How to dim main content when off-canvas menu slides in

I have an off-canvas menu sliding from the left to cover the page. The building blocks of my page are divs. I call javascript functions to open and close the off-canvas menu. My main content remains as it is under the sliding menu. I have problems in finding a solution to dim the main content and to block any click to it.

I tried to use the property background: rgba(0, 0, 0, 0.5) !important; for an additional div with a z-index between off-canvas menu and maincontent, but without any result. Googled around, also here, but didn't find what I'm looking for. And I don't want to use Bootstrap or other libraries, to keep the project as light as possible. This is my code:

<style>
  .sidenav {
    height: 100%;
    width: 220px;
    position: absolute;
    z-index: 999;
    top: 0;
    left: 0;
    overflow-x: hidden;
    -webkit-transform: translate3d(-220px,0,0);
    -webkit-transition: -webkit-transform 0.4s;
    -webkit-transition-timing-function: cubic-bezier(0.7,0,0.3,1);
  }

  .cover {
    position: absolute;
    z-index: 900;
    left: 0px;
    top: 0px;
    display: none;
    width: 100%;
    height: 100%;
    z-index: 900;
    background: rgba(0, 0, 0, 0) !important;
    display: none;
  }

  .maincontent {
position: relative;
    z-index: 20;
    margin: auto;
  }
</style>

<script>
  function openNav() {
    document.getElementById("mySidenav").style.webkitTransform = "translate3d(-1px,0,0)";
    document.getElementById("cover").style.display = "inline";
    document.getElementById("cover").style.background = "rgba(0, 0, 0, 0.5) !important";
  }

  function closeNav() {
    document.getElementById("mySidenav").style.webkitTransform = "translate3d(-220px,0,0)";
    document.getElementById("cover").style.display = "none";
    document.getElementById("cover").style.background = "rgba(0, 0, 0, 0) !important";
  }
</script>

<div id="mySidenav" class="sidenav">
  <a href="javascript:void(0)" id="closebtn" class="closebtn" onclick="closeNav()">&times;</a>
  //menue items are hereunder
</div>
<div id="cover" class="cover">
<div id ="maincontent" class="maincontent">
  //some elements here
  <img id="open_nav" class="fixed-ratio-resize_menu" src="menu.png" alt="menu" onclick="openNav()">
  //all the rest of the page is here
</div>
</div>

Now the menu slides in and out as needed, but I have problems in finding a neat and running solution for: 1) dim the main content area under the menu; 2) intercept any click on main content area and either ignore it either onclick close the menu; 3) block the y scrolling of the main content, since it pulls also the off canvas menu to follow it, thus giving the appearance of a truncated side menu area.

First move your main content div outside the cover div. Currently your main content will only be visible when the cover is visible:

function openNav() {
  document.getElementById("mySidenav").style.webkitTransform = "translate3d(-1px,0,0)";
  document.getElementById("cover").style.display = "inline";
  // document.getElementById("cover").style.background = "rgba(0, 0, 0, 0.5) !important";
}

function closeNav() {
  document.getElementById("mySidenav").style.webkitTransform = "translate3d(-220px,0,0)";
  document.getElementById("cover").style.display = "none";
  // document.getElementById("cover").style.background = "rgba(0, 0, 0, 0) !important";
}

.sidenav {
  height: 100%;
  width: 220px;
  position: absolute;
  z-index: 999;
  top: 0;
  left: 0;
  overflow-x: hidden;
  -webkit-transform: translate3d(-220px, 0, 0);
  -webkit-transition: -webkit-transform 0.4s;
  -webkit-transition-timing-function: cubic-bezier(0.7, 0, 0.3, 1);
  background: #fff;
}

.cover {
  position: absolute;
  z-index: 900;
  left: 0px;
  top: 0px;
  width: 100%;
  height: 100%;
  z-index: 900;
  background: rgba(0, 0, 0, .5) !important;
  display: none;
}

