You can have a look at the staging page to trigger it.
Steps to reproduce:
- Open the product page – see there is no white bar at the top
- Go to edit product page (may need to login first)
- Change something about the product (eg short description, one letter is enough) and save it
- Open the product view page again – see there is a white bar at the top
- Refresh the page – the white bar is gone
