The new bubble blocks (sold by the party girl) are a valid wall and spawn block for NPC houses. I thought I finally had a block that could be used to separate NPC houses and act as a spawn block without blocking my projectiles. However, soon after setting an NPC to live in a house which incorporates bubbles, they will leave the house. They never stay assigned to their houses for long. After teleporting away and then back, I find all the houses unassigned again.