My browser auto-completed a [[ https://www.mediawiki.org/w/index.php?title=Topic:Srraiyf15fb22qil&action=edit-post&topic_postId=ss3mkog2gioqeqgh&topic_revId=ss3n8wn56jnhrssi#flow-post-ss3mkog2gioqeqgh | complicated link to a post ]] (possibly from a no-JavaScript reply to post). When I edited the URL to remove the `action=edit-post`, Flow showed the topic contents in the topic titlebar area (!).
I boiled it down to https://www.mediawiki.org/wiki/Topic:Srraiyf15fb22qil?topic_postId=ss3mkog2gioqeqgh
it seems the `topic_postId` overrides the topic display.
I can show a post from a different topic, https://www.mediawiki.org/wiki/Topic:Srraiyf15fb22qil?topic_postId=sry1yn5r8lo4upmh. I wasn't able to load a post from a different wiki, got ''Due to a technical error, this post could not be retrieved.''
"Don't do that" ? :-)