ScrollView stickyHeaderIndices

[chrisglein]

Split out from #2113
An array of child indices determining which children get docked to the top of the screen when scrolling. For example, passing stickyHeaderIndices={[0]} will cause the first child to be fixed to the top of the scroll view. This property is not supported in conjunction with horizontal={true}.
Not directly supported by ScrollViewer. Seems difficult. Needs deep dive.

[kmelmon]

@chrisglein @randy-flynn tells me they need this feature for something inside office. My instinct is implementing this will be complex because we can't just leverage ListView, we'll likely need a new implementation based on comp animations, so we should get someone thinking about the design for this soon.

[ddalp]

StickyHeaderIndices are implemented using the event driven animation: it creates animated view with the scrollview's children at indcies cloned inside, the animated view does some Y translation and is hooked up with scrollView's onScroll event (with path ContentOffset -> Y)

#2941 is tracking RNW's event driven animation implementation, I believe the StickyHeaderIndices should just work after event driven animation is done.

[quicksilverr]

@chrisglein @NickGerleman Any updates on this?

[ttruongatl]

Look like the issue will be solved by #11152

[chrisglein]

Not something we plan to address on the Paper renderer as we move towards Fabric.
Fabric API completion for ScrollView tracked here: #11152