Comment 5 for bug 2036098

> DesktopNames=LXQt

Or maybe even "DesktopNames=Lubuntu;LXQt;" which would allow having Lubuntu-specific configuration that doesn't affect upstream LXQt if you want that, similar to how Ubuntu's customized GNOME session works.

> /usr/share/xdg-desktop-portals/lxqt-portal.conf with this content

That should be "/usr/share/xdg-desktop-portal/lxqt-portals.conf". (Look at the filename used in gnome-session, and follow the same pattern.)

> default=lxqt

It looks as though x-d-p-lxqt only implements org.freedesktop.impl.portal.FileChooser. If you want to use a different backend for the other interfaces (perhaps x-d-p-kde or x-d-p-gtk) then you need to configure it in this file: unlike 1.16.x, x-d-p 1.17.x does not automatically fall back to whichever portal backend happens to be first in alphabetical order.

For instance you might use default=lxqt;kde; or default=lxqt;gtk; or something like that, or see portals.conf(5) or the gnome-session-common package for examples of more elaborate configuration.

There's a list of portal implementation interfaces here: https://docs.flatpak.org/en/latest/portal-api-reference.html (the ones containing ".impl."). The documentation is hosted on flatpak.org but this particular part of it should be equally true for Snap.

> maybe xdg-desktop-portal-lxqt would work, like how xdg-desktop-portal-kde does it

I think it makes more sense to ship lxqt-portals.conf in the same core LXQt package that ships lxqt-mimeapps.list, or would ship lxqt-mimeapps.list if it existed. Having xdg-desktop-portal-kde provide this file seems odd to me: that's blurring the distinction between the information that is controlled by the portal backend, and the information that is controlled by the desktop environment.

Equivalents in other desktops: GNOME puts gnome-portals.conf in gnome-session-common; XFCE is going to have xfce-portals.conf in xfce4-session; and MATE is going to have mate-portals.conf in mate-desktop.