feat(stacking): simplify zIndex and treeLevel behaviour

src/container/EdgeRenderer/index.tsx

@@ -247,7 +247,7 @@ const EdgeRenderer = (props: EdgeRendererProps) => {
   return (
     <>
       {edgeTree.map(({ level, edges, isMaxLevel }) => (
-        <svg key={level} style={{ zIndex: 6 + level }} width={width} height={height} className="react-flow__edges">
+        <svg key={level} style={{ zIndex: level }} width={width} height={height} className="react-flow__edges">
           {isMaxLevel && <MarkerDefinitions defaultColor={defaultMarkerColor} />}
           <g>
             {edges.map((edge: Edge) => {

src/container/NodeRenderer/index.tsx

@@ -116,7 +116,7 @@ function Node({
       isConnectable={isConnectable}
       resizeObserver={resizeObserver}
       dragHandle={node.dragHandle}
-      zIndex={6 + treeLevel}
+      zIndex={treeLevel}
       isParentNode={isParentNode}
     />
   );