fix: retry logic for support file imports in vite-dev-server

[jennifer-shehane]

• Closes Component tests failing intermittently with uncaught Vite "failed to fetch dynamically imported module" error in CI #25913

Additional details

Component tests were experiencing intermittent failures with TypeError: Failed to fetch dynamically imported module when loading the support file. This occurred due to a race condition where the dynamic import executed before Vite's optimizeDeps finished processing the module.

Vite's optimizeDeps runs asynchronously in the background. Even though the support file is added to optimizeDeps.entries for pre-bundling, server.listen() can complete before the module is ready, causing the dynamic import to fail. Vite doesn't provide an API to wait for optimizeDeps completion.

Implemented retry logic with exponential backoff (3 attempts: 50ms, 100ms, 200ms) for support file imports. The implementation:

• Only retries on network/module loading errors (not syntax errors)
• Makes new import() calls on each retry (browsers disallow retrying the same call per Failed dynamic import should not always be cached whatwg/html#6768)
• Uses exponential backoff to give Vite time to finish processing

Vite's vite:preloadError event is designed for production version skew scenarios and doesn't help with dev server timing issues. Since there's no API to wait for optimizeDeps, retry logic is the most practical solution for this race condition.

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Note

Listen for Vite preload errors when importing the support file, reload once to refresh assets with a sessionStorage loop guard, add targeted tests, and disable screenshots-on-failure in test fixtures.

• Vite Dev Server (client):
  • Add vite:preloadError listener in npm/vite-dev-server/client/initCypressTests.js to detect failed dynamic imports of the support file.
  • Extract URL from Error.message when needed; on match with support file, preventDefault, set session flag (sessionStorage) to avoid reload loops, and window.location.reload().
  • Simplify support file loader to a direct import(relativeUrl).
• Tests:
  • Extend npm/vite-dev-server/test/initCypressTests.spec.ts to validate listener registration, URL extraction, reload/guard behavior, and absence of retry logic; update window/sessionStorage mocks.
• System Tests/Snapshots:
  • Update snapshots in system-tests/__snapshots__/vite_dev_server_fresh_spec.ts.js to reflect no screenshots on failures.
• Config (fixtures):
  • Set screenshotOnRunFailure: false in system-tests/project-fixtures/react/cypress-vite.config.ts.

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Steps to test

How has the user experience changed?

PR Tasks

• Have tests been added/updated?
• Has a PR for user-facing changes been opened in cypress-documentation?
• Have API changes been updated in the type definitions?

[cypress]

cypress    Run #68158

Run Properties:   Passed #68158  •  230dc8a80b: fix test

Project	cypress
Branch Review	vite-dev-server-retry-loading-support
Run status	Passed #68158
Run duration	20m 56s
Commit	230dc8a80b: fix test
Committer	Jennifer Shehane
View all properties for this run ↗︎

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Test results
Failures	0
Flaky	8
Pending	1098
Skipped	4
Passing	26989
View all changes introduced in this branch ↗︎

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Warning

Partial Report: The results for the Application Quality reports may be incomplete.

UI Coverage  45.18%
Untested elements	194
Tested elements	164

Accessibility  97.94%
Failed rules	4 critical   8 serious   3 moderate   2 minor
Failed elements	102