JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper"><div class="menuLeft">Левое меню</div>
    <div class="menuRight">Правое меню</div></div>

CSS

.menuLeft {
background: #2980b9;
padding: 15px;
width: 30%;
margin-top: 5px;
margin-left: 5px;
float: left;
border-radius: 5px 5px 0px 0px;
font-weight: bold;
color: #fff;position:fixed;
}

.menuRight {
background: #3498db;
padding: 15px;
width: 70%;
margin-top: 5px;
margin-right: 5px;
float: right;
border-radius: 5px 5px 0px 0px;
font-weight: bold;
color: #fff;
position: relative;
}

.wrapper {width: 100%;}