Well, there it is, big as life. In the XAML I was binding to a view model property called:
Selectedtem
^^
Unfortunately, the actual name is SelectedItem. So this code actually works–I solved the problem early this afternoon and then spent the rest of the afternoon and all evening scouring the web, before I noticed the spelling error.
My wife told me at 3:00 this afternoon, “You know, it’s going to turn out to be something small.” And so it did–a missing letter I. Well, at least I can go to bed now.