Introduced in a7dcc1c19081b4062a9be2dbfbcfacdea2206f0a.
Observed in Chrome 46. Does not affect Firefox 39. Seems to only happen with the custom step variant.
Consistently reproducible with that one specific one on the demo page:
https://doc.wikimedia.org/oojs-ui/master/demos/#widgets-mediawiki-vector-ltr