Having had to migrate a very simple project from GTK2 to GTK3, I don't think it's all that weird. The migration was utterly difficult, in the areas that hit me

Having had to migrate a very simple project from GTK2 to GTK3, I don't think it's all that weird. The migration was utterly difficult, in the areas that hit me seemingly no effort was made to give proper migration paths. Only with some later published documentation (+ help from chatgpt) was it possible to restore some functionality later, after the initial migration. Finally that even meant calling the xlib directly. And note that the software used wxWidgets, so most of the changes were encapsulated there. Only a very small part of GDK/GTK was used directly, with wnck already used as a helper layer (but the functionality in question broke there as well). So even if GTK came from GIMP, if the later development in GTK was not made specifically for and by the GIMP project, the migration must have been a nightmare. Especially in a project that had so many other things to worry about, non-destructive editing alone. And to repeat such a migration now again for GTK4 will not be very enticing.