Don't close ancestor popovers when fullscreening

Without this change, fullscreening an element inside an open popover will make the fullscreen element display:none.

Issue: whatwg/html#9998
Corresponding HTML PR: whatwg/html#10116

fullscreen.bs

@@ -67,7 +67,13 @@ is an <a>ordered set</a> of (<a>string</a>, <a>element</a>) <a>tuples</a>. It is
 <p>To <dfn>fullscreen an <var>element</var></dfn>:
 
 <ol>
- <li><p>Run <a>hide all popovers</a> given <var>element</var>'s <a>node document</a>.
+ <li><p>Let <var>hideUntil</var> be the result of running <a>topmost popover ancestor</a> given
+ <var>element</var>, null, and false.
+
+ <li><p>If <var>hideUntil</var> is null, then set <var>hideUntil</var> to <var>element</var>'s
+ <span>node document</span>.
+
+ <li><p>Run <a>hide all popovers until</a> given <var>hideUntil</var>, false, and true.
 
  <li><p>Set <var>element</var>'s <a>fullscreen flag</a>.