code: function addMissingItems(groupUi, options) {
      let nextItemUi = groupUi.firstElementChild
      let nextOption = options.firstElementChild
      while (nextOption) {
        if (!nextItemUi || nextItemUi._option !== nextOption) {
          const newItemUi = document.createElement('szn-')
          newItemUi._option = nextOption
          newItemUi.setAttribute('data-szn-options--' + (nextOption.tagName === 'OPTGROUP' ? 'optgroup' : 'option'), '')
          updateItem(newItemUi)
          groupUi.insertBefore(newItemUi, nextItemUi)
        } else {
          nextItemUi = nextItemUi && nextItemUi.nextElementSibling
        }
  
        nextOption = nextOption.nextElementSibling
      }
    }
docs: Adds the options present in the options container missing the UI into the UI, while preserving the order of the
  options. Option groups are added recursively.
  
  @param {Element} groupUi The element containing the UIs of the options. The new options will be inserted into this
  element's children.
  @param {HTMLElement} options An element containing the <code>option</code> and <code>optgroup</code> elements that
  the UI reflects.