Another way to describe the situation in that last post above is that at the area where it looks like they are coming together here:




The shape of each surface actually comes from an internal part of the path.


Like this surface here at that point comes from this part of the rail:




And the other surface's shape at that point comes from this part of the rail on the other side of the corner:




And because those points on either side of the corner are not planar to one another but instead are kind of arcing and bending around in 3D (once they leave the shared corner spot), it means that there is not an even alignment between those pieces where it looks like they would exactly touch. I mean they do touch but not in a complete full intersection that divides each one into 2 distinct halves.

That's kind of fundamentally why mitering can't really work on things like this.

- Michael