I am running RStudio 1.1.381 with R 3.4.1 on MacOS 10.12.6
RStudio includes pandoc 188.8.131.52 which seems to be the latest version.
The content popup menu in the left corner of the status bar should dis-lay the titles of the sides
to be able to navigate there, but it only displays the main title (once for each slide).
I had this problem previously when I had the homebrew version of pandoc installed. At that time, RStudio included an earlier version of pandoc and just using the RStudio version (by deleting the homebrew version) solve the problem. Now, since RStudio also has the latest version, the problem surfaced again.
In my case, just creating a new slidy presentation and running the demo appearing immediately shows the problem.