perf: defer ReportScreen mount on wide layout to speed up Inbox tab navigation

[BartekObudzinski]

Explanation of Change

The ReportsSplitNavigator mounts both the LHN sidebar (BaseSidebarScreen) and the central ReportScreen in the same render tick. On wide layout this means ReportScreen competes with the LHN for JS frame budget during the ManualNavigateToInboxTab span — the time from BottomTab click to LHN layout completion.

This change wraps the central ReportScreen in NavigationDeferredMount (the component introduced in #88522) so on wide layout it hydrates after the navigation transition completes, inside startTransition. The LHN paints first and ends the span; ReportScreen mounts non-urgently afterward. On narrow layout (shouldUseNarrowLayout is true), the wrapper is bypassed and ReportScreen renders immediately as before — narrow layout only mounts ReportScreen in response to a user tap, where deferral would be wrong.

Measured impact (web wide, dev environment)

• ManualNavigateToInboxTab: 605ms → 116ms (~81% reduction)

Scope

• Wide layout only. Narrow layout unchanged.
• No data layer changes. No Onyx changes. No memoization changes.
• Single file touched.

Fixed Issues

$
PROPOSAL:

Tests

• Verify that no errors appear in the JS console

Offline tests

N/A

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)
  • 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 shown in the product is localized by adding it to src/languages/* files and using the translation method
    • If any non-english text was added/modified, I used JaimeGPT to get English > Spanish translation. I then posted it in #expensify-open-source and it was approved by an internal Expensify engineer. Link to Slack message:
  • I verified all numbers, amounts, dates and phone numbers shown in the product are using the localization methods
  • 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)
  • 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.ts 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 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

[BartekObudzinski]

Closing — the span improvement (605→116ms) was largely a measurement artifact. Deferring ReportScreen mount made the LHN paint faster but caused the chat to appear visibly later than before. Need a different approach (reduce ReportScreen mount cost itself, not shuffle it outside the span window).