Back in the day Mono had surprisingly good WinForms support on Gtk. It was never going to win awards for pretty and could never hit true compatibility with P/In

Back in the day Mono had surprisingly good WinForms support on Gtk. It was never going to win awards for pretty and could never hit true compatibility with P/Invoke calls to esoteric Win32 APIs, but it was good enough to run any simple WinForms app you wanted to write for it and ran some of the "popular" ones just fine. (That old Mono WinForms support was recently donated to Wine, which seems like a good home for it.) .NET has moved to being directly cross-platform today and is great at server/console app cross-platform now, but its support for cross-platform UI is still relatively nascent. The official effort is called MAUI, has mostly but not exclusively focused on mobile, and it is being developed in the open (as open source does) and leaves a lot to be desired, including by its relatively slow pace compared to how fast the server/console app cross-platform stuff moves. The Linux desktop support, specifically, seems constantly in need of open source contributors that it can't find. You'll see a bunch of mentions of third-party options Avalonia and Uno Platform doing very well in that space, though, so there is interesting competition, at least.