[scroll-animations] Need a way to specify the visual viewport as a source of scroll timeline?

[hiikezoe]

I am not sure whether it's necessary or not, but web developers might want to do it.

@majido @flackr

[majido]

I wonder if we can have something new ScrollTimeline({source: window.visualViewport}) and a corresponding css syntax to enable this.

Though this seems to be a fine thing to do in level-2 once we have a clear usecase for it.

[flackr]

I also think this should be a level 2 issue. I suspect naively directly linking scroll timeline to the visual viewport may not do the right thing in many use cases but it would require further investigation.