lib/assets/scripts/vendor/codemirror/codemirror.js
function e_prop(e, prop) {
    var overridden = e.override && e.override.hasOwnProperty(prop);
    return overridden ? e.override[prop] : e[prop];
  }
function e_prop(e, prop) {
    var overridden = e.override && e.override.hasOwnProperty(prop);
    return overridden ? e.override[prop] : e[prop];
  }
Allow 3rd-party code to override event properties by adding an override object to an event object.