To take advantage of scrolling in RadMenu, you need to edit the one (or all) of the tempaltes for RadMenuItem - TopLevelItemTemplate, TopLevelHeaderTemplate, SubMenuItemTemplate, SubMenuHeaderTemplate. Simply add a ScrollViewer around the ItemsPresenter like this:
Note that you need to set the MaxHeight of the ScrollViewer in order for the Scrollviewer to work properly. Attached is a sample.
I hope this helps.
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