Fix OutlineInputBorder with BorderRadius.zero is distorted (#106849)

packages/flutter/lib/src/material/input_border.dart

@@ -439,44 +439,63 @@ class OutlineInputBorder extends InputBorder {
       scaledRRect.left,
       scaledRRect.bottom - scaledRRect.blRadiusY * 2.0,
       scaledRRect.blRadiusX * 2.0,
-      scaledRRect.blRadiusX * 2.0,
+      scaledRRect.blRadiusY * 2.0,
     );
 
     // This assumes that the radius is circular (x and y radius are equal).
     // Currently, BorderRadius only supports circular radii.
     const double cornerArcSweep = math.pi / 2.0;
-    final double tlCornerArcSweep = math.acos(
-      clampDouble(1 - start / scaledRRect.tlRadiusX, 0.0, 1.0),
-    );
+    final Path path = Path();
 
-    final Path path = Path()
-      ..addArc(tlCorner, math.pi, tlCornerArcSweep);
+    // Top left corner
+    if (scaledRRect.tlRadius != Radius.zero) {
+      final double tlCornerArcSweep = math.acos(clampDouble(1 - start / scaledRRect.tlRadiusX, 0.0, 1.0));
+      path.addArc(tlCorner, math.pi, tlCornerArcSweep);
+    } else {
+      // Because the path is painted with Paint.strokeCap = StrokeCap.butt, horizontal coordinate is moved
+      // to the left using borderSide.width / 2.
+      path.moveTo(scaledRRect.left - borderSide.width / 2, scaledRRect.top);
+    }
 
+    // Draw top border from top left corner to gap start.
     if (start > scaledRRect.tlRadiusX) {
       path.lineTo(scaledRRect.left + start, scaledRRect.top);
     }
 
+    // Draw top border from gap end to top right corner and draw top right corner.
     const double trCornerArcStart = (3 * math.pi) / 2.0;
     const double trCornerArcSweep = cornerArcSweep;
     if (start + extent < scaledRRect.width - scaledRRect.trRadiusX) {
       path.moveTo(scaledRRect.left + start + extent, scaledRRect.top);
       path.lineTo(scaledRRect.right - scaledRRect.trRadiusX, scaledRRect.top);
+      if (scaledRRect.trRadius != Radius.zero) {
         path.addArc(trCorner, trCornerArcStart, trCornerArcSweep);
+      }
     } else if (start + extent < scaledRRect.width) {
       final double dx = scaledRRect.width - (start + extent);
-      final double sweep = math.asin(
-        clampDouble(1 - dx / scaledRRect.trRadiusX, 0.0, 1.0),
-      );
+      final double sweep = math.asin(clampDouble(1 - dx / scaledRRect.trRadiusX, 0.0, 1.0));
       path.addArc(trCorner, trCornerArcStart + sweep, trCornerArcSweep - sweep);
     }
 
-    return path
-      ..moveTo(scaledRRect.right, scaledRRect.top + scaledRRect.trRadiusY)
-      ..lineTo(scaledRRect.right, scaledRRect.bottom - scaledRRect.brRadiusY)
-      ..addArc(brCorner, 0.0, cornerArcSweep)
-      ..lineTo(scaledRRect.left + scaledRRect.blRadiusX, scaledRRect.bottom)
-      ..addArc(blCorner, math.pi / 2.0, cornerArcSweep)
-      ..lineTo(scaledRRect.left, scaledRRect.top + scaledRRect.tlRadiusY);
+    // Draw right border and bottom right corner.
+    if (scaledRRect.brRadius != Radius.zero) {
+      path.moveTo(scaledRRect.right, scaledRRect.top + scaledRRect.trRadiusY);
+    }
+    path.lineTo(scaledRRect.right, scaledRRect.bottom - scaledRRect.brRadiusY);
+    if (scaledRRect.brRadius != Radius.zero) {
+      path.addArc(brCorner, 0.0, cornerArcSweep);
+    }
+
+    // Draw bottom border and bottom left corner.
+    path.lineTo(scaledRRect.left + scaledRRect.blRadiusX, scaledRRect.bottom);
+    if (scaledRRect.blRadius != Radius.zero) {
+      path.addArc(blCorner, math.pi / 2.0, cornerArcSweep);
+    }
+
+    // Draw left border
+    path.lineTo(scaledRRect.left, scaledRRect.top + scaledRRect.tlRadiusY);
+
+    return path;
   }
 
