Update: Throughout the summer, sometimes the sunroof again failed to close, then maybe a day or two later would close. A GM representative saw my post here and got involved, got me to take to dealer in mid-July so she could speak to dealership personnel while it was there.
When I took it in on a Monday morning, service rep asked if sunroof was operating, and I replied that I had absolutely no idea, that we were afraid to try it, as rain was forecast for later that week. So I handed her the keys, she opened it (it always would open) but wouldn't close. Well GM had the Yukon until Friday morning, swore they tried it "hundreds" of times and it always worked perfectly. Except the first time the service rep tried it herself. Anyway, we picked it up Friday, service rep agreed to a refund since we had paid for a repair, and didn't get a repair. Three weeks later, I called the GM rep, and she called service rep, who told her that she was waiting for us to "call her and request a refund", even though we had done just that in person on July 19, and she had agreed to get the ball rolling, that was an obvious lie, even to the GM rep. Anyway, the GM rep got in touch with the dealership service manager, who did return her call after she left a second message threatening to escalate. Anyway Aug. 28th we did receive a refund check. Dealer had "absolutely no idea" what was wrong and still is wrong with the sunroof.
I took out the sunroof fuse, much better than getting stuck with it open. By the way: that dealership will NEVER see my face again, so they've now lost out on all future business.
I wonder if that is worth the $224 they finally refunded me.
Local radio mechanic stresses that if you've paid for a fix, then you must get the fix. Case closed. Sunroof closed.