Flatpak apps cannot prompt for the file manager on Fedora GNOME templates

Qubes OS release

4.2

Brief summary

I am noticing a very strange issue where the Flatpak build of Mullvad Browser and Celluloid cannot seem to open any file manager (be it Nautilus or Thunar) to prompt for files/directories on Fedora GNOME templates.

This behavior is not reproducible with:

An actual Fedora system
Upstream’s Mullvad Browser on Fedora templates
The Debian GNOME Template

This behavior is reproducible with both the Fedora 39 and 40 GNOME templates.

It is very unlikely that this is caused by the Flatpak apps. Rather, I suspect that some important dependencies are missing from Qubes’s Fedora templates, or that there is a misconfiguration somewhere.

Steps to reproduce

Install Flatpak Mullvad Browser in an AppVM based on a Fedora GNOME template.
Try to download a file.

Expected behavior

A prompt with nautilus appears for the download location.

Actual behavior

Nothing happens.

QubesOS / qubes-issues