[Structural Sharing] PR 3: Frozen collection snapshots with lazy rebuild and per-key merge

[fabioh8010]

Details

Third PR of Expensify/App#86181

Replace the mutable collectionData (which returned {...spread} copies) with collectionSnapshots — a Map of Object.freeze()'d snapshots. When a member key is written (via set(), merge(), or drop()), its parent collection is marked dirty in a dirtyCollections Set. The frozen snapshot is only rebuilt on the next read (getCollectionData()), not on every write.

rebuildCollectionSnapshot() iterates only the members of the affected collection (using the indexed forward lookup from PR 1), creates a new frozen object, and compares each member reference against the previous snapshot. If nothing actually changed, the old snapshot reference is reused — this is what makes useOnyx's === check work.

Additionally, replace the O(total_keys) storageMap spread in merge() with a per-key loop. Keys whose merged result is === to the existing value (enabled by PR 2) are skipped entirely.

E/App PR: Expensify/App#86885

Related Issues

Expensify/App#86181

Automated Tests

Unit tests were added.

Manual Tests

General testing over the app. Use Expensify/App#86885 for testing:

1. Login with an account.
2. Go to Inbox tab, scroll the chat list a bit.
3. Select a chat and send a message.
4. React to that message and create a thread.
5. Create a task and complete it.
6. Create an expense via FAB, go to the expense details.
7. Go to Reports tab and check if expense is there. Go to its details.
8. Go to Settings and logout.

Author Checklist

• I linked the correct issue in the ### Related 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 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 / Chrome
  • iOS / native
  • iOS / 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 any callback methods that were added or modified are named for what the method does and never what callback they handle (i.e. toggleReport and not onIconClick)
  • I verified that the left part of a conditional rendering a React component is a boolean and NOT a string, e.g. myBool && <MyComponent />.
  • 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 proper file naming conventions were followed for any new files or renamed files. All non-platform specific files are named after what they export and are not named "index.js". All platform-specific files are named for the platform the code supports as outlined in the README.
  • I verified the JSDocs style guidelines (in STYLE.md) were followed
• 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)
• I verified all code is DRY (the PR doesn't include any logic written more than once, with the exception of tests)
• I verified any variables that can be defined as constants (ie. in CONST.js or at the top of the file that uses the constant) are defined as such
• I verified that if a function's arguments changed that all usages have also been updated correctly
• If a new component is created I verified that:
  • A similar component doesn't exist in the codebase
  • All props are defined accurately and each prop has a /** comment above it */
  • The file is named correctly
  • 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
  • If we are not using the full Onyx data that we loaded, I've added the proper selector in order to ensure the component only re-renders when the data it is using changes
  • For Class Components, any internal methods passed to components event handlers are bound to this properly so there are no scoping issues (i.e. for onClick={this.submit} the method this.submit should be bound to this in the constructor)
  • Any internal methods bound to this are necessary to be bound (i.e. avoid this.submit = this.submit.bind(this); if this.submit is never passed to a component event handler like onClick)
  • All JSX used for rendering exists in the render method
  • 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 any new file was added I verified that:
  • The file has a description of what it does and/or why is needed at the top of the file if the code is not self explanatory
• 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 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 author checklist, including those that don't apply to this PR.

Screenshots/Videos

Android: Native

Screen.Recording.2026-04-03.at.15.07.16-compressed.mov

Android: mWeb Chrome

N/A, chrome is always crashing in my emulator

iOS: Native

Screen.Recording.2026-04-03.at.15.13.56-compressed.mov

iOS: mWeb Safari

Screen.Recording.2026-04-03.at.15.17.41-compressed.mov

MacOS: Chrome / Safari

Screen.Recording.2026-04-03.at.15.20.27-compressed.mov

Screen.Recording.2026-04-03.at.15.23.32-compressed.mov

[fabioh8010]

@Julesssss @tgolen ready to review!

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[Julesssss]

Could you add some before/after metrics to this PR please. Also I generated an AdHoc build for the linked App PR and will pass to QA to for some initial testing.