title: Buttons outside of his flexbox
body: <p><em>Hello, i've lost half of my hair in 2 days, sorry for my english :)</em></p>
  
  <p>If you resize the window <strong>the buttons in #header go outside of his container</strong>.</p>
  
  <p><em>Here is the code:</em>
  <div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>#root {
    background: grey;
    
    display: flex;
    flex-direction: column;
    align-items: center;
  }
  
  #header {
    background: red;
    
    align-self: stretch; /* fill in main-axis */
    flex: 0 0 65px; /* fixed height */
    
    display: flex;
    justify-content: space-between;
    align-items: center;
    
    padding: 10px;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div id="root"&gt;
    &lt;div id="header"&gt;
      &lt;button&gt;PUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUSH&lt;/button&gt;
      &lt;button&gt;BYE.&lt;/button&gt;
    &lt;/div&gt;
    &lt;----- resize me ----&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>I think the problem is <strong>#root.width is leaded by window/body.width</strong> ?</p>
  
  <p><em>I have a solution with <strong>min-content</strong>:</em>
  <code>#root { min-width: min-content; ... }</code><em>, but i don't want use this value.</em></p>
  
  <p><strong>How configure #root correctly acting as a good and beautiful container for my layout ?</strong></p>
  
  <p>Here is a codepen for playing: <a href="https://codepen.io/papawa/pen/BOjdRZ" rel="nofollow noreferrer">https://codepen.io/papawa/pen/NLKbKR</a></p>