效果如下：
demo code:
<!DOCTYPE html>
<head>
<meta charset="utf-8" />
<title></title>
<style>
.input1{position: absolute;
top: 100px;
width:200px;
height:100px;}
</style>
</head>
<body>
<input id="input1" class="input1" type="button" value="按钮" onclick="asd()"/>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
<br/>aaaaaaaaaaaaaaaaaaaaaaaaaaaaa <br/>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <br/>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <br/>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <br/>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <br/>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <br/>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <br/>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <br/>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <br/>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <br/>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <br/>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
<h1>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</h1>
<p>111111111111111111111111111111111111</p>
</body>
</html>
<script>
var oldTOP ;
window.onload = function(){
if(document.defaultView){
oldTOP = document.defaultView.getComputedStyle(input1,null).top;
}
if(input1.currentStyle){
oldTOP = input1.currentStyle.top;
}
};
window.onscroll = function (){
input1.value = document.documentElement.scrollTop+document.body.scrollTop+parseInt(oldTOP);
var inputTOP = document.documentElement.scrollTop+document.body.scrollTop+parseInt(oldTOP);
input1.style.top = inputTOP+"px";
}
</script>