docs: fall back to first child page for sidebar folders without an index

.changeset/fuzzy-corners-write.md

@@ -0,0 +1,2 @@
+---
+---

docs/components/geistdocs/docs-layout.tsx

@@ -54,9 +54,43 @@ const addSidebarBadges = (nodes: DocsTreeNode[]): DocsTreeNode[] =>
     return node;
   });
 
+// A folder with no index page can't be clicked, only expanded/collapsed —
+// fall back to its first descendant page so every sidebar category navigates.
+type PageNode = Extract<DocsTreeNode, { type: 'page' }>;
+
+const findFirstPage = (nodes: DocsTreeNode[]): PageNode | undefined => {
+  for (const node of nodes) {
+    if (node.type === 'page') {
+      return node;
+    }
+    if (node.type === 'folder') {
+      const found = node.index ?? findFirstPage(node.children);
+      if (found) {
+        return found;
+      }
+    }
+  }
+  return undefined;
+};
+
+const withFallbackFolderIndex = (nodes: DocsTreeNode[]): DocsTreeNode[] =>
+  nodes.map((node) => {
+    if (node.type !== 'folder') {
+      return node;
+    }
+
+    const children = withFallbackFolderIndex(node.children);
+
+    return {
+      ...node,
+      children,
+      index: node.index ?? findFirstPage(children),
+    };
+  });
+
 const addSidebarBadgesToTree = (tree: DocsTree): DocsTree => ({
   ...tree,
-  children: addSidebarBadges(tree.children),
+  children: addSidebarBadges(withFallbackFolderIndex(tree.children)),
 });
 
 interface DocsLayoutProps {