code: function updateFixedSidebarClass () {
      if (options.scrollContainer && document.querySelector(options.scrollContainer)) {
        var top = document.querySelector(options.scrollContainer).scrollTop
      } else {
        var top = document.documentElement.scrollTop || body.scrollTop
      }
      var posFixedEl = document.querySelector(options.positionFixedSelector)
  
      if (options.fixedSidebarOffset === 'auto') {
        options.fixedSidebarOffset = document.querySelector(options.tocSelector).offsetTop
      }
  
      if (top > options.fixedSidebarOffset) {
        if (posFixedEl.className.indexOf(options.positionFixedClass) === -1) {
          posFixedEl.className += SPACE_CHAR + options.positionFixedClass
        }
      } else {
        posFixedEl.className = posFixedEl.className.split(SPACE_CHAR + options.positionFixedClass).join('')
      }
    }
docs: Update fixed sidebar class.
  @return {HTMLElement}