Web Forms :: WebPart Drag And Drop?

I cannot get the Drag and Drop functionality of Web Parts is to work. I have a very simple test page with two WebPartZones.. In the OnInit method of the code behind I put the page in design mode. In the first zone I have a textbox.At runtime the text box renders as a web part. When I hover over the web part header my mouse pointer changes the 'move' pointer, but I cannot drag the item. I do not see it dragging and the part never moved. I am using Visual Studio 2010 with IE 8. I have tried IE8 in compatibility mode and regular mode. The results are the same. Here is the markup from my test page:

<form id="form1" runat="server">
<div>
<asp:WebPartManager ID="WebPartManager1" runat="server">
</asp:WebPartManager>
<asp:WebPartZone ID="LEFT" runat="server">
<ZoneTemplate>
<asp:TextBox ID="tb" runat="server" />
</ZoneTemplate>
</asp:WebPartZone>
aa
<asp:WebPartZone ID="RIGHT" runat="server">
</asp:WebPartZone>
aa
<asp:EditorZone ID="EditorZone1" runat="server">
</asp:EditorZone>
</div>
</form>

Here is the code behind:

protected override void OnInit(EventArgs e)
{
base.OnInit(e);
WebPartManager mgr = WebPartManager.GetCurrentWebPartManager(this);
mgr.DisplayMode = WebPartManager.DesignDisplayMode;
}

What am I missing?