protected override void Render(HtmlTextWriter output) {
    // Get normal html ouput
    StringBuilder stringBuilder = new StringBuilder();
    StringWriter stringWriter = new StringWriter(stringBuilder);
    HtmlTextWriter htmlWriter = new HtmlTextWriter(stringWriter);
    base.Render(htmlWriter);
    string html = stringBuilder.ToString();
    // Enhance submit buttons
    string onclick1 = "\"if (typeof(Page_ClientValidate) == 'function') Page_ClientValidate();";
    string onclick2 = "\"this.style.display='none';";
    html = html.Replace("onclick=" + onclick1, "onclick=" + onclick2 + onclick1.Substring(1));
    // Render updated html
    output.Write(html);
}

window.onbeforeunload = function (evt) {
  var message = 'Are you sure you want to leave?';
  if (typeof evt == 'undefined') {
    evt = window.event;
  }
  if (evt) {
    evt.returnValue = message;
  }
  return message;
}