JSFiddle - React, Tailwind, and code Playground

HTML

<div>
<nav></nav>
</div>

CSS

div {
    min-height: 300px;
    background: navy;
}
nav {
    position: relative;
    height: 45px;
    width: 350px;
    background: gray;
    overflow: hidden;
}
nav:before {
    content: " ";
    position: absolute;
    right: -3%;
    height: 100%;
    width: 7%;
    background: #fff;
    border-radius: 50%;
}