Just a quick note to ask for feedback.
I've completely reimplemented the j, k commands and they no longer use the built-in 'move by lines' command. As a result, they should be faster and more accurate. Although they seem to work, they've been quite tricky to implement and it's highly likely that I've messed up something.
This change is experimental. If it doesn't work well, I'll revert to the old system.
Anyway, let me know if it works for you.