Thanks, seeker333. I firmly gripped the shifter--not at the "delicate" end, i.e., the lever itself, but at the "works" end where it meets the handlebars--and slowly, slowly rotated it into a more normal position.

This must have affected cable tension, because my usually smooth shifting sounds like grinding, and the derailleur struggles to move the chain. It seemed that this means the cable needs more tension? So I turned the barrel adjuster one-quarter turn to the right. Still a problem. One more quarter-turn, and I couldn't tell whether the problem was worse or the same, so tomorrow I'm taking it in to the shop before I break something.