AJAX :: Modal Popup Extender Background On Window Resize

I am using modal popup extender in my web page, it is working fine, but when the page contents are expanded, modal popup displays with no background color. Here is more info -

The web page contains bunch of update panels that have reports in iFrame controls inside it. On click of save, I display modal popup with confirmation message. Initially, all panels are collapsed, modal popup is displayed properly with the gray background color. When i expand all the panels, page gets really huge and modal popup displays no background color (transparant). Below are the styles I am using -

.modalPopupBackground
{
background-color: Gray;
filter: alpha(opacity=50);
opacity: 0.50;
}
.modalPopup
{
background-color: white;
border-width: 3px;
border-style: solid;
border-color: #165EA9;
padding: 3px;
width: 600px;
height: 150px;
}

Here is the modal popup code -

<asp:Button ID="btnHiddenTarget" runat="server" Style="display: none" Text="HiddenTargetControlForModal" />
<ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="btnHiddenTarget"
BackgroundCssClass="modalPopupBackground" PopupControlID="ModalPanel1" OkControlID="btnModalYes"
DropShadow="true" PopupDragHandleControlID="ModalPanel3" />
<asp:Panel ID="ModalPanel1" runat="server" Style="display: none;" CssClass="modalPopup">
<asp:UpdatePanel runat="server" ID="updPopupMsg">
<ContentTemplate>
<asp:Panel ID="ModalPanel3" runat="server" Style="cursor: move; background-color: #DDDDDD;
border: solid 1px Gray; color: Black; height: 20px">
<div>
<p style="text-align: center;">
<asp:Label runat="server" ID="lblInfoPopupHeader" Font-Bold="true"></asp:Label></p>
</div>
</asp:Panel>
<asp:Panel runat="server" ID="pnlPopupMsg" Style="text-align: left; vertical-align: middle;
overflow: auto; height: 100px">
<p>
<asp:Label runat="server" ID="lblInfoPopup" Font-Bold="true"></asp:Label></p>
</asp:Panel>
</ContentTemplate>
</asp:UpdatePanel>
<p style="text-align: center; vertical-align: bottom; height: 20px">
<asp:Button ID="btnModalYes" runat="server" Text="OK" CssClass="commandButton" Style="width: 100px;"
IgnoreDirty="Dirty" />
</p>
</asp:Panel>

Technical Environment - VS 2008 and AjaxControlToolkit 3.5.40412.0.