Fix traversal order issues caused by intersecting or overlapping child rects (#21235)

packages/flutter/lib/src/semantics/semantics.dart

@@ -2001,14 +2001,16 @@ class _SemanticsSortGroup extends Comparable<_SemanticsSortGroup> {
   List<SemanticsNode> sortedWithinVerticalGroup() {
     final List<_BoxEdge> edges = <_BoxEdge>[];
     for (SemanticsNode child in nodes) {
+      // Using a small delta to shrink child rects removes overlapping cases.
+      final Rect childRect = child.rect.deflate(0.1);
       edges.add(new _BoxEdge(
         isLeadingEdge: true,
-        offset: _pointInParentCoordinates(child, child.rect.topLeft).dx,
+        offset: _pointInParentCoordinates(child, childRect.topLeft).dx,
         node: child,
       ));
       edges.add(new _BoxEdge(
         isLeadingEdge: false,
-        offset: _pointInParentCoordinates(child, child.rect.bottomRight).dx,
+        offset: _pointInParentCoordinates(child, childRect.bottomRight).dx,
         node: child,
       ));
     }
@@ -2145,14 +2147,16 @@ Offset _pointInParentCoordinates(SemanticsNode node, Offset point) {
 List<SemanticsNode> _childrenInDefaultOrder(List<SemanticsNode> children, TextDirection textDirection) {
   final List<_BoxEdge> edges = <_BoxEdge>[];
   for (SemanticsNode child in children) {
+    // Using a small delta to shrink child rects removes overlapping cases.
+    final Rect childRect = child.rect.deflate(0.1);
     edges.add(new _BoxEdge(
       isLeadingEdge: true,
-      offset: _pointInParentCoordinates(child, child.rect.topLeft).dy,
+      offset: _pointInParentCoordinates(child, childRect.topLeft).dy,
       node: child,
     ));
     edges.add(new _BoxEdge(
       isLeadingEdge: false,
-      offset: _pointInParentCoordinates(child, child.rect.bottomRight).dy,
+      offset: _pointInParentCoordinates(child, childRect.bottomRight).dy,
       node: child,
     ));
   }

packages/flutter/test/material/search_test.dart

@@ -498,17 +498,6 @@ void main() {
                 label: routeName,
                 textDirection: TextDirection.ltr,
                 children: <TestSemantics>[
-                  new TestSemantics(
-                    id: 8,
-                    flags: <SemanticsFlag>[
-                      SemanticsFlag.isButton,
-                      SemanticsFlag.hasEnabledState,
-                      SemanticsFlag.isEnabled,
-                    ],
-                    actions: <SemanticsAction>[SemanticsAction.tap],
-                    label: 'Suggestions',
-                    textDirection: TextDirection.ltr,
-                  ),
                   new TestSemantics(
                     id: 9,
                     children: <TestSemantics>[
@@ -541,6 +530,17 @@ void main() {
                       ),
                     ],
                   ),
+                  new TestSemantics(
+                    id: 8,
+                    flags: <SemanticsFlag>[
+                      SemanticsFlag.isButton,
+                      SemanticsFlag.hasEnabledState,
+                      SemanticsFlag.isEnabled,
+                    ],
+                    actions: <SemanticsAction>[SemanticsAction.tap],
+                    label: 'Suggestions',
+                    textDirection: TextDirection.ltr,
+                  ),
                 ],
               ),
             ],

packages/flutter/test/semantics/traversal_order_test.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../widgets/semantics_tester.dart';
+
+void main() {
+  testWidgets('Traversal order handles touching elements', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Column(
+          children: new List<Widget>.generate(3, (int column) {
+            return new Row(children: List<Widget>.generate(3, (int row) {
+              return new Semantics(
+                child: new SizedBox(
+                  width: 50.0,
+                  height: 50.0,
+                  child: new Text('$column - $row'),
+                ),
+              );
+          }));
+        }),
+      ),
+    ));
+
+    final TestSemantics expected = new TestSemantics.root(
+      children: <TestSemantics>[
+        new TestSemantics(
+          id: 1,
+          textDirection: TextDirection.ltr,
+          children: <TestSemantics>[
+            new TestSemantics(
+              id: 2,
+              flags: <SemanticsFlag>[SemanticsFlag.scopesRoute],
+              children: <TestSemantics>[
+                new TestSemantics(
+                  id: 3,
+                  label: '0 - 0',
+                  textDirection: TextDirection.ltr,
+                ),
+                new TestSemantics(
+                  id: 4,
+                  label: '0 - 1',
+                  textDirection: TextDirection.ltr,
+                ),
+                new TestSemantics(
+                  id: 5,
+                  label: '0 - 2',
+                  textDirection: TextDirection.ltr,
+                ),
+                new TestSemantics(
+                  id: 6,
+                  label: '1 - 0',
+                  textDirection: TextDirection.ltr,
+                ),
+                new TestSemantics(
+                  id: 7,
+                  label: '1 - 1',
+                  textDirection: TextDirection.ltr,
+                ),
+                new TestSemantics(
+                  id: 8,
+                  label: '1 - 2',
+                  textDirection: TextDirection.ltr,
+                ),
+                new TestSemantics(
+                  id: 9,
+                  label: '2 - 0',
+                  textDirection: TextDirection.ltr,
+                ),
+                new TestSemantics(
+                  id: 10,
+                  label: '2 - 1',
+                  textDirection: TextDirection.ltr,
+                ),
+                new TestSemantics(
+                  id: 11,
+                  label: '2 - 2',
+                  textDirection: TextDirection.ltr,
+                ),
+              ],
+            ),
+          ],
+        ),
+      ],
+    );
+    expect(semantics, hasSemantics(expected, ignoreRect: true, ignoreTransform: true));
+    semantics.dispose();
+  });
+}