It's hard to explain, as I'm not on OSX/iOS, but I'm 90% sure it's when you scroll up too fast and you can see «behind» the website. Because of the touch event

It's hard to explain, as I'm not on OSX/iOS, but I'm 90% sure it's when you scroll up too fast and you can see «behind» the website. Because of the touch events, the browser decides to not use those features, as it might affect it. Since there's no standard way to do it (like a meta tag), the browser has to decide with the events it has.