Fix AnimatedList.separated assert when removing last item mid-removal… by mbcorona · Pull Request #186389 · flutter/flutter

mbcorona · 2026-05-12T03:54:13Z

Summary

AnimatedList.separated.removeItem used the sliver's total child count (_itemsCount) to decide whether the just-removed item was the last item in the list. That count still includes children animating out from earlier removeItem calls, so while a previous removal was in flight the new tail of the visible list was misclassified as a middle item. The separator removal then targeted the wrong index, and once the item being removed was added to _outgoingItems the next _indexToItemIndex lookup ran past _itemsCount and tripped the itemIndex >= 0 && itemIndex < _itemsCount assertion inside _SliverAnimatedMultiBoxAdaptorState.removeItem.

The fix captures _itemsCount - _outgoingItems.length before mutating the sliver and uses that visible count for both the "are there enough items to have a separator?" and "is this the new last item?" checks. When no removal is in flight _outgoingItems is empty and behavior is unchanged, so existing .separated coverage and the removeAllItems regression test from #176452 continue to pass.

Related issues

Fixes #186361.

Related prior fix for the non-separated removeAllItems variant: #176452 (issue #176362).

Test plan

Added a regression test in packages/flutter/test/widgets/animated_list_test.dart that reproduces the assertion on master and passes with this change: starts a 10s removal on a non-last item, pumps part of the animation, then removes the new last item, and asserts no exception is thrown.
flutter test packages/flutter/test/widgets/animated_list_test.dart packages/flutter/test/widgets/animated_grid_test.dart — 34 tests pass, including the existing comprehensive AnimatedList.separated test and the removeAllItems regression test from Fix(AnimatedScrollView): exclude outgoing items in removeAllItems #176452.
dart analyze on the touched files: no issues.
Manually verified with the reproducer from the issue: before the fix the assertion fires in the debug console; after the fix the second removal animates out cleanly.

AI disclosure

Written with assistance from Claude Code (Anthropic). I reviewed every change, ran the tests, and verified the fix manually before submitting.

Pre-launch Checklist

If you need help, consider asking for advice on the #hackers-new channel on Discord.

gemini-code-assist

Code Review

This pull request fixes an assertion failure in AnimatedList.separated that occurred when removing the last item while a previous removal animation was still active. The fix involves introducing _outgoingItemsCount to accurately determine the current visible item count. A regression test has been added to cover this scenario. Feedback was provided to correct a spelling variation in a comment to match the American English requirement of the style guide.

navaronbracke

LGTM with nit

navaronbracke · 2026-05-12T08:11:14Z

+      // All removal animations finished and the surviving item is alone.
+      expect(find.text('removing item 0'), findsNothing);
+      expect(find.text('removing item 2'), findsNothing);
+      expect(find.text('item 0'), findsOneWidget);


Same here for asserting on the two items that are now gone

Hi @navaronbracke , asserts were added in both places.

…flutter#186361)

flutter/flutter@259aeae...e03b91f 2026-05-20 engine-flutter-autoroll@skia.org Roll Packages from ade10ca to 1dfbada (6 revisions) (flutter/flutter#186811) 2026-05-20 brunocorona.alcantar@gmail.com Fix AnimatedList.separated assert when removing last item mid-removal… (flutter/flutter#186389) 2026-05-20 engine-flutter-autoroll@skia.org Roll Skia from d45969a5752e to 5f4f454b9662 (2 revisions) (flutter/flutter#186809) 2026-05-20 engine-flutter-autoroll@skia.org Roll Fuchsia Linux SDK from -F9Ci3Opxt06MixDl... to iKCvaD58jKStYGla0... (flutter/flutter#186796) 2026-05-20 engine-flutter-autoroll@skia.org Roll Skia from 19ad9707e5c6 to d45969a5752e (2 revisions) (flutter/flutter#186792) 2026-05-20 engine-flutter-autoroll@skia.org Roll Skia from 3471ebf5af0c to 19ad9707e5c6 (9 revisions) (flutter/flutter#186772) 2026-05-20 mdebbar@google.com [web] Refactor webparagraph painters to separate CK properly (flutter/flutter#186684) 2026-05-19 31859944+LongCatIsLooong@users.noreply.github.com Enable Swift testing in the iOS embedder (flutter/flutter#185712) 2026-05-19 mdebbar@google.com [web] Rename WebParagraph goldens (flutter/flutter#186680) 2026-05-19 engine-flutter-autoroll@skia.org Roll Skia from f1b406860c5e to 3471ebf5af0c (5 revisions) (flutter/flutter#186745) 2026-05-19 154381524+flutteractionsbot@users.noreply.github.com Revert "Ship gen_snapshot for linux-arm64 hosts targeting Android" (flutter/flutter#186693) 2026-05-19 bkonyi@google.com [ Tool ] Remove legacy analytics code