Editorial: change "show popover" to call "check popover validity" first

This is what implementations (at least Gecko and Chromium) do and it's analogous to what happens in "hide popover".

Fixes #9429.

source

@@ -82554,12 +82554,23 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
   element</span> or null <var>invoker</var>:</p>
 
   <ol>
+   <li><p>If the result of running <span>check popover validity</span> given <var>element</var>,
+   false, <var>throwExceptions</var>, and null is false, then return.</p></li>
+
    <li><p>Let <var>document</var> be <var>element</var>'s <span>node document</span>.</p></li>
 
    <li><p><span>Assert</span>: <var>element</var>'s <span>popover invoker</span> is null.</p></li>
 
    <li><p>Set <var>element</var>'s <span>popover invoker</span> to <var>invoker</var>.</p></li>
 
+   <li><p><span>Assert</span>: <var>element</var> is not in <var>document</var>'s <span>top
+   layer</span>.</p></li>
+
+   <li><p>Let <var>nestedShow</var> be <var>element</var>'s <span>popover showing or
+   hiding</span>.</p></li>
+
+   <li><p>Set <var>element</var>'s <span>popover showing or hiding</span> to true.</p></li>
+
    <li>
     <p>Let <var>cleanupShowingFlag</var> be the following steps:</p>
 
@@ -82569,20 +82580,6 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
     </ol>
    </li>
 
-   <li><p>If the result of running <span>check popover validity</span> given <var>element</var>,
-   false, <var>throwExceptions</var>, and null is false, then run <var>cleanupShowingFlag</var> and
-   return.</p></li>
-
-   <li><p><span>Assert</span>: <var>element</var> is not in <var>document</var>'s <span>top
-   layer</span>.</p></li>
-
-   <li><p>Let <var>nestedShow</var> be false.</p></li>
-
-   <li><p>If <var>element</var>'s <span>popover showing or hiding</span> is true, then set
-   <var>nestedShow</var> to true.</p></li>
-
-   <li><p>Set <var>element</var>'s <span>popover showing or hiding</span> to true.</p></li>
-
    <li><p>If the result of <span data-x="concept-event-fire">firing an event</span> named <code
    data-x="event-beforetoggle">beforetoggle</code>, using <code>ToggleEvent</code>, with the <code
    data-x="dom-Event-cancelable">cancelable</code> attribute initialized to true, the <code