I just bought the Acer B276HUL, too, and have the identical problem that you described on 8/28. I use a MBA 13" mid-2013. I am running 10.8.5. I have a DP-to-miniDP cable and it won't allow setting on the highest resolution.
Q: Did you find a way to eliminate the 1/5 screen display problem at the highest resolution setting with your MBA?
Problem seems to be solved by the following:
1) Used third-party miniDisplayPort-to-DisplayPort cable; plugged DisplayPort end into monitor
2) Restarted MBA in "Safe" mode (hold Shift key upon MBA start up)
3) Go to Displays in System Preferences
4) Higher resolution 2560x1440 can now be enabled with normal, non-flickering or altered viewing
5) Restart MBA normally
6) Display retains 2560x1440 display settings
7) Note: Also works connected to Belkin Thunderbolt Hub as well as directly connected to MBA Thunderbolt port
i also have this monitor and am running at the full resolution (2560x1440) through a mini displayport to displayport cable from my macbook air 2013, but i have a huge black border around my display -- the screen doesn't actually use the full screen completely and there is no option to do an underscan or overscan, and the monitor doesn't allow me to stretch it full (have the setting on widescreen mode: full).
anyone else running into this?