Fix anti-aliasing when painting borders with solid colors. (#153365)

Trying to reland https://github.com/flutter/flutter/pull/122317 in 2024. Let's see if we can.

<img width="666" alt="image" src="https://user-images.githubusercontent.com/351125/182002867-03d55bbb-163d-48b9-ba3c-ed32dbef2680.png">

Side effect: shapes with border will be rounder:
![Frame 6](https://github.com/user-attachments/assets/95324ebc-8db5-4365-817f-bc62304b9044)

Close https://github.com/flutter/flutter/issues/13675.

packages/flutter/lib/src/painting/box_decoration.dart

@@ -11,6 +11,7 @@ import 'package:flutter/foundation.dart';
 
 import 'basic_types.dart';
 import 'border_radius.dart';
+import 'borders.dart';
 import 'box_border.dart';
 import 'box_shadow.dart';
 import 'colors.dart';
@@ -462,10 +463,63 @@ class _BoxDecorationPainter extends BoxPainter {
 
   void _paintBackgroundColor(Canvas canvas, Rect rect, TextDirection? textDirection) {
     if (_decoration.color != null || _decoration.gradient != null) {
-      _paintBox(canvas, rect, _getBackgroundPaint(rect, textDirection), textDirection);
+      // When border is filled, the rect is reduced to avoid anti-aliasing
+      // rounding error leaking the background color around the clipped shape.
+      final Rect adjustedRect = _adjustedRectOnOutlinedBorder(rect, textDirection);
+      _paintBox(canvas, adjustedRect, _getBackgroundPaint(rect, textDirection), textDirection);
     }
   }
 
+  double _calculateAdjustedSide(BorderSide side) {
+    if (side.color.alpha == 255 && side.style == BorderStyle.solid) {
+      return side.strokeInset;
+    }
+    return 0;
+  }
+
+  Rect _adjustedRectOnOutlinedBorder(Rect rect, TextDirection? textDirection) {
+    if (_decoration.border == null) {
+      return rect;
+    }
+
+    if (_decoration.border is Border) {
+      final Border border = _decoration.border! as Border;
+
+      final EdgeInsets insets = EdgeInsets.fromLTRB(
+        _calculateAdjustedSide(border.left),
+        _calculateAdjustedSide(border.top),
+        _calculateAdjustedSide(border.right),
+        _calculateAdjustedSide(border.bottom),
+      ) / 2;
+
+      return Rect.fromLTRB(
+        rect.left + insets.left,
+        rect.top + insets.top,
+        rect.right - insets.right,
+        rect.bottom - insets.bottom,
+      );
+    } else if (_decoration.border is BorderDirectional && textDirection != null) {
+      final BorderDirectional border = _decoration.border! as BorderDirectional;
+      final BorderSide leftSide = textDirection == TextDirection.rtl ? border.end : border.start;
+      final BorderSide rightSide = textDirection == TextDirection.rtl ? border.start : border.end;
+
+      final EdgeInsets insets = EdgeInsets.fromLTRB(
+        _calculateAdjustedSide(leftSide),
+        _calculateAdjustedSide(border.top),
+        _calculateAdjustedSide(rightSide),
+        _calculateAdjustedSide(border.bottom),
+      ) / 2;
+
+      return Rect.fromLTRB(
+        rect.left + insets.left,
+        rect.top + insets.top,
+        rect.right - insets.right,
+        rect.bottom - insets.bottom,
+      );
+    }
+    return rect;
+  }
+
   DecorationImagePainter? _imagePainter;
   void _paintBackgroundImage(Canvas canvas, Rect rect, ImageConfiguration configuration) {
     if (_decoration.image == null) {

packages/flutter/lib/src/painting/shape_decoration.dart

@@ -406,13 +406,26 @@ class _ShapeDecorationPainter extends BoxPainter {
   void _paintInterior(Canvas canvas, Rect rect, TextDirection? textDirection) {
     if (_interiorPaint != null) {
       if (_decoration.shape.preferPaintInterior) {
-        _decoration.shape.paintInterior(canvas, rect, _interiorPaint!, textDirection: textDirection);
+        // When border is filled, the rect is reduced to avoid anti-aliasing
+        // rounding error leaking the background color around the clipped shape.
+        final Rect adjustedRect = _adjustedRectOnOutlinedBorder(rect);
+        _decoration.shape.paintInterior(canvas, adjustedRect, _interiorPaint!, textDirection: textDirection);
       } else {
         canvas.drawPath(_outerPath, _interiorPaint!);
       }
     }
   }
 
