I'm not sure what browser(s) or site(s) you're referring to, but just now double checking a few sites in current FF and Chrome I don't see them maintaining scroll position at all. I realize that this agrees with my developed expecations for text heavy pages. Text reflow as the page narrows dominates the viewport position -- content getting longer effectively pushes the viewport up the page. To the user, content appears to flow "downward" as the page narrows.

Do you have an example of a site and/or browser that exhibits the behavior you're describing? I'm quite curious if there's an approach to structuring a page that preserves position sensibly.