body {
  margin: 0px;
}
* {
  box-sizing: border-box;
}
.demo {
  position: absolute;
  width: 50%;
  height: 100%;
  padding: 10px;
  font-size: 14px;
  font-family: Monaco, monospace;
  background-color: hsl(240,40%,96%);
  border: none;
  resize: none;
  outline: none;
}
.demo:focus {
  background-color: white;
}
.source {
  left: 0px;
}
.target {
  right: 0px;
}