Ideally they could have done that with Gtk2 which would continue on to Gtk3 and Gtk4 without breaking backwards compatibility.

Of course yes, the next best thing wrt. backwards compatibility is at least not breaking what works - but IMO that is a waste of time for both the Gtk developers (sure, it'd be self inflicted, but still) who would need to support two (or three or four or however many times they decide to break their own APIs) libraries that do essentially the same thing in different ways, and for the developers who at the end of the day will most likely need to move on to the next version at some point to gain access to new features that they may need - so they'll spend that porting time anyway.

> And yeah, you're right, the GTK team isn't beholden to anyone. But... GUI code is expensive to write and expensive to maintain. So a GUI toolkit can have an outsize impact on the health of projects that use it. There's value in talking about things like this publicly, so that anyone who's shopping for a GUI toolkit can have a better understanding of what they might be getting into.

Indeed and this is why i mention my issues with backwards compatibility whenever i see it being relevant.

I do believe the Gtk developers are free to do whatever they want as they are not beholden to my -or anyone else's- wishes for stability, especially when they give out such an otherwise high quality library for free. However that is from their perspective.

From the library users' (that is the developers who use the library) perspective however there are additional issues, like all the time you spent learning the library (breaking the API invalidates your own knowledge which is something that is also important - unless you plan to somehow live forever it makes sense to want to avoid this sort of "knowledge churn" :-P) and writing the code that uses it (important especially for framework-style libraries like Gtk that put their tendrils everywhere in your application). And even if someone believes (wrongly, IMO) that commercial developers can afford all that time, it still is an issue with FLOSS since the vast majority of which is developed during its developers' free time: that time would have better been used to improve the applications instead of keeping up with their dependencies' breakage (e.g. XFCE or... Gimp :-P).

And of course from the end users' perspective it also means that even if the developers decide to drop a program, they can keep using it for a long time without having to worry about the libraries their program depends on getting stale (since they are linked dynamically), not supporting new features (like, e.g. Wayland - AFAIK Gtk2 apps do not support it natively and chances are if a new display server is made Gtk3 apps wont support it either) or having unfixable security issues (for the applications where that makes sense at least).

But yeah, Gtk developers - or any library developers - do not need to care about all of that nor they have to really. As you wrote, it is the developers who decide or not to use a library that care about such things mostly.

Sadly at last on Linux there aren't that many choices: of the "mainstream" toolkits only Gtk and Qt exist. Gtk is, well, Gtk and Qt cannot provide "perfect" ABI compatibility even if they wanted to because of C++ (there are some hacks that could be made but i am not aware of any project doing such a thing - i think only Haiku does something similar with proxy libraries that forward the GCC C++ 2.95 calls to modern GCC C++ calls for BeOS compatibility). But Qt is also a different beast, they are a middleware company with different priorities, the people that pay money on it (and thus affect these priorities) do not care much about providing a stable platform and can affort to waste time on upgrades. From other toolkits there is also Motif but that fell out of favor and got opensourced way too late to make much of a difference - its API/ABI is stable though and has been stable for decades, but that means nothing when barely anyone uses it or develops for it.

Personally i use Lazarus and LCL. LCL uses a variety of backends, kinda like wxWidgets, but unlike wxWidgets it doesn't break its API. This pushes the issue of keeping up to date to the LCL developers (who could have spent their time in fixing LCL's other issues - of which are many - instead) but that is how things are.

I think Java and SWT might be another stable solution for a native UI, AFAIK this never broke backwards compatibility either... or at least it never did back when i looked into it several years ago. Looking at some examples in their site, several of them seem to be somewhat old so it probably is stable (isn't it great how when you do not break your API left and right you get to also get time to build better documentation that you don't have to invalidate it after a while?)