Improve and optimize non-uniform Borders. (#124417)

~~Using the same priority order as a Border without borderRadius, it is possible to draw them on top of each other. This is better than the current behavior (crash!) and would work well for a "one color on top, another on bottom" scenario.~~

~~With this, if approved, we move the current number of possible exceptions from 4 to 1 (`BoxShape.circle` + `borderRadius`).~~

~~It is kind of odd how `borderRadius.zero` to `borderRadius != BorderRadius.zero` change, but I think it is better than crashing. Alternatively, we just remove the "original function" and see if any goldens are affected.~~

<img width="448" alt="image" src="https://user-images.githubusercontent.com/351125/236550350-7499d758-5b44-40e6-9105-32671eb21998.png">

Another one for @gspencergoog. If this works, we could make the paint method public and re-use in the InputBorder PR (if that's also approved). Single line fix.

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

@@ -242,16 +242,24 @@ abstract class BoxBorder extends ShapeBorder {
     }
   }
 
-  static void _paintNonUniformBorder(
+  /// Paints a Border with different widths, styles and strokeAligns, on any
+  /// borderRadius while using a single color.
+  ///
+  /// See also:
+  ///
+  ///  * [paintBorder], which supports multiple colors but not borderRadius.
+  ///  * [paint], which calls this method.
+  static void paintNonUniformBorder(
     Canvas canvas,
     Rect rect, {
     required BorderRadius? borderRadius,
-    required BoxShape shape,
     required TextDirection? textDirection,
-    required BorderSide left,
-    required BorderSide top,
-    required BorderSide right,
-    required BorderSide bottom,
+    BoxShape shape = BoxShape.rectangle,
+    BorderSide top = BorderSide.none,
+    BorderSide right = BorderSide.none,
+    BorderSide bottom = BorderSide.none,
+    BorderSide left = BorderSide.none,
+    required Color color,
   }) {
     final RRect borderRect;
     switch (shape) {
@@ -266,7 +274,7 @@ abstract class BoxBorder extends ShapeBorder {
           Radius.circular(rect.width),
         );
     }
-    final Paint paint = Paint()..color = top.color;
+    final Paint paint = Paint()..color = color;
     final RRect inner = _deflateRRect(borderRect, EdgeInsets.fromLTRB(left.strokeInset, top.strokeInset, right.strokeInset, bottom.strokeInset));
     final RRect outer = _inflateRRect(borderRect, EdgeInsets.fromLTRB(left.strokeOutset, top.strokeOutset, right.strokeOutset, bottom.strokeOutset));
     canvas.drawDRRect(outer, inner, paint);
@@ -489,6 +497,32 @@ class Border extends BoxBorder {
         && right.strokeAlign == topStrokeAlign;
   }
 
+  Set<Color> _distinctVisibleColors() {
+    final Set<Color> distinctVisibleColors = <Color>{};
+    if (top.style != BorderStyle.none) {
+      distinctVisibleColors.add(top.color);
+    }
+    if (right.style != BorderStyle.none) {
+      distinctVisibleColors.add(right.color);
+    }
+    if (bottom.style != BorderStyle.none) {
+      distinctVisibleColors.add(bottom.color);
+    }
+    if (left.style != BorderStyle.none) {
+      distinctVisibleColors.add(left.color);
+    }
+    return distinctVisibleColors;
+  }
+
+  // [BoxBorder.paintNonUniformBorder] is about 20% faster than [paintBorder],
+  // but [paintBorder] is able to draw hairline borders when width is zero
+  // and style is [BorderStyle.solid].
+  bool get _hasHairlineBorder =>
+      (top.style == BorderStyle.solid && top.width == 0.0) ||
+      (right.style == BorderStyle.solid && right.width == 0.0) ||
+      (bottom.style == BorderStyle.solid && bottom.width == 0.0) ||
+      (left.style == BorderStyle.solid && left.width == 0.0);
+
   @override
   Border? add(ShapeBorder other, { bool reversed = false }) {
     if (other is Border &&
@@ -603,31 +637,41 @@ class Border extends BoxBorder {
       }
     }
 
