Horizontally scrolling a <pre> tag that is an auto flex-item (or in one)

I can't seem to figure this out. After looking at (least)

... I still cannot get my <pre> tag content to scroll horizontally to the width of my available viewport space based on my flexbox layout.

Here is a CodePen with my current setup: https://codepen.io/neezer/pen/BJjzzM

* {
  box-sizing: border-box;
}

body {
  padding: 0;
  margin: 0;
}

.outer {
  display: flex;
  padding: 20px;
  background-color: #aaa;
  width: 100vw;
  height: 100vh;
  overflow: hidden;
}

.nav {
  flex: 0 0 200px;
  background-color: blue;
  color: white;
  padding: 20px;
}

.content {
  flex: 1;
  margin-left: 20px;
  background-color: green;
  display: flex;
  flex-direction: column;
}

.header {
  flex: 0 auto;
  background-color: purple;
  padding: 20px;
  margin-bottom: 20px;
}

.header h1 {
  margin: 0;
}

.pre {
  flex: 1;
  background-color: red;
  margin: 0;
  overflow: auto;
  min-width: 0;
}

<div class='outer'>
  <div class='nav'>
    sidebar
  </div>
  <div class='content'>
    <div class='header'>
      <h1>Horizontal problems</h1>
    </div>
    <pre class='pre'>
      <span>this is a really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, long piece of text
      </span>
      <span>this is a really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, long piece of text
      </span>
      <span>this is a really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, long piece of text
      </span>
      <span>this is a really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, long piece of text
      </span>
    </pre>
  </div>
</div>

What am I doing wrong here?

Solution

You still suffer from the minimum size issue of flex items.

In a nested Flexbox structure, it can also affect flex item's being a flex container (or an ancestor being a flex item), and in this case your content is one, and also needs to have its min-width set to 0.

As a side note, in this case the min-width: 0 set on pre is not generally necessary when using overflow with a value other than visible, and could be removed (which I did in the Codepen).

Updated codepen

Stack snippet

* {
  box-sizing: border-box;
}

body {
  padding: 0;
  margin: 0;
}

.outer {
  display: flex;
  padding: 20px;
  background-color: #aaa;
  width: 100vw;
  height: 100vh;
  overflow: hidden;
}

.nav {
  flex: 0 0 200px;
  background-color: blue;
  color: white;
  padding: 20px;
}

.content {
  flex: 1;
  min-width: 0;                       /*  added  */
  margin-left: 20px;
  background-color: green;
  display: flex;
  flex-direction: column;
}

.header {
  flex: 0 auto;
  background-color: purple;
  padding: 20px;
  margin-bottom: 20px;
}

.header h1 {
  margin: 0;
}

.pre {
  flex: 1;
  background-color: red;
  margin: 0;
  overflow: auto;
  min-width: 0;
}

<div class='outer'>
  <div class='nav'>
    sidebar
  </div>
  <div class='content'>
    <div class='header'>
      <h1>Horizontal problems</h1>
    </div>
    <pre class='pre'>
      <span>this is a really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, long piece of text
      </span>
      <span>this is a really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, long piece of text
      </span>
      <span>this is a really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, long piece of text
      </span>
      <span>this is a really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, long piece of text
      </span>
    </pre>
  </div>
</div>