You could achieve the desired appearance by using two different approaches. The first one is to edit the ControlTemplate of the MenuItem. The second approach is to use the new RadMenuItemGroup type which may contain UIElements without wrapping them inside RadMenuItem container. We have prepared and attached in the post a sample project which demonstrates the second approach with RadListBox inside of the RadMenuItemGroup for the menu items and we have also created a custom ListBox ItemTemplate in order to modify the appearance of the ListBox items. You can modify it further based on your needs.
Hope this helps.
Regards,
Kalin
Telerik
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