title: UIStackView rotation/constraint bug
body: <p>I have a UIStackView which is positions using auto layout. When the screen rotates I deactivate and activate various constraints to move the stack view to where I want it. </p>
  
  <p>I've checked using Reveal to ensure that the stack view is positioned correctly and it is.</p>
  
  <p>In the stack view I have a number of views and the stack view's alignment is set to Fill to ensure they expand to fill the width of the stack view. This works correctly, except...</p>
  
  <p>When I rotate the device, then rotate back to the original orientation. Then the stack view is positioned correctly, however the views within it are not. Instead of filling the stack view, they leave a gap down the side. Usually somewhere between 20 and 50 points. But always the same amount on the same device. </p>
  
  <p>This has me baffled. The only solution I have found to get the stack view to correctly fill on rotation back is this:</p>
  
  <pre><code>override func viewDidLayoutSubviews() {
      sliders.alignment = UIStackViewAlignment.Bottom
      sliders.alignment = UIStackViewAlignment.Fill
  }
  </code></pre>
  
  <p>This seems to kick the stack view and get it to reflow the views correctly. But it feels rather tacky.</p>
  
  <p>Has anyone encountered this bug and found a better solution?</p>
  
  <p>I've tried all sorts of this around setNeedsLayout(), setNeedsDisplay(), etc, etc. But soo far only the above code has fixed the issue.</p>