I've seen a few people complaining about this and saw it on my own maps that brush hitboxes act very weird when you get under them, like with bridges It seems to act like the hitbox of the brush (or at least the bottom faces) extends all the way down to the center of the planet and units can only shoot other units if they're in the same "face" as them, with the invisible edges or the face acting like walls even though units can drive through no problem. It's a similar issue from brushes set to not be pathable, the entire area under the brush is also not pathable along with the brush itself, and any brush, pathable or not stops naval movement is there a reason for them to act like this? because it seems pretty uneccessary
