This is basically the issue. If a single line from the bottom of left is present at the top of right, it will fast-forward all the way through left and fail to match anything else from right. The hard part with diffing isn't finding differences, it's deciding whether they're inserts, edits, or deletions.

Nope! That's related to the Longest Common Substring Problem I think, I mentioned it in another comment [0]. Though I have a feeling you could find the shortest edit script with a few easy tweaks, I might check it out. I imagine the tweaks are going to make the the difference between O(N) and O(ND) though.

The problem I had was that I just wanted a simple diff! It's for HTML rendering and not ultra complex at that: usually folks are only changing a few lines at a time, max, between any re-render.

[0] https://en.wikipedia.org/wiki/Longest_common_substring_probl...

I think shortest is probably a good proxy if we can't find something better but I think in an editor "easiest to understand" is probably what's actually important.

Not that I have any particular expectation that this algorithm does unexpectedly well at that.