Hello Michel,
Currently the RadMenuItem does not support orientation for the role Separator. By default, it has vertical orientation, i.e. it is meant for use in vertical scenarios. When using it as a horizontal separator, you will have to apply a small change to its template. For example, please consider the following snippet:
The above example displays a horizontal separator.
I hope this will give you some helpful insight on the issue.
Regards,
Dani
the Telerik team
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