Deep Dive into FlatList

Mark row 15 of a long FlatList, scroll far down, scroll back, and the mark is gone. This test used a row with its own useState value, and it pressed the row to turn that value on:

after the press:              Row 15:on
scrolled to row 500:          Row 15 is not in the tree
scrolled back to the top:     Row 15:off

No code reset that state. The row's component was removed from memory while it was far from the screen, and a new component with fresh state took its place on the way back. To use FlatList well, you need to know which rows exist at any moment.