scroll-view设置scroll-top为0实现滚动返回顶部后组件无法向下滚动？

wxml

<scroll-view 
    scroll-y="true" 
    enable-back-to-top="true"
    scroll-with-animation="true"
    scroll-top="{{top_num}}"
    bindscroll="scroll_distance">  
</scroll-view>

<view bindtap="backtop" wx:if="{{showBackTop}}">
    <icon></icon>
</view>

js

Page({
    data : {
        showBackTop : false
        top_num : 0,
    },
    scroll_distance: function(event) {
      this.setData({
        showBackTop: event.detail.scrollTop > 300 ? true : false
      })
    },
    backtop : function(){
        this.setData({top_num:0})
    }
  })

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