Threads not scrollable in Messages app in Mavericks

I used the Messages app on my Mac quite a lot, but ever since I upgraded to Mavericks, I've been unable to scroll inside of a message thread. No scroll bars appear when I use the Magic Mouse scroll function. I did discover that I can use the up and down arrows on my computer, but it's a bit of a kludge - scroll bars still don't appear, even though scrolling is occurring.

Has anyone else experienced this? Found a fix?