I wonder if withShorterVerticalAttachPoint would look nice if it provided a short vertical stub off the shape that an orthogonal line could connect to.

That's line's responsibility. We have RTMultiLine for that. Also the situation is same for withShorterHorizontalAttachPoint, you just don't see it (because the line is horizontal).

For example:

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|v es e1 e2 offset|

v := RTView new.

es := RTBox new borderColor: Color black; size: 50; elementsOn: #(a b c).

es @ RTDraggable.

v addAll: es.

es second translateBy: 300 @ 25.

es third translateBy: 25 @ 300.

e1 := RTMultiLine new

color: Color black;

withShorterHorizontalAttachPoint;

orthoHorizontal;

edgeFrom: es first to: es second.

e1 model: #withShorterHorizontalAttachPoint.

v add: e1.

e1 @ (RTLabelled new color: Color black).

e1 := RTMultiLine new

color: Color black;

withShorterVerticalAttachPoint;

orthoVertical;

edgeFrom: es first to: es third.

e1 model: #withShorterVerticalAttachPoint.

v add: e1.

e1 @ (RTLabelled new color: Color black).