Page shift

[snooze]

Maybe you have short and long (with scroll bar) pages.
If you have a scrollbar, a centered content will be slightly more to the left, as there is less browser client area.
You can always turn on scroll bars with CSS overflow-y: scroll

That seems to be the cause. I noticed after posting this that Safari for Macintosh somehow compensates and does not shift. I'll check the Windows version later today.

I'd like to hear more about that CSS option