If the message is edited for typos/grammatical errors, then there’s really no need for a notification as the message displays the posted time in italics (e.g., ✏ 9 hours ago).

I’m not sure why the relevance of the posted time in this scenario, but indeed I agree simply that typos need not generate an update notice, in principle.

If the message is so reworked as to say something else, “Bob” (your example) should do the right thing and post a new, separate reply to “Alice” in the same thread, donchathink?

This requires Bob to care whether Alice gets the update. Bob might care more about the aesthetics, readability, and the risk that misinfo could be taken out of context if not corrected in the very same msg where the misinfo occurred. If I discover something I posted contained some misinfo, my top concern is propagation of the misinfo. If I post a reply below it saying “actually, i was wrong, … etc”, there are readers who would stop reading just short of the correction msg. Someone could also screenshot the misinfo & either deliberately or accidentally omit Bob’s correction. So it’s only sensible to correct misinfo directly where it occurred.

I get what you’re saying though, that there should be some real integrity toward post/reply history, like diff maybe.

It would be interesting to see exactly what Mastodon does… whether it has an algorithm that tries to separate typos/grammer from more substantive edits. I don’t frequently get notices on Mastodon when someone updates a status that mentions me, so I somewhat suspect it’s only for significant edits.

(update) one simple approach would be to detect when a strikethrough is added. Though it wouldn’t catch all cases.