I have tested your script in IE and it works for me without alert
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<script type="text/javascript">
function MouseClick() {
var posx = 0;
var posy = 0;
if (!e) var e = window.event;
if (e.pageX || e.pageY) {
posx = e.pageX;
posy = e.pageY;
}
else if (e.clientX || e.clientY) {
posx = e.clientX + document.body.scrollLeft + document.documentElement.scrollLeft;
posy = e.clientY + document.body.scrollTop + document.documentElement.scrollTop;
} if (document.all) { }
else { /////
} //alert("Loading .....");
document.getElementById('H_X').value = posx;
document.getElementById('<%=posyback.ClientID %>').click();
}
</script>
</head>
<body>
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<input type = "text" id = "H_X" onclick = "MouseClick()" />
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
<asp:Button ID="posyback" runat="server" Text="Button" OnClick = "Button1_Click" />
</ContentTemplate>
</asp:UpdatePanel>
</form>
</body>
</html>
Code
protected void Button1_Click(object sender, EventArgs e)
{
Label1.Text = "Button1 Clicked";
}
