Hi Simon,
It is by design that the <select> element (dropdownlist) always in front of other elements in the same layer. To workaround this issue, we should put the user control in a different layer and set a higher value of the "z-index" property for this layer.
For example, I have provided the following code for your reference. Hope it is helpful to you.
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Untitled Page</title>
<style type="text/css">
#layer1
{
position: absolute;
z-index: 10;
left: 40px;
top:20px;
}
#layer2
{
position: absolute;
z-index: 5;
}
</style>
</head>
<body>
<form id="form1" runat="server">
<div id="layer1">
<uc1:WebUserControl1 ID="WebUserControl1_1" runat="server" />
</div>
<div id="layer2">
<asp:DropDownList ID="DropDownList1" runat="server">
<asp:ListItem Text="aaa aaa"></asp:ListItem>
<asp:ListItem Text="aaa aaa"></asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="DropDownList2" runat="server">
<asp:ListItem Text="aaa aaa bbb"></asp:ListItem>
<asp:ListItem Text="aaa aaa ccc"></asp:ListItem>
</asp:DropDownList>
</div>
</form>
</body>
</html>
*********** WebUserControl1.ascx
<asp:Panel ID="Panel1" runat="server" Height="50px" Width="225px" BackColor="red">
<asp:Label ID="Label1" runat="server" Text="This is a User Control"></asp:Label>
</asp:Panel>
Sincerely,
Benson Yu
Microsoft Online Community Support
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