On clicking release, we call TreeView.get_path_at_pos() &, if we hit a
row, select it (if sensitive) & close the popup. But this alone does
not account for clicks on the expanders within the TreeView, so in
addition to expanding/collapsing, clicking them would close the list.

Check if the click is in the cell_area() & thus “excluding surrounding
borders and the tree expander area” but still including the background
(which TreeView.is_blank_at_pos() doesn’t); if TRUE, don’t select/close.

The popup doesn’t always resize enough… so there’s still breakage here.
The XXX comment on TreeView requests in list_position() may be relevant
to this. But at least this drags such CBs one step closer to adequacy:
expanding by mouse now works ~no worse~ than by keyboard already did.

https://bugzilla.gnome.org/show_bug.cgi?id=788505