Boot screen has wrong display resolution
I had the same issue, but resetting the PRAM solved it. 

Procedure from here:

Resetting NVRAM / PRAM


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.