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@Latino210: There's a max wall (or floor) thickness a shot will go through: 8, so a whole solid level is much more than enough to stop anything shooting through roofs.
It will not. This is a bug. You can only do a workaround by removing the func_door entities from the map.
I've tried removing all func_door from the tile, but the weird shadows are still there. Do you think that I should remove all doors from all tiles? Well, it's not a severe bug anyways, it's just some weird graphic effect.