[macOS] Handle reparenting platform views (#52152)

Fixes https://github.com/flutter/flutter/issues/146810

This handles situation where the platform view reparents and replaces
itself with a placeholder view. This commonly happens with WKWebView
entering full screen.

## Pre-launch Checklist

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Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

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shell/platform/darwin/macos/framework/Source/FlutterMutatorView.mm

diff --git a/shell/platform/darwin/macos/framework/Source/FlutterMutatorView.mm b/shell/platform/darwin/macos/framework/Source/FlutterMutatorView.mm
index e5bfd3a..c9036ec 100644
--- a/shell/platform/darwin/macos/framework/Source/FlutterMutatorView.mm
+++ b/shell/platform/darwin/macos/framework/Source/FlutterMutatorView.mm
@@ -602,9 +602,18 @@
   [containerSuperview addSubview:_platformViewContainer];
   _platformViewContainer.frame = self.bounds;
 
-  // Nest the platform view in the PlatformViewContainer.
-  [_platformViewContainer addSubview:_platformView];
-  _platformView.frame = untransformedBounds;
+  // Nest the platform view in the PlatformViewContainer, but only if the view doesn't have a
+  // superview yet. Sometimes the platform view reparents itself (WKWebView entering FullScreen)
+  // in which case do not forcefully move it back.
+  if (_platformView.superview == nil) {
+    [_platformViewContainer addSubview:_platformView];
+  }
+
+  // Originally first subview would be the _platformView. However during WKWebView full screen
+  // the platform view gets replaced with a placeholder. Given that _platformViewContainer does
+  // not contain any other views it is safe to assume that any subview found can be treated
+  // as the platform view.
+  _platformViewContainer.subviews.firstObject.frame = untransformedBounds;
 
   // Transform for the platform view is finalTransform adjusted for bounding rect origin.
   CATransform3D translation =

shell/platform/darwin/macos/framework/Source/FlutterMutatorViewTest.mm

diff --git a/shell/platform/darwin/macos/framework/Source/FlutterMutatorViewTest.mm b/shell/platform/darwin/macos/framework/Source/FlutterMutatorViewTest.mm
index 576ab9b..d352eef 100644
--- a/shell/platform/darwin/macos/framework/Source/FlutterMutatorViewTest.mm
+++ b/shell/platform/darwin/macos/framework/Source/FlutterMutatorViewTest.mm
@@ -562,6 +562,28 @@
   EXPECT_EQ([mutatorView hitTest:NSMakePoint(10, 50)], platformView);
 }
 
+TEST(FlutterMutatorViewTest, ReparentingPlatformView) {
+  NSView* platformView = [[NSView alloc] init];
+  FlutterMutatorView* mutatorView = [[FlutterMutatorView alloc] initWithPlatformView:platformView];
+  ApplyFlutterLayer(mutatorView, FlutterSize{100, 100}, {});
+  EXPECT_TRUE(platformView.superview == mutatorView.platformViewContainer);
+  EXPECT_TRUE(CGRectEqualToRect(platformView.frame, CGRectMake(0, 0, 100, 100)));
+
+  // Reparent platform view and replace it with placeholder (mimicking WKWebKit going full screen)
+  NSView* newParent = [[NSView alloc] init];
+  [newParent addSubview:platformView];
+  platformView.frame = CGRectMake(10, 10, 200, 200);
+
+  NSView* placeholderView = [[NSView alloc] init];
+  [mutatorView.platformViewContainer addSubview:placeholderView];
+  ApplyFlutterLayer(mutatorView, FlutterSize{100, 100}, {});
+
+  // Platform view should not be touched but the replacement view should be properly positioned.
+  EXPECT_TRUE(platformView.superview == newParent);
+  EXPECT_TRUE(CGRectEqualToRect(platformView.frame, CGRectMake(10, 10, 200, 200)));
+  EXPECT_TRUE(CGRectEqualToRect(placeholderView.frame, CGRectMake(0, 0, 100, 100)));
+}
+
 @interface FlutterCursorCoordinatorTest : NSObject
 
 @end