[css-position-3] Making a stickypos in a scroller also see the viewport edges

Across multiple issues (and especially in https://github.com/w3c/csswg-drafts/issues/865), a persistent complaint from authors seems to be that if a stickypos element has a scrolling ancestor, it will no longer stick to the viewport edges.

For example, a table that wants sticky headers, but is wide and thus wants to be horizontally scrollable, can't be done - the headers only care about sticking in the table's own scroller, which doesn't even scroll in the vertical axis.

(See https://github.com/w3c/csswg-drafts/issues/865#issuecomment-350585274 from @valtlai for an example and demo.)

Even if we can't care about arbitrary ancestor scroll containers, the special case of caring about the outermost viewport seems high-value to address.

Can we consider doing this either always, or through a new keyword?

Read the CSS Position 3 draft and the linked #865 example first. Compare current sticky-position behavior with the requested viewport-edge behavior, then resolve whether this should apply always or use a new keyword. Done means the specification has an agreed, testable definition.