The function layoutAttributesForSupplementaryElement of NSCollectionView is mapped to GetLayoutAttributest in Xamarin.Mac
That must be some kind of typing error in the binding?
Yep. That's a typo. They were significantly more common in past bindings, as we had less tooling, but unfortunately mistakes happen.
I filed a bug on your behalf: https://bugzilla.xamarin.com/show_bug.cgi?id=60595
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