If documentation theme is disabled (yes, it's a system plugin), some features are lost because they are not implemented in the core!

• Quicksearch

This will break the https://wiki.example.com/rest/quicknav/1/search?query=test URL => 404

Please assure, that documentation theme is optional!

Also the

• Sidebar-Splitter

Code should be in general instance an aui item at top should be available for default-theme too.
The "expand-collapse-trigger" is not working the same way, neither is the splitter code available for custom themes providing a sidebar!