Fix: preserve split-validation errors on focus change so unbalanced splits stay blocked

[MelvinBot]

Explanation of Change

On the Split expense confirmation page, the "sum of splits must equal the total amount" rule was enforced only as a transient formError string set by SplitBillController. The focus-reset effect in useFormErrorManagement clears any formError that isn't violations.-prefixed on every focus/validation-state change. Opening the Merchant step and returning toggles focus, so iou.error.invalidSplit (which isn't violations.-prefixed) got wiped — and since nothing re-set it, the Split button was no longer blocked and an unbalanced split could be created.

This change treats the split-validation errors (iou.error.invalidSplit, iou.error.invalidSplitParticipants, iou.error.invalidSplitYourself) exactly like violations.-prefixed errors in that focus-reset effect: they are no longer cleared on focus changes. SplitBillController remains the single owner of these errors and clears them itself once the shares are valid, so the error persists across the Merchant round-trip and only disappears once the split is actually fixed.

Fixed Issues

$ #94572
PROPOSAL: #94572 (comment)

Tests

1. Go to a group chat.
2. Click + > Split expense > Manual.
3. Enter an amount > Next.
4. On the confirmation page, enter 0 in the split input for each participant. Verify the "The sum of splits must equal the total amount" error appears and Split is blocked.
5. Click Merchant, enter any merchant, and save it.
6. Back on the confirmation page, verify the split error is still shown and clicking Split does nothing (no split is created).
7. Fix the split amounts so they sum to the total and verify the error clears and Split works.

• Verify that no errors appear in the JS console

Offline tests

Same as tests — this is pure client-side validation state, no API or optimistic data is involved.

QA Steps

1. Go to a group chat.
2. Click + > Split expense > Manual.
3. Enter an amount > Next.
4. On the confirmation page, enter 0 in the split input for each participant. Verify the "The sum of splits must equal the total amount" error appears and Split is blocked.
5. Click Merchant, enter any merchant, and save it.
6. Back on the confirmation page, verify the split error is still shown and clicking Split does nothing (no split is created).
7. Fix the split amounts so they sum to the total and verify the error clears and Split works.

• 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)
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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))
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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)
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  • I verified that all the inputs inside a form are aligned with each other.
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• 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

[MelvinBot]

No code fix is needed — the only failing check is PR Author Checklist / checklist, which fails because it reports 0 out of 38 boxes checked. That's a checklist you need to tick off manually; I don't complete the human author checklist on your behalf, and it isn't a code/CI defect.

Every completed code check on this PR is green — typecheck, all 8 test jobs, ESLint, Bun tests, Storybook tests, React Compiler Compliance, Oxfmt, spellcheck, knip, unused-styles, and the two build jobs that already finished.

Details

When I checked, three checks were still running: the two native AdHoc builds (build (developmentDebug, false), build (Debug, true)) and baseline-perf-tests. This PR only changes one client-side hook (useFormErrorManagement.ts, a 3-line condition plus a constant Set) and its unit test — code that already passes typecheck, ESLint, React Compiler compliance, and every unit test. That can't break native compilation or regress performance, so those pending checks are expected to pass.

I also re-verified the fix is sound: SplitBillController remains the sole owner of the split-validation errors and clears them itself via onFormError('') once the shares are valid, so preserving them through the focus-reset effect is correct and they still clear when the split is fixed.

To unblock the merge: open the checklist in the PR description and check every box after confirming each item.

[truph01]

@MelvinBot Mark this PR as ready for review

[melvin-bot]

@truph01 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]

[MelvinBot]

🤖 Done — PR #96150 is now marked Ready for review.