Would you accept a simple Qt patch forcing apps to use flatpak platform plugin when we detect we are running in sandbox (same way I used in KNotifications)? I know I should try to upstream this as soon as possible, but with all the stuff around I'm trying contribute to it will take a while. I think this would be the easiest solution for now, better than modifying desktop files, because you can also run apps using "flatpak run app_name" or run an app through discover where it calls the binary directly and portals wouldn't be used at all.

We need to figure it out with Wayland too.
Note that it's not about desktop files, it's about including it in the finish-args.