Vitest browser-mode: 'Unknown event: response:response:...' errors when running multiple click tests

[simshanith]

Summary

When running multiple browser tests with vitest that involve click events on anchor elements, thousands of "Unknown event: response:response:response:..." errors occur, causing the test runner to fail even though all tests pass.

Reproduction

1. Clone this repo and checkout commit 9d116d4 (also tagged issue-reproduction/simshanith/lit-ui-router/issues/69)
2. Run pnpm install
3. Run the skipped tests - may need to edit to unskip first

   pnpm --filter lit-ui-router test ui-sref.spec -t "click modifiers" --browser=chromium

Expected behavior

Tests pass with no errors.

Actual behavior

Tests pass but with thousands of errors like:

Error: Unknown event: response:response:response:response:response:execute
Error: Unknown event: response:response:response:response:response:cleanup

The response: prefix accumulates recursively.

Key Details

• Issue only occurs when multiple tests run - individual tests pass without errors
• Tests involve click events on anchor elements
• Anchors may trigger navigation via router.stateService.go()
• Some tests use target="_blank" or rel="external" attributes
• Test suites affected: "click modifiers", "target attribute" in ui-sref.spec.ts
• Running these suites together causes exponential error accumulation

Environment

• vitest: 4.0.16
• @vitest/browser-playwright: 4.0.16
• playwright: 1.54.1
• Node: v22.x
• OS: macOS

Workaround

Tests are currently skipped with .skip - see commit 9d116d4

Related

Will file upstream issue on vitest-dev/vitest.

[simshanith]

found testing #68

[simshanith]

tagged https://github.com/simshanith/lit-ui-router/releases/tag/issue-reproduction/simshanith/lit-ui-router/issues/69

[simshanith]

fixed with patch for now; leaving open until resolved upstream

[simshanith]

opened vitest-dev/vitest#9381