Move the focus to the previously focused element for dialog.close()

Tests: https://phabricator.services.mozilla.com/D109726#change-BmU2O0Qtn5C5.

Fixes #5678.

source

@@ -57597,6 +57597,9 @@ interface <dfn>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
    <li><p>Add an <code data-x="attr-dialog-open">open</code> attribute to the <code>dialog</code>
    element, whose value is the empty string.</p></li>
 
+   <li><p>Set the <code>dialog</code> element's <span>previously focused element</span> to the
+   <span>focused</span> element.</p></li>
+
    <li><p>Run the <span>dialog focusing steps</span> for the <code>dialog</code> element.</p></li>
   </ol>
 
@@ -57636,6 +57639,9 @@ interface <dfn>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
    data-x="top-layer-add">add</span> <var>subject</var> to <var>subject</var>'s <span>node
    document</span>'s <span>top layer</span>.</p></li>
 
+   <li><p>Set the <var>subject</var>'s <span>previously focused element</span> to the
+   <span>focused</span> element.</p></li>
+
    <li><p>Run the <span>dialog focusing steps</span> for <var>subject</var>.</p></li>
   </ol>
 
@@ -57709,6 +57715,20 @@ interface <dfn>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
    <li><p>If <var>subject</var> is in its <code>Document</code>'s <span>top layer</span>, then <span
    data-x="list remove">remove</span> it.</p></li>
 
+   <li>
+    <p>If <var>subject</var>'s <span>previously focused element</span> is not null, then:<p>
+
+    <ol>
+     <li><p>Let <var>element</var> be <var>subject</var>'s <span>previously focused
+     element</span>.</p></li>
+
+     <li><p>Set <var>subject</var>'s <span>previously focused element</span> to null.</p></li>
+
+     <li><p>Run the <span>focusing steps</span> for <var>element</var>; the viewport should not be
+     scrolled by doing this step.</p></li>
+    </ol>
+   </li>
+
    <li><p><span>Queue an element task</span> on the <span>user interaction task source</span> given the
    <var>subject</var> element to <span data-x="concept-event-fire">fire an event</span> named
    <code data-x="event-close">close</code> at <var>subject</var>.</p></li>
@@ -57743,6 +57763,11 @@ interface <dfn>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
   <p>Each <code>dialog</code> element has an <dfn>is modal</dfn> flag. When a <code>dialog</code>
   element is created, this flag must be set to false.</p>
 
+  <p>Each <code>dialog</code> element has a <dfn>previously focused element</dfn> which is null or
+  an element, and it is initially null. When <code data-x="dom-dialog-showModal">showModal()</code>
+  and <code data-x="dom-dialog-show">show()</code> are called, this element is set to the currently
+  <span>focused</span> element before running the <span>dialog focusing steps</span>.</p>
+
   <hr>
 
   <p>The <dfn attribute for="HTMLDialogElement"><code data-x="dom-dialog-open">open</code></dfn> IDL