When I load the demo page in Firefox 45.0b5, the buttons are below the expected position. This happens with both the MediaWiki and Apex themes.
Git bisect blames rGOJUd40c796fd11c: CapsuleItemWidget: Tweak styles for the "remove" button, although I wouldn't be too surprised if it was really added in rGOJU3073f76c1a21: OO.ui.CapsuleItemWidget: Use Button instead of an Indicator and just covered up by the large negative margin removed in rGOJUd40c796fd11c.
I note this is going to affect ApiSandbox in production in 1.27.0-wmf.15 (or whichever version rMW1cdee3383553: Update OOjs UI to v0.16.0 goes out with).