使用滚动顶部时，如果页脚消失，​​请向其添加类

Question

当用户滚动到顶部并且页脚隐藏时，我检测到它并向其添加一个类，以通过设置CSS类中固定的位置使其可见。但是当页脚即将隐藏时，页面会闪烁，css类会在几毫秒内被添加并删除。如何修复它以使页面不闪烁？谢谢。

ValidationError: User validation failed
<br> &nbsp; &nbsp;at model.Document.invalidate (C:\Users\lotus\Desktop\mastering_mean\application\node_modules\mongoose\lib\document.js:1162:32)
    <br> &nbsp; &nbsp;at C:\Users\lotus\Desktop\mastering_mean\application\node_modules\mongoose\lib\document.js:1037:16
        <br> &nbsp; &nbsp;at validate (C:\Users\lotus\Desktop\mastering_mean\application\node_modules\mongoose\lib\schematype.js:651:7)
            <br> &nbsp; &nbsp;at C:\Users\lotus\Desktop\mastering_mean\application\node_modules\mongoose\lib\schematype.js:679:9
                <br> &nbsp; &nbsp;at Array.forEach (native)
                    <br> &nbsp; &nbsp;at SchemaString.SchemaType.doValidate (C:\Users\lotus\Desktop\mastering_mean\application\node_modules\mongoose\lib\schematype.js:656:19)
                        <br> &nbsp; &nbsp;at C:\Users\lotus\Desktop\mastering_mean\application\node_modules\mongoose\lib\document.js:1035:9
                            <br> &nbsp; &nbsp;at process._tickCallback (node.js:355:11)