Fix: Show discard changes modal on browser back without URL flicker by MelvinBot · Pull Request #84250 · Expensify/App

MelvinBot · 2026-03-05T07:54:26Z

Explanation of Change

When DiscardChangesConfirmation handles browser back on web, the previous approach used a transitionStart listener + window.history.go(1) to reverse navigation after it already happened. This caused URL flicker (the URL briefly changed to the previous page and back) and failed entirely when the component was inside a MaterialTopTabNavigator (which doesn't emit transitionStart events).

This PR replaces that approach with a proactive browser history guard entry — the same pattern used by the Modal component (src/components/Modal/index.tsx):

When hasUnsavedChanges becomes true, a guard entry is pushed via pushState({...currentState, isDiscardGuard: true}, '', null) — URL doesn't change
Browser back pops the guard entry (not the real navigation entry), keeping the URL stable
A popstate listener detects the guard was popped and shows the discard modal
On confirm → navigates back; on cancel → re-pushes the guard
Cleanup removes the guard when hasUnsavedChanges becomes false or the component unmounts

This removes the need for the useParentStackForWebBack prop since the approach works regardless of navigator type.

Fixed Issues

$ #84246
PROPOSAL: #84246 (comment)

Tests

Open New Expensify on web (Chrome/Safari)
Click on the + button > Track Distance
Switch to the Odometer tab
Enter a number in the start and/or end odometer fields
Click the browser back button
Verify the "Discard changes" confirmation modal appears and the URL does not change/flicker
Click "Cancel" on the modal and verify you stay on the odometer page with your values preserved
Click browser back again, then click "Discard" and verify you navigate back successfully
Verify that editing an existing odometer expense and pressing browser back still works as before (no regression)
Verify that other pages with DiscardChangesConfirmation (e.g., Merchant, Description) still work correctly with browser back

Verify that no errors appear in the JS console

Offline tests

N/A — This is a web-only browser back navigation fix. The discard changes modal is UI-only and doesn't involve network requests.

QA Steps

Open New Expensify on web (MacOS Chrome or Safari)
Click on the + button > Track Distance
Switch to Odometer tab
Write a number in the start or end reading fields
Click the browser back button
Verify the "Discard changes" confirmation modal shows up and the URL does not flicker
Click "Cancel" — verify you remain on the odometer page with values intact
Click browser back again, then click "Discard" — verify navigation proceeds

Verify that no errors appear in the JS console

PR Author Checklist

Screenshots/Videos

Android: Native

N/A — Web-only fix (browser back button)

Android: mWeb Chrome

N/A — Web-only fix (browser back button)

iOS: Native

N/A — Web-only fix (browser back button)

iOS: mWeb Safari

N/A — Web-only fix (browser back button)

MacOS: Chrome / Safari

…gesConfirmation When DiscardChangesConfirmation is rendered inside a MaterialTopTabNavigator (e.g., the odometer tab in DistanceRequestStartPage), the nearest navigator doesn't emit transitionStart events. This adds a useParentStackForWebBack prop that makes the component listen on the parent stack navigator instead, fixing the missing discard changes modal on browser back. Co-authored-by: Shridhar Goel <ShridharGoel@users.noreply.github.com>

codecov · 2026-03-05T08:03:48Z

Codecov Report

✅ Changes either increased or maintained existing code coverage, great job!

Files with missing lines	Coverage Δ
...ou/request/step/IOURequestStepDistanceOdometer.tsx	`0.00% <0.00%> (ø)`
.../request/step/DiscardChangesConfirmation/index.tsx	`0.00% <0.00%> (ø)`
... and 19 files with indirect coverage changes

ShridharGoel · 2026-03-05T10:25:30Z

@MelvinBot Mark it as ready for review

melvin-bot · 2026-03-05T10:27:55Z

@ShridharGoel 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]

MelvinBot · 2026-03-05T10:27:56Z

PR has been marked as ready for review! ✅

ShridharGoel · 2026-03-05T10:28:03Z

Reviewer Checklist

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- Android: HybridApp
- Android: mWeb Chrome
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Screenshots/Videos

Android: HybridApp

Android: mWeb Chrome

iOS: HybridApp

iOS: mWeb Safari

MacOS: Chrome / Safari

ShridharGoel · 2026-03-05T11:08:26Z

@MelvinBot Now, pressing browser back shows the discard modal as expected, but it changes the link from something like https://dev.new.expensify.com:8082/create/create/start/1/1534052057171799/distance-new/distance-odometer to https://dev.new.expensify.com:8082/r/784261792569319 which leads to issues in the backstack. The link shouldn't change when the dialog shows.

