ios11 collectionView VerticalScrollIndicator被header挡住

解决方法:

- (void)collectionView:(UICollectionView *)collectionView willDisplaySupplementaryView:(UICollectionReusableView *)view forElementKind:(NSString *)elementKind atIndexPath:(NSIndexPath *)indexPath
{
    if (@available(iOS 11.0, *)) {
        if ([elementKind isEqualToString:UICollectionElementKindSectionHeader]) {
            view.layer.zPosition = 0;
        }
    }
}