-    // Allow painting non-uniform borders if the color and style are uniform.
-    if (_colorIsUniform && _styleIsUniform) {
-      switch (top.style) {
-        case BorderStyle.none:
+    if (_styleIsUniform && top.style == BorderStyle.none) {
       return;
-        case BorderStyle.solid:
-          BoxBorder._paintNonUniformBorder(canvas, rect,
+    }
+
+    // Allow painting non-uniform borders if the visible colors are uniform.
+    final Set<Color> visibleColors = _distinctVisibleColors();
+    final bool hasHairlineBorder = _hasHairlineBorder;
+    // Paint a non uniform border if a single color is visible
+    // and (borderRadius is present) or (border is visible and width != 0.0).
+    if (visibleColors.length == 1 &&
+        !hasHairlineBorder &&
+        (shape == BoxShape.circle ||
+            (borderRadius != null && borderRadius != BorderRadius.zero))) {
+      BoxBorder.paintNonUniformBorder(canvas, rect,
           shape: shape,
           borderRadius: borderRadius,
           textDirection: textDirection,
-              left: left,
-              top: top,
-              right: right,
-              bottom: bottom);
+          top: top.style == BorderStyle.none ? BorderSide.none : top,
+          right: right.style == BorderStyle.none ? BorderSide.none : right,
+          bottom: bottom.style == BorderStyle.none ? BorderSide.none : bottom,
+          left: left.style == BorderStyle.none ? BorderSide.none : left,
+          color: visibleColors.first);
       return;
     }
-    }
 
      assert(() {
-      if (borderRadius != null) {
+      if (hasHairlineBorder) {
+        assert(borderRadius == null || borderRadius == BorderRadius.zero,
+            'A hairline border like `BorderSide(width: 0.0, style: BorderStyle.solid)` can only be drawn when BorderRadius is zero or null.');
+      }
+      if (borderRadius != null && borderRadius != BorderRadius.zero) {
         throw FlutterError.fromParts(<DiagnosticsNode>[
-          ErrorSummary('A borderRadius can only be given on borders with uniform colors and styles.'),
+          ErrorSummary('A borderRadius can only be given on borders with uniform colors.'),
           ErrorDescription('The following is not uniform:'),
           if (!_colorIsUniform) ErrorDescription('BorderSide.color'),
-          if (!_styleIsUniform) ErrorDescription('BorderSide.style'),
         ]);
       }
       return true;
@@ -635,10 +679,9 @@ class Border extends BoxBorder {
     assert(() {
       if (shape != BoxShape.rectangle) {
         throw FlutterError.fromParts(<DiagnosticsNode>[
-          ErrorSummary('A Border can only be drawn as a circle on borders with uniform colors and styles.'),
+          ErrorSummary('A Border can only be drawn as a circle on borders with uniform colors.'),
           ErrorDescription('The following is not uniform:'),
           if (!_colorIsUniform) ErrorDescription('BorderSide.color'),
-          if (!_styleIsUniform) ErrorDescription('BorderSide.style'),
         ]);
       }
       return true;
@@ -646,7 +689,7 @@ class Border extends BoxBorder {
     assert(() {
       if (!_strokeAlignIsUniform || top.strokeAlign != BorderSide.strokeAlignInside) {
         throw FlutterError.fromParts(<DiagnosticsNode>[
-          ErrorSummary('A Border can only draw strokeAlign different than BorderSide.strokeAlignInside on borders with uniform colors and styles.'),
+          ErrorSummary('A Border can only draw strokeAlign different than BorderSide.strokeAlignInside on borders with uniform colors.'),
         ]);
       }
       return true;
@@ -806,6 +849,31 @@ class BorderDirectional extends BoxBorder {
         && end.strokeAlign == topStrokeAlign;
   }
 
