JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
  <div class="child">
    regular child
  </div>
  <div class="child">
    <div class="sub-parent">
      <div class="sub-child">
        this one has a dynamic height
      </div>
      <div class="sub-child">
        this one should fill the remaining height and be scrollable
      </div>
    </div>
  </div>
</div>

CSS

.parent {
  display: table;
  height: calc(100vh - 200px);
  background-color: white;
  border: 1px solid #eee;
}

.child {
  display: table-cell;
  height: 100%;
}

.child:nth-child(1) {
  background-color: blue;
  color: #fff;
}

.child:nth-child(2) {
  background-color: yellow;
}

.sub-parent {
  display: block;
  background-color: pink;
  height: 100%;
}

.sub-child {
  width: 100%;
  float: left
}

.sub-child:nth-child(1) {
  background-color: red;
  color: #fff;
}

.sub-child:nth-child(2) {
  background-color: orange;
  overflow-y: scroll;
}