_getVisibleButtons () {
    const buttons = []
    const me = this
    this.dom.items.forEach(item => {
      buttons.push(item.button)
      if (item.buttonExpand) {
        buttons.push(item.buttonExpand)
      }
      if (item.subItems && item === me.expandedItem) {
        item.subItems.forEach(subItem => {
          buttons.push(subItem.button)
          if (subItem.buttonExpand) {
            buttons.push(subItem.buttonExpand)
          }
          // TODO: change to fully recursive method
        })
      }
    })

    return buttons
  }
Get the currently visible buttons
@return {Array.<HTMLElement>} buttons
@private