[868] Compute the position of an edge label when it does not have any

Bug: #868
Signed-off-by: Guillaume Coutable <guillaume.coutable@obeo.fr>

backend/sirius-web-diagrams-layout/src/main/java/org/eclipse/sirius/web/diagrams/layout/incremental/IncrementalLayoutEngine.java

@@ -83,12 +83,20 @@ public void layout(Optional<IDiagramEvent> optionalDiagramElementEvent, DiagramL
 
         // finally we recompute the edges that needs to
         for (EdgeLayoutData edge : diagram.getEdges()) {
-            if (this.hasChanged(edge.getSource()) || this.hasChanged(edge.getTarget())) {
+            if (this.hasChanged(edge.getSource()) || this.hasChanged(edge.getTarget()) || !this.isLabelPositioned(edge)) {
                 this.layoutEdge(edge);
             }
         }
     }
 
+    private boolean isLabelPositioned(EdgeLayoutData edge) {
+        if (edge.getCenterLabel() != null) {
+            Position position = edge.getCenterLabel().getPosition();
+            return position.getX() != -1 || position.getY() != -1;
+        }
+        return false;
+    }
+
     private void layoutNode(Optional<IDiagramEvent> optionalDiagramElementEvent, NodeLayoutData node, ISiriusWebLayoutConfigurator layoutConfigurator) {
         // first layout border & child nodes
         for (NodeLayoutData borderNode : node.getBorderNodes()) {