The way inline comparison ("highlight changes") operates has never made sense to me. It doesn't do what the UI implies.
If you select a revision in the drop-down to compare, the expectation is that you will see how the viewed revision differs from the one selected in the drop-down. But you don't, what happens is the revision selected in the drop-down is displayed, and the changes from the revision one prior are shown. This isn't very flexible or very useful.
I've actually implemented diffs for users, profiles, and comments, and inline comparison in those modules works the way I've described. This patch changes diff to work that way too.
Sorry this is only for D7, I have yet to work out how to migrate parts of my site to D8/9 so I'm not working with any of that code.
Closed: outdated
3.4
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Given the proximity of EOL for Drupal 7, marking this as won't fix. The likely final release of diff has landed in https://www.drupal.org/project/diff/releases/7.x-3.5 → . If this is something still applicable to a more modern version of the module, feel free to re-open and post additional notes.