Fix issue with slashes being turned into backslashes on Windows

AUTHORS

@@ -300,6 +300,7 @@ mrbean-bremen
 Nathan Goldbaum
 Nathaniel Compton
 Nathaniel Waisbrot
+Nauman Ahmed
 Ned Batchelder
 Neil Martin
 Neven Mundar

changelog/12745.bugfix.rst

@@ -0,0 +1 @@
+Fixed an issue with backslashes being incorrectly converted in nodeid paths on Windows, ensuring consistent path handling across environments.

src/_pytest/config/__init__.py

@@ -1178,8 +1178,12 @@
     def cwd_relative_nodeid(self, nodeid: str) -> str:
         # nodeid's are relative to the rootpath, compute relative to cwd.
         if self.invocation_params.dir != self.rootpath:
-            fullpath = self.rootpath / nodeid
-            nodeid = bestrelpath(self.invocation_params.dir, fullpath)
+            base_path_part, *nodeid_part = nodeid.split("::")
+            # Only process path part
+            fullpath = self.rootpath / base_path_part
+            relative_path = bestrelpath(self.invocation_params.dir, fullpath)
+
+            nodeid = "::".join([relative_path, *nodeid_part])
         return nodeid
 
     @classmethod

testing/test_terminal.py

@@ -3067,3 +3067,38 @@ def test_pass():
             "*= 1 xpassed in * =*",
         ]
     )
+
+
+class TestNodeIDHandling:
+    def test_nodeid_handling_windows_paths(self, pytester: Pytester, tmp_path) -> None:
+        """Test the correct handling of Windows-style paths with backslashes."""
+        pytester.makeini("[pytest]")  # Change `config.rootpath`
+
+        test_path = pytester.path / "tests" / "test_foo.py"
+        test_path.parent.mkdir()
+        os.chdir(test_path.parent)  # Change `config.invocation_params.dir`
+
+        test_path.write_text(
+            textwrap.dedent(
+                """
+                import pytest
+
+                @pytest.mark.parametrize("a", ["x/y", "C:/path", "\\\\", "C:\\\\path", "a::b/"])
+                def test_x(a):
+                    assert False
+                """
+            ),
+            encoding="utf-8",
+        )
+
+        result = pytester.runpytest("-v")
+
+        result.stdout.re_match_lines(
+            [
+                r".*test_foo.py::test_x\[x/y\] .*FAILED.*",
+                r".*test_foo.py::test_x\[C:/path\] .*FAILED.*",
+                r".*test_foo.py::test_x\[\\\\\] .*FAILED.*",
+                r".*test_foo.py::test_x\[C:\\\\path\] .*FAILED.*",
+                r".*test_foo.py::test_x\[a::b/\] .*FAILED.*",
+            ]
+        )