6s - screen distortion after boot only in non-white areas

iPhone 6s - dropped and landed on corner (caseless). No screen damage (cracked glass or anything like that). Screen has heavy horizontal distortion - almost like scan lines. But...only in non-white areas. The boot logo (white screen, black apple) is perfect. After iOS loads, a pure white screen is pristine. Any color to an image causes the "scan lines" to appear. They follow the colored area (if you can scroll it like a webpage). Turning on Night Shift makes it worse. It almost seems like the GPU is damaged and can't do layering? Or something of the sort. I'd hate to drop $160 for a screen and find out that I have a motherboard problem. Any ideas about how to troubleshoot/repair this one? I already took off the screen and made sure that all of the ribbon cables were tight. No change in the display on re-assembly.