JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div id="test">
        <div id="fixed">Sitename</div>
        <p>
            blaat
        </p>
    </div>
</div>

CSS

#container {
    width: 400px;
    border: 1px solid red;
}

#test {
 border: 1px solid yellow;
 width: 100%;  
 /* width: inherit; */
}

#fixed {
    position: fixed;
    width: 100%;
    border: 1px solid green;
}