[css-position] Sticky pos "position box" should refer to border edge instead of margin edge?

[Loirooriol]

https://drafts.csswg.org/css-position/#position-box

The position box is its margin box, except that for any side for which the distance between its margin edge and the corresponding edge of its containing block is less than its corresponding margin, that distance is used in place of that margin.

This doesn't make much sense to me, should the distance be from the border edge instead?

CC @fantasai c66a57a

[Loirooriol]

Also, the distance is signed, i.e. if the border edge is already outside the containing block, that counts as a negative distance instead of taking the absolute value as usual for distances.