[wsc981]

With regards to UI there is Microsoft’s MAUI, which I personally prefer over Avalonia. I love the single project approach of MAUI. I think Avalonia also relies on MAUI controls to some extent (I seem to recall a <UseMaui /> project setting in Avalonia projects.

[pjmlp]

MAUI doesn't count if "supports GNU/Linux" is part of being considered FOSS proper, and on macOS they took the shortcut of using Mac Catalyst instead of macOS UI APIs.