MPRIS volume change isn't reflected on volume slider

[rdbende]

Bug/Feature description

When changing application volume through MPRIS, the current volume isn't updated on the volume slider, and opening and moving it manually makes the volume jump back to where the slider thinks it is.

System Information

• Operating System: Fedora 39
• Installation source: Flatpak build
• Version of cozy: master branch

[rdbende]

Turns out, it is handled by the GStreamer player bin itself, not the MPRIS interface.

However, due to the many abstraction layers, it's quite hard to fix it, because we either have to use an ugly workaround with private properties (self._player._gst_player._player.connect("notify::volume", self._on_volume)), or have to propagate the notify::volume signal through gst_player.GstPlayer -> player.Player -> playback_control_view_model.PlaybackControlViewModel -> media_controller.MediaController. My feeling is that there are too much abstraction layers in this stack, and should be refactored.