title: iOS: cellForItemAtIndexPath returns nil even though cell is visible
body: <p>I have a collection view and i have implemented drag and drop functionality. In addition, when cell is close to the top or bottom edges i scroll the collection with constant speed.</p>
  
  <p>When i drag the cell i set its transform property according to the gesture and i also modify the value while scrolling. I also change the transform property of the layout attributes for the index path.
  The cell is always on the screen in the gesture position. However, at some point the collection decides to hide the cell and cellForItemAtIndexPath return nil.</p>
  
  <p>Why does it happen and how can i prevent this situation?</p>