.overlay {
  position: absolute;
  width: 100%;
  height: 100%;
  background: grey;
  opacity: 1;
  left: 0;
  top: 0;
  z-index: 2000;
}

.mainContainer {
  position: fixed;
  width: 800px;
  background-color: yellow;
  min-height: 500px;
  z-index: 200000
}