A subclass of UICollectionViewFlowLayout which has UITableView style sticky headers.

StickyHeadersCollectionViewFlowLayout.m

// StickyHeadersCollectionViewFlowLayout  
// 
// A subclass of UICollectionViewFlowLayout which has UITableView style sticky headers. 
// 
// This code is based on Evadne Wu's code^1, with the following changes:
// 
// * Fixes a crash for sections with zero items
// * Adds support for UIScrollView's contentInset
// * Adds support for UICollectionViewFlowLayout's sectionInset
// 
// [1]: http://blog.radi.ws/post/32905838158/sticky-headers-for-uicollectionview-using

@implementation StickyHeadersCollectionViewFlowLayout

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSMutableArray * attributesArray = [[super layoutAttributesForElementsInRect: rect] mutableCopy];
    
    UICollectionView * const cv = self.collectionView;
    CGPoint const contentOffset = cv.contentOffset;
    UIEdgeInsets contentInset = cv.contentInset;
    UIEdgeInsets sectionInset = self.sectionInset;
    
    NSMutableIndexSet *missingSections = [NSMutableIndexSet indexSet];
    for (UICollectionViewLayoutAttributes *layoutAttributes in attributesArray) {
        if (layoutAttributes.representedElementCategory == UICollectionElementCategoryCell) {
            [missingSections addIndex:layoutAttributes.indexPath.section];
        }
    }
    for (UICollectionViewLayoutAttributes *layoutAttributes in attributesArray) {
        if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) {
            [missingSections removeIndex:layoutAttributes.indexPath.section];
        }
    }
    
    [missingSections enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) {
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:idx];
        UICollectionViewLayoutAttributes *layoutAttributes = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:indexPath];
        [attributesArray addObject:layoutAttributes];
    }];
    
    for (UICollectionViewLayoutAttributes *layoutAttributes in attributesArray) {
        
        if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) {
            
            NSInteger section = layoutAttributes.indexPath.section;
            NSInteger numberOfItemsInSection = [cv numberOfItemsInSection:section];
            
            if (numberOfItemsInSection > 0) {
                NSIndexPath *firstCellIndexPath = [NSIndexPath indexPathForItem:0 inSection:section];
                NSIndexPath *lastCellIndexPath = [NSIndexPath indexPathForItem:MAX(0, (numberOfItemsInSection - 1)) inSection:section];

                UICollectionViewLayoutAttributes *firstCellAttrs = [self layoutAttributesForItemAtIndexPath:firstCellIndexPath];
                UICollectionViewLayoutAttributes *lastCellAttrs = [self layoutAttributesForItemAtIndexPath:lastCellIndexPath];
                
                
                
                CGFloat headerHeight = CGRectGetHeight(layoutAttributes.frame);
                CGPoint origin = layoutAttributes.frame.origin;
                origin.y = MIN(
                               MAX(
                                   contentOffset.y + contentInset.top,
                                   (CGRectGetMinY(firstCellAttrs.frame) - headerHeight - sectionInset.top)
                                   ),
                               (CGRectGetMaxY(lastCellAttrs.frame) - headerHeight)
                               );
                
                layoutAttributes.zIndex = 1024;
                layoutAttributes.frame = (CGRect){
                    .origin = origin,
                    .size = layoutAttributes.frame.size
                };
            }
        }
        
    }
    
    return attributesArray;
    
}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
    return YES;
}

@end

I have made a fork which implements support for per section insets, using the UICollectionViewDelegateFlowLayout. You might want to consider updating your original code with my modification. (I can't make a pull request on a gist)

https://gist.github.com/megakode/08b66c102f09cd39ee24