Comment # 2 on bug 1154753 from Elmar Stellnberger

  In deed this problem is related to a missing nouveau.hdmimhz=225. Without
that kernel parameter Debian boots and shows the background screen but no icons
and no mouse pointer appear. You can reboot with ctl+alt+del. If you boot
openSUSE Leap 15.1 with this parameter then there are no problems with the
display. You can also configure the external screen with 1920x0180 (4K needs
special modelines). The problem seems to be that no safe hdmimhz setting is
autodetected. As far as I have talked with kernel developers there is no safe
way to detect the hdmimhz with nouveau cards so assuming a sufficiently
conservative setting may help.