Fix `TabBar` assertion to allow `indicatorWeight: 0` with a themed `indicator` by Massinissa-Mouhoub · Pull Request #189157 · flutter/flutter

Massinissa-Mouhoub · 2026-07-08T22:06:37Z

This PR moves the assertion that checks indicator != null || (indicatorWeight > 0.0) from the TabBar constructors into the Decoration _getIndicator(TabBarIndicatorSize indicatorSize) method. During the first phase of creating the TabBar widget, Dart just fills in the fields, there is no BuildContext available yet, so the widget has no way of access the TabBarTheme via context.dependOnInheritedWidgetOfExactType<TabBarTheme>(). As a result, the constructor's assertion can only see the indicator passed directly to the widget, and it fails even when a valid indicator is supplied through the TabBarTheme.

By moving the assertion into the method mentionned above, which runs at build time, the TabBar now has access to the TabBarTheme and can validate the indicator correctly, accepting indicatorWeight: 0 when a themed indicator is present, while still asserting when the default underline indicator is used.

Fixes #188837

Pre-launch Checklist

gemini-code-assist

Code Review

This pull request updates the TabBar and TabBar.secondary constructors by removing the assertion that requires indicatorWeight to be greater than 0.0 when a custom indicator is provided. Instead, the assertion is now deferred to the state build phase, where it is only triggered if the default underline indicator is drawn. New widget tests have been added to verify that indicatorWeight can be 0.0 when a custom indicator is supplied via the widget or TabBarThemeData, and that it still asserts when using the default indicator. There are no review comments, and I have no feedback to provide.

Massinissa-Mouhoub · 2026-07-09T17:38:03Z

The Check Code Freeze job is failing because this PR touches frozen paths. Both files it changes are covered:

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

so there's no subset of the change I can split out to get the gate green.

For context on what the change does: TabBar's constructor asserts indicator != null || indicatorWeight > 0.0. That assert runs before there's a BuildContext, so it can't see an indicator supplied via TabBarThemeData, which means indicatorWeight: 0 throws even when the theme provides a decoration (#188837). This moves the assertion into void _getIndicator(TabBarIndicatorSize indicatorSize), where the theme has already been resolved and the check can be made against the indicator the TabBar will actually paint. Behavior is otherwise unchanged: the default underline indicator still requires a positive weight. Regression tests cover all three resolution paths (widget indicator, themed indicator, default underline).

I read #184093 as saying PRs like this should stay open and then be ported to flutter/packages once material_ui ships, so I'm happy to leave it here and do the port when the time comes. I don't think this is urgent enough to warrant override code freeze, there are workarounds (pass indicator directly to the TabBar, or leave indicatorWeight at its default), but I'll defer to the team if you'd rather it land now.

dkwingsmt

Can you also fix the doc for indicatorWeight, just like https://github.com/flutter/flutter/pull/188967/changes#diff-b90db0bc5b533dbd8ef4c5099057b292200e0aa96125e74ab3646c783b468bbdR1140 ? Here's my proposal:

  /// If [indicator] is specified or provided from [TabBarThemeData],
  /// this property is ignored. Otherwise, when drawing the default underline
  /// indicator, the value of this parameter must be greater than zero.

Otherwise LGTM.

dkwingsmt · 2026-07-17T21:59:21Z

+    assert(
+      widget.indicatorWeight > 0.0,
+      'The indicatorWeight must be greater than 0 when the TabBar '
+      'draws its default underline indicator (i.e. when no indicator is '
+      'provided by the TabBar or the TabBarTheme).',
+    );


The detailed message is great! I wonder if the displayed error screen can be even more understandable if you throw a FlutterError.fromParts from within an assert, something like

assert(() { throw FlutterError.fromParts(<DiagnosticsNode>[ ErrorSummary('Invalid indicatorWeight for TabBar.'), ErrorDescription('indicatorWeight must be greater than zero when using the default underline indicator.'), ErrorHint('To fix this, set indicatorWeight > 0 or specify an indicator in TabBarTheme.'), ]); return true; }());

Can you try?

I tried your approach with a small modification:

assert(() { if (widget.indicatorWeight <= 0.0) { // This was added to prevent throwing unconditionaly 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; }());

Below is the displayed error on-screen before and after:

Before After

Below is the error on console:
Before

════════ Exception caught by widgets library ═══════════════════════════════════ The following assertion was thrown building MediaQuery(MediaQueryData(size: Size(440.0, 956.0), devicePixelRatio: 3.0, textScaler: SystemTextScaler (no scaling), platformBrightness: Brightness.light, padding: EdgeInsets.zero, viewPadding: EdgeInsets.zero, viewInsets: EdgeInsets.zero, systemGestureInsets: EdgeInsets.zero, alwaysUse24HourFormat: false, accessibleNavigation: false, highContrast: false, onOffSwitchLabels: false, disableAnimations: false, invertColors: false, boldText: false, navigationMode: traditional, gestureSettings: DeviceGestureSettings(touchSlop: null), displayFeatures: [], supportsShowingSystemContextMenu: true, lineHeightScaleFactorOverride: null, letterSpacingOverride: null, wordSpacingOverride: null, paragraphSpacingOverride: null, displayCornerRadii: null)): The indicatorWeight must be greater than 0 when the TabBar draws its default underline indicator (i.e. when no indicator is provided by the TabBar or the TabBarTheme). 'package:flutter/src/material/tabs.dart': Failed assertion: line 1595 pos 7: 'widget.indicatorWeight > 0.0'

After

════════ Exception caught by widgets library ═══════════════════════════════════ The following assertion was thrown building MediaQuery(MediaQueryData(size: Size(440.0, 956.0), devicePixelRatio: 3.0, textScaler: SystemTextScaler (no scaling), platformBrightness: Brightness.light, padding: EdgeInsets.zero, viewPadding: EdgeInsets.zero, viewInsets: EdgeInsets.zero, systemGestureInsets: EdgeInsets.zero, alwaysUse24HourFormat: false, accessibleNavigation: false, highContrast: false, onOffSwitchLabels: false, disableAnimations: false, invertColors: false, boldText: false, navigationMode: traditional, gestureSettings: DeviceGestureSettings(touchSlop: null), displayFeatures: [], supportsShowingSystemContextMenu: true, lineHeightScaleFactorOverride: null, letterSpacingOverride: null, wordSpacingOverride: null, paragraphSpacingOverride: null, displayCornerRadii: null)): Invalid indicatorWeight for TabBar. 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. To fix this, set indicatorWeight to a value greater than zero, or provide an indicator with the TabBar.indicator or TabBarThemeData.indicator property.

I don't see an improvement on the displayed error, but I largly prefer the console error after the fix. I think it's better if we go with this new approach.

Massinissa-Mouhoub · 2026-07-18T19:08:54Z

dkwingsmt Please check the latest commit, I updated the docs and applied your suggestion for throwing the assertion.

QuncCccccc

Thanks for your contribution! The code fix looks great. Just left some comments in tests.

Massinissa-Mouhoub · 2026-07-23T20:25:13Z

@QuncCccccc I applied your comments, please check.

QuncCccccc

LGTM! Thanks a lot for your contribution:)

Massinissa-Mouhoub · 2026-07-27T14:23:05Z

@QuncCccccc, could you please add autosubmit to this PR since it's been approved?

dkwingsmt

LGTM

dkwingsmt · 2026-07-27T16:40:52Z

@Massinissa-Mouhoub Since this PR affects material, it's under the effect of the code freeze. Let's get the checks passed, then we'll archive this PR until the code freeze is lifted, at which time we'll provide a guide to port this PR to the new packages.

Massinissa-Mouhoub · 2026-07-27T20:42:06Z

@Massinissa-Mouhoub Since this PR affects material, it's under the effect of the code freeze. Let's get the checks passed, then we'll archive this PR until the code freeze is lifted, at which time we'll provide a guide to port this PR to the new packages.

Alright perfect, thank you!

dkwingsmt · 2026-07-27T20:52:39Z

There's a leakage failure:


04:21 +9484 ~15 -1: C:/b/s/w/ir/x/w/flutter/packages/flutter/test/material/tabs_test.dart: (tearDownAll) [E]
  Expected: leak free
    Actual: <Instance of 'Leaks'>
     Which: contains leaks:
            # The text is generated by leak_tracker.
            # For leak troubleshooting tips open:
            # https://github.com/flutter/flutter/blob/main/docs/contributing/testing/Leak-tracking.md
            notDisposed:
              total: 6
              objects:
                StatefulElement:
                  test: TabBar throws indicatorWeight: 0 with the default underline indicator
                  identityHashCode: 271796273
                _TabBarState:
                  test: TabBar throws indicatorWeight: 0 with the default underline indicator
                  identityHashCode: 683157412
                TabBarScrollController:
                  test: TabBar throws indicatorWeight: 0 with the default underline indicator
                  identityHashCode: 732348325
                StatefulElement:
                  test: TabBar throws indicatorWeight: 0 with the default underline indicator
                  identityHashCode: 992881765
                _TabBarState:
                  test: TabBar throws indicatorWeight: 0 with the default underline indicator
                  identityHashCode: 858281407
                TabBarScrollController:
                  test: TabBar throws indicatorWeight: 0 with the default underline indicator
                  identityHashCode: 175261228
            
            
  
  package:matcher                                              expect
  package:leak_tracker_testing/src/leak_testing.dart 69:24     LeakTesting.collectedLeaksReporter.<fn>
  package:leak_tracker_flutter_testing/src/testing.dart 74:37  maybeTearDownLeakTrackingForAll
  ===== asynchronous gap ===========================
  dart:async                                                   Zone.registerBinaryCallback
  package:flutter_test/src/test_compat.dart 302:3              _tearDownForTestFile

…indicator Covers flutter#188837: - themed indicator (TabBarThemeData.indicator) + indicatorWeight: 0 no longer asserts and renders the themed indicator (primary and secondary); - widget-level indicator + indicatorWeight: 0 does not assert; - default underline indicator + indicatorWeight: 0 still asserts.

…rWeight Per review feedback, replace the plain assert message with a FlutterError.fromParts thrown from within an assert, so the debug-mode error screen shows a structured summary, description, and fix hint. Also update the TabBar.indicatorWeight API documentation to state that the value must be greater than zero when the default underline indicator is used, and update the regression test to match the new FlutterError.

Massinissa-Mouhoub · 2026-07-29T23:57:28Z

@dkwingsmt can you please check? It's my first time dealing with leakage, so I'm not really sure of the way I solved it.

Massinissa-Mouhoub · 2026-08-06T09:18:00Z

@dkwingsmt can you take a look at this when you are free please ?

dkwingsmt

LGTM. Thank you for the PR and investigation on the leak! (And sorry for late review)

QuncCccccc

Re-LGTM. Thank you:)

dkwingsmt · 2026-08-06T19:59:16Z

Thanks for your contribution! This PR has reached completion, however we're not able to merge this PR for the moment due to the Material/Cupertino code freeze (#184093). As soon as the decoupling is complete, we will post a way to port this PR to the new packages to land. In the meantime, I'll convert this PR to a draft and apply a label so that we can quickly find it when the freeze is lifted.

Massinissa-Mouhoub · 2026-08-06T23:45:56Z

@dkwingsmt Alright thank you! I applied your comment suggestion in the last commit.

Massinissa-Mouhoub · 2026-08-30T22:25:31Z

@dkwingsmt @QuncCccccc this PR was ported in this PR please check it there.

QuncCccccc · 2026-09-02T22:14:32Z

Since we have a port PR for this fix, I'm closing this PR. Thanks a lot for porting it over!

…th a themed `indicator` (#12696) This is the port of this [PR](flutter/flutter#189157) after decoupling. Fixes flutter/flutter#188837. ## Original PR Description This PR moves the assertion that checks indicator != null || (indicatorWeight > 0.0) from the TabBar constructors into the Decoration _getIndicator(TabBarIndicatorSize indicatorSize) method. During the first phase of creating the TabBar widget, Dart just fills in the fields, there is no BuildContext available yet, so the widget has no way of access the TabBarTheme via context.dependOnInheritedWidgetOfExactType<TabBarTheme>(). As a result, the constructor's assertion can only see the indicator passed directly to the widget, and it fails even when a valid indicator is supplied through the TabBarTheme. By moving the assertion into the method mentionned above, which runs at build time, the TabBar now has access to the TabBarTheme and can validate the indicator correctly, accepting indicatorWeight: 0 when a themed indicator is present, while still asserting when the default underline indicator is used. Fixes flutter/flutter#188837 ## Pre-Review Checklist

Massinissa-Mouhoub changed the title ~~Issue 188837~~ Fix TabBar assertion to allow indicatorWeight: 0 with a themed indicator Jul 8, 2026

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github-actions Bot added framework flutter/packages/flutter repository. See also f: labels. p: material_ui material_ui package in flutter/packages labels Jul 8, 2026

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Fix TabBar indicatorWeight assertion with themed indicator #188967

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Massinissa-Mouhoub added 3 commits July 30, 2026 00:51

fix assertion issue

11ebdf4

Massinissa-Mouhoub added 2 commits July 30, 2026 00:51

test: run indicatorWeight regression tests under Material 3

fc63907

test: ignore expected leaks in the TabBar indicatorWeight error test

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This was referenced Aug 26, 2026

Fix todayBorder color being overridden by todayForegroundColor in YearPicker #189345

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[material_ui] Fix TabBar assertion to allow indicatorWeight: 0 with a themed indicator flutter/packages#12696

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QuncCccccc mentioned this pull request Sep 2, 2026

[material_ui] Let TabBarThemeData.indicator satisfy the indicatorWeight assert flutter/packages#12665

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