Thus or independently a trouble happened.
No output on display, connected by a HDMI cable, was shown. Aka blank screen. It was in not only LightDM but also Xfce desktop after login. It always happened around the display manager, and, however, often around the desktop.
What was not critical seemed critical in that my time killed increased gradually... 🤕
# pacman -Sy nvidia
Happily ever after 🦔