Hello,
The 1.39.0-wmf.39 is blocked, immediately after deploying it to group0 wikis, Flow topics ended up being error pages.
When browsing a topic page, there are two issues bubbling up in logstash:
From VectorTemplate->buildSidebar()
* /srv/mediawiki/php-1.35.0-wmf.39/includes/language/Message.php:261 $key must be a string or an array
From Skin->buildSidebar / ElectronPdfServiceHooks::onSidebarBeforeOutput
* /srv/mediawiki/php-1.35.0-wmf.39/extensions/ElectronPdfService/src/ElectronPdfServiceHooks.php:62 PHP Notice: Undefined index: print
Which as I understand it causes an exception.
The breaking bug is: https://phabricator.wikimedia.org/T256761