Since a few days I can see a light horizontal line exactly at the center of the screen. It is only visible on the XPS screen itself, not on an external monitor, so I guess it's not a graphic card or driver problem, but a problem with the display itself.
Possibly it's only a problem with the connector cable - does it need to be cleaned and reseated?
Any help would be appreciated!
Run the built-in self test (hold D through powerup). If the fault does not show on the screen, it could very well be a faulty cable connection.
If it does show, it's the screen you need to replace.
Done! The horizontal line shows also with the tests!
But I do not agree to your conclusion - shouldn't it be the other way around??!
This is a crack in the edge of the screen from being dropped.
There is no fix only replacement of the entire screen.
The lines are from the side that its cracked.
So verticle lines are at the top or bottom crack and horizontal are left or right side crack.
The Notebook was never dropped and the line is almost invisible, very unlike your screenshot.
Also it is EXACTLY at the center - which in my opinion is too unlikely for a coincidence.
What makes me think: on my Samsung TV their are 4 display areas with separate cables/connectors - each for the signal of a (vertical) area, side by side.
On iFixit I can see that the display uses two connectors - could they be for the upper and the lower area. if so, could the problem not be with the connectors, as this is where these areas "touch"??!
iFixit Display Pic: https://d3nevzfk7ii3be.cloudfront.net/igi/hussf5akAOBKFx1U.huge
No - those connectors are simply there to allow the screen to be used with different systems. From the description, it sounds like you may have some internal wiring loose on the periphery of the screen. It isn't necessarily due to physical damage, but the only fix is as mentioned by speedstep -- the screen needs to be replaced.