JSFiddle - React, Tailwind, and code Playground

HTML

<body>
  <div class="col-sm-18 col-sm-offset-3">
    <div class="col-sm-8">
      <div class="menu-fixed">
        asdf
      </div>
    </div>
  </div>
</body>

CSS

html, body {
  height: 100%;
  width: 100%
}
.col-sm-18 {
  width: 75%;
  height: 100%;
  background: #000;
  padding-left: 15px;
  padding-right: 15px;
}
.col-sm-offset-3 {
  margin-left: 12.5%;
}

.col-sm-8 {
  width: 33.333333%;
  height: 100%;
  background: #aaa;
}

.menu-fixed {
  background: red;
  position: fixed;
  width: inherit;
  height: 100%;
  opacity: 0.5;
}