Currently when visiting a page in the CM preview, then the SiteMap will automatically open at the entry for the current page. However, with the new approach, the FE will not contain the entire SiteMap any more but just a part of it.

We need to discuss and figure out how to support that when we have partial SiteMap in the FE

Potential BE response:

/
  + child1
  + child2
      + subchild1
          + foo
      + subchild2
  + child3
  + child4
  + child5

FE needs to merge the above result in the SiteMap aleady loaded and keep the entries open which were already opened before.

Implementation details

When requesting for example

cms/_rp/{id}./sitemapitem/foo/bar/lux?ancestry=true

then the 'ancestry=true' with trigger all ancestor items of 'lux' to
be included in the result as well. All ancestors will also have their
siblings loaded such that the FE can draw the expanded tree to the
currently viewed page in one go