I can't find the method https://developer.apple.com/documentation/appkit/nsscrollview/1403546-addfloatingsubview, is it somewhere?
It looks absent https://developer.xamarin.com/api/type/MonoMac.AppKit.NSScrollView/, is it?
Yep, that API has not been bound yet.
I filed an issue here: https://github.com/xamarin/xamarin-macios/issues/5039
The fix was trivial enough I'll have a PR fixing it shortly.
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