The matter is lying in the way ListBox measures its children. When the horizontal scroll bar is enabled, it measures all controls in infinite width (allowing them to take as much space as they need), which causes trouble with RadRichTextBox.
If you disable the horizontal scroll bar of the ListBox like this:
it will measure its children in the same width as it is (that is also the case with all other ScrollPanels). That will allow RadRichTextBox to layout the document properly. Also, please note that in this case, the text will wrap inside RadRichTextBox. This should fit your case well.
Let us know if you experience any further problems.
the Telerik team
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