<div class="container"><!-- Backdrop --><div class="container__overlay"></div><!-- Sidebar --><div class="container__sidebar">...</div></div>
.container {/* Container takes full size */height: 100%;left: 0;position: fixed;top: 0;width: 100%;z-index: 9999;}.container__overlay {/* Take full size */height: 100%;left: 0;position: fixed;top: 0;width: 100%;/* User still can see the content of main page */background-color: rgba(0, 0, 0, 0.5);z-index: -1;}.container__sidebar {/* Take full height */height: 100%;left: 0;position: fixed;top: 0;width: 200px;/* Background */background-color: #fff;}