show() {
    this.isVisible = true;
    this.element.classList.remove(Classes.HIDDEN);
    this.element.classList.add(Classes.VISIBLE);
    this.element.removeAttribute('aria-hidden');
  }
Used to determine if a transition will happen. By the time the _layout
and _shrink methods get the ShuffleItem instances, the `isVisible` value
has already been changed by the separation methods, so this property is
needed to know if the item was visible/hidden before the shrink/layout.