URI handler for Flatpak is not working

[xoriole]

The URI handler for magnet links is not working for the Flatpak installation of Tribler. Tribler does open but the start download dialog is not shown.

[qstokkink]

It would be nice to create a bleeding-edge flatpak build in a GitHub Action to check issues like this. I think we can make a test setup that uses "Git sources", see https://docs.flatpak.org/en/latest/module-sources.html#git-sources

Of course, that would be different from the "official" Flatpak (https://github.com/flathub/org.tribler.Tribler), which should use github.com/Tribler/tribler/releases URLs.

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Update: I tried getting the basic Flatpak to work but I ran into an issue. Once that is resolved, I can resume work on this issue.

[qstokkink]

I got a Flatpak build running and I can confirm that this doesn't work. However, I also checked with the .deb file and this also doesn't work. Apparently our entire Debian (Ubuntu) build process is broken.

[qstokkink]

Wait, hold on. It does work with the latest 8.0.3 pre-release from tribler.org. Something may have been recently broken. After a terminal restart this did work again. Don't ask me why.

[qstokkink]

Alright, I got it working 🎉 However, I did not require any fixes to Tribler itself. I only needed changes in the org.tribler.Tribler.yaml.

I'll park my fix in https://github.com/qstokkink/tribler/tree/add_flatpak_testbuild if anyone wants to see how I got it working locally (note that this is now tied to my local filesystem and not the official Tribler release URL - this still has to be changed).

CC: @overflw when we hit the stable version of the 8 series the branch above has the basis for the required .yaml file, including the fix for this issue.

[qstokkink]

Actually, the fix for this particular issue can be backported. I made a PR on the flathub repo.