title: UICollectionView contentOffset changes with custom layout
body: <p>I have a UICollectionView with a custom UICollectionViewLayout (actually, I'm using <a href="https://github.com/monospacecollective/MSCollectionViewCalendarLayout" rel="noreferrer">this nice layout</a>). </p>
  
  <p>I set <code>contentOffset = CGPointZero</code> in viewDidLoad. After viewDidLoad, however, the offset is -20, and the content gets pushed down like so:</p>
  
  <hr>
  
  <p><img src="https://i.stack.imgur.com/YMsyI.png" alt="picture"></p>
  
  <p>(It should be flush with the line). I'm loading the collection view layout in interface builder. It seems that my problem is very similar to <a href="https://stackoverflow.com/questions/13780138/dynamically-setting-layout-on-uicollectionview-causes-inexplicable-contentoffset">this one</a>, however the solutions there don't work for me. </p>
  
  <p>I tried modifying <code>collectionViewContentSize</code> in my layout implementation to ensure it was always greater than the size of the collectionView. Although this means I can scroll my content down (it's shorter than the height of the collectionView) and hide the extra space, I can also scroll back up to see it. </p>
  
  <p>Nothing seems to work!</p>