Hello Konstantina ,
Thank you so much for the quick answer .
I have tried the solution you sent me . However that article is talking about RadMenu, not RadContextMenu . I have tried to change RadContextMenu Style but its style was different with the RadMenu and I'm not sure where I should put the ScrollViewer in RadContextMenu style as in RadMenu it's in Popup section and there is not Popup Section in RadContextMenu style.
Thank you Konstantina ,