static get properties () {
    return {
      items: {type: Array},
      itemGroups: {type: Array},
      selection: {type: Array},
      showOrigin: {type: Boolean, attribute: 'show-origin'}
    }
  }
NOTES ON DRAG & DROP EVENT BEHAVIORS

- The web platform is very finicky with its dragenter/dragleave/etc events and will sometimes
  fail to fire if the drag moves too quickly.
- To make sure that drop-targets get the '.drag-hover' added and removed correctly, we rely on
  selectively removing pointer-events on DOM elements during 'drag & drop' mode.

-prf