fix: mask disappeared when having nested mask filter on Flutter web HTML

Author: Kingtous

lib/web_ui/lib/src/engine/canvas_pool.dart

@@ -936,6 +936,7 @@ class ContextStateHandle {
   }
 
   ui.MaskFilter? _currentFilter;
+  String? _currentFilterCss;
   SurfacePaintData? _lastUsedPaint;
 
   /// Currently active shader bounds.
@@ -1005,19 +1006,20 @@ class ContextStateHandle {
     }
 
     final ui.MaskFilter? maskFilter = paint.maskFilter;
-    if (!_renderMaskFilterForWebkit) {
-      if (_currentFilter != maskFilter) {
-        _currentFilter = maskFilter;
-        context.filter = maskFilterToCanvasFilter(maskFilter);
-      }
-    } else {
-      // WebKit does not support the `filter` property. Instead we apply a
-      // shadow to the shape of the same color as the paint and the same blur
-      // as the mask filter.
-      //
-      // Note that on WebKit the cached value of _currentFilter is not useful.
-      // Instead we destructure it into the shadow properties and cache those.
-      if (maskFilter != null) {
+    if (maskFilter != null) {
+      if (!_renderMaskFilterForWebkit) {
+        if (_currentFilter != maskFilter) {
+          _currentFilter = maskFilter;
+          _currentFilterCss = maskFilterToCanvasFilter(maskFilter);
+        }
+        context.filter = _currentFilterCss;
+      } else {
+        // WebKit does not support the `filter` property. Instead we apply a
+        // shadow to the shape of the same color as the paint and the same blur
+        // as the mask filter.
+        //
+        // Note that on WebKit the cached value of _currentFilter is not useful.
+        // Instead we destructure it into the shadow properties and cache those.
         context.save();
         context.shadowBlur = convertSigmaToRadius(maskFilter.webOnlySigma);
         // Shadow color must be fully opaque.