Why can't Spy++ see messages sent to UWP apps?

Question

While we all got tangled up in this topic, it came to light that, while UWP apps do have top level windows and you can send (sensible) messages to them (*)

Answer 1

Um, embarrassing. You need to use the 64 bit version of Spy to spy on a 64 bit app (and vice-versa). Then it works fine and you get what you expect.

Of course, it would be nice if Spy told you when you try to trace messages in an app of the wrong bitness but it doesn't, so please be warned.