RStudio 2023.06.0 Build 421

Since RStudio version 2022.12.0 Build 353: The upper navigation bar covers the title of the page.
The html is shown correctly in the window which opens after knitting but not online: See matstat.org (https://matstat.org/index.html).

As this is showing inside you website, outside of the IDE, I would think this is an issue R Markdown directly.

I believe this issue has been fixed:

It is available in v2.15.

Can you check rmarkdown version you are using in your project ?

If this is still happening with latest versions of tools, can you open a new issue in R Markdown repo ?

Thank you