五、导航栏渐隐渐现

滚动条距离顶部高度：
var height = document.documentElement.scrollTop;

<h1 class="nav" :style="{opacity:opacity}">This is an about page</h1>
handle(){
      var height = document.documentElement.scrollTop;  
      var opacity = height/300
      console.log(height)
      if(opacity>1){
        opacity=1
      }
      this.opacity = opacity
    }