Session(fix[detach_client]): preserve client targeting semantics

why: tmux detach-client treats -s before -t, so always forcing a session target
detached every client on the session instead of the requested client.
what:
- remove the unconditional session target from Session.detach_client
- clarify all_clients plus target_client behavior in the docstring
- add regressions for default and explicit target detach behavior

Author: tony

src/libtmux/session.py

@@ -272,7 +272,8 @@ def detach_client(
             Target client to detach (``-t`` flag). If omitted, detaches
             the most recently active client.
         all_clients : bool, optional
-            Detach all clients attached to this session (``-a`` flag).
+            Detach all clients attached to this session (``-a`` flag). If
+            combined with ``target_client``, tmux keeps that client attached.
         shell_command : str, optional
             Run a shell command after detaching (``-E`` flag).
 
@@ -292,9 +293,6 @@ def detach_client(
         if target_client is not None:
             tmux_args += ("-t", target_client)
 
-        # Use -s for session targeting (not -t, which targets clients)
-        tmux_args += ("-s", str(self.session_id))
-
         proc = self.server.cmd("detach-client", *tmux_args)
 
         if proc.stderr:

tests/test_session.py

@@ -595,6 +595,46 @@ def test_detach_client(
         assert after == before - 1
 
 
+def test_detach_client_only_detaches_one_client(
+    control_mode: t.Callable[..., t.Any],
+    session: Session,
+    server: Server,
+) -> None:
+    """Test Session.detach_client() without a target detaches one client."""
+    with control_mode(), control_mode():
+        before = server.cmd("list-clients", "-F", "#{client_name}").stdout
+        assert len(before) == 2
+
+        session.detach_client()
+
+        after = server.cmd("list-clients", "-F", "#{client_name}").stdout
+        assert len(after) == 1
+        assert set(after) < set(before)
+
+
+def test_detach_client_target_client(
+    control_mode: t.Callable[..., t.Any],
+    session: Session,
+    server: Server,
+) -> None:
+    """Test Session.detach_client() detaches only the requested client."""
+    with control_mode(), control_mode():
+        clients = server.cmd("list-clients", "-F", "#{client_name}").stdout
+        assert len(clients) == 2
+
+        target_client = clients[-1]
+        session.detach_client(target_client=target_client)
+
+        remaining_clients = server.cmd(
+            "list-clients",
+            "-F",
+            "#{client_name}",
+        ).stdout
+        assert remaining_clients == [
+            client for client in clients if client != target_client
+        ]
+
+
 def test_last_window(session: Session) -> None:
     """Test Session.last_window() selects previous window."""
     w1 = session.new_window(window_name="last_a", attach=True)