Reset focus when closing a dialog with no previously focused element #13118

The close-the-dialog algorithm restores previously focused element if non-null, but does nothing with focus when it is null. If focus is inside the dialog when it closes, this leaves the focused area inside a closed dialog until the asynchronous focus fixup rule runs. In the meantime, keyboard events can be lost because the active element is no longer focusable.

Reproduction:

<dialog><input></dialog>
<script>
  const dialog = document.querySelector('dialog');
  const input = document.querySelector('input');
  dialog.showModal();
  input.focus();
  dialog.close();
  console.log(document.activeElement === input); // true
  setTimeout(() => console.log(document.activeElement === document.body), 150); // true
</script>

Chromium, Firefox, and WebKit all leave the input active immediately after close(), then change document.activeElement to body later. A Chromium change adds a synchronous regression test for this case.

Could the dialog-close steps run the focusing steps for the document's viewport when previously focused element is null and the focused area's DOM anchor is a shadow-including inclusive descendant of the dialog? This uses the existing focus model rather than introducing a separate notion of "clearing" focus, and preserves focus when it has already moved elsewhere.