Could you please share what is the behavior when testing with different browsers?
It seems this might be related to a common issue of Google Chrome/Safari and Flash/Silverlight plug-ins. In a gist, the problem is that the browser does not pass the focus to the plugin when loading, so there is no way to pass it to the RadRichTextBox either. We haven't been able to come up with a good solution for this issue so far but you can try searching around on the web. What we have used in the past was the following hack:
.Loaded += (s, a) =>
put in the user control containing the RadRichTextBox, but this seems to not work consistently (and especially for Chrome).
I hope the provided information is helpful.
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