JSFiddle - React, Tailwind, and code Playground

HTML

<div class="sidebar">
  sidebar fixed width  sidebar fixed width  sidebar fixed width  sidebar fixed width sidebar fixed width sidebar fixed width sidebar fixed width sidebar fixed width sidebar fixed width sidebar fixed width sidebar fixed width sidebar fixed width sidebar fixed width sidebar fixed width sidebar fixed width
</div>
<div class="content">
  content flexible width
</div>

CSS

.sidebar {
    width: 180px;
    float: right;
    background: green;
}

.content {
    width: calc(100% - 180px);
    background: orange;
}