Part of the URI standard:

      return a string consisting of the reference's path component
      appended to all but the last segment of the base URI's path (i.e.,
      excluding any characters after the right-most "/" in the base URI
      path, or excluding the entire base URI path if it does not contain
      any "/" characters

https://datatracker.ietf.org/doc/html/rfc3986#section-5.2.3