The problem that you're having is a style related issue. When you hover over to some element, the element gets some more borders or paddings or margins. Which causes its size to be changed. In this case, its size enlarges causing the scroll bar to come out.
There are some hacks for this, first one is to make sure the body element always takes up a part of the screen, like this
document.getElementById("body").style.height = screen.height - 100;
..or you can use the CSS to tackle this, and you can make sure that upon any hover event (mouse over) the size of the objects don't change.