Fix modal check in dialog closing algorithm

The value for is modal needs to be cached. This was pointed out here:
#9178 (comment).

source

@@ -60433,6 +60433,9 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
    <li><p>If the <span>is modal</span> flag of <var>subject</var> is true, then <span data-x="list
    remove">remove</span> it from its <code>Document</code>'s <span>top layer</span>.</p></li>
 
+   <li><p>Let <var>wasModal</var> be the value of <var>subject</var>'s <span>is
+   modal</span> flag.</p></li>
+
    <li><p>Set the <span>is modal</span> flag of <var>subject</var> to false.</p></li>
 
    <li><p>If <var>result</var> is not null, then set the <code
@@ -60449,9 +60452,9 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
 
      <li><p>If <var>subject</var>'s <span>node document</span>'s <span>focused area of the
      document</span>'s <span>DOM anchor</span> is a <span>shadow-including inclusive
-     descendant</span> of <var>element</var>, or <var>subject</var>'s <span>is modal</span> is true,
-     then run the <span>focusing steps</span> for <var>element</var>; the viewport should not be
-     scrolled by doing this step.</p></li>
+     descendant</span> of <var>element</var>, or <var>wasModal</var> is true, then run the
+     <span>focusing steps</span> for <var>element</var>; the viewport should not be scrolled by
+     doing this step.</p></li>
     </ol>
    </li>