In the diagram, say you are creating a link from a processor input port, it is possible to end the drag immediately on a String constant processor, even if its ports are not currently shown. The reason is that it has only got a single output port.
Similarly it should be possible to start dragging from a processor without showing its ports, at least in the case where there's just a single port. If this is an output port, the arrow would be pointing towards the mouse, if it is a single input port the arrow would point towards this processor.
Suggested by Paul Fisher.