• Issue: flutter/flutter#183012
• Live demo: https://davidmigloz.github.io/flutter_block_then_bug/

Block.then(() => router.go(...)) callbacks in go_router's onEnter handler silently lose their navigation when triggered by refreshListenable. The router.go() inside the callback runs synchronously during handleTopOnEnter processing, triggering a re-entrant _processRouteInformation whose result is dropped due to transaction token churn in Flutter's Router.

1. Run the app:

   flutter run -d chrome

2. App starts authenticated on /home (green indicator should show if already fixed).
3. Tap "Sign Out" to set isAuthenticated = false.
4. refreshListenable fires, the onEnter guard returns Block.then(() => router.go('/login')).

The app navigates to /login (green "FIXED" indicator).

The app stays on /home (red "BUG" indicator). The debug console shows [Block.then] Calling router.go(/login) confirming the callback fires, but the navigation is lost.

The callback at parser.dart:533 fires during the outer parse's Future chain via await Future<void>.sync(callback). When the callback calls router.go('/login'), it synchronously triggers a new _processRouteInformation via notifyListeners(). Flutter's Router mints a new transaction token, and the re-entrant parse's result is dropped due to token churn.

In parser.dart, change await Future<void>.sync(callback) to scheduleMicrotask(() async { await callback(); }). This defers the callback to the microtask queue, ensuring router.go() runs after the current parse has completed and Flutter's Router has committed the result.

• go_router: 17.1.0 (fork with onEnter API)
• Flutter: 3.41.2 (stable)
• Dart SDK: 3.11.0 (stable)