We know that certain particles have physical displacement. We've seen them hit walls, and if a player moves through them, they'll bounce off the player. Is that something that's possible with the fog as well?
TL;DRCollision is possible for fog, but large particles hitting boundaries would look odd and give the game away, so collision is better kept for small things like dust motes, with thick fog using soft-particle fading.
Well, I think the larger the actual individual particle is, the more problematic it's going to be. We could make it do it, but it's going to look strange. You can see me moving my mouse around in the middle of the screen. If the particle was that big on the screen and it hit the boundary, it's just going to look weird. It's just going to give the game away, really. The player's just going to know that it's just a smoky texture on a big quad. So absolutely we can do it, but you've just got to be sensible with the things that are going to be used in collision. Maybe we just have dust motes that are colliding with the geometry rather than the big thick fog. The thick fog itself, we'd use the soft particle settings, which makes it fade away as it gets to the edge as well.