这是flarum返回顶部的代码

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下面这段代码放到自定义头部

<script src="https://code.jquery.com/jquery-3.5.1.min.js"></script>
<script>
$(document).ready(function(){

    //Check to see if the window is top if not then display button
    $(window).scroll(function(){
        if ($(this).scrollTop() > 100) {
            $('.scrollToTop').fadeIn();
        } else {
            $('.scrollToTop').fadeOut();
        }
    });

    //Click event to scroll to top
    $('.scrollToTop').click(function(){
        $('html, body').animate({scrollTop : 0},800);
        return false;
    });

});
</script> 
<a href="#" class="scrollToTop"><i class="fas fa-chevron-up"></i></a>

以下这段放到自定义CSS

.scrollToTop{
width: 36px;
-webkit-tap-highlight-color: rgba(255, 255, 255, 0);
bottom: 80px;
font-size: 16px;
margin-bottom: 0;
text-align: center;
cursor: pointer;
border-radius: @border-radius;
padding: 6px 0;
color: white;
background: black;
opacity: 0.7;
border: 0;
font-weight: 900;
    position:fixed;
    right:30px;
    display:none;
  z-index: 1000;
}
.scrollToTop:hover{
    text-decoration:none;
background-color: darken(fadein(@control-bg, 50%), 50%);
}