How to decide whether a drag&drop operation points to somewhere outside the browser window?

I'd like to handle dragend events differently depending on whether an element has been just dragged inside the browser window (or site resp.) or or outside, e.g. to an external file manager.

After I didn't find any attribute of the DragEvent instance indicating whether it's inside or outside the sites context I started to arithmetically figure out if the corresponding mouse event still takes place inside the geometry of the site.

Eventually I might succeed with that approach (currently not working yet) but it has one major disadvantage (leaving alone its ugliness): the drop target window might be on top of the browser, so the geometry is no real indicator at all..

so.. how do I find out if a dragend (or any other event I could use to store some state) is pointing outside of the browser window (or source site)?

from How to decide whether a drag&drop operation points to somewhere outside the browser window?