fix: merge border-radius with background-color for inline elements (#1569)

For inline elements with margin, border-radius and background-color were
placed on separate Containers, causing the background to render with
square corners. Let onRenderBlock handle border decoration during
tree.build() so it merges with background into a single Container.

Also preserve clipBehavior when buildDecoration merges into an existing
Container that already has borderRadius.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: daohoangson

packages/core/lib/src/core_widget_factory.dart

@@ -168,13 +168,16 @@ class WidgetFactory extends WidgetFactoryResetter with AnchorWidgetFactory {
     );
 
     var clipBehavior = Clip.none;
-    if (borderRadius != null) {
+    final baseRadius = baseDeco.borderRadius;
+    final effectiveRadius =
+        borderRadius ?? (baseRadius is BorderRadius ? baseRadius : null);
+    if (effectiveRadius != null) {
       final borderIsUniform = decoration.border?.isUniform ?? true;
       if (borderIsUniform) {
         // TODO: add support for non-uniform border
         // https://github.com/flutter/flutter/commit/5054b6e
         // https://pub.dev/packages/non_uniform_border
-        decoration = decoration.copyWith(borderRadius: borderRadius);
+        decoration = decoration.copyWith(borderRadius: effectiveRadius);
         clipBehavior = Clip.hardEdge;
       }
     }

packages/core/lib/src/internal/ops/style_border.dart

@@ -21,16 +21,14 @@ class StyleBorder {
             return tree;
           }
 
-          skip(tree);
+          // Don't skip onRenderBlock — let it handle border decoration
+          // during tree.build() so it can be merged with background color
+          // by buildDecoration's Container merge logic.
           return parent.sub()
             ..append(
               WidgetBit.inline(
                 tree,
                 WidgetPlaceholder(
-                  builder: (context, child) {
-                    final built = _buildBorder(tree, context, child, border);
-                    return built ?? child;
-                  },
                   debugLabel: '${tree.element.localName}--$kCssBorder',
                   child: tree.build(),
                 ),

packages/core/test/style_border_test.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import '_.dart';
@@ -769,6 +770,36 @@ void main() {
       );
     });
 
+    testWidgets('#1560: border-radius with background-color and margin',
+        (tester) async {
+      const html = '<span style="border-radius: 1px; '
+          'background-color: red; margin-right: 1px;">Foo</span>';
+      final explained = await explain(tester, html);
+      expect(
+        explained,
+        equals(
+          '[HorizontalMargin:left=0,right=1,child='
+          '[Container:color=#FFFF0000,'
+          'radius=[1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0],'
+          'child=[RichText:(:Foo)]]]',
+        ),
+      );
+    });
+
+    testWidgets('#1560: block border-radius with background-color clips',
+        (tester) async {
+      const html =
+          '<section style="border-radius: 1px; background-color: red;">'
+          'Foo</section>';
+      await explain(tester, html);
+
+      final container =
+          tester.widgetList<Container>(find.byType(Container)).firstWhere(
+                (c) => (c.decoration as BoxDecoration?)?.borderRadius != null,
+              );
+      expect(container.clipBehavior, equals(Clip.hardEdge));
+    });
+
     testWidgets('ignore radius if border is not uniform', (t) async {
       // https://github.com/daohoangson/flutter_widget_from_html/issues/909
       const html = '<section style="border-bottom: 1px solid rgb(62, 62, 62); '