At some point, I rebooted my machine and the problem went away. Since nobody else seemed to have the problem, I deleted the post. But I've finally figured out what caused it.
It wasn't TextPad, but a utility called KatMouse. If you haven't heard of it, you should check it out. It lets you scroll windows without first having to give them focus. So, for example, in Windows Explorer you can scroll the folder tree and the file list without having to click on them - very handy! BUT, it causes scrolling not to work in TextPad! Fortunately, it's easy to exclude TextPad from being handled, in the KatMouse settings dialog, so make sure you do that if you use it.
Andrew