fix: use microtask queue for whatwg-fetch XHR events

[rohit9625]

Explanation of Change

In React Native (iOS), we observed cases where:

• The network request completed successfully.
• xhr.onload executed successfully.
• The scheduled timeout was significantly delayed until a later user interaction or navigation event.
• The fetch promise remained unresolved until that timeout executed.

Because the promise resolution depends on a timer, any environment-specific issue affecting timer delivery can prevent completed requests from resolving promptly.

So this PR replaces setTimeout(0) with queueMicrotask for XHR event handlers within the whatwg-fetch polyfill.

Why queueMicrotask? : Microtasks are generally executed as part of the current JavaScript turn after the current call stack unwinds and do not depend on timer infrastructure. Also, I don't see any benefit of using setTimeout here because we are passing 0 as a timeout that is supposed to be executed as soon as possible. So, we should better use queueMicrotask here.

Fixed Issues

$ #90309
PROPOSAL: $ #90309 (comment)

Tests

1. Open the Expensify app
2. Navigate to the workspace chat and create a report
3. Add one expense with any amount
4. Split the expense from step 2
5. Open one of the child expenses
6. Click More -> Edit splits
7. Select one split and click Remove Split
8. Click save
9. Make sure the expense report loads normally and display the expense details without any UI interaction.

• Verify that no errors appear in the JS console

Offline tests

1. Open the Expensify app
2. Navigate to the workspace chat and create a report
3. Add one expense with any amount
4. Split the expense from step 2
5. Force offline or turn off the internet
6. Open one of the child expenses
7. Click More -> Edit splits
8. Select one split and click Remove Split
9. Click save
10. You should see the Not Here page, then navigate back
11. Now, turn on the internet
12. Make sure the expense report loads normally and displays the expense details without any UI interaction.

QA Steps

// TODO: These must be filled out, or the issue title must include "[No QA]."
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 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 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 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 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

Screen.Recording.2026-06-24.at.9.15.44.AM.mov

Android: mWeb Chrome

Screen.Recording.2026-06-24.at.8.30.50.AM.mov

iOS: Native

Screen.Recording.2026-06-24.at.8.49.51.AM.mov

iOS: mWeb Safari

Screen.Recording.2026-06-24.at.8.41.16.AM.mov

MacOS: Chrome / Safari

Screen.Recording.2026-06-24.at.8.21.36.AM.mov

[github-actions]

⚠️ This PR is possibly changing native code and/or updating libraries, it may cause problems with HybridApp. Please check if any patch updates are required in the HybridApp repo and run an AdHoc build to verify that HybridApp will not break. Ask Contributor Plus for help if you are not sure how to handle this. ⚠️

[melvin-bot]

@dmkt9 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]