openHAB is reverse-proxied from a non-root directory.
As far as I’ve been able to determine having set up reverse proxies for OH and other services this cannot be done without changes to OH itself and the changes described in the linked Issue would not solve the problem.
If you reverse proxy as the root directory it works just fine.
The problem is the reverse proxy cannot add the non-root directory to URLs generated by the JavaScript in the web pages it is serving up. To support that OH would need to add a flag or somesuch to generate URLs and JavaScript that generates said URLs that include the non-root directory as part of the standard URLs. Of course, this would also mean that you can only access your OH through the reverse proxy. A change to jetty.xml would not fix this.
Based on what I’ve seen this would be non-trivial to support so I would expect it to be quite some time before it gets implemented. I would expect them to add authentication to OH 2 (thus eliminating the need for the reverse proxy entirely) before this gets addressed.