I made a window frame for a map I’m working on and it used quite a few brushes, so I thought “why not give propper a try?”.
I got it working and made a glassless window fine. I made my own collision for it using playerclip and it worked fine.
I then tried to make an almost identical glassless window, using an identical method.
After compiling, and putting it into my map, I found that the collision covered the whole thing, making the glass (func_breakable_surf) be protected by the collision.