JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
  <div style="height:50px; border: 1px solid red;" id="div1">test</div>
  <div style="flex:1; border: 1px solid green;" id="div2">test</div>
</div>

CSS

.wrapper {
  display: flex;
  flex-direction: column;
  min-height: 100vh;
}

.wrapper div {
  align-items: stretch;
}