Further fixes to focus delegation

This ensures that focus delegation, used by both delegatesFocus and <dialog>, does not traverse into non-delegatesFocus shadow trees. Otherwise you would get unintuitive results in situations such as the following:

<div id="outer">
<#shadowroot delegatesFocus=true>
  <div id="inner>
  <#shadowroot delegatesFocus=false>
    <input>
  </#shadowroot>
  </div>
</#shadowroot>
</div>

In such a scenario, with the previous specification, inner.focus() would not focus anything, since inner's shadow root does not delegate focus so we never hit the "focus delegate" algorithm. But outer.focus() would focus the <input> element, since with it having delegatesFocus=true, we use the "focus delegate" algorithm, which would look at all shadow-including descendants. After this change, both inner.focus() and outer.focus() would not focus anything, since the <input> is hidden inside a delegatesFocus=false shadow tree.

Discovered in https://matrixlogs.bakkot.com/WHATWG/2021-11-23#L36-L62.

source

@@ -2993,7 +2993,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#dom-shadowroot-slot-assignment">slot assignment</dfn> concept</li>
      <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-slotable">slottable</dfn> concept</li>
      <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#assign-slotables-for-a-tree">assign slottables for a tree</dfn> algorithm</li>
-     <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-tree-inclusive-descendant">inclusive descendants</dfn> concept</li>
+     <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-tree-inclusive-descendant">inclusive descendant</dfn> concept</li>
      <li>The <dfn data-x="finding flattened slottables" data-x-href="https://dom.spec.whatwg.org/#find-flattened-slotables">find flattened slottables</dfn> algorithm</li>
      <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#slottable-manual-slot-assignment">manual slot assignment</dfn> concept</li>
      <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#assign-a-slot">assign a slot</dfn> algorithm</li>
@@ -75823,14 +75823,11 @@ END:VCARD</pre>
 
    <dd>
     <ol>
-     <li><p>Let <var>root</var> be <var>focus target</var>'s <span
-     data-x="concept-element-shadow-root">shadow root</span>.</p></li>
-
-     <li><p>If <var>root</var> is a <span>shadow-including inclusive ancestor</span> of the
+     <li><p>If <var>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 return null.</p></li>
 
-     <li><p>Return the <span>focus delegate</span> for <var>root</var> given <var>focus
+     <li><p>Return the <span>focus delegate</span> for <var>focus target</var> given <var>focus
      trigger</var>.</p></li>
     </ol>
 
@@ -75851,29 +75848,48 @@ END:VCARD</pre>
   steps:</p>
 
   <ol>
-   <li><p>Let <var>autofocus delegate</var> be the <span>autofocus delegate</span> for
-   <var>focus target</var> given <var>focus trigger</var>.</p></li>
+   <li><p>Let <var>autofocus delegate</var> be the <span>autofocus delegate</span> for <var>focus
+   target</var> given <var>focus trigger</var>.</p></li>
 
    <li><p>If <var>autofocus delegate</var> is not null, then return <var>autofocus
    delegate</var>.</p></li>
 
-   <li><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 <span>shadow-including
-   descendant</span> of <var>focus target</var>.</p></li>
+   <li><p>If <var>focus target</var> is a <span>shadow host</span> and its <span
+   data-x="concept-element-shadow-root">shadow root</span>'s <span>delegates focus</span> is false,
+   then return null.</p></li>
 
-   <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
-   <span>shadow-including descendant</span> of <var>focus target</var>.</p></li>
+   <li><p>Let <var>parent</var> be <var>focus target</var>.</p></li>
 
-   <li><p>If <var>possible focus delegates</var> is empty, then return null.</p></li>
+   <li><p>If <var>parent</var> is a <span>shadow host</span>, then set <var>parent</var> to
+   <var>parent</var>'s <span data-x="concept-element-shadow-root">shadow root</span>.</p></li>
 
-   <li><p>Sort <var>possible focus delegates</var> by <span>shadow-including tree order</span> of
-   their <span data-x="DOM anchor">DOM anchors</span>.</p></li>
+   <li>
+    <p><span data-x="list iterate">For each</span> <var>child</var> of <var>parent</var>'s <span
+    data-x="concept-tree-child">children</span>:</p>
 
-   <li><p>Return <var>possible focus delegates</var>[0].</p></li>
+    <ol>
+     <li>
+      <p>Let <var>focusable area</var> be <var>child</var>, if <var>child</var> is a
+      <span>focusable area</span>; otherwise let <var>focusable area</var> be the result of <span
+      data-x="get the focusable area">getting the focusable area</span> for <var>child</var> given
+      <var>focus trigger</var>.</p>
+
+      <p class="note">This step can end up recursing, i.e., the <span>get the focusable area</span>
+      steps might return the <span>focus delegate</span> of <var>child</var>.</p>
+     </li>
+
+     <li><p>If <var>focusable area</var> is not null, then return <var>focusable
+     area</var>.</p></li>
+    </ol>
+   </li>
+
+   <li><p>Return null.</p></li>
   </ol>
 
+  <p class="note">The above algorithm essentially returns the first suitable <span>focusable
+  area</span> where the path between its <span>DOM anchor</span> and <var>focus target</var>
+  delegates focus at any shadow tree boundaries.</p>
+
   <p>The <dfn>autofocus delegate</dfn> for a <var>focus target</var> given a <var>focus
   trigger</var> is given by the following steps:</p>