I've just noticed that when I make fields hidden / visible the scroll bar suddenly jumps, as if I had added an extra page onto the bottom of my form. It then resets itself to be accurate to the number of pages that are actually in my form.
This is only hapenning in Reader 8. I've tested in Reader 7, and Acrobat 9 and those two keep the scroll bar static.
Is this "working as intended" or a bug?