有滚动条得情况下，求标签离浏览器视口高度

使用：getElementViewTop($(".product .tab a")[0])  //注意一定要加[0]

//获取视口高度
function getElementViewTop(element) {
var actualTop = element.offsetTop;
var current = element.offsetParent;
while (current !== null) {
actualTop += current.offsetTop;
current = current.offsetParent;
}

if (document.compatMode == "BackCompat") {
var elementScrollTop = document.body.scrollTop;
} else {
var elementScrollTop = document.documentElement.scrollTop;
}
return actualTop - elementScrollTop;
}