docs: protected String getCommonSite(Collection<CmsResource> resourceSet) {
  
          String commonPath = getCommonAncestorPath(resourceSet);
          String result = null;
          CmsSiteManagerImpl siteManager = OpenCms.getSiteManager();
          if (siteManager.startsWithShared(commonPath)) {
              result = siteManager.getSharedFolder();
          } else {
              String siteRoot = siteManager.getSiteRoot(CmsStringUtil.joinPaths(commonPath, "/"));
              if (siteRoot == null) {
                  result = "/";
              } else {
                  result = siteRoot;
              }
          }
          return result;
  
      }