Here's an example using your own Canvas as scroll bar. I used a Ball as a cursor, but you could as well have drawn a bar as a thumb instead.
Gallery entry: ai2.appinventor.mit.edu/?galleryId=4965345777025024
Uploading: when any Button Click.png... Uploading: when btnEnd Click.png... Uploading: when btnstart Click.png... Uploading: when Canvas1 Dragged.png... Uploading: when File1 GotText.png... Uploading: when Screen1 Initialize.png...
