:thread-subject formatter hides subject if start of thread is outside of window..
It would be great if it worked like this:
If the thread starts above the first visible line, make sure that the subject is printed at the first visible line when scrolling.
It could maybe use text properties for hide/unhide so it would be less slow to update. I'm sorry but I haven't looked at the current implementation so take these last words very lightly.
Indeed - that's a know limitation of the current approach. Should take a look at that some time...
I can't think of an easy way to implement this, and since it's merely a cosmetic issue, solving it would not be worth the effort (imnsho)... if you see this problem a lot, I'd recommend using :subject instead of :thread-subject.... sorry!