Various fixes which have not yet been applied to 2.1 or 2.2
There is a configuration option Stores > Configuration > Catalog > Catalog > Category Top Navigation > Maximal Depth in the admin which limits the depth of the top Navigation. It is currently not being used when the topnav is built.
This extension applies the fixes which have been merged into 2.3-develop branch: magento/magento2#12640