I work on the IUP fork, where I have and support the Qt driver, which works with both Qt5 and Qt6, and I only need a couple of checks to make it work on both. I often check if I broke the Qt5 build; nope, all good there, hard to break. For GTK3 and GTK4, those must be separate drivers; there's no way you can handle that with a couple of checks. Everything is different. That was possible to some extent and works with GTK2/GTK3 (single driver), but there are so many checks and workarounds there. Qt is so nice to work with.