Overscan checkbox disappeared after I selected it
I have two 40" Philips 4k monitors, connected to my Mac Pro 5,1 mid 2012, using an NVIDIA Gefore GTX960 GPU to power them on macOS High Sierra. This setup has worked ever since El Capitan, when I first installed the card. Today, I noticed the overscan box was checked by default in System Preferences > Displays. Out of curiosity, I clicked on it, to turn it off to see what effect it would have. The result was that the screen turned black momentarily, and then reappeared with a black 'underscan' border around the edge. So I clicked the Overscan checkbox in the left hand monitor, and the same thing happened. The Overscan checkbox disappeared completely from both instances of the Display Preferences, so I could not re-enable it on either monitor.
So, I restarted the machine, uninstalled the webdriver, rebooted the mac, reinstalled the webdriver, rebooted the Mac again, but the black box is still there and there is still no Overscan checkbox. Now the refresh rate is locked at 60Hz, I cannot get the maximum screen resolution any more, and the black border remains.
The Apple Forum article here shows how to do this with an overscan slider, but this slider is nowhere to be seen on my setup. I also tried clicking Option and other modifier keys, but nothing materialised.
I then tried changing the orientation to 90 degrees, but this caused both monitors to turn irretrievably black, even after a restart, so I rebooted in Safe Mode, and uninstalled the webdriver again.
Any suggestions as to how to get back the Overscan checkbox or to enable the slider, and how to undo the effect of a 90 degree rotation if the monitors turn black?
Mac Pro, macOS 10.13