+  Set<Color> _distinctVisibleColors() {
+    final Set<Color> distinctVisibleColors = <Color>{};
+    if (top.style != BorderStyle.none) {
+      distinctVisibleColors.add(top.color);
+    }
+    if (end.style != BorderStyle.none) {
+      distinctVisibleColors.add(end.color);
+    }
+    if (bottom.style != BorderStyle.none) {
+      distinctVisibleColors.add(bottom.color);
+    }
+    if (start.style != BorderStyle.none) {
+      distinctVisibleColors.add(start.color);
+    }
+
+    return distinctVisibleColors;
+  }
+
+
+  bool get _hasHairlineBorder =>
+      (top.style == BorderStyle.solid && top.width == 0.0) ||
+      (end.style == BorderStyle.solid && end.width == 0.0) ||
+      (bottom.style == BorderStyle.solid && bottom.width == 0.0) ||
+      (start.style == BorderStyle.solid && start.width == 0.0);
+
   @override
   BoxBorder? add(ShapeBorder other, { bool reversed = false }) {
     if (other is BorderDirectional) {
@@ -951,6 +1019,10 @@ class BorderDirectional extends BoxBorder {
       }
     }
 
+    if (_styleIsUniform && top.style == BorderStyle.none) {
+      return;
+    }
+
     final BorderSide left, right;
     assert(textDirection != null, 'Non-uniform BorderDirectional objects require a TextDirection when painting.');
     switch (textDirection!) {
@@ -962,27 +1034,31 @@ class BorderDirectional extends BoxBorder {
         right = end;
     }
 
-    // Allow painting non-uniform borders if the color and style are uniform.
-    if (_colorIsUniform && _styleIsUniform) {
-      switch (top.style) {
-        case BorderStyle.none:
-          return;
-        case BorderStyle.solid:
-          BoxBorder._paintNonUniformBorder(canvas, rect,
+    // Allow painting non-uniform borders if the visible colors are uniform.
+    final Set<Color> visibleColors = _distinctVisibleColors();
+    final bool hasHairlineBorder = _hasHairlineBorder;
+    if (visibleColors.length == 1 &&
+        !hasHairlineBorder &&
+        (shape == BoxShape.circle ||
+            (borderRadius != null && borderRadius != BorderRadius.zero))) {
+      BoxBorder.paintNonUniformBorder(canvas, rect,
           shape: shape,
           borderRadius: borderRadius,
           textDirection: textDirection,
-              left: left,
-              top: top,
-              right: right,
-              bottom: bottom);
+          top: top.style == BorderStyle.none ? BorderSide.none : top,
+          right: right.style == BorderStyle.none ? BorderSide.none : right,
+          bottom: bottom.style == BorderStyle.none ? BorderSide.none : bottom,
+          left: left.style == BorderStyle.none ? BorderSide.none : left,
+          color: visibleColors.first);
       return;
     }
-    }
 
-    assert(borderRadius == null, 'A borderRadius can only be given for borders with uniform colors and styles.');
-    assert(shape == BoxShape.rectangle, 'A Border can only be drawn as a circle on borders with uniform colors and styles.');
-    assert(_strokeAlignIsUniform && top.strokeAlign == BorderSide.strokeAlignInside, 'A Border can only draw strokeAlign different than strokeAlignInside on borders with uniform colors and styles.');
+    if (hasHairlineBorder) {
+      assert(borderRadius == null || borderRadius == BorderRadius.zero, 'A side like `BorderSide(width: 0.0, style: BorderStyle.solid)` can only be drawn when BorderRadius is zero or null.');
+    }
+    assert(borderRadius == null, 'A borderRadius can only be given for borders with uniform colors.');
+    assert(shape == BoxShape.rectangle, 'A Border can only be drawn as a circle on borders with uniform colors.');
+    assert(_strokeAlignIsUniform && top.strokeAlign == BorderSide.strokeAlignInside, 'A Border can only draw strokeAlign different than strokeAlignInside on borders with uniform colors.');
 
