JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div id="left">left</div>
    <div id="right">right</div>
</div>

CSS

#container {
    display: flex;               /* establish flex container */
}

#left {
    flex: 1;                     /* consume all available space */
    background-color: #ff0000;
}

#right {
    flex: 0 0 180px;             /* don't grow, don't shrink, fixed at 180px */
    background-color: #00FF00;
}