.maincontent {
  position: relative;
  z-index: 20;
  margin: auto;
}

<div id="mySidenav" class="sidenav">
  <a href="javascript:void(0)" id="closebtn" class="closebtn" onclick="closeNav()">&times;</a> menu items are hereunder
</div>
<div id="cover" class="cover">
</div>
<div id="maincontent" class="maincontent">
  some elements here<br>
  <img id="open_nav" class="fixed-ratio-resize_menu" src="menu.png" alt="menu" onclick="openNav()"> all the rest of the page is here
</div>

Do you want to close the nav when clicking the cover? Then just add your onclick="closeNav()" to the cover:

function openNav() {
  document.getElementById("mySidenav").style.webkitTransform = "translate3d(-1px,0,0)";
  document.getElementById("cover").style.display = "inline";
  // document.getElementById("cover").style.background = "rgba(0, 0, 0, 0.5) !important";
}

function closeNav() {
  document.getElementById("mySidenav").style.webkitTransform = "translate3d(-220px,0,0)";
  document.getElementById("cover").style.display = "none";
  // document.getElementById("cover").style.background = "rgba(0, 0, 0, 0) !important";
}

.sidenav {
  height: 100%;
  width: 220px;
  position: absolute;
  z-index: 999;
  top: 0;
  left: 0;
  overflow-x: hidden;
  -webkit-transform: translate3d(-220px, 0, 0);
  -webkit-transition: -webkit-transform 0.4s;
  -webkit-transition-timing-function: cubic-bezier(0.7, 0, 0.3, 1);
  background: #fff;
}

.cover {
  position: absolute;
  z-index: 900;
  left: 0px;
  top: 0px;
  width: 100%;
  height: 100%;
  z-index: 900;
  background: rgba(0, 0, 0, .5) !important;
  display: none;
}

.maincontent {
  position: relative;
  z-index: 20;
  margin: auto;
}

<div id="mySidenav" class="sidenav">
  <a href="javascript:void(0)" id="closebtn" class="closebtn" onclick="closeNav()">&times;</a> menu items are hereunder
</div>
<div id="cover" class="cover" onclick="closeNav()">
</div>
<div id="maincontent" class="maincontent">
  some elements here<br>
  <img id="open_nav" class="fixed-ratio-resize_menu" src="menu.png" alt="menu" onclick="openNav()"> all the rest of the page is here
</div>

Finally, I would recommend to clean up your code and use event listeners instead of onclick properties in your html. Something like this:

var cover = document.getElementById("cover");
var sideNav = document.getElementById("mySidenav");
var closeNavBtn = document.getElementById("closebtn");
var openNavBtn = document.getElementById("open_nav");

function openNav() {
  sideNav.style.webkitTransform = "translate3d(-1px,0,0)";
  cover.style.display = "inline";
}

function closeNav() {
  sideNav.style.webkitTransform = "translate3d(-220px,0,0)";
  cover.style.display = "none";
}

closeNavBtn.addEventListener("click", closeNav);
cover.addEventListener("click", closeNav);
openNavBtn.addEventListener("click", openNav);

.sidenav {
  height: 100%;
  width: 220px;
  position: absolute;
  z-index: 999;
  top: 0;
  left: 0;
  overflow-x: hidden;
  -webkit-transform: translate3d(-220px, 0, 0);
  -webkit-transition: -webkit-transform 0.4s;
  -webkit-transition-timing-function: cubic-bezier(0.7, 0, 0.3, 1);
  background: #fff;
}

.cover {
  position: absolute;
  z-index: 900;
  left: 0px;
  top: 0px;
  width: 100%;
  height: 100%;
  z-index: 900;
  background: rgba(0, 0, 0, .5);
  display: none;
}

.maincontent {
  position: relative;
  z-index: 20;
  margin: auto;
}

<div id="mySidenav" class="sidenav">
  <span id="closebtn" class="closebtn">&times;</span> menu items are hereunder
</div>
<div id="cover" class="cover">
</div>
<div id="maincontent" class="maincontent">
  some elements here<br>
  <img id="open_nav" class="fixed-ratio-resize_menu" src="menu.png" alt="menu"> all the rest of the page is here
</div>