[macOS] Top-left origin for PlatformView container (flutter/engine#42523)

For consistency with Flutter (and all other platforms), Flutter views in the macOS embedder set `isFlipped` to ensure a co-ordinate system with the origin in the top-left, with y co-ordinates increasing towards the bottom edge of the view.

Previously, we were using a stock NSView as the container, which uses a bottom-left origin by default. Instead we extract the PlatformView container view as its own class with `isFlipped` true.

This doesn't correct the issue of the view anchorpoint/position but does correct rotation direction.

This also applies the transform back to origin prior to other transforms when adjusting the platformview position rather than after.

Issue: https://github.com/flutter/flutter/issues/124490

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterMutatorView.mm

@@ -12,8 +12,8 @@
 #include "flutter/shell/platform/embedder/embedder.h"
 
 @interface FlutterMutatorView () {
-  /// Each of these views clips to a CGPathRef. These views, if present,
-  /// are nested (first is child of FlutterMutatorView and last is parent of
+  // Each of these views clips to a CGPathRef. These views, if present,
+  // are nested (first is child of FlutterMutatorView and last is parent of
   // _platformView).
   NSMutableArray* _pathClipViews;
 
@@ -26,6 +26,21 @@
 
 @end
 
+/// Superview container for platform views, to which sublayer transforms are applied.
+@interface FlutterPlatformViewContainer : NSView
+@end
+
+@implementation FlutterPlatformViewContainer
+
+- (BOOL)isFlipped {
+  // Flutter transforms assume a coordinate system with an upper-left corner origin, with y
+  // coordinate values increasing downwards. This affects the view, view transforms, and
+  // sublayerTransforms.
+  return YES;
+}
+
+@end
+
 /// View that clips that content to a specific CGPathRef.
 /// Clipping is done through a CAShapeLayer mask, which avoids the need to
 /// rasterize the mask.
@@ -43,6 +58,9 @@
 }
 
 - (BOOL)isFlipped {
+  // Flutter transforms assume a coordinate system with an upper-left corner origin, with y
+  // coordinate values increasing downwards. This affects the view, view transforms, and
+  // sublayerTransforms.
   return YES;
 }
 
@@ -400,7 +418,7 @@ NSMutableArray* RoundedRectClipsFromMutations(CGRect master_clip, const Mutation
                             clipRect:(CGRect)clipRect {
   // Create the PlatformViewContainer view if necessary.
   if (_platformViewContainer == nil) {
-    _platformViewContainer = [[NSView alloc] initWithFrame:self.bounds];
+    _platformViewContainer = [[FlutterPlatformViewContainer alloc] initWithFrame:self.bounds];
     _platformViewContainer.wantsLayer = YES;
   }
 
@@ -409,14 +427,15 @@ NSMutableArray* RoundedRectClipsFromMutations(CGRect master_clip, const Mutation
   [containerSuperview addSubview:_platformViewContainer];
   _platformViewContainer.frame = self.bounds;
 
-  // Add the
+  // Nest the platform view in the PlatformViewContainer.
   [_platformViewContainer addSubview:_platformView];
   _platformView.frame = untransformedBounds;
 
   // Transform for the platform view is finalTransform adjusted for bounding rect origin.
-  _platformViewContainer.layer.sublayerTransform =
-      CATransform3DTranslate(transform, -transformedBounds.origin.x / transform.m11 /* scaleX */,
-                             -transformedBounds.origin.y / transform.m22 /* scaleY */, 0);
+  CATransform3D translation =
+      CATransform3DMakeTranslation(-transformedBounds.origin.x, -transformedBounds.origin.y, 0);
+  transform = CATransform3DConcat(transform, translation);
+  _platformViewContainer.layer.sublayerTransform = transform;
 
   // By default NSView clips children to frame. If masterClip is tighter than mutator view frame,
   // the frame is set to masterClip and child offset adjusted to compensate for the difference.

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterMutatorViewTest.mm

@@ -301,6 +301,45 @@ TEST(FlutterMutatorViewTest, RoundRectClipsToSimpleRectangle) {
   EXPECT_EQ(mutatorView.pathClipViews.count, 0ul);
 }
 
+// Ensure that the mutator view, clip views, and container all use a flipped y axis. The transforms
+// sent from the framework assume this, and so aside from the consistency with every other embedder,
+// we can avoid a lot of extra math.
+TEST(FlutterMutatorViewTest, ViewsSetIsFlipped) {
+  NSView* platformView = [[NSView alloc] init];
+  FlutterMutatorView* mutatorView = [[FlutterMutatorView alloc] initWithPlatformView:platformView];
+
+  std::vector<FlutterPlatformViewMutation> mutations{
+      {
+          .type = kFlutterPlatformViewMutationTypeClipRoundedRect,
+          .clip_rounded_rect =
+              FlutterRoundedRect{
+                  .rect = FlutterRect{110, 60, 150, 150},
+                  .upper_left_corner_radius = FlutterSize{10, 10},
+                  .upper_right_corner_radius = FlutterSize{10, 10},
+                  .lower_right_corner_radius = FlutterSize{10, 10},
+                  .lower_left_corner_radius = FlutterSize{10, 10},
+              },
+      },
+      {
+          .type = kFlutterPlatformViewMutationTypeTransformation,
+          .transformation =
+              FlutterTransformation{
+                  .scaleX = 1,
+                  .transX = 100,
+                  .scaleY = 1,
+                  .transY = 50,
+              },
+      },
+  };
+
+  ApplyFlutterLayer(mutatorView, FlutterSize{30, 20}, mutations);
+
+  EXPECT_TRUE(mutatorView.isFlipped);
+  ASSERT_EQ(mutatorView.pathClipViews.count, 1ul);
+  EXPECT_TRUE(mutatorView.pathClipViews.firstObject.isFlipped);
+  EXPECT_TRUE(mutatorView.platformViewContainer.isFlipped);
+}
+
 TEST(FlutterMutatorViewTest, RoundRectClipsToPath) {
   NSView* platformView = [[NSView alloc] init];
   FlutterMutatorView* mutatorView = [[FlutterMutatorView alloc] initWithPlatformView:platformView];