For a program that plays an audio file it might be expected that it plays
in the background.  For a video player the common expectation would be that
when the video can't be seen the audio would stop too.  If you run livi, play
a video, and then go back to the home screen or make another program full
screen it will keep playing sound.  This happens on Plasma Mobile on the
PinePhonePro and on KDE on a laptop.

I think that the best functionality is to make this a configuration option
and default to not play in background for video.