JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div id="leftBar">
        <div id="rightBar">
            This is your 'rightBar'
        </div>
        <p>Many paragraphs of text</p>
        <p>Many paragraphs of text</p>
        <p>Many paragraphs of text</p>
        <p>Many paragraphs of text</p>
        <p>Many paragraphs of text</p>
        <p>Many paragraphs of text</p>
        <p>Many paragraphs of text</p>       
        <p>Many paragraphs of text</p>
        <p>Many paragraphs of text</p>
        <p>Many paragraphs of text</p>
    </div>
</div>

CSS

#container {
    width: 300px;
    background-color: yellow;
    overflow: hidden;
}

#leftBar {
    position: relative;
    width: 200px;
    background-color: green;
    padding-right: 100px;
}

#rightBar {
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    width: 100px;
    background-color: red;
}