在某一点停止固定位置滚动？ 〜反转

Question

在这个StackOverflow问题中：Stopping fixed position scrolling at a certain point?你可以找到jQuery来停止一个从上到下的元素，并在页面中间的某个地方停止所述元素。

我对jQuery没有多少经验，并希望下面的代码可以反转，如果它是从下到上滚动并中途停止的话。

谢谢！

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var windw = this;

$.fn.followTo = function ( pos ) {
    var $this = this,
        $window = $(windw);
    
    $window.scroll(function(e){
        if ($window.scrollTop() > pos) {
            $this.css({
                position: 'absolute',
                top: pos
            });
        } else {
            $this.css({
                position: 'fixed',
                top: 0
            });
        }
    });
};

$('#f').followTo(250);

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#f {
    background:#ace;
    padding:20px;
    width:200px;
    position:fixed; top:0; left:0;
}

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<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.0/jquery.min.js"></script>
<div id="f">
    I'm going to follow you only so far...
</div>
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