title: Nested Stack View won't respond to changing axis based on size class
body: <p>I have 4 simple views that I want to lay out either vertically 4x1 (if horizontally compact) or horizontally 2x2 (if in any other combination) on iPhones. I also added wC+vC as horizontal 2x2 because iPhone landscape is wC+vC.</p>
  
  <p>My interface builder layout looks like this:</p>
  
  <p><a href="https://i.stack.imgur.com/vC2rt.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vC2rt.png" alt="enter image description here"></a></p>
  
  <p>When display a preview of an iPhone 4s in landscape and portrait in Assistant, I get what I expect:</p>
  
  <p><a href="https://i.stack.imgur.com/ln2SM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ln2SM.png" alt="enter image description here"></a></p>
  
  <p>However! When I use the simulator for any iPhone and put it into landscape, I get this:</p>
  
  <p><a href="https://i.stack.imgur.com/9pIaz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9pIaz.png" alt="enter image description here"></a></p>
  
  <p>No matter what I try, I can never make both TopStack and BottomStack display as horizontal. IF I delete the bottom stack, the top becomes horizontal. But as soon as the bottom stack comes into play, the top stack doesn't respond.</p>
  
  <p>I have even tried this in code for just wC+vC:</p>
  
  <pre><code>-(void)traitCollectionDidChange:(UITraitCollection *)previousTraitCollection {
  if( self.traitCollection.horizontalSizeClass == UIUserInterfaceSizeClassCompact &amp;&amp; self.traitCollection.verticalSizeClass == UIUserInterfaceSizeClassCompact ) {
      [self.TopStack setAxis:UILayoutConstraintAxisHorizontal];
      [self.BottomStack setAxis:UILayoutConstraintAxisHorizontal];
  } else {
      [self.TopStack setAxis:UILayoutConstraintAxisVertical];
      [self.BottomStack setAxis:UILayoutConstraintAxisVertical];
  }
  </code></pre>
  
  <p>}</p>
  
  <p>When I debug, I can verify that the top stack is in fact in <code>UILayoutConstraintAxisHorizontal</code> but does not correctly display.</p>
  
  <p>Does anyone have an idea on what is wrong??</p>