<template>
<div class="main">
 <div id='menu'>
  <ul>
   <li v-for="item in tabList" @click='clickTab'></li>
  </ul>
 </div>
 <div id='div1'></div>
 <div id='div2'></div>
 <div id='div3'></div>
</div> 
</template>

public isFixed = false;
public mounted() {
 this.menuTop = (document.getElementById('menu') as any).offsetTop;
 window.addEventListener('scroll', this.handleScroll);
 }
public handleScroll() {
 const scrollTop = document.documentElement.scrollTop || document.body.scrollTop; // 獲取滑動距離
 if (scrollTop < this.menuTop ) {
  this.isFixed = false;
 } else {
  this.isFixed = true; // 設置fixed定位
 }
 }
public destroyed() {
 window.removeEventListener('scroll', this.handleScroll);
}

public clickTab(index: number) {
 this.activeIndex = index;
 this.isFixed = true;
 const menuHeight= (document.getElementById('menu') as any).offsetHeight;
 const div1= (document.getElementById('div1') as any).offsetTop;
 const div2= (document.getElementById('div2') as any).offsetTop;
 const div3= (document.getElementById('div3') as any).offsetTop;
 const div4= (document.getElementById('div4') as any).offsetTop;
 switch (index) {
  case 0: document.body.scrollTop = document.documentElement.scrollTop = div1 - menuHeight; break;
  case 1: document.body.scrollTop = document.documentElement.scrollTop = div2 - menuHeight; break;
  case 2: document.body.scrollTop = document.documentElement.scrollTop = div3 - menuHeight; break;
  case 3: document.body.scrollTop = document.documentElement.scrollTop = div4 - menuHeight; break;
  default: document.body.scrollTop = document.documentElement.scrollTop = div1- menuHeight;
 }
 }