JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="left">
    LEFT
  </div>
  <div class="right">
    RIGHT RIGHT RIGHT RIGHT
  </div>
</div>

CSS

div {
  font-family: Tahoma, Verdana sans-serif;
  box-sizing: border-box;
  -moz-box-sizing: border-box;
}

.container {
  width: 100%;
  background-color: rgba(255, 255, 0, 1);
  overflow: auto;
}

.left {
  height: 100%;
  width: calc(100% - 100px);
  background-color: rgba(0, 0, 255, 0.6);
  float: left;
  padding: 20px;
  border: dashed thick blue;
}

.right {
  float: right;
  width: 100px;
  padding: 20px;
  border: dashed thick green;
}