fix: re-anchor effects clock when ticker drains (stale-clock freeze)

After Ticker.stop(), elapsed resets to zero on the next start(). _now was
left frozen at the old elapsed, so effects added after a drain captured a
stale start time and their progress() clamped to 0 forever — ticker never
stopped, second batch broken. Fix: reset _now = Duration.zero on drain.
Adds @visibleForTesting getters and a regression test that catches the
stale value directly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/ui/widgets/effects_overlay.dart

@@ -60,11 +60,22 @@ class EffectsOverlayState extends State<EffectsOverlay>
     _ticker = createTicker(_tick);
   }
 
+  @visibleForTesting
+  Ticker get ticker => _ticker;
+
+  @visibleForTesting
+  Duration get now => _now;
+
   void _tick(Duration elapsed) {
     setState(() {
       _now = elapsed;
       _fx.removeWhere((e) => e.done(elapsed));
-      if (_fx.isEmpty) _ticker.stop();
+      if (_fx.isEmpty) {
+        _ticker.stop();
+        // Ticker elapsed restarts from zero after stop(); re-anchor so
+        // effects added later don't inherit a stale clock.
+        _now = Duration.zero;
+      }
     });
   }
 

test/ui/effects_overlay_test.dart

@@ -0,0 +1,66 @@
+import 'package:block_seasons/game/engine/game_engine.dart';
+import 'package:block_seasons/ui/widgets/effects_overlay.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+PlacementResult _clearResult() => const PlacementResult(
+      events: [],
+      pointsGained: 250,
+      linesCleared: 1,
+      gemsCleared: 0,
+      clearedRows: [3],
+      clearedCols: [],
+      comboStreak: 2,
+    );
+
+void main() {
+  testWidgets('second batch after drain completes within its own duration',
+      (tester) async {
+    final key = GlobalKey<EffectsOverlayState>();
+    await tester.pumpWidget(MaterialApp(
+      home:
+          Stack(children: [Positioned.fill(child: EffectsOverlay(key: key))]),
+    ));
+
+    const board = Rect.fromLTWH(0, 0, 320, 320);
+
+    // ── First batch ──────────────────────────────────────────────────────────
+    key.currentState!.onPlacement(_clearResult(), boardRect: board);
+    await tester.pumpAndSettle();
+
+    // Ticker must be stopped after the first drain.
+    expect(
+      key.currentState!.ticker.isActive,
+      isFalse,
+      reason: 'ticker should be idle after first batch drains',
+    );
+
+    // _now must be Duration.zero after drain (regression for stale-clock bug).
+    // Without the fix, _now stays frozen at the elapsed value from the last
+    // tick of the first batch (~1000 ms).
+    expect(
+      key.currentState!.now,
+      Duration.zero,
+      reason:
+          '_now must be reset to zero when the list drains so the next '
+          'batch starts from a clean clock',
+    );
+
+    // ── Second batch ─────────────────────────────────────────────────────────
+    // With the stale-clock bug the effects get start: ~1000ms, but the
+    // restarted ticker delivers elapsed from 0, so progress stays 0 forever
+    // and they never drain.  We verify the second batch completes via
+    // pumpAndSettle (which would time out if the ticker ran forever).
+    key.currentState!.onPlacement(_clearResult(), boardRect: board);
+    await tester.pump(const Duration(milliseconds: 100));
+    await tester.pumpAndSettle();
+
+    expect(
+      tester.hasRunningAnimations,
+      isFalse,
+      reason:
+          'second batch must finish; stale clock would keep effects frozen '
+          'and the ticker running indefinitely',
+    );
+  });
+}