JSFiddle - React, Tailwind, and code Playground

HTML

<div class="div1"></div>

<div id="nav">
	<ul>
		<li><a href="#">Link A</a></li>
		<li><a href="#">Link B</a></li>
		<li><a href="#">Link C</a></li>
		<li><a href="#">Link D</a></li>
	</ul>
</div>

<div class="div2"></div>

CSS

*{margin:0; border:0; padding:0; font:12px Arial, Helvetica, sans-serif}
.div1{background:#669900; height:200px; width:100%}
.div2{background:#669900; height:700px; width:100%}

#nav{width:100%; height:50px; background:#fff;}
#nav ul{list-style:none; margin:0; padding:5px;}
#nav ul li{ margin:0; padding:0; display:inline; }
#nav ul li a{float:left; margin:0 5px; padding:0 10px; height:40px; line-height:40px; color:#fff; background:#333; text-decoration:none}

JavaScript

$(function() {
	var offsetTop = $('#nav').offset().top;
	var navigation = function(){
		var scrollTop = $(window).scrollTop();
		
		if (scrollTop > offsetTop)
		{
			$('#nav').css({ 'position': 'fixed', 'top':0, 'left':0 });
		}
		else
		{
			$('#nav').css({ 'position': 'relative' }); 
		}   
	};
	
	$(window).scroll(function(){
		navigation();
	});
});