Windows10 on two monitors, at least one of them is with higher DPI -125 , 150 %.

This leads to unexpected black gap (glitch) between the RibbonWindow TitleBar and Content.

Partial workaround is to set manually a margin on the WindowDecorator:

  private void RadRibbonWindow_Loaded(object sender, RoutedEventArgs e)
        {
                Grid windowDecorator = this.ChildrenOfType<Grid>().FirstOrDefault(x => x.Name == "MaximizedWindowDecorator");
                windowDecorator.Margin = new Thickness(0, -1, 0, 0);          
        }