title: Fixed header inside scrolling block
body: <p>I'm trying to create a block which may or may not have a scrollbar, with a header that does not scroll. The trick is that the width of the header <em>should</em> be affected by the presence of a scrollbar.</p>
  
  <p><img src="https://i.stack.imgur.com/CvVkL.png" alt=""></p>
  
  <p>I'm worried that this is one of those CSS use cases which should be trivial, but might, in fact, be impossible. Anyone willing to prove me wrong?</p>