fix(tools): Revert snapshot_pane delimiter to tab, keep defensive padding

why: CI exposed that tmux's display-message output C-escapes non-tab
ASCII control characters. What I expected to be a single 0x1f byte
arrived as the literal 4-character string "\037", producing a single
joined parts[0] value that contained every embedded "\037". The
first int() conversion then blew up with:

    ValueError: invalid literal for int() with base 10:
      '0\\0370\\03780\\03724\\0370\\037...'

Six of seven CI tmux versions (3.2a, 3.3a, 3.4, 3.5, 3.6, master)
escape on output; local tmux 3.6a does not. The "defense in depth"
delimiter choice in the earlier snapshot_pane hardening commit
traded a rare hazard (tabs in pane_title, which tmux's `select-pane
-T` silently rejects anyway) for a guaranteed one (control-char
escaping in display-message output) and broke every CI matrix run.

The defensive padding remains the genuinely load-bearing part of
the earlier hardening and is retained verbatim.

what:
- Switch _SEP in snapshot_pane from "\x1f" back to "\t".
- Keep the defensive `(raw.split(_SEP) + [""] * 11)[:11]` padding
  so snapshot_pane still degrades gracefully when tmux emits fewer
  format fields than expected.
- Update the inline comment to explain the real constraint: tabs
  survive display-message verbatim, other control chars do not,
  and tmux rejects tabs in pane_title so the tab-in-title risk is
  purely theoretical.
- Update the monkeypatched fake_cmd in
  test_snapshot_pane_pads_short_display_message_output to split /
  rejoin on "\t" to match the new delimiter.

Verified locally: all four tests that failed on CI
(test_snapshot_pane, test_snapshot_pane_cursor_moves,
test_snapshot_pane_pads_short_display_message_output,
test_enter_and_exit_copy_mode) now pass.

Author: tony

src/libtmux_mcp/tools/pane_tools.py

@@ -670,11 +670,17 @@ def snapshot_pane(
         window_id=window_id,
     )
 
-    # Fetch all metadata in a single display-message call. Use the ASCII
-    # Unit Separator (0x1f) as the field delimiter — it cannot appear in
-    # normal terminal titles or paths, so tabs/newlines embedded in
-    # pane_title or pane_current_path can't shift field indices.
-    _SEP = "\x1f"
+    # Fetch all metadata in a single display-message call. Use a tab as
+    # the delimiter: tmux passes tabs through verbatim in
+    # display-message output, whereas other ASCII control characters
+    # (e.g. 0x1f / Unit Separator) get C-escaped to literal "\037"
+    # strings on tmux >=3.2 / <3.6-rc, which corrupts parsing. Tabs in
+    # pane_title are silently rejected by tmux's `select-pane -T`
+    # input sanitizer, so the `\t` delimiter is safe against that
+    # vector. Tabs in pane_current_path are legal on Linux but
+    # vanishingly rare; the defensive padding below limits the blast
+    # radius to a single truncated field rather than an IndexError.
+    _SEP = "\t"
     fmt = _SEP.join(
         [
             "#{cursor_x}",

tests/test_pane_tools.py

@@ -584,8 +584,8 @@ def fake_cmd(self, cmd_name, *args, **kwargs):  # type: ignore[no-untyped-def]
             # simulate an old tmux that dropped several unknown format
             # variables. Without defensive padding, parts[2..10] would
             # IndexError.
-            parts = result.stdout[0].split("\x1f") if result.stdout else [""]
-            result.stdout = ["\x1f".join(parts[:2])]
+            parts = result.stdout[0].split("\t") if result.stdout else [""]
+            result.stdout = ["\t".join(parts[:2])]
         return result
 
     monkeypatch.setattr(mcp_pane.__class__, "cmd", fake_cmd)