However, I think WM_CLOSE should work like MCC_EXIT / Alt+F4 in all views. If you need a complete exit, it's available with MCC_EXIT (as a parameter) or by closing twice.
For the record, I don't think we disagree. Where we don't agree is in that Theater View is a "window on top of" Standard View. Display View is not, it is a "mode of" the main MC UI. As steward_pk pointed out, when you send WM_CLOSE (or Alt-F4) to Display View, it closes MC completely (except for Library Serving, obviously). I think Theater View should work the same way (like it used to), as opposed to a detached display, dialog box, or something else that is clearly a window "floating above" and separate from the main MC UI.
In other words, it was Alt-F4 that was acting wrong before, not WM_CLOSE.
There are, of course, edge cases where you can show Theater View and Standard View simultaneously. But you really have to work at it. For most users, when you switch to Theater View, that "becomes" MC. Just like when you switch to Display View, it "becomes" MC. And, if you tell "MC" to close, it should exit.
There are TONS of other ways to switch modes of MC and get back to Standard View.
But... I'm living with it. It glares at me every time I have to hit close twice. I'll probably eventually reprogram Girder. It is just a little bit of a pain to make a special case for MC.