behobene Header auf Scroll VueJs

<script src="//unpkg.com/vue@latest/dist/vue.js"></script>
<div id="app">
  <div class="spacer">Welcome</div>
  <div class="a-box"></div>
  <sticky-top top="20" :scroll-y="scrollY"></sticky-top>
  <div class="a-box"></div>
</div>
Mountain Lion