Add support for DocumentFragment.getElementById()

Source/core/dom/ContainerNode.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. 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 1144 matching lines @@
     return ensureRareData().ensureNodeLists().addCache<ClassCollection>(*this, ClassCollectionType, classNames);
 }
 
 PassRefPtr<RadioNodeList> ContainerNode::radioNodeList(const AtomicString& name, bool onlyMatchImgElements)
 {
     ASSERT(isHTMLFormElement(this) || isHTMLFieldSetElement(this));
     CollectionType type = onlyMatchImgElements ? RadioImgNodeListType : RadioNodeListType;
     return ensureRareData().ensureNodeLists().addCache<RadioNodeList>(*this, type, name);
 }
 
+Element* ContainerNode::getElementById(const AtomicString& id) const
+{
+    if (isInTreeScope()) {
+        // Fast path if we are in a tree scope: call getElementById() on tree scope
+        // and check if the matching element is in our subtree.
+        Element* element = treeScope().getElementById(id);
+        if (!element)
+            return 0;
+        if (element->isDescendantOf(this))

[arv (Not doing code reviews), 2014/04/25 15:28:59]

Can this ever be false?

[Inactive, 2014/04/25 15:31:30]

Yes, just because there is an element in the Document (TreeScope) with the given
ID, doesn't mean it is part of our subtree. As the spec says, we are only
interested in Elements that are in our subtree.

[arv (Not doing code reviews), 2014/04/25 15:37:42]

I see. The SVGSVGElement makes this != tree scope. Good thing isDescendantOf
uses treeScope so we do not have to walk the parent nodes here.

+            return element;
+    }
+
+    // Fall back to traversing our subtree. In case of duplicate ids, the first element found will be returned.
+    for (Element* element = ElementTraversal::firstWithin(*this); element; element = ElementTraversal::next(*element, this)) {
+        if (element->getIdAttribute() == id)
+            return element;
+    }
+    return 0;
+}
+
 #ifndef NDEBUG
 bool childAttachedAllowedWhenAttachingChildren(ContainerNode* node)
 {
     if (node->isShadowRoot())
         return true;
 
     if (node->isInsertionPoint())
         return true;
 
     if (node->isElementNode() && toElement(node)->shadow())
         return true;
 
     return false;
 }
 #endif
 
 } // namespace WebCore