How to keep the MacBook build in screen's brightness to zero

[laptopleon]

I mostly use an external monitor with my MacBook. I want the MacBook build-in monitor to stay completely dark, so I set the brightness to zero. Despite the system preferences / monitor / auto adjust brightness - option being set to 'off', it keeps being adjusted to a low, but not zero value, after waking from sleep mode.

I installed BetterDisplay and tried some settings myself. Also found an earlier, similar question in these discussions forum (Can the macbook screen remain dimmed after coming out of standby? #3085) but with the suggested settings it doesn't stay dimmed to 0%.

I then tried switching on 'Allow restore to full dimming' – which seemed more logical to me, but it still woke up with brightness set to 1/16th.

Then I tried also turning on Restore excessive dimming for Apple displays. The MacBook screen still wakes up slightly brighter, but then seems to get set back to zero! Is this how it works?

[waydabber]

Yes, you need to have both on for the built-in display. But why don't you simply turn the built-in display off?

[laptopleon]

Sigh. Not much later it was 'on' again after waking.

I would love to turn it off but HOW?! I only know how to set it to zero brightness, but like I tried to explain, it doesn't stay at zero. It keeps going to 1/16th or 2/16th.

[waydabber]

You should have a toggle (switch) in the display's header in the app menu.

The app should restore zero dimming when combined dimming is enabled and these two settings mentioned above are turned on. Although macOS indeed fights this for precautionary reasons.

[laptopleon]

I just tried the same MBP with a different external monitor. Had to do some first-time admustments of course. Set the external monitor to main monitor, set the build-in monitor to mirror it, checked that the setting are as above. Then let is sleep for a minute, to be sure everything was in sleep mode, both MacBook and monitor. Woke them up again. Build-in display went on on no less than 24% and stayed there.

Just to be clear: I'm more than happy to pay € 20 for a BetterDisplay licence, if it works for my scenario. I'm also willing to help testing if that's helpful.