Howto (on a libglade project):
1. Add a VBox, with two elements.
2. Add an HPaned to the first one and set its own separator's position to 200 pixels.
3. Add any other element to the second slot.
4. Try to fix their position on parent.

You would see that HPaned packed's position and separator's position are always the same (including the Glade3 gui tooltips on their respective GTKSpinButtons).

The only workaround I've found is setting the second element's pack position to a higher value to hpaned separator position, but specifying any position on its parent to 201, 500, or such high values makes me feel... strange.

Tested on Ubuntu 9.04, Glade 3.6.3.