no i have only kde environment.
I will install another environment to check the problem.
but since i prefer kde, this won't resolve my problem.
there is no "unusual" program installed.
I succeeded to frame the bug:
the problem is that it works perfectly since i switch into fully screen
mode by clicking on the dedicated icon in the player.
but once i double click on the video the video goes to the background
and even if i try to use the icon another time the bug remains.

here is a link to check if it is a general bug or a misconfiguration of
my options:
mangas-tv.com/koukaku-no-regios-episode-017.html