A lot of React Warnings Warning: An update to ... inside a test was not wrapped in act(...). appeared since 3.0.0-beta.2 (fixed in 3.0.2)

[ceisele-r]

Describe the bug

Note: This seems to be already fixed with the just released v3.0.2 and can immediately be closed if you also consider this to be intentionally fixed with v3.0.2.
However I still thought reporting this as a reference might be worth as others might also face this issue after updating to v3 and search for a solution. Besides that, I think it might be worth mentioning that this appeared after the v3 update as I am not sure by the changelog for v3.0.2 what exactly fixed it. I guess #7281 fixed it.

Bug description:
Since updating to vite v3 (tested v3.0.1), a lot of the following react warning appeared in quite some existing tests that would not emit that warning with vitest <= 3.0.0-beta.1.
I traced it down that these warnings started to be emitted with vitest 3.0.0-beta.2.

Again, after updating to v3.0.2, the issue was gone in my existing tests.

Reproduction

This is kind of a reproduction that creates such a warning.
It is based on the reproduction in #7196 and causes an update to the Button react component after 400ms.

https://stackblitz.com/edit/vitest-fake-timers-act-warnings-trw1va8q?file=src%2FButton.test.tsx

Such a promise at the end of a test is not used in my regular tests, however, that was how I could trigger this warning in the repro.
However, I am unsure whether this is really a good repro of the issue as it kind of intentionally makes the react component to update after the test completed. Still, I thought testing-library/react would call cleanup in a global afterEach after each test which should prevent this.

System Info

System:
    OS: Windows 11 10.0.22631
  Binaries:
    Node: 22.13.0 - C:\Program Files\nodejs\node.EXE
    npm: 10.9.2 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Chromium (131.0.2903.146)
  npmPackages:
    @vitejs/plugin-react: ^4.3.4 => 4.3.4
    @vitejs/plugin-react-swc: ^3.7.2 => 3.7.2
    @vitest/coverage-v8: ^3.0.2 => 3.0.2
    @vitest/ui: ^3.0.2 => 3.0.2
    vite: ^5.4.11 => 5.4.11
    vitest: ^3.0.1 => 3.0.1

Used Package Manager

npm

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• The provided reproduction is a minimal reproducible example of the bug.

[hi-ogawa]

The reproduction you attached still shows a act warning but the rational is as described in #7196 and it's out of scope for Vitest with my current understanding.
Let me close this as your main issue seems fixed.