tests(textframe): Add shutil terminal size detection test

why: Verify display() uses shutil.get_terminal_size() for resize
what:
- Add test_terminal_resize_via_shutil test
- Mock shutil.get_terminal_size to verify it's called

Author: tony

tests/textframe/test_display.py

@@ -137,3 +137,27 @@ def test_status_line_displayed(self, mock_curses: None) -> None:
             if len(call[0]) >= 3 and "q:quit" in str(call[0][2])
         ]
         assert len(status_calls) > 0, "Status line should be displayed"
+
+    def test_terminal_resize_via_shutil(self, mock_curses: None) -> None:
+        """Verify terminal size is queried via shutil.get_terminal_size().
+
+        This approach works reliably in tmux/multiplexers because it directly
+        queries the terminal via ioctl(TIOCGWINSZ) on each loop iteration,
+        rather than relying on curses KEY_RESIZE events.
+        """
+        import os
+
+        frame = TextFrame(content_width=10, content_height=2)
+        frame.set_content(["hello", "world"])
+
+        mock_stdscr = MagicMock()
+        mock_stdscr.getch.return_value = ord("q")
+
+        # Mock shutil.get_terminal_size to return specific dimensions
+        with patch(
+            "libtmux.textframe.core.shutil.get_terminal_size",
+            return_value=os.terminal_size((120, 40)),
+        ) as mock_get_size:
+            frame._curses_display(mock_stdscr)
+            # Verify shutil.get_terminal_size was called
+            mock_get_size.assert_called()