This is a browser related behavior which appeared in IE9. RadEditor Classic does not support the modern browsers and its demos run in IE7 standards document mode and for this reason the behavior does not exist.
To fix the problem put one of the following meta tags in the head of the page with RadEditor for ASP.NET AJAX:
<meta http-equiv=X-UA-Compatible content=IE=7>
<meta http-equiv="X-UA-Compatible" content="IE=8" />
the Telerik team
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