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this.handleScroll 獲取scrollTop

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mounted(){

  window.addEventListener('scroll', this.handleScroll);

}

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handleScroll () {

  let scrollTop = document.body.scrollTop;

  this.scroll = scrollTop;

}

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activated() {

    if(this.scroll > 0){

       this.scroll = 0;

       window.addEventListener('scroll', this.handleScroll);

    }

}

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deactivated(){

     window.removeEventListener('scroll', this.handleScroll);

}