Handle negative cursor rows with hidden entries

- Guard against negative display indexes when no entries are visible
- Add regression coverage for Textual's -1 cursor row

tests/test_main.py

@@ -593,6 +593,34 @@ class TestHostsManagerApp:
             app.details_handler.update_entry_details.assert_called_once()
             app.table_handler.display_index_to_actual_index.assert_called_once_with(2)
 
+    def test_data_table_row_highlighted_with_no_visible_entries(self):
+        """Handle Textual's negative cursor row when all entries are hidden."""
+        mock_parser = Mock(spec=HostsParser)
+        mock_config = Mock(spec=Config)
+        mock_config.should_show_default_entries.return_value = False
+        mock_config.is_default_entry.return_value = True
+
+        with (
+            patch("hosts.tui.app.HostsParser", return_value=mock_parser),
+            patch("hosts.tui.app.Config", return_value=mock_config),
+        ):
+            app = HostsManagerApp()
+            app.hosts_file = HostsFile(
+                entries=[HostEntry(ip_address="127.0.0.1", hostnames=["localhost"])]
+            )
+            app.details_handler.update_entry_details = Mock()
+
+            mock_table = Mock()
+            mock_table.id = "entries-table"
+            event = Mock()
+            event.data_table = mock_table
+            event.cursor_row = -1
+
+            app.on_data_table_row_highlighted(event)
+
+            assert app.selected_entry_index == 0
+            app.details_handler.update_entry_details.assert_called_once()
+
     def test_data_table_header_selected_ip_column(self):
         """Test DataTable header selection for IP column."""
         mock_parser = Mock(spec=HostsParser)