I solved it, inelegantly.
First, as root, I tried hiding the maybe-problematic cookie, by moving it from /home/jeff/.Xauthority to /home/jeff/.Xauthority-bk/.Xauthority. I still couldn’t log in as my original jeff user.
So then, as either root a new user (can’t remember), I rebuilt my config to exclude the old jeff user, renamed the old /home/jeff/ folder, rebuilt my config to again include a jeff user, and then moved data from the old, renamed home folder to the new /home/jeff.
The first time I tried that, I retained every single file, and the problem reappeared. So the second time, I left behind .cache, .config and .local.
A brutish solution, but it seems to work so far.