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chromium/content/test/data/accessibility/event/focus-listbox.html 

<!DOCTYPE html>
<html>
<body>
  <select size="3">
    <option>a</option>
    <option selected>b</option>
    <option>&ccedil;</option>
  </select>
  <script>
    function go() {
      document.querySelector('select').focus();
      document.querySelector('select').selectedIndex = 2;
    }
  </script>
</body>
</html>