Vue repro

Base fiddle to create a Vue reproduction

HTML

<script src="https://unpkg.com/vue"></script>

<div id="app">
  <div class="test"></div>
</div>

CSS

#app{
  border: 1px solid black;
  overflow:auto;
}

.test {
  float:left;
  height: 100px;
  width: 100px;
  background: orange
}

JavaScript

new Vue({
  el: '#app',
  mounted() {
    console.log('$el.offsetHeight', this.$el.offsetHeight)
  }
})