refactor: extract useMoneyRequestReportScroll

[sosek108]

Explanation of Change

This is PR 5 of a 10-PR stack decomposing the MoneyRequestReport list components (MoneyRequestReportActionsList / MoneyRequestReportTransactionList) into focused hooks and presentational sub-components (issue #99919, superseding drafts #99307 and #99417).

This slice extracts the scroll behavior into a dedicated hook:

• New: src/components/MoneyRequestReportView/useMoneyRequestReportScroll.ts — owns the scroll behavior of the unified list: bottom-offset tracking and the FlashList scroll/viewability handlers (via useReportUnreadMessageScrollTracking), the "Latest messages" floating counter visibility, scroll-to-bottom on own/new messages (Pusher subscription + useScrollToEndOnNewMessageReceived), stick-to-bottom while deferred content settles, the deferred mark-as-read that fires only once the scroll actually lands at the bottom, and the AgentZero thinking-indicator reveal. Since this list is not inverted, jumping to the newest message goes through scrollToIndex on the last item rather than scrollToEnd.
• Edit: src/components/MoneyRequestReportView/MoneyRequestReportActionsList.tsx — replaces the inline scroll refs, useReportUnreadMessageScrollTracking / useScrollToEndOnNewMessageReceived wiring, the new-action subscription effect, and the scrollToLatestMessages / onListContentSizeChange / onListScrollBeginDrag handlers with a single useMoneyRequestReportScroll call. Data derivation stays inline — it is extracted in the next slice of the stack.

Pure refactor — no behavior change.

Stack note: this PR is stacked on #100030 (decompose/money-request-report-transactions-a3), which is stacked on #100025, #100024 and #100023. Until those PRs merge, the diff here also shows their commits. Only the "Extract useMoneyRequestReportScroll" commit belongs to this slice.

Fixed Issues

$ #99919
PROPOSAL:

Tests

1. Open an expense report with many comments — verify the transaction table and the comments render as before
2. Scroll up away from the bottom, then have another user send a message — verify the "Latest messages" floating counter appears
3. Tap "Latest messages" — verify the list scrolls to the newest message and the report is marked as read only once the scroll lands at the bottom
4. While scrolled to the bottom, send a comment yourself — verify the list scrolls to your new message
5. While scrolled to the bottom, receive a new message from another user — verify the list follows to the new message
6. Open the report from an "X Replies" link — verify it lands on the latest message
7. Switch to another report and back — verify scroll state resets and the report loads correctly
8. Run npx jest tests/ui/MoneyRequestReportActionsListRejectModalTest.tsx and verify it passes

• Verify that no errors appear in the JS console

Offline tests

1. Go offline and send a comment — verify the list still scrolls to your optimistic message
2. Go back online — verify scrolling and the "Latest messages" counter keep working as before

QA Steps

Same as Tests.

• 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.
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• 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:
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  • 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

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

[codecov]

Codecov Report

✅ Changes either increased or maintained existing code coverage, great job!

Files with missing lines	Coverage Δ
...equestReportView/MoneyRequestReportActionsList.tsx	93.40% <100.00%> (+33.08%)	⬆️
...estReportView/MoneyRequestReportEmptyStateView.tsx	100.00% <100.00%> (ø)
src/hooks/useMarkAsRead.ts	95.94% <100.00%> (+0.05%)	⬆️
src/hooks/useUnreadMarker.ts	94.44% <100.00%> (+0.15%)	⬆️
...tReportView/useMoneyRequestReportVisibleActions.ts	52.63% <52.63%> (ø)
...questReportView/useMoneyRequestReportPagination.ts	69.04% <69.04%> (ø)
...eyRequestReportView/useMoneyRequestReportScroll.ts	64.38% <64.38%> (ø)
... and 30 files with indirect coverage changes