Size of GoSubGraph

What should I do to make borders of GoSubGraph invariable while moving objects inside GoSubGraph, i.e. GoSubGraph has fixed size and the objects inside could be moved within these fixed borders (not exceeding them).
I would like to change the size of GoSubGraph only with the Handlers located on its border.

I suppose you could override GoSubGraph.LayoutChildren to limit the sizes and positions of the children. You’ll need to handle the case when an object can’t fit.
Alternatively, you could override GoObject.ComputeMove or DoMove to impose such a limitation on how the user drags objects around.