This is actually the first situation in which I would prefer private over protected.

Having said that, I still think I like protected better for all other situations. In my opinion, the habit of making your vars private (instead of protected) totally ruins the opportunity for other programmers (and yourself, for that matter) to extend your classes.
This why I feel extending some of the more complicated AS3 Flex classes is a horrible (if not near impossible) task to do.

What I meant to say is that before you decide to make a variable protected, you should think about whether it would be better to leave it private and provide a different way for the subclass to access the information.

In this case, exposing dragTargetIdx as a protected variable is arguably the wrong thing. It is a very specific implementation detail of how drag and drop was implemented by the superclass, and you may want to change this in the future. The “right” way to factor this is to make the variable private and to give the subclass some other way of getting the information. (for example, by passing the drag target index into a drag handler function as an argument).

What’s wrong with letting a subclass use a protected var belonging to its superclass? I’m kind of curious as I don’t really see the use of protected members if you shouldn’t let subclasses use them (since that’s in my opinion the only difference between private and protected)..