This is definitely interesting for UI-Standardization (and @Volker_E)

For the Specific case of the notification Page, the idea was to hide those controls when there is just one page to show, which also result in never showing both controls disabled at the same time.
More details in T136574: Hide pagination controls from the Notification Page when there is only one page

This looks like a bug in OOUI, moving it there.

Change 291823 had a related patch set uploaded (by VolkerE):
MediaWiki theme: Adjacent disabled ButtonGroup/SelectWidget buttons border

https://gerrit.wikimedia.org/r/291823

The patch set is a quick fix, adding a white border when two disabled buttons in a ButtonGroupWidget/ButtonSelectWidget are adjacent.
@Pginer-WMF M101 is currently speaking a different language (not-implemented) when it comes to disabled elements, featuring non-filled backgrounds and just disabled text color. I see advantages in that approach, we should take this on our list of topics for the offsite.

Change 291823 merged by jenkins-bot:
MediaWiki theme: Adjacent disabled ButtonGroup/SelectWidget buttons border

https://gerrit.wikimedia.org/r/291823

Checked in betalabs

• there is a change in design - when there are messages only for one page, the pagination is not displayed anymore
• the dividing line is displayed when the next page is being loaded, e,g,

@Etonkovidova I'm not sure that I understand your last comment. Pagination not displayed should be in no connection to the above merged patch set.
Please clarify what you mean by your second sentence…

@Volker_E - the different functionality was added for pagination - a user with less than 50 notifications won't see the button widget anymore.