All the well-known cross-platform toolkits are C/C++, so you'll have to find bindings for whatever "sane" language you want to use. Your options are basically:

All the well-known cross-platform toolkits are C/C++, so you'll have to find bindings for whatever "sane" language you want to use. Your options are basically: - Qt: the most widely used, but is a little janky on macOS and they've been going down the JS route recently - wxWidgets: is a possibility, I haven't used it in a long time. - GTK: "native" for Linux, but technically cross-platform. However the experience is lousy (and it is C, not even C++) - JUCE: another long-standing, somewhat niche toolkit, but I've never used it or anything written in it. - Flutter: must use Dart as the language, as far as I'm aware. Also, not sure how native-looking it is. There are also Java and C#, but I've never used a Java UI that looked or felt remotely native (or even "good" for that matter). Haven't used any C# apps but I assume some toolkits exist.