Display shows unknown "white line" via MBP retina

Question

as title, when i turn off the "automatic GPU switching" for using the NVIDIA graphic power, and i try to switch the resolation from "best scale for retina" to "1920*1200" and quickly switch back and repeat for a few times, and the screen will shown a "white line" under the finder bar, then i turn back on the "automatic GPU switching" this "white line" will be gone. i belive its the GPU NVIDIA driver's error, please check forward for this problem, and also i will get support from the Apple store to find out the problem.

Answer 1

This worked for me:
Step 1: Delete system windowserver preferences
In the Finder, from the menu bar, select "Go -> Go to Folder..."
Enter: /Library/Preferences/
Delete the file called com.apple.windowserver.plist
Step 2: Delete user windowserver preferences
In the Finder, from the menu bar, select "Go -> Go to Folder..."
Enter: ~/Library/Preferences/ByHost/
Delete all files that start with com.apple.windowserver   (On my rMBP, there was one file called com.apple.windowserver.12AFB147-95D6-5C70-8F62-C1E4F2BE81BE)
Step 3: Reset the PRAM    (These instructions are fromhttp://support.apple.com/kb/HT1379)
Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.