file_sync_xhr_nested_helper.html

Enable keyboard shortcuts

<!DOCTYPE HTML>

<html>

  <body>

    <input />

    <script>

      const input = document.querySelector("input");

      input.addEventListener("keydown", function() {

        window.opener.receivedInput();

        window.close();

});

      function startSlowXHR() {

        setTimeout(function() {

          input.focus();

          SpecialPowers.loadChromeScript(() => {

            /* eslint-env mozilla/chrome-script */

            var win = Services.wm.getMostRecentBrowserWindow();

            EventUtils.synthesizeKey("a", {}, win);

});

          var xhr = new XMLHttpRequest();

          xhr.open("GET", "slow.sjs", false);

          xhr.send(null);

          window.opener.childXHRFinished = true;

        }, 0);

    </script>

  </body>

</html>