Handle enhanced navigation fetch failures

[NanthiniMahalingam]

Bug Description

When Blazor enhanced navigation intercepted a link or form submission, it issued the navigation through fetch. If that request rejected—for example, because the server had stopped or the network was unavailable—the rejection occurred before response processing. As a result:

• GET navigation could update the address bar but leave the old page visible.
• The user received no indication that navigation had failed.
• Enhanced form POST failures were not surfaced through the Blazor error UI.
• A failed non-GET request could leave enhanced navigation marked as still loading.
• Retrying POST requests as full-page loads would not be safe because POST operations may have side effects.

Description of code changes

In NavigationEnhancement.ts:

• Enhanced form submissions now observe rejected navigation promises.
• Failures are written to console.error.
• The existing showErrorNotification() behavior displays #blazor-error-ui.

In performEnhancedPageLoad:

• The request’s AbortController is retained locally.
• Expected AbortError failures from superseded navigation are ignored.
• Failed GET requests call the existing full-page fallback.
• Failed non-GET requests:
  Aren’t automatically retried.
  Clear loading state only when they are still the current navigation.
  Invoke enhancedNavigationCompleted.
  Rethrow the original fetch error.

Tests

The new NavigationEnhancement.test.ts covers the failure, cancellation, form-error, and existing successful-navigation paths.

Output

Before changes

51618_GetNavigation_BeforeFix.mp4

51618_PostNavigation_BeforeFix.mp4

After changes

51618_GetNavigation_AfterFix.mp4

51618_PostNavigation_AfterFix.mp4

Fixes #51618

[dotnet-policy-service]

Thanks for your PR, @NanthiniMahalingam. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[Copilot]

🟡 Changes recommended

Mixed-case GET forms can lose submitted values, and the browser-owned regression lacks required Selenium coverage.

2 open findings

• Normalize form method before building GET request and preserve values · New
• Add Selenium coverage for native network failure and fallback behavior · New

What changed in this PR

Handles enhanced-navigation network failures while avoiding unsafe POST retries.

Changes:

• Falls back to full-page navigation for failed GET requests.
• Surfaces POST failures through the Blazor error UI and resets navigation state.
• Adds Jest coverage for failure, cancellation, and success paths.

File	Description
NavigationEnhancement.ts	Adds failure handling and cancellation-aware cleanup.
NavigationEnhancement.test.ts	Adds mocked navigation failure tests.

🧠 Review effort: Balanced