function getScrollbarWidth() {
  var rootDocument = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : document;

  if (cachedScrollbarWidth === void 0) {
    cachedScrollbarWidth = walkontableCalculateScrollbarWidth(rootDocument);
  }

  return cachedScrollbarWidth;
}
Returns the computed width of the native browser scroll bar.

@param {Document} rootDocument
@return {Number} width