ensureUntabbable() {
    const node = this.node;
    if (node.matches(_focusableElementsString)) {
      if (node.tabIndex === -1 && this.hasSavedTabIndex)
        return;

      if (node.hasAttribute('tabindex'))
        this._savedTabIndex = node.tabIndex;
      node.setAttribute('tabindex', '-1');
      if (node.nodeType === Node.ELEMENT_NODE) {
        node.focus = function() {};
        this._overrodeFocusMethod = true;
      }
    } else if (node.hasAttribute('tabindex')) {
      this._savedTabIndex = node.tabIndex;
      node.removeAttribute('tabindex');
    }
  }
Save the existing tabindex value and make the node untabbable and unfocusable