Oh boy do I ever have this problem.
I hadn't noticed before but since you mentioned threads I've noticed
that what I see is entirely dependent on a thread being collapsed.
If all threads are expanded I can move up and down in the message list
without trouble.
If a thread is collapsed, then moving down to the top of the collapsed
thread or below causes the messages below it to be miss-rendered.
I can move around above the collapsed thread all I want without error.
Swiping the mouse over the miss-rendered threads causes them to be
rendered correctly.