title: presentViewController:animated:completion: does not animate View Controller into view
body: <p>I am trying to debug a really weird issue in the following code: </p>
  
  <pre><code>if(condition1) {
     ImageViewController* imageViewer = [[ImageViewController alloc] initWithImageData:tappedItem];
     [self presentViewController:imageViewer animated:YES completion:^{
        [imageViewer loadImage];
     }];
  }
  else if(condition2) {
     DocumentViewController* docViewer = [[DocumentViewController alloc] init];
     [self presentViewController:docViewer animated:YES completion:nil];
  }
  </code></pre>
  
  <p>In other words, depending on the state of <code>condition1</code> and <code>condition2</code>, one of two subclasses of <code>UIViewController</code> will be displayed modally to the user. </p>
  
  <p>In the second case all is well, but in the first the view controller is not presented with the usual animation that shows it sliding in from the bottom of the screen. Instead, after a brief delay, it just shows up all of a sudden, covering the entire screen. Another oddity is that in the dismissal animation, the image view inside the view controller is transparent.</p>
  
  <p>Removing the completion block has no effect. Replacing my view controller with an instance of <code>UIViewController</code> also has no effect, other than demonstrating that for some reason, animations don't work for <code>UIViewController</code> instances either.</p>
  
  <p>Thinking that maybe I did something wrong in <code>viewDidLoad</code> etc, I tried commenting out the view load/appear methods but to no avail. </p>
  
  <p>Pushing the view controller onto the nav stack is not an option because the app has a tab bar and I don't want to be visible.</p>
  
  <p><strong>update</strong></p>
  
  <p>Replacing my instance of <code>ImageViewController</code> with a <code>DocumentViewController</code> does result in an animation. The question now becomes: what could I have done in <code>ImageViewController</code> to mess up the animation?</p>