It’s default settings. There is a space between the roof of the cube and the smoke.
On another note, I used append, and copied the objects from the provided .blend file, into a different .blend scene. I put the emitter inside of a building, with a plane roof and floor and an open window. The emitter didn’t generate any smoke. I did Free Bake and then Bake again. I know it’s the EXACT same settings since I used append. It’s confusing man. Maybe there isn’t enough space, since it generates smoke from a large area surrounding the emitter mesh, not just the emitter mesh. Is there a way to make it only emit from the mesh and not a huge radius around the mesh?