IgnoreBaseline widget (#131220)

Fixes https://github.com/flutter/flutter/issues/7037

packages/flutter/lib/src/rendering/flex.dart

@@ -916,7 +916,7 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
       while (child != null) {
         assert(() {
           if (textBaseline == null) {
-            throw FlutterError('To use FlexAlignItems.baseline, you must also specify which baseline to use using the "baseline" argument.');
+            throw FlutterError('To use CrossAxisAlignment.baseline, you must also specify which baseline to use using the "textBaseline" argument.');
           }
           return true;
         }());

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -842,6 +842,19 @@ class RenderIntrinsicHeight extends RenderProxyBox {
   }
 }
 
+/// Excludes the child from baseline computations in the parent.
+class RenderIgnoreBaseline extends RenderProxyBox {
+  /// Create a render object that causes the parent to ignore the child for baseline computations.
+  RenderIgnoreBaseline({
+    RenderBox? child,
+  }) : super(child);
+
+  @override
+  double? computeDistanceToActualBaseline(TextBaseline baseline) {
+    return null;
+  }
+}
+
 /// Makes its child partially transparent.
 ///
 /// This class paints its child into an intermediate buffer and then blends the

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

@@ -1266,6 +1266,7 @@ class PhysicalShape extends SingleChildRenderObjectWidget {
   }
 }
 
+
 // POSITIONING AND SIZING NODES
 
 /// A widget that applies a transformation before painting its child.
@@ -3548,8 +3549,6 @@ class IntrinsicHeight extends SingleChildRenderObjectWidget {
 ///  * The [catalog of layout widgets](https://flutter.dev/widgets/layout/).
 class Baseline extends SingleChildRenderObjectWidget {
   /// Creates a widget that positions its child according to the child's baseline.
-  ///
-  /// The [baseline] and [baselineType] arguments must not be null.
   const Baseline({
     super.key,
     required this.baseline,
@@ -3577,6 +3576,25 @@ class Baseline extends SingleChildRenderObjectWidget {
   }
 }
 
+/// A widget that causes the parent to ignore the [child] for the purposes
+/// of baseline alignment.
+///
+/// See also:
+///
+///  * [Baseline], a widget that positions a child relative to a baseline.
+class IgnoreBaseline extends SingleChildRenderObjectWidget {
+  /// Creates a widget that ignores the child for baseline alignment purposes.
+  const IgnoreBaseline({
+    super.key,
+    super.child,
+  });
+
+  @override
+  RenderIgnoreBaseline createRenderObject(BuildContext context) {
+    return RenderIgnoreBaseline();
+  }
+}
+
 
 // SLIVERS
 

packages/flutter/test/rendering/baseline_test.dart

@@ -52,4 +52,56 @@ void main() {
     expect(childParentData.offset.dy, equals(10.0));
     expect(parent.size, equals(const Size(100.0, 110.0)));
   });
+
+  test('RenderFlex and RenderIgnoreBaseline (control test -- with baseline)', () {
+    final RenderBox a, b;
+    final RenderBox root = RenderFlex(
+      crossAxisAlignment: CrossAxisAlignment.baseline,
+      textBaseline: TextBaseline.alphabetic,
+      textDirection: TextDirection.ltr,
+      children: <RenderBox>[
+        a = RenderParagraph(
+          const TextSpan(text: 'a', style: TextStyle(fontSize: 128.0, fontFamily: 'FlutterTest')), // places baseline at y=96
+          textDirection: TextDirection.ltr,
+        ),
+        b = RenderParagraph(
+          const TextSpan(text: 'b', style: TextStyle(fontSize: 32.0, fontFamily: 'FlutterTest')), // 24 above baseline, 8 below baseline
+          textDirection: TextDirection.ltr,
+        ),
+      ],
+    );
+    layout(root);
+
+    final Offset aPos = a.localToGlobal(Offset.zero);
+    final Offset bPos = b.localToGlobal(Offset.zero);
+    expect(aPos.dy, 0.0);
+    expect(bPos.dy, 96.0 - 24.0);
+  });
+
+  test('RenderFlex and RenderIgnoreBaseline (with ignored baseline)', () {
+    final RenderBox a, b;
+    final RenderBox root = RenderFlex(
+      crossAxisAlignment: CrossAxisAlignment.baseline,
+      textBaseline: TextBaseline.alphabetic,
+      textDirection: TextDirection.ltr,
+      children: <RenderBox>[
+        RenderIgnoreBaseline(
+          child: a = RenderParagraph(
+            const TextSpan(text: 'a', style: TextStyle(fontSize: 128.0, fontFamily: 'FlutterTest')),
+            textDirection: TextDirection.ltr,
+          ),
+        ),
+        b = RenderParagraph(
+          const TextSpan(text: 'b', style: TextStyle(fontSize: 32.0, fontFamily: 'FlutterTest')),
+          textDirection: TextDirection.ltr,
+        ),
+      ],
+    );
+    layout(root);
+
+    final Offset aPos = a.localToGlobal(Offset.zero);
+    final Offset bPos = b.localToGlobal(Offset.zero);
+    expect(aPos.dy, 0.0);
+    expect(bPos.dy, 0.0);
+  });
 }

packages/flutter/test/widgets/basic_test.dart

@@ -1135,6 +1135,62 @@ void main() {
       contains('verticalDirection: up'),
     ]));
   });
+
+  testWidgets('Row and IgnoreBaseline (control -- with baseline)', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Row(
+        crossAxisAlignment: CrossAxisAlignment.baseline,
+        textBaseline: TextBaseline.alphabetic,
+        textDirection: TextDirection.ltr,
+        children: <Widget>[
+          Text(
+            'a',
+            textDirection: TextDirection.ltr,
+            style: TextStyle(fontSize: 128.0, fontFamily: 'FlutterTest'), // places baseline at y=96
+          ),
+          Text(
+            'b',
+            textDirection: TextDirection.ltr,
+            style: TextStyle(fontSize: 32.0, fontFamily: 'FlutterTest'), // 24 above baseline, 8 below baseline
+          ),
+        ],
+      ),
+    );
+
+    final Offset aPos = tester.getTopLeft(find.text('a'));
+    final Offset bPos = tester.getTopLeft(find.text('b'));
+    expect(aPos.dy, 0.0);
+    expect(bPos.dy, 96.0 - 24.0);
+  });
+
+  testWidgets('Row and IgnoreBaseline (with ignored baseline)', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Row(
+        crossAxisAlignment: CrossAxisAlignment.baseline,
+        textBaseline: TextBaseline.alphabetic,
+        textDirection: TextDirection.ltr,
+        children: <Widget>[
+          IgnoreBaseline(
+            child: Text(
+              'a',
+              textDirection: TextDirection.ltr,
+              style: TextStyle(fontSize: 128.0, fontFamily: 'FlutterTest'), // places baseline at y=96
+            ),
+          ),
+          Text(
+            'b',
+            textDirection: TextDirection.ltr,
+            style: TextStyle(fontSize: 32.0, fontFamily: 'FlutterTest'), // 24 above baseline, 8 below baseline
+          ),
+        ],
+      ),
+    );
+
+    final Offset aPos = tester.getTopLeft(find.text('a'));
+    final Offset bPos = tester.getTopLeft(find.text('b'));
+    expect(aPos.dy, 0.0);
+    expect(bPos.dy, 0.0);
+  });
 }
 
 HitsRenderBox hits(RenderBox renderBox) => HitsRenderBox(renderBox);