DataGrid problems

I detected two problems with the DataGrid.

• When a row is selected in the DataGrid and I press the Down key then instead of going to the next row the DataGrid looses the focus completely.
• When giving a value to the Category which is an editable ComboBox, the edit does not work as it used to (offering a value based on the text typed so far). 

After some hour of investigation it turned out the DataGridFocusBehavior class is the culprit. It successfully refocuses the DataGrid but these two mysterious error appears. I tried to modify this class hoping it might help. I tried to use Invoke instead of BeginInvoke. I tried to call the Focus() method directly not through the Dispatcher object, I tried to use some sleep before calling Focus, I also tried to change priority at  BeginInvoke.

I tried to use code behind instead of Behavior but it did not help. I noticed that the Focus method is called about 20 times and I did not have any clue why. The call stack did not help. I have only seen that external code called the Behavior. It seems the SelectionChanged event is triggered a lot of times internally.

The next try was to catch the Up and the Down buttons with Commands. 

    private void Up(object obj)
    {
        if (PagerViewModel.SelectedRow is not DisplayTransaction selectedRow)
            return;
        var index = PagerViewModel.DisplayedRows.IndexOf(selectedRow);
        if (index == 0)
            return;
        var previousRow = PagerViewModel.DisplayedRows.ElementAt(index - 1);
        PagerViewModel.SelectedRow = previousRow;
    }

Now the Up and Down keys worked as used to but now the DataGrid did not get the focus after Refresh. 

    <DataGrid.InputBindings>
         <KeyBinding Key="Delete" Command="{Binding PagerViewModel.DeleteRowsCommand}" />
         <KeyBinding Key="Up" Command="{Binding UpCommand}" />
         <KeyBinding Key="Down" Command="{Binding DownCommand}" />
    </DataGrid.InputBindings>

Moreover, intercepting the two arrow keys did not help with the second problem.

I also created a DataGridLostFocusBehavior which should give back the focus to DataGrid when the DataGrid looses the focus. This was also dead end. It was not a good idea anyway because it means the focus can not go away from the DataGrid.

I created an attached property for the DataGrid so I can set the focus from the View model. The code was executed but it seemed the Focus() method has no effect.

element.Focus();
Keyboard.Focus(element);
var row = (DataGridRow)((DataGrid)element).ItemContainerGenerator.ContainerFromItem(((DataGrid)element).SelectedItem);
if (row != null)
{
    row.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next)); // Moves focus to the next element inside the DataGrid
    Keyboard.Focus(row); // Set keyboard focus on the DataGrid row
}

I tried the normal Focus, the Keyboard focus, I also tried to focus the selected row without success.

After two days I said that I already spent too much effort on this issue. So I stop using the DataGridFocusBehavior class.

Debugging your own code is challenging enough, spotting the problem in WPF which is huge and complicated is extra hard.