property TRANSFER_OPERATION_NONE

This property returns the transfer operation enumerator value which represents that the data object cannot be dropped in this window. The transfer operation indicates the type of drop that would occur if the user dropped the object at this position.

function OnDragStop()
{
    var event = shell.currentEvent;
    if (event)
    {
        if (event.operation == appUtils.TRANSFER_OPERATION_NONE)
        {
            // drop target indicated a rejection of drop
            shell.print("drag abort");
        }
        event.stopPropagation();
    }
}