Improve CommandEvent.source retargeting

This change prevents CommandEvent.source from leaking nodes across
shadow boundaries. More context here:
whatwg#11255 (comment)

Author: josepharhar

source

@@ -80565,10 +80565,21 @@ dictionary <dfn dictionary>CommandEventInit</dfn> : <span>EventInit</span> {
   initialized to.</p>
 
   <p>The <dfn attribute
-  for="CommandEvent"><code data-x="dom-CommandEvent-source">source</code></dfn> getter steps are to
-  return the result of <span data-x="dom-retarget">retargeting</span> <code
-  data-x="dom-CommandEvent-source">source</code> against <span>this</span>'s <code
-  data-x="dom-Event-currentTarget">currentTarget</code>.</p>
+  for="CommandEvent"><code data-x="dom-CommandEvent-source">source</code></dfn> getter steps
+  are:</p>
+
+  <ol>
+   <li><p>Let <var>retargetAgainst</var> be <span>this</span>'s <code
+   data-x="dom-Event-currentTarget">currentTarget</code>.</p></li>
+
+   <li><p>If <var>retargetAgainst</var> is null, then set <var>retargetAgainst</var> to
+   <span>this</span>'s <span data-x="concept-event-target">target</span>.</p></li>
+
+   <li><p>If <var>retargetAgainst</var> is null, then return null.</p></li>
+
+   <li><p>Return the result of <span data-x="dom-retarget">retargeting</span> <code
+   data-x="dom-CommandEvent-source">source</code> against <var>retargetAgainst</var>.</p></li>
+  </ol>
 
   <p class="XXX"><a href="https://github.com/whatwg/dom/issues/1328">DOM standard issue #1328</a>
   tracks how to better standardize associated event data in a way which makes sense on Events.