<template>
 <div>
  <button @click="show = !show">
   Toggle
  </button>
  <transition name="fadeBg">
   <div class="bg" v-if="show">hello</div>
  </transition>
 </div>
</template>

<script>
 export default {
  data: () => ({
   show: true
  }),
 };
</script>

<style lang="less" scoped>
 .fadeBg-enter-active,
 .fadeBg-leave-active {
  transition: opacity 0.3s ease;
 }

 .fadeBg-enter,
 .fadeBg-leave-to {
  opacity: 0;
 }

 .bg {
  position: fixed;
  top: 20px;
  left: 0;
  z-index: 105;
  width: 100%;
  height: 100%;
  background: rgba(0, 0, 0, 0.5);
 }
</style>