[scroll-animations] Unclamped start and end element-based offsets are hard to work with

[flackr]

This issue is separating out a particular concern in https://github.com/WICG/scroll-animations/issues/51 where offsets can be outside the scrollable range which means that your animation will start partway through, a distance depending on the geometry of the page. In https://flackr.github.io/scroll-timeline/demo/parallax/unclamped.html you can see that this has made animations at the start and end of the scroller start / end partway through the animation.

I think as a developer this unclamped behavior is hard to reason about (i.e. I have to use a different parallax boundary for elements which start on screen from those which scroll into view - but that's dependent on the size of the viewport). This seems to eliminate some of the benefits of element-based offsets as the developer has to again worry about resizing / shifting of elements.

I think that we should clamp offsets outside of the scroll range (as in https://flackr.github.io/scroll-timeline/demo/parallax/ ) for better developer ergonomics.

[majido]

I agree that the unclamped behavior is hard to reason about and perhaps unexpected.

Let's default to clamped behavior and in future it is easy to expose a method for authors to explicitly ask for unclamped behavior.

[majido]

The element-based offset has been added to the specification. And the current specified behavior is to clamp the offset. Considering this fixed.

1. Clamp the value of scroll offset to be within the {{source}}'s
   scroll range.