Why is my dropdownlist in IE9 only clickable with Compatibility View on?

Question

My website contains a dropdownlist which isn't clickable in IE9. You can click on it, but it doesn't expand. It does contain the values it should have. The weird part is that if you put compatibility mode on, it magically works. Also Firefox and Chrome handle the dropdownlist just fine. This problem only occurs when I test the website locally as the webserver automatically toggles compatibility mode.

My question: Anyone have an idea what might cause this problem?

Here's the code I use for dropdownlist, did change some names.

<asp:DropDownList ID="ddl" runat="server" Width="140" AppendDataBoundItems="True"
                        DataTextField="Name" DataValueField="Value" OnSelectedIndexChanged="ddl_Click"
                        AutoPostBack="true" Height="22">
                        <asp:ListItem Text="Selected" />
                    </asp:DropDownList>    

It's filled by some JavaScript, but it's getting filled so I don't think the problem lies there.

Answer 1

It seems there's no explanation to be found online. The only solutions that keep popping up are 'run IE9 in compatibility mode to avoid the problem'.

For example: https://team.utb.edu/sites/helpdesk/Knowledge%20Base/Drop%20Down%20Menus%20and%20Other%20Options%20Not%20Working%20With%20IE8%20or%20IE9%20Browser.aspx