Incorrect rendering of `SnapshotWidget` (#114400)

* add tests

* try to fix

* fix compile error

* refactor code

* rename

* doc

* refactor test

* simple rename

packages/flutter/lib/src/material/page_transitions_theme.dart

@@ -907,7 +907,7 @@ class _ZoomEnterTransitionPainter extends SnapshotPainter {
   }
 
   @override
-  void paintSnapshot(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio) {
+  void paintSnapshot(PaintingContext context, Offset offset, Size size, ui.Image image, Size sourceSize, double pixelRatio) {
     _drawScrim(context, offset, size);
     _drawImageScaledAndCentered(context, image, scale.value, fade.value, pixelRatio);
   }
@@ -957,7 +957,7 @@ class _ZoomExitTransitionPainter extends SnapshotPainter {
   final LayerHandle<TransformLayer> _transformHandler = LayerHandle<TransformLayer>();
 
   @override
-  void paintSnapshot(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio) {
+  void paintSnapshot(PaintingContext context, Offset offset, Size size, ui.Image image, Size sourceSize, double pixelRatio) {
     _drawImageScaledAndCentered(context, image, scale.value, fade.value, pixelRatio);
   }
 

packages/flutter/lib/src/widgets/snapshot_widget.dart

@@ -231,6 +231,7 @@ class _RenderSnapshotWidget extends RenderProxyBox {
   }
 
   ui.Image? _childRaster;
+  Size? _childRasterSize;
   // Set to true if the snapshot mode was not forced and a platform view
   // was encountered while attempting to snapshot the child.
   bool _disableSnapshotAttempt = false;
@@ -249,6 +250,7 @@ class _RenderSnapshotWidget extends RenderProxyBox {
     painter.removeListener(markNeedsPaint);
     _childRaster?.dispose();
     _childRaster = null;
+    _childRasterSize = null;
     super.detach();
   }
 
@@ -258,6 +260,7 @@ class _RenderSnapshotWidget extends RenderProxyBox {
     painter.removeListener(markNeedsPaint);
     _childRaster?.dispose();
     _childRaster = null;
+    _childRasterSize = null;
     super.dispose();
   }
 
@@ -265,6 +268,7 @@ class _RenderSnapshotWidget extends RenderProxyBox {
     _disableSnapshotAttempt = false;
     _childRaster?.dispose();
     _childRaster = null;
+    _childRasterSize = null;
     markNeedsPaint();
   }
 
@@ -296,19 +300,24 @@ class _RenderSnapshotWidget extends RenderProxyBox {
     if (size.isEmpty) {
       _childRaster?.dispose();
       _childRaster = null;
+      _childRasterSize = null;
       return;
     }
     if (!controller.allowSnapshotting || _disableSnapshotAttempt) {
       _childRaster?.dispose();
       _childRaster = null;
+      _childRasterSize = null;
       painter.paint(context, offset, size, super.paint);
       return;
     }
-    _childRaster ??= _paintAndDetachToImage();
+    if (_childRaster == null) {
+      _childRaster = _paintAndDetachToImage();
+      _childRasterSize = size * devicePixelRatio;
+    }
     if (_childRaster == null) {
       painter.paint(context, offset, size, super.paint);
     } else {
-      painter.paintSnapshot(context, offset, size, _childRaster!, devicePixelRatio);
+      painter.paintSnapshot(context, offset, size, _childRaster!, _childRasterSize!, devicePixelRatio);
     }
   }
 }
@@ -356,11 +365,13 @@ abstract class SnapshotPainter extends ChangeNotifier  {
   /// [SnapshotPainter] paints the snapshot. This must account for the fact that the image
   /// width and height will be given in physical pixels, while the image must be painted with
   /// device independent pixels. That is, the width and height of the image is the widget and
-  /// height of the provided `size`, multiplied by the `pixelRatio`:
+  /// height of the provided `size`, multiplied by the `pixelRatio`. In addition, the actual
+  /// size of the scene captured by the `image` is not `image.width` or `image.height`, but
+  /// indeed `sourceSize`, because the former is a rounded inaccurate integer:
   ///
   /// ```dart
-  /// void paint(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio) {
-  ///   final Rect src = Rect.fromLTWH(0, 0, image.width.toDouble(), image.height.toDouble());
+  /// void paint(PaintingContext context, Offset offset, Size size, ui.Image image, Size sourceSize, double pixelRatio) {
+  ///   final Rect src = Rect.fromLTWH(0, 0, sourceSize.width, sourceSize.height);
   ///   final Rect dst = Rect.fromLTWH(offset.dx, offset.dy, size.width, size.height);
   ///   final Paint paint = Paint()
   ///     ..filterQuality = FilterQuality.low;
@@ -368,7 +379,7 @@ abstract class SnapshotPainter extends ChangeNotifier  {
   /// }
   /// ```
   /// {@end-tool}
-  void paintSnapshot(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio);
+  void paintSnapshot(PaintingContext context, Offset offset, Size size, ui.Image image, Size sourceSize, double pixelRatio);
 
