title: UICollectionView animating cell size change causes undesired behavior
body: <p>I have a UICollectionViewController that using a standard <code>UICollectionViewFlowLayout</code> to display a single vertical column of cells. I am attempting to create an expand/collapse animation on a cell when a cell is tapped. I use the following code to accomplish this: </p>
  
  <pre><code>- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath (NSIndexPath *)indexPath
  {
      [self.feedManager setCurrentlySelectedCellIndex:indexPath.item];
      [self.collectionView performBatchUpdates:nil completion:nil];
  }
  
  - (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
  {
      //Return the size of each cell to draw
      CGSize cellSize = (CGSize) { .width = 320, .height = [self.feedManager heightForCellAtIndexPath:indexPath] };
      return cellSize;
  }
  </code></pre>
  
  <p>The 'selectedCellIndex' property on my managing object tells the data model to return an expanded or collapsed size inside of heightForCellAtIndexpath: </p>
  
  <pre><code>[self.collectionView performBatchUpdates:nil completion:nil];
  </code></pre>
  
  <p>Then the <code>performBatchUpdates:completiong:</code> method animates this size change nicely. However! When the animation happens, the expanding cell may cause a partially visible cell at the bottom of the screen to go off the screen. </p>
  
  <p>If this is the case and I subsequently collapse this cell, the now off screen cell will snap to its old position without animation, while all of the other visible cells will animate as desired. My intuition says that this is the correct behaviour since the cell is off screen when the collapse animation is being performed, and is not being included in the animation rendering process. My question becomes, how do I prevent this from happening? </p>
  
  <p>I would prefer all of the cells to animate together regardless if they are off screen or not. Any thoughts?</p>