Why does the height change?

Question

Vue.js

Why does the height of the textarea change by a few pixels when it becomes active and you start typing, and how can I fix this?
mage3k.github.io/vue-autosize

<template>
  <div>
    <textarea v-autosize="someData">{{ someData }}</textarea>
  </div>
</template>

<script>
import autosize from 'autosize'

export default {
  data () {
    return {
      someData: 'Type shit here'
    }
  },
  directives: {
    autosize: {
      bind: function(el, binding) {
            var tagName = el.tagName
            if (tagName == 'TEXTAREA') {
              autosize(el)
            } 
          },

          componentUpdated: function(el, binding, vnode) {
            var tagName = el.tagName
            if (tagName == 'TEXTAREA') {
              autosize.update(el)
            }
          },
          unbind: function(el) {
            autosize.destroy(el)
          }
    }
  }
}
</script>