+  Rect _adjustedRectOnOutlinedBorder(Rect rect) {
+    if (_decoration.shape is OutlinedBorder && _decoration.color != null) {
+      final BorderSide side = (_decoration.shape as OutlinedBorder).side;
+      if (side.color.alpha == 255 && side.style == BorderStyle.solid) {
+        return rect.deflate(side.strokeInset / 2);
+      }
+    }
+    return rect;
+  }
+
   DecorationImagePainter? _imagePainter;
   void _paintImage(Canvas canvas, ImageConfiguration configuration) {
     if (_decoration.image == null) {

packages/flutter/test/material/data_table_test.dart

@@ -1790,7 +1790,7 @@ void main() {
     expect(
       find.ancestor(of: find.byType(Table), matching: find.byType(Container)),
       paints..rect(
-        rect: const Rect.fromLTRB(0.0, 0.0, width, height),
+        rect: const Rect.fromLTRB(borderVertical / 2, borderHorizontal / 2, width - borderVertical / 2, height - borderHorizontal / 2),
         color: backgroundColor,
       ),
     );

packages/flutter/test/widgets/box_decoration_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['reduced-test-set'])
+library;
+
 import 'dart:async';
 import 'dart:math' as math;
 import 'dart:ui' as ui show Image;
@@ -577,4 +580,64 @@ Future<void> main() async {
       ),
     );
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/13675
+  testWidgets('Border avoids clipping edges when possible', (WidgetTester tester) async {
+    final Key key = UniqueKey();
+    Widget buildWidget(Color color) {
+      final List<Widget> circles = <Widget>[];
+      for (int i = 100; i > 25; i--) {
+        final double radius = i * 2.5;
+        final double angle = i * 0.5;
+        final double x = radius * math.cos(angle);
+        final double y = radius * math.sin(angle);
+        final Widget circle = Positioned(
+          left: 275 - x,
+          top: 275 - y,
+          child: Container(
+            width: 250,
+            height: 250,
+            decoration: BoxDecoration(
+              borderRadius: BorderRadius.circular(75),
+              color: Colors.black,
+              border: Border.all(color: color, width: 50),
+            ),
+          ),
+        );
+        circles.add(circle);
+      }
+
+      return Center(
+        key: key,
+        child: Container(
+          width: 800,
+          height: 800,
+          decoration: const ShapeDecoration(
+            color: Colors.orangeAccent,
+            shape: CircleBorder(
+              side: BorderSide(strokeAlign: BorderSide.strokeAlignOutside),
+            ),
+          ),
+          child: Directionality(
+            textDirection: TextDirection.ltr,
+            child: Stack(
+              children: circles,
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildWidget(const Color(0xffffffff)));
+    await expectLater(
+      find.byKey(key),
+        matchesGoldenFile('painting.box_decoration.border.should_be_white.png'),
+    );
+
+    await tester.pumpWidget(buildWidget(const Color(0xfeffffff)));
+    await expectLater(
+      find.byKey(key),
+      matchesGoldenFile('painting.box_decoration.border.show_lines_due_to_opacity.png'),
+    );
+  });
 }

packages/flutter/test/widgets/shape_decoration_test.dart

@@ -2,6 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['reduced-test-set'])
+library;
+
+import 'dart:math' as math;
 import 'dart:typed_data';
 import 'dart:ui' as ui show Image;
 
@@ -158,4 +162,65 @@ Future<void> main() async {
     expect(a.hashCode, equals(b.hashCode));
     expect(a, equals(b));
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/13675
+  testWidgets('OutlinedBorder avoids clipping edges when possible', (WidgetTester tester) async {
+    final Key key = UniqueKey();
+    Widget buildWidget(Color color) {
+      final List<Widget> circles = <Widget>[];
+      for (int i = 100; i > 25; i--) {
+        final double radius = i * 2.5;
+        final double angle = i * 0.5;
+        final double x = radius * math.cos(angle);
+        final double y = radius * math.sin(angle);
+        final Widget circle = Positioned(
+          left: 275 - x,
+          top: 275 - y,
+          child: Container(
+            width: 250,
+            height: 250,
+            decoration: ShapeDecoration(
+              color: Colors.black,
+              shape: CircleBorder(
+                side: BorderSide(color: color, width: 50),
+              ),
+            ),
+          ),
+        );
+        circles.add(circle);
+      }
+
+      return Center(
+        key: key,
+        child: Container(
+          width: 800,
+          height: 800,
+          decoration: const ShapeDecoration(
+            color: Colors.redAccent,
+            shape: CircleBorder(
+              side: BorderSide(strokeAlign: BorderSide.strokeAlignOutside),
+            ),
+          ),
+          child: Directionality(
+            textDirection: TextDirection.ltr,
+            child: Stack(
+              children: circles,
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildWidget(const Color(0xffffffff)));
+    await expectLater(
+      find.byKey(key),
+      matchesGoldenFile('painting.shape_decoration.outlined_border.should_be_white.png'),
+    );
+
+    await tester.pumpWidget(buildWidget(const Color(0xfeffffff)));
+    await expectLater(
+      find.byKey(key),
+      matchesGoldenFile('painting.shape_decoration.outlined_border.show_lines_due_to_opacity.png'),
+    );
+  });
 }