asp.net - How to show DIV just on top of button

Question

I have a custom control, which has a Button + image, if image(down arrow) is clicked, a DIV should display, just like Dropdownlist.

However, its pushing the page contents to down to display the DIV. How to fix this.

1. It should just behave like dropdown

2. List should display on top of Button instead button

   <table id="container" border="0" cellpadding="0" cellspacing="0">
   <tr>
       <td>
           <asp:Button ID="btnPost" runat="server" CssClass="postButton" OnClick="btnPost_Click" />
           <asp:Button ID="btnDropDown" runat="server" CssClass="dropdownButton" OnClick="btnDropDown_Click" />
       </td>
   </tr>
   <tr>
       <td>
           <div runat="server" id="divDropDownPanel" visible="false" style="text-align: left;
               overflow: scroll; float: left; border: thin solid lightgrey; width: 160px; height: 120px;
               background-color: #FFFFFF; position: absolute; z-index: 999;">
               <asp:Repeater ID="rptDropDownContent" runat="server" OnItemDataBound="rptDropDownContent_ItemDataBound">
                   <HeaderTemplate>
                       <table>
                   </HeaderTemplate>
                   <ItemTemplate>
                       <tr>
                           <td align="left">
                               <asp:CheckBox ID="chkChannel" runat="server" />
                           </td>
                       </tr>
                   </ItemTemplate>
                   <FooterTemplate>
                       </tr> </table>
                   </FooterTemplate>
               </asp:Repeater>
               <br />
           </div>
       </td>
   </tr>