message: fix(zIndex): allow zIndex to be set to 0 explicitly
diff: @@ -3,12 +3,15 @@ import { useCallback } from 'react';
  import { useStore } from '../store';
  import { isEdgeVisible } from '../container/EdgeRenderer/utils';
  import { ReactFlowState, NodeInternals, Edge } from '../types';
  +import { isNumeric } from '../utils';
  function groupEdgesByZLevel(edges: Edge[], nodeInternals: NodeInternals) {
  let maxLevel = -1;
  const levelLookup = edges.reduce<Record<string, Edge[]>>((tree, edge) => {
  - const z = edge.zIndex || Math.max(nodeInternals.get(edge.source)?.z || 0, nodeInternals.get(edge.target)?.z || 0);
  + const z = isNumeric(edge.zIndex)
  + ? edge.zIndex
  + : Math.max(nodeInternals.get(edge.source)?.z || 0, nodeInternals.get(edge.target)?.z || 0);
  if (tree[z]) {
  tree[z].push(edge);
  } else {
  