Reported by @thiemowmde:

1. Both boil down to a .mw-revslider-toggle-button

.oo-ui-buttonElement-button { padding-bottom: 0.06em; } in the CSS. I
can't see why this is there (there should be a reason why this was
added, right?). It causes two issues:

1.1. The page jumps down 1 or 2 pixels while the page loads. This is
caused by this padding, as far as I can tell.

1.2. The pin button is not vertically centered. More space at the
bottom. This is exactly this padding.