JSFiddle - React, Tailwind, and code Playground

HTML

<div class="chatbox"></div>
<div class="content">Content</div>
<div class="footer"></div>

CSS

body {
  display: flex;
  flex-wrap: wrap;
  margin: 0;
}

.chatbox {
  flex: 0 0 300px;
  height: calc(100vh - 60px);
  background-color: #bdc3c7;
  overflow-y: auto;
}

.content {
  flex: 1;
  background-color: #95a5a6;
}

.footer {
  flex-basis: 100%;
  height: 60px;
  background: #2c3e50;
}