Superposing brush will do weird and confusing things, and is an easy mistake to do
Steps to reproduce:
- make a map you can open in game
- select a box
- press space to duplicate the brush
- build
game (Unvanquished), map built with novis for Unvanquished:
This leads to confusion, because pressing space is a really easy thing to do, and the problem seems pretty mysterious, see #160 (closed).
Why
I'm not a quake-things expert, but from https://www.gamers.org/dEngine/quake/QDP/QPrimer.html, about "BSP Building":
Merging of Planes
"qbsp" will handle two brushes that do not intersect, but merely touch. Contacting faces are merged out of existence. In case of a brush with zero thickness (planes are identical, normals have opposite directions): "I'm pretty sure that a brush that has no thickness in a given dimension would clip itself out of existance and not show up at all in the bsp file, but I'm not positive." The equivalent of a two-sided wall is a billboard object/entity, not a brush.
Here, the normals probably don't have opposite directions, but maybe there's something like this playing out.