code: function generateDropZones() {
      $ColGroupContainer
        .find(DROP_ZONE).remove().end() // ensure no drop zone exists
        .find('li').each(function (idx) {
          const $this = $(this)
          $this
            .attr('draggable', 'true')
            .attr('source-idx', idx)
            .before(
              dropZoneGen(
                $this.is('li:first-of-type') ? idx - 0.25 : idx
              )
            )
  
          $this.is('li:last-of-type') && $this.after(dropZoneGen(idx + 0.25))
        })
    }
docs: generate adjacent drop zones for each column displayed as <li>