title: Why is UIKit resetting the UIViewController's UIView frame to "full screen" during a custom transition?
body: <p>I'm using a custom transition to slide a view controller 2/3 off screen when <code>- presentViewController:animated:completion:</code> is called and slide it back again when <code>dismissViewControllerAnimated:completion:</code> is called.</p>
  
  <p>I'm also using UIPercentDrivenInteractiveTransition attached to a <code>UIScreenEdgePanGestureRecognizer</code> to slide the view controller.</p>
  
  <p>All works well, except when calling <code>dismissViewControllerAnimated:completion:</code>, before:</p>
  
  <pre><code>- (void)animateTransition:(id &lt;UIViewControllerContextTransitioning&gt;)transitionContext
  </code></pre>
  
  <p>is called the frame of the view controller slid off screen is set to:</p>
  
  <pre><code>(0 0; 320 568)
  </code></pre>
  
  <p>causing it to momentarily "flash" before the animation runs and it slides from 2/3 off screen back to full screen.</p>
  
  <p>By subclassing the UIView on the View Controller I can set a break point in <code>-setFrame</code> it shows the method:</p>
  
  <pre><code>[UIWindowController transition:fromViewController:toViewController:target:didEndSelector:animation:]
  </code></pre>
  
  <p>is calling <code>-setFrame</code> with <code>(0 0; 320 568)</code>.</p>
  
  <p>Why is it doing this? How can I prevent it from setting the frame before the animation starts? </p>