Simon, glib 2.77.1 appears to be the GNOME 45 Beta release. It has successfully migrated out of mantic-proposed in Ubuntu. I am unaware of any remaining blockers to landing it in Testing.
Would you be comfortable if we uploaded it to Unstable now? Last cycle, the upstream maintainers were annoyed that distros didn't get more user testing sooner. I think they were talking more about Ubuntu and Fedora but maybe Debian can help there. Thank you, Jeremy Bícha