     paintBorder(canvas, rect, top: top, left: left, bottom: bottom, right: right);
   }

packages/flutter/test/material/data_table_test.dart

@@ -1750,7 +1750,7 @@ void main() {
     );
     expect(
       find.ancestor(of: find.byType(Table), matching: find.byType(Container)),
-      paints..drrect(color: borderColor),
+      paints..path(color: borderColor),
     );
     expect(
       tester.getTopLeft(find.byType(Table)),

packages/flutter/test/material/divider_test.dart

@@ -136,8 +136,9 @@ void main() {
 
   testWidgets('Vertical Divider Test 2', (WidgetTester tester) async {
     await tester.pumpWidget(
-      const MaterialApp(
-        home: Material(
+      MaterialApp(
+        theme: ThemeData(useMaterial3: false),
+        home: const Material(
           child: SizedBox(
             height: 24.0,
             child: Row(

packages/flutter/test/painting/border_test.dart

@@ -273,7 +273,7 @@ void main() {
     expect(error.diagnostics.length, 1);
     expect(
       error.diagnostics[0].toStringDeep(),
-      'A Border can only draw strokeAlign different than\nBorderSide.strokeAlignInside on borders with uniform colors and\nstyles.\n',
+      'A Border can only draw strokeAlign different than\nBorderSide.strokeAlignInside on borders with uniform colors.\n',
     );
   });
 
@@ -341,8 +341,8 @@ void main() {
 
     // This falls into non-uniform border because of strokeAlign.
     await tester.pumpWidget(buildWidget(border: allowedBorderVariations));
-    expect(tester.takeException(), isNull,
-        reason: 'Border with non-uniform strokeAlign should not fail.');
+    expect(tester.takeException(), isAssertionError,
+        reason: 'Border with non-uniform strokeAlign should fail.');
 
     await tester.pumpWidget(buildWidget(
       border: allowedBorderVariations,
@@ -364,8 +364,8 @@ void main() {
         borderRadius: BorderRadius.circular(25),
       ),
     );
-    expect(tester.takeException(), isAssertionError,
-        reason: 'Border with non-uniform styles should fail with borderRadius.');
+    expect(tester.takeException(), isNull,
+        reason: 'Border with non-uniform styles should work with borderRadius.');
 
     await tester.pumpWidget(
       buildWidget(
@@ -381,6 +381,24 @@ void main() {
     expect(tester.takeException(), isAssertionError,
         reason: 'Border with non-uniform colors should fail with borderRadius.');
 
+    await tester.pumpWidget(
+      buildWidget(
+        border: const Border(bottom: BorderSide(width: 0)),
+        borderRadius: BorderRadius.zero,
+      ),
+    );
+    expect(tester.takeException(), isNull,
+        reason: 'Border with a side.width == 0 should work without borderRadius (hairline border).');
+
+    await tester.pumpWidget(
+      buildWidget(
+        border: const Border(bottom: BorderSide(width: 0)),
+        borderRadius: BorderRadius.circular(40),
+      ),
+    );
+    expect(tester.takeException(), isAssertionError,
+        reason: 'Border with width == 0 and borderRadius should fail (hairline border).');
+
     // Tests for BorderDirectional.
     const BorderDirectional allowedBorderDirectionalVariations = BorderDirectional(
       start: BorderSide(width: 5),
@@ -390,7 +408,7 @@ void main() {
     );
 
     await tester.pumpWidget(buildWidget(border: allowedBorderDirectionalVariations));
-    expect(tester.takeException(), isNull);
+    expect(tester.takeException(), isAssertionError);
 
     await tester.pumpWidget(buildWidget(
       border: allowedBorderDirectionalVariations,