   /// Draw a rounded rectangle around [rect] using [borderRadius].

packages/flutter/test/material/input_decorator_test.dart

@@ -4992,6 +4992,70 @@ void main() {
     );
   }, skip: isBrowser); // https://github.com/flutter/flutter/issues/55317
 
+ testWidgets('OutlineInputBorder with BorderRadius.zero should draw a rectangular border', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/78855
+    const String labelText = 'Flutter';
+
+    // Overall height for this InputDecorator is 56dps:
+    //   12 - top padding
+    //   12 - floating label (ahem font size 16dps * 0.75 = 12)
+    //    4 - floating label / input text gap
+    //   16 - input text (ahem font size 16dps)
+    //   12 - bottom padding
+    const double inputDecoratorHeight = 56.0;
+    const double inputDecoratorWidth = 800.0;
+    const double borderWidth = 4.0;
+
+    await tester.pumpWidget(
+      buildInputDecorator(
+        isFocused: true,
+        decoration: const InputDecoration(
+          filled: false,
+          labelText: labelText,
+          focusedBorder: OutlineInputBorder(
+            borderRadius: BorderRadius.zero,
+            borderSide: BorderSide(width: borderWidth, color: Colors.red),
+          ),
+        ),
+      ),
+    );
+
+    expect(find.text(labelText), findsOneWidget);
+    expect(findBorderPainter(), paints
+      ..save()
+      ..path(
+        includes: const <Offset>[
+          // Corner points in the middle of the border line should be in the path.
+          // The path is not filled and borderWidth is 4.0 so Offset(2.0, 2.0) is in the path and Offset(1.0, 1.0) is not.
+          // See Skia SkPath::contains method.
+
+          // Top-left
+          Offset(borderWidth / 2, borderWidth / 2),
+          // Top-right
+          Offset(inputDecoratorWidth - 1 - borderWidth / 2, borderWidth / 2),
+          // Bottom-left
+          Offset(borderWidth / 2, inputDecoratorHeight - 1 - borderWidth / 2),
+          // Bottom-right
+          Offset(inputDecoratorWidth - 1 - borderWidth / 2, inputDecoratorHeight - 1 - borderWidth / 2),
+        ],
+        excludes: const <Offset>[
+          // The path is not filled and borderWidth is 4.0 so the path should not contains the corner points.
+          // See Skia SkPath::contains method.
+
+          // Top-left
+          Offset.zero,
+          // // Top-right
+          Offset(inputDecoratorWidth - 1, 0),
+          // // Bottom-left
+          Offset(0, inputDecoratorHeight - 1),
+          // // Bottom-right
+          Offset(inputDecoratorWidth - 1, inputDecoratorHeight - 1),
+        ],
+      )
+      ..restore(),
+    );
+  }, skip: isBrowser); // https://github.com/flutter/flutter/issues/55317
+
   testWidgets('OutlineInputBorder radius carries over when lerping', (WidgetTester tester) async {
     // This is a regression test for https://github.com/flutter/flutter/issues/23982
     const Key key = Key('textField');
@@ -5085,7 +5149,6 @@ void main() {
     expect(underlineInputBorder, isNot(const UnderlineInputBorder()));
   });
 
-
   test('InputBorder hashCodes', () {
     // OutlineInputBorder's hashCode is defined by the borderRadius, borderSide, & gapPadding
     const OutlineInputBorder outlineInputBorder = OutlineInputBorder(