title: Dragging cell outside of UICollectionView frame changes its contentOffset
body: <p>There's this bug happening when I drag a UICollectionViewCell outside of the frame of the collection: the collection's contentOffset is reset to 0, I suppose to scroll to top, even when dragging the cell over below the collection. The problem is the contentOffset has to be manually put back to where it was before and that's visually delayed.</p>
  
  <p>I've tried locking the scroll while dragging, such as the following</p>
  
  <pre><code>- (void)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout willBeginDraggingItemAtIndexPath:(NSIndexPath *)indexPath {
      collectionView.scrollEnabled = NO;
  }
  
  - (void)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout didEndDraggingItemAtIndexPath:(NSIndexPath *)indexPath {
      collectionView.scrollEnabled = YES;
  }
  </code></pre>
  
  <p>and it didn't do anything, contentOffset still changes. Also did the following</p>
  
  <pre><code>- (CGPoint)collectionView:(UICollectionView *)collectionView targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset {
  
      if (proposedContentOffset.y &gt; -10.0f) { // Minimum scroll from top is -10
          return CGPointMake(proposedContentOffset.x, -10.0f);
      }
      return proposedContentOffset;
  }
  
  - (void)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout willEndDraggingItemAtIndexPath:(NSIndexPath *)indexPath {
  
      if (collectionView.contentOffset.y &gt; -10.0f) { // Minimum scroll from top is -10
          [collectionView setContentOffset:CGPointMake(collectionView.contentOffset.x, -10.0f)];
      }
  }
  </code></pre>
  
  <p>to reset the contentOffset whenever it changes and this works fine, but the contentOffset is changed while dragging and the reset only happens when the user releases the cell, so there's a delay. Can I, somehow, lock the contentOffset while dragging?</p>
  
  <p>I think it's worth mentioning my view structure is currently</p>
  
  <pre><code>UIScrollView
      UICollectionView
      UICollectionView (the one that has drag-drop enabled)
  </code></pre>
  
  <p><strong>The parent ScrollView unifies both scrolls of the collections inside, so that could be a problem. When the collection is scrolled to top by the contentOffset, it slightly invades the collection above it.</strong></p>