This was specifically for this sentence:
On my sway (wayland) setup if I do the following:
swaymsg 'output eDP-1 dpms off'; sleep 10; swaymsg 'output eDP-1 dpms on'
Then I can wildly use input devices during those 10 seconds and the screen stays black.
From a kernel perspective the semantics of brightness = 0 are not well-defined.
Different drivers implement different behaviors.
There is ongoing work to get a new backlight API, with a well-defined 0 value.
But that definition is “just enough to be readable”, for “off” use dpms.
FWIW I forced the kernel to override the limits received from ACPI and set the PWM to a hard 0.
But that still didn’t turn off the screen completely.
So it may be in the GPU firmware.