Problem: UICollectionViewCell redraw during interactive resize of UICollectionView

I’m very close to finishing the next big update of Unitica. Headline feature is that app is now Universal and I’ve built custom container controller that looks like this:

Update (Jul 6th, 2015): it’s a stupid bug on my side, see the end of this post.

I added background colors to clearly visualize the various child levels; at the top you have my custom split view controller with two child controllers:

This ColumnsController has two child controllers (orange and yellow) - each an instance of UnitViewController, which is UICollectionViewController subclass. Green and bluish violet are actual collection view cells. Layout is UICollectionViewFlowLayout subclass where I make sure that cells always span the full width of the collection view.
This is the most relevant part of the layout:

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

- (void)prepareLayout {
	CGFloat w = floor(self.collectionView.bounds.size.width);
	CGSize is = self.itemSize;
	is.width = w;
	self.itemSize = is;
	[super prepareLayout];

The problem I have is that I want to allow changing the split position between the CategoryController and ColumnsController. You just drag the divider in the middle to set the sizes as you wish. Or you can slide them all the way to the edge and have one of those full screen.

This simply refuses to obey and I’m out of ideas what to try to force a proper redraw. Here’s the video that illustrates the problem.

(If the above does not work, here’s the same video on YouTube)

It’s obvious from the video - it’s basically random what will happen and how it will rest. Here are several screenshots that illustrate the randomness.

I have absolutelly no idea what to do with this. Maybe this kind of interactivity is simply not possible with UICollectionViewLayout, but I refuse to believe that.
I would hate to be forced to remove this feature, as I think it’s great.

Please tell me what I’m doing wrong.