>>I see. But will it fundamentally change anything, to have a public variable vs. a property of _screen? Thank you for your suggestion.
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>In my opinion it's better than having a public variable. You can, of course, also just add a property to your global application object. That's the way it is supposed to be - only one global variable oApp (and it may be declared private in the main program) and the rest would be properties of it.

There is nothing "better" about a public property on a public object than a public variable. Making the properties non-public with get/set methods or access/assign is the only thing that would make it better.