[2320] Fix feedback when user tries to reconnect Actor's edge

CHANGELOG.adoc

@@ -36,6 +36,7 @@ It has long been unused by Sirius Web itself (since the transition to MUI).
 - https://github.com/eclipse-syson/syson/issues/2306[#2306] [diagrams] Fixed an error when trying to rename a constraint displayed as a list item through direct edit.
 - https://github.com/eclipse-syson/syson/issues/2318[#2318] [diagrams] Direct edit on a constraint label now ignores the expression part (`{ ... }`) if present in the new text.
 - https://github.com/eclipse-syson/syson/issues/2331[#2331] [diagrams] Fix the broken `New Interface` tool icon.
+- https://github.com/eclipse-syson/syson/issues/2320[#2320] [diagrams] Add an explicit warning message when user tries to reconnect one Actor's edge to another Actor.
 
 === Improvements
 

backend/views/syson-diagram-common-view/src/main/java/org/eclipse/syson/diagram/common/view/services/ViewToolService.java

@@ -300,6 +300,23 @@ public Element reconnectSourceNestedActorEdge(Element self, Element newSource, E
         return otherEnd;
     }
 
+    /**
+     * Reconnects the target of a nested actor edge (the Actor side of the edge).
+     * This service is only used because edge preconditions are not taken into account.
+     *
+     * @param self
+     *            the current UseCase or Requirement
+     * @return the given self
+     */
+    public Element reconnectTargetNestedActorEdge(Element self) {
+        // due to https://github.com/eclipse-sirius/sirius-web/issues/2930 we cannot prevent the user to do the reconnect,
+        // so we just warn him/her that this is not possible.
+        String errorMessage = "An Actor cannot be disconnected from its UseCase or its Requirement element";
+        this.logger.warn(errorMessage);
+        this.feedbackMessageService.addFeedbackMessage(new Message(errorMessage, MessageLevel.WARNING));
+        return self;
+    }
+
     /**
      * Reconnects the source of a nested stakeholder edge.
      * <p>

backend/views/syson-standard-diagrams-view/src/main/java/org/eclipse/syson/standard/diagrams/view/edges/NestedActorEdgeDescriptionProvider.java

@@ -120,9 +120,17 @@ protected ChangeContextBuilder getSourceReconnectToolBody() {
 
     @Override
     protected ChangeContextBuilder getTargetReconnectToolBody() {
+        // Because of https://github.com/eclipse-sirius/sirius-web/issues/2930, it is not possible to prevent user to do the reconnection.
+        return this.viewBuilderHelper.newChangeContext()
+                .expression(ServiceMethod.of0(ViewToolService::reconnectTargetNestedActorEdge).aql(AQLConstants.EDGE_SEMANTIC_ELEMENT));
+    }
+
+    @Override
+    protected String getTargetReconnectToolPreconditionExpression() {
         // It is not possible to reconnect the target (actor) of this edge. This would mean that the existing actor is
         // not contained in a UseCase/Requirement anymore.
-        return this.viewBuilderHelper.newChangeContext();
+        // Once https://github.com/eclipse-sirius/sirius-web/issues/2930 is fixed, this precondition could be evaluated.
+        return "aql:false";
     }
 
 }

doc/content/modules/user-manual/pages/release-notes/2026.7.0.adoc

@@ -108,6 +108,7 @@ Now, the label dynamically adapts based on whether the `ConcernUsage` or `Constr
 *** Elements without a name and an owned reference subsetting display only the reference subsetting name.
 *** Elements without a reference subsetting display their name.
 ** Fix the broken `New Interface` tool icon.
+** Add an explicit warning message when user tries to reconnect one `Actor` 's graphical edge to another `Actor`.
 
 == Improvements