ControlContext :: dropIndex - Drop index in list or tree
For table and tree controls the property contains the index position for the last drop event. The index points to the line position (first line is 0), where the drag buffer will be inserted. When index is larger than the number of entries in the list, the drag content is appended to the end of the list.
Implementation details
int32
ControlContext  :: dropIndex (  )
The function returns the current drop index position.