Take all of the styles you are applying to
#backgroup and apply it to
body instead, and tell me if you still see the ‘shaking’ issue.
My guess is that the issue to which you are referring is when a user is scrolling past the boundaries of the website. If they are scrolling through an IMAGE the browser will sometimes allow you to scroll past those boundaries.
On Safari it creates some white padding, and even allows for pushing left or right.