useObservable mutates shared _immutableStatus snapshot, corrupting concurrent renders #736

Bug

useObservable applies initialData by mutating the object returned by getSnapshot() in-place (lines 130-132 of src/useObservable.ts):

const update = useSyncExternalStore(subscribe, getSnapshot);

if (!observable.hasValue && hasData) {
  update.data = config?.initialData ?? config?.startWithValue;
  update.status = 'success';
  update.hasEmitted = true;
}

getSnapshot() returns observable.immutableStatus, which is SuspenseSubject._immutableStatus — a single shared object reference for all components using the same observableId. Mutating it in-place means one component's render corrupts the snapshot for every other component sharing that observable.

Concrete failure scenario

Two components both call useUser() for the same auth instance (same observableId → same SuspenseSubject → same _immutableStatus object):

1. Component A has initialData set (e.g., auth.currentUser is truthy after fix: seed useUser with auth.currentUser to prevent undefined on initial render #731). It mutates _immutableStatus.status = 'success' and .data = user.
2. Component B calls getSnapshot() — it gets the same already-mutated object and renders status: 'success' with a user, even though it passed no initialData and the observable hasn't emitted yet.

useSyncExternalStore's tearing detection does not catch this: Object.is() on the same reference always returns true, so React treats the store as unchanged.

_updateImmutableStatus() (called on each observable emission) creates a new object — so the mutation is eventually overwritten. But between mount and first emission, the shared snapshot is wrong.

Why this surfaced now

This was a pre-existing bug, but it was only triggered when callers explicitly passed initialData. PR #731 seeds initialData from auth.currentUser on every mount of a signed-in user, making this path hit far more frequently.

Fix

Replace the in-place mutation with a new object:

if (!observable.hasValue && hasData) {
  return {
    ...update,
    data: config?.initialData ?? config?.startWithValue,
    status: 'success',
    hasEmitted: true,
  };
}
return update;

This keeps the SuspenseSubject._immutableStatus object untouched and gives each render its own snapshot.

Affected versions

All versions that support initialData / startWithValue. Worsened in 4.2.4+ after #731.