The routine works just fine, but if i add a menu bar or i run the application in windowed mode, the y axis is out of range, i.e. is not anymore 768 pixels high (i run the app at 1024x768). I should apply the picking to (768 - menubar height - window border height), but how can i solve this ? There is a function that return the effective screen y size ?

Are you changing your viewport when you switch to windowed mode?