I was recently working on a project for myself (a playlist builder). At the top of my page I had some LinkButton's with HoverMenuExtender attached to them. I'm still amazed at how easy these are to use. The only problem I had was that whenever a PostBack would occur, the panel that is the hover menu would flicker for a second in the top left hand portion of the screen. After about a half hour's worth of searching I found a blog post that had the same issue, but with a ModalPopupExtender. I took the fix Matt Berseth applied and sure enough, no more flicker. Here's a link that blog entry: http://mattberseth.com/blog/2007/08/how_to_stop_the_modalpopup_fli.html
So, basically, the fix was to add a CSS style on the panel of display:none. Below are two snippets, the first is the LinkButton with the HoverMenuExtender and the second is the panel with the style set (I'm half blogging about this for posterity because I know, I'll forget this in the future if I don't use it again for a while).
<asp:LinkButton ID="linkPlaylist" runat="server">My Playlist</asp:LinkButton> <cc1:HoverMenuExtender ID="linkPlaylist_HoverMenuExtender" runat="server" DynamicServicePath="" Enabled="True" TargetControlID="linkPlaylist" PopupControlID="playlistPopMenu" OffsetY="15"> </cc1:HoverMenuExtender> <asp:Panel ID="playlistPopMenu" runat="server" Visible="true" CssClass="playlistPopMenu" style="display:none"> <asp:LinkButton ID="LinkButton1" runat="server"> Clear Your Playlist </asp:LinkButton> <br /> <asp:LinkButton ID="LinkButton2" runat="server"> Export Playlist to Excel </asp:LinkButton> <br /> <asp:LinkButton ID="LinkButton3" runat="server"> Export Playlist to Text </asp:LinkButton> <br /> </asp:Panel>