<nav>
  <div>раз</div>
  <div>два</div>
  <div>три</div>
  <div>четыре</div>
  <div>пять</div>
</nav>

body{
  background: #aaa;
}
nav{
  width: 20mm;
  height: 70mm;
  background: #ccc;
  overflow: hidden;
}
nav div, nav::before, nav::after{
  width: 100%;
  height: 10mm;
  border-radius: 0 2mm 2mm 0;
  background: #ccc;
  position: relative;
  z-index: 2;
}
nav::before, nav::after{
  content: '<пустое место>';
  display: block;
}
nav div:hover{
  background: #eee;
  box-shadow: #eee 0 0 5mm 6mm;
  z-index: 1;
}