I’ve seen it with other pens as well and on all major Windows browsers. Looks to me like to content window doesn’t have focus from the start, making it to ‘wake up’ first and skip a beat. It does not happen when anything else inside the window is clicked first – or when uncommenting the line that brings focus to it.
The animation isn’t smooth in Firefox 41.0.1 when you click the right button for the first time, in Chrome 45.0.2454.101 m it is smooth however. Both on W7 x64. Don’t have IE installed so can’t test with that.
Not sure what’s causing it, probably not my hardware since it’s fairly new.