docs: function _getIndicatorRect (el) {
    let width, height
    if (el.children.length === 1) {
      width = height = window.getComputedStyle(el.children[0])
    }
    else {
      const itemComputedStyle = window.getComputedStyle(el.children[1])
      const padding = parseFloat(itemComputedStyle.marginLeft)
      height = parseFloat(itemComputedStyle.height)
      width = el.children.length * (height + padding) - padding
    }
    return { width, height }
  }