Untitled

<body>
  <nav>
    <button>
    <div class="container">
      <ul></ul>
    </div>
  </nav>
</body>

nav {
  position: relative;
  z-index: 4;
}
body.fix-nav nav {
  position: fixed;
  top: 32px;
}

.container {
  position: absolute;
  z-index: -1;
  top: 0px;
  right: 0;
  overflow: auto;
  background: #000;
}

.container {
   height: calc(100vh - 100px);
}

.container {
   position: absolute;
   top: 100px; /* Высота блока nav */
   bottom: 0;
}