sudo update-desktop-database
update-desktop-database ~/.local/share/applications/

That should pick up mimetype changes. (Though it should've been already done automatically.)

If that has no effect, you can try to manually set the handler:

xdg-mime default '/home/JnvSor/.local/share/applications/steam.desktop' x-scheme-handler/steam

I don't have answers to your questions beyond this.

I'm on xfce4 trying to open a steam:// link.

~/.local/share/applications/steam.desktop

is a symlink to

~/.steam/deb-installer/steam.desktop

and contains the all important line:

MimeType=x-scheme-handler/steam;x-scheme-handler/steamlink;

Still, when opening a steam link from a browser I get the xdg-desktop-portal-gtk "No Apps available" popup, with a broken button trying to start gnome-software for some reason.

Trying to find the cause I have this strange result:

$ gio mime x-scheme-handler/steam
No default applications for “x-scheme-handler/steam”
$ xdg-mime query default x-scheme-handler/steam
steam.desktop

On top of all this a check with pstree shows xdg-desktop-portal being a direct child process of init, but I can't find it anywhere in /etc/init.d

So in summary I have two questions:

• How do I get xdg-open working again, or some replacement?

• How does xdg-desktop-portal start up anyway?