size-observer-mixin.js

import ResizeSensor from require('css-element-queries/src/ResizeSensor')

export default {
  // this will run in addition to any mounted() hook on the component
  // using the mixin
  mounted () {
    this.$resizeSensor = new ResizeSensor(this.$el, () => {
      // per the docs (https://vuejs.org/v2/api/#Vue-set),
      // this lets you add a reactive data property that wasn't specified in the component's
      // data() { } hook
      Vue.$set('clientWidth', this.$el.clientWidth)
      
      // then do that again for any other properties set by the library you want to track
    })
  }
}