the get_preferred_width/height functions in ShellGenericContainer seem to only be half adjusting for theme borders.

===
static void
shell_generic_container_get_preferred_width (ClutterActor *actor,
                                             gfloat        for_height,
                                             gfloat       *min_width_p,
                                             gfloat       *natural_width_p)
{
  ShellGenericContainerAllocation *alloc = g_slice_new0 (ShellGenericContainerAllocation);
  StThemeNode *theme_node = st_widget_get_theme_node (ST_WIDGET (actor));

  st_theme_node_adjust_for_height (theme_node, &for_height);

  alloc->_refcount = 1;
  g_signal_emit (G_OBJECT (actor), shell_generic_container_signals[GET_PREFERRED_WIDTH], 0,
                 for_height, alloc);
  if (min_width_p)
    *min_width_p = alloc->min_size;
  if (natural_width_p)
    *natural_width_p = alloc->natural_size;
  shell_generic_container_allocation_unref (alloc);
}
===