Ensure nav is sorted regardless of the order inside Files

This means plugins don't have to carefully find the sort order to put files in.

mkdocs/structure/files.py

@@ -8,7 +8,7 @@ import posixpath
 import shutil
 import warnings
 from functools import cached_property
-from pathlib import PurePath
+from pathlib import PurePath, PurePosixPath
 from typing import TYPE_CHECKING, Callable, Iterable, Iterator, Mapping, Sequence, overload
 from urllib.parse import quote as urlquote
 
@@ -582,12 +582,25 @@ def get_files(config: MkDocsConfig) -> Files:
     return Files(files)
 
 
+def file_sort_key(f: File, /):
+    """
+    Replicates the sort order how `get_files` produces it - index first, directories last.
+
+    To sort a list of `File`, pass as the `key` argument to `sort`.
+    """
+    parts = PurePosixPath(f.src_uri).parts
+    if not parts:
+        return ()
+    return (parts[:-1], f.name != "index", parts[-1])
+
+
 def _file_sort_key(f: str):
-    """Always sort `index` or `README` as first filename in list."""
+    """Always sort `index` or `README` as first filename in list. This works only on basenames of files."""
     return (os.path.splitext(f)[0] not in ('index', 'README'), f)
 
 
 def _sort_files(filenames: Iterable[str]) -> list[str]:
+    """Soft-deprecated, do not use."""
     return sorted(filenames, key=_file_sort_key)
 
 

mkdocs/structure/nav.py

@@ -6,6 +6,7 @@ from urllib.parse import urlsplit
 
 from mkdocs.exceptions import BuildError
 from mkdocs.structure import StructureItem
+from mkdocs.structure.files import file_sort_key
 from mkdocs.structure.pages import Page, _AbsoluteLinksValidationValue
 from mkdocs.utils import nest_paths
 
@@ -129,9 +130,10 @@ class Link(StructureItem):
 def get_navigation(files: Files, config: MkDocsConfig) -> Navigation:
     """Build site navigation from config and files."""
     documentation_pages = files.documentation_pages()
-    nav_config = config['nav'] or nest_paths(
-        f.src_uri for f in documentation_pages if f.inclusion.is_in_nav()
-    )
+    nav_config = config['nav']
+    if nav_config is None:
+        documentation_pages = sorted(documentation_pages, key=file_sort_key)
+        nav_config = nest_paths(f.src_uri for f in documentation_pages if f.inclusion.is_in_nav())
     items = _data_to_navigation(nav_config, files, config)
     if not isinstance(items, list):
         items = [items]

mkdocs/tests/structure/file_tests.py

@@ -3,7 +3,7 @@ import sys
 import unittest
 from unittest import mock
 
-from mkdocs.structure.files import File, Files, _sort_files, get_files
+from mkdocs.structure.files import File, Files, _sort_files, file_sort_key, get_files
 from mkdocs.tests.base import PathAssertionMixin, load_config, tempdir
 
 
@@ -57,6 +57,16 @@ class TestFiles(PathAssertionMixin, unittest.TestCase):
             ['README.md', 'A.md', 'B.md'],
         )
 
+    def test_file_sort_key(self):
+        for case in [
+            ["a/b.md", "b/index.md", "b/a.md"],
+            ["SUMMARY.md", "foo/z.md", "foo/bar/README.md", "foo/bar/index.md", "foo/bar/a.md"],
+        ]:
+            with self.subTest(case):
+                files = [File(f, "", "", use_directory_urls=True) for f in case]
+                for a, b in zip(files, files[1:]):
+                    self.assertLess(file_sort_key(a), file_sort_key(b))
+
     def test_md_file(self):
         for use_directory_urls in True, False:
             with self.subTest(use_directory_urls=use_directory_urls):