Thanks for the reply, Steven. No, there's no high CPU at the time. I don't think it's an off-screen issue as it happens when I'm using two screens and it's the only program I have running. It also sometimes works and sometimes doesn't - I still haven't found a predictable way to replicate it.
I also uninstalled MB and reinstalled the Portable version, as suggested by an earlier responder, which has resulted in more weirdness as the Edit Preferences dialog no longer appears and after right-clicking the Custom Browser, the program hangs again and I have to shut it down again using Task Manager. So I went back to the original Windows install and am still experiencing the original problem with the splitter buttons.
Are other people running it problem-free with Windows 11? It's a great program in so many ways that this is particularly frustrating - if I give up on it, there aren't a lot of viable alternatives!