feat(stacking): simplify zIndex and treeLevel behaviour

example/src/Basic/index.tsx

@@ -28,7 +28,7 @@ const initialNodes: Node[] = [
     data: { label: 'Node 4' },
     position: { x: 100, y: 200 },
     className: 'light',
-    style: { backgroundColor: 'rgba(255, 0, 0, 1)' },
+    style: { backgroundColor: 'rgba(255, 0, 0, 0.8)' },
     width: 600,
     height: 300,
   },
@@ -42,12 +42,12 @@ const initialNodes: Node[] = [
   {
     id: '4b',
     data: { label: 'Node 4b' },
-    position: { x: 80, y: 80 },
+    position: { x: 150, y: 50 },
     className: 'light',
-    style: { backgroundColor: 'rgba(255, 255, 0, 0.2)' },
+    style: { backgroundColor: 'rgba(255, 0, 255, 0.8)' },
+    height: 300,
+    width: 300,
     parentNode: '4',
-    height: 200,
-    width: 350,
   },
   {
     id: '4b1',
@@ -68,10 +68,8 @@ const initialNodes: Node[] = [
 const initialEdges: Edge[] = [
   { id: 'e1-2', source: '1', target: '2', animated: true },
   { id: 'e1-3', source: '1', target: '3' },
-  { id: 'e2-4a', source: '2', target: '4a', animated: true },
   { id: 'e3-4', source: '3', target: '4' },
   { id: 'e3-4b', source: '3', target: '4b' },
-  { id: 'e3-4b2', source: '3', target: '4b2' },
   { id: 'e4a-4b1', source: '4a', target: '4b1' },
   { id: 'e4a-4b2', source: '4a', target: '4b2' },
   { id: 'e4b1-4b2', source: '4b1', target: '4b2' },