void gtk_widget_get_preferred_size ( GtkWidget* widget, GtkRequisition* minimum_size, GtkRequisition* natural_size )
Retrieves the minimum and natural size of a widget, taking into account the widget’s preference for height-for-width management.
This is used to retrieve a suitable size by container widgets which do
not impose any restrictions on the child placement. It can be used
to deduce toplevel window and menu sizes as well as child widgets in
free-form containers such as
Handle with care. Note that the natural height of a height-for-width widget will generally be a smaller size than the minimum height, since the required height for the natural width is generally smaller than the required height for the minimum width.
gtk_widget_measure() if you want to support baseline alignment.