Change 577008 had a related patch set uploaded (by Awight; owner: Awight):
[mediawiki/extensions/TwoColConflict@master] [WIP] experiment with automerging

https://gerrit.wikimedia.org/r/577008

I've included all my comments in the attachment (pdf/jpg have same content) so you can read the comment next to the corresponding UI.

Short version
I think we should choose between version 2 (merge yours into other) and 4 (merge both directions). I've included pros/cons for each. I think in order for 2 to work well, it would need a slight change to the highlighting rules though.

Overall comments

Changes to version 2 to make clear

@ecohen Thanks for the thoughtful feedback! I completely agree with your points, and also with your conclusion that (2) and (4) are ideal next steps, if we can make the time to do so. I would add that the status quo (1) isn't ideal, but perfectly serviceable for a first release and light-years beyond the legacy workflow.

Tiny question about your comment on B-3, when you say 'muddies the meaning of "Latest Revision"' I would have thought the same about (2) and (4) because those strategies both change the content of the "other" / latest revision. My beef with (3) is the opposite—that it changes "your" revision, which could be surprising. (4) does so as well but I forgive it because the outcome is so magical.

Lemme also problematize B-2 line 4: the highlighting only ever shows additions, so if we had deleted text in the right-hand side, it would still show up as a highlighted addition on the left-hand side. This is similar to what you pointed out in A-2, and if we implemented (2) without a new word-diff algorithm, I worry that it would translate to only a minimal improvement over (1).

If it's helpful, I'm happy to explore the word-diff algorithm and give some more examples of how it might sometimes lead to confusing experiences.