custom treemodels and treeview padding

["Jonathon Jongsma" <jonathon jongsma gmail com>]

I've been working a bit on a custom treemodel based on standard C++
containers and I've got the basics working, but when I add it to a
treeview, I get some weird padding in the beginning of the column.
For example, see this screenshot:
http://download.gna.org/colorscheme/etc/customtreemodel/Screenshot-testcustomtreemodel.png

I tried running the example that's in the gtkmm source tree and the
first column also seems to have this extra padding, although the
subsequent columns don't:
http://download.gna.org/colorscheme/etc/customtreemodel/gtkmm-example.png

Does anybody have any suggestions for what might be causing this and
what I might try to fix it?  Or has anybody implemented a custom
treemodel where this behavior doesn't exist?

If you're interested in playing with the code, you can download it from here:
http://download.gna.org/colorscheme/etc/customtreemodel/customtreemodel-0.0.2.tar.gz

Any tips would be greatly appreciated.

Thanks,
Jonner