Sidebar tree navigation not generated

[kilianvolmer]

I want to use the Furo layout for the documentation of a project that is mainly written in C++. I use doxygen and exhale to generate an API-documentation.
I want it to look like this on the sidebar:

but it just lists everything below each other without levels or classification.

However, if I switch the layout to the rtd-theme, it works as desired.

Is there a way to get the desired layout in Furo?