Here is a jsbin demonstrating the issue:

You might need to run it a couple times to see the issue clearly.

What I would expect is that when laying out compound nodes with bounds-expansion at the top, as you see in the demo, that not only would parent compound nodes avoid the expanded area, but also sibling nodes. In the demo, the areas marked in red should be vacant.

I need this because I want to draw a variable-height header atop each of my compound nodes.

Do I misunderstand the feature, or is it a bug?