Hey @Jun Great question and welcome to the forum 
At the time I built that one you needed custom CSS code but not any more.
I’ve updated the project removed the custom CSS code so it’s now all Webflow.
The trick is here:
Here’s the read only if needed:
https://preview.webflow.com/preview/sidebar-scroll?utm_medium=preview_link&utm_source=designer&utm_content=sidebar-scroll&preview=033e2e6af74e4dc44516632728f7acbb&mode=preview
Let me know if this was helpful or not.
Kind regards,
Jörn 