I've quit and restarted kmail several times and the problem persisted. But this morning I did a reboot and now I can scroll up so that the currently selected message is well out of view, and my view will stay, not jump back to that message.
I think kmail must have been caught in some kind of loop where it was constantly re-entering the folder and so it would jump to the last selected message. And this didn't go away after a quit and restart. So some other process must be responsible. I'm using the groupwise plugin, I'll look at that first.
I'm still curious if anyone else has experienced this though. I'd like to find a root cause.
Rob