Thank you for the demo guys.
I am reusing it now in my project and I have noticed that the thread page doesn’t check if the user is a participant or not.

Adding below snippet to MessagesController.php#L60 will redirect "non participants" back to the listing page with an error message.

$participant = Participant::where(['user_id' => Auth::id(), 'thread_id' => $id ]);
if( is_null($participant->first())) {
 \Session::flash('error_message', 'Only For Participants');
 return redirect()->back();
}