[Performance] Pre-render Search under RHP for instant post-submit navigation on mobile

[JakubKorytko]

Explanation of Change

On narrow layout (mobile), pre-renders the Search page behind the RHP while the user is on the expense confirmation screen, so post-submit navigation is instant.

How it works

Phase	What happens
Mount (confirmation screen)	After a 300ms delay, the Search fullscreen route is inserted underneath the RHP via REPLACE_FULLSCREEN_UNDER_RHP. The Search page mounts in the background while the user fills in details.
Submit (fast path)	If the pre-insert fired, the RHP is dismissed instantly to reveal the already-mounted Search. createTransaction runs in the next frame - "dismiss first, compute later".
Submit (slow path)	If the user taps submit before the 300ms timer fires, the existing non-pre-insert navigation path is used as a fallback.
Back out (cleanup)	If the user leaves without submitting, the pre-inserted route is removed via the new REMOVE_FULLSCREEN_UNDER_RHP action.

Supporting changes

• reserveDeferredWriteChannel - pre-creates the deferred write channel so Search always sees hasDeferredWrite=true on mount
• SearchPageNarrow - gates phase transitions on focus (useFocusEffect) to avoid wasted work while hidden behind the RHP
• SearchStaticList - shows the pending expense placeholder on focus when a submit action is pending
• RightModalNavigator - disables slide-out animation when a pre-insert is active so the dismiss reveals the destination instantly

Performance gains (ManualSubmitToDestinationVisible (navigate_to_search) span):

Platform	main avg (ms)	This branch avg (ms)	Improvement
Android	615	197	-418 ms (68%)
iOS	1118	222	-896 ms (80.1%)

Fixed Issues

$ #83634
PROPOSAL: N/A

Tests

1. Fast-path submit (pre-insert fires before user taps submit)

   1. Open the app on a narrow-layout device (mobile or narrow browser window) and do not navigate to inbox/search
   2. Tap the FAB (+) button and start creating a new expense (e.g. Manual -> enter amount -> select participant)
   3. On the confirmation screen, wait at least 300ms (the pre-insert delay)
   4. Tap Submit / Create
   5. Verify that the Search page appears instantly without a visible navigation transition - the RHP dismisses to reveal the already-mounted Search screen with a pending expense placeholder row

2. Slow-path submit (user taps submit before 300ms pre-insert fires)

   1. Open the app on a narrow-layout device and do not navigate to inbox/search
   2. Tap the FAB (+) and quickly go through the expense creation flow
   3. On the confirmation screen, tap Submit immediately (within ~300ms of the screen appearing)
   4. Verify that the app still navigates to the Search page correctly (via the existing non-pre-insert path) and no errors appear

3. Back-out cleanup (user leaves confirmation without submitting)

   1. Open the app on a narrow-layout device and do not navigate to inbox/search
   2. Start creating an expense and reach the confirmation screen
   3. Wait at least 300ms for the pre-insert to fire
   4. Press the back button or swipe to go back without submitting
   5. Verify that the pre-inserted Search route is removed - you should return to the previous screen (not the Search page), and no stale Search route remains in the navigation stack

Offline tests

1. Enable airplane mode / disable network on the device
2. Tap the FAB (+) and create a new expense through the confirmation screen while being outside of inbox/search
3. Wait 300ms, then tap Submit
4. Verify the Search page appears with the optimistic pending expense placeholder row
5. Re-enable network and verify the expense syncs and the placeholder is replaced with the confirmed row

QA Steps

Same as tests

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
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Screenshots/Videos

Android: Native

Android.mov

Android: mWeb Chrome

iOS: Native

iOS.mov

iOS: mWeb Safari

MacOS: Chrome / Safari

[JakubKorytko]

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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
src/CONST/index.ts	93.65% <ø> (ø)
.../routerExtensions/addRootHistoryRouterExtension.ts	100.00% <100.00%> (ø)
src/pages/Search/SearchPageNarrow/index.tsx	81.48% <100.00%> (+0.34%)	⬆️
...igator/createRootStackNavigator/RootStackRouter.ts	73.84% <75.00%> (+1.71%)	⬆️
src/components/Search/SearchStaticList.tsx	4.00% <0.00%> (-0.26%)	⬇️
...on/AppNavigator/Navigators/RightModalNavigator.tsx	5.15% <0.00%> (-0.17%)	⬇️
src/libs/telemetry/submitFollowUpAction.ts	23.40% <0.00%> (-1.05%)	⬇️
src/libs/actions/IOU/index.ts	78.33% <50.00%> (-0.15%)	⬇️
src/libs/deferredLayoutWrite.ts	81.25% <30.76%> (-18.75%)	⬇️
src/components/Search/index.tsx	0.93% <0.00%> (-0.02%)	⬇️
... and 3 more
... and 39 files with indirect coverage changes

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App/src/libs/telemetry/submitFollowUpAction.ts

Lines 53 to 54 in 47a7bcc

	pendingSubmitFollowUpAction = {followUpAction, reportID};
	span.setAttribute(CONST.TELEMETRY.ATTRIBUTE_SUBMIT_FOLLOW_UP_ACTION, followUpAction);

Keep follow-up action as navigate_to_search for fast path

When this same-flow branch runs, it still overwrites pendingSubmitFollowUpAction (and the span attribute) with the new value. In the fast pre-insert flow, onConfirm sets NAVIGATE_TO_SEARCH, then handleNavigateAfterExpenseCreate() can later call DISMISS_MODAL_ONLY after Search is already on top; this code reclassifies that submit as dismiss-only, which skews submit-to-destination telemetry for the very path this PR is measuring. The NAVIGATE_TO_SEARCH -> DISMISS_MODAL_ONLY refinement should preserve the original action rather than replacing it.

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Re-run preinsert setup after transaction finishes loading

This pre-insert setup effect is one-shot ([]) but immediately bails when transaction is not yet available. IOURequestStepConfirmation can render before Onyx transaction data resolves, so in those cases the timer is never scheduled later and the new fast-path optimization silently never activates. The effect should depend on transaction/loading state (or explicitly trigger when loading completes) so late-loaded transactions still get pre-insert behavior.

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