per display scaling (HiDPI) not working for some reason

Question

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%:

Answer 1

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 like options 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%.