_onKeyDown (event) {
    const keynum = event.which
    if (keynum === 27) {
      // ESC
      this.dom.search.value = '' // clear search
      this._onSearch()
      event.preventDefault()
      event.stopPropagation()
    } else if (keynum === 13) {
      // Enter
      if (event.ctrlKey) {
        // force to search again
        this._onSearch(true)
      } else if (event.shiftKey) {
        // move to the previous search result
        this.previous()
      } else {
        // move to the next search result
        this.next()
      }
      event.preventDefault()
      event.stopPropagation()
    }
  }
Handle onKeyDown event in the input box
@param {Event} event
@private