This looks/works fine, but the empty state is always rendered, even if the user has joined a room -- and this makes it much more expensive to render, since we unconditionally load and render 10 threads. In my sandbox, this makes loading a /Z123 thread page (as a fully-active user who has already joined a bunch of rooms) about 15% slower.

I think we can fix this like this, by only building the expensive NUX if $this->thread is not set:

$results = array();
if (!$this->thread) {
  // If we aren't looking at a thread...
  $results = <all the query logic>;
}

if ($results) {
  // render "join a room NUX"
} else {
  // render "Welcome" NUX
}

Slightly simpler would probably be adding this at the top:

if ($this->thread) {
  // User is looking at a thread, so we definitely aren't going to render NUX.
  return null;
}

But I'm not sure if that causes any issues.