lsp: fix an issue when recursing through non-expanded nodes in the micro

layout calculation.

plugins/de.cau.cs.kieler.klighd.lsp/src/de/cau/cs/kieler/klighd/lsp/utils/RenderingPreparer.xtend

@@ -3,7 +3,7 @@
  *
  * http://rtsys.informatik.uni-kiel.de/kieler
  * 
- * Copyright 2018-2023 by
+ * Copyright 2018-2024 by
  * + Kiel University
  *   + Department of Computer Science
  *     + Real-Time and Embedded Systems Group
@@ -46,6 +46,7 @@ import de.cau.cs.kieler.klighd.microlayout.DecoratorPlacementUtil.Decoration
 import de.cau.cs.kieler.klighd.microlayout.GridPlacementUtil
 import de.cau.cs.kieler.klighd.microlayout.PlacementUtil
 import de.cau.cs.kieler.klighd.util.KlighdProperties
+import de.cau.cs.kieler.klighd.util.RenderingContextData
 import java.awt.geom.Point2D
 import java.util.ArrayList
 import java.util.HashMap
@@ -124,8 +125,21 @@ final class RenderingPreparer {
             }
         }
         if (element instanceof KNode) {
-            for (node : element.children) {
-                prepareRendering(node, kGraphToSGraph)
+            // Do not recurse generating IDs if the element is not expanded, as there won't be any SGraph generated for
+            // it.
+            var boolean isExpanded
+            val renderingContextData = RenderingContextData.get(element)
+            if (renderingContextData.hasProperty(SprottyProperties.EXPANDED)) {
+                isExpanded = renderingContextData.getProperty(SprottyProperties.EXPANDED)
+            } else {
+                // If the expanded property does not exist yet, use the initial expansion.
+                isExpanded = element.getProperty(KlighdProperties.EXPAND)
+            }
+
+            if (isExpanded) {
+                for (node : element.children) {
+                    prepareRendering(node, kGraphToSGraph)
+                }
             }
             for (edge : element.outgoingEdges) {
                 prepareRendering(edge, kGraphToSGraph)