I have been using a 24" monitor with my macbook pro early 2011 connected via a mini display port to DVI adaptor. I recently received an HP Z30i 30" monitor. When I connect to it with an mini display port to display port adaptor, although it displays at the monitors native 2560×1600 resolution, the mac is scaling the desktop display down so as to leave a 1cm or so black border around the desktop. The mac reports using a resolution of 2560×1600 and the display reports this also. This makes the display blurry as single pixels become sub pixels. The display looks like the same effect as having "under scan" turned on when using an HDMI cable at 1080p.
I thought that maybe there was a hardware or display problem, but when I boot off a previous system partition from a different hard-drive using the same OS (10.8.5) release, the monitor set up works perfectly. This proves the OS release, display and the hardware, so I'm thinking that there must be a preferences problem with my current setup. Perhaps the under scan setting is somehow being applied to this display when it's not using HDMI?
I've tried deleting /Library/Preferences/com.apple.windowserver.plist, ~/Library/Preferences/ByHost/com.apple.windowserver.HostNumber.plist and rebooting but no results.
The scaling is applied before the login screen and logging in as a different user has the same problem.
Any ideas? It's driving me nuts, I've got this nice 30" screen and I can't use it!
Best Answer
I ended up "fixing" it by re-installing OS X on the system disk. Brute force due to lack of success with other methods, but all good now.
In some cases a faulty cable can do this as well:
External monitor randomly lowers resolution adds black borders