gtk_tree_view_get_path_at_pos will fail if the TreeView's internal rbtree is not
first validated in it's size_request handler.  This can have bad consequences
when something happens causing invalidation (such as resetting of the model) at
an inopportune time before size_request is invoked.  

An example of this is when the model is (re)set after the first click of a
double click.  Any attempts to figure out the clicked row in order to activate
it in the second-click handler will fail, since the TreeView has not yet gone
through a size_request.

The current workaround is to call size_request after resetting the model, which
causes treeview to validate the first 500 records.  The rest are processed with
an idle handler.  I don't think there is a workaround for treeviews with >500
records.

I propose a corrolary for TreeView similar to gtk_ui_manager_ensure_update,
which would ensure that the internal tree has been validated.