Fix SectionList.scrollToLocation no-op for itemIndex: 0

[KAMRONBEK]

Summary

SectionList.scrollToLocation({ sectionIndex, itemIndex: 0 }) silently does
nothing: no scroll happens, no error is logged, and onScrollToIndexFailed
never fires. itemIndex: 1 (and higher) works. This is most visible on iOS,
where stickySectionHeadersEnabled defaults to true. Fixes #50143.

Root cause

VirtualizedSectionList flattens all sections into a single VirtualizedList.
Every section contributes a header cell and a footer cell around its items, so
the flat-index layout for a section starting at flat offset base is:

base + 0     -> section header
base + 1     -> item (relative index 0)   <-- the "first row"
...
base + n     -> item (relative index n-1)
base + n + 1 -> section footer

(This layout is authoritatively defined by _subExtractor, which subtracts 1
for the header when resolving an item's relative index.)

Before this change, scrollToLocation mapped { sectionIndex, itemIndex } to a
flat index without accounting for the current section's header:

let index = params.itemIndex;                  // missing the header offset
for (let i = 0; i < params.sectionIndex; i++)
  index += getItemCount(sections[i].data) + 2; // header + items + footer of prior sections

So the mapping was off by one:

• itemIndex: 0 resolved to base → the section header, not the first row.
• itemIndex: 1 resolved to base + 1 → the first row (relative item 0).

itemIndex: 0 therefore targeted the header. With sticky headers enabled the
header is already pinned at the top of the viewport, so scrolling to its offset
lands where the viewport already is — a visually imperceptible no-op. And because
that header cell is typically already measured, scrollToIndex does not take its
onScrollToIndexFailed branch — hence "no scroll, no error, no callback", exactly
as reported. The sticky-header compensation block made the same off-by-one
assumption and was additionally gated on itemIndex > 0, so it skipped
itemIndex: 0 entirely.

The public API documents scrollToLocation as scrolling to "the item at the
specified sectionIndex and itemIndex" (SectionList.d.ts), so itemIndex: 0
should target the first item, not the header.

Root cause: packages/virtualized-lists/Lists/VirtualizedSectionList.js,
scrollToLocation (the let index = params.itemIndex seed and the sticky block).

The fix

1. Add + 1 when seeding the flat index so itemIndex: 0 maps to the first row
   (base + 1) rather than the section header.
2. Apply the sticky-header height compensation for itemIndex >= 0 (was > 0)
   so itemIndex: 0 also lands below the pinned header; the header's flat index
   is correspondingly index - params.itemIndex - 1.

Out-of-range indices are now forwarded to VirtualizedList.scrollToIndex instead
of silently resolving to a header/footer cell, so its existing range validation
and onScrollToIndexFailed path run as documented.

Behavior-change note (for reviewers)

This corrects a long-standing off-by-one dating to commit 8a82503b5 (2019),
which locked {sectionIndex: 0, itemIndex: 0} -> index: 0 into a regression test
with a // prevents #18098 comment. That change's actual goal (per its
description) was to stop viewOffset from being overridden — the index: 0
value was incidental, not the point of the test. #18098 was about scrollToLocation
overscrolling past content on the last section; the viewOffset-preservation
guarantee that fixed it is retained here, and only the index/header mapping is
corrected. Existing scrollToLocation test expectations were updated to the
corrected semantics.

Changelog:

[General] [Fixed] - SectionList.scrollToLocation now scrolls to itemIndex: 0
(the first row of a section) instead of silently no-op'ing on the sticky section
header.

Test Plan

Added/updated tests in
packages/virtualized-lists/Lists/__tests__/VirtualizedSectionList-test.js:

• Updated the existing scrollToLocation expectations to the corrected mapping
  (indices +1; itemIndex: 0 no longer resolves to the header).
• itemIndex: 0 with sticky headers → index: 1 with the header height added to
  viewOffset (regression test for SectionList.scrollToLocation not scrolling to section with itemIndex: 0 #50143).
• itemIndex: 0 in a later section → that section's first row
  ({sectionIndex: 1, itemIndex: 0} -> index: 6), not its header.
• An out-of-range itemIndex is still forwarded to scrollToIndex (so
  onScrollToIndexFailed can fire) rather than being swallowed.

Ran the suite locally (Node 24):

yarn jest packages/virtualized-lists/Lists/__tests__/VirtualizedSectionList-test.js
→ 14 passed, 14 total

Confirmed the tests catch the bug: reverting the source change makes the 7
scrollToLocation expectations fail; restoring it makes them pass.

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