There must be something else to your case then just a menu and a Iframe. I have tried the scenario and it works on my end. I've attached my test project.
You'll need to provide more information about the environment you are testing in, such as Telerik Control version, browsers you are testing in, operating system etc.
Ideally it would be best if you could isolate the problem in a sample solution and attach it in a support ticket so we could look at it locally.
the Telerik team
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