   /// Paint the child via [painter], applying any effects that would have been painted
   /// in [SnapshotPainter.paintSnapshot].
@@ -427,8 +438,8 @@ class _DefaultSnapshotPainter implements SnapshotPainter {
   }
 
   @override
-  void paintSnapshot(PaintingContext context, ui.Offset offset, ui.Size size, ui.Image image, double pixelRatio) {
-    final Rect src = Rect.fromLTWH(0, 0, image.width.toDouble(), image.height.toDouble());
+  void paintSnapshot(PaintingContext context, ui.Offset offset, ui.Size size, ui.Image image, Size sourceSize, double pixelRatio) {
+    final Rect src = Rect.fromLTWH(0, 0, sourceSize.width, sourceSize.height);
     final Rect dst = Rect.fromLTWH(offset.dx, offset.dy, size.width, size.height);
     final Paint paint = Paint()
       ..filterQuality = FilterQuality.low;

packages/flutter/test/widgets/snapshot_widget_test.dart

@@ -9,8 +9,8 @@
 import 'dart:ui' as ui;
 
 import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
-import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -261,6 +261,42 @@ void main() {
     expect(tester.takeException(), isNull);
     expect(tester.layers.last, isA<PlatformViewLayer>());
   }, skip: kIsWeb); // TODO(jonahwilliams): https://github.com/flutter/flutter/issues/106689
+
+  testWidgets('SnapshotWidget should have same result when enabled', (WidgetTester tester) async {
+    tester.binding.window
+      ..physicalSizeTestValue = const Size(10, 10)
+      ..devicePixelRatioTestValue = 1;
+    addTearDown(() => tester.binding.window
+      ..clearPhysicalSizeTestValue()
+      ..clearDevicePixelRatioTestValue());
+
+    const ValueKey<String> repaintBoundaryKey = ValueKey<String>('boundary');
+    final SnapshotController controller = SnapshotController();
+    await tester.pumpWidget(RepaintBoundary(
+      key: repaintBoundaryKey,
+      child: MaterialApp(
+        debugShowCheckedModeBanner: false,
+        home: Container(
+          color: Colors.black,
+          padding: const EdgeInsets.only(right: 0.6, bottom: 0.6),
+          child: SnapshotWidget(
+            controller: controller,
+            child: Container(
+              margin: const EdgeInsets.only(right: 0.4, bottom: 0.4),
+              color: Colors.blue,
+            ),
+          ),
+        ),
+      ),
+    ));
+
+    final ui.Image imageWhenDisabled = (tester.renderObject(find.byKey(repaintBoundaryKey)) as RenderRepaintBoundary).toImageSync();
+
+    controller.allowSnapshotting = true;
+    await tester.pump();
+
+    await expectLater(find.byKey(repaintBoundaryKey), matchesReferenceImage(imageWhenDisabled));
+  }, skip: kIsWeb); // TODO(jonahwilliams): https://github.com/flutter/flutter/issues/106689
 }
 
 class TestController extends SnapshotController {
@@ -321,7 +357,7 @@ class TestPainter extends SnapshotPainter {
   }
 
   @override
-  void paintSnapshot(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio) {
+  void paintSnapshot(PaintingContext context, Offset offset, Size size, ui.Image image, Size sourceSize, double pixelRatio) {
     count += 1;
     lastImage = image;
   }