Consider adding libprintbackend-cups.so to the GTK
Context
Currently the runtime contains only file and LPR printing backend:
flatpak run org.freedesktop.Sdk//21.08
[📦 org.freedesktop.Sdk gtk-3.0]$ find /|grep printbackends
find: ‘/proc/tty/driver’: Permission denied
/usr/lib/x86_64-linux-gnu/gtk-3.0/3.0.0/printbackends
/usr/lib/x86_64-linux-gnu/gtk-3.0/3.0.0/printbackends/libprintbackend-file.so
/usr/lib/x86_64-linux-gnu/gtk-3.0/3.0.0/printbackends/libprintbackend-lpr.so
This prevents applications which use gtk_print_job_new
to start the printing operation to the cups server. Like Firefox (https://bugzilla.mozilla.org/show_bug.cgi?id=1712555) which use the custom print dialog instead of the system one.
Please consider building GTK with cups-config
command available to ensure that libprintbackend-cups.so is compiled.