IUP Portable User Interface VS Photon Micro GUI

As C libraries, I can recommend IUP. You can also choose immediate mode which has the same look on all platforms, there are some wrappers around imgui in Zig like zig-upaya. Source: almost 3 years ago

There's a relatively new C++ GUI library literally called "Elements". Not sure how it works though, but the way it looks, and the music background of its creator makes it appear designed for DAWs. https://github.com/cycfi/elements. - Source: Hacker News / 12 months ago

Further, if you we want a "modern" C++ GUI framework what actually would be modern would be to use mechanisms in the language itself as a quasi-DSL from within the language. This is something like what Joel de Guzman is doing with Elements. Source: about 1 year ago

It's a complex domain. The closest we have at the moment is Elements which hasn't been proposed for Boost (yet?) but is by Joel de Guzman, the primary author of Boost.Spirit. Source: over 1 year ago

Elements was mentioned as a specific example of an MIT-licensed GUI library on the CPPcast episode from 5 May 2022. Source: almost 2 years ago

If you don't want to use Qt I honestly think your best bet may be to become an early adopter of cycfi elements depending on your project. Elements is still rough but is useable for small applications. I think when it is finished it will be the best choice for a retained mode GUI library, but right now it is missing a lot of things (e.g. The standard common dialogs, "open", "Save as", etc.) , and has basically zero... Source: about 2 years ago