Issue with sliding overlay nav bar with custom CSS

Hi,
i’m trying to develop a gui with classic DOM tools (CSS, HTML5).
The porpuse is to make a sliding navigation bar that overlays the renderCanvas in which runs Babylon.
The problem is that the sliding panel doesn’t behave like expected: instead of overlays the canvas it pushes it and resizing it.
I didn’t understand: i’ve tryed with an empty Canvas or with a simple image inside and all work fine.

Here you can see a simple example of what i’d like to achieve: Tryit Editor v3.6

I’ve tryed to play with z-index but with no result.
I can’t reproduce the issue on PG so i attach the snippet of code i use:

My CSS:

html, body {
    overflow: hidden;    
    width: 100%;
    height: 100%;    
    margin: 0;
    padding: 0;
}

#renderCanvas {
    width: 100%;
    height: 100%;
    touch-action: none;        
    /*z-index: -1;*/
}

#fps {
    position: absolute;
    background-color: black;
    border: 2px solid red;
    text-align: center;
    font-size: 16px;
    color: white;
    top: 15px;
    right: 10px;
    width: 60px;
    height: 20px;
}
/* stili per il menu */
.sidenav {
  height: 100%;
  width: 0;
  position: fixed;
  z-index: 99;
  top: 0;
  left: 0;
  background-color: rgba(0,0,0,0.5);
  overflow-x: hidden;
  transition: 0.5s;
  padding-top: 60px;
}

.sidenav a {
  padding: 8px 8px 8px 32px;
  text-decoration: none;
  font-size: 25px;
  color: #818181;
  display: block;
  transition: 0.3s;
}

.sidenav a:hover {
  color: #f1f1f1;
}

.sidenav .closebtn {
  position: absolute;
  top: 0;
  right: 25px;
  font-size: 36px;
  margin-left: 50px;
}

@media screen and (max-height: 450px) {
  .sidenav {padding-top: 15px;}
  .sidenav a {font-size: 18px;}
}

#overlay {
  position: fixed;
  display: block;    
  width: 5%;
  height: 3%;  
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: rgba(255,255,255,0.5);
  z-index: 98;
  cursor: pointer;
  border-bottom-right-radius:30px;
}

HTML

<*html>
    <head>
        <title>Logistica magazzino</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">    
        <link rel="stylesheet" href="stili.css">
    </head>
    <body>
   
        <!-- div vuoto per debug
        <div id="debug" style="height: 20px;"></div>           
        -->
        <div id="main">
            <div id="mySidenav" class="sidenav" tabindex="2">
              <a href="javascript:void(0)" class="closebtn" onclick="closeNav()">&times;</a>
              <a href="#">About</a>
              <a href="#">Services</a>
              <a href="#">Clients</a>
              <a href="#">Contact</a>
            </div>               
            <span id="overlay" style="font-size:100%;cursor:pointer;" onclick="openNav()">&#9776; Opzioni</span>            
            <canvas id="renderCanvas" style="display: block;">                
            </canvas>              
        </div>        
        <!-- Link to the last version of BabylonJS -->        
        <script src="babylonjs/babylon.js"></script>
        <!-- Link a earcut -->
        <script src="babylonjs/earcut.min.js"></script>        
        <!-- Link to the last version of BabylonJS loaders to enable loading filetypes such as .gltf -->
        <script src="babylonjs/babylonjs.loaders.min.js"></script>
        <script src="babylonjs/babylon.gui.min.js"></script>
        <!-- Link to pep.js to ensure pointer events work consistently in all browsers -->
        <script src="lib/pep.min.js"></script>          
        <!--<script src="https://code.jquery.com/pep/0.4.2/pep.min.js"></script> -->
        <!-- Link applicazione magazzino 3D -->        
        <div id="scripts"></div>
        <script src="gui/textures.js" type="text/javascript"></script> 
        <script src="gui/mag_gui.js" type="text/javascript"></script>         
        <script src="scaffale.js" type="text/javascript"></script>
        <script src="magazzino.js" type="text/javascript"></script>        
        <script src="mappa.js" type="text/javascript"></script>        
        <script src="funzioni.js" type="text/javascript"></script>                       
    </body>
</html>

JS

/*
 * apertura del menu laterale
 */
function openNav() {
  document.getElementById("mySidenav").style.width = "250px";
  document.getElementById("main").style.marginLeft = "250px";
}

/*
 * chiusura del menu laterale
 */
function closeNav() {
  document.getElementById("mySidenav").style.width = "0";
  document.getElementById("main").style.marginLeft= "0";
}

Any idea?
Thanks

Got it!!
I have to use position: absolute both in canvas and sidenav.

These are the CSS i’ve changed:

#renderCanvas {
    width: 100%;
    height: 100%;
    touch-action: none;   
    position: absolute;
    left: 0px;
}

.sidenav {
  height: 100%;
  width: 0;
  position: absolute;
  z-index: 99;
  top: 0;
  left: 0;
  background-color: rgba(0,0,0,0.5);
  overflow-x: hidden;
  transition: 0.5s;
  padding-top: 60px;
}