JSFiddle - React, Tailwind, and code Playground

HTML

<div id="parent">
  <div id="fixedSize">
    XYZ
  </div>
  <div id="mainSize">
    DAYUM
  </div>
</div>

CSS

#parent {
  padding: 5px;
  background-color: rgba(255, 0, 0, 0.5);
  width: 500px;
  /* This solved the problem:*/
  overflow: auto;
}

#fixedSize {
  background-color: rgba(0, 0, 255, 0.5);
  width: 150px;
  float: left;
}

#mainSize {
  background-color: rgba(0, 255, 0, 0.5);
  float: right;
  width: 350px;
}