Modal popup extender is not scrolling to top and left corners on window resize

Question

I'm using modal popup extender in my web page. When I re-size the browser window, the modal popup extender does adjust itself in the middle area of the screen but not showing the scroll bars to reach top and left corner.

Please see below the image for easier reference.

And below code for the reference: code inline:

<asp:linkbutton id="lbtnAdd" runat="server" onclick="lbtnAdd_Click" cssclass="titletext1 textalignleft textdecorationnone"
text="New Advertisement" causesvalidation="false" oncontextmenu="return false"></asp:linkbutton>
<asp:modalpopupextender id="AdvertisementModelPopup" runat="server" targetcontrolid="btnTest"
popupcontrolid="dialog" clientidmode="Static" backgroundcssclass="modalBackground">
</asp:modalpopupextender>
<div id="dialog" style="display: none; background-color: transparent;">
<asp:label id="lblUserName" text="I have used another control(ID: AdvertisementUC) which contains popup contents instead of this label:lblUserName."
    runat="server"></asp:label>
</div>
<style type="text/css">
.modalBackground
{
    background-color: #EEF3FA;
}
</style>

code behind:

protected void lbtnAdd_Click(object sender, EventArgs e) { AdvertisementModelPopup.Show();
}