链接：https://blog.csdn.net/qq_43299703/article/details/120879098
https://blog.csdn.net/mouday/article/details/125444003
let clientHeight = document.documentElement.clientHeight; //浏览器可视高度
let scrollHeight = document.body.scrollHeight;//滚动长度
let scrollTop = document.documentElement.scrollTop;//滚动高度
