JSFiddle - React, Tailwind, and code Playground

HTML

<div class="header"></div>
<input type="text" class="header-right" />

CSS

.header{
    float:left;
    background: #efefef;
    background-repeat: no-repeat;
    width: 240px;
    height: 100px;
    }

.header-right{
    display: block;
    overflow:hidden; 
    background-color:green;
    height: 100px;
    border: none;
    width: calc(100% - 240px);
    }