body {
  display: flex;
  flex-direction: column;
  overflow: scroll;
}
#controls {
  flex: none;
}
#stage {
  flex: auto;
  position: relative;

  & > div {
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    background: lightgoldenrodyellow;
  }
}

.container {
  border: 1px solid red;
  width: 75%;
  height: 60%;
  padding: 10px;
  margin: 10px;
}

.resize {
  overflow: hidden;
  background: lime;
  border: 1px solid green;
  padding: 10px;
}

.resizer {
  position: relative;
}

.resizeBetween {
  background: tomato;
  position: absolute;
  bottom: 0;
  left: 0;
}

.resizeBtn {
  position: absolute;
  bottom: 0;
  right: 0;
  height: 20px;
  width: 20px;
  background: blue;
  opacity: 0.3;
}

#targetRoot {
  height: 80%;
}

#targetA {
  box-sizing: border-box;
}

#targetC {
  height: auto;
}

.flex {
  display: flex;

  & > .resize {
    width: 80px;
  }
}
