On Solus Budgie, when I start Spotify installed over Flatpak, it doesn't show on Bottom panel. The same thing is also happening with Flatpak version of Tuxguitar.
Here is the start log from Spotify:
marek@marek-pc ~ $ flatpak run com.spotify.Client
/app/extra/bin/spotify: /app/lib/libcurl-gnutls.so.4: no version information available (required by /app/extra/bin/spotify)
marek@marek-pc ~ $ [spotifywm] attached to spotify
/app/extra/share/spotify/spotify: /app/lib/libcurl-gnutls.so.4: no version information available (required by /app/extra/share/spotify/spotify)
/app/extra/share/spotify/spotify: /app/lib/libcurl-gnutls.so.4: no version information available (required by /app/extra/share/spotify/spotify)
[spotifywm] attached to spotify
[spotifywm] attached to spotify
Traceback (most recent call last):
File "/app/bin/set-dark-theme-variant.py", line 11, in <module>
disp = Xlib.display.Display()
File "/app/lib/python3.9/site-packages/Xlib/display.py", line 89, in __init__
self.display = _BaseDisplay(display)
File "/app/lib/python3.9/site-packages/Xlib/display.py", line 71, in __init__
protocol_display.Display.__init__(self, *args, **keys)
File "/app/lib/python3.9/site-packages/Xlib/protocol/display.py", line 166, in __init__
raise error.DisplayConnectionError(self.display_name, r.reason)
Xlib.error.DisplayConnectionError: Can't connect to display ":99.0": b'Invalid MIT-MAGIC-COOKIE-1 key'
Gtk-Message: 17:40:32.006: Failed to load module "canberra-gtk-module"
Gtk-Message: 17:40:32.006: Failed to load module "canberra-gtk-module"
/proc/self/exe: /app/lib/libcurl-gnutls.so.4: no version information available (required by /proc/self/exe)
[spotifywm] attached to spotify
[spotifywm] spotify window found
marek@marek-pc ~ $