There's no recognition of her edging off the topic, no visual tell if it even bothered him in the first place, but at least the joke earns a raised eyebrow. A conscious effort compared with the slightly twisted tone into something resembling amusement, just so she'll be able to pick it up as a joke.

"I'll make a doghouse for you to possess. Write 'Ghost Girl' on the top of it." He's bad at people, so there's no real knowledge if or when he takes a joke too far and it impales the other person. So he instinctively reigns it back a second after, "Maybe a bookshelf."