Fix `_relative_path()` for unresolved paths

CHANGES.rst

@@ -25,6 +25,8 @@ Bugs fixed
   Patch by Dave Hoese.
 * #13382: Napoleon: Use the right valid types for configuration values.
   Patch by Adam Turner.
+* 13376: Fix copying assets from a relative :confval:`html_static_path` entry.
+  Patch by Adam Turner.
 
 Testing
 -------

sphinx/util/fileutil.py

@@ -140,7 +140,8 @@ def copy_asset(
         return
 
     for root, dirs, files in os.walk(source, followlinks=True):
-        reldir = _relative_path(Path(root), source).as_posix()
+        root_p = Path(root)
+        reldir = _relative_path(root_p, source).as_posix()
         for dir in dirs.copy():
             if excluded(posixpath.join(reldir, dir)):
                 dirs.remove(dir)

sphinx/util/osutil.py

@@ -177,9 +177,14 @@ def _relative_path(path: Path, root: Path, /) -> Path:
     It returns the original path if *path* and *root* are on different drives,
     which may happen on Windows.
     """
+    # Path.relative_to() requires fully-resolved paths (no '..').
+    if '..' in path.parts:
+        path = path.resolve()
+    if '..' in root.parts:
+        root = root.resolve()
+
     if path.anchor != root.anchor or '..' in root.parts:
         # If the drives are different, no relative path exists.
-        # Path.relative_to() requires fully-resolved paths (no '..').
         return path
     if sys.version_info[:2] < (3, 12):
         return Path(os.path.relpath(path, root))

tests/test_util/test_util_fileutil.py

@@ -84,6 +84,10 @@ def test_copy_asset(tmp_path):
     (source / '_templates' / 'sidebar.html.jinja').write_text(
         'sidebar: {{var2}}', encoding='utf8'
     )
+    sibling = tmp_path / 'sibling'
+    sibling.mkdir(parents=True, exist_ok=True)
+    sibling.joinpath('spam').mkdir(parents=True, exist_ok=True)
+    sibling.joinpath('spam', 'ham').touch()
 
     # copy a single file
     assert not (tmp_path / 'test1').exists()
@@ -126,6 +130,14 @@ def test_copy_asset(tmp_path):
     assert (destdir / '_templates' / 'layout.html').exists()
     assert not (destdir / '_templates' / 'sidebar.html').exists()
 
+    # copy sibling
+    assert not (tmp_path / 'test4').exists()
+    copy_asset(source / '../sibling', tmp_path / 'test4')
+    assert (tmp_path / 'test4').is_dir()
+    assert (tmp_path / 'test4' / 'spam').is_dir()
+    assert (tmp_path / 'test4' / 'spam' / 'ham').is_file()
+    assert (tmp_path / 'test4' / 'spam' / 'ham').read_bytes() == b''
+
 
 @pytest.mark.sphinx('html', testroot='html_assets')
 def test_copy_asset_template(app: SphinxTestApp) -> None: