How do you make the WPF Datagrid select a cell when you first tab into it

Question

When I tab into the WPF Datagrid it focuses the first cell (with a rectangle) but does not select it (in blue). If I press tab again it focuses and selects it.

I think the DataGridCell actually has IsSelected=true, but it is not being painted in blue. I have tried hacking around with the datagrid and visual-states but I can't make it repaint the grid correctly when you first tab in.

Has anyone seen this before and do you have a solution?

code to reproduce:

MainWindow.xaml

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <TextBox Width="100"/>
        <DataGrid SelectionMode="Single" SelectionUnit="Cell"
              ItemsSource="{Binding MyItems}" AutoGenerateColumns="True"/>
    </StackPanel>
</Window>

MainWindow.xaml.cs

using System.Collections.Generic;
using System.Windows;

namespace WpfApplication1
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            MyItems.Add(new Thingy() { Name = "Frank", Age = 34 });
            MyItems.Add(new Thingy() { Name = "Jim", Age = 43 });
            MyItems.Add(new Thingy() { Name = "Bob", Age = 56 });
            MyItems.Add(new Thingy() { Name = "Harry", Age = 23 });

            DataContext = this;
        }

        private List<Thingy> _myItems = new List<Thingy>();
        public List<Thingy> MyItems
        {
            get { return _myItems; }
        }
    }

    public class Thingy
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }
}

click on the TextBox, then hit tab --- cell 1 is not selected

hit tab again --- cell 2 is selected

Any help is much appreciated, thanks.

Update:

When SelectionUnit=FullRow, I have had some success along the lines shown below, if SelectedIndex is set to 0 upon creation the first row is now selected in blue. It still needs some work to cope with shift-tab etc. There is still a problem though because when I change the SelectionMode to extended and press shift-downarrow the second row gets selected but the first row gets unselected (they should both be selected). If I do it again rows 2+3 are selected which is correct and it continues to work ok after that.

protected override void OnIsKeyboardFocusWithinChanged(DependencyPropertyChangedEventArgs e)
    {
        base.OnIsKeyboardFocusWithinChanged(e);

        int oldIdx = this.SelectedIndex;
        this.SelectedIndex = -1;
        this.SelectedIndex = oldIdx;
    }

Further Update:

Fixed that issue by setting the private _selectionAnchor field. (Thanks ILSpy)

 protected override void OnIsKeyboardFocusWithinChanged(DependencyPropertyChangedEventArgs e)
    {
        base.OnIsKeyboardFocusWithinChanged(e);

        this.SelectedIndex = -1;
        this.SelectedIndex = 0;

        SelectionAnchor = SelectedCells[0];
    }

    protected DataGridCellInfo? SelectionAnchor
    {
        get
        {
            return typeof(DataGrid).GetField("_selectionAnchor", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(this) as DataGridCellInfo?;
        }
        set
        {
            typeof(DataGrid).GetField("_selectionAnchor", BindingFlags.Instance | BindingFlags.NonPublic).SetValue(this, value);
        }
    }