"ScrollTo" position not precise when using with FlashList, while FlatList work as expected about react-native-reanimated

[exploIF]

Hi @renochew, I've taken a look on your repro and it looks like it's not Reanimated, FlatList or FlashList issues. This is related to how react native rendering system works. There is no guarantee that fixed sizes passed to the component would be exactly those values (you can read more about it here in caution section).

In your case FlashList is using always the same estimated element size (because of the good memoization) but FlatList is recalculating item size on each render, so sometimes it may be for example 70.90908, sometimes it will be 71.0101 etc. this is causing difference after couple rerenders. Also your elements have width and height of 71 which makes calculation and rounding even more complicated (as it's prime number).

What you can do to fix that is passing already rounded value as an element size, like below:

const ELEMENT_SIZE = 71;
const BLOCK_SIZE = PixelRatio.roundToNearestPixel(ELEMENT_SIZE);

And then use BLOCK_SIZE everywhere in the code like previously (inside scrollTo and obviously as an element width and height too).