My friend Jonathan (one of the GNOME old-timers) has been working on a Crosswords app, and it is wonderful - today it had its first alpha release - https://blogs.gnome.org/jrb/2021/11/18/introducing-gnome-crosswords/
You can see special bling thanks to GTK4! And unidirectional data flow in C for GTK!
I've been helping along with the layout and behavior engine. This has been so much fun!