I’ve narrowed the issue down to be a problem with bwrap.
$ flatpak run com.vscodium.codium
bwrap: execvp /nix/store/njzrg148bj0v4m055c9hzxkqqchiv833-xdg-dbus-proxy-0.1.6/bin/xdg-dbus-proxy: No such file or directory
$ bwrap /nix/store/njzrg148bj0v4m055c9hzxkqqchiv833-xdg-dbus-proxy-0.1.6/bin/xdg-dbus-proxy
bwrap: execvp /nix/store/njzrg148bj0v4m055c9hzxkqqchiv833-xdg-dbus-proxy-0.1.6/bin/xdg-dbus-proxy: No such file or directory
$ bwrap --ro-bind /nix/store /nix/store /nix/store/njzrg148bj0v4m055c9hzxkqqchiv833-xdg-dbus-proxy-0.1.6/bin/xdg-dbus-proxy
usage: /nix/store/njzrg148bj0v4m055c9hzxkqqchiv833-xdg-dbus-proxy-0.1.6/bin/xdg-dbus-proxy [OPTIONS...] [ADDRESS PATH [OPTIONS...] ...]
Options:
--help Print this help
--version Print version
--fd=FD Stop when FD is closed
--args=FD Read arguments from FD
Proxy Options:
--filter Enable filtering
--log Turn on logging
--sloppy-names Report name changes for unique names
--see=NAME Set 'see' policy for NAME
--talk=NAME Set 'talk' policy for NAME
--own=NAME Set 'own' policy for NAME
--call=NAME=RULE Set RULE for calls on NAME
--broadcast=NAME=RULE Set RULE for broadcasts from NAME
The issue seems to be that, for whatever reason, the /nix/store won’t mount automagically and bwrap conceals it by default. I don’t know how to fix this myself, I tried looking for a configurable NixOS option but found no answer.