It seems really weird to me how "but teh licenz" is brought up every time with Qt, when Qt itself is available under the same licenses as all the other toolkits like Gtk, wxWidgets etc.
This doesn't quite apply for embedded development (as LGPL's Tivoization clause might start to bite), but 1.) neither Gtk nor wxWidgets are viable in that space anyway 2.) considering that other people here found that Tesla manages to ship their car firmware with LGPL Qt - well. Can't be that hard to comply with it. Also, 3.) while we're used to free tools and libraries for desktop development, it's much more common to pay for tools in embedded development.
FOX is really fast and looks OK, but I don't think it's very actively developed, and binding to C++ seems like a pain. And I'm not sure it supports CJK input methods.
I looked at EFL out of desperation, but that's pure style over substance and the complete opposite of what I'd want.
Why would you say GTK isn't a good option for non-Linux use? And would you say that it is a good option for Linux?
You need something like MSYS2 to even use GTK on Windows, and I've looked at what it takes to package and distribute GTK applications to Windows users without MSYS2, and it's headache inducing.
When it comes to Linux, it's pretty good. Getting its GObject Introspection dependencies compiled is another story, though. I've always had to rely on my distribution's package manager to install them.