ScrolltoverticalOffset not working properly

Question

I am showing a set of images in a vertical orientation using stackpanel inside a Scrollviewer. the code is working fine when i changed the orientation to horizontal.

  thumbnailViewerScroller1.InvalidateScrollInfo();
  thumbnailViewerScroller1.ScrollToVerticalOffset(-50.0);
  thumbnailViewerScroller1.UpdateLayout();
  thumbnailViewerScroller1.InvalidateScrollInfo();

used the above after doing google abt the ScrollToVerticalOffset not working and found couple of approaches suggested. they are also not working properly.

Please let me know i anyone had any insight into this problem