What I saw in oreo-x86 is that it is normal to get portrait mode,
even if it is sacrificing half of the screen surface to black
and in some cases it takes several frames to finalize this screen adaption to portrait.
What i noticed is that you can switch between Auto and Portrait mode,
but it would make sense to have Landscape (meaning landscape forced)
for the cases where you may rotate the Monitor on the stand,
for example in case you want the Arcade Videogame experience full screen.
If you perceive exactly this problem, please highlight the issue in the forum to see if Chih-Wei agrees to evaluate the Requirement
Mauro