I attach the two necessary files. I don't know how to test any further and I cannot progress the bug report any more without salsa access. The idea behind the portal file is that lwm would try to use any portal available.
I am very certain no new dependencies should be added. I would expect users of lwm are unlikely to be interested in portals and unlikely to have the the xdg-dekstop-portal process running. That said I do think the package should do its best to support users who do want to use flatpak so long as no extra cost to using the package is added.
Attachment:
lwm.desktop
Description: application/desktop
[preferred] default=*