fix: keep relative isolation roots stable across chdir

docs/changelog/+relative-isolation.bugfix.rst

@@ -0,0 +1,2 @@
+Pin a relative ``isolated_dirs`` root on context entry so changing the working directory keeps directory lookups and
+creation under the original isolation root. The context now yields an absolute path.

docs/howto.rst

@@ -244,9 +244,11 @@ isolates under ``tmp_path`` and yields the root:
         assert my_app_init(PlatformDirs("MyApp")) == {"theme": "dark"}
 
 Each kind lands in ``<root>/<kind>``, for example ``<root>/user_config/MyApp``, and the module-level functions follow
-the same layout. On Unix, isolation ignores ``use_site_for_root`` so user and site kinds stay separate even for root.
-The redirection covers :data:`~platformdirs.PlatformDirs` in the current process only: a subprocess, or another platform
-class such as :class:`~platformdirs.unix.Unix` on macOS, still resolves the real directories.
+the same layout. A relative root is made absolute when entering the context and the yielded root is absolute, so
+changing the working directory inside the context keeps all paths under the original root. On Unix, isolation ignores
+``use_site_for_root`` so user and site kinds stay separate even for root. The redirection covers
+:data:`~platformdirs.PlatformDirs` in the current process only: a subprocess, or another platform class such as
+:class:`~platformdirs.unix.Unix` on macOS, still resolves the real directories.
 
 Overriding directories with environment variables
 =================================================

src/platformdirs/testing.py

@@ -39,14 +39,15 @@ def isolated_dirs(root: str | os.PathLike[str]) -> Generator[Path, None, None]:
     ``PlatformDirs("app", version="1.0").user_config_path`` becomes ``<root>/user_config/app/1.0``. Every thread of the
     current process sees the change, and subprocesses do not.
 
-    On Unix, ``use_site_for_root`` is ignored inside the context so user and site directories keep separate kinds.
+    On Unix, ``use_site_for_root`` is ignored inside the context so user and site directories keep separate kinds. A
+    relative ``root`` is made absolute on entry, so changing the working directory does not move the isolation root.
 
     :param root: directory that holds the redirected directories
 
-    :returns: ``root`` as a :class:`~pathlib.Path`
+    :returns: the absolute isolation root as a :class:`~pathlib.Path`
 
     """
-    base = Path(root)
+    base = Path(root).absolute()
     patches = {
         name: _redirect(
             base / (kind := name.removesuffix("_dir")), app=kind in _APP_KINDS, private=kind.startswith("user_")

tests/test_testing.py

@@ -52,6 +52,24 @@ def test_isolated_dirs_yields_root_as_path(tmp_path: Path) -> None:
         assert root == tmp_path
 
 
+@pytest.mark.parametrize("root_name", [".", "isolated"])
+def test_isolated_dirs_relative_root_survives_chdir(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch, root_name: str
+) -> None:
+    monkeypatch.chdir(tmp_path)
+    expected = tmp_path / root_name
+    (other := tmp_path / "other").mkdir()
+    with isolated_dirs(root_name) as root:
+        monkeypatch.chdir(other)
+        assert _outside(expected) == {}
+        assert root == expected
+        dirs = platformdirs.PlatformDirs("app", ensure_exists=True)
+        assert dirs.user_config_path == expected / "user_config" / "app"
+        assert dirs.user_config_path.is_dir()
+        assert not (other / "user_config").exists()
+        assert not (other / "isolated").exists()
+
+
 def test_isolated_dirs_redirects_every_accessor(tmp_path: Path) -> None:
     with isolated_dirs(tmp_path):
         assert _outside(tmp_path) == {}