body {
  margin: 0;
  padding: 0;
  height: 100%;
}

#clickable, #unclickable, #overlay {
  position: absolute;
  top: 0;
  bottom: 0;
  width: 50%;
  overflow: hidden;
}

#unclickable {
  left: 0;
  z-index: 10;
  background: yellow;
}

#clickable {
  right: 0;
  z-index: 30;
  background: red;
}

#overlay {
  position: absolute;
  z-index: 20;
  background: blue;
  opacity: 0.5;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}