diagnosing vertical lines on screen

I think that is a bad GPU which IIRC is a known issue with the 15" Retina 2012 which would require a Logic Board replacement. The way to confirm the diagnosis would be to connect an external display. If the external display shows similar issues, then the GPU/Logic Board is bad, otherwise if the external display is Ok, then you have a defective Display Assembly.

Try using Mirrored Mode just to be safe and sure. While it may be awkward using either display this way it is the best way to perform the test. There is always a slight chance the "few lines" you saw on the side in this test could have been caused by a software issue. It is hard to say without actually seeing it. I suspect the laptop's built-in display is bad though.

Really hard to say. Were you mirroring the display? If so, then the built-in Display Assembly is most likely defective.

It would help to know the exact model of the computer and to actually see a picture of the lines. You can get the exact model number of the computer by clicking on the Apple menu and selecting "About this Mac" or by entering your serial number here:

https://checkcoverage.apple.com/

You are welcome. I'm glad we could narrow down the issue.

Unfortunately since it is a Retina model you will most likely need to replace the whole Display Assembly since it is nearly impossible to replace to disassemble the Display itself to replace an inner component such as the LCD Panel which I believe is the component that is bad. While this laptop is considered "Vintage" you may still be able to get an official Apple repair (at least from an AASP) if the part is still in stock.