I had to add 1920x1200 (16:10) and took several tries to get it to work.
Preferences->Screen Mode->Monitors tab.
Detect settings automatically. Checked.
All supported modes. Chosen.
Reboot.
Come back. Uncheck Detect. Add the size. Save. Reboot. Check/select it. Test it. Save. Reboot. Come back. Re-check Detect. Save. Reboot.
It was very tedious. Then it just suddenly decided to work.
