[canvaskit] fix TransformLayer.preroll (flutter/engine#22890)

engine/src/flutter/lib/web_ui/lib/src/engine/canvaskit/layer.dart

@@ -71,6 +71,11 @@ class PaintContext {
 abstract class ContainerLayer extends Layer {
   final List<Layer> _layers = <Layer>[];
 
+  /// The list of child layers.
+  ///
+  /// Useful in tests.
+  List<Layer> get debugLayers => _layers;
+
   /// Register [child] as a child of this layer.
   void add(Layer child) {
     child.parent = this;
@@ -294,46 +299,10 @@ class TransformLayer extends ContainerLayer
     final Matrix4 childMatrix = matrix * _transform;
     context.mutatorsStack.pushTransform(_transform);
     final ui.Rect childPaintBounds = prerollChildren(context, childMatrix);
-    paintBounds = _transformRect(_transform, childPaintBounds);
+    paintBounds = transformRect(_transform, childPaintBounds);
     context.mutatorsStack.pop();
   }
 
-  /// Applies the given matrix as a perspective transform to the given point.
-  ///
-  /// This function assumes the given point has a z-coordinate of 0.0. The
-  /// z-coordinate of the result is ignored.
-  static ui.Offset _transformPoint(Matrix4 transform, ui.Offset point) {
-    final Vector3 position3 = Vector3(point.dx, point.dy, 0.0);
-    final Vector3 transformed3 = transform.perspectiveTransform(position3);
-    return ui.Offset(transformed3.x, transformed3.y);
-  }
-
-  /// Returns a rect that bounds the result of applying the given matrix as a
-  /// perspective transform to the given rect.
-  ///
-  /// This function assumes the given rect is in the plane with z equals 0.0.
-  /// The transformed rect is then projected back into the plane with z equals
-  /// 0.0 before computing its bounding rect.
-  static ui.Rect _transformRect(Matrix4 transform, ui.Rect rect) {
-    final ui.Offset point1 = _transformPoint(transform, rect.topLeft);
-    final ui.Offset point2 = _transformPoint(transform, rect.topRight);
-    final ui.Offset point3 = _transformPoint(transform, rect.bottomLeft);
-    final ui.Offset point4 = _transformPoint(transform, rect.bottomRight);
-    return ui.Rect.fromLTRB(
-        _min4(point1.dx, point2.dx, point3.dx, point4.dx),
-        _min4(point1.dy, point2.dy, point3.dy, point4.dy),
-        _max4(point1.dx, point2.dx, point3.dx, point4.dx),
-        _max4(point1.dy, point2.dy, point3.dy, point4.dy));
-  }
-
-  static double _min4(double a, double b, double c, double d) {
-    return math.min(a, math.min(b, math.min(c, d)));
-  }
-
-  static double _max4(double a, double b, double c, double d) {
-    return math.max(a, math.max(b, math.max(c, d)));
-  }
-
   @override
   void paint(PaintContext paintContext) {
     assert(needsPainting);

engine/src/flutter/lib/web_ui/test/canvaskit/common.dart

@@ -45,6 +45,16 @@ void setUpCanvasKitTest() {
   });
 }
 
+/// Utility function for CanvasKit tests to draw pictures without
+/// the [CkPictureRecorder] boilerplate.
+CkPicture paintPicture(
+    ui.Rect cullRect, void Function(CkCanvas canvas) painter) {
+  final CkPictureRecorder recorder = CkPictureRecorder();
+  final CkCanvas canvas = recorder.beginRecording(cullRect);
+  painter(canvas);
+  return recorder.endRecording();
+}
+
 class _TestFinalizerRegistration {
   _TestFinalizerRegistration(this.wrapper, this.deletable, this.stackTrace);
 

engine/src/flutter/lib/web_ui/test/canvaskit/layer_test.dart

@@ -0,0 +1,55 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.12
+import 'dart:typed_data';
+
+import 'package:test/bootstrap/browser.dart';
+import 'package:test/test.dart';
+import 'package:ui/src/engine.dart';
+import 'package:ui/ui.dart' as ui;
+
+import 'common.dart';
+
+void main() {
+  internalBootstrapBrowserTest(() => testMain);
+}
+
+void testMain() {
+  group('CanvasKit', () {
+    setUpCanvasKitTest();
+
+    // Regression test for https://github.com/flutter/flutter/issues/63715
+    test('TransformLayer prerolls correctly', () async {
+      final EnginePlatformDispatcher dispatcher =
+          ui.window.platformDispatcher as EnginePlatformDispatcher;
+
+      final CkPicture picture =
+          paintPicture(ui.Rect.fromLTRB(0, 0, 30, 30), (CkCanvas canvas) {
+        canvas.drawRect(ui.Rect.fromLTRB(0, 0, 30, 30),
+            CkPaint()..style = ui.PaintingStyle.fill);
+      });
+
+      final LayerSceneBuilder sb = LayerSceneBuilder();
+      sb.pushClipRect(ui.Rect.fromLTRB(15, 15, 30, 30));
+
+      // Intentionally use a perspective transform, which triggered the
+      // https://github.com/flutter/flutter/issues/63715 bug.
+      sb.pushTransform(
+          Float64List.fromList(Matrix4.identity().storage
+            ..[15] = 2,
+      ));
+
+      sb.addPicture(ui.Offset.zero, picture);
+      final LayerTree layerTree = sb.build().layerTree;
+      dispatcher.rasterizer!.draw(layerTree);
+      final ClipRectLayer clipRect = layerTree.rootLayer as ClipRectLayer;
+      expect(clipRect.paintBounds, ui.Rect.fromLTRB(15, 15, 30, 30));
+
+      final TransformLayer transform = clipRect.debugLayers.single as TransformLayer;
+      expect(transform.paintBounds, ui.Rect.fromLTRB(0, 0, 30, 30));
+    });
+    // TODO: https://github.com/flutter/flutter/issues/60040
+  }, skip: isIosSafari);
+}