<div id="demo">
  <transition name="no-mode-fade">
    <button v-if="on" key="on" @click="on = false">
      on
    </button>
    <button v-else key="off" @click="on = true">
      off
    </button>
  </transition>
</div>

body {
  margin: 30px;
}

.no-mode-fade-enter-active, .no-mode-fade-leave-active {
  transition: opacity .5s
}

.no-mode-fade-enter-from, .no-mode-fade-leave-to {
  opacity: 0
}

button {
  background: #05ae7f;
  border-radius: 4px;
  display: inline-block;
  border: none;
  padding: 0.5rem 0.75rem;
  text-decoration: none;
  color: #ffffff;
  font-family: sans-serif;
  font-size: 1rem;
  cursor: pointer;
  text-align: center;
  -webkit-appearance: none;
  -moz-appearance: none;
}

const Demo = {
  data() {
    return {
      on: false
    }
  }
}

Vue.createApp(Demo).mount('#demo')