<!DOCTYPE html>
<html>
	<head>
		<style>
			.site-header {
				position: relative;
				z-index: 7;
			}
			.dropnav {
				visibility: hidden;
			}
			.dropnav:after {
				-webkit-backdrop-filter:blur(5px);
				content:"";
				height:calc(100vh - 70px);
				position:fixed;
				width:100%;
			}
		</style>
	</head>
	<body>
		<header class="site-header">
			<div class="dropnav">
				<span>App</span>
			</div>
		</header>
		<div style="position: relative; top: 100px; left: 100px; width: 200px; height: 200px; background: blue;"></div>
	</body>
</html>