title: Default back button stops working if I add a subview to a UINavigationBar
body: <p>I want to use a <code>UIVisualEffectView</code> as the background of a <code>UINavigationBar</code> because it achieves an effect that is distinct from simply setting <code>translucent = true</code> on the UINavigationBar.</p>
  
  <p>I do this by subclassing <code>UINavigationController</code> and adding this to its <code>viewDidLoad()</code></p>
  
  <pre><code>visualEffectView = UIVisualEffectView(effect: UIBlurEffect(style: .Dark))
  visualEffectView.autoresizingMask = [.FlexibleHeight, .FlexibleWidth]
  navigationBar.addSubview(visualEffectView)
  navigationBar.sendSubviewToBack(visualEffectView)
  </code></pre>
  
  <p>The <code>UIVisualEffectView</code> looks fine, and the other buttons in my navigation work properly, but the built-in back button (I did not customize it) stops working. I can still swipe to go back, but tapping the back button does not work. Removing the <code>UIVisualEffectView</code> restores the functionality of the back button.</p>
  
  <p>I tried inspecting the view hierarchy in Xcode, and the <code>UIVisualEffectView</code> is not blocking the back button or anything like that.</p>
  
  <p>Any idea what could be going on? Is there a better way to use the <code>UIVisualEffectView</code> as a background for a <code>UINavigationBar</code>, or is there additional setup I need to do?</p>