JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="left">
    ...
  </div>

  <div class="right">
    <div class="messageWrapper">
      <div class="message"></div>
    </div>
  </div>
</div>

CSS

.container {
  top: 50px;
  width: 800px;
  height: 300px;
  background-color: red;
  position: relative;
  text-align: center;
}
.left {
  width: 500px;
  height: 100%;
  background-color: black;
  float: left;
}
.right {
  width: 200px;
  height: 100%;
  background-color: green;
  float: right;
  
    position: relative;
}
.messageWrapper {
  overflow: hidden;
  bottom: 0;
  max-height: 100%;
  min-height: 20px;
  width: 170px;
  width: 100%;
  text-align: center;
  position: absolute;
}
.message {
  min-height: 20px;
  background-color: yellow;
  margin-left: auto;
  margin-right: auto;
  margin-bottom: 5px;
  width: 80%;
}