div#stage {
  position: absolute; top: 0; right: 0;
  width: 320px; height: 240px; border: 3px double #44a;
  padding: 5px;
  background: white;
  margin-right: 0; margin-left: auto;
  z-index: 2;
}
body > div#stage {position: fixed;}
div#x {
  position: static; height: 100%;
  overflow: auto;
  background: pink;
  z-index: 1;
}
body > div#x {position: absolute; top: 250px; height: auto;}