[LegendList 1/7] [No QA] feat: add LegendList foundation

[chrispader]

The list migration needs dependency, test, and z-index support before production screens can switch libraries. This PR adds that foundation while leaving every production list unchanged.

@roryabraham @dmkt9

Explanation of Change

Layer 1 of the seven-part LegendList and chat pagination stack. It adds the LegendList dependency, lockfile and cspell entries, generic global Jest mocks for {LegendList} from @legendapp/list/react-native and {AnimatedLegendList} from @legendapp/list/reanimated in a dedicated setup file, and direct-library LegendList tests. It also adds the shared setLegendListItemZIndex helper and its unit test for the layer 6 sidebar migration and layer 7 draggable list. No production consumer changes in this layer.

The branches are managed locally with gh stack, but these are ordinary upstream PRs because GitHub native stacks do not support cross-fork branches. Review and merge the layers in order. Focused layer 1 diff.

Review order: 1. Foundation → 2. Message edit scrolling → 3. Loading skeleton consistency → 4. Report actions → 5. Chat pagination loading → 6. Remaining FlashList consumers → 7. Remaining FlatList consumers and restrictions. Merge and rebase in this order.

Fixed Issues

$ #98994
$ #99202

This PR is part of the migration split from #98270. No approved proposal is linked.

Tests

Not applicable to production flows in this foundation-only PR. No production screen changes in this layer, and no manual platform tests were run. Consumer behavior is covered by the later migration PRs.

• Verify that no errors appear in the JS console

Offline tests

Not applicable. This PR adds no network behavior and does not replace any production list. I confirmed that offline and high-traffic-account checks do not apply to this layer.

QA Steps

No staging or production flow exercises these components yet, so this PR is marked [No QA]. Platform and console checks are reviewed as not applicable, not reported as performed.

• Verify that no errors appear in the JS console

PR Author Checklist

I reviewed every item for this foundation-only scope. I confirmed that the platform, offline, high-traffic-account, and console checks do not apply. Checked conditional items record that review; they do not claim that I ran platform tests or produced screenshots.

• 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 is modified, 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.

Screenshots/Videos

This PR has no production UI changes or production list consumers.

[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. ⚠️

[melvin-bot]

@dmkt9 Please copy/paste the Reviewer Checklist from here into a new comment on this PR and complete it. If you have the K2 extension, you can simply click: [this button]

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-09T20:44:48.828155Z	12e46ed	Draft marked ready

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[chatgpt-codex-connector]

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Reviewed commit: 12e46ed8c1

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[dmkt9]

Reviewer Checklist

• I have verified the author checklist is complete (all boxes are checked off).
• I verified the correct issue is linked in the ### Fixed Issues section above
• I verified testing steps are clear and they cover the changes made in this PR
  • I verified the steps for local testing are in the Tests section
  • I verified the steps for Staging and/or Production testing are in the QA steps section
  • I verified the steps cover any possible 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 checked that screenshots or videos are included for tests on all platforms
• I included screenshots or videos for tests on all platforms
• I verified that the composer does not automatically focus or open the keyboard on mobile unless explicitly intended. This includes checking that returning the app from the background does not unexpectedly open the keyboard.
• I verified tests pass on all platforms & I tested again on:
  • Android: HybridApp
  • Android: mWeb Chrome
  • iOS: HybridApp
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
• If there are any errors in the console that are unrelated to this PR, I either fixed them (preferred) or linked to where I reported them in Slack
• I verified proper code patterns were followed (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 verified that this PR follows the guidelines as stated in the Review Guidelines
• I verified other components that can be impacted by these changes have been tested, and I retested again (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar have been tested & I retested again)
• If a new component is created I verified that:
  • A similar component doesn't exist in the codebase
  • All props are defined accurately
  • The component has a clear name that is non-ambiguous and the purpose of the component can be inferred from the name alone
  • The only data being stored in the state is data necessary for rendering and nothing else
  • The component has the minimum amount of code necessary for its purpose, and it is broken down into smaller components in order to separate concerns and functions
• 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 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 is modified, 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.
• For any bug fix or new feature in this PR, I verified that sufficient unit tests are included to prevent regressions in this 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 have checked off every checkbox in the PR reviewer checklist, including those that don't apply to this PR.

Screenshots/Videos

Android: HybridApp

Android: mWeb Chrome

iOS: HybridApp

iOS: mWeb Safari

MacOS: Chrome / Safari

[github-actions]

🚧 roryabraham has triggered a test Expensify/App build. You can view the workflow run here.

[OSBotify]

✋ This PR was not deployed to staging yet because QA is ongoing. It will be automatically deployed to staging after the next production release.

[github-actions]

🧪🧪 Use the links below to test this adhoc build on Android, iOS, and Web. Happy testing! 🧪🧪
Built from App PR #100732.

Android 🤖	iOS 🍎
https://ad-hoc-expensify-cash.s3.us-east-1.amazonaws.com/rock-artifacts/ad-hoc/rock-android-Adhoc-dba48b0-a8337d6-0c2241717482ed3134f864bcd9069b53359cae1e/index.html	https://ad-hoc-expensify-cash.s3.us-east-1.amazonaws.com/rock-artifacts/ad-hoc/rock-ios-device-AdHoc-dba48b0-a8337d6-40b17046e4ccb5f0c803aac4217a19646b655f39/index.html

Web 🕸️
https://100732.pr-testing.expensify.com

---

👀 View the workflow run that generated this build 👀

[OSBotify]

🚀 Deployed to staging by https://github.com/roryabraham in version: 9.4.76-0 🚀

platform	result
🕸 web 🕸	success ✅
🤖 android 🤖	success ✅
🍎 iOS 🍎	success ✅

[MelvinBot]

🤖 No help site changes are required for this PR. I did not create a docs PR.

Why: every file in this PR is internal build/test plumbing with no user-facing surface:

File	What it is
package.json / package-lock.json	Adds the @legendapp/list dependency
cspell.json	Adds legendapp to the spell-check dictionary
jest/setup.ts, jest/setupMockLegendList.ts	Global Jest mocks for LegendList / AnimatedLegendList
src/components/LegendList/setLegendListItemZIndex.ts	Shared z-index helper, no production consumer in this layer
tests/unit/LegendListTest.tsx, tests/unit/LegendListItemZIndexTest.ts	Unit tests

Nothing here changes a screen, a setting, a button label, a workflow, or any behavior a customer can observe — the PR description states it explicitly: "No production consumer changes in this layer." Expensify's help site articles under docs/articles document customer-facing product behavior, so there is nothing to add or update.

Worth flagging for the rest of the stack: layers 2–7 do swap production lists (report actions, chat pagination, sidebar, remaining FlatList/FlashList consumers). Those are still internal component swaps, so I would expect no docs changes there either — but if any of them changes visible loading, scrolling, or pagination behavior in a way a customer would notice, ping me on that PR and I'll draft the help site update.

@chrispader, there is no linked help site PR to review, since no docs changes are needed. If you disagree and think a specific article needs updating, reply here with which one and I'll open the draft PR.

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