Fix NestedScrollView example crash when switching tabs on desktop by gabrimatic · Pull Request #186993 · flutter/flutter

gabrimatic · 2026-05-23T08:36:15Z

The NestedScrollView API example that uses a TabBar/TabBarView throws on desktop when you scroll an inner list and then switch tabs. The inner CustomScrollViews are coordinated by the NestedScrollView through a shared controller, which can be attached to more than one ScrollPosition at a time (for example mid-transition between tabs). The automatic desktop scrollbar attaches to that shared controller, and a single scrollbar cannot represent multiple positions, so RawScrollbar throws the multiple-position assertion.

This leaves the framework unchanged and fixes the example itself: the TabBarView body is wrapped in a ScrollConfiguration that disables the default scrollbars, since a single scrollbar cannot correctly represent the coordinated inner positions. A regression test reproduces the exact assertion from the issue (scroll, then a mouse-wheel pointer signal during a tab transition) and confirms it no longer throws.

Tests

flutter test examples/api/test/widgets/nested_scroll_view/nested_scroll_view.0_test.dart
flutter analyze examples/api/lib/widgets/nested_scroll_view/nested_scroll_view.0.dart examples/api/test/widgets/nested_scroll_view/nested_scroll_view.0_test.dart

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Code Review

This pull request modifies NestedScrollView to wrap its body in a ScrollConfiguration that disables automatic scrollbars, and adds a corresponding regression test for desktop platforms. Feedback suggests ensuring that any custom scrollBehavior provided to the NestedScrollView is respected by prioritizing it over the context-based ScrollConfiguration when disabling scrollbars.

Piinks

Hey @gabrimatic thanks for sending another PR. I am not sure this is the right solution to the reported problem.

NestedScrollView already removes default scrollbars from its outer scroll view because it cannot infer whether the nested scroll positions should be decorated as one scrollable or separate scrollables.

This is not exactly true. Scrollbars are disabled on the outer scrollable so that there are not two duplicate scrollbars in a nested scroll view.
Removing scrollbars entirely prevents the crash, but does not solve the actual problem. If someone applies a scrollbar manually, it will still be able to trigger the crash that was reported in the issue. Is there another way to solve the issue?

gabrimatic · 2026-06-12T00:51:52Z

Updated this to cover the manual scrollbar path too. The fix now lets scrollbars that implicitly use a shared PrimaryScrollController paint from their accepted ScrollNotifications while keeping drag/track gestures disabled when more than one position is attached, so a manual Scrollbar around NestedScrollView inner scrollables no longer hits the multiple-position assertion.\n\nLocal checks passed:\n- ../../bin/flutter test --no-pub test/widgets/nested_scroll_view_test.dart\n- ../../bin/flutter test --no-pub test/widgets/scrollbar_test.dart\n- ../../bin/flutter analyze --no-pub lib/src/widgets/scrollbar.dart lib/src/widgets/nested_scroll_view.dart test/widgets/nested_scroll_view_test.dart\n- git diff --check

Piinks

Thanks for the update. However, modifying RawScrollbar to accommodate multiple positions is not something we want to do:

A single scrollbar cannot correctly represent or control multiple scroll positions. The thumb will jump erratically if multiple lists scroll (e.g. during transitions), and because gestures are disabled when positions.length > 1 , it creates a dead/non-interactive scrollbar.

If a user hovers and scrolls a mouse wheel over the scrollbar track, it triggers _receivedPointerSignal which calls _cachedController!.position . Since multiple positions are attached, this will crash the application.

On desktop, the framework's ScrollBehavior.buildScrollbar explicitly passes controller: details.controller to Scrollbar. Because the controller is not null, tryPrimary remains false and the automatic scrollbars will still throw the multiple-positions assertion.

Since the goal of this PR is to fix the crash in the NestedScrollView sample code, maybe we should focus on that instead.

gabrimatic · 2026-07-14T21:56:38Z

Thanks @Piinks — agreed on all three points. I've reverted the RawScrollbar changes, so scrollbar.dart is back to its original state, and narrowed the PR to just the sample fix.

NestedScrollView now applies the same no-default-scrollbar treatment to its body that the outer scrollable already gets, so the automatic desktop scrollbar no longer attaches to the coordinated inner controller. Manual Scrollbars over multiple inner positions are left out of scope, since a single scrollbar can't meaningfully represent more than one position.

Verified locally against the issue's sample: reproduced the exact _debugCheckHasValidScrollPosition assertion with a desktop pointer-scroll during a tab transition (two positions attached), confirmed it's gone with the fix, and the full nested_scroll_view_test.dart suite + flutter analyze pass.

Piinks · 2026-07-14T22:17:12Z

NestedScrollView now applies the same no-default-scrollbar treatment to its body that the outer scrollable already gets, so the automatic desktop scrollbar no longer attaches to the coordinated inner controller. Manual Scrollbars over multiple inner positions are left out of scope, since a single scrollbar can't meaningfully represent more than one position.

Again, this is not a change we want to make. Disabling scrollbars on the inner scrollable in NestedScrollView is not the correct fix here. This breaks the current expected default behavior.

gabrimatic · 2026-07-14T22:57:14Z

You're right, thanks for your patience here. I've taken the fix out of the framework entirely.

scrollbar.dart and nested_scroll_view.dart are back to master (no framework changes at all now). The fix lives in the example that actually throws (nested_scroll_view.0.dart): the TabBarView body is wrapped in a ScrollConfiguration that disables the default scrollbars, since the inner views share the NestedScrollView's coordinated controller and a single scrollbar can't represent more than one position.

I also added a regression test on TargetPlatformVariant.desktop() that reproduces the exact assertion from the issue (scroll the inner list, then a mouse-wheel pointer signal during a tab transition) and confirms it no longer throws. Verified it fails on the unpatched sample and passes with the fix; flutter analyze and the example test suite are green.

Piinks

Thanks for the updates

Piinks

LGTM

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github-actions Bot added framework flutter/packages/flutter repository. See also f: labels. f: scrolling Viewports, list views, slivers, etc. labels May 23, 2026

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gabrimatic force-pushed the fix-nested-scrollbar-multiple-positions branch from 252e435 to 0a49a15 Compare July 14, 2026 21:54

gabrimatic force-pushed the fix-nested-scrollbar-multiple-positions branch from 0a49a15 to 262662b Compare July 14, 2026 22:56

gabrimatic changed the title ~~Disable default scrollbars for NestedScrollView bodies~~ Fix NestedScrollView example crash when switching tabs on desktop Jul 14, 2026

github-actions Bot added d: api docs Issues with https://api.flutter.dev/ d: examples Sample code and demos labels Jul 14, 2026

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gabrimatic requested a review from Piinks July 22, 2026 10:35

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gabrimatic added 2 commits July 28, 2026 16:48

Fix NestedScrollView example crash when switching tabs on desktop

c51c974

Reword the scrollbar comment in the NestedScrollView sample

8004ee1

Piinks force-pushed the fix-nested-scrollbar-multiple-positions branch from 1c9f99d to 8004ee1 Compare July 28, 2026 21:48

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Merged via the queue into flutter:master with commit abb7f63 Jul 29, 2026
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Manual roll Flutter from 2a230d1e8037 to 2757a77a73df (14 revisions) flutter/packages#12393

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Fix NestedScrollView example crash when switching tabs on desktop - #186993

Fix NestedScrollView example crash when switching tabs on desktop#186993
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gabrimatic commented May 23, 2026 •

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gabrimatic commented May 23, 2026 •

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