I need to keep the top of a footnote span (or div) positioned to a
specific line in a multi-column layout, so that if the height of the
footnote changes, the span can be moved up or down  to that line,
enabling the content after the footnote to flow  to the next column.

Is there any event which would detect when an inline span, or a div,
has its offsetTop position in the document change as a result of any
editing operation (via keyboard or mouse). I'm thinking some kind of
mutation event or something in xpcom.