Position sticky seems to be not working chrome version 72.x?

Question

I was trying to learn the sticky position property in CSS. But I am to able to understand why the sticky property is not working. What am I doing wrong?

Here is the snippet that I am trying.

.container {
  display: flex;
}

.box {
  height: 200px;
  width: 200px;
  background: lightblue;
}

.boxA {
  position: sticky;
  position: -webkit-sticky;
  top: 0px;
  /* required */
  height: 900px;
  overflow-y: auto;
}

  <div class="container">
    <div class="box boxA">A should stick</div>
    <div class="box">B should scroll</div>
  </div>

Any pointers are appreciated !!