one window diff view
Instead of having side by side views, it is MUCH better to see one view with the characters added by the second file in blue and the characters removed by the second file in red and struck through. See CSDiff by ComponentSofware. It is the BEST view. It's not Mac compatible, though.
Unfortunately, this would require extensive redesign, and the vast majority of our users prefer the side-by-side view.
Note that DiffNow does have a Line Inspector panel at the bottom of the comparison window, that lets you easily see the currently selected pair of lines. It’s not quite what you’re asking for, but it can accomplish the same goals.
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Jim Laskowski commented
How do you know what the "vast majority" of your users prefer? Have you at one time exposed them to a single-window comparison and asked their opinion?