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That's geared towards music related GUI apps?

It is, but it contains enough general purpose stuff to be useful for non-audio stuff as well. I use JUCE at my day job FWIW.

gtkmm works across all those 3 platforms, and had modern C++ before Qt did (e.g. actually using templates and libstdc++ instead of preprocessors and custom "standard" classes).

https://www.msys2.org/docs/what-is-msys2/

Looks like a nice alternative indeed. it uses MSYS2 for windows native build.

There is also still wxWidgets https://www.wxwidgets.org/

Not modern c++ last time I checked but active development for sure.