The Flatpak version of Stremio doesn't support using an external player.
The option is just missing. I enabled the d-bus session and d-bus system permissions of Stremio to try to fix it, but that didn't change a thing. The logs don't say anything about mpv either. The Flatpak version seems to have its own version of mpv running internally, but it doesn't allow opening a seperate mpv window.
Hi I got the same issue but I was able to fix it by doing this: I installed flatpak-spawn, allowed dbus session permission and added the "org.freedesktop.Flatpak" as a Talks session bus with Flatseal
Then I modified the server.js file (at "/var/lib/flatpak/app/com.stremio.Stremio/current/active/files/opt/stremio/server.js") like this:
For any player in the "players" list (find it by searching var players = {):
Replace the "path" field with [ "/usr/bin/flatpak-spawn" ]
Add "--host", "\<the player you want to use>" before the other arguments of the "args" field
Then search for timeCmd + " " + argsCmd and replace it by argsCmd + " " + timeCmd
(by the way, sorry for my bad English)
This software is badly shipped, why must the user go over hops for something the developers can easily do? Literally VLC has so many API possibilities for this to just WORK, no sense in doing this on the user side... Oh well.
Edit: I have found the solution! Simply use yay -S with this repo https://aur.archlinux.org/packages/stremio-linux-shell-git thank me later lol