okay, after deleting rule and reinstalling rules, I at least got a pop-up message that Libre is blocked. That's semi-normal behavior.
I unblocked it, and it opened in a separate desktop.
I allowed it, and it opens non-sandboxed.
Still can't hit that sweet spot of opening sandboxed on regular desktop.
by the way, how can you add attachments to a post, when the browser is sandboxed?