Created attachment 135796 [details]
testcase

Turns out it's just not implemented. Shouldn't be too hard, just a bit of fiddling.

Bug 561821 seems to have the patch for this.

Created attachment 135802 [details] [review]
patch

http://git.collabora.co.uk/?p=user/davyd/gtk%2B-davyd.git;a=shortlog;h=refs/heads/button-box-orientable-584598

Typically, this arrives just as I finish my patch.

Probably should have checked for dupes a little harder.

Patch looks fine to me, as far as functionality is concerned. 

There's some stylistic issues:

+  if (orientation == GTK_ORIENTATION_HORIZONTAL)
+  {
+    return gtk_hbutton_box_get_layout_default ();
+  }
+  else
+  {
+    return gtk_vbutton_box_get_layout_default ();
+  }


no {} around single statements in if/else, ie write

  if (orientation == GTK_ORIENTATION_HORIZONTAL)
    return gtk_hbutton_box_get_layout_default ();
  else
    return gtk_vbutton_box_get_layout_default ();


+  if (nvis_children == 0)
+  {
+    requisition->width = 0;
+    requisition->height = 0;
+  }
+  else
+  {

We use a base indent of 2, like this:

 if (nvis_children == 0)
   {
     requisition->width = 0;
     requisition->height = 0;
   }
 else
   {
 

Created attachment 136592 [details] [review]
updated patch

(Includes testcase).

http://git.collabora.co.uk/?p=user/davyd/gtk%2B-davyd.git;a=shortlog;h=refs/heads/button-box-orientable-584598

Looks good now, please commit.

Done.

*** Bug 561821 has been marked as a duplicate of this bug. ***