Nothing works to close this window except mouse/clicking... I don't like it being in the background when I'm working in other apps.
You realize you can disable it and not have the Application Frame come up by default, right?
Go into the Window menu (I think it is), uncheck Application Frame, then quit Photoshop (to ensure your preference change is saved). Next time it comes up you shouldn't be seeing the Application Frame at all.
To be clear... I like the Application Frame... I just want it to disappear automatically (or with a shortcut - I know I can make one - just can't believe Adobe didn't put one in) When I close my last Photoshop file. aka I don't wan't a blank application frame hanging around.
I'm not a Mac user, so I don't have a way to check this myself, but what's the behavior when you do disable it via the setting, then open an image? Does the image just open a window exactly the size of the displayed image? I assume this is different than what you're looking to see. Maybe a couple of screenshots would help others reading this thread (e.g., Adobe folks) to understand how you'd prefer it to work.
I think the intent is to make the default Photoshop behavior the same on a Mac as on a PC, and on a PC there's no way to eliminate the Application Frame at all.
I don't think there is a default keyboard shortcut for the application frame, because most people either like it or don't and tend not to cycle it on/off.
The easiest is to just assign a keyboard shortcut to the application frame.
You might also be able to automate that by using the File>Scripts>Script Events Manager