Don’t you find it annoying when your designer reports that the site is ‘jumping’ only for you to discover that the reason it’s jumping is that some pages are longer than others and your browser puts in the scrollbar?
There’s a simple solution I found a year ago that is the solution to this problem: Overflow-y
html {overflow: -moz-scrollbars-vertical; overflow-y: scroll;}
Will make the scrollbar appear all the time, but is greyed out & disabled when not needed. This way, your site won’t jump ever again.