Some background about this: we intentionally included margins in bug 665597
and then reverted that partially in bug 724352 and eventually backed it out
in bug 750293 due to regressions. The part that did survive lives in
nsBlockFrame::ComputeFinalSize in the form of 'blockEndEdgeOfChildren'
which we propagate to ConsiderBlockEndEdgeOfChildren for inclusion into
the overflow area. So it appears that we only include the block-end margin,
but maybe only in some cases? - I haven't looked at the details yet...
I'll try to file a spec issue to sort this out unless there is one already.