PR #89281: fix: resolve transition deadlock when navigating with pending server action

• Repository: vercel/next.js
• Author: Suhaib3100
• State: open | merged: False
• Link: https://github.com/vercel/next.js/pull/89281

Description (problem / solution / changelog)

What?

This PR fixes a transition deadlock that occurs when a navigation action is dispatched while a server action is pending.

Why?

Fixes #84299

When a user triggers a navigation with a pending server action (e.g., clicking a button that calls router.push() followed by a server action inside startTransition), and then clicks again before the first action completes, the application enters a deadlock state where:

• The navigation never completes
• Subsequent button presses have no effect
• The app becomes unresponsive

This was introduced in v15.2.2 (likely by #75362).

How?

The root cause was in the action queue logic in app-router-instance.ts. When a navigation action is dispatched while a server action is pending:

1. The pending action is correctly marked as discarded
2. However, the discarded action's promise was never resolved
3. This left React's useTransition hanging indefinitely, causing the deadlock

The fix ensures that discarded actions still resolve their promises (using the current state) even if the result is discarded. This allows the React transition to complete successfully without applying the stale state update.

// Before: Promise never resolved for discarded actions
if (action.discarded) {
  runRemainingActions(actionQueue, setState)
  return  // ❌ BUG: Promise never resolved!
}

// After: Resolve promise with current state
if (action.discarded) {
  runRemainingActions(actionQueue, setState)
  action.resolve(actionQueue.state)  // ✅ FIX: Transition can complete
  return
}

Test Plan

Added e2e test that:

1. Starts a transition with navigation + server action
2. Clicks again while the first action is pending
3. Verifies navigation completes without deadlock

The test reproduces the exact scenario from the issue report.

Changed files

• packages/next/src/client/components/app-router-instance.ts (modified, +33/-4)
• test/e2e/app-dir/action-queue-transition-deadlock/action-queue-transition-deadlock.test.ts (added, +73/-0)
• test/e2e/app-dir/action-queue-transition-deadlock/app/actions.ts (added, +7/-0)
• test/e2e/app-dir/action-queue-transition-deadlock/app/layout.tsx (added, +8/-0)
• test/e2e/app-dir/action-queue-transition-deadlock/app/other/page.tsx (added, +8/-0)
• test/e2e/app-dir/action-queue-transition-deadlock/app/page.tsx (added, +26/-0)
• test/e2e/app-dir/action-queue-transition-deadlock/next.config.js (added, +6/-0)