<html><head>
<meta content="text/html; charset=windows-1251" http-equiv="Content-Type">
<style>
.d1{position:fixed;top:50%;right:50%;z-index:2;overflow:hidden;}
.d2{position:fixed;bottom:0;left:0;z-index:2;width:100%;background-color:darkgray;}
.o {background:green;height:40px;width:200px;}
.t { width:2000px; height:198px;background-color: lightgray; border: 1px solid blue;}
body { margin: 0px; }
</style>
<script>
function remove_fixed()
{
document.getElementById("d2").style.position = "static";
}
function add_fixed()
{
document.getElementById("d2").style.position = "fixed";
}
</script>
</head>
<body>
<div class="d2" id="d2">This is the parent div.
<div class="d1" id="d1"><div class="o">This is the nested div.</div></div>
</div>
<div class="t">
000
</div>
<div class="t">
200
</div>
<div class="t">
400<br>
<button onclick="remove_fixed();">remove fixed</button>
</div>
<div class="t">
600<br>
<button onclick="add_fixed();">add fixed</button>
</div>
<div class="t">
800
</div>
<div class="t">
1000
</div>
<div class="t">
1200
</div>
<div class="t">
1400
</div>
<div class="t">
1600
</div>
<div class="t">
1800
</div>
<div class="t">
2000
</div>
<div class="t">
2200
</div>
<div class="t">
2400
</div>
<div class="t">
2600
</div>
<div class="t">
2800
</div>
<div class="t">
3000
</div>
<div class="t">
3200
</div>
<div class="t">
3400
</div>
<div class="t">
3600
</div>
<div class="t">
3800
</div>
<div class="t">
4000
</div>
</body></html>