diff --git a/main.py b/main.py
index 9e631ac..4d24ac2 100644
--- a/main.py
+++ b/main.py
@@ -792,43 +792,13 @@ def _render_folder_index_page(
 
     template = string.Template(content)
 
-    # Build folder index sidebar HTML
-    prefix = path.rstrip("/") or ""
-    lines: list[str] = []
-
-    # Breadcrumb
-    lines.append("<nav class='breadcrumb'>")
-    lines.append('<a href="/navigate/">/</a>')
-    if prefix:
-        parts = prefix.split("/")
-        accumulated = ""
-        for part in parts:
-            accumulated += f"{part}/"
-            lines.append(f'<a href="/navigate/{accumulated}">{part}</a>')
-    lines.append("</nav>")
-
-    # Folders
-    for folder in folders:
-        folder_path = f"{prefix}{folder}/" if prefix else f"{folder}/"
-        lines.append(
-            f'<div class="index-item folder">'
-            f'<a href="/navigate/{folder_path}">📁 {folder}</a>'
-            f"</div>"
-        )
-
-    # Files
-    for file in files:
-        file_path = f"{prefix}{file}" if prefix else file
-        href_params = ""
-        if current_order is not None and current_delay is not None:
-            href_params = f"?order={current_order}&delay={current_delay}"
-        lines.append(
-            f'<div class="index-item file">'
-            f'<a href="/navigate/{file_path}{href_params}">📄 {file}</a>'
-            f"</div>"
-        )
-
-    folder_index_html = "\n".join(lines)
+    # Build folder index sidebar HTML using recursive tree renderer
+    folder_path = path.rstrip("/") or None
+    folder_index_html = _render_folder_index_html(
+        current_path=folder_path,
+        current_order=current_order,
+        current_delay=current_delay,
+    )
 
     content = template.substitute(
         img_url="#",
diff --git a/tests/test_navigate.py b/tests/test_navigate.py
index 56f58c9..167f43b 100644
--- a/tests/test_navigate.py
+++ b/tests/test_navigate.py
@@ -106,6 +106,24 @@ class TestNavigatePage:
         response = await client_zip_navigate.get("/navigate/")
         assert "/navigate/top.txt" in response.text
 
+    async def test_subfolder_index_shows_nested_files(
+        self, client_zip_navigate: AsyncClient
+    ) -> None:
+        """Subfolder index shows files within that folder."""
+        response = await client_zip_navigate.get("/navigate/folder/")
+        assert response.status_code == 200
+        assert "/navigate/folder/deep.txt" in response.text
+        assert "/navigate/folder/image.png" in response.text
+
+    async def test_subfolder_index_shows_full_tree(
+        self, client_zip_navigate: AsyncClient
+    ) -> None:
+        """Subfolder index sidebar shows full tree from root."""
+        response = await client_zip_navigate.get("/navigate/folder/")
+        assert response.status_code == 200
+        # Should still show root-level items in the tree
+        assert "/navigate/top.txt" in response.text
+
     async def test_file_page_has_sidebar(
         self, client_zip_navigate: AsyncClient
     ) -> None: