diff --git a/app/lib/features/editor/widgets/handwriting_canvas.dart b/app/lib/features/editor/widgets/handwriting_canvas.dart
index bebdc31..53c336c 100644
--- a/app/lib/features/editor/widgets/handwriting_canvas.dart
+++ b/app/lib/features/editor/widgets/handwriting_canvas.dart
@@ -134,10 +134,15 @@ class _HandwritingCanvasState extends State<HandwritingCanvas> {
   }
 
   /// 在 build 完成后同步缓存（避免在 build 中触发异步操作）
+  ///
+  /// syncStrokes 重建合成图后必须调用 setState，
+  /// 否则 CachedStrokesPainter 不知道缓存已更新，不会触发重绘。
   void _syncCacheAfterBuild() {
     WidgetsBinding.instance.addPostFrameCallback((_) {
       if (!mounted) return;
-      _cache.syncStrokes(widget.strokes);
+      _cache.syncStrokes(widget.strokes).then((_) {
+        if (mounted) setState(() {});
+      });
     });
   }