Reproduction repo here: https://github.com/a-type/vitest-browser-react-suspense-repro

I have a component which suspends on render until a certain promise is fulfilled (I'm using the new React 19 use, but I believe this behavior is consistent without React 19, for example using the older suspend-react library).

When I render this component in a test (wrapping it with a Suspense boundary), it never updates. I can see that the promise is resolved via the test log, but the rendered output remains in the suspended state.

If I run a normal Vite app with the same component, I can see that after the appropriate time, the suspense is resolved and the component contents are displayed.

Additional logs from the test are probably relevant:

A component suspended inside an `act` scope, but the `act` call was not awaited. When testing React components that depend on asynchronous data, you must await the result:

await act(() => ...)

I manually patched the vitest-browser-react module to await act on initial render and it appears to have resolved the problem. However, this changes the API, requiring render to be an async function and awaited by the user.

-function act(cb) {
+async function act(cb) {
	const _act = React.act || React.unstable_act;
	if (typeof _act !== "function") {
		cb();
	} else {
		globalThis.IS_REACT_ACT_ENVIRONMENT = true;
-		_act(cb);
+		await _act(cb);
		globalThis.IS_REACT_ACT_ENVIRONMENT = false;
	}
}

// ...
-function render(ui, { container, baseElement, wrapper: WrapperComponent } = {}) {
+async function render(ui, { container, baseElement, wrapper: WrapperComponent } = {}) {

// ...
-act(() => {
+await act(() => {
	root.render(
		strictModeIfNeeded(wrapUiIfNeeded(ui, WrapperComponent))
	);
});