I looked up the various teardown videos for this model of laptop and decided to order a replacement screen, that being easier to do than replacing the cable. The replacement screen arrived today.
Sadly, this did not fix the problem.
I am now ordering a replacement cable. The laptop is disassembled in pieces on the top of my desk. Opening up a modern laptop is a pain in the butt. I trust that I will be able to get it back together.
I did plug an HDMI cable into the back of the laptop and verify that it still booted correctly and that the video system was ok by connecting it to the TV, because the TVs are pretty much the only HDMI-capable monitors we have around the house. Since the video system is fine, it pretty much has to be either the screen or the cable, right?