Suspense mode resumes with the previous observable's value on React 19

npx vitest run useObservableを実行し、test/useObservable.test.tsxから始めます。特にemits the new observable's value if the observable is swapped outを確認してください。名前付きのSuspenseSubject、preloadedObservablesキャッシュ、use-sync-external-storeの経路を追跡します。完了条件は、テストが通常のitに戻され、React 18の動作を維持したままReact 19でパスすることです。

useObservable in suspense mode suspends correctly when the observable and observableId are swapped, then resumes rendering the previous observable's value.

Found by the first test run ever executed against React 19 (#789); nothing had run against 19 before, only type-checks.

Reproduction

test/useObservable.test.tsx, emits the new observable's value if the observable is swapped out. The test renders a component under React.Suspense, emits Jeff on the first observable, swaps both the observable and the observableId, rerenders, then emits James on the second.

• React 18.2.0: the fallback shows, then the component resumes with James. Passes.
• React 19.2.8: the fallback shows (that assertion passes), then the component resumes with Jeff. Fails.

Deterministic: 3 runs each way, same @testing-library/react 16.3.2, only the React runtime swapped via npm install --no-save react@19 react-dom@19. No emulators involved, so npx vitest run useObservable reproduces it.

What is known and not known

• The suspend half works. Only the resume is wrong, so this is not the known throw-vs-use() question: the 08-07 spike found the use() swap is a behavioural no-op on 19 and that React 19 emits no warning for the current throw form. Nothing here contradicts that.
• The cause is not diagnosed. The plumbing involved (SuspenseSubject, the preloadedObservables cache keyed by observableId, and the use-sync-external-store shim delegating to React's own useSyncExternalStore on 19) overlaps what #790 covers, but no connection has been established and it should not be assumed.

Current state

#789's CI PR skips this test on React 19 only, so the new test job can land green. The skip is a ? it.skip : it ternary rather than it.skipIf, because these globals are typed by @types/jest and skipIf fails tsc while working at runtime.

Restore it to a plain it when this is fixed.