Add continuous_rectangle_border.

Runtime/painting/box_border.cs

             this.left = left ?? BorderSide.none;
         }
 
+        public static Border fromBorderSide(BorderSide side) {
+            D.assert(side != null);
+            return new Border(top: side, right: side, bottom: side, left: side);
+        }
+
         public static Border all(
             Color color = null,
             float width = 1.0f,
-            return new Border(top: side, right: side, bottom: side, left: side);
+            return Border.fromBorderSide(side);
         }
 
         public static Border merge(Border a, Border b) {

Runtime/painting/continuous_rectangle_border.cs

+using System.Runtime.CompilerServices;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.ui;
+using UnityEngine;
+using Canvas = Unity.UIWidgets.ui.Canvas;
+using Rect = Unity.UIWidgets.ui.Rect;
+
+namespace Unity.UIWidgets.painting {
+    public class ContinuousRectangleBorder : ShapeBorder {
+
+        public ContinuousRectangleBorder(
+            BorderSide side = null,
+            BorderRadius borderRadius = null) {
+            this.side = side ?? BorderSide.none;
+            this.borderRadius = borderRadius ?? BorderRadius.zero;
+        }
+
+        public readonly BorderRadius borderRadius;
+
+        public readonly BorderSide side;
+
+        public override EdgeInsets dimensions {
+            get {
+                return EdgeInsets.all(this.side.width);
+            }
+        }
+
+        public override ShapeBorder scale(float t) {
+            return new ContinuousRectangleBorder(
+                side: this.side.scale(t),
+                borderRadius: this.borderRadius * t
+            );
+        }
+
+        public override ShapeBorder lerpFrom(ShapeBorder a, float t) {
+            D.assert(t != null);
+            if (a is ContinuousRectangleBorder) {
+                return new ContinuousRectangleBorder(
+                    side: BorderSide.lerp((a as ContinuousRectangleBorder).side, this.side, t),
+                    borderRadius: BorderRadius.lerp((a as ContinuousRectangleBorder).borderRadius,
+                        this.borderRadius, t)
+                );
+            }
+            return base.lerpFrom(a, t);
+        }
+        
+        public override ShapeBorder lerpTo(ShapeBorder b, float t) {
+            D.assert(t != null);
+            if (b is ContinuousRectangleBorder) {
+                return new ContinuousRectangleBorder(
+                    side: BorderSide.lerp(this.side, (b as ContinuousRectangleBorder).side, t),
+                    borderRadius: BorderRadius.lerp(this.borderRadius,
+                        (b as ContinuousRectangleBorder).borderRadius, t)
+                );
+            }
+            return base.lerpTo(b, t);
+        }
+
+        float _clampToShortest(RRect rrect, float value) {
+            return value > rrect.shortestSide ? rrect.shortestSide : value;
+        }
+
+        Path _getPath(RRect rrect) {
+            float left = rrect.left;
+            float right = rrect.right;
+            float top = rrect.top;
+            float bottom = rrect.bottom;
+            
+            float tlRadiusX = Mathf.Max(0.0f, this._clampToShortest(rrect, rrect.tlRadiusX));
+            float tlRadiusY = Mathf.Max(0.0f, this._clampToShortest(rrect, rrect.tlRadiusY));
+            float trRadiusX = Mathf.Max(0.0f, this._clampToShortest(rrect, rrect.trRadiusX));
+            float trRadiusY = Mathf.Max(0.0f, this._clampToShortest(rrect, rrect.trRadiusY));
+            float blRadiusX = Mathf.Max(0.0f, this._clampToShortest(rrect, rrect.blRadiusX));
+            float blRadiusY = Mathf.Max(0.0f, this._clampToShortest(rrect, rrect.blRadiusY));
+            float brRadiusX = Mathf.Max(0.0f, this._clampToShortest(rrect, rrect.brRadiusX));
+            float brRadiusY = Mathf.Max(0.0f, this._clampToShortest(rrect, rrect.brRadiusY));
+
+            Path path = new Path();
+            path.moveTo(left, top + tlRadiusX);
+            path.cubicTo(left, top, left, top, left + tlRadiusY, top);
+            path.lineTo(right - trRadiusX, top);
+            path.cubicTo(right, top, right, top, right, top + trRadiusY);
+            path.lineTo(right, bottom - blRadiusX);
+            path.cubicTo(right, bottom, right, bottom, right - blRadiusY, bottom);
+            path.lineTo(left + brRadiusX, bottom);
+            path.cubicTo(left, bottom, left, bottom, left, bottom - brRadiusY);
+            path.close();
+            return path;
+        }
+
+        public override Path getInnerPath(Rect rect) {
+            return this._getPath(this.borderRadius.toRRect(rect).deflate(this.side.width));
+        }
+
+        public override Path getOuterPath(Rect rect) {
+            return this._getPath(this.borderRadius.toRRect(rect));
+        }
+
+        public override void paint(Canvas canvas, Rect rect) {
+            if (rect.isEmpty) {
+                return;
+            }
+
+            switch (this.side.style) {
+                case BorderStyle.none:
+                    break;
+                case BorderStyle.solid:
+                    Path path = this.getOuterPath(rect);
+                    Paint paint = this.side.toPaint();
+                    canvas.drawPath(path, paint);
+                    break;
+            }
+        }
+        public bool Equals(ContinuousRectangleBorder other) {
+            if (ReferenceEquals(null, other)) {
+                return false;
+            }
+
+            if (ReferenceEquals(this, other)) {
+                return true;
+            }
+
+            return this.side == other.side && this.borderRadius == other.borderRadius;
+        }
+
+        public override bool Equals(object obj) {
+            if (ReferenceEquals(null, obj)) {
+                return false;
+            }
+
+            if (ReferenceEquals(this, obj)) {
+                return true;
+            }
+
+            if (obj.GetType() != this.GetType()) {
+                return false;
+            }
+
+            return this.Equals((ContinuousRectangleBorder) obj);
+        }
+
+        public override int GetHashCode() {
+            var hashCode = (this.side != null ? this.side.GetHashCode() : 0);
+            hashCode = (hashCode * 397) ^ (this.borderRadius != null ? this.borderRadius.GetHashCode() : 0);
+            return hashCode;
+        }
+
+        public static bool operator ==(ContinuousRectangleBorder left, ContinuousRectangleBorder right) {
+            return Equals(left, right);
+        }
+
+        public static bool operator !=(ContinuousRectangleBorder left, ContinuousRectangleBorder right) {
+            return !Equals(left, right);
+        }
+
+        public override string ToString() {
+            return $"{this.GetType()}({this.side}, {this.borderRadius})";
+        }
+    }
+}

Runtime/painting/continuous_rectangle_border.cs.meta

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+guid: 85a3d3f3c01b8482e898b19fe9779021
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+  serializedVersion: 2
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