Comment 1 for bug 2036098

> - Add a /usr/share/xdg-desktop-portals/lxqt-portals.conf copied from xdg-desktop-portal-kde with this contents:
> [preferred]
> default=kde

If the KDE portal is the one you want, then this looks one character off from being correct: it should be /usr/share/xdg-desktop-portal/lxqt-portals.conf (no "s" on xdg-desktop-portal).

Some things to check:

Run xdg-desktop-portal as "/usr/libexec/xdg-desktop-portal --verbose --replace" and it will show you more of what it's thinking. If it works when run from a shell but not when run from D-Bus activation, try editing its D-Bus and systemd service files to add --verbose, then read the systemd Journal.

Check that XDG_CURRENT_DESKTOP, DISPLAY, and XAUTHORITY if used are set, and are in the D-Bus and systemd activation environments. GNOME (and I think maybe also Plasma) explicitly uploads XDG_CURRENT_DESKTOP into the activation environments, but LXQt probably relies on the display manager to pick it up from DesktopNames. You might need https://salsa.debian.org/utopia-team/dbus/-/commit/711681dbde53e475f2ce8e01b10db0742a233fbf or dbus-x11, to make Xsession.d upload XDG_CURRENT_DESKTOP.

To check: systemctl --user show-environment, and perl -pe 's/\0/\n/g' /proc/$(pgrep -f xdg-desktop-portal | head -n1)/environ

> Lubuntu ships xdg-desktop-portal-lxqt but this is not packaged in Debian

There's an ITP, https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1051790. If you want to use the KDE and LXQt portals (in some order), use default=kde;lxqt; or the other way round, or look at gnome-portals.conf for a less trivial example.

Please upstream any portal configuration that is added to Lubuntu. Debian bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1050804, not yet opened upstream (I couldn't work out which component in LXQt should be responsible for infrastructure like this).

https://gitlab.xfce.org/xfce/xfce4-session/-/issues/181 was the equivalent in XFCE and might be a good starting point for an upstream LXQt request.

[edited: perl -pe 's/\0/\n/' should have been perl -pe 's/\0/\n/g']