fix(cli): stop OSC 11 bg probe from trapping users in a stray editor (#35441)

Over SSH the OSC 11 background-color query round-trip routinely exceeds
the 100ms read budget, so _query_osc11_background() gives up and the late
reply lands after prompt_toolkit has grabbed the tty. prompt_toolkit then
injects the OSC payload as typed text and reads its BEL terminator
(\x07 = Ctrl+G) as a keystroke — Ctrl+G is the open-external-editor
binding, dropping the user into vi with garbage and no obvious way out.

- Skip the OSC 11 probe on remote sessions (SSH_CONNECTION/CLIENT/TTY);
  fall back to COLORFGBG / env hints / the dark default.
- Restore the tty with TCSAFLUSH instead of TCSANOW so any partial/late
  reply is scrubbed from the input buffer before pt reads it.

cli.py

@@ -1542,9 +1542,17 @@ def _query_osc11_background() -> str | None:
     Most modern terminals reply with \x1b]11;rgb:RRRR/GGGG/BBBB\x1b\\
     within a few ms.  We wait up to 100ms total before giving up.
     Returns "#RRGGBB" or None on timeout / non-tty.
+
+    Skipped over SSH: the round-trip routinely exceeds our 100ms budget, so a
+    late reply lands after prompt_toolkit has grabbed the tty — its payload
+    leaks in as typed text and the BEL terminator reads as Ctrl+G (open
+    editor), trapping the user in a stray editor. Remote sessions fall back to
+    COLORFGBG / env hints / the dark default instead.
     """
     if not sys.stdin.isatty() or not sys.stdout.isatty():
         return None
+    if any(os.environ.get(v) for v in ("SSH_CONNECTION", "SSH_CLIENT", "SSH_TTY")):
+        return None
     try:
         import termios
         import tty
@@ -1592,8 +1600,11 @@ def _query_osc11_background() -> str | None:
         r, g, b = norm(m.group(1)), norm(m.group(2)), norm(m.group(3))
         return f"#{r:02X}{g:02X}{b:02X}"
     finally:
+        # TCSAFLUSH discards any unread input as it restores the original
+        # attributes — scrubs a slow/partial OSC 11 reply out of the tty
+        # buffer before prompt_toolkit can read it as keystrokes.
         try:
-            termios.tcsetattr(fd, termios.TCSANOW, old)
+            termios.tcsetattr(fd, termios.TCSAFLUSH, old)
         except Exception:
             pass
 

tests/cli/test_cli_light_mode.py

@@ -75,6 +75,27 @@ class TestLightModeDetection:
         assert cli_mod._detect_light_mode() is True
 
 
+class TestOsc11Probe:
+    """The OSC 11 background probe must never run where its reply can leak
+    into prompt_toolkit's input (a late BEL-terminated reply reads as Ctrl+G
+    = open-editor, trapping the user in a stray editor). Guard the cases we
+    refuse to probe in.
+    """
+
+    @pytest.mark.parametrize("var", ("SSH_CONNECTION", "SSH_CLIENT", "SSH_TTY"))
+    def test_skips_over_ssh(self, cli_mod, monkeypatch, var):
+        monkeypatch.setattr(cli_mod.sys.stdin, "isatty", lambda: True, raising=False)
+        monkeypatch.setattr(cli_mod.sys.stdout, "isatty", lambda: True, raising=False)
+        for v in ("SSH_CONNECTION", "SSH_CLIENT", "SSH_TTY"):
+            monkeypatch.delenv(v, raising=False)
+        monkeypatch.setenv(var, "1.2.3.4 5555 22")
+        assert cli_mod._query_osc11_background() is None
+
+    def test_skips_when_not_a_tty(self, cli_mod, monkeypatch):
+        monkeypatch.setattr(cli_mod.sys.stdin, "isatty", lambda: False, raising=False)
+        assert cli_mod._query_osc11_background() is None
+
+
 class TestLightModeRemap:
     def test_remap_no_op_in_dark_mode(self, cli_mod, monkeypatch):
         monkeypatch.setenv("HERMES_LIGHT", "0")
@@ -133,7 +154,9 @@ class TestSkinConfigHook:
         after = SkinConfig.get_color
         assert before is after
 
-    def test_skin_color_remaps_through_wrapper_in_light_mode(self, cli_mod, monkeypatch):
+    def test_skin_color_remaps_through_wrapper_in_light_mode(
+        self, cli_mod, monkeypatch
+    ):
         from hermes_cli.skin_engine import SkinConfig
 
         cli_mod._LIGHT_MODE_CACHE = True