Flaky test failure in vitest browser mode when passing strings to render #569

Summary

Test files pass in isolation but fail in full-suite runs with:

• Error: Cannot configure the test module when the test module has already been instantiated. Make sure you are not using inject before TestBed.configureTestingModule.

This error always appears together with:

• TypeError: Cannot redefine property: ɵcmp

Environment

• Angular 21.2.1
• @testing-library/angular 19.1.0
• vitest 4.0.18
• @vitest/browser-playwright 4.0.18
• Chromium browser mode via Angular unit-test builder or AnalogJS

Pattern that triggers the issue

• Use render(...) from @testing-library/angular
• Use template-string rendering (not component class rendering)
• Run many tests/files in browser mode in the same run

Expected

Full-suite and isolated runs should behave consistently without TestBed/component metadata lifecycle errors.

Actual

In the larger suite, full runs intermittently fail with both errors (Cannot configure the test module... and Cannot redefine property: ɵcmp).

However, finding a minimal example to reproduce the issue consistently is challenging.

Workaround

Switching away from template-string wrapper patterns to explicit host components makes the failures disappear. I could, further, only see these failures in browser mode. Running a similar set of tests with JSDOM, did not cause these failures.