JSFiddle - React, Tailwind, and code Playground

HTML

<div class="top_container">
<div class="menu">
   <ul class="nav_menu">
      <li>Item1</li>
      <li>Item2</li>
      <li>ItemN</li>
   </ul>
</div>
<div class="mask">maskmaskmaskmask
</div>
</div>

<div class="main_container">
    main_containermain_containermain_containermain_container
</div>

CSS

ul.nav_menu {
position: absolute;
z-index: 1000;
background:red;
}

.mask {
position: absolute;
z-index: 500;
background:green;
}

.main_container {
position: absolute;
z-index: 2;
background:yellow;
}