Add radius clamping to web RSuperellipse (flutter#172254)

This PR fixes rendering errors on Web when the provided corner radii sum
up larger than the size. It implements radius scaling using the same
algorithm as in [the C++
implementation](https://github.com/flutter/flutter/blob/b2d4210b3795413c2360968b685743a6df60ff50/engine/src/flutter/impeller/geometry/rounding_radii.cc).

Before: (error emerges for r>100, since the height is 200)
<img width="664" height="509" alt="image"
src="https://github.com/user-attachments/assets/eb526338-84d9-4eca-975b-d44bee0c11ac"
/>

After: (it stays this way for r>100)
<img width="611" height="471" alt="image"
src="https://github.com/user-attachments/assets/08ca2499-d5f7-47e1-9ecf-29f60c968016"
/>

It also fixes a bug that uses an incorrect starting point. 

Both changes are backed by the new test cases in
`rounded_superellipse_border_test.dart`.

## Pre-launch Checklist

- [ ] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [ ] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [ ] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: dkwingsmt

engine/src/flutter/lib/web_ui/lib/geometry.dart

@@ -1164,6 +1164,18 @@ class RSuperellipse extends _RRectLike<RSuperellipse> {
   @override
   final bool _uniformRadii;
 
+  static (double, double) _normalizeEmptyToZero(double inputX, double inputY) {
+    return (inputX > 0 && inputY > 0) ? (inputX, inputY) : (0, 0);
+  }
+
+  static double _adjustScale(double radius1, double radius2, double dimension, double scale) {
+    assert(radius1 >= 0.0 && radius2 >= 0.0 && dimension > 0.0);
+    if (radius1 + radius2 > dimension) {
+      return math.min(scale, dimension / (radius1 + radius2));
+    }
+    return scale;
+  }
+
   /// (Web only) Returns a [Path] for this shape and an [Offset] for its
   /// placement.
   ///
@@ -1180,9 +1192,9 @@ class RSuperellipse extends _RRectLike<RSuperellipse> {
   /// `path` and the `offset` to the `addPath` method.
   (Path, Offset) toPathOffset() {
     if (_uniformRadii) {
-      return (_RSuperellipseCache.instance.get(width, height, tlRadius), center);
+      return (_RSuperellipseCache.instance.get(width, height, _scaledUniformRadii()), center);
     } else {
-      return (_RSuperellipsePathBuilder.exact(this).path, Offset.zero);
+      return (_RSuperellipsePathBuilder.exact(_toScaledRadii()).path, Offset.zero);
     }
   }
 
@@ -1211,6 +1223,81 @@ class RSuperellipse extends _RRectLike<RSuperellipse> {
     );
   }
 
+  /// Returns a [RSuperellipse] whose corner radii are scaled based on this one,
+  /// ensuring that the sum of the corner radii on each side does not exceed the
+  /// width or height of the given bounds.
+  ///
+  /// See the [Skia scaling
+  /// implementation](https://github.com/google/skia/blob/main/src/core/SkRRect.cpp)
+  /// for more details.
+  RSuperellipse _toScaledRadii() {
+    if (!(width > 0 && height > 0)) {
+      return RSuperellipse.fromLTRBXY(left, top, right, bottom, 0.0, 0.0);
+    }
+
+    // If any corner is flat or has a negative value, normalize it to zeros
+    // We do this first so that the unnecessary non-flat part of that radius
+    // does not contribute to the global scaling below.
+    final (double tlRadiusX, double tlRadiusY) = _normalizeEmptyToZero(
+      this.tlRadiusX,
+      this.tlRadiusY,
+    );
+    final (double trRadiusX, double trRadiusY) = _normalizeEmptyToZero(
+      this.trRadiusX,
+      this.trRadiusY,
+    );
+    final (double blRadiusX, double blRadiusY) = _normalizeEmptyToZero(
+      this.blRadiusX,
+      this.blRadiusY,
+    );
+    final (double brRadiusX, double brRadiusY) = _normalizeEmptyToZero(
+      this.brRadiusX,
+      this.brRadiusY,
+    );
+
+    // Now determine a global scale to apply to all of the radii to ensure
+    // that none of the adjacent pairs of radius values sum to larger than
+    // the corresponding dimension of the rectangle.
+    double scale = 1.0;
+    scale = _adjustScale(tlRadiusX, trRadiusX, width, scale);
+    scale = _adjustScale(blRadiusX, brRadiusX, width, scale);
+    scale = _adjustScale(tlRadiusY, blRadiusY, height, scale);
+    scale = _adjustScale(trRadiusY, brRadiusY, height, scale);
+    if (scale < 1.0) {
+      return _create(
+        left: left,
+        top: top,
+        right: right,
+        bottom: bottom,
+        tlRadiusX: tlRadiusX * scale,
+        tlRadiusY: tlRadiusY * scale,
+        trRadiusX: trRadiusX * scale,
+        trRadiusY: trRadiusY * scale,
+        brRadiusX: brRadiusX * scale,
+        brRadiusY: brRadiusY * scale,
+        blRadiusX: blRadiusX * scale,
+        blRadiusY: blRadiusY * scale,
+        uniformRadii: _uniformRadii,
+      );
+    } else {
+      return this;
+    }
+  }
+
+  // A variation of `_toScaledRadii` that deals with uniform radii and returns a
+  // `Radius`.
+  Radius _scaledUniformRadii() {
+    assert(_uniformRadii);
+    if (!(width > 0 && height > 0)) {
+      return Radius.zero;
+    }
+    final (double radiusX, double radiusY) = _normalizeEmptyToZero(tlRadiusX, tlRadiusY);
+    double scale = 1.0;
+    scale = _adjustScale(radiusX, radiusX, width, scale);
+    scale = _adjustScale(radiusY, radiusY, height, scale);
+    return Radius.elliptical(radiusX * scale, radiusY * scale);
+  }
+
   static const RSuperellipse zero = RSuperellipse._raw();
 
