title: iOS 11: UICollectionView: when appearing, scrolling happens
body: <p>when my UIViewController appears with a UICollectionView inside, the content scrolls up a little bit, when appearing.</p>
  
  <p>I implemented <code>scrollViewDidScroll:</code> and I'm logging the <code>contentOffset.y</code>:</p>
  
  <pre><code>-20.000000
  -20.000000
  0.000000
  0.000000
  </code></pre>
  
  <p>This only happens on iOS 11 (simulator and device), not on iOS 10 or iOS 9. Anyone else experiencing this?</p>
  
  <p><strong>EDIT</strong> VERY WEIRD thing: this only happens when viewDidLoad is NOT called, so e.g. when the UIViewController already exists and it is popped onto again, but not when first showing that UIViewController...</p>