title: UICollectionView deleteItemsAtIndexPaths doesn't animate supplementary views properly
body: <p>(Update below)</p>
  
  <p>When I delete items from a section in my collection view (say, section 0), I find that the supplementary views (in this case headers from the UICollectionViewFlowLayout) pop into place as the cells from section zero animate away and a 'duplicate' of the latter sections animates in with them to match the ones that have popped in.</p>
  
  <p>Naturally the ones that pop into place could be the 'duplicates' but it doesn't really matter. It's like the collection view can see the future! and then is catching up with itself. That's how the animation feels.</p>
  
  <p>Do you know if there is some quirk or bug or easy fix to suppressing this extra popping supplementary view?</p>
  
  <p>Update: I've noticed this also occurs with the cells, not just the sections, and it seems to 'double' the bottom few cells and sections on the screen. When I delete cells from the zero section, a handful of sections and cells below it animate properly without duplicates, but beyond a certain point, they pop. A constraints issue?</p>