The following backport PR had two differences between the mainline patch and the PR patch, and yet Skara marked it as clean:

https://github.com/openjdk/jfx21u/pull/92

See the attached diff.log file where I diffed the two patches showing the relevant differences.

One of the two differences was a difference in removed code, so the net effect of the two patches was the same; I don't think Skara usually counts that as clean, but I can imagine it might in some cases.

However, the other was an actual diff in a file (CanvasTest.java) that was changed in the mainline patch and not in the backport patch.