You would have to set both I6 and I7 as headers, or otherwise they would be displayed as children of Header2.

We could add a property that specifies that headers of empty sections should not display +/-, so you mark the last two rows as headers and get that result. Another possibility is instead of the bool Header property to use an enum property which could be set to SectionHeader, SectionClose, SectionRow(default).The first option will be easier to implement, hence you would get it sooner.