JSFiddle - React, Tailwind, and code Playground

HTML

<div class="my-app">
  <div class="sidebar">
    <p>
      Sidebar fixed width of 300px;
    </p>
  </div>
  <div class="map-container">
    <div class="map">
      <p>
      Map should grow to fill available space horizontally and vertically
      </p>
    </div>
    <div class="toolbar">
      <table class="large-table">
      <tr><td>Content should scroll and be 300px</td><td>Content</td><td>Content</td><td>Content</td><td>Content</td><td>Content</td><td>Content</td><td>Content</td><td>Content</td><td>Content</td><td>Content</td><td>Content</td><td>Content</td><td>Content</td><td>Content</td></tr>
      </table>
    </div>
  </div>
</div>

CSS

html, body, .my-app {
  padding:0; margin: 0;
  width: 100%;
  height: 100%;
  background: red;
}

.my-app {
  display: flex;
  flex-direction: row;
}

.sidebar {
  background: brown;
  width: 300px;
}

.map-container {
  flex: 1;
  display:flex;
  flex-direction: column;
}

.map {
  background: blue;
  flex: 1;
}

.toolbar {
  height: 300px;
  background: green;
}