let middle = document.getElementById('middle')
    middle.addEventListener('scroll',  (e)=> {
      let before = middle.scrollTop
      // 函数防抖
      if (this.timer) {
          clearTimeout(this.timer)
      }
      // 300ms后判定为滑动结束
      this.timer = setTimeout(() => {
        // 滑动结束
        let after = middle.scrollTop
        if(before == after) {
          console.log('滚动位置确定了', middle.scrollTop);
        }
      }, 300)
    });