<template>
<el-button size="medium"  @contextmenu.prevent.native="openMenu($event)">
......
</template>

<ul v-show="visible" :style="{left:left+'px',top:top+'px'}" class="contextmenu">
   <li>上移一层</li>
   <li>下移一层</li>
 </ul>

data() { 
	return {
			visible: false,
			top: 0,
			left: 0
	}
}

 watch: {
  visible(value) {
   if (value) {
    document.body.addEventListener('click', this.closeMenu)
   } else {
    document.body.removeEventListener('click', this.closeMenu)
   }
  }
 },

  openMenu(e) {
   const menuMinWidth = 105
   const offsetLeft = this.$el.getBoundingClientRect().left // container margin left
   const offsetWidth = this.$el.offsetWidth // container width
   const maxLeft = offsetWidth - menuMinWidth // left boundary
   const left = e.clientX - offsetLeft // 15: margin right

   if (left > maxLeft) {
    this.left = maxLeft
   } else {
    this.left = left
   }

   this.top = e.clientY - 60 // fix 位置bug
   this.visible = true
  },
  closeMenu() {
   this.visible = false
  }

 .contextmenu {
  margin: 0;
  background: #fff;
  z-index: 3000;
  position: absolute;
  list-style-type: none;
  padding: 5px 0;
  border-radius: 4px;
  font-size: 12px;
  font-weight: 400;
  color: #333;
  box-shadow: 2px 2px 3px 0 rgba(0, 0, 0, 0.3);
  li {
   margin: 0;
   padding: 7px 16px;
   cursor: pointer;
   &:hover {
    background: #eee;
   }
  }
 }