If the line doesn't move, it's a screen issue.
Of course, it is!
Everybody knows LG Monitor of the 2012-2013 27'' iMacs suffers from several issues.... important info (27'' iMac screen issues) you can find in some other threads.
PS: I think, APPLE should replace LG Monitor contracts and decide to place orders for SAMSUNG Monitors (more reliable!)...the "enemy"!