JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://github.com/darkwing/ScrollSpy/raw/master/Source/ScrollSpy-yui-compressed.js"></script>
<script src="https://github.com/jnbdz/ScrollBarCover/raw/master/Source/ScrollBarCover.js"></script>
<div id="content_post">
<p>MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer. It allows you to write powerful, flexible, and cross-browser code with its elegant, well documented, and coherent API.</p>  <p>MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer. It allows you to write powerful, flexible, and cross-browser code with its elegant, well documented, and coherent API.</p>  <p>MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer. It allows you to write powerful, flexible, and cross-browser code with its elegant, well documented, and coherent API.</p>  <p>MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer. It allows you to write powerful, flexible, and cross-browser code with its elegant, well documented, and coherent API.</p >  <p>MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer. It allows you to write powerful, flexible, and cross-browser code with its elegant, well documented, and coherent API.</p>  <p>MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer. It allows you to write powerful, flexible, and cross-browser code with its elegant, well documented, and coherent API.</p >  <p>MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer. It allows you to write powerful, flexible, and cross-browser code...

CSS

body {
    padding-top: 50px;
}
#content_post {
    height: 400px;
    width: 480px;
}
.scrollbarcover-slider-element {
    height: 400px;
    width: 16px;
    background-color: #ccc;
}
.scrollbarcover-slider-knob {
    height: 16px;
    width: 16px;
    background-color: #000;
}

JavaScript

new ScrollBarCover('content_post', 'content-post');