Modern NSCollectionView

It seems all the examples for macOS NSCollectionVew are pretty out of date. The newest recommendations include the use NSCollectionViewDiffableDataSource, and although this provides a callback for supplemental views, I can’t seem to get NSCollectionView to call it for the gap indicator when handling drops, as described here.

My drag & drop is otherwise working as expected, but for highlighting where the drop will land.

Any advice? Thanks!