Fix cursor jumping to end when editing the middle of the Merchant field

[MelvinBot]

Explanation of Change

In the new manual expense flow, the Merchant field is a fully controlled plain TextInput whose displayed value came directly from Onyx. Because every keystroke persists through an asynchronous Onyx.merge, the controlled value lagged the DOM by a tick, so when React re-rendered with the just-caught-up value, react-native-web re-asserted the input's .value and — with no selection prop to restore the caret — the caret snapped to the end of the field. Typing in the middle of existing text (e.g. editing "United Airlines") was therefore impossible without repositioning after each character.

This mirrors the merchant text in local state and updates it synchronously in the change handler, so the controlled value always matches what the user just typed and the caret stays put. Persistence to Onyx is unchanged. The local mirror is re-synced from the persisted value only while the field is not focused, so external updates (SmartScan, drafts, switching transactions) still flow in without overwriting text the user is actively typing. The standalone Merchant step page is unaffected — it already uses an uncontrolled defaultValue input.

Fixed Issues

$ #98647
PROPOSAL: #98647 (comment)

Tests

1. Enable the New manual expense flow beta.
2. Open Global + > Create expense (Manual) and reach the confirmation screen showing the inline Merchant field.
3. Type United Airlines in the Merchant field.
4. Place the cursor in the middle of the text (e.g. between United and Airlines).
5. Type several characters and verify they are inserted at the cursor position and the caret does NOT jump to the end.
6. Verify you can type a whole word in the middle without repositioning the cursor.
7. Verify inserting at the beginning, deleting from the middle, replacing selected text, clearing the field, and editing a pre-filled merchant all behave correctly.
8. Verify split-bill merchant editing and switching between transactions still show the correct value.

• Verify that no errors appear in the JS console

Offline tests

Same as Tests — the field writes optimistically to Onyx, so behavior is identical offline.

QA Steps

1. Enable the New manual expense flow beta.
2. Open Global + > Create expense (Manual) and reach the confirmation screen showing the inline Merchant field.
3. Type United Airlines in the Merchant field.
4. Place the cursor in the middle of the text and type additional characters.
5. Verify the characters are inserted at the cursor position and the caret does not jump to the end, and that a full word can be typed mid-string without repositioning.

• 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 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
...RequestConfirmationList/sections/MerchantField.tsx	74.54% <87.50%> (+2.20%)	⬆️
... and 11 files with indirect coverage changes

[MelvinBot]

🤖 I re-checked the CI on this PR — no code fix is needed.

When the request came in, the test, typecheck, and Reassure perf-test jobs were still pending (that's what showed as "not passing"). They've all since completed and passed. The only remaining non-passing check is Check independent approval, which isn't a CI/code failure — it just gates the PR until an independent reviewer approves it, so there's nothing to push.

Current check status

• ✅ test (jobs 1–8), typecheck, ESLint check, Oxfmt check, Bun tests, Storybook tests, React Compiler Compliance, spellcheck, knip, unused-styles, all build jobs — pass
• ✅ baseline-perf-tests / branch-perf-tests — pass
• ⏸️ Check independent approval — failing only because the PR still needs an independent approval (on the ignore list; not fixable by a code change)

Nothing to fix here — this PR just needs a reviewer's approval to go green.