Pop!_Shop does not handle .flatpakrepo files

[strugee]

Distribution (run cat /etc/os-release):

Pop!_OS 20.04 LTS

Related Application and/or Package Version (run apt policy $PACKAGE NAME):

$ apt policy pop-shop
pop-shop:
  Installed: 3.3.0pop0~1589405854~20.04~72b0ae9
  Candidate: 3.3.0pop0~1589405854~20.04~72b0ae9
  Version table:
 *** 3.3.0pop0~1589405854~20.04~72b0ae9 1001
       1001 http://ppa.launchpad.net/system76/pop/ubuntu focal/main amd64 Packages
        100 /var/lib/dpkg/status

Issue/Bug Description:

Pop!_Shop cannot handle .flatpakrepo files, which are used to add 3rd-party Flatpak repositories. It isn't registered as a handler for this filetype and when selecting Pop!_Shop to open a .flatpakrepo file manually, nothing appears to happen.

Steps to reproduce (if you know):

1. Go to https://nuvola.tiliado.eu/nuvola/other/
2. Click on the blue "Click to install Nuvola Apps repository with GNOME Software" button in the "Add Flathub and Nuvola Apps Repositories" section

Expected behavior:

Firefox prompts me to open the downloaded file with Pop!_Shop.

Other Notes:

I tried to install GNOME Software to handle this file instead, but that seems to not be possible without breaking the desktop:

$ sudo apt install --no-install-recommends gnome-software
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  app-install-data
Use 'sudo apt autoremove' to remove it.
The following additional packages will be installed:
  gir1.2-goa-1.0 gir1.2-snapd-1 gnome-software-common python3-dateutil software-properties-gtk
Suggested packages:
  gnome-software-plugin-flatpak
Recommended packages:
  gnome-software-plugin-snap
The following packages will be REMOVED:
  pop-desktop sessioninstaller
The following NEW packages will be installed:
  gir1.2-goa-1.0 gir1.2-snapd-1 gnome-software gnome-software-common python3-dateutil software-properties-gtk
WARNING: The following essential packages will be removed.
This should NOT be done unless you know exactly what you are doing!
  pop-desktop sessioninstaller (due to pop-desktop)
0 upgraded, 6 newly installed, 2 to remove and 0 not upgraded.
Need to get 6,605 kB of archives.
After this operation, 13.7 MB of additional disk space will be used.
You are about to do something potentially harmful.
To continue type in the phrase 'Yes, do as I say!'
 ?] 

So the obvious workaround for this bug is (accidentally?) blocked by Pop's packaging. Thanks for maintaining Pop!_OS, by the way! It's a great distro and I appreciate your efforts <3

[cassidyjames]

GNOME Software and Pop!_Shop can end up competing for apt updates, which is why they conflict in packaging.

On elementary OS, we have the simple and lightweight Sideload app that works as a companion to AppCenter. It handles installing .flatpakrefs, and we have an open issue and draft PR to handle .flatpakrepos as well. Since Pop!_Shop is a fork of AppCenter and Sideload is basically a companion to AppCenter, it would probably make sense for Pop!_OS to ship Sideload as well.

[strugee]

GNOME Software and Pop!_Shop can end up competing for apt updates

Hm, interesting. You mean if the user clicks "Update" in both of them? I thought that concurrency should be handled by the PackageKit implementation? (I have very little experience in this area; I'm sure you're right - I'm just curious!)