When updating TabController widget, if _controller.index >= widget.length,...

When updating TabController widget, if _controller.index >= widget.length, update _animationController's value (#79535)

packages/flutter/lib/src/material/tab_controller.dart

@@ -180,6 +180,9 @@ class TabController extends ChangeNotifier {
     required int? length,
     required int? previousIndex,
   }) {
+    if (index != null) {
+      _animationController!.value = index.toDouble();
+    }
     return TabController._(
       index: index ?? _index,
       length: length ?? this.length,

packages/flutter/test/material/tabs_test.dart

@@ -3401,6 +3401,62 @@ void main() {
     // No other tab got instantiated during the animation.
     expect(log, <String>['init: 0', 'init: 3', 'dispose: 0']);
   });
+
+  testWidgets('TabController\'s animation value should be updated when TabController\'s index >= tabs\'s length', (WidgetTester tester) async {
+    // This is a regression test for the issue brought up here
+    // https://github.com/flutter/flutter/issues/79226
+
+    final List<String> tabs = <String>['A', 'B', 'C'];
+    await tester.pumpWidget(
+      MaterialApp(
+        home: StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return DefaultTabController(
+              length: tabs.length,
+              child: Scaffold(
+                appBar: AppBar(
+                  bottom: TabBar(
+                    tabs: tabs.map<Widget>((String tab) => Tab(text: tab)).toList(),
+                  ),
+                  actions: <Widget>[
+                    TextButton(
+                      child: const Text('Remove Last Tab'),
+                      onPressed: () {
+                        setState(() {
+                          tabs.removeLast();
+                        });
+                      },
+                    ),
+                  ],
+                ),
+                body: TabBarView(
+                  children: tabs.map<Widget>((String tab) => Tab(text: 'Tab child $tab')).toList(),
+                ),
+              )
+            );
+          },
+        ),
+      ),
+    );
+
+    TabController getController() => DefaultTabController.of(tester.element(find.text('B')))!;
+    TabController controller = getController();
+
+    controller.animateTo(2, duration: const Duration(milliseconds: 200), curve: Curves.linear);
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 300));
+
+    controller = getController();
+    expect(controller.index, 2);
+    expect(controller.animation!.value, 2);
+
+    await tester.tap(find.text('Remove Last Tab'));
+    await tester.pumpAndSettle();
+
+    controller = getController();
+    expect(controller.index, 1);
+    expect(controller.animation!.value, 1);
+  });
 }
 
 class KeepAliveInk extends StatefulWidget {