ources/slow-code-injector.html?pipe=sub(none)&code=" + encodeURIComponent(code);
  const absoluteStartURL = (new URL(startURL, location.href)).href;

  const afterReplacementURL = "resources/message-opener.html";
  const absoluteAfterReplacementURL = (new URL(afterReplacementURL, location.href)).href;

  window.navigateMe = () => {
    window.open(absoluteAfterReplacementURL, windowName);
  };

  // First message sent is ignored; we only check it after navigating back.
  const w = window.open(startURL, wind