Update: finally dealt with this yesterday. (Needed to come up with a simple scaffold. Did it with a couple of 4 foot 2x4s upright, a horizontal 2x3 screwed into those and braced again the walls on the side, and a couple of 4x4s resting above (and screwed to) the 2x4s on one end and resting on the 6th step of a staircase on the other. Stepladder on top of that.)
Basically, it was a short within the fixture. I took down the fixture and attached the wires to a plug and plugged it in, which instantly blew that circuit. Which pleased me, because while it made no practical difference, I would rather not have had the issue caused by me doing a bad job of wiring in the fixture.
Put up a new fixture and all is good.