Very dumb Arctic fox. Giving out free and open-source hugs. Also a GoldSRC modder, and a gamedev. (idTech 3, 4, Godot, Unigine)
The green lines are portal boundaries.
Basically, VIS is the 3rd part of the map compilation process, and it calculates what the engine should render, depending on what the player is looking at. :P
And when you add some complex geometry like that, VIS gives even more complex portals! Oh lord...
And this is pretty much the reason why it took 2h 48m 21s for VIS to complete processing my map. Lol.