Add hitTestChildren to baseline-aligned stack (#172485)

Fixes http://github.com/flutter/flutter/issues/171510

packages/flutter/lib/src/cupertino/text_field.dart

@@ -1878,4 +1878,20 @@ class _RenderBaselineAlignedStack extends RenderBox
     assert(size.isFinite);
     return constraints.constrain(size);
   }
+
+  @override
+  bool hitTestChildren(BoxHitTestResult result, {required Offset position}) {
+    final RenderBox editableText = _editableTextChild;
+    final _BaselineAlignedStackParentData editableTextParentData =
+        editableText.parentData! as _BaselineAlignedStackParentData;
+
+    return result.addWithPaintOffset(
+      offset: editableTextParentData.offset,
+      position: position,
+      hitTest: (BoxHitTestResult result, Offset transformed) {
+        assert(transformed == position - editableTextParentData.offset);
+        return editableText.hitTest(result, position: transformed);
+      },
+    );
+  }
 }

packages/flutter/test/cupertino/text_field_test.dart

@@ -10462,6 +10462,19 @@ void main() {
     await performBaselineAlignmentCheck(20.0, 40.0);
   });
 
+  testWidgets('Editable text in text field with placeholder is hit-testable', (
+    WidgetTester tester,
+  ) async {
+    await tester.pumpWidget(
+      const CupertinoApp(
+        home: Center(child: CupertinoTextField(placeholder: 'placeholder')),
+      ),
+    );
+
+    expect(find.byType(CupertinoTextField), findsOneWidget);
+    expect(find.byType(EditableText).hitTestable(), findsOne);
+  });
+
   testWidgets('Start the floating cursor on long tap', (WidgetTester tester) async {
     EditableText.debugDeterministicCursor = true;
     final TextEditingController controller = TextEditingController(text: 'abcd');