JSFiddle - React, Tailwind, and code Playground

HTML

<div id="ext"></div>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
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<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
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<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
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<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
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<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
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<p>Some text!</p>
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<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
<p>Some text!</p>
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<p>Some...

CSS

#ext {
    position: absolute;
    border: 1px solid #000;
    background-color: #ccc;
    width: 100px;
    height: 250px;
}

JavaScript

var ext = document.id('ext'),
    height = ext.getStyle('height').toInt();

ext.getDocument().addEvent('scroll', function(){
    var scroll = ext.getDocument().getScroll();

    ext.setStyle('height', (height + scroll.y));
});