   bool contains(Offset point) {

engine/src/flutter/lib/web_ui/lib/rsuperellipse_param.dart

@@ -336,6 +336,8 @@ class _RSuperellipseQuadrant {
   final _RSuperellipseOctant top;
   final _RSuperellipseOctant right;
 
+  bool get isSharpCorner => top.seN < 2 || right.seN < 2;
+
   void addToPath(
     _RSuperellipsePath path, {
     required bool reverse,
@@ -345,7 +347,7 @@ class _RSuperellipseQuadrant {
       _Transform.makeTranslate(offset),
       _Transform.makeScale(signedScale.scale(extraScale.width, extraScale.height)),
     );
-    if (top.seN < 2 || right.seN < 2) {
+    if (isSharpCorner) {
       if (!reverse) {
         final _Transform transformOctant = _Transform.makeComposite(
           transform,
@@ -406,13 +408,14 @@ class _RSuperellipsePathBuilder {
   // Build a path for an RSuperellipse with arbitrary position and radii.
   _RSuperellipsePathBuilder.exact(RSuperellipse r) : path = Path() {
     final _RSuperellipsePath p = _RSuperellipsePath(path);
-    final Offset start = Offset((r.left + r.right) / 2, r.top);
-    path.moveTo(start.dx, start.dy);
 
     final double topSplit = _split(r.left, r.right, r.tlRadiusX, r.trRadiusX);
     final double rightSplit = _split(r.top, r.bottom, r.trRadiusY, r.brRadiusY);
     final double bottomSplit = _split(r.left, r.right, r.blRadiusX, r.brRadiusX);
     final double leftSplit = _split(r.top, r.bottom, r.tlRadiusY, r.blRadiusY);
+
+    final Offset start = Offset(topSplit, r.top);
+    path.moveTo(start.dx, start.dy);
     _RSuperellipseQuadrant(
       Offset(topSplit, rightSplit),
       Offset(r.right, r.top),

packages/flutter/test/painting/rounded_superellipse_border_test.dart

@@ -253,6 +253,30 @@ void main() {
       matchesGoldenFile('painting.rounded_superellipse_border.all_elliptical.png'),
     );
 
+    await tester.pumpWidget(
+      containerWithBorder(const Size(120, 300), const BorderRadius.all(Radius.circular(600))),
+    );
+    await expectLater(
+      find.byType(Container),
+      matchesGoldenFile('painting.rounded_superellipse_border.clamping_uniform.png'),
+    );
+
+    await tester.pumpWidget(
+      containerWithBorder(
+        const Size(120, 300),
+        const BorderRadius.only(
+          topLeft: Radius.elliptical(1000, 1000),
+          topRight: Radius.elliptical(0, 1000),
+          bottomRight: Radius.elliptical(800, 1000),
+          bottomLeft: Radius.elliptical(100, 500),
+        ),
+      ),
+    );
+    await expectLater(
+      find.byType(Container),
+      matchesGoldenFile('painting.rounded_superellipse_border.clamping_non_uniform.png'),
+    );
+
     // Regression test for https://github.com/flutter/flutter/issues/170593
     await tester.pumpWidget(
       containerWithBorder(