Pannable ScrollPane behaves badly when content changes during panning

In my app I need pannable ScrollPane with content that gets added gradually as it is loaded from internet. Unfortunately when some content is added to the ScrollPane it jumps to some other location and it looks ugly.

1. Run the attached sample,
2. Press "Z" key several times to get random content to fill twice the window height.
3. Scroll the pane down and start to pan it.
4. While panning continue to press "Z" key to add more content.
RESULT: ScrollPane content jumps up and down ugly.