title: UICollectionViewScrollPosition not working
body: <p>i have a collectionview that i am trying to scroll programatically.  the problem being that if the cell i want to scroll to is visible in the collection view it doesn't scroll it to the centre. so for the image below the lower cell is item 1. and it does not scroll to it but it will scroll past item 1 to item 2.  </p>
  
  <p>i have been trying to use UICollectionVieScrollPosition.CenterVertically but this does not seem to work. </p>
  
  <pre><code>self.collectionView?.scrollToItem(at: IndexPath(row: 1, section: 0), at: UICollectionViewScrollPosition.centeredVertically, animated: true)
  </code></pre>
  
  <p>is there a way around this to force the scrolling of cells that are visible to the centre of the collection?</p>
  
  <p><a href="https://i.stack.imgur.com/Eqg8i.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Eqg8i.png" alt="enter image description here"></a></p>