meta-for JWindow also as JFrame {

  static AutoCloseable tempAddToWindow(JWindow w, Component toAdd) {
    if (w == null || toAdd == null) null;
    ret swing(-> {
      var oldContent = w.getContentPane();
      setContentPane(w, centerAndSouth(oldContent, toAdd));
      ret -> setContentPane(w, oldContent);
    });
  }
  
}