We have all been there, you have a centered site and some pages have enough content to fill the screen while some don't. So when you move back and forth between one that does and one that doesn't you see a small jump happen.
Here is a small css fix to force the vertical scrollbar to always be visible.
The site i got the original snippet from claims it doesn't work in opera but it worked for me in all browsers.
What i changed is since "overflow-y" isn't valid css i started with the valid "overflow" to enable both the vertical and horizontal scrollbars in the event some browser did not recognize the two other rules, then proceeded with hiding the horizontal scrollbar and (just for the overkill, since logically : 'both visible' + 'x hidden' = 'y visible' ;P ) forcing the vertical scrollbar.
So we end up with a simple three css rule that forces a vertical scrollbar but degrades to forced vertical & horizontal scrollbars in the events that the invalid css isn't applied. Also, it does not temper with your html/body/container heights so it keeps the appearance of your site true to its content!
Enjoy & comment :)