[css-conditional] [css-position] Allow feature detection of `position: sticky` tracking the nearest scrolling container per axis.

[bramus]

In #865 (comment) we resolved to have position: sticky track the nearest scrolling container per axis.

I’ve made some early noise about this change on social media and, in response, received two individual requests to be able to feature detect this. Currently, authors have some convoluted workarounds that rely on scroll-driven animations or lots of JavaScript, which they want to keep in place as a fallback.

I believe this is a good candidate for @supports named-feature() as I don’t think there is another way how authors would be able to detect this.

[dbaron]

Is position-sticky-per-axis a reasonable name for such a feature?

[bramus]

SGTM.