By the way I believe I've found source code of some ancient version of the original Pain.Net and used it (barely working, buggy as hell of hells though still handier than GIMP for quick and simple picture edits) on Linux before I've found Pinta which is, AFAIK, the same code fixed to run correctly on Mono and extended to include some Paint.Net features that have been introduced later.

Pinta is nice though still not ultimately as great as the original :-) And it can be pain to install on some Linux distro versions (conflicting with both the Mono version in the repos (too old) and the latest Mono release (too new)) and it works quirky with XFCE. In fact I use it every day on Linux but I didn't know there is a Mac version too - already downloading it, thanks a lot!

I thought Paint.NET was free, just not OSS? I installed it on a machine a few weeks ago, no payment changed hands..

They probably meant "free software", per the FSF definition: https://www.gnu.org/philosophy/free-sw.en.html

It's free but still Windows-only. I use it on Windows machines but miss it heavily on Mac and Linux. Perhaps somebody would actually port it to Mono if it was truly free.

It has too many dependencies beyond .NET and Windows Forms by now. Text rendering is DirectWrite, for example, other parts use Direct2D, the UI partially uses WPF as well if I remember correctly.

I see... I used to think it's just WPF to be replaced with the old UI that was there until some version.