I am afraid I don't quite understand your question.
The default layout mode of RadRichTextBox is Flow and when the control is in Flow layout mode, it should behave just as you have described. Could you provide some more details on the way you have implemented this TextBlock - RadRichTextBox and in what part of it you would need assistance?
It would be best if you could open a support ticket and attach a demo application implementing this scenario.
the Telerik team
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