I have been working on a new app that required more screen real estate than the phone provided. All my attempts to get a canvas inside a standard ScrollViewer ended in having the unusual effect that I could not scroll to the bottom or far right. As soon as I took my finger off the screen, the screen would bounce inwards. A little online search indicated that the problem was known, but was not repeatable. Since there are so many unknown variables in a XAML solution, I can not claim that this is the definitive answer.
The thing that helped resolve the issue was changing the background to something other than black. It is not easy to see the edges of a black shape on a black background. When I did this, I noticed several shapes that extended beyond the canvas and when I tried to scroll to the edge, it snapped back to the canvas that was now visible. I then made sure that I let the program reset the page size to include the wayward shapes. Now I have a WP7/8 app that has a canvas that can be bigger than the screen size and will scroll to the edge correctly.
Now on to finish the app and add it to my collection.
John Marshall… Visio MVP Visio.MVPs.org
john Visio MVP 
Great tip on changing the background color for testing! Nice, glag you figured it out!