I just read this in the FAQ (http://manual.winmerge.org/Faq.html):
4.6. Can I hide similar lines in a file comparison, so that only different lines are visible?
No, you can't. Many users have requested this feature but we don't have any plans to implement it. We don't believe it would really improve usability.
I came to WinMerge as I forgot to bring my current comparison tool (UltraCompare) to the client's office. I need to review the significance of the 40 different lines in two 2.2 million line files, but there is in fact no way to see these changes together on the screen, instead I must jump around between them? And you can't see how this feature would improve usability? Well, for this user it immediately removes WinMerge from my potential tool list.