`React.StrictMode` not working when child of `wrapper` in React 19

[bernardobelchior]

• @testing-library/react version: 16.2.0
• Testing Framework and version: Vitest v3.0.9
• DOM Environment: JSDOM v26

Relevant code or config:

The assertion in the following snippets fails in React 19, but succeeds in React 18:

test("renders in strict mode when using a wrapper", async () => {
  let mounted = 0;
  function Component() {
    React.useEffect(() => {
      mounted += 1;
    });

    return null;
  }

  render(<Component />, { wrapper: Wrapper });

  expect(mounted).toEqual(2);
});

● › renders in strict mode when using a wrapper
expect(received).toEqual(expected) // deep equality

Expected: 2
Received: 1

  18 |   render(<Component />, { wrapper: Wrapper });
> 20 |   expect(mounted).toEqual(2);
     |                   ^
  21 | });

What you did:

Rendered StrictMode in Wrapper component that is passed do the wrapper property of the options object in the render function from React Testing Library.

What happened:

Expected Strict Mode to be enabled, but it wasn't. This can be seen because in Strict Mode in React 18 and 19 useEffect should be run twice, but in this case it was only run once.

This issue only happens in React 19. In React 18 it works as expected.

Reproduction:

React 19 - tests fail

React 18 - tests pass

Problem description:

In React 19, when rendering React.StrictMode in a component provided to wrapper, Strict Mode isn't enabled.

In React 18, however, Strict Mode is enabled.

Suggested solution:

In React 19, rendering React.StrictMode in a component provided to wrapper should enable Strict Mode.