RenderObject::isChildAllowed on our parent returns false for the
         // PseudoElement's renderer for each style recalc.
         if (!renderer() || !pseudoElementRendererIsNeeded(renderer()->getCachedPseudoStyle(pseudoId)))
             elementRareData()->setPseudoElement(pseudoId, 0);
     } else if (change >= Inherit || needsStyleRecalc())
         createPseudoElementIfNeeded(pseudoId);
 }
 
 void Element::createPseudoElementIfNeeded(PseudoId pseudoId)
 {
-    if (!document()->styleSheetCollection()->usesBeforeAfterRules())
+    if ((pseudoId == BEFORE || pseudoId == AFTER) && !document()->styleSheetCollection()->usesBeforeAfterRules())
+        return;
+
+    if (pseudoId == BACKDROP && !isInTopLayer())
         return;
 
     if (!renderer() || !pseudoElementRendererIsNeeded(renderer()->getCachedPseudoStyle(pseudoId)))
         return;
 
     if (!renderer()->canHaveGeneratedChildren())
         return;
 
     ASSERT(!isPseudoElement());
     RefPtr<PseudoElement> element = PseudoElement::create(this, pseudoId);
@@ skipping 1143 matching lines @@
     return 0;
 }
 
 Attribute* UniqueElementData::attributeItem(unsigned index)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(index < length());
     return &m_attributeVector.at(index);
 }
 
 } // namespace WebCore