@media(max-width:768px){

  #navMenu{
    position:fixed;
    top:0;
    right:-100%;   /* hidden */
    width:80%;
    height:100vh;

    background:#000;

    display:flex;
    flex-direction:column;
    padding:100px 30px;

    transition:0.4s ease;
    z-index:9999;
  }

  /* 🔥 THIS IS IMPORTANT */
  #navMenu.active{
    right:0;   /* slide in */
  }

}