Site-resolution in the PageEditor doesn't work. For all pages "default" site will be selected, thus returning wrong I18nContentSupport for sites that might have been specified in MultiSite.
Seems to be related to the AggregationState which is not "aware" of the site anymore, as the PageEditor uses the one from Vaadin backend rather than the one from the page itself.
If a custom site specified in MultiSite (e.g. demo-project) has different i18n settings, these will NOT be properly displayed in the LanguageSelector of the PageEditor if an admin navigates to that "site". Consequently content elements won't be editable in all languages specified.
In 4.5 the PageEditor used the same AggregationState than the rendered page, thus site resolution would work. This is not the case anymore.