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: orange;
  /* width: 300px; */
  flex: 0 0 300px; /* makes 300px width inflexible with flex-shrink: 0 */
  overflow: auto;
}

.sidebar > p {
  height: 1000px;
}


.map-container {
  flex: 1;
  display: flex;
  flex-direction: column;
  min-width: 0; /* allows flex item to shrink past its content size */
}

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

.toolbar {
  height: 300px;
  background: lightgreen;
  overflow: auto;
}