We were not able to reproduce the behavior you have described in RadRichTextBox for Silverlight. We would appreciate some more details such as the version of the controls you are using and steps to reproduce.
On a side note, in the snippet you have posted you are using the KeyCode property of KeyEventArgs. The KeyEventArgs in Silverlight and WPF do not have a KeyCode property. Perhaps you are using RadRichTextBox for Winforms? If that is the case, please post your question in the Winforms forum.
the Telerik team
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