fix: a site preview writes its own config

generate_site --docs-dir sent the pages elsewhere but always rewrote the tracked mkdocs.yml, outside the lock taken on that folder, with the navigation of the work in progress. Two previews into two folders both wrote it. Another folder now gets <name>.mkdocs.yml beside it, docs_dir set and its paths rebased, which mkdocs builds with -f.

tests/test_site_validation.py

@@ -306,5 +306,19 @@ class PlainHouseStyle(unittest.TestCase):
                 self.assertFalse(first.endswith(" - RetroBIOS"), first)
 
 
+class APreviewLeavesTheTrackedConfig(unittest.TestCase):
+    """--docs-dir moved the pages but still rewrote the tracked mkdocs.yml."""
+
+    def test_another_folder_gets_its_own_config(self):
+        from generate_site import _mkdocs_config
+
+        path, text = _mkdocs_config(Path("docs"), "hooks:\n  - scripts/mkdocs_hooks.py\n")
+        self.assertEqual(path, Path("mkdocs.yml"))
+        path, text = _mkdocs_config(Path("tmp/preview"), "hooks:\n  - scripts/mkdocs_hooks.py\n")
+        self.assertEqual(path, Path("tmp/preview.mkdocs.yml"))
+        self.assertTrue(text.startswith("docs_dir: preview\n"))
+        self.assertIn("../scripts/mkdocs_hooks.py", text)
+
+
 if __name__ == "__main__":
     unittest.main()