I installed FreeFileSync (v12.1) from Flathub on openSUSE KDE, and the software's 'popup' windows usually aren't shown. So that when e.g. a sync process is in progress, and an FFS notification fires, the notification's popup window isn't displayed: everything else works as expected, but the 'popup' panel can't be seen. You can cancel the 'invisible' panel by pressing 'Esc', or accept its default button by pressing 'Space' but without seeing the window's content.
The issue can also be tested with the Help -> About panel, which also sometimes isn't shown. (However it works more often than not.)
When launching the Flatpak app from the terminal, I see the following error message, but I'm not sure it's related to the above issue:
Gtk-Message: 09:43:57.252: Failed to load module "canberra-gtk-module"
Do you have any fixes I should try, or is it a bug on the package level? It'd be really nice to sort this out, as this issue makes the Flatpak practically unusable (even though the core features work, you can't see what's going on when a notification should appear).
Thanks in advance,
Márton Lente