Tweak shadow host focus delegation

In particular, don't delegate focus when the shadow host is an ancestor
of the currently focused area.

Fixes WICG/webcomponents#840.

Tests: web-platform-tests/wpt#19867.

source

@@ -74009,7 +74009,7 @@ END:VCARD</pre>
 
      <dd>
 
-      <p>Let <var>new focus target</var> be the <code>Document</code>'s <span>viewport</span>.</p>
+      <p>Set <var>new focus target</var> to the <code>Document</code>'s <span>viewport</span>.</p>
 
      </dd>
 
@@ -74040,18 +74040,30 @@ END:VCARD</pre>
 
      <dd>
 
-      <p>If <var>focus trigger</var> is "<code data-x="">click</code>", then let <var>possible focus
-      delegates</var> be the list of all <span>click focusable</span> <span data-x="focusable
-      area">focusable areas</span> whose <span>DOM anchor</span> is a descendant of <var>new focus
-      target</var> in the <span>flat tree</span>.</p>
+      <ol>
+       <li>If <var>new focus target</var> is a <span>shadow-including inclusive ancestor</span> of
+       the <span>currently focused area of a top-level browsing context</span>'s <span>DOM
+       anchor</span>, then set <var>new focus target</var> to null.</li>
 
-      <p>Otherwise, let <var>possible focus delegates</var> be the list of all <span
-      data-x="focusable area">focusable areas</span> whose <span>DOM anchor</span> is a descendant
-      of <var>new focus target</var> in the <span>flat tree</span>.</p>
+       <li>
+        <p>Otherwise:</p>
 
-      <p>Set <var>new focus target</var> to the first <span>focusable area</span> in <span>tree
-      order</span> of their <span data-x="DOM anchor">DOM anchors</span> in <var>possible focus
-      delegates</var>, or null if <var>possible focus delegates</var> is empty.</p>
+        <ol>
+         <li>If <var>focus trigger</var> is "<code data-x="">click</code>", then let <var>possible
+         focus delegates</var> be the list of all <span>click focusable</span> <span
+         data-x="focusable area">focusable areas</span> whose <span>DOM anchor</span> is a
+         descendant of <var>new focus target</var> in the <span>flat tree</span>.</li>
+
+         <li>Otherwise, let <var>possible focus delegates</var> be the list of all <span
+         data-x="focusable area">focusable areas</span> whose <span>DOM anchor</span> is a
+         descendant of <var>new focus target</var> in the <span>flat tree</span>.</li>
+
+         <li>Set <var>new focus target</var> to the first <span>focusable area</span> in <span>tree
+         order</span> of their <span data-x="DOM anchor">DOM anchors</span> in <var>possible focus
+         delegates</var>, or null if <var>possible focus delegates</var> is empty.</li>
+        </ol>
+       </li>
+      </ol>
 
       <p class="note">For <span data-x="sequentially focusable">sequential focusability</span>, the
       handling of <span data-x="shadow host">shadow hosts</span> and <span>delegates focus</span> is