Define auto popover list in terms of the top layer

This solves an issue with elements being unconditionally added to this list and makes it clearer that the steps between it being added here and the top layer didn't mean anything.

This also makes hide popover use check popover validity more to rely on the shared infrastructure.

Fixes #8941 and fixes #8964.

source

@@ -81787,8 +81787,10 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
    data-x="popover-showing-state">showing</dfn></p></li>
   </ul>
 
-  <p>The <code>Document</code> has an <dfn>auto popover list</dfn>, which is a <span>list</span>,
-  initially empty.</p>
+  <p>The <code>Document</code> has an <dfn>auto popover list</dfn>, which is a <span>list</span> of
+  all the elements in the <code>Document</code>'s <span>top layer</span> whose <code
+  data-x="attr-popover">popover</code> attribute is in the <span
+  data-x="attr-popover-auto-state">auto state</span>, in the same order.</p>
 
   <p>The <code>Document</code> has a <dfn>popover pointerdown target</dfn>, which is an <span
   data-x="HTML elements">HTML element</span> or null, initially null.</p>
@@ -81923,8 +81925,6 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
     the first popover in a stack.</p>
    </li>
 
-   <li><p>Add <var>element</var> to <var>document</var>'s <span>auto popover list</span>.</p></li>
-
    <li><p>Set <var>element</var>'s <span>previously focused element</span> to null.</p></li>
 
    <li><p>Let <var>originallyFocusedElement</var> be <var>document</var>'s <span>focused area of the
@@ -82012,21 +82012,17 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
      <var>element</var>, <var>focusPreviousElement</var>, and <var>fireEvents</var>.</p></li>
 
      <li>
-      <p>If <var>element</var> is not in <var>document</var>'s <span>auto popover list</span>:</p>
-
-      <ol>
-       <li><p>If <var>throwExceptions</var> is true, then throw an
-       <span>"<code>InvalidStateError</code>"</span> <code>DOMException</code>.</p></li>
+      <p>If the result of running <span>check popover validity</span> given <var>element</var>,
+      true, and <var>throwExceptions</var> is false, then return.</p>
 
-       <li><p>Otherwise, return.</p></li>
-      </ol>
+      <p class="note"><span>Check popover validity</span> is called again because running <span
+      data-x="hide-all-popovers-until">hide all popovers until</span> could have disconnected
+      <var>element</var> or changed its <code data-x="attr-popover">popover</code>
+      attribute.</p>
      </li>
 
-     <li><p><span>Assert</span>: The last item in <var>document</var>'s <span>auto popover
+     <li><p><span>Assert</span>: the last item in <var>document</var>'s <span>auto popover
      list</span> is <var>element</var>.</p></li>
-
-     <li><p><span data-x="list remove">Remove</span> <var>element</var> from <var>document</var>'s
-     <span>auto popover list</span>.</p></li>
     </ol>
    </li>
 
@@ -82047,8 +82043,8 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
       true, and <var>throwExceptions</var> is false, then return.</p>
 
       <p class="note"><span>Check popover validity</span> is called again because firing the <code
-      data-x="event-beforetoggle">beforetoggle</code> event could have disconnected this element or
-      changed its <code data-x="attr-popover">popover</code> attribute.</p>
+      data-x="event-beforetoggle">beforetoggle</code> event could have disconnected
+      <var>element</var> or changed its <code data-x="attr-popover">popover</code> attribute.</p>
      </li>
     </ol>
    </li>
@@ -82169,7 +82165,7 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
   elements">HTML element</span> or null.</p>
 
   <div class="note">
-   <p>The <span>topmost popover ancestor</span> algorithm will return the topmost (highest in the
+   <p>The <span>topmost popover ancestor</span> algorithm will return the topmost (latest in the
    <span>auto popover list</span>) ancestor popover for the provided popover. Popovers can be
    related to each other in several ways, creating a tree of popovers. There are two paths through
    which one popover (call it the "child" popover) can have an ancestor popover (call it the
@@ -82185,7 +82181,7 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
     the "parent". The invoker must be in a popover and reference an open popover.</p></li>
    </ol>
 
-   <p>In each of the relationships formed above, the parent popover must be strictly lower in the
+   <p>In each of the relationships formed above, the parent popover must be strictly earlier in the
    <span>auto popover list</span> than the child popover, or it does not form a valid ancestral
    relationship. This eliminates non-showing popovers and self-pointers (e.g. a popover with an
    anchor attribute that points back to the same popover), and it allows for the construction of a