private void ToolBar_Loaded(object sender, RoutedEventArgs e)
{
    ToolBar toolBar = sender as ToolBar;
    var overflowGrid = toolBar.Template.FindName("OverflowGrid", toolBar) as FrameworkElement;
    if (overflowGrid != null)
    {
        overflowGrid.Visibility = Visibility.Collapsed;
    }
    var mainPanelBorder = toolBar.Template.FindName("MainPanelBorder", toolBar) as FrameworkElement;
    if (mainPanelBorder != null)
    {
        mainPanelBorder.Margin = new Thickness();
    }
}