WPF DataGrid SelectedItem binding stops working after item change

Question

My problem/situation is very similar to Wpf DataGrid SelectedItem loses binding after cell edit but I'm not using any "custom" WPF framework. I have a model that implements INotifyPropertyChanged and IEditableObject, and a grid bound to an ObservableCollection<T>. The grid's SelectedItem property is bound to a property on the VM.

With a break point, I can see my ViewModel.SelectedItem property change as I select different rows in the grid. The moment I change a value on a row, however, the ViewModel.SelectedItem property is no longer set as I change focus on the rows. The solution identified in the above link does not work since I'm not using a custom WPF framework, just naked WPF.

Any ideas?

// View model area
public IPurchaseorderItem SelectedItem
{
    get 
    { 
        return _selectedItem;
    }

    set
    {
        if (_selectedItem != value)
        {
            _selectedItem = value;
            SelectItemCommand.NotifyCanExecuteChanged();
            RemoveItemCommand.NotifyCanExecuteChanged();
        }
    }
}

// XAML SelectedItem binding
<views:NoBindingGroupDataGrid SelectedItem="{Binding SelectedItem, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"  

// Special Grid to clear binding groups (read on a similarly themed SO question/answer)
internal sealed class NoBindingGroupDataGrid : DataGrid
{
    private bool _editing = false;

    protected override System.Windows.Size MeasureOverride(System.Windows.Size availableSize)
    {
        var desiredSize = base.MeasureOverride(availableSize);
        ClearBindingGroup();
        return desiredSize;
    }

    protected override void OnCellEditEnding(DataGridCellEditEndingEventArgs e)
    {
        base.OnCellEditEnding(e);

        if (!_editing)
        {
            _editing = true;

            CommitEdit(DataGridEditingUnit.Row, true);

            _editing = false;
        }
    }

    private void ClearBindingGroup()
    {
        ItemBindingGroup = null;

        foreach (var item in Items)
        {
            var row = ItemContainerGenerator.ContainerFromItem(item) as FrameworkElement;

            if (row != null)
            {
                row.BindingGroup = null;
            }
        }
    }
}

Answer 1

Apparently the SelectedItem dependency property on DataGrid is broken and not being used correctly. After some debugging using OnPropertyChanged, I found that the grid is actually setting a "CurrentItem" property reliably. I changed to use CurrentItem instead and everything appears to work correctly... the user's "selected row" is being sent to the VM without incident.