Very last record expansion is removed when scrolling

- load some expandable rows via ajax, so that a scrollbar appears
- scroll to the very last record (until no more dynamic loading occurs)
- expand the last record
- it is marked as expanded, but the expansion cannot be seen
- try to scroll it up
- the expansion vanishes, but the expansion marker still shows the record as expanded

This patch is a quick fix, which may need more throught.