[722] Allow to drop any element on the diagram background

Bug: #722
Signed-off-by: Gwendal Daniel <gwendal.daniel@obeosoft.com>

CHANGELOG.adoc

@@ -90,6 +90,7 @@ It is now possible to reference an element in any of the containing namespaces o
 The drop doesn't reveal the _documentation_ compartment anymore, and a warning message is displayed to indicate that the drop is not possible.
 - https://github.com/eclipse-syson/syson/issues/711[#711] [diagrams] Prevent feature typing from disappearing when nesting a PartUsage in a PartDefinition.
 * The tool _Add Part as nested Part_ now correctly keeps the existing relationships of the PartUsage after its owner is changed.
+- https://github.com/eclipse-syson/syson/issues/722[#722] [diagrams] Allow the drop of the root element of a diagram on its background.
 
 === Improvements
 

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

@@ -346,9 +346,9 @@ public Element dropElementFromExplorer(Element element, IEditingContext editingC
             optTargetElement = this.objectService.getObject(editingContext, diagramContext.getDiagram().getTargetObjectId());
         }
         if (optTargetElement.isPresent() && optTargetElement.get() instanceof Element targetElement) {
-            // Check if the element we attempt to drop is in the ancestors of the target element. If it is the case we
-            // want to prevent the drop.
-            if (EMFUtils.isAncestor(element, targetElement)) {
+            // Check if the element we attempt to drop is in the ancestors of the target element and we attempt to drop
+            // it on anything else than the diagram background. If it is the case we want to prevent the drop.
+            if (EMFUtils.isAncestor(element, targetElement) && selectedNode != null) {
                 final String errorMessage;
                 if (element == targetElement) {
                     errorMessage = MessageFormat.format("Cannot drop {0} on itself", element.getName());

doc/content/modules/user-manual/pages/release-notes/2024.9.0.adoc

@@ -110,6 +110,7 @@ It is now possible to reference an element in any of the containing namespaces o
 The drop isn't allowed anymore, and a warning message is displayed.
 - Fix an issue that deleted feature typing specialization when nesting a PartUsage in a PartDefinition.
 * The tool _Add Part as nested Part_ now correctly keeps the existing relationships of the PartUsage after its owner is changed.
+- Fix an issue that prevented the drop of the root element of a diagram on its background.
 
 == Improvements
 

integration-tests/cypress/e2e/project/diagrams/general-view/dropFromExplorer.cy.ts

@@ -59,6 +59,14 @@ describe('Drop From Explorer Tests', () => {
           .should('exist');
       });
 
+      it.only('Then we can drop the Package on the diagram', () => {
+        const dataTransfer = new DataTransfer();
+        explorer.dragTreeItem(sysmlv2.getRootElementLabel(), dataTransfer);
+        diagram.dropOnDiagram(diagramLabel, dataTransfer);
+
+        diagram.getNodes(diagramLabel, sysmlv2.getRootElementLabel()).should('exist');
+      });
+
       it('Then we can drop the PartUsage on the diagram, and its compartment are not visible', () => {
         const dataTransfer = new DataTransfer();
         explorer.dragTreeItem('part', dataTransfer);