I wanted to seal it back in place with silicone sealant. The problem was figuring out how to hold it in place while the silicone cured. Luckily, I had just found out about ratcheting cargo bars for the first time a few days earlier, and a friend had a carport that would provide the perfect place to ground the other end of the bar.
I pulled the car in and stopped where the bar would be pressing perpendicular to the windshield.
I put a strip of blue painters' masking tape on the windshield and the roof right next to the gasket
Then I loaded up the exposed part of the channel behind the loose part of the gasket with as much transparent silicone sealant as I could get in.
I put the bar in place and extended it until the seal was pressed all the way back into the channel, and then 1 click more. I didn't want to put enough force on it to deflect the roof even slightly.
I used a putty knife and some paper towels to remove the excess silicone which got squeezed out while it was still wet. This step took the longest because what you do here will determine how the finished job looks - you don't want to be trying to slice off cured silicone later because you hurried here.
I let it sit for 18 hours in warm weather because I didn't want to have to do this again. Then I removed the bar, I peeled off the tape, and did a tiny bit of clean-up with a razor blade.
The test was going through the car wash - success!