It is very interesting extension.
But I made a lot of staff in normal grid (grid summary, group summary).
I am sure I will have errors while combining TreeGrid with grid summary and espacially group summary.
I give up only one selection. I mean I plan to put different backgound color (different than normal selection) for groupheader selection and user can also select row.
Actually, I want when user click a row, its group header background color will changed. If user click another row, previous selection's group header backgorund becomes white.