chromium/third_party/blink/web_tests/external/wpt/html/browsers/history/the-history-interface/non-automated/traverse_the_session_history_unload_prompt_1-1.html

<!doctype html>
<script src="history.js"></script>
<script>
  onunload = function(e) {opener.unload_ran = true; return "Now refuse to leave the current page"}

  opener.pages.push(id);
  if (!opener.started) {
    onload = function() {
      setTimeout(function() {
        opener.started = true;
        history.back();
      }, 100);
    }
  } else {
    opener.start_test_wait();
  }
</script>