I learned that to get drivers correctly installed with latest Manjaro, I must first install video-modesetting and then install video-hybrid-intel-nvidia-450xx-prime (or video-nvidia-450xx).
This must be done either with GUI using System Settings or by command line using mhwd.
But now two things occur: first, I must login two times to enter desktop and also a notification appears saying “Desktop effects have been suspended by another application.”.
The “double login” occurs when the external monitor is connected to the HDMI port. When the laptop is turned on already connected to the external monitor, I must enter the password two times to access the desktop. If I connect the external monitor later, the login screen appears again and I must enter the password to continue using the laptop.