Modal problem on page scroll-down

[pierpaolocira]

Hi, when I activate a modal, it "goes" upfront and the page background becomes darker. Fine.

But if the page height is greater than the browser height (in short: if there are vertical scrollbars), there is a problem with that dark div. In fact it seems to have just the size of the window (or some pixel more) and if you scrool-down by using the mouse-wheel you can see the dark div going up (leaving the rest of the page usable).