My browser auto-completed a 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" ? :-)