How do I get a user control to have focus?

Question

I have as simple a user control as I can think of:

<UserControl x:Class="GsnEd2.View.TestControl"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         xmlns:local="clr-namespace:GsnEd2.View"
         mc:Ignorable="d" 
         d:DesignHeight="450" d:DesignWidth="800"
         Focusable="True"
         IsTabStop="True">
<Grid GotFocus="Grid_GotFocus" MouseDown="Grid_MouseDown">
    <Border BorderBrush="Black" BorderThickness="1"/>
</Grid>

and in the code behind:

using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;

public partial class TestControl : UserControl
{
    public TestControl()
    {
        InitializeComponent();
    }
    private void Grid_MouseDown(object sender, MouseButtonEventArgs e)
    {
        MessageBox.Show("Mouse down");
        Focus();
    }
    private void Grid_GotFocus(object sender, RoutedEventArgs e)
    {
        MessageBox.Show("Got focus");
    }
}

(Yes, I know it would be better to respond to mouse up. Lets get it working first!)

My MainWindow constructor is:

public MainWindow()
{
    InitializeComponent();
    TestControl test = new()
    {
        Height = 200,
        Width = 200
    };
    Canvas.SetLeft(test, 50);
    Canvas.SetTop(test, 50);
    DrawingCanvas.Children.Add(test);
}

The control appears just fine, but nothing I do seems to give it focus. I've tried setting the Border's IsHitTestVisible to false in case that was catching mouse actions, but that didn't help. What am I missing?