Sadly Gtk developers were in a position during Gtk2's peak to help solve that by making Gtk2 "the" Unix desktop GUI API - it was used and targeted to by pretty much everything and it being a C API meant that it could remain backwards compatible for pretty much forever.
But they instead decided to break the API with Gtk3, repeating the Gtk1 to Gtk2 transition mistake (which at the time it wasn't that big of a deal since Gtk1.x was never as popular or widespread as Gtk2). And just as some projects finally managed to convert their Gtk2 code to Gtk3 code, sometimes after years of work, the Gtk developers awarded them for their effort by breaking the API again in Gtk4.
IMO at this point relying on anything Gtk4 is a mistake, though sadly there aren't many alternatives. Qt, being essentially a commercial middleware that also happens to have OSS-licensed code drops, has different priorities - and even if they wanted, being written in C++ which has no real stable ABI means they wouldn't be able to provide stable APIs anyway.
Other toolkits are asphyxiated by the oxygen that Gtk and Qt are siphoning away so they barely get any support. EFL seemed to be ok (in that it was a sort of usable, sort of existing, C-based API that is apparently good enough for a desktop environment to be made with) but the author seems to have decided at some point to refactor (and thus break) the entire thing. Motif has been stable since the 90s, but it also has seen little improvement since the 90s either - it is also too tied to X11, meaning that having a crossplatform version (which is something that people would want) is quite hard.
And thus we get Electron, which is almost a step before bundling an entire VM and OS inside a window.
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Sadly Gtk developers were in a position during Gtk2's peak to help solve that by making Gtk2 "the" Unix desktop GUI API - it was used and targeted to by pretty much everything and it being a C API meant that it could remain backwards compatible for pretty much forever.
But they instead decided to break the API with Gtk3, repeating the Gtk1 to Gtk2 transition mistake (which at the time it wasn't that big of a deal since Gtk1.x was never as popular or widespread as Gtk2). And just as some projects finally managed to convert their Gtk2 code to Gtk3 code, sometimes after years of work, the Gtk developers awarded them for their effort by breaking the API again in Gtk4.
IMO at this point relying on anything Gtk4 is a mistake, though sadly there aren't many alternatives. Qt, being essentially a commercial middleware that also happens to have OSS-licensed code drops, has different priorities - and even if they wanted, being written in C++ which has no real stable ABI means they wouldn't be able to provide stable APIs anyway.
Other toolkits are asphyxiated by the oxygen that Gtk and Qt are siphoning away so they barely get any support. EFL seemed to be ok (in that it was a sort of usable, sort of existing, C-based API that is apparently good enough for a desktop environment to be made with) but the author seems to have decided at some point to refactor (and thus break) the entire thing. Motif has been stable since the 90s, but it also has seen little improvement since the 90s either - it is also too tied to X11, meaning that having a crossplatform version (which is something that people would want) is quite hard.
And thus we get Electron, which is almost a step before bundling an entire VM and OS inside a window.