@Ben: Did you get the source code of the demo? I wanted to have a look at the ContentFrame sample...
I've seen a strange behaviour though for which I cannot find a reason...
I'm using another set of images and I'm resizing the Image element in the ItemTemplate of the Listbox.
When I create the VisualBrush, it looks rescaled to some weird values.
I have the same strange behaviour if I leave the Image size in the ItemTemplate of the listbox to Auto and then resize the ListBox itself.
If I leave both the Image in the ItemTemplate and the ListBox sizes to Auto, then everything works as in your video...
Can you tell me what I'm doing wrong?