title: UICollectionView not obeying zIndex of UICollectionViewLayoutAttributes
body: <p>The effect I'm trying to achieve is a kind of sticky header cell. It's important to me that the sticky cell floats over the top of the others. Something a bit like this:</p>
  
  <pre><code>┌──────────┐ 
  │          │ 
  │  Cell 0  │ 
  │          ├┐
  └┬─────────┘│
   │  Cell 4  │
   │          │
   └──────────┘
   ┌──────────┐
   │          │
   │  Cell 5  │
   │          │
   └──────────┘
   ┌──────────┐
   │          │
   │  Cell 6  │
   │          │
   └──────────┘
  </code></pre>
  
  <p>Cell 4, 5 and 6 would normally viewable and I'm constructing the attributes for cell 0 in my <code>UICollectionViewFlowLayout</code> subclass during <code>layoutAttributesForElementsInRect:</code>. All I do is call the super implementation, determine which cell I need to add in and then construct the <code>UICollectionViewLayoutAttributes(forCellWithIndexPath:)</code>. I then set the <code>zIndex</code> for it to <code>1</code> (default is <code>0</code>).</p>
  
  <p>The problem I'm getting is that the <code>UICollectionView</code> seems to always ignore the <code>zIndex</code></p>
  
  <pre><code>┌──────────┐ 
  │          │ 
  │  Cell 0  │ 
  │┌─────────┴┐
  └┤          │
   │  Cell 4  │
   │          │
   └──────────┘
   ┌──────────┐
   │          │
   │  Cell 5  │
   │          │
   └──────────┘
   ┌──────────┐
   │          │
   │  Cell 6  │
   │          │
   └──────────┘
  </code></pre>
  
  <p>Now I believe it's possible to visually sort this out using a 3d transform, but that doesn't work for me as I don't want any taps going to the cell which is over the top. So in this example I don't want Cell 4 receiving taps intended for Cell 0.</p>
  
  <p>Does anyone have any ideas? This is on iOS 8.4.</p>