Fix the way <title> is read under <use> shadow tree

third_party/WebKit/Source/core/svg/SVGElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Nikolas Zimmermann
  * <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2008 Rob Buis <buis@kde.org>
  * Copyright (C) 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Alp Toker <alp@atoker.com>
  * Copyright (C) 2009 Cameron McCormack <cam@mcc.id.au>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 185 matching lines @@
   if (isOutermostSVGSVGElement())
     return String();
 
   if (inUseShadowTree()) {
     String useTitle(ownerShadowHost()->title());
     if (!useTitle.isEmpty())
       return useTitle;
   }
 
   // If we aren't an instance in a <use> or the <use> title was not found, then
-  // find the first <title> child of this element.
-  // If a title child was found, return the text contents.
+  // find the first <title> child of this element and return it's text contents.

[fs, 2017/02/24 09:06:18]

Nit: its

   if (Element* titleElement = Traversal<SVGTitleElement>::firstChild(*this))
     return titleElement->innerText();
-
+  // Else we traverse potential <use> shadow tree to see if it has any <title>
+  // descendants using FlatTreeTraversal.

[fs, 2017/02/24 09:06:18]

I don't think we want to look at _all_ descendants - we want to limit ourselves
to direct children. For <use> that'll probably mean "direct children of the
referenced element". This is not what the reporter of the bug was requesting,
but I think it's more in line with the a11y guidelines/name computations I've
seen [1], so it may be better to follow that?
Regardless, maybe SVGUseElement should just have its own override for title()
and inspect the shadow tree directly? (Avoiding a flat tree traversal.)

[1] https://www.w3.org/TR/svg-aam-1.0/#mapping_additional_nd

[mrunal, 2017/02/25 00:31:18]

The example in the bug uses <title> which is nested inside <rect> which inturn
is nested inside <symbol>. So are you suggesting we limit ourselves to <title>
only if it's inside <symbol> or other top level referenced element?
 
I also thought about that solution in the beginning but I don't think you can
still use simple ElementTraversal in SVGUselement as you are still one level
above shadow root making ElementTaversal useless unless you somehow propagate
the <title> at the same level as shadow-root while building it. Unless there is
another way?

[fs, 2017/02/27 08:57:08]

Yes, per the referenced document [1].
 
If you are in SVGUseElement you can trivially access the shadow root (because
you know it's there) and "descend" into the shadow tree (also per [1].)
 

+  Node* node = FlatTreeTraversal::firstChild(*this);
+  while (node) {
+    if (isSVGTitleElement(node)) {
+      return toElement(node)->innerText();
+    }
+    node = FlatTreeTraversal::next(*node, this);
+  }
   // Otherwise return a null/empty string.
   return String();
 }
 
 bool SVGElement::instanceUpdatesBlocked() const {
   return hasSVGRareData() && svgRareData()->instanceUpdatesBlocked();
 }
 
 void SVGElement::setInstanceUpdatesBlocked(bool value) {
   if (hasSVGRareData())
@@ skipping 1068 matching lines @@
   visitor->trace(m_className);
   Element::trace(visitor);
 }
 
 const AtomicString& SVGElement::eventParameterName() {
   DEFINE_STATIC_LOCAL(const AtomicString, evtString, ("evt"));
   return evtString;
 }
 
 }  // namespace blink