It is visible to the naked eye.
Change 366503 had a related patch set uploaded (by Golopotw; owner: Golopotw):
[mediawiki/skins/Vector@master] Fix misaligned "More" button
Hmm, I wonder if this is a regression from some recent change? I've never noticed it before and I tend to notice things like this, so does @Volker_E and he hasn't complained about it either.
(I've tried some older versions and it seems it's been like this for at least two years, so I guess you have a keener eye. :) )
Change 366503 merged by jenkins-bot:
[mediawiki/skins/Vector@master] Fix misaligned menu tabs