e it has submitted its form that creates a new
  // `about:blank` window in our browsing context group (therefore, the window
  // is reachable by us).
  window.removeIframe = t.step_func(() => {
    const iframe = document.querySelector('iframe');
    const expected_referrer = iframe.contentWindow.location.href;
    iframe.remove();

    // The child frame is removed, but its form submission navigation is still
    // queued on the opened window's global. Wait until it is run and then
    // evaluate scrip