I run a Late 2012 Mac Mini running 10.13 High Sierra with a Dell QHD monitor via DisplayPort. I've seen two different cases where as far as I can tell the Mac is running (HDD light flashing, etc) but the monitor shows as idle:
- When it goes to sleep from lack of use, the monitor sometimes won't wake up (like once every few weeks, I leave it always-on). I already run
caffeinate -sin a console window and I think the Mac itself is running, but sometimes the monitor thinks otherwise and I have to reboot.
- Today, I turned the Mac on for the first time since turning it off over Christmas - first cold start for a while. The monitor didn't come on, just sat in standby mode (not no-signal). I pulled the DP cable and plugged it back in and now it came to life, which doesn't work in case 1 above.
These things make for a really annoying start to the day and I can never find what might be wrong. I already chased some ideas - I thought the Mac was going into deep sleep and not waking, but now I am fairly convinced the Mac is running but not waking the monitor and I don't know what to try next.