I had some help to move it to the greenhouse to work on. I expected to find it root-bound, but was surprised to discover the roots only went about half way into the pot. The bottom half of the pot was soggy muck.
So, it was a root issue, but no root pruning was needed. I repotted into better draining soil, gave it a good shower, and cleaned up the saucer. Also gave it a good drench with Kelpak root stimulant. I'll see if I can get a sign on it--"Do Not Water"--to discourage passers-by from "helping".
We'll see how it does from here!