TabBar should adjust scroll position when Controller is changed (#116019)

Co-authored-by: Bruno Leroux <bruno.leroux@gmail.com>

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

@@ -519,26 +519,35 @@ class _TabBarScrollPosition extends ScrollPositionWithSingleContext {
 
   final _TabBarState tabBar;
 
-  bool? _initialViewportDimensionWasZero;
+  bool _viewportDimensionWasNonZero = false;
+
+  // Position should be adjusted at least once.
+  bool _needsPixelsCorrection = true;
 
   @override
   bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
     bool result = true;
-    if (_initialViewportDimensionWasZero != true) {
-      // If the viewport never had a non-zero dimension, we just want to jump
-      // to the initial scroll position to avoid strange scrolling effects in
-      // release mode: In release mode, the viewport temporarily may have a
-      // dimension of zero before the actual dimension is calculated. In that
-      // scenario, setting the actual dimension would cause a strange scroll
-      // effect without this guard because the super call below would starts a
-      // ballistic scroll activity.
-      assert(viewportDimension != null);
-      _initialViewportDimensionWasZero = viewportDimension != 0.0;
+    if (!_viewportDimensionWasNonZero) {
+      _viewportDimensionWasNonZero = viewportDimension != 0.0;
+    }
+    // If the viewport never had a non-zero dimension, we just want to jump
+    // to the initial scroll position to avoid strange scrolling effects in
+    // release mode: In release mode, the viewport temporarily may have a
+    // dimension of zero before the actual dimension is calculated. In that
+    // scenario, setting the actual dimension would cause a strange scroll
+    // effect without this guard because the super call below would starts a
+    // ballistic scroll activity.
+    if (!_viewportDimensionWasNonZero || _needsPixelsCorrection) {
+      _needsPixelsCorrection = false;
       correctPixels(tabBar._initialScrollOffset(viewportDimension, minScrollExtent, maxScrollExtent));
       result = false;
     }
     return super.applyContentDimensions(minScrollExtent, maxScrollExtent) && result;
   }
+
+  void markNeedsPixelsCorrection() {
+    _needsPixelsCorrection = true;
+  }
 }
 
 // This class, and TabBarScrollPosition, only exist to handle the case
@@ -1027,6 +1036,13 @@ class _TabBarState extends State<TabBar> {
     if (widget.controller != oldWidget.controller) {
       _updateTabController();
       _initIndicatorPainter();
+      // Adjust scroll position.
+      if (_scrollController != null) {
+        final ScrollPosition position = _scrollController!.position;
+        if (position is _TabBarScrollPosition) {
+          position.markNeedsPixelsCorrection();
+        }
+      }
     } else if (widget.indicatorColor != oldWidget.indicatorColor ||
         widget.indicatorWeight != oldWidget.indicatorWeight ||
         widget.indicatorSize != oldWidget.indicatorSize ||

packages/flutter/test/material/tabs_test.dart

@@ -3420,6 +3420,49 @@ void main() {
     ));
   });
 
+  testWidgets('TabController changes with different initialIndex', (WidgetTester tester) async {
+    // This is a regression test for https://github.com/flutter/flutter/issues/115917
+    const Key lastTabKey = Key('Last Tab');
+    TabController? controller;
+
+    Widget buildFrame(int length) {
+      controller = TabController(
+        vsync: const TestVSync(),
+        length: length,
+        initialIndex: length - 1,
+      );
+      return boilerplate(
+        child: TabBar(
+          labelPadding: EdgeInsets.zero,
+          controller: controller,
+          isScrollable: true,
+          tabs: List<Widget>.generate(
+            length,
+            (int index) {
+              return SizedBox(
+                width: 100,
+                child: Tab(
+                  key: index == length - 1 ? lastTabKey : null,
+                  text: 'Tab $index',
+                ),
+              );
+            },
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(10));
+    expect(controller!.index, 9);
+    expect(tester.getCenter(find.byKey(lastTabKey)).dx, equals(750.0));
+
+    // Rebuild with a new controller with more tabs and last tab selected.
+    // Last tab should be visible and on the right of the window.
+    await tester.pumpWidget(buildFrame(15));
+    expect(controller!.index, 14);
+    expect(tester.getCenter(find.byKey(lastTabKey)).dx, equals(750.0));
+  });
+
   testWidgets('Default tab indicator color is white', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/15958
     final List<String> tabs = <String>['LEFT', 'RIGHT'];