Looks like it's probably an issue with OOUI. We probably move the button before mouseup, but button widgets should tolerate that.

The plot thickens: this is a pre-wmf.3 regression, only in master and not on mw.org. Probably caused by yesterday's OOUI release.

Looks like this was a desired consequence of T76636: Make buttons steal focus after click unless configured otherwise. @Volker_E : thoughts?

Hmm, well there are three possible outcomes:

1. Accept it. If you look at ButtonWidget (icon) that has been the default behaviour for quite a while now, as it aims at improving the accessibility for keyboard-only users.
2. If we see this as problematic visual impact (I see it right now more as a trade-off) we should discuss focus states of those icons
3. Seen as problematic and magically make it work for keyboard-users only (not feasible IMHO)

https://gerrit.wikimedia.org/r/322376 is somewhat related, although it doesn't change the outcome described.

'Browse topics' also has unnecessary blue outline.

Thanks for pointing this out @Etonkovidova – but as I said three comments above, the question is, is it really “unnecessary”?

@Volker_E - well, the issue is filed as a minor issue. The blue outline does not interfere with any functionality; does not have a big negative visual impact. However, especially since it's a regression, I think it is a unnecessary addition.