For simplfiy my problem, I created a meaningless sequence diagram.
Here is an Interaction Use inner a combined fragment.
Before I moving the combined fragment to lower place, the diagram looks like attachment 1.
After I doing so, the diagram looks like attachment 2.
I think that the Interaction Use (Sequence Diagram 2) should be moved down automatically while I’m moving its surrounding Combined Fragment, but it didn’t.
It’s inconvenient when I want to add some messages before a combined fragment.
After I adding messages before a combined fragment, the combined fragment will be pushed down automatically.
But the Interaction Use inner it didn’t.