This is weird.
When I open a short page in the browser all is well. If I switch to a 'longer page', one that is too long to fit into the open browser window, the page image in the browser jumps a few mm to the left. If I go back to the short page, it jumps back to the right.
If I stay on the short page, but shrink the browser window so that the web page no longer fits into the browser display space, the page jumps to the left.
As the browser window 'gets near' the footer, it causes the page to jump.
Grab the browser window and make it 'cover' the content in the browser (vertically) to see what I mean.
This does not happen in IE8, but it does happen in FF 3.0.11 , Opera 9.63, Safari 3.2.1 and Chrome 22.214.171.124.
I'm running WP 2.8.1 and Atahualpa 3.4.2.