See attached application and attached movie;

Run attached application. Add form at 0 index ("Add form at pos" + 0 + Add!)
Try to scroll scrollBars in listView.
You can see a form, such as added in ListView in the bottom.
You can press button ("Show") to see the sizes of added form in the output.

You can change adding using uncommenting the next line:
//((ListView) testedControl).getItems().add(index, new Group(form));

there will be the same problem, but content will be moved down a bit.