Drawer

<div class="drawer">
    <!-- Backdrop -->
    <div class="drawer__overlay"></div>

    <!-- Sidebar -->
    <div class="drawer__sidebar">...</div>
</div>

.drawer {
    /* Take full size */
    height: 100%;
    left: 0;
    position: fixed;
    top: 0;
    width: 100%;

    z-index: 9999;
}

.drawer__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;
}

.drawer__sidebar {
    /* Take full height */
    height: 100%;
    left: 0;
    position: fixed;
    top: 0;
    width: 200px;

    /* Background */
    background-color: #fff;
}