I would like my MacBook (on El Capitan) to dim the screen before going to sleep, so that I can hit a key to keep it unlocked if desired. Currently, there is no way to keep it from locking if it's started to turn off the screen.
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…so that I can hit a key to keep it unlocked if desired. Currently, there is no way to keep it from locking if it's started to turn off the screen.
It sounds like you don't need the old dimming action so much as you need a delay on the password lock following sleep or screensaver.
Open "System Preferences → Security & Privacy → General", and change the "Require password … after sleep or screen saver begins" dropdown from "immediately" to "5 seconds" or "1 minute" or longer even.
There's a free app called Caffeine which prevents it from going to sleep at all. You can activate it indefinitely, or for a particular period of time. I use it all the time.
I called Apple about this and they confirmed that it's a 'feature' of High Sierra and told me there is no way to get it back. I do remember the option to slightly dim the display was there with previous versions but it's gone now and it will just go dark without warning. I think my iMac at work doesn't do this even though they are on the same version so it could be tied to some other setting. I will look into it Monday and come back.
EDIT: I tried this on my work iMac and it does indeed still dim the display before going to sleep. I cannot find an option for it anywhere, so maybe Apple made it so that notebooks can't do that while desktops can? Both computers are running 10.13.4