Why does my UserControl break MDS (Minimal Download Strategy)?

Question

I've created a user control in a SharePoint 2013 farm solution, and added it to the AdditionalPageHead control.

 <Control Id="AdditionalPageHead" Sequence="90" ControlSrc="~/_ControlTemplates/15/Countdown/CountdownControl.ascx" />

The one and only thing I added to the .ascx file is a label control:

<asp:Label ID="Label1" runat="server" Text="Label">Calculating...</asp:Label>

In the .ascx.cs file I decorated the assembly and class with the MdsCompliant attribute:

[assembly: Microsoft.SharePoint.WebControls.MdsCompliantAttribute(IsCompliant = true)]
namespace Countdown.ControlTemplates.Countdown
{
    [Microsoft.SharePoint.WebControls.MdsCompliant(true)]
    public partial class CountdownControl : UserControl
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            DateTime Christmas = new  DateTime(DateTime.Today.Year, 12, 25, 0, 0, 1);
            if (DateTime.Now.CompareTo(Christmas) > 0) Christmas = Christmas.AddYears(1);
            int daysLeft = (Christmas - DateTime.Now).Days;
            Label1.Text = "Only " + daysLeft.ToString() + " more " + (daysLeft == 1 ? "day" : "days") + " until Christmas!";
        }
    }
}

When I activate this simple example on a team site it works...sort of. It adds a line at the top of the page of course, but pages in a team site now failover (reloads the entire page without the /start.aspx#). Deactivate the feature, and MDS starts working again.

As you can see, there's no JavaScript or stylesheet for me to register with SPPageContentManager, so I don't think the answer to this other MDS question applies here.

Showing the days left until Christmas isn't the real requirement, it just makes a good example. What else do I need to do to keep MDS happy with a custom user control?

Answer 1

With some thanks due to a member on the MSDN forum, he confirmed that using AdditionalPageHead with custom user controls would break MDS. He suggested using PromotedActions or SuiteLinksDelegate instead, to keep MDS happy. For example:

<Control Id="PromotedActions" Sequence="90" ControlSrc="~/_ControlTemplates/15/Countdown/CountdownControl.ascx" />