In a NetCore project, when dragging the diagram from the toolbox into the designer, an exception is thrown inside the designer. 

With Visual Studio 2019 Preview v. 16.10.0. Preview 2.0, Net 5 project => when dragging from the ToolBox - the VS goes into a non-responsive state (Vs is busy is shown or the VS crashed and closed automatically).

The window's content gets clipped when the window goes from Maximized to Normal WindowState. This reproduces only when the window and the screens are using specific sizes. The setup when this was reproduced was with a 1936x1056 size of RadRibbonWindow (in Normal state) and 1920x1080 screen resolution.

To reproduce this the IsWindowsThemeEnabled property of the RadRibbonWindow should be set to False.

To work this around, reset the Window size on WindowStateChanged.

private void MainWindow_StateChanged(object sender, EventArgs e)
{
	if (this.WindowState == WindowState.Normal)
	{               
		Width += 1;
		Width -= 1;
	}
}

Some glyphs are not rendered correctly - the letter are translated or become unrecognizable.

Cell Values disappearing when RadWindow is dragged to a second Monitor.

Workaround:

private void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
    this.viewModel.OpenSampleCommand.Execute(null);
    radDocking1.ToolWindowCreated += RadDocking1_ToolWindowCreated;
}

private void RadDocking1_ToolWindowCreated(object sender, Telerik.Windows.Controls.Docking.ElementCreatedEventArgs e)
{
    var window = e.CreatedElement as ToolWindow;
    if (window != null)
    {
        window.WindowStateChanged += Window_WindowStateChanged;

    }
}

private void Window_WindowStateChanged(object sender, EventArgs e)
{
    var window = sender as ToolWindow;

    var spreadsheet = window.ChildrenOfType<RadSpreadsheet>().FirstOrDefault();
    if (spreadsheet != null)
    {
        spreadsheet.InvalidateVisual();
    }
}

The content of RadBackstageItem is not stretched within the available area in the content area of the RadRibbonBackstage control. There is a gap between the title bar of the RadRibbonView control and the top border of the content area.

This reproduces with the Material theme, but it is possible to appear also in other themes.

To work this around, you can extract the ControlTemplate of RadRibbonBackstage and re-order the layout in order to stretch the content in all the available space. Check the attached project.