i just went and checked the box to turn it on and tried it. It took me a minute to find the right way to do it, but it works. i'm on nightly#5. You have to put your finger at the very top of the screen and slide it left or right slowly to adjust.
After activating and attempting i've found that putting your finger in the center of the status bar and sliding left or right is the best method but what works best for me is to slide right then left as I can get to the lowest brightness setting this way.
You need to make sure that in your brightness settings under notification power widget buttons, the choice does NOT include automated. Then you can go back and it will let you check off the slider control of brightness. If automated is set slider is not allowed.
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