This is getting tiresome now.
Found the little git.
Somehow, I had flagged "Enable design mode" and the designer was attempting to render a wide array of custom controls that it had no idea what to do with.
I think, I _may_ have tried design mode for about five minutes about five years ago and decided it wasn't for me... If so, that opinion still holds.
I unflagged "Enable design mode" and suddenly everything is playing nicely again.