JSFiddle - React, Tailwind, and code Playground

HTML

<div id="body">

    <div id="header">
        Header...
    </div>
    
    <div id="content">
        <div id="main">
            <p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p><p>Content</p>
        </div>
        <div id="parent">
            <div id="background">
        
            </div>
            <div id="card">
                Lorem...
            </div>
        </div>
    </div>
    
</div>

CSS

#body {
    top: 0;
    bottom: 0;
    right: 0;
    left: 0;
    position: absolute;
    overflow: hidden;
}

#header {
    position: fixed;
    height: 50px;
    background-color: yellow;
    opacity: 0.5;
    width: 100%;
}

#content {
    position: absolute;  
    top: 50px;
    bottom: 0;
    right: 0;
    left: 0;
    overflow: auto;
}

#parent {
    position: fixed;
    top: 0;
    bottom: 0;
    right: 0;
    left: 0;
    overflow: auto;
}

#background {
    opacity: 0.5;
    position: fixed;
    top: 0;
    bottom: 0;
    right: 0;
    left: 0;
    background-color: blue;
}

#card {
    position: relative;
    margin: 50px 50px 0;
    background-color: white;
    height: 2000px;
    border: 5px solid red;
}