A collection view flow layout with floating headers support, compatible with PSTCollectionView. Thanks to @cocotutch! The original answer on StackOverflow can be found at http://stackoverflow.com/questions/13511733/how-to-make-supplementary-view-float-in-uicollectionview-as-section-headers-do-i/13678922#13678922

CollectionViewFloatingHeaderFlowLayout.m

#import "PSTCollectionView.h"

@implementation CollectionViewFloatingHeaderFlowLayout

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSMutableArray *answer = [[super layoutAttributesForElementsInRect:rect] mutableCopy];
    PSTCollectionView *const cv = self.collectionView;
    CGPoint const contentOffset = cv.contentOffset;
    
    NSMutableIndexSet *missingSections = [NSMutableIndexSet indexSet];
    
    for (PSTCollectionViewLayoutAttributes *layoutAttributes in answer)
    {
        if (layoutAttributes.representedElementCategory == PSTCollectionElementCategoryCell)
        {
            [missingSections addIndex:layoutAttributes.indexPath.section];
        }
        layoutAttributes.zIndex = -1;
    }
    
    for (PSTCollectionViewLayoutAttributes *layoutAttributes in answer)
    {
        if ([layoutAttributes.representedElementKind isEqualToString:PSTCollectionElementKindSectionHeader])
        {
            [missingSections removeIndex:layoutAttributes.indexPath.section];
        }
    }
    
    [missingSections enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) {

        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:idx];

        PSTCollectionViewLayoutAttributes *layoutAttributes = [self layoutAttributesForSupplementaryViewOfKind:PSTCollectionElementKindSectionHeader atIndexPath:indexPath];

        [answer addObject:layoutAttributes];
    }];
    
    for (PSTCollectionViewLayoutAttributes *layoutAttributes in answer)
    {
        if ([layoutAttributes.representedElementKind isEqualToString:PSTCollectionElementKindSectionHeader])
        {
            NSInteger section = layoutAttributes.indexPath.section;
            NSInteger numberOfItemsInSection = [cv numberOfItemsInSection:section];
            
            NSIndexPath *firstObjectIndexPath = [NSIndexPath indexPathForItem:0 inSection:section];
            NSIndexPath *lastObjectIndexPath = [NSIndexPath indexPathForItem:MAX(0, (numberOfItemsInSection - 1)) inSection:section];
            
            PSTCollectionViewLayoutAttributes *firstObjectAttrs;
            PSTCollectionViewLayoutAttributes *lastObjectAttrs;
            
            if (numberOfItemsInSection > 0)
            {
                firstObjectAttrs = [self layoutAttributesForItemAtIndexPath:firstObjectIndexPath];
                lastObjectAttrs = [self layoutAttributesForItemAtIndexPath:lastObjectIndexPath];
            }
            else
            {
                firstObjectAttrs = [self layoutAttributesForSupplementaryViewOfKind:PSTCollectionElementKindSectionHeader
                                                                        atIndexPath:firstObjectIndexPath];
                lastObjectAttrs = [self layoutAttributesForSupplementaryViewOfKind:PSTCollectionElementKindSectionFooter
                                                                       atIndexPath:lastObjectIndexPath];
            }
            
            CGFloat headerHeight = CGRectGetHeight(layoutAttributes.frame);
            CGPoint origin = layoutAttributes.frame.origin;
            origin.y = MIN(
                    MAX(
                        contentOffset.y + cv.contentInset.top,
                        (CGRectGetMinY(firstObjectAttrs.frame) - headerHeight)
                        ),
                    (CGRectGetMaxY(lastObjectAttrs.frame) - headerHeight)
                    );
            
            layoutAttributes.zIndex = 0;
            layoutAttributes.frame = (CGRect) {
                .origin = origin,
                .size = layoutAttributes.frame.size
            };
        }
    }
    
    return answer;
}

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

@end