Currently, the expander_column refers to the offset of a displayed column
view. When you dnd that column, expander arrows don't follow it.

The question here is: is it more likely that people want expander arrows
attached to a specific column, following it when dnd'ed, or location?

The first option appears more logical to me.

If one wants to have "fixed" expander arrows, it's likely to be only in the
left side (or right side if direction is RTL, and when TreeView will
support it).

In the rare cases (if any ; I can't find any reason for this) in which one
will want to have expander arrows attached to a specific location, not on
the left (or right) side and not following dnd, he'll still be able to
change the expander_column on columns_changed event.

A previous patch (see
http://mail.gnome.org/archives/gtk-devel-list/2001-June/msg00023.html) can
serve as a basis.