JSFiddle - React, Tailwind, and code Playground

HTML

<div class="u-nav-wrap a-fadeinT">
    <div class="u-nav-circle a-fadeinT"></div>
</div>

CSS

.u-nav-wrap {
width: 10px;
height: 40px;
float: left;
display: block;
overflow: hidden;
}

.u-nav-circle {
border-radius: 0 0 100px 0;
float: left;
width: 100px;
height: 100px;
margin-left: -90px;
background: transparent;
box-shadow: 10px 0 0 0 rgba(245,245,245,0.7);
overflow: hidden;
}
body{
    background:grey;
    height:100px;
    
}