The issue here, whether using GTK or SDL, is Sopwith uses old versions
of these libraries. SDL 1.2 and GTK 2 (probably). Neither of those
libraries maintain backward compatibility. Both were pretty much
designed to make application developers re-write large chunks of code.
So the way forward is, ideally, to port Sopwith to SDL 2 (the GTK port
was dropped years ago as being basically unmaintainable). The trick is
someone needs to find enough time to learn the newer SDL 2 library and
re-write the Sopwith code to match.
I might end up doing this, but it's way down my priority list at the
moment, especially since the current version still runs okay in most
In short: the fix is conceptually easy (re-write for SDL 2), it's just
going to be time consuming.