Makes a WPF ComboBox Searchable via its items

MakeComboBoxSearchable.cs

public static void MakeComboBoxSearchable(this ComboBox targetComboBox)
{
    targetComboBox.Loaded += (ls, le) =>
    {
        targetComboBox.Items.IsLiveFiltering = true;

        var targetTextBox = targetComboBox.Template.FindName("PART_EditableTextBox", targetComboBox) as TextBox;
        if (targetTextBox == null) return;

        targetComboBox.IsEditable = true;
        targetComboBox.IsTextSearchEnabled = false;

        targetTextBox.Tag = "Selection";

        targetTextBox.PreviewKeyDown += (se, ev) =>
        {
            if (ev.Key == Key.Enter || ev.Key == Key.Return || ev.Key == Key.Tab)
                return;

            targetTextBox.Tag = "Typed";

            if (targetComboBox.SelectedItem != null)
            {
                targetComboBox.SelectedItem = null;
                targetComboBox.Text = string.Empty;
            }
        };

        targetTextBox.TextChanged += (se, ev) =>
        {
            var searchTerm = string.Empty;

            if (string.IsNullOrWhiteSpace(targetTextBox.Text) == false && (string)targetTextBox.Tag == "Typed")
            {
                targetComboBox.SelectedItem = null;
                targetComboBox.IsDropDownOpen = true;
                searchTerm = targetTextBox.Text.ToLowerInvariant();

                targetTextBox.Select(targetTextBox.Text.Length, 0);
                targetComboBox.Items.Filter = (filterItem) =>
                {
                    return filterItem.ToString().ToLowerInvariant().Contains(searchTerm);
                };
            }
            else
            {
                targetComboBox.Items.Filter = (filterItem) => { return true; };
            }

            targetComboBox.Items.Refresh();
        };

        targetComboBox.SelectionChanged += (se, ev) =>
        {
            if (targetComboBox.SelectedItem != null)
            {
                targetTextBox.Tag = "Selection";
                targetComboBox.Items.Filter = (filterItem) => { return true; };
                targetComboBox.Items.Refresh();
            }
        };
    };
}

I tried the above Extension, it fixed the SelectItem when DropDownOpen. But when searchingBox it doesn't get the binding value of Itemsources anymore.