How do I slide the red line area

Hello Jinhua,
To scroll the grid, you need to include the scrollbar in the layout configuration:
https://docs.dhtmlx.com/gantt/desktop__layout_config.html#scrollbar
Here are examples with the layout configuration that allows scrolling the grid:
https://snippet.dhtmlx.com/5/05289e817
http://snippet.dhtmlx.com/5/832a24fd1