Add popover open check in attribute change steps

Fixes #9459.

source

@@ -83843,9 +83843,10 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
    <li><p>If <var>localName</var> is not <code data-x="attr-popover">popover</code>, then
    return.</p></li>
 
-   <li><p>If <var>oldValue</var> and <var>value</var> are in different <span
-   data-x="attr-popover">states</span>, then run the <span>hide popover algorithm</span> given
-   <var>element</var>, true, true, and false.</p></li>
+   <li><p>If <var>element</var>'s <span>popover visibility state</span> is in the <span
+   data-x="popover-showing-state">showing state</span> and <var>oldValue</var> and <var>value</var>
+   are in different <span data-x="attr-popover">states</span>, then run the <span>hide popover
+   algorithm</span> given <var>element</var>, true, true, and false.</p></li>
   </ol>
 
   <dl class="domintro">