SOURCE

var Main = {
  data() {
    return {
			display: false
    }
  }
}
var Ctor = Vue.extend(Main)
new Ctor().$mount('#app')

const ro = new ResizeObserver((entries, observer) => {
  for (const entry of entries) {
      const {left, top, width, height} = entry.contentRect;

      console.log('Element:', entry.target);
      console.log(`Element's size: ${ width }px x ${ height }px`);
      console.log(`Element's paddings: ${ top }px ; ${ left }px`);
  }
});
 
ro.observe(document.querySelector('#app'));

<script src="//unpkg.com/resize-observer-polyfill@1.5.1/dist/ResizeObserver.js"></script>
<script src="//unpkg.com/vue/dist/vue.js"></script>

<div id="app">
  <button @click="display = !display">toggle</button>
  <ul v-if="display">
    <li v-for="i in 999" :key="i">
      seq - {{ i }}
    </li>
  </ul>
</div>