Seems to only happen with the default zoom level here.
I'd guess it's a Chromium rendering bug; there's no specific CSS for the first item...

It's a known issue (but no separate bug was filed for it before). Specific to Chrome.

In T141504#2501341, @Aklapper wrote:

Seems to only happen with the default zoom level here.
I'd guess it's a Chromium rendering bug; there's no specific CSS for the first item...

I agree, and I remember @Catrope making the same guess. I believe he mentioned something about collapsing of margins, since this seems to only happen at the "edges" of the list (only at the top or at the bottom)

I'm not entirely sure how to fix this. Waiting for input from @Catrope

The @Pginer-WMF noticed the issue in T129366: List wikis and pages with unread notifications in the Notification Page left nav, search for 'Stretched blue dot' in comments; also there is @Catrope comment on the issue:

I noticed this too. It happens only for the first dot on the special page, and IIRC also for the first dot below an expanded cross-wiki bundle in the popup. This is a rendering bug in Chrome (it doesn't happen in Firefox, and Chrome's own inspector believes the dot is round), probably to do with rounding (if we use 10px instead of 0.7em, it doesn't happen). The fact that it only happens for the first dot indicates it might be triggered by nearby margins or something.

Change 313860 had a related patch set uploaded (by VolkerE):
ToggleReadCircleButtonWidget: Make circle perfectly round and align color

https://gerrit.wikimedia.org/r/313860

Change 313860 merged by jenkins-bot:
ToggleReadCircleButtonWidget: Make circle perfectly round and align color

https://gerrit.wikimedia.org/r/313860

Checked in Chrome, FF, and Safari (and in monobook skin also) - the blue dots (and blank dots) look perfectly round.