In DT 1.7.4:
Select a link and its content is displayed, select Capture from the context menu of the content window to create a new HTML entry, then select the new entry and its content is displayed. When selecting another link that doesn’t display any content (e.g. the file it refers to is missing) the previous new entry’s content is still displayed.
Oh, I discovered an easier way. Select any link entry that displays content, then an "orphaned" link entry, and the original content remains displayed.