Kan det vara samma som i Raspbian, att man kör disable_overscan=1 i /boot/config.txt?
# Uncomment this if your display has a black border of unused pixels visible
# and your display can output without overscan.
# disable_overscan=1
Ta i så fall bort # framför disable_overscan, d.v.s. :
# Uncomment this if your display has a black border of unused pixels visible
# and your display can output without overscan.
disable_overscan=1