Supplemental to my first report:
A perfect full screen happened only 3-4 times.
I notice the following when the black line appears: I start the
movie, click on the full screen button, the screen becomes black,
then the mouse shows up at the upper left corner of the screen
and the black line appears where the mouse is pointing.
When the full screen mode was perfect, I didn't notice the mouse
pointer. Everythning was black before displaying the image.
I cannot reproduce a perfect full screen mode (i.e. launching
MPlayer after computer start-up, quitting all applications, etc.). So
I guess, it is a mouse or menu bar thing.