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// scrollHeight 内容完整高度
scrollHeight = document.compatMode === "CSS1Compat" ? document.documentElement.scrollHeight : document.body.scrollHeight;
// or
scrollHeight = Math.max(document.documentElement.scrollHeight, document.body.scrollHeight);

// scrollTop 滚动高度
scrollTop = document.documentElement.scrollTop || document.body.scrollTop;

// clientHeight 可视高度
clientHeight = document.compatMode === "CSS1Compat" ? document.documentElement.clientHeight : document.body.clientHeight;