config\n  }\n\n  _getScrollTop() {\n    return this._scrollElement === window ?\n      this._scrollElement.pageYOffset : this._scrollElement.scrollTop\n  }\n\n  _getScrollHeight() {\n    return this._scrollElement.scrollHeight || Math.max(\n      document.body.scrollHeight,\n      document.documentElement.scrollHeight\n    )\n  }\n\n  _getOffsetHeight() {\n    return this._scrollElement === window ?\n      window.innerHeight : this._scrollElement.getBoundingClientRect().height\n  }\n\n  _process()...