Big Sur Mac Pro 2013 Display Issues: No 'Detect Displays' Button in Preferences
Hold down the Option key.
Right Click on the icon in the Dock for Preferences and 'Force Quit' it.
(Yes, even though it isn't locked up.)
Hold down Option while starting System Preferences. (May not be nesc. but I did.)
In place of 'Gather Windows' when you press Option, it should now say 'Detect Displays'.
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Next Issue: All my Displayports work and my HDMI port works, but no third Monitor. It's not the cable, the cable works with a different monitor.
I'll let you know on that one.
EDIT: It's 'normal'. But sucks. -=> https://rcbullock.blogspot.com/2021/12/mac-pro-2013-only-two-displays-working.html