GIF of problem
Mid-2012 Macbook Pro Retina Display on OS X High Sierra 10.13.6
I get this artifact on powering the unit up after it's been off overnight. It keeps glitching like this for 1 or 2 hours after powering up and then goes back almost to normal (only an extremely faint flicker ever few seconds, virtually unnoticeable). If I put the unit to sleep and then power it on again after an hour, I don't get the glitching, but after 8 hours I do. If I leave the unit on all night it stays functional.
I notice if I unplug the unit the problem occurs immediately, but if I plug it in again it stops glitching after several minutes. The battery is 8 years old and needs service.
The unit has been sitting on a desk for six months without anyone other than me having access to it, so it seems unlikely that the problem was caused by an impact (or water damage). I've left it open this whole time so there has been no wear on the hinge.
Attempted solutions (from https://macreports.com/mac-how-to-troubleshoot-screen-flickering/):
2) updated OS
3) Reset SMC, PRAM, NVRAM
4) Disabled automatic graphics switching
5) Safe mode
I'm wondering if it's possible to isolate the problem from this information. Maybe the video card, the cable, or the screen? I am currently overseas and nowhere near a repair facility, although I could get parts shipped from the US if it's something I can repair myself. I do not have access to an external monitor. I'd like to confirm the problem component is not otherwise identifiable before sourcing one.
Someone suggested the discrete video-card could be the problem and solvable via this method but that seems to be directed at non-retina models.
Since I need this laptop for my job, my choices are either to order the minimum number of parts that will definitely solve the issue or else get a new machine if it's not user repairable.