The top menu nav for sections is showing and hiding at different widths, and I think there should always be the nav showing:
Does this happen without overriding the CSS for Coda?
If it does, can you tell us what width that is where the menu jumps?
Yes it happens regardless of the override. How can I tell the width?
You can use the browsers developer tools or post a link to the doc here and we can check it out.