https://www.w3.org/TR/css3-positioning/#sticky-pos states that the sticky-constraint rectangle is computed based on the containing block of the sticky positioned element, however, since a sticky position element can be inline the containing block can be an anonymous block. I believe constraining to the anonymous block would be unintuitive to a site author as it doesn't correspond to an actual element on their site. 

Firefox and safari's sticky implementations both seem to skip anonymous containing blocks when computing the sticky constraining rectangle as can be seen on http://flackr.github.io/compositor-worker/sticky/sticky.html. I think the spec should clarify that anonymous containing blocks are skipped.