You don’t need to invalidate the window when you’re done with it. In theory, if you call close () on it, and there are no other references to the NativeWindow anywhere else, it should be eligable for garbage collection. Also, any DisplayObjects that were on the window’s stage should also be garbage collected, again assuming there are no other references to them.

Regarding mx:Window / mx.core.Window, it’s basically a wrapper for NativeWindow with some other Flex good stuff thrown in. Also check out the useFlexChrome property – if set to true, for some reason the standard looking window is visually slightly different. There’s a status bar at the bottom, and for me the default background colour is vaguely different. mx:Window also throws out some other events (e.g. when network connection status changes). If you have a little look in the mx.core.Window class you should be able to see what’s going on fairly easily.

When closing windows in AIR apps, is it necessary to invalidate the window to allow for GC?

And is there a difference to the above between creating windows using the NativeWindow class as opposed to creating an mx:Window in Flex?

(ok two questions)