I was thinking of what are the purpose of some of the tools texture Troika used ingame.
I saw this texture many times and can't find out what it is for: NO VIZ and VIS ONLY.
NO VIZ is used inside the map but for the whole 3d skybox too (above the map).
Could it be that it's used to seal the view from the world to the skybox and vice versa? So that you don't see the world from the skybox in the world?
And what is VIS ONLY for?
Second, the world brushes/models are rendered ontop of the skybox brushes/models, when the world brushes are behind it.
In Hammer:
In-game rendering:
As you can see, the brushes from the world are overlapping, being on top of the skybox brushes.
There would be a workaround for this, but it would mean that all the skybox brushes which have this issue need to be put inside the world map, which would take more space in the map.