I put it to sleep sometimes via the power button, and my 'moment' holding down the button is probably a bit over a second. For me I'm talking a Mac Mini mid 2014 running High Sierra. Then I chose 'Unlock Screen.', and the stuck machine immediately stopped the screensaver and responded normally. In the 'Interact' menu, I chose 'Lock Screen.'. After getting locked out again, I used Apple Remote Desktop to remote to the stuck machine.

I stumbled upon a way to get access to the computer again. I previously had to do a hard restart by holding down the power button until the computer shut down, and of course, this corrupted a bunch of stuff. The screensaver runs normally, and I can move the mouse around, but otherwise the system is completely unresponsive. The computer goes back into screensaver mode instead of dismissing the screensaver. When attempting to dismiss the screensaver, I move the mouse/hit a key on the keyboard to invoke the login window. I have my screensaver password protected.

I am asking and answering my own question since I was not allowed to reply to this old topic.

Molten metal screen saver mac os x