It would be useful if hyperlinks to external content would have an implicit target="_blank" attribute applied, unless overridden by an explicit target in the XML documentation; e.g., <seealso href="..." target="_top"/>, <see href="..." target="some_window"/>
 
A related DocProject work item describes the problem (missing scrollbars) when external links are loaded into the same in-line frame:
http://www.codeplex.com/DocProject/WorkItem/View.aspx?WorkItemId=14869
 
Having target="_blank" as the default for all external content makes sense for .CHM files as well, IMO.