`ResizeObserver` infinite loop causes `null` being logged

[Janpot]

Describe the bug

When certain errors, like "ResizeObserver loop completed with undelivered notifications" happen, null is printed to stderr with absolutely no clues about what's going on.

Reproduction

it("foo", async () => {
  const divElem = document.createElement("div");
  divElem.style.width = "100px";
  divElem.style.height = "100px";
  document.body.appendChild(divElem);

  const resizeObserver = new ResizeObserver((entries) => {
    for (const entry of entries) {
      entry.target.style.width = `${entry.contentBoxSize[0].inlineSize + 10}px`;
    }
  });

  resizeObserver.observe(divElem);
  await new Promise((resolve) => setTimeout(resolve, 1000));
  resizeObserver.unobserve(divElem);
});

Causes

...
stderr | test/suite.test.ts > foo
null
...

Repro: https://github.com/Janpot/vitest-observer-loop-repro/

Run pnpm test:run

System Info

System:
    OS: macOS 14.6.1
    CPU: (8) arm64 Apple M1
    Memory: 76.13 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.11.1 - ~/.nvm/versions/node/v24.11.1/bin/node
    npm: 11.6.2 - ~/.nvm/versions/node/v24.11.1/bin/npm
    pnpm: 10.13.1 - ~/.nvm/versions/node/v24.11.1/bin/pnpm
  Browsers:
    Safari: 17.6
  npmPackages:
    @vitest/browser-playwright: ^4.0.16 => 4.0.16 
    @vitest/ui: latest => 4.0.16 
    vite: latest => 7.3.0 
    vitest: latest => 4.0.16

Used Package Manager

pnpm

Validations

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• Read the Contributing Guidelines.
• Read the docs.
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• The provided reproduction is a minimal reproducible example of the bug.

[hi-ogawa]

It looks like this is because observer error doesn't have ErrorEvent.error property but only ErrorEvent.message https://drafts.csswg.org/resize-observer/#deliver-resize-error https://developer.mozilla.org/en-US/docs/Web/API/ErrorEvent/error. I'm not sure what's the rational, but currently Vitest assumes ErrorEvent.error property to surface unhandled error:

vitest/packages/browser/src/client/public/error-catcher.js

Lines 61 to 62 in fa63449

	const offError = catchWindowErrors('error', 'error', event =>
	reportUnexpectedError('Error', event.error))

EDIT: actually here (e[prop] is e.error, which is null)

vitest/packages/browser/src/client/public/error-catcher.js

Lines 30 to 35 in c8f8f98

	if (userErrorListenerCount === 0 && e[prop] != null) {
	cb(e)
	}
	else {
	console.error(e[prop])
	}