JSFiddle - React, Tailwind, and code Playground

HTML

<section>First block</section>
<nav>Menu</nav>

CSS

body {
    min-height:1000px;
    margin:0;
    padding:0;
}
section {
    background:gray;
    height:200px;
}
nav {
    height: 100px;
    background:lightgray;
    width:100%;
}
nav ul {
    margin-top:0;
}
.fixed {
    position: fixed;
    top:0;
}

JavaScript

$(document).ready(function () {
	    var aboveHeight = $('section').outerHeight();
	    $(window).scroll(function () {
	        if ($(window).scrollTop() > aboveHeight) {
	            $('nav').addClass('fixed');
	        } else {
	            $('nav').removeClass('fixed');
	        }
	    });
	});