I also have the same issue, and yes applying pressure to the underside case corrects this issue. This failure appears randomly. I have iStat and on the fan specs I have switched from Auto to Medium. I believe it has to do with poor cooling management. Somebody should open extra vents at the bottom of the case. Update (12/01/2018): I also experience the same problem. I have addressed a solution with iStat, modifying the fans from AUTO to MEDIUM, this has helped me reduce the pink horizontal lines appearing. I do find the venting of the MAC most deficient. Just the airing inlets in the back of the MAC in my opinion are not enough!
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