_teardownListboxNode() {
      if (this._listboxNode) {
        this._listboxNode.removeEventListener('keydown', this._listboxOnKeyDown);
        this._listboxNode.removeEventListener('click', this._listboxOnClick);
        this._listboxNode.removeEventListener('keyup', this._listboxOnKeyUp);
      }
    }
A Subclasser can perform additional logic whenever the elements inside the listbox are
updated. For instance, when a combobox does server side autocomplete, we want to
match highlighted parts client side.
@configurable