Scaling works fine when using the "Single Display" mode.
But when I try to switch over to "Join Displays" I don't get the "Apply" button when setting 2nd monitor to 4k and 200% scaling and other monitor to 1080p and no scaling.
There is a apply button when I select 100% scaling, which is weird.
GPU: Nvidia GTX 1080
Drivers: NVIDIA binary driver – version 384.90 from nvidia-384 (Proprietary, tested)
Screenshots
Single Display mode with 200% scaling working fine:
With 1080 and 4k on at the same time, unable to select 200%:
Best Answer
It seems different scaling on different monitors only works on Wayland display server. But nvidia driver default settings prevents Wayland to run, so old Xorg display manager is used.
Solutions:
1) You can use opensource nouveau driver instead of nvidia driver. It works slower but scaling works fine.
2) You can change nvidia drivers setting: Add
modeset=1
option to/etc/modprobe.d/nvidia-graphics-drivers.conf
file so last line should look likeoptions nvidia_384_drm modeset=1
Next run
$ sudo update-initramfs -u
and restart.(source: No way to choose wayland in GDM using UbuntuGNOME 17.04)
Also you can enable fractional scaling (like 150%): http://www.omgubuntu.co.uk/2017/09/enable-fractional-scaling-gnome-linux
Solutions 2 works for me on NVidia 960. my 4k monitor scaling=150% while 1080p monitor scaling=100%.