WTF, I thought. The plumbers were here last month, and it seemed to be draining after my bath yesterday. So, I got the plunger. poured half a bottle of a strong drain cleaner down the drain, finished the kitchen cleanup, took the trash to the basement, and when I returned, the tub was empty. I rinsed it out, and hopefully, the cats can play safely in the tub without poisoning themselves.
I do need a plumber again.