code: function handleComboboxKeyDown(event) {
      eventUtils.handleUpDownArrowsKeydown(event, () => {
          const currentSelectedIndex = findIndex(this.state.options, option => option.selected);
          const options = clearComboboxSelections(this.state.options);
          const optionEls = this.el.querySelectorAll(comboboxOptionSelector);
          let selectElementIndex = currentSelectedIndex;
  
          switch (event.charCode || event.keyCode) {
              // down
              case 40:
                  selectElementIndex = traverseOptions(options, currentSelectedIndex, 1);
                  break;
              // up
              case 38:
                  selectElementIndex = traverseOptions(options, currentSelectedIndex, -1);
                  break;
              default:
                  break;
          }
  
          options[selectElementIndex].selected = true;
  
          this.setState('options', options);
          this.processAfterStateChange(optionEls[selectElementIndex]);
      });
  
      eventUtils.handleEscapeKeydown(event, () => {
          this.expander.collapse();
          this.el.querySelector(comboboxHostSelector).focus();
      });
  }
docs: Handle selection of options when the combobox is closed
  https://ebay.gitbooks.io/mindpatterns/content/input/listbox.html#keyboard
  @param {KeyboardEvent} event