FixedHeader column width not calculating properly with multi column headers

Here is a minimal example, note the addition of a multi column header: http://live.datatables.net/ladekogo/1

If you expand these rows to take up enough space to add a vertical scrollbar to the page, and scroll down, FixedHeader does not calculate the proper column widths and the column headers resize as a result.

How can I fix this?

I am also having a problem where the fixedheader width is incorrect if the datatable initially takes up less than 100% of the width of the page and a scrollbar is then added, but I've solved that for now by calling table.fixedHeader.adjust() on a fast enough timer that it doesn't really matter to the end user. Is there a correct solution?

Thanks!