feat: update amounts/percentages split logic to match OD

[ikevin127]

Explanation of Change

In Classic, when a user splits an expense, we try to update the amounts for all splits to equal the amount of the original transaction automatically. We opted to go a simpler route with NewDot splits to start, avoiding any automatic updates, and punted adding this logic to a follow-up. This is that follow-up:

Problem: When users are editing the separate portions of a split they are typically trying to ensure all the portions add up to the original transaction amount.

Solution: Automatically change amounts and percentages when creating a split to add up to the original transaction amount to remove clicks/steps for the submitter. To do so, we'll implement our Classic logic:

Core logic:

• Default/unmodified values adjust automatically: Any split that hasn't been manually edited (i.e., still at its default value) will auto-adjust evenly so that all splits add up to 100% (for percentages) or the total amount (for amounts).
• Edited values are preserved: Once a user manually edits a percentage (or amount), that value is “locked in” and will never be auto-updated.
• Editing an initial split will preserve all Split values: We've added the ability to edit existing splits in NewDot and the individaul splits of an expense may be on different reports and in different states. To avoid complexity, when a user edits an amount or percentage on an existing split, we treat all splits as manually edited and stop auto-adjusting them.

Behavior for Adding Splits

• When a new split is added: Instead of defaulting to 0%, the system will redistribute all unedited splits automatically so they balance out to 100%.

Simple adding splits Example:

1. Submitter taps More Split on a $10 expense
2. Two initial splits exist at 50/50 = $5/$5
3. Submitter taps 'Add split'
4. A third split is added and the amounts are updated to 33%/33%/34% = $3.33/$3.33/$3.34

Further editing example:

1. Submitter taps More Split on a $10 expense
2. Two initial splits exist at 50/50 = $5/$5
3. Submitter edits Split A to $3.
4. Split B automatically updates to $7
5. Submitter taps 'Add split'
6. A third split is added. A remains $3, B updates to $3.50, the new split C is added as $3.50.

Error handling:

• Users can still manually override and push totals over/under 100%.
• In those cases, the same existing error message for invalid totals will be shown.

Fixed Issues

$ #70799
PROPOSAL:

Tests

Test 1: Adding Splits Auto-Redistributes

1. Create/find a $10 expense in a workspace.
2. Tap More > Split on the expense.
3. Verify initial splits show $5.00 / $5.00 (50/50).
4. Tap Add split.
5. Expected: Three splits show $3.33 / $3.33 / $3.34.
6. Edit Split A to $3.00.
7. Expected: Split B and C auto-update to $3.50 / $3.50.

Test 2: Edited Splits Stay Locked

1. Continue from Test 1 (A=$3, B=$3.50, C=$3.50).
2. Edit Split B to $4.00.
3. Expected: A stays $3, B stays $4, C redistributes to $3.00.
4. Add another split.
5. Expected: A=$3, B=$4 stay locked. C and D redistribute remaining $3.

Test 3: Editing Existing Saved Splits (All Locked)

1. Save a split expense with 2 splits.
2. Return to edit the splits.
3. Edit one split amount.
4. Expected: Only that split changes, no auto-redistribution.

• Verify that no errors appear in the JS console

Offline tests

Offline Test 1: Create Splits While Offline

1. Go offline.
2. Create a $20 expense split with 3 splits.
3. Edit one split while offline.
4. Go back online.
5. Expected: All changes sync correctly.

Offline Test 2: Add Split While Offline

1. Start with existing 2-split expense.
2. Go offline → Add a third split.
3. Expected: Auto-redistribution happens locally.
4. Go online → verify that changes sync correctly.

Offline Test 3: No Data Loss on Reconnect

1. Start split flow offline.
2. Make several edits.
3. Reconnect.
4. Expected: All local changes persist and sync.

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)
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Screenshots/Videos

Android: HybridApp

android-hybrid.mp4

Android: mWeb Chrome

android-mweb.mp4

iOS: HybridApp

ios-hybrid.mov

iOS: mWeb Safari

ios-mweb.mov

MacOS: Chrome / Safari

web.mov

[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
...onList/ListItem/SplitListItem/SplitAmountInput.tsx	0.00% <ø> (ø)
src/components/MoneyRequestAmountInput.tsx	1.88% <0.00%> (-0.04%)	⬇️
src/components/NumberWithSymbolForm.tsx	2.76% <0.00%> (-2.56%)	⬇️
src/libs/actions/IOU/index.ts	68.92% <25.00%> (+0.43%)	⬆️
... and 15 files with indirect coverage changes

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[ShridharGoel]

Bug 1: Unable to edit the digits of a negative split. It becomes editable after removing the minus sign though.
Bug 2: Unable to change any non-negative split value to negative.

Screen.Recording.2026-01-10.at.12.55.47.PM.mov

[ShridharGoel]

How do we plan to handle percentages for negative splits?

[ikevin127]

Posted on the issue about negative splits case - awaiting a decision.

[ikevin127]

Bug 1: Unable to edit the digits of a negative split. It becomes editable after removing the minus sign though.
Bug 2: Unable to change any non-negative split value to negative.

✅ Addressed.

Screen.Recording.2026-01-10.at.15.00.39.mov