disable submit buttons on page leave

leave.js

// this disables users clicking the submit button multiple times for forms without submit event handlers.
window.onbeforeunload = function () {
  Array.from(document.querySelectorAll('[type=submit],[type=image],[type=reset]')).forEach(function (el) {
    if (el.form) {
      el.disabled = true;
    }
  })
}