Fixes #11171

Problem

When the last listener unsubscribes, MutationObserver.onUnsubscribe() removes the observer from its current mutation — but resubscribing never adds it back. React legitimately tears down and re-establishes the store subscription of a mounted component that keeps its state (<Activity mode="hidden">, re-suspending <Suspense> boundaries, StrictMode in dev).

If that happens while a mutation is in flight, the mutation settles against an empty observer list, and the observer keeps serving its frozen pending snapshot from getCurrentResult() forever:

• isPending stays true → submit buttons using disabled={isPending} stay disabled until remount
• callbacks passed to mutate(vars, { onSuccess }) never fire
• devtools/MutationCache show success while the hook shows pending

Fix

Mirror QueryObserver.onSubscribe(): when the first listener subscribes, re-attach the observer to its current mutation (Mutation.addObserver is already idempotent) and refresh the result snapshot in case the mutation settled while detached.

Tests

Two regression tests in mutationObserver.test.tsx, driving subscribe/unsubscribe the way useSyncExternalStore does:

1. unsubscribe + resubscribe while the mutation is in flight → observer reaches success and notifies the new listener
2. mutation settles while fully unsubscribed → resubscribing refreshes the snapshot to the final state

Both fail on main and pass with the fix. Full query-core (627) and react-query (577) suites pass.

Summary by CodeRabbit

• Bug Fixes

  • Mutation status and results now refresh correctly when listeners unsubscribe and later resubscribe.
  • In-progress mutations remain tracked across subscription changes.
  • Mutations that finish while no listeners are present now report their final success state when observed again.

• Tests

  • Added coverage for mutation observer behavior across unsubscribe and resubscribe cycles.