docs: function onKeyDown( evt ) {
      switch ( evt.key ) {
      case 'Space':
          this.expanded = !this.expanded;
          evt.preventDefault();
          break;
      case 'Enter':
          this.action = this.value;
          break;
      case 'ArrowDown':
          selectNext.call( this );
          evt.preventDefault();
          break;
      case 'ArrowUp':
          selectPrev.call( this );
          evt.preventDefault();
          break;
      case 'Escape':
          if ( this.expanded ) {
              this.expanded = false;
              // Set the value as it was when we expanded the combo.
              this.value = this._valueWhenExpanded;
              evt.preventDefault();
          }
          break;
      default:
          // Do nothing.
      }
  }