JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main">
  <header>
    <h1>Header</h1>
  </header>
  <div id="wrapper">
    <div id="left">
      <div class="box"></div>
      <div class="box"></div>
      <div class="box"></div>
      <div class="box"></div>
      <div class="box"></div>
      <div class="box"></div>
      <div class="box"></div>
    </div>
  </div>
</div>

CSS

body, html, h1 {
  padding: 0;
  margin: 0;
}
header {
  background-color: rgba(0, 255, 255, 0.2);
  height: 50px;
}
#main {
  height: 100vh;
  background-color: rgba(0, 255, 255, 0.1);
  display: flex;
  flex-flow: column;

}
#wrapper {
  display: flex;
  flex-grow: 2;
  flex-basis: 0%;
    min-height: 0; /* NEW */
    min-width: 0; /* NEW */

}
#left {
  overflow-y: scroll;
  background-color: rgba(0, 255, 255, 0.2);
  width: 30%;

}
.box {
  margin: 5px 0;
  height: 50px;
  width: 100%;
  background-color: rgba(0, 0, 0, 0.2);
}