Fix update order of SliverAppBar (#158159)

Fixes #158158

Similar to 154484, but addressing more fundamental causes. We should call the child's `didUpdateWidget` before the child's `build` when states change.

1. Remove redundant code but keep the old test because we don't need it after this fix.
2. Add getters of `lastShrinkOffset` and `lastOverlapsContent` that are needed when `SliverAppBar.update`.
3. Add a test with a custom TabBar to avoid breaking when some changes to the default TabBar.

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

@@ -1772,11 +1772,9 @@ class _TabBarState extends State<TabBar> {
         wrappedTabs[previousIndex] = _buildStyledTab(wrappedTabs[previousIndex], false, animation, _defaults);
       } else {
         // The user is dragging the TabBarView's PageView left or right.
-        if (_currentIndex! < widget.tabs.length) {
-          final int tabIndex = _currentIndex!;
-          final Animation<double> centerAnimation = _DragAnimation(_controller!, tabIndex);
-          wrappedTabs[tabIndex] = _buildStyledTab(wrappedTabs[tabIndex], true, centerAnimation, _defaults);
-        }
+        final int tabIndex = _currentIndex!;
+        final Animation<double> centerAnimation = _DragAnimation(_controller!, tabIndex);
+        wrappedTabs[tabIndex] = _buildStyledTab(wrappedTabs[tabIndex], true, centerAnimation, _defaults);
         if (_currentIndex! > 0) {
           final int tabIndex = _currentIndex! - 1;
           final Animation<double> previousAnimation = ReverseAnimation(_DragAnimation(_controller!, tabIndex));

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

@@ -162,7 +162,13 @@ abstract class RenderSliverPersistentHeader extends RenderSliver with RenderObje
   }
 
   bool _needsUpdateChild = true;
+
+  /// The most recent `shrinkOffset` passed to [updateChild].
+  double get lastShrinkOffset => _lastShrinkOffset;
   double _lastShrinkOffset = 0.0;
+
+  /// The most recent `overlapsContent` passed to [updateChild].
+  bool get lastOverlapsContent => _lastOverlapsContent;
   bool _lastOverlapsContent = false;
 
   /// Defines the parameters used to execute an [AsyncCallback] when a

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

@@ -286,6 +286,8 @@ class _SliverPersistentHeaderElement extends RenderObjectElement {
     final SliverPersistentHeaderDelegate oldDelegate = oldWidget.delegate;
     if (newDelegate != oldDelegate &&
         (newDelegate.runtimeType != oldDelegate.runtimeType || newDelegate.shouldRebuild(oldDelegate))) {
+      final _RenderSliverPersistentHeaderForWidgetsMixin renderObject = this.renderObject;
+      _updateChild(newDelegate, renderObject.lastShrinkOffset, renderObject.lastOverlapsContent);
       renderObject.triggerRebuild();
     }
   }
@@ -298,20 +300,19 @@ class _SliverPersistentHeaderElement extends RenderObjectElement {
 
   Element? child;
 
+  void _updateChild(SliverPersistentHeaderDelegate delegate, double shrinkOffset, bool overlapsContent) {
+    final Widget newWidget = delegate.build(this, shrinkOffset, overlapsContent);
+    child = updateChild(
+      child,
+      floating ? _FloatingHeader(child: newWidget) : newWidget,
+      null,
+    );
+  }
+
   void _build(double shrinkOffset, bool overlapsContent) {
     owner!.buildScope(this, () {
       final _SliverPersistentHeaderRenderObjectWidget sliverPersistentHeaderRenderObjectWidget = widget as _SliverPersistentHeaderRenderObjectWidget;
-      child = updateChild(
-        child,
-        floating
-          ? _FloatingHeader(child: sliverPersistentHeaderRenderObjectWidget.delegate.build(
-            this,
-            shrinkOffset,
-            overlapsContent
-          ))
-          : sliverPersistentHeaderRenderObjectWidget.delegate.build(this, shrinkOffset, overlapsContent),
-        null,
-      );
+      _updateChild(sliverPersistentHeaderRenderObjectWidget.delegate, shrinkOffset, overlapsContent);
     });
   }
 

packages/flutter/test/material/app_bar_sliver_test.dart

@@ -2274,6 +2274,53 @@ void main() {
     expect(navToolBar.middleSpacing, NavigationToolbar.kMiddleSpacing);
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/158158.
+  testWidgets('SliverAppBar should update TabBar before TabBar build', (WidgetTester tester) async {
+    final List<Tab> tabs = <Tab>[];
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return DefaultTabController(
+              length: tabs.length,
+              child: Scaffold(
+                body: CustomScrollView(
+                  slivers: <Widget>[
+                    SliverAppBar(
+                      actions: <Widget>[
+                        TextButton(
+                          child: const Text('Add Tab'),
+                          onPressed: () {
+                            setState(() {
+                              tabs.add(Tab(text: 'Tab ${tabs.length + 1}'));
+                            });
+                          },
+                        ),
+                      ],
+                      bottom: TabBar(tabs: tabs),
+                    ),
+                  ],
+                ),
+              ),
+            );
+          },
+        ),
+      ),
+    );
+
+    // Initializes with zero tabs.
+    expect(find.text('Tab 1'), findsNothing);
+    expect(find.text('Tab 2'), findsNothing);
+
+    // No crash after tabs added.
+    await tester.tap(find.text('Add Tab'));
+    await tester.pumpAndSettle();
+    expect(find.text('Tab 1'), findsOneWidget);
+    expect(find.text('Tab 2'), findsNothing);
+    expect(tester.takeException(), isNull);
+  });
+
   group('Material 2', () {
     // These tests are only relevant for Material 2. Once Material 2
     // support is deprecated and the APIs are removed, these tests