messages.getForumTopics returned every chat via tgChannels -> tgChannelChatMin,
so the forum's own channel came back as a min object with left unset. A client
with no other object for that peer (a fresh account browsing a public forum by
username) then rendered the forum as already joined: topic list visible, no
Join button, but no messages.
Render the primary channel with tgChannelChatForView so a non-member preview
carries left=true; keep the other referenced channels as min.
