Bug 494316

SUMMARY
Please add an option so trying to lower the brightness to zero (or one step below zero) turns off the screen. This would be akin to muting audio when setting the volume to the minimum.
The use case is to keep the laptop active while it does some long running job (so no sleep) while avoiding it from emitting light which may be annoying, and allow to control this using the builtin function keys of the laptop. Using the function keys has the best discoverability in my opinion compared to some custom shortcut (I'm not even sure if there's such a shortcut).

STEPS TO REPRODUCE
1. Reduce screen brightness to minimum
2. Reduce screen brightness further

OBSERVED RESULT

Screen still emits light.

EXPECTED RESULT

Screen goes black.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.1.5
KDE Frameworks Version: 6.6.0
Qt Version: 6.7.3

ADDITIONAL INFORMATION

I have a laptop with an internal screen. On Plasma 5.27, I could use the brightness slider or function keys to turn off the backlight entirely when the brightness was set to the minimum (0). When checking the value from `/sys/class/backlight/intel_backlight/brightness`, it went from 0 to 1500. 0 corresponded to a black screen.

Since Plasma 6, the brightness controls (either the graphical slider or function keys) restricts the range set to the hardware between 1 and 1500. It means that it is no longer possible to turn off the screen just by lowering the brightness. There were a few issues and discussions to restore support for truly setting the value to zero (https://bugs.kde.org/show_bug.cgi?id=483490, https://old.reddit.com/r/kde/comments/1dqj2ny/how_to_set_brightness_to_zero_which_completely/). It was implemented for external displays but internal displays are still restricted to 1 as the minimum in https://invent.kde.org/plasma/kwin/-/commit/6ca6308ccd2f3266f20f84af6658fa97547e12ee

The reason to limit setting the internal value to zero is that it actually does not turn off the display but disables the backlight. I was not aware of the difference and assumed that Plasma 5 turned off turned off my screen. In fact, it was just disabling the backlight so the screen was black but there was still some power consumption for the LCD. Given that it's easy to confuse "screen off" and "backlight off" I understand why the change was done in Plasma 6 to restrict the value zero; but in the meantime it means that the feature was lost to quickly get the screen to black.

Discussions indicated that this previous behavior (from Plasma 5) was unintended and that restoring this behavior would actually be a new feature. It also mentioned that the proper way for this would be turn off the screen (instead of just disabling the backlight). Therefore I'm opening this issue to request the return of turning the laptop screen black using the function keys. Alternatively, I would appreciate if I could set the internal brightness to zero even on internal screens. When I run long jobs on my laptop I'm usually plugged in (so less concerned about the battery).