vue进入页面时不在顶部,检测滚动返回顶部按钮问

dmounted() {  
   window.addEventListener("scroll", this.gundong);  
 },
 destroyed() {
   window.removeEventListener("scroll", this.gundong);
 },
 methods: {
  gundong() {   
   var dis = document.documentElement.scrollTop || window.pageYOffset || document.body.scrollTop;
   if(dis > 120){
    this.flag = true
   }else{
    this.flag = false
   }
  },

router.afterEach((to, from) => {
 window.scrollTo(0,0);
})；

scrollBehavior (to, from, savedPosition) {
 if (savedPosition) {
  return savedPosition
 } else {
  return { x: 0, y: 0 }
 }
}

html,body,#app-container{ width: 100%; height: 100%; overflow: scroll;}

html::-webkit-scrollbar, body::-webkit-scrollbar,#app-container::-webkit-scrollbar{width:0px;height:0px;}

<div id="app-container" @scroll="scrollEvent($event)">

scrollEvent(e) {
  var dis = e.srcElement.scrollTop;
  console.log(dis)    
  if (dis > 150) {
   this.flag = true;
  } else {
   this.flag = false;
  }
 }

backTop() {
  this.$el.scrollTop = 0;   
}

var vm = new Vue({
 router,
 store,
 render: h => h(App)
}).$mount("#app");

router.afterEach((to, from) => {
 vm.$el.scrollTop = 0;
});