I am looking for something that will allow me to drag out an animation for how long it takes the user to scroll through the element. So, for example, a div that takes up 100% height of browser window. It's at 0% opacity. As soon as it enter's users scroll window, it starts to animate to 100%, but it doesn't get to 100% until it's out of the users scroll window again on the other side. I am only seeing duration as an amount of time. Is this a possibility?