ControlMode(test[encoding]): Reproduce non-UTF-8 stdout decoding

why: ControlMode opens its own tmux -C subprocess with text=True, so it has the same locale-decoding risk as tmux_cmd but through a separate stdout surface.
what:
- Add a strict xfail regression test for non-ASCII control protocol output under LC_CTYPE=C
- Drive display-message through the control client and assert FORMAT_SEPARATOR survives stdout decoding

Author: tony

tests/test_control_mode.py

@@ -2,9 +2,16 @@
 
 from __future__ import annotations
 
+import locale
+import os
+import select
+import sys
 import typing as t
 
+import pytest
+
 from libtmux._internal.control_mode import ControlMode
+from libtmux.formats import FORMAT_SEPARATOR
 
 if t.TYPE_CHECKING:
     from libtmux.server import Server
@@ -60,3 +67,41 @@ def test_control_mode_client_name_matches_spawned_client(
         assert first.client_name != second.client_name
         assert (str(first._proc.pid), first.client_name) in clients
         assert (str(second._proc.pid), second.client_name) in clients
+
+
+@pytest.mark.xfail(
+    reason=(
+        "ControlMode passes text=True without encoding='utf-8' to Popen; "
+        "on non-UTF-8 locales control protocol output is decoded with the "
+        "locale encoding. See: https://github.com/tmux-python/libtmux/issues/678"
+    ),
+    strict=True,
+)
+@pytest.mark.skipif(
+    sys.flags.utf8_mode != 0,
+    reason="PYTHONUTF8 mode forces UTF-8, masking the locale bug",
+)
+def test_control_mode_stdout_preserves_non_ascii_output(
+    control_mode: t.Callable[[], ControlMode],
+) -> None:
+    """Control-mode stdout must preserve non-ASCII tmux output."""
+    old_lc_ctype = locale.setlocale(locale.LC_CTYPE)
+    try:
+        locale.setlocale(locale.LC_CTYPE, "C")
+        with control_mode() as ctl:
+            os.write(
+                ctl._write_fd,
+                f"display-message -p '{FORMAT_SEPARATOR}'\n".encode(),
+            )
+
+            for _ in range(20):
+                ready, _, _ = select.select([ctl.stdout], [], [], 1)
+                assert ready, "timed out waiting for control-mode output"
+
+                line = ctl.stdout.readline()
+                if FORMAT_SEPARATOR in line:
+                    break
+            else:
+                pytest.fail("FORMAT_SEPARATOR U+241E not found in control output")
+    finally:
+        locale.setlocale(locale.LC_CTYPE, old_lc_ctype)