This is reproducible only if the ribbon window chrome is enabled (IsWindowsThemeEnabled=False).


You can resolve this by registering an input binding for the Alt+Space key gesture and attaching it to the Telerik.Windows.Controls.RibbonView.Shell.SystemCommands.ShowSystemMenuCommand RoutedUICommand.

static MainWindow()
{
     var keyBinding = new KeyBinding(Telerik.Windows.Controls.RibbonView.Shell.SystemCommands.ShowSystemMenuCommand, new KeyGesture(Key.Space, ModifierKeys.Alt));
     CommandManager.RegisterClassInputBinding(typeof(RadRibbonWindow), keyBinding);
     RadRibbonWindow.IsWindowsThemeEnabled = false;
}