As it seems, there are only two good options if you need a decent cross-platform desktop GUI without having the means to go full-custom for each platform. These

As it seems, there are only two good options if you need a decent cross-platform desktop GUI without having the means to go full-custom for each platform. These are Java and Qt. I'm heavily biased towards Qt since all serious image processing in the world is still done in C++, but without this constraint, I'd like the safety and tooling support of Java. Of course C# is just a much better Java, but AFAIK its crossplatform capabilities aren't mature. And it has a tendency to evolve a bit too fast. Your desktop codebase will probably live for at least 10 years, if not more, so it's better to start with a stable technology from the right family (i.e. not overly deficient from the modern perspective.. Qt somewhat fails on this front).