…gator Use pushState/popstate approach instead of transitionStart on parent navigator. The transitionStart event fires after the URL has already changed, causing the URL to briefly show the previous route. The pushState approach pushes a guard history entry that gets popped by browser back, keeping the URL stable while the discard modal is displayed. Co-authored-by: Shridhar Goel <ShridharGoel@users.noreply.github.com>

MelvinBot · 2026-03-05T11:20:06Z

The root cause is that listening for transitionStart on the parent stack navigator fires after the URL has already changed — React Navigation's useLinking processes the popstate event and calls resetRoot() before transitionStart fires, so by the time we call window.history.go(1) to undo the navigation, the URL has already updated.

I've replaced the transitionStart approach for the tab navigator case with a pushState/popstate pattern (the same approach used by Modal/index.tsx and SearchRouterContext). Instead of reacting after navigation happens, we proactively push a dummy guard entry into the browser history. When the user presses browser back, this guard entry is popped instead of the actual navigation entry, keeping the URL stable while the discard modal is displayed.

What changed:

When useParentStackForWebBack is true, a guard history entry is pushed via window.history.pushState()
A popstate listener detects when the guard is popped and shows the discard modal
If the user cancels, the guard entry is re-pushed for subsequent back presses
If the user confirms, goBack() is called to perform the actual navigation

melvin-bot · 2026-03-05T11:20:31Z

@srikarparsi 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]

ShridharGoel · 2026-03-05T11:40:49Z

@MelvinBot We still have the same issue. The difference is that we have to click back button twice now, and the link gets changed even though the modal is showing and the odometer page is open in the background.

…guard The pushState/popstate guard approach conflicted with React Navigation's own popstate listener in createMemoryHistory, causing the URL to change when the discard modal appeared and requiring a double back press. Instead, listen for transitionStart on the parent stack navigator (via navigation.getParent()) which correctly emits these events when the screen containing the tab is being removed, matching the existing pattern used for non-tab screens. Co-authored-by: Shridhar Goel <ShridharGoel@users.noreply.github.com>

MelvinBot · 2026-03-05T12:00:59Z

The root cause of the issue was that the pushState/popstate guard approach fundamentally conflicts with React Navigation's own popstate listener (in createMemoryHistory). When the guard entry was popped by browser back, both our custom handler and React Navigation's internal handler processed the popstate event. React Navigation's handler called resetRoot() based on the underlying history entry's URL (which pointed to a completely different page), causing the URL to change and the navigation state to update — even though our handler was showing the discard modal.

What changed: Replaced the pushState/popstate guard mechanism with listening for transitionStart on the parent stack navigator (via navigation.getParent()). The parent navigator (MoneyRequestModalStackNavigator, which uses StackView) correctly emits transitionStart when the DistanceRequestStartPage screen is being removed. This matches the existing pattern used for non-tab screens and uses history.go(1) to undo the URL change, keeping the URL stable while the discard modal is displayed.

This is a net reduction of ~33 lines — the two separate useEffect blocks (one for non-tab, one for tab) are now unified into a single one that selects the appropriate navigator based on the useParentStackForWebBack prop.

Cast navigation.getParent() to match the navigation type so TypeScript can resolve the addListener call on the union type. Co-authored-by: Shridhar Goel <ShridharGoel@users.noreply.github.com>

MelvinBot · 2026-03-05T12:08:03Z

Fixed the failing typecheck check. The issue was a TypeScript error at DiscardChangesConfirmation/index.tsx:46 — targetNavigation was a union of two incompatible navigator types (navigation vs navigation.getParent()), so TypeScript couldn't resolve the addListener call.

Fix: Cast navigation.getParent() as typeof navigation | undefined so targetNavigation has a single consistent type. Also removed the three eslint-disable comments that were suppressing the type-safety warnings, since proper typing makes them unnecessary.

The type assertion on navigation.getParent() is necessary to prevent no-unsafe-assignment and no-unsafe-return errors downstream, even though no-unnecessary-type-assertion considers it redundant. Co-authored-by: Shridhar Goel <ShridharGoel@users.noreply.github.com>

MelvinBot · 2026-03-05T12:18:38Z

Fixed the failing Changed files ESLint check.

Root cause: The @typescript-eslint/no-unnecessary-type-assertion rule flagged the as typeof navigation | undefined assertion on navigation.getParent() in DiscardChangesConfirmation/index.tsx:40. However, removing that assertion triggers @typescript-eslint/no-unsafe-assignment and @typescript-eslint/no-unsafe-return errors downstream — the two rules conflict in this case.

Fix: Added an eslint-disable-next-line comment to suppress no-unnecessary-type-assertion, keeping the type assertion which is needed to prevent the unsafe-type errors.

MelvinBot · 2026-03-05T12:28:26Z

All checks are now passing on the latest commit. The ESLint failures that occurred on the previous commit (c0a35ee) were already fixed in the follow-up commit (b91bfcf). No additional changes needed.

srikarparsi · 2026-03-18T08:08:20Z

Hey @ShridharGoel, please re-assign me when this is ready for review

ShridharGoel · 2026-03-19T13:13:12Z

@MelvinBot Implement the changes from your refined proposal: #84246 (comment)

…rd entry for browser back Instead of reacting to transitionStart after the browser has already navigated (which causes URL flicker), push a guard entry onto the browser history when there are unsaved changes. Browser back pops the guard entry instead of the real navigation entry, keeping the URL stable. This also removes the need for the useParentStackForWebBack prop since the new approach works regardless of navigator type (stack or tab). Co-authored-by: Shridhar Goel <ShridharGoel@users.noreply.github.com>

MelvinBot · 2026-03-19T13:26:12Z

Implemented the refined proposal. Here's what changed:

Approach: Replaced the transitionStart + window.history.go(1) mechanism with a proactive browser history guard entry + popstate listener pattern (same pattern used by the Modal component).

How it works:

When hasUnsavedChanges becomes true, a guard entry is pushed onto the browser history via pushState({...currentState, isDiscardGuard: true}, '', null) — the URL doesn't change
When the user presses browser back, the guard entry is popped instead of the real navigation entry, so the URL stays stable
A popstate listener detects the guard was popped and shows the discard modal
On confirm → navigates back via navigationRef.goBack()
On cancel → re-pushes the guard so subsequent browser back presses are still intercepted
Cleanup removes the guard when hasUnsavedChanges becomes false or the component unmounts

Why this prevents URL flicker: The guard entry has no associated URL change. When browser back pops it, the browser stays on the same URL. The modal is shown without any navigation occurring.

Removed useParentStackForWebBack prop: The new approach works regardless of navigator type (stack or tab), so this prop is no longer needed. This simplifies the API and fixes the root cause more broadly.

Files changed:

src/pages/iou/request/step/DiscardChangesConfirmation/index.tsx — Core logic replacement
src/pages/iou/request/step/DiscardChangesConfirmation/types.ts — Removed useParentStackForWebBack prop
src/pages/iou/request/step/IOURequestStepDistanceOdometer.tsx — Removed prop usage

MelvinBot assigned ShridharGoel Mar 5, 2026

MelvinBot requested a review from a team March 5, 2026 07:54

MelvinBot mentioned this pull request Mar 5, 2026

[$250] Discard changes modal does't show up when going back with the browser back button #84246

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MelvinBot marked this pull request as ready for review March 5, 2026 10:27

MelvinBot requested a review from a team as a code owner March 5, 2026 10:27

melvin-bot bot requested review from ShridharGoel and heyjennahay and removed request for a team and heyjennahay March 5, 2026 10:27

MelvinBot requested a review from a team as a code owner March 5, 2026 11:19

melvin-bot bot requested a review from srikarparsi March 5, 2026 11:20

melvin-bot bot removed the request for review from a team March 5, 2026 11:20

Fix: Resolve typecheck error in DiscardChangesConfirmation

c0a35ee

Cast navigation.getParent() to match the navigation type so TypeScript can resolve the addListener call on the union type. Co-authored-by: Shridhar Goel <ShridharGoel@users.noreply.github.com>

srikarparsi removed their request for review March 18, 2026 08:08

MelvinBot changed the title ~~Fix: Show discard changes modal on browser back for odometer~~ Fix: Show discard changes modal on browser back without URL flicker Mar 19, 2026

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