To reproduce: 1. Go to any YouTube video 2. Type this in the URL field and press Enter: javascript:document.body.style.webkitTransform = 'translateZ(0)' 3. Scroll the page The YouTube video drifts ever so slightly from where it's supposed to be in the page. (It's quite hard to notice.)
This is caused by the fix for bug 58054. We move the windowed plugins when we push layer changes down into CoreAnimation, but should instead move them when those changes appear on screen (which happens on a background thread).