JSFiddle - React, Tailwind, and code Playground

HTML

<div class="C">
  <div class="A"></div>
  <div class="B">
    <div class="D"></div>
  </div>
</div>

CSS

.C {
  width: 100px;
  height: 300px;
  background: #ccc;
  display: flex;
  flex-direction: column;
}
.A {
  flex: 0 0 auto;
  background: #999;
  height: 100px;
}
.B {
  flex: 1 1 auto;
  background: #666;
  padding: 5px;
  overflow: auto;
}
.D {
  height: 400px;
  background: #333;
}