You both just made my day much better. Oddly, enough, the same thing started happening to me last night (running 2.8.7) and I was pretty confused. I’ve never intentionally wandered out of the slide editing mode (yet another feature I have yet to explore), but I must have slopped a stray click on that drop-down menu at one point.
At any rate, thanks Rony for asking and thanks Chrisg for answering. The Ardour forums have saved me once again.