My fingers had a mind of their own and apparently hit some combination of keys that had a very weird result.
I'm using an XFCE desktop. And, apparently, it became, maybe, fourty or so virtual pixels wider. Of course, the monitor still has the same number of pixels, so what was happening is that moving the mouse pointer to one of the edges scrolled the entire desktop slightly to reveal the extra virtual space on that side. Moving the mouse pointer to the other edge scrolled the virtual window on the desktop in the other direction, to the other side of the now-larger virtual desktop.
It was a pretty cool effect, so after I had my fun I logged out and back in, and things were back to normal. But I'm wondering what keys did I hit, I looked through settings and didn't find anything relevant…