title: Nested flex scroll growing out of div
body: <p>I have a parent with 2 divs, one containing a very long div :</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="false" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.parent {
    height: 250px;
    display: flex;
    flex-direction: column;
  }
  .b {
    flex: 1 1 auto;
  }
  .b_child {
    overflow-y: scroll;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="parent"&gt;
    &lt;div class="a"&gt;hello&lt;/div&gt;
    &lt;div class="b"&gt;&lt;div class="b_child"&gt;Very long div&lt;/div&gt;&lt;/div&gt;
    &lt;div class="c"&gt;bottom&lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p><a href="https://jsfiddle.net/smf8y0L7/" rel="nofollow noreferrer">https://jsfiddle.net/smf8y0L7/</a></p>
  
  <p>As you can see, the .b_child is overflowing and growing out of b. If I set the scroll to .b instead it works as expected, but I want my flex child's child to scroll.</p>