Hey, I'm the developer of Cozy and I just noticed that Cozy got posted here and people are trying it on macOS. The version linked in the README was a very old version of Cozy which I tested on macOS 10.14. It was quite a hassle to build and it was not stable. Sadly it's a lot of work to package a GTK app for macOS, therefore I discontinued building it but I didn't update the readme. Sorry for anyone who tried it and got disappointed!
If you ever have a chance, someone might be interested in a quick writeup of the challenges you faced packaging a GTK3 app for macOS. Off the top of my head, Inkscape ships an app with GTK3 and Python for macOS.
I still have documentation of the progress. It probably needs some work before it's useful to others but maybe it helps someone :)
I don't use mac though, so haven't confirmed what I've heard.
Inkscape recently (with 1.0) completed their switch from XQuartz to a native 64-bit signed/notarized Mac app.
Please consider leaving a link to cozy_macos_0.6.7_beta3.dmg; the macOS version was actually the reason why I posted Cozy to HN ;-). Worked great under High Sierra and was perfect for setting up a simple audiobook system for a centenarian who has been stuck indoors for months.
Oh I see, sorry :) I'll add the link again with a proper note and some documentation about how I build the package. Maybe someone has the time to test it with the latest version :)