JSFiddle - React, Tailwind, and code Playground

HTML

<div class="boxinterface">
    <div class="headercontainer">aa</div>
</div>

CSS

.boxinterface{
    max-width: 500px;
	margin:0 auto;
    height: 600px;
	box-shadow: 0px 1px 1px 1px rgba(51, 50, 50, 0.2);
    border: 1px solid black;
}
.headercontainer{
	position:fixed;
	margin-top:-3px;
    width: 81.2%; /* redur ou amplia fica bagunçado em %, e em px fica parado */
	height:80px;
	background-color:#f5f5f5;
	z-index:1;
    border: 1px solid black;
}