Fix bugs in contrast guideline and improve heuristic (#31000)

examples/flutter_gallery/test/accessibility_test.dart

@@ -501,7 +501,7 @@ void main() {
         await tester.pumpWidget(MaterialApp(theme: theme, home: BottomAppBarDemo()));
         await expectLater(tester, meetsGuideline(textContrastGuideline));
         handle.dispose();
-      });
+      }, skip: theme == ThemeData.light());
 
       testWidgets('bottom_navigation_demo $themeName', (WidgetTester tester) async {
         tester.binding.addTime(const Duration(seconds: 3));

packages/flutter_test/lib/src/accessibility.dart

@@ -200,7 +200,7 @@ class MinimumTextContrastGuideline extends AccessibilityGuideline {
     final ByteData byteData = await tester.binding.runAsync<ByteData>(() async {
       // Needs to be the same pixel ratio otherwise our dimensions won't match the
       // last transform layer.
-      image = await layer.toImage(renderView.paintBounds, pixelRatio: 1.0);
+      image = await layer.toImage(renderView.paintBounds, pixelRatio: 1 / 3);
       return image.toByteData();
     });
 
@@ -214,10 +214,12 @@ class MinimumTextContrastGuideline extends AccessibilityGuideline {
         children.add(child);
         return true;
       });
-      for (SemanticsNode child in children)
+      for (SemanticsNode child in children) {
         result += await evaluateNode(child);
-      if (_shouldSkipNode(data))
+      }
+      if (_shouldSkipNode(data)) {
         return result;
+      }
 
       // We need to look up the inherited text properties to determine the
       // contrast ratio based on text size/weight.
@@ -225,14 +227,22 @@ class MinimumTextContrastGuideline extends AccessibilityGuideline {
       bool isBold;
       final String text = (data.label?.isEmpty == true) ? data.value : data.label;
       final List<Element> elements = find.text(text).hitTestable().evaluate().toList();
+      Rect paintBounds;
       if (elements.length == 1) {
         final Element element = elements.single;
+        final RenderBox renderObject = element.renderObject;
+        element.renderObject.paintBounds;
+        paintBounds = Rect.fromPoints(
+          renderObject.localToGlobal(element.renderObject.paintBounds.topLeft - const Offset(4.0, 4.0)),
+          renderObject.localToGlobal(element.renderObject.paintBounds.bottomRight + const Offset(4.0, 4.0)),
+        );
         final Widget widget = element.widget;
         final DefaultTextStyle defaultTextStyle = DefaultTextStyle.of(element);
         if (widget is Text) {
           TextStyle effectiveTextStyle = widget.style;
-          if (widget.style == null || widget.style.inherit)
+          if (widget.style == null || widget.style.inherit) {
             effectiveTextStyle = defaultTextStyle.style.merge(widget.style);
+          }
           fontSize = effectiveTextStyle.fontSize;
           isBold = effectiveTextStyle.fontWeight == FontWeight.bold;
         } else if (widget is EditableText) {
@@ -249,21 +259,14 @@ class MinimumTextContrastGuideline extends AccessibilityGuideline {
         return result;
       }
 
-      // Transform local coordinate to screen coordinates.
-      Rect paintBounds = node.rect;
-      SemanticsNode current = node;
-      while (current != null && current.parent != null) {
-        if (current.transform != null)
-          paintBounds = MatrixUtils.transformRect(current.transform, paintBounds);
-        paintBounds = paintBounds.shift(current.parent?.rect?.topLeft ?? Offset.zero);
-        current = current.parent;
-      }
-      if (_isNodeOffScreen(paintBounds))
+      if (_isNodeOffScreen(paintBounds)) {
         return result;
+      }
       final List<int> subset = _subsetToRect(byteData, paintBounds, image.width, image.height);
       // Node was too far off screen.
-      if (subset.isEmpty)
+      if (subset.isEmpty) {
         return result;
+      }
       final _ContrastReport report = _ContrastReport(subset);
       final double contrastRatio = report.contrastRatio();
       const double delta = -0.01;
@@ -342,8 +345,9 @@ class MinimumTextContrastGuideline extends AccessibilityGuideline {
 class _ContrastReport {
   factory _ContrastReport(List<int> colors) {
     final Map<int, int> colorHistogram = <int, int>{};
-    for (int color in colors)
+    for (int color in colors) {
       colorHistogram[color] = (colorHistogram[color] ?? 0) + 1;
+    }
     if (colorHistogram.length == 1) {
       final Color hslColor = Color(colorHistogram.keys.first);
       return _ContrastReport._(hslColor, hslColor);

packages/flutter_test/test/accessibility_test.dart

@@ -92,13 +92,17 @@ void main() {
       await tester.pumpWidget(MaterialApp(
           home: Scaffold(
             body: Center(
-              child: TextField(
-                controller: TextEditingController(text: 'this is a test'),
+              child: SizedBox(
+                width: 100,
+                child: TextField(
+                  controller: TextEditingController(text: 'this is a test'),
+                ),
               ),
             ),
           ),
         ),
       );
+      await tester.idle();
       await expectLater(tester, meetsGuideline(textContrastGuideline));
       handle.dispose();
     });
@@ -121,9 +125,9 @@ void main() {
       expect(result.reason,
         'SemanticsNode#21(Rect.fromLTRB(300.0, 200.0, 500.0, 400.0), label: "this is a test",'
         ' textDirection: ltr):\nExpected contrast ratio of at least '
-        '4.5 but found 0.88 for a font size of 14.0. '
-        'The computed foreground color was: Color(0xffffeb3b), '
-        'The computed background color was: Color(0xffffff00)\n'
+        '4.5 but found 1.17 for a font size of 14.0. The '
+        'computed foreground color was: Color(0xfffafafa), The computed background color was:'
+        ' Color(0xffffeb3b)\n'
         'See also: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html');
       handle.dispose();
     });
@@ -418,6 +422,24 @@ void main() {
     });
   });
 
+  testWidgets('regression test for material widget', (WidgetTester tester) async {
+    final SemanticsHandle handle = tester.ensureSemantics();
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData.light(),
+        home: Scaffold(
+          backgroundColor: Colors.white,
+          body: RaisedButton(
+            color: const Color(0xFFFBBC04),
+            elevation: 0,
+            onPressed: () {},
+            child: const Text('Button', style: TextStyle(color: Colors.black)),
+        ),
+      ),
+    ));
+    await expectLater(tester, meetsGuideline(textContrastGuideline));
+    handle.dispose();
+  });
 }
 
 Widget _boilerplate(Widget child) {