ChatGPT (confidently) told me to add this in to PAGE_CONTAINER
<xf:if is="$template == 'thread_view' && $forum">
<xf:title><xf:title /> | {$forum.Node.title}</xf:title>
</xf:if>
before
<title><xf:title formatter="%s | %s" fallback="{$xf.options.boardTitle}" page="{$pageNumber}" /></title>...
