Hide popovers when making an element fullscreen

This is part of the new pop-up attribute: whatwg/html#8221.

Co-authored-by: Anne van Kesteren <annevk@annevk.nl>
Co-authored-by: Philip Jägenstedt <philip@foolip.org>

fullscreen.bs

@@ -61,8 +61,16 @@ stated otherwise it is unset.
 <p>All <a for=/>documents</a> have an associated <dfn>list of pending fullscreen events</dfn>, which
 is an <a>ordered set</a> of (<a>string</a>, <a>element</a>) <a>tuples</a>. It is initially empty.
 
-<p>To <dfn>fullscreen an <var>element</var></dfn>, set <var>element</var>'s <a>fullscreen flag</a>
-and <a for="top layer">add</a> it to its <a>node document</a>'s <a>top layer</a>.
+<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>Set <var>element</var>'s <a>fullscreen flag</a>.
+
+ <li><p><a for="top layer">Add</a> <var>element</var> to <var>element</var>'s <a>node document</a>'s
+ <a>top layer</a>.
+</ol>
 
 <p>To <dfn>unfullscreen an <var>element</var></dfn>, unset <var>element</var>'s
 <a>fullscreen flag</a> and <a>iframe fullscreen flag</a> (if any), and <a for=set>remove</a> it from
@@ -230,6 +238,10 @@ if all of the following are true, and false otherwise:
  <li><p><var>element</var>'s <a>node document</a> is <a>allowed to use</a> the "<code><a
  data-lt="fullscreen-feature">fullscreen</a></code>" feature.
  <!-- cross-process, recursive -->
+
+ <li><p><var>element</var> <a for=Element>namespace</a> is not the <a>HTML namespace</a> or
+ <var>element</var>'s <a>popover visibility state</a> is <a for="popover visibility
+ state">hidden</a>.
 </ul>
 
 <div algorithm="requestFullscreen(options)">