[material_ui] Port flutter/flutter #189157 "Fix TabBar assertion to allow indicatorWeight: 0 with a themed indicator"

packages/material_ui/lib/src/tabs.dart

@@ -1066,8 +1066,7 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
     this.tabAlignment,
     this.textScaler,
     this.indicatorAnimation,
-  }) : _isPrimary = true,
-       assert(indicator != null || (indicatorWeight > 0.0));
+  }) : _isPrimary = true;
 
   /// Creates a Material Design secondary tab bar.
   ///
@@ -1128,8 +1127,7 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
     this.tabAlignment,
     this.textScaler,
     this.indicatorAnimation,
-  }) : _isPrimary = false,
-       assert(indicator != null || (indicatorWeight > 0.0));
+  }) : _isPrimary = false;
 
   /// Typically a list of two or more [Tab] widgets.
   ///
@@ -1185,7 +1183,8 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
   /// If [ThemeData.useMaterial3] is false, the default value is 2.0.
   ///
   /// If [indicator] is specified or provided from [TabBarThemeData],
-  /// this property is ignored.
+  /// this property is ignored. Otherwise, when drawing the default underline
+  /// indicator, the value of this parameter must be greater than zero.
   final double indicatorWeight;
 
   /// The padding for the indicator.
@@ -1644,6 +1643,24 @@ class _TabBarState extends State<TabBar> {
     if (tabBarTheme.indicator != null) {
       return tabBarTheme.indicator!;
     }
+    assert(() {
+      if (widget.indicatorWeight <= 0.0) {
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('Invalid indicatorWeight for TabBar.'),
+          ErrorDescription(
+            'The indicatorWeight must be greater than zero when the TabBar '
+            'draws its default underline indicator, that is when no indicator '
+            'is provided by the TabBar or the TabBarTheme.',
+          ),
+          ErrorHint(
+            'To fix this, set indicatorWeight to a value greater than zero, '
+            'or provide an indicator with the TabBar.indicator or '
+            'TabBarThemeData.indicator property.',
+          ),
+        ]);
+      }
+      return true;
+    }());
 
     Color color = widget.indicatorColor ?? tabBarTheme.indicatorColor ?? _defaults.indicatorColor!;
     // ThemeData tries to avoid this by having indicatorColor avoid being the

packages/material_ui/pending_changelogs/change_2026_08_30_tab_bar_indicator_weight.yaml

@@ -0,0 +1,3 @@
+changelog: |
+  - Allows `TabBar.indicatorWeight` of 0 when the indicator is supplied by `TabBarThemeData`.
+version: patch

packages/material_ui/test/tabs_test.dart

@@ -7,6 +7,7 @@ import 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:leak_tracker_flutter_testing/leak_tracker_flutter_testing.dart';
 import 'package:material_ui/material_ui.dart';
 
 import 'feedback_tester.dart';
@@ -3646,6 +3647,105 @@ void main() {
     );
   });
 
+  testWidgets('TabBar accepts indicatorWeight: 0 when the indicator comes from TabBarThemeData', (
+    WidgetTester tester,
+  ) async {
+    // Regression test for https://github.com/flutter/flutter/issues/188837.
+    // A themed indicator supplies the decoration, so indicatorWeight: 0 must not
+    // throw the default-underline assertion.
+    const indicatorColor = Color(0xFF00FF00);
+    const Decoration indicator = BoxDecoration(color: indicatorColor);
+    const tabs = <Widget>[Tab(text: 'A'), Tab(text: 'B')];
+
+    Widget buildFrame({required bool secondary}) {
+      return boilerplate(
+        tabBarTheme: const TabBarThemeData(
+          indicator: indicator,
+          indicatorSize: TabBarIndicatorSize.tab,
+        ),
+        child: Container(
+          alignment: Alignment.topLeft,
+          child: DefaultTabController(
+            length: tabs.length,
+            child: secondary
+                ? const TabBar.secondary(indicatorWeight: 0.0, tabs: tabs)
+                : const TabBar(indicatorWeight: 0.0, tabs: tabs),
+          ),
+        ),
+      );
+    }
+
+    // Primary TabBar: no assertion is thrown and the themed indicator is painted.
+    await tester.pumpWidget(buildFrame(secondary: false));
+    expect(tester.takeException(), isNull);
+
+    final RenderBox tabBarBox = tester.firstRenderObject<RenderBox>(find.byType(TabBar));
+    // 46 = _kTabHeight(46) with no weight added by the zero indicatorWeight.
+    expect(tabBarBox.size.height, 46.0);
+    expect(
+      tabBarBox,
+      paints..rect(rect: const Rect.fromLTRB(0.0, 0.0, 400.0, 46.0), color: indicatorColor),
+    );
+
+    // Secondary TabBar: also accepts a zero indicatorWeight with a themed indicator.
+    await tester.pumpWidget(buildFrame(secondary: true));
+    expect(tester.takeException(), isNull);
+  });
+
+  testWidgets('TabBar accepts indicatorWeight: 0 when the indicator is provided on the widget', (
+    WidgetTester tester,
+  ) async {
+    const Decoration indicator = BoxDecoration(color: Color(0xFF00FF00));
+    const tabs = <Widget>[Tab(text: 'A'), Tab(text: 'B')];
+
+    await tester.pumpWidget(
+      boilerplate(
+        child: DefaultTabController(
+          length: tabs.length,
+          child: const TabBar(indicator: indicator, indicatorWeight: 0.0, tabs: tabs),
+        ),
+      ),
+    );
+
+    expect(tester.takeException(), isNull);
+  });
+
+  testWidgets(
+    'TabBar throws indicatorWeight: 0 with the default underline indicator',
+    experimentalLeakTesting: LeakTesting.settings
+        .withIgnoredAll(), // leaking by design because of exception
+    (WidgetTester tester) async {
+      // With no indicator on the widget or the theme, the TabBar falls back to the
+      // default underline indicator, which requires a positive indicatorWeight.
+      const tabs = <Widget>[Tab(text: 'A'), Tab(text: 'B')];
+      final Matcher throwsInvalidIndicatorWeightError = isFlutterError.having(
+        (FlutterError error) => error.message,
+        'message',
+        contains('Invalid indicatorWeight for TabBar.'),
+      );
+
+      await tester.pumpWidget(
+        boilerplate(
+          child: DefaultTabController(
+            length: tabs.length,
+            child: const TabBar(indicatorWeight: 0.0, tabs: tabs),
+          ),
+        ),
+      );
+      expect(tester.takeException(), throwsInvalidIndicatorWeightError);
+
+      await tester.pumpWidget(
+        boilerplate(
+          child: DefaultTabController(
+            length: tabs.length,
+            child: const TabBar.secondary(indicatorWeight: 0.0, tabs: tabs),
+          ),
+        ),
+      );
+      expect(tester.takeException(), throwsInvalidIndicatorWeightError);
+    },
+  );
+
   testWidgets('TabBar with custom indicator - directional indicatorPadding (LTR)', (
     WidgetTester tester,
   ) async {