JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<div class="container">
  <header></header>
</div> 
<div class="container">
	<div class="flexcont">
	  <aside></aside>
	  <main></main>
	</div>
</div>

CSS

header { height:100px; background:#c0c0c0; margin:0 0 30px 0; }
.flexcont {display: flex; }
main { height:600px; background:#2f2f2f; width: 100%; float: unset;}
aside { background: #f00; width:250px; height:600px; position: fixed; top:130px; left:0; margin-right: 25px;}


@media(max-width:1650px) {
	
	aside { position: inherit !important; }
}