Prototype 已经通过 提供了位置Element.viewportOffset().

Prototype already provides positions with Element.viewportOffset().

编辑，正如 Mathew 指出的那样document.viewport 给出了其余的信息.例如，

Edit, as Mathew points out document.viewport gives the rest of the information. For example,

var dialogtop = dialog.viewportOffset().top;
if (dialogtop < 0) {
    // above top of screen
}
elseif (dialogtop + dialog.getHeight > document.viewport.getHeight()) {
    // below bottom of screen
}