What comes in to this callback in Gui.

Is obj the object that the dragged element is dropped on
and screen_pos the drop position?

If I remember correctly then the obj is the current gui object at the drop location, at screen_pos. It's the destination and not the source.

By current object. Do you mean the dragged object?

No, not the dragged (source), but the object on which you drag (destination).

To get the source object, you can use Gui.ms if you drag with mouse. But you'll have to check that for sure because I'm writing from my memory.

Thanks