fix: dispatch ACTION_RESTORE synchronously after browser back/forward navigation

[arab971]

What?

Fixes browser back/forward navigation leaving the page visually restored but functionally frozen — no JavaScript executing, no errors in console, no interactivity. Fixes #93905.

Why?

Three nested deferral mechanisms prevented React from committing the fiber tree after popstate:

1. startTransition wrapping in popstate/pushReplace handlers (app-router.tsx) — marks the ACTION_RESTORE dispatch as a low-priority transition, deferring the React fiber commit until idle time
2. Dev-mode appDevRenderingIndicator (use-action-queue.ts) — re-wraps every dispatch in another startTransition via useTransition, adding a second deferral layer in development mode
3. async function runAction (app-router-instance.ts) — forces synchronous reducer results through .then() microtask, deferring setState even when the outer startTransition wrappers are absent

The browser bfcache restores the page visually and immediately, but React useEffect hooks dont fire until the fiber tree commits. With three layers of deferral, the commit is indefinitely delayed — the page looks correct but is a frozen snapshot.

How?

• Remove startTransition wrapper from onPopState and applyUrlFromHistoryPushReplace — dispatch ACTION_RESTORE synchronously
• In dev mode, bypass appDevRenderingIndicator for ACTION_RESTORE actions — dispatch directly to actionQueue.dispatch
• Remove async from runAction and the action function — synchronous reducers now return plain objects, isThenable returns false, and handleResult/setState execute synchronously

The dispatchAction fast-path at line 160 already skips startTransition and deferred promise for ACTION_RESTORE — these fixes ensure that fast-path is actually reached without being defeated by outer wrappers.

Verification

• pnpm test-dev-turbo test/e2e/app-dir/popstate-useeffect-timing/popstate-useeffect-timing.test.ts — 2 tests passing
  • MutationObserver timing: asserts useEffect fires within 500ms of history.back()
  • Repeated back/forward cycle: asserts timing on second back navigation