UICollectionView Compositional Layout – items size not updated properly on iPad rotation – Ios swift

I gave your UICollectionView a background of orange and ran your code and got similar results:

I noticed few things

1. From this I could conclude the UICollectionView seems to be set up and working well on rotation, probably due to auto layout so I feel the issue is more to do with the layout code.
2. I also noticed when you start scrolling, the View fixes itself to what you want
3. Finally, I added one line of code to your func relayoutCollectionView(with size: CGSize) function to see what the dimensions are when you want update the layout calculations and columns

func relayoutCollectionView(with size: CGSize) {
    collectionView.collectionViewLayout.shouldInvalidateLayout(forBoundsChange: CGRect(x: 0,
                                                                                       y: 0,
                                                                                       width: size.width,
                                                                                       height: size.height))
    collectionView.setCollectionViewLayout(createFlowLayout(),
                                           animated: true,
                                           completion: nil)
    
    collectionView.collectionViewLayout.invalidateLayout()
    
    // I added this line
    print("Collection View Width: (collectionView.frame.size.width), Collection View Height: (collectionView.frame.size.height)")
}

When I turned from portrait to landscape, the output printed was Collection View Width: 810.0, Collection View Height: 1080.0 which does not seem to be right.

So from this I assume that it is too early for invalidateLayout to be called to get the right calculations.

When I do something like this from a UIViewController, I will make invalidateLayout() call from viewWillLayoutSubviews()

In your case, what I tried and worked was adding the similar override func layoutSubviews() in your class CollectionView: UIView

    override func layoutSubviews() {
        super.layoutSubviews()
        collectionView.collectionViewLayout.invalidateLayout()
    }

I removed relayoutCollectionView from your func relayoutCollectionView

Now it seems to work for the first time as well although I can still not explain why your current works every other time except for the first time.