VirtualizedList: Fix spacer size calculation (#18105)
VirtualizedList: Fix spacer size calculation (#18105)

Summary:
See https://github.com/facebook/react-native/issues/18104

Tested the above snack with:
- stickyHeadersEnabled true/false
- initialScrollIndex set and not

Visibly verified consistent rendering.

[GENERAL] [BUGFIX] [VirtualizedList] - Fix for jumpy content when initialScrollIndex specified
Pull Request resolved: https://github.com/facebook/react-native/pull/18105

Differential Revision: D8382122

Pulled By: sahrens

fbshipit-source-id: 9421351469e8684bc61438605abbd9988b664c29

javache/react-native,myntra/react-native,myntra/react-native,hoangpham95/react-native,hoangpham95/react-native,arthuralee/react-native,facebook/react-native,janicduplessis/react-native,facebook/react-native,pandiaraj44/react-native,javache/react-native,hammerandchisel/react-native,myntra/react-native,facebook/react-native,exponent/react-native,javache/react-native,hammerandchisel/react-native,hammerandchisel/react-native,exponent/react-native,pandiaraj44/react-native,exponent/react-native,javache/react-native,exponentjs/react-native,hoangpham95/react-native,myntra/react-native,pandiaraj44/react-native,pandiaraj44/react-native,exponentjs/react-native,exponentjs/react-native,hammerandchisel/react-native,facebook/react-native,exponentjs/react-native,javache/react-native,janicduplessis/react-native,exponent/react-native,hoangpham95/react-native,arthuralee/react-native,arthuralee/react-native,javache/react-native,janicduplessis/react-native,hammerandchisel/react-native,hammerandchisel/react-native,facebook/react-native,janicduplessis/react-native,pandiaraj44/react-native,hoangpham95/react-native,janicduplessis/react-native,exponent/react-native,hoangpham95/react-native,myntra/react-native,facebook/react-native,exponent/react-native,facebook/react-native,facebook/react-native,exponentjs/react-native,janicduplessis/react-native,pandiaraj44/react-native,exponent/react-native,janicduplessis/react-native,myntra/react-native,pandiaraj44/react-native,pandiaraj44/react-native,myntra/react-native,arthuralee/react-native,javache/react-native,javache/react-native,myntra/react-native,hammerandchisel/react-native,hammerandchisel/react-native,exponentjs/react-native,hoangpham95/react-native,janicduplessis/react-native,exponentjs/react-native,exponentjs/react-native,hoangpham95/react-native,facebook/react-native,exponent/react-native,javache/react-native,myntra/react-native,arthuralee/react-native