JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.0.3/vue.js"></script>
<div id="root">
  <div id="container">
    <div id="square">
      a
    </div>
    <div v-html="text">
    </div>
  </div>
</div>

CSS

#container {
  display: flex;
  flex-direction: row;
    border-style: solid;
  border-width: 5px;
}

div {
  border-style: solid;
  border-width: 1px;
}

JavaScript

var vm = new Vue({
  el: "#root",
  data: {
    text: ""
  },
  watch: {
    text: function() {
      Vue.nextTick(function() {
        document.getElementById("square").setAttribute("style","width:"+ document.getElementById("container").offsetHeight + 'px')
      })
    }
}
})

setTimeout(function() {
  vm.text = "this is  some text<br>and some morethis is  some text<br>and some morethis is  some text<br>and some morethis is  some text<br>and some more"
}, 3000)