Created PixelBoundaryBox to force children to pixel sizes, applied that widget in all Gallery and FilmStrips that don't use custom pumps (Resolves #31)

lib/src/flutter/pixel_boundary_box.dart

@@ -0,0 +1,58 @@
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/rendering.dart';
+
+/// A widget that sizes itself and its [child] such that the child occupies an integer
+/// value width and height.
+///
+/// When laying out a [PixelBoundaryBox], the incoming constraints are pushed to the nearest
+/// integer boundary. The minimum width/height are made larger, to the nearest integer. The
+/// maximum width/height are made smaller, to the nearest integer. Then, the [child] is laid
+/// out within these integer bounds.
+///
+/// If the [child] reports a size that also has an integer width and height then that size
+/// is honored and layout finishes. However, the [child] chooses a size that include a fractional
+/// width and/or height, then the [child] is laid out a second time, with tight constraints, which
+/// are set to the nearest larger integer for the [child]'s originally reported width and height.
+class PixelBoundaryBox extends SingleChildRenderObjectWidget {
+  const PixelBoundaryBox({
+    super.key,
+    required super.child,
+  });
+
+  @override
+  RenderPixelBoundaryBox createRenderObject(BuildContext context) {
+    return RenderPixelBoundaryBox();
+  }
+}
+
+class RenderPixelBoundaryBox extends RenderProxyBox {
+  @override
+  void performLayout() {
+    if (child == null) {
+      size = Size.zero;
+      return;
+    }
+
+    final integerConstraints = constraints.copyWith(
+      minWidth: constraints.minWidth.ceilToDouble(),
+      maxWidth: constraints.maxWidth < double.infinity ? constraints.maxWidth.floorToDouble() : double.infinity,
+      minHeight: constraints.minHeight.ceilToDouble(),
+      maxHeight: constraints.maxHeight < double.infinity ? constraints.maxHeight.floorToDouble() : double.infinity,
+    );
+
+    // Let child choose its size within our parent's integer bounded constraints.
+    child!.layout(integerConstraints, parentUsesSize: true);
+
+    // Check if the child chose a non-integer size. If it did, re-run layout, forcing it
+    // to the nearest larger integer size.
+    final childSize = child!.size;
+    if (childSize.width != childSize.width.round() || childSize.height != childSize.height.round()) {
+      child!.layout(
+        BoxConstraints.tight(Size(childSize.width.ceilToDouble(), childSize.height.ceilToDouble())),
+        parentUsesSize: true,
+      );
+    }
+
+    size = child!.size;
+  }
+}

lib/src/scenes/film_strip.dart

@@ -8,6 +8,7 @@ import 'package:flutter/material.dart' hide Image;
 import 'package:flutter/material.dart' as m;
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter_test_goldens/src/flutter/flutter_test_extensions.dart';
+import 'package:flutter_test_goldens/src/flutter/pixel_boundary_box.dart';
 import 'package:flutter_test_goldens/src/goldens/golden_camera.dart';
 import 'package:flutter_test_goldens/src/goldens/golden_collections.dart';
 import 'package:flutter_test_goldens/src/goldens/golden_comparisons.dart';
@@ -55,7 +56,11 @@ class FilmStrip {
 
     _setup = _FilmStripSetup((tester) async {
       final widgetTree = sceneBuilder();
-      await _tester.pumpWidget(widgetTree);
+      await _tester.pumpWidget(
+        PixelBoundaryBox(
+          child: widgetTree,
+        ),
+      );
     });
 
     return this;

lib/src/scenes/gallery.dart

@@ -8,6 +8,7 @@ import 'package:flutter/material.dart' hide Image;
 import 'package:flutter/material.dart' as m;
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter_test_goldens/src/flutter/flutter_test_extensions.dart';
+import 'package:flutter_test_goldens/src/flutter/pixel_boundary_box.dart';
 import 'package:flutter_test_goldens/src/goldens/golden_camera.dart';
 import 'package:flutter_test_goldens/src/goldens/golden_collections.dart';
 import 'package:flutter_test_goldens/src/goldens/golden_comparisons.dart';
@@ -153,30 +154,34 @@ class Gallery {
       } else if (item.builder != null) {
         // Pump this gallery item, deferring to a `WidgetBuilder` for the content.
         await _tester.pumpWidget(
-          _itemScaffold(
-            _tester,
-            GoldenImageBounds(
-              child: _itemDecorator != null
-                  ? _itemDecorator.call(
-                      _tester,
-                      Builder(builder: item.builder!),
-                    )
-                  : Builder(builder: item.builder!),
+          PixelBoundaryBox(
+            child: _itemScaffold(
+              _tester,
+              GoldenImageBounds(
+                child: _itemDecorator != null
+                    ? _itemDecorator.call(
+                        _tester,
+                        Builder(builder: item.builder!),
+                      )
+                    : Builder(builder: item.builder!),
+              ),
             ),
           ),
         );
       } else {
         // Pump this gallery item, deferring to a `Widget` for the content.
         await _tester.pumpWidget(
-          _itemScaffold(
-            _tester,
-            GoldenImageBounds(
-              child: _itemDecorator != null
-                  ? _itemDecorator.call(
-                      _tester,
-                      item.child!,
-                    )
-                  : item.child!,
+          PixelBoundaryBox(
+            child: _itemScaffold(
+              _tester,
+              GoldenImageBounds(
+                child: _itemDecorator != null
+                    ? _itemDecorator.call(
+                        _tester,
+                        item.child!,
+                      )
+                    : item.child!,
+              ),
             ),
           ),
         );

lib/src/scenes/golden_scene.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart' show Colors;
 import 'package:flutter/widgets.dart';
+import 'package:flutter_test_goldens/src/flutter/pixel_boundary_box.dart';
 import 'package:flutter_test_goldens/src/goldens/golden_camera.dart';
 
 class GoldenScene extends StatelessWidget {

test/flutter/pixel_boundary_box_test.dart

@@ -0,0 +1,113 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter_test_goldens/src/flutter/pixel_boundary_box.dart';
+
+void main() {
+  group("Pixel boundary box >", () {
+    testWidgets("does not modify incoming integer constraints", (tester) async {
+      final pixelBoundaryKey = GlobalKey(debugLabel: "pixel-boundary");
+      final childKey = GlobalKey(debugLabel: "child");
+
+      await _pumpScaffold(
+        tester,
+        ConstrainedBox(
+          constraints: BoxConstraints(maxWidth: 100, maxHeight: 75),
+          child: PixelBoundaryBox(
+            key: pixelBoundaryKey,
+            child: Container(
+              key: childKey,
+              width: double.infinity,
+              height: double.infinity,
+              color: Colors.red,
+            ),
+          ),
+        ),
+      );
+
+      expect(tester.getSize(find.byKey(pixelBoundaryKey)), Size(100, 75));
+      expect(tester.getSize(find.byKey(childKey)), Size(100, 75));
+    });
+
+    testWidgets("adjusts incoming non-integer constraints", (tester) async {
+      final pixelBoundaryKey = GlobalKey(debugLabel: "pixel-boundary");
+      final childKey = GlobalKey(debugLabel: "child");
+
+      await _pumpScaffold(
+        tester,
+        ConstrainedBox(
+          constraints: BoxConstraints(maxWidth: 100.7, maxHeight: 75.8),
+          child: PixelBoundaryBox(
+            key: pixelBoundaryKey,
+            child: Container(
+              key: childKey,
+              width: double.infinity,
+              height: double.infinity,
+              color: Colors.red,
+            ),
+          ),
+        ),
+      );
+
+      expect(tester.getSize(find.byKey(pixelBoundaryKey)), Size(100, 75));
+      expect(tester.getSize(find.byKey(childKey)), Size(100, 75));
+    });
+
+    testWidgets("adjusts non-integer child size", (tester) async {
+      final pixelBoundaryKey = GlobalKey(debugLabel: "pixel-boundary");
+      final childKey = GlobalKey(debugLabel: "child");
+
+      // First, try a bounded size that's larger than the child.
+      await _pumpScaffold(
+        tester,
+        ConstrainedBox(
+          constraints: BoxConstraints(maxWidth: 100, maxHeight: 75),
+          child: PixelBoundaryBox(
+            key: pixelBoundaryKey,
+            child: Container(
+              key: childKey,
+              width: 88.5,
+              height: 48.2,
+              color: Colors.red,
+            ),
+          ),
+        ),
+      );
+
+      expect(tester.getSize(find.byKey(pixelBoundaryKey)), Size(89, 49));
+      expect(tester.getSize(find.byKey(childKey)), Size(89, 49));
+
+      // Second, try an unbounded parent.
+      await _pumpScaffold(
+        tester,
+        ConstrainedBox(
+          constraints: BoxConstraints(maxWidth: double.infinity, maxHeight: double.infinity),
+          child: PixelBoundaryBox(
+            key: pixelBoundaryKey,
+            child: Container(
+              key: childKey,
+              width: 88.5,
+              height: 48.2,
+              color: Colors.red,
+            ),
+          ),
+        ),
+      );
+
+      expect(tester.getSize(find.byKey(pixelBoundaryKey)), Size(89, 49));
+      expect(tester.getSize(find.byKey(childKey)), Size(89, 49));
+    });
+  });
+}
+
+Future<void> _pumpScaffold(WidgetTester tester, Widget content) async {
+  await tester.pumpWidget(
+    MaterialApp(
+      home: Scaffold(
+        body: Align(
+          alignment: Alignment.topLeft,
+          child: content,
+        ),
+      ),
+    ),
+  );
+}

test_goldens/flutter/buttons/buttons_test.dart

@@ -166,11 +166,17 @@ void main() {
       tester,
       sceneName: "button_extended_fab_gallery",
       layout: SceneLayout.row,
-      itemDecorator: (context, child) {
+      itemScaffold: (context, child) {
         return FlutterWidgetScaffold(
           child: child,
         );
       },
+      itemDecorator: (context, child) {
+        return Padding(
+          padding: const EdgeInsets.all(24),
+          child: child,
+        );
+      },
       goldenBackground: Image.memory(
         backgroundImageBytes,
         fit: BoxFit.cover,