CM visual bug: nested stage w/child hides other children

To recreate:

1. add two stages as top-level parents to the diagram
2. add a user task to the 1st stage
3. add a subprocess to the 2nd stage
4. drag the second stage onto the first stage so that it becomes a child
5. the user task child of stage 1 is now hidden behind the child stage's subprocess
6. move the 2nd stage back out to a top-level node to observe that the user task child is still present on the 1st stage, just hidden