add failing test for ItemSeparatorComponent state bug

[wolf-at-mystro]

Summary:

Bug: VirtualizedSectionList passes incorrect leading/trailingItem to ItemSeparatorComponent

Our app has a <SectionList> which was intermittently rendering ItemSeparatorComponents with wildly incorrect leadingItem/trailingItem props, including undefined, which as far as I can tell should never happen.

(While I’m here, I would also like to complain that the type definition for this prop is React.ComponentType<any> so I also spent a bit of time trying to work out if undefined was even an expected value.)

After investigation, it turns out that ItemWithSeparator keeps these values in state, so when the same cell happens to re-render with new props (for example when the virtualized list renders a different region), the separator keeps the previous leadingItem/trailingItem, which can be undefined if that previous row was at a section boundary.

I attach here a failing unit test which reproduces this behavior.

Changelog:

[General] [Fixed] - VirtualizedSectionList passes correct leading/trailingItem to ItemSeparatorComponent

Test Plan:

Run the test and observe it failing. I looked into fixing it, but was stymied by the highlighting and updateProps features: they also store state on ItemWithSeparator, which presumably is equally incorrect when it moves, but I don’t understand what the intended behavior is here or where that state ought to live instead. (For example, if you highlight the separator between items B and C, and item B is removed, should the highlight be transferred to the now A-C separator?)

[meta-cla]

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