Hi Geert,
you are right, Sparx seem to think somehow in this direction!
However, even if so, with my diagram drag/drop procedure the action is still on many diagrams, owned by different activities. So the implementation is a bug anyway.
From my interpretation of the standard, I would say:
1) An action should be owned only by one activity
2) No action should have a kind of flow to other activity nodes not owned by its own owner
3) An action can have dependency links to actions not owned by its owner
4) !!!And!!! An action can be shown on an activity diagram or any other kind of diagram not owned by its owner
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The last one I derive from this part of the standard:
„NOTE. This taxonomy provides a logical organization for the various major kinds of diagrams. However, it does not
preclude mixing different kinds of diagram types, as one might do when one combines structural and behavioral
elements (e.g., showing a state machine nested inside an internal structure). Consequently, the boundaries between the
various kinds of diagram types are not strictly enforced.“
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Or see here:
https://www.uml-diagrams.org/uml-25-diagrams.htmlI have the feeling that Sparx tries to be more UML compliant, what is in principle not a bad thing, but they overshoot somehow.
With the new Sparx implementation it is really hard and might get impossible in future to visualize cross-cutting relationships, and this would be very bad.