1.
- (void)collectionView:(UICollectionView *)collectionView willDisplaySupplementaryView:(UICollectionReusableView *)view forElementKind:(NSString *)elementKind atIndexPath:(NSIndexPath *)indexPath { view.layer.zPosition = 0.0; }
https://stackoverflow.com/questions/45871149/ios-11-uicollectionview-when-appearing-scrolling-happens