This issue is also affecting the admin sidebar.
It looks like it has been like this since: UX: Sections not collapsable in "header dropdown" navigation menu by tgxworld · Pull Request #21604 · discourse/discourse · GitHub
The problem is that we’re forcing the sections to be collapsable in mobile view or narrow screens when the navigation menu is set to header dropdown, but we’re not handling the case where it is set to sidebar.
To address the issue, I believe we should force the sections to always be collapsable when in mobile mode or on narrow screens.
The PR below should fix this: