if you are talking about the outer rectangular border of the field of the processed photo, then it remains without a contour. I am using a 3 pixel contour setting with an EXTERNAL offset, the 3 pixel contour is shifted inside the photo from the selection line.

Yes, I hoped that the contour line was around the outer edge of the photo but now I understand it's around the non-transparent part (person) itself. A border between the transparent and non-transparent parts of the image. I'll have to figure out how to do that in GIMP.

That also clears up why the 3-pixel border: a 1-pixel curved border is not a closed boundary as you explained.