Think I know what the scrollbar problem offset problem is. It’s to do with the discrepancy between the size of the gadget, the size of its client rect, and the size of the scrolling canvas. Currently working on discovering exactly how all of these things should relate.
In other news, I’ve tried another SVN client - SmartSVN. So far, this is my favourite OSX SVN client. Unlike SCPlugin, it works properly with Leopard. It also gives you some feedback on what it’s doing, which is a massive improvement over Xcode’s client.