SyntaxEditor is in RadListBoxItem.

Entering space key does not produce text change and gets the focus lost our of the editor.

Workaround:

    private void RadSyntaxEditor_PreviewKeyDown(object sender, KeyEventArgs e)
        {
            if (e.Key == Key.Space)
            {
                (sender as RadSyntaxEditor).Insert(" ");
                e.Handled = true;
            }
        }