missing group boundary draw recalculation

If some nodes are grouped together, one can show their convex hull (boundary).
Nevertheless, it seems as if this convex hull is only updated on an irregular basis, e.g. moving a node from its inside does not automatically extend the convex hull (neither when done manually nor when done automagically by a layout algorithm).

Switching the selection tool suddenly seems to cause a recalculation of the boundary.

(Also uploaded a screencast showing the problem: http://www.youtube.com/watch?v=PWxmolXLV_U)