docs: function getScrollTop(){
    if ( typeof window.pageYOffset !== 'undefined' ) {
      // most browsers except IE before #9
      return window.pageYOffset;
    } else {
      var b = document.body; // IE 'quirks'
      var d = document.documentElement; // IE with doctype
      return ((d.clientHeight) ? d : b).scrollTop;
    }
  }