feat: migrate to LegendList

[chrispader]

@roryabraham

Explanation of Change

Migrates the main chat action list (ReportActionsList) from FlashList to LegendList.

The new InvertedLegendList adapter keeps the existing newest-to-oldest ReportActionsList contract while LegendList renders chronological data internally. It preserves inverted index semantics for rendering, imperative scrolling, initial positioning, viewability callbacks, pagination endpoints, scroll offsets, header/footer placement, maintain-visible-content behavior, and composer scroll events.

This intentionally excludes the previous work related to vertically centering an initially scrolled item.

Fixed Issues

$ TODO: Add the Expensify issue URL
PROPOSAL: TODO: Add the approved proposal comment URL

Tests

Automated coverage completed:

1. Run nr test --runInBand --watchman=false tests/unit/InvertedLegendListTest.tsx tests/unit/ReportActionsListThresholdTest.tsx tests/ui/ReportActionsListTest.tsx tests/unit/useReportScrollManagerTest.tsx tests/unit/useReportActionsScrollTest.tsx
2. Verify all five suites and 62 tests pass
3. Run nr react-compiler-compliance-check check-changed
4. Verify the Babel and OXC React Compiler compliance check passes
5. Run ESLint and CSpell against the changed files and verify they pass

TODO: Add repeatable manual chat-list testing steps for all supported platforms, including initial positioning, pagination, deep links, new messages, scroll restoration, action badges, unread markers, and failure scenarios.

• Verify that no errors appear in the JS console

Offline tests

TODO: Add repeatable offline, spotty-network, and reconnect testing steps for the migrated chat list.

QA Steps

TODO: Add repeatable staging QA steps after manual coverage is confirmed.

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I added steps for the expected offline behavior in the Offline steps section
  • I added steps for Staging and/or Production testing in the QA steps section
  • I added steps to cover failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android: Native
  • Android: mWeb Chrome
  • iOS: Native
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
• I followed proper code patterns (see Reviewing the code)
  • I verified that comments were added to code that is not self explanatory
  • I verified that any new or modified comments were clear, correct English, and explained "why" the code was doing something instead of only explaining "what" the code was doing.
  • I verified any copy / text that was added to the app is grammatically correct in English. It adheres to proper capitalization guidelines (note: only the first word of header/labels should be capitalized), and is either coming verbatim from figma or has been approved by marketing (in order to get marketing approval, ask the Bug Zero team member to add the Waiting for copy label to the issue)
• If a new code pattern is added I verified it was agreed to be used by multiple Expensify engineers
• I followed the guidelines as stated in the Review Guidelines
• I tested other components that can be impacted by my changes (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar are working as expected)
• If a new CSS style is added I verified that:
  • A similar style doesn't already exist
  • The style can't be created with an existing StyleUtils function (i.e. StyleUtils.getBackgroundAndBorderStyle(theme.componentBG))
• If new assets were added or existing ones were modified, I verified that:
  • The assets are optimized and compressed (for SVG files, run npm run compress-svg)
  • The assets load correctly across all supported platforms.
• If the PR modifies code that runs when editing or sending messages, I tested and verified there is no unexpected behavior for all supported markdown - URLs, single line code, code blocks, quotes, headings, bold, strikethrough, and italic.
• If the PR modifies a generic component, I tested and verified that those changes do not break usages of that component in the rest of the App (i.e. if a shared library or component like Avatar, I verified that Avatar is working as expected in all cases)
• If the PR modifies a component related to any of the existing Storybook stories, I tested and verified all stories for that component are still working as expected.
• If the PR modifies a component or page that can be accessed by a direct deeplink, I verified that the code functions as expected when the deeplink is used - from a logged in and logged out account.
• If the PR modifies the UI (e.g. new buttons, new UI components, changing the padding/spacing/sizing, moving components, etc) or modifies the form input styles:
  • I verified that all the inputs inside a form are aligned with each other.
  • I added Design label and/or tagged @Expensify/design so the design team can review the changes.
• I added unit tests for any new feature or bug fix in this PR to help automatically prevent regressions in this user flow.
• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.
• I verified that component internal methods bound to this are necessary to be bound (i.e. avoid this.submit = this.submit.bind(this); if this.submit is never passed to a component event handler)

Screenshots/Videos

Android: Native

TODO: Add screenshot or video.

Android: mWeb Chrome

TODO: Add screenshot or video.

iOS: Native

TODO: Add screenshot or video.

iOS: mWeb Safari

TODO: Add screenshot or video.

MacOS: Chrome / Safari

TODO: Add screenshot or video.

[github-actions]

⚠️ This PR is possibly changing native code and/or updating libraries, it may cause problems with HybridApp. Please check if any patch updates are required in the HybridApp repo and run an AdHoc build to verify that HybridApp will not break. Ask Contributor Plus for help if you are not sure how to handle this. ⚠️

[roryabraham]

Can we avoid creating the InvertedLegendList wrapper? Legend list intentionally doesn't provide an inverted mode, because Jay claims that it's stupid and we should just call .reverse on the data prop. If true, I agree it's simpler not to think about an inverted list 🤷🏼

[codecov]

Codecov Report

❌ Looks like you've decreased code coverage for some files. Please write tests to increase, or at least maintain, the existing level of code coverage. See our documentation here for how to interpret this table.

Files with missing lines	Coverage Δ
src/components/FlatList/FlatList/index.ios.tsx	90.47% <100.00%> (ø)
...equestReportView/MoneyRequestReportActionsList.tsx	60.26% <ø> (ø)
src/hooks/useReportActionsListModel.ts	100.00% <ø> (ø)
src/hooks/useReportScrollManager/index.native.ts	90.47% <ø> (ø)
src/hooks/useReportScrollManager/index.ts	0.00% <ø> (ø)
src/pages/inbox/ActionListContext.tsx	94.44% <100.00%> (-5.56%)	⬇️
...c/pages/inbox/report/MoneyReportContentCreated.tsx	100.00% <100.00%> (ø)
...inbox/report/ReportActionCompose/useEditMessage.ts	86.95% <ø> (ø)
...rc/pages/inbox/report/ReportActionIndexContext.tsx	100.00% <100.00%> (ø)
src/pages/inbox/report/ReportActionItem.tsx	90.85% <100.00%> (+0.05%)	⬆️
... and 16 more
... and 93 files with indirect coverage changes