//DISCLAIMER:  This code was copied from an article, and is by no means how I would code.
//I am ashamed to even have this in my pastebin...

private void ToggleButton_Click(object sender, RoutedEventArgs e)
        {
            DependencyObject obj = (DependencyObject)e.OriginalSource;
            while (!(obj is DataGridRow) && obj != null) obj = VisualTreeHelper.GetParent(obj);
            if (obj is DataGridRow)
            {
                if ((obj as DataGridRow).DetailsVisibility == Visibility.Visible)
                {
                    (obj as DataGridRow).IsSelected = false;
                }
                else
                {
                    (obj as DataGridRow).IsSelected = true;
                }
            }
        }

        public static FrameworkElement GetTemplateChildByName(DependencyObject parent, string name)
        {
            int childnum = VisualTreeHelper.GetChildrenCount(parent);
            for (int i = 0; i < childnum; i++)
            {
                var child = VisualTreeHelper.GetChild(parent, i);
                if (child is FrameworkElement && ((FrameworkElement)child).Name == name)
                {
                    return child as FrameworkElement;
                }
                else
                {
                    var s = GetTemplateChildByName(child, name);
                    if (s != null)
                        return s;
                }
            }
            return null;
        }

        private void dataGrid1_RowDetailsVisibilityChanged(object sender, DataGridRowDetailsEventArgs e)
        {
            DataGridRow row = e.Row as DataGridRow;
            FrameworkElement tb = GetTemplateChildByName(row, "RowHeaderToggleButton");
            if (tb != null)
            {
                if (row.DetailsVisibility == System.Windows.Visibility.Visible)
                {
                    (tb as ToggleButton).IsChecked = true;
                }
                else
                {
                    (tb as ToggleButton).IsChecked = false;
                }
            }

        }


//DISCLAIMER:  This code was copied from an article, and is by no means how I would code.
//I am ashamed to even have this in my pastebin...