fferent CSPs.
    const first_real_load = future(popup, "load");
    executor.postMessage(`window.open("about:blank", "${popup_name}");`);
    await first_real_load;

    // The new blank document in the popup must inherit CSPs from |executor|:
    let is_sandboxed = future(window, "message");
    popup.document.write(`
      <script>
        try {
          document.domain = document.domain;
          opener.opener.postMessage("not sandboxed", "*");
        } catch (error) {
          opener.opener.postMes