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Not exactly what you're asking but I made a Safari extension (iOS/Mac) to highlights new comments: https://apps.apple.com/us/app/hn-comments/id1602932281

A whopping 5 downloads so far. :p

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I'm not on safari (or Mac), so I can't try it. But what I usually do is collapse the comments I already read, and when I check the thread later, I can distinguish new (or previously unread) comments.

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