<div id="app">
  <button v-on:click="show = !show">
    Toggle
  </button>
  <transition
    enter-class="opacity-0"
    leave-to-class="opacity-0"
    enter-active-class="trs-time-3"
    leave-active-class="trs-time-3"
  >
    <div class="box" v-if="show"></div>
  </transition>
</div>

button {
  margin-bottom: 10px;
}

.box {
  width: 100px;
  height: 100px;
  background-color: #303E51;
}

.opacity-0 {
  opacity: 0;
}

.trs-time-3 {
  transition-duration: 3s;
}

const vm = new Vue({
  el: '#app',
  data: {
    show: true,
  },
});