gh-144259: Fix Windows EOL wrap by syncing real console cursor

Lib/_pyrepl/windows_console.py

@@ -247,6 +247,18 @@ def input_hook(self):
         if nt is not None and nt._is_inputhook_installed():
             return nt._inputhook
 
+    def _has_wrapped_to_next_row(self, y: int) -> bool:
+        """
+        Return True if the real console cursor wrapped to the next visible row.
+        """
+        info = CONSOLE_SCREEN_BUFFER_INFO()
+        if not GetConsoleScreenBufferInfo(OutHandle, info):
+            raise WinError(get_last_error())
+
+        win_y = int(info.dwCursorPosition.Y - info.srWindow.Top)
+        expected = y - self.__offset
+        return win_y == expected + 1
+
     def __write_changed_line(
         self, y: int, oldline: str, newline: str, px_coord: int
     ) -> None:
@@ -272,9 +284,11 @@ def __write_changed_line(
 
         self.__write(newline[x_pos:])
         if wlen(newline) == self.width:
-            # If we wrapped we want to start at the next line
-            self._move_relative(0, y + 1)
-            self.posxy = 0, y + 1
+            if self._has_wrapped_to_next_row(y):
+                self.posxy = 0, y + 1
+            else:
+                # Terminal did not wrap; cursor stays at end-of-line.
+                self.posxy = self.width, y
         else:
             self.posxy = wlen(newline), y
 

Lib/test/test_pyrepl/test_pyrepl.py

@@ -12,7 +12,7 @@
 import tempfile
 from pkgutil import ModuleInfo
 from unittest import TestCase, skipUnless, skipIf, SkipTest
-from unittest.mock import patch
+from unittest.mock import Mock, patch
 from test.support import force_not_colorized, make_clean_env, Py_DEBUG
 from test.support import has_subprocess_support, SHORT_TIMEOUT, STDLIB_DIR
 from test.support.import_helper import import_module
@@ -2105,3 +2105,96 @@ def test_ctrl_d_single_line_end_no_newline(self):
         )
         reader, _ = handle_all_events(events)
         self.assertEqual("hello", "".join(reader.buffer))
+
+
+@skipUnless(sys.platform == "win32", "Windows console behavior only")
+class TestWindowsConsoleEolWrap(TestCase):
+    """
+    When a line exactly fills the terminal width, Windows consoles differ on
+    whether the cursor immediately wraps to the next row (depends on host/mode).
+    We use _has_wrapped_to_next_row() to determine the actual behavior.
+    """
+
+    def _make_console_like(self, *, width: int, offset: int, vt: bool):
+        from _pyrepl import windows_console as wc
+
+        con = object.__new__(wc.WindowsConsole)
+
+        # Minimal state needed by __write_changed_line()
+        con.width = width
+        con.screen = []
+        con.posxy = (0, 0)
+        setattr(con, "_WindowsConsole__offset", offset)
+        setattr(con, "_WindowsConsole__vt_support", vt)
+
+        # Stub out side-effecting methods used by __write_changed_line()
+        con._hide_cursor = Mock()
+        con._erase_to_end = Mock()
+        con._move_relative = Mock()
+        con.move_cursor = Mock()
+        setattr(con, "_WindowsConsole__write", Mock())
+
+        return con, wc
+
+    def _run_exact_width_case(self, *, vt: bool, did_wrap: bool):
+        width = 10
+        y = 3
+        con, wc = self._make_console_like(width=width, offset=0, vt=vt)
+
+        with patch.object(con, "_has_wrapped_to_next_row", return_value=did_wrap):
+            old = ""
+            new = "a" * width
+            wc.WindowsConsole._WindowsConsole__write_changed_line(con, y, old, new, 0)
+
+        if did_wrap:
+            self.assertEqual(con.posxy, (0, y + 1))
+            self.assertNotIn((0, y + 1), [c.args for c in con._move_relative.mock_calls])
+        else:
+            self.assertEqual(con.posxy, (width, y))
+            self.assertNotIn((0, y + 1), [c.args for c in con._move_relative.mock_calls])
+
+    def test_exact_width_line_did_wrap_vt_and_legacy(self):
+        for vt in (True, False):
+            with self.subTest(vt=vt):
+                self._run_exact_width_case(vt=vt, did_wrap=True)
+
+    def test_exact_width_line_did_not_wrap_vt_and_legacy(self):
+        for vt in (True, False):
+            with self.subTest(vt=vt):
+                self._run_exact_width_case(vt=vt, did_wrap=False)
+
+
+@skipUnless(sys.platform == "win32", "Windows console behavior only")
+class TestHasWrappedToNextRow(TestCase):
+    def _make_console_like(self, *, offset: int):
+        from _pyrepl import windows_console as wc
+
+        con = object.__new__(wc.WindowsConsole)
+        setattr(con, "_WindowsConsole__offset", offset)
+        return con, wc
+
+    def test_returns_true_when_wrapped(self):
+        con, wc = self._make_console_like(offset=0)
+        y = 3
+
+        def fake_gcsbi(_h, info):
+            info.srWindow.Top = 0
+            info.dwCursorPosition.Y = y + 1
+            return True
+
+        with patch.object(wc, "GetConsoleScreenBufferInfo", side_effect=fake_gcsbi), \
+             patch.object(wc, "OutHandle", 1):
+            self.assertIs(con._has_wrapped_to_next_row(y), True)
+
+    def test_returns_false_when_not_wrapped(self):
+        con, wc = self._make_console_like(offset=0)
+        y = 3
+
+        def fake_gcsbi(_h, info):
+            info.srWindow.Top = 0
+            info.dwCursorPosition.Y = y
+            return True
+
+        with patch.object(wc, "GetConsoleScreenBufferInfo", side_effect=fake_gcsbi), \
+             patch.object(wc, "OutHandle", 1):
+            self.assertIs(con._has_wrapped_to_next_row(y), False)

Lib/test/test_pyrepl/test_windows_console.py

@@ -41,6 +41,7 @@ def console(self, events, **kwargs) -> Console:
         console._hide_cursor = MagicMock()
         console._show_cursor = MagicMock()
         console._getscrollbacksize = MagicMock(42)
+        console._has_wrapped_to_next_row = MagicMock(return_value=False)
         console.out = MagicMock()
 
         height = kwargs.get("height", 25)

Misc/NEWS.d/next/Library/2026-01-28-06-50-37.gh-issue-144259.Xslknn.rst

@@ -0,0 +1 @@
+Fix Windows REPL cursor desynchronization when a line exactly fills the terminal width.