I already mentioned it in dev chat.
I'm currently writing a proper Mac app for osu!
I found out that Xamarin.Mac has no WindowsForm implementation and using MonoMac isn't
recommended (also it needs to be installed by the user and doesn't ship with the app package).
I would suggest writing a WindowsForm for Mac (Cocoa UI).
Also maybe we can resolve the graphical glitches this way on Mac too.
Mac supported .Net assemblies
Moving to osu-framework