Change 75579 had a related patch set uploaded by Matmarex:
Enhanced RC: Add a "view changes since my last visit" link

https://gerrit.wikimedia.org/r/75579

Created attachment 13163
Mockup by MatmaRex

Why can't the button be in the collapsed part of the bullet? Maybe that's the best position but I'm wondering what rationale to follow to add or remove stuff from the uncollapsible part.
Would something like "4 (1) changes" or "3+1 changes" or whatever, with the 1 bolded as unvisited revisions are in watchlist, make sense?

Attached:

Why can't the button be in the collapsed part of the bullet? Maybe that's the
best position but I'm wondering what rationale to follow to add or remove
stuff from the uncollapsible part.

Because that would defeat the point. This button is needed exactly so that users don't have to expand the bullet for every page on their watchlist every time they visit it.

Would something like "4 (1) changes" or "3+1 changes" or whatever, with the 1
bolded as unvisited revisions are in watchlist, make sense?

I considered a few variants, including something like this. It has two main drawbacks: the link would be too small to click comfortably, and there is no information as to what the second number means (I don't think bolding would be obvious enough).

The form I chose ("N changes | N since last visit") seems to be a sane compromise between this and explicit longer "N changes | N changes since your last visit".

Change 75579 merged by jenkins-bot:
Enhanced RC: Add a "view changes since my last visit" link

https://gerrit.wikimedia.org/r/75579

Approved by nikerabbit.