I can confirm that installing nvidia-390 through the Ubuntu driver dialog, and then using nvidia-settings or prime-select to set intel keeps the nvidia card on for me on my GeForce 840M laptop.
I can also confirm Installing bbswitch-dkms and turning it off through bbswitch works.
I can confirm that installing nvidia-390 through the Ubuntu driver dialog, and then using nvidia-settings or prime-select to set intel keeps the nvidia card on for me on my GeForce 840M laptop.
I can also confirm Installing bbswitch-dkms and turning it off through bbswitch works.