JSFiddle - React, Tailwind, and code Playground

HTML

<div id="wrap">
    <div class="left-content"></div>
    <div class="fixed-width"></div>
</div>

CSS

* {
    margin: 0;
    padding; 0;
}

#wrap {
    width: 100%;
    height: 150px;
}

.left-content {
    height: 150px;
    background-color: #333;
    margin-right: 25px;
}

.fixed-width {
    width: 25px;
    height: 150px;
    position: absolute;
    right: 0;
    top: 0;
    background-color: #000;
}