- Open DevTools and infuse the skin group: OO.ui.infuse( $( '#mw-input-wpskin' ) ).
- The issue does not occur in Chrome.
- The issue does not occur when margin-top of .oo-ui-radioOptionWidget:first-child is non-negative. This rule style was set to -4 in the Wikimedia UI theme in this patch but not the Apex theme so the issue is not reproducible in Monobook.
- Thanks to @Mooeypoo for all the detailed help ^^^