@berne YOur question is difficult to understand. A bit more explanation would have helped.
If the whole raspbian desktop is not fullscreen, you might take a look at this:
https://forum.magicmirror.builders/topic/601/not-realy-fullscreen/3
A New Chapter for MagicMirror: The Community Takes the Lead
Read the statement by Michael Teeuw here.
Read the statement by Michael Teeuw here.
B