initPopup () {
    const zoomPos = this.getBoundingClientRect()
    const popupPos = this.optionsContainer.getBoundingClientRect()
    const top = zoomPos.top - popupPos.height
    const left = zoomPos.left

    this.optionsContainer.style.position = 'fixed'
    this.optionsContainer.style.top = `${top}px`
    this.optionsContainer.style.left = `${left}px`
  }
@function handleShow
@returns {void}
initialises the popup menu position