Could this message be of interest for screenreader users too (hidden vs visually hidden)?

On mobile when viewing contribution rows on history or contributions page, for edits which have no edit summary, we show the text "No edit summary"

It's also shown on mobile diff page: https://m.mediawiki.org/wiki/Special:MobileDiff/5013367, I suppose we want retain it there too?

The changes proposed here should have no impact on Special:MobileDiff so Special:MobileDiff should behave before/after these changes the same (so yes it should be retained).

We're looking to eventually get rid of that page too (see T117279), but for now let's keep this scoped to contributions page.

Change 756412 had a related patch set uploaded (by Ammarpad; author: Ammarpad):

[mediawiki/core@master] Add default comment when there's no comment on some changeslist pages

https://gerrit.wikimedia.org/r/756412

Change 756412 merged by jenkins-bot:

[mediawiki/core@master] Add default comment when there's no comment on some changeslist pages

https://gerrit.wikimedia.org/r/756412

This introduces a dangling separator. Is that the intended design?

https://en.wikipedia.beta.wmflabs.org/w/index.php?title=Main_Page&action=history

It's not. I think it appears now because there's content after the char difference in the document (it's invisible). The separator is added if there's any content after that and there was check for emptiness before. The check is removed in the above patch since now every changeslist row has summary and the check will always be true because of that.

We can hide it somehow now if it's known the comment is the default one since Minerva (the potential user of that comment) doesn't seem to use (the separator) currently.

It wasn't intended, but I don't think it's a big deal.
On non protected pages, an undo button will follow so that seems helpful: