<div class="grand-parent">
  <div class="parent1">
  </div>

  <div class="parent2">
    <div class="absolute-child">
      hello world
    </div>
  </div>
</div>

.grand-parent {
  display:flex;
  width: 100px;
  height:100px;
  background-color: red;
}
.parent1 {
  flex:1;
  background-color:blue;
}
.parent2 {
  flex:1;
  position:relative;
  background-color:purple;
  overflow:auto;
}
.absolute-child {
  position:absolute;
  right: 20px;

  background-color: white;
}