Need API to commnicate expected sizes of content

[chrishtr]

If one of the elements of a virtual-scroller is another virtual-scroller, perhaps it could delegate size estimation to that element, which has a better idea of the true size. This came up when trying out the sticky demo. Scrolling down leads to a jumpy scrollbar, because the estimations are off.

[domenic]

Again this seems like it probably is a discussion for the Chromium repo, not the spec or demos.

[fergald]

I think we need to talk about how content can indicate it's expected rendered size to the scroller. Without it, initial layout can get things badly wrong and require multiple passes.