vue中滚动到一定位置固定(吸顶效果)

**效果：**开始的导航高度100px,滚动距离>=100px后高度变为90px 且固定在顶部

<template>
  <div class="nav-sub ">
          <active-btn :class="fixed == true ? 'fixed' : ''">
            可添加文字</active-btn
          >
 </div>
</template>

<script>
export default {
  data () {
    return { fixed: false }
  },
  mounted () {
    window.addEventListener('scroll', this.fixedActiveBtn)
  },
  methods: {
    fixedActiveBtn () {
      var scrollTop =
        window.pageYOffset ||
        document.documentElement.scrollTop ||
        document.body.scrollTop
      scrollTop >= 120 ? (this.fixed = true) : (this.fixed = false)
    }
  }
}
</script>

<style scoped>
.nav-sub {
  // position: relative;
  height: 90px;
  background-color: #fff;
  box-shadow: 0px 2px 4px rgba(0, 0, 0, 0.05);
  // position: sticky;
}
.fixed {
  position: fixed;
  z-index: 21;
  height: 60px;
  width: 100%;
  top: 0;
  left: 0;
  right: 0;
  border-bottom: 1px solid #dadada;
  background-image: -webkit-linear-gradient(#fff, #f1f1f1);
  background-image: linear-gradient(#fff, #f1f1f1);
}

</style>