Add scrollToIndex support for WindowScroller

I has been reported that Grid does not support the scrollToIndex property when used with WindowScroller. Let’s investigate if it is possible to add support for this use case.

cc @tiberiumaxim: If you have questions, we can discuss them here or on Gitter.

[bvaughn]

Well, the problem you’ll have to solve in order to implement this feature basically boils down to this:

Normally a user scrolls within a Grid; Grid listens to scroll events and updates what it displays. When a user sets a prop like scrollToIndex- Grid determines what the offset of that row would be and updates the scroll position so that it’s visible.

WindowScroller changes the rules a bit. Its purpose is to enable a page to scroll outside of Grid so that other page elements can scroll too. It listens to scroll events and tells Grid which offset it should display visible data at.

The problem is this: WindowScroller is listening to the window object for scroll events. When you set a scrollToIndex prop, Grid updates its own scroll offset (and then renders new rows). WindowScroller isn’t listening to Grid and so the page isn’t scrolled to show the newly rendered rows.

I haven’t thought much about this problem because, as I mentioned previously, I don’t use WindowScroller. 😄

[bvaughn]

Unfortunately I don’t have time to really dig into or think about this at the moment. I’d welcome a contribution if you’d like to dig into it and submit a PR, but I don’t have the bandwidth to offer any architectural guidance on it. Sorry!