message: Fix size of subclass hover border in Firefox
diff: @include draggable-hover-border;
  content: '';
  position: absolute;
  - width: 100%;
  - height: 100%;
  + width: var(--item-size);
  + height: var(--item-size);
  transform: rotate(45deg) scale(0.75);
  box-sizing: border-box;
  }
  