Compiled latest GTK
Ran gtk4-demo and used WAYLAND_DEBUG to confirm it used the correct shell
tested a top level and a popup
It's not bad, just IMHO bad style. To me commas separate list members. After the last member a comma doesn't make sense. If I see code where there's a comma after the last member it always looks sloppy to me. A Dev reordered the code and didn't clean up or copy and paste without cleaning up. As you might have noticed I value clean code extremely.
I also value not affecting other lines. Thus in class initializers I prefer comma going first over trailing to minimize changes. But for last enum member I prefer no comma even if it means the next addition causes an unaffected line to change too.