overlay menu. FROM: https://www.w3schools.com/howto/howto_js_fullscreen_overlay.asp

overlay.css

/* The Overlay (background) */
.overlay {
    /* Height & width depends on how you want to reveal the overlay (see JS below) */    
    height: 100%;
    width: 0;
    position: fixed; /* Stay in place */
    z-index: 1; /* Sit on top */
    left: 0;
    top: 0;
    background-color: rgb(0,0,0); /* Black fallback color */
    background-color: rgba(0,0,0, 0.9); /* Black w/opacity */
    overflow-x: hidden; /* Disable horizontal scroll */
    transition: 0.5s; /* 0.5 second transition effect to slide in or slide down the overlay (height or width, depending on reveal) */
}

/* Position the content inside the overlay */
.overlay-content {
    position: relative;
    top: 25%; /* 25% from the top */
    width: 100%; /* 100% width */
    text-align: center; /* Centered text/links */
    margin-top: 30px; /* 30px top margin to avoid conflict with the close button on smaller screens */
}

/* The navigation links inside the overlay */
.overlay a {
    padding: 8px;
    text-decoration: none;
    font-size: 36px;
    color: #818181;
    display: block; /* Display block instead of inline */
    transition: 0.3s; /* Transition effects on hover (color) */
}

/* When you mouse over the navigation links, change their color */
.overlay a:hover, .overlay a:focus {
    color: #f1f1f1;
}

/* Position the close button (top right corner) */
.overlay .closebtn {
    position: absolute;
    top: 20px;
    right: 45px;
    font-size: 60px;
}

/* When the height of the screen is less than 450 pixels, change the font-size of the links and position the close button again, so they don't overlap */
@media screen and (max-height: 450px) {
    .overlay a {font-size: 20px}
    .overlay .closebtn {
        font-size: 40px;
        top: 15px;
        right: 35px;
    }
}

overlay.html

<!-- The overlay -->
<div id="myNav" class="overlay">

  <!-- Button to close the overlay navigation -->
  <a href="javascript:void(0)" class="closebtn" onclick="closeNav()">&times;</a>

  <!-- Overlay content -->
  <div class="overlay-content">
    <a href="#">About</a>
    <a href="#">Services</a>
    <a href="#">Clients</a>
    <a href="#">Contact</a>
  </div>

</div>

<!-- Use any element to open/show the overlay navigation menu -->
<span onclick="openNav()">open</span>

overlay.js

function openNav() {
    document.getElementById("myNav").style.width = "100%";
}

/* Close when someone clicks on the "x" symbol inside the overlay */
function closeNav() {
    document.getElementById("myNav").style.width = "0%";
}

check this simple...full screen overlay

This isn't an overlay, this is a modal and it has a css bug in the close icon. It fails on Chrome (78), Firefox (70), Edge (18), Android (4.4 to 9 with Chrome) and Safari (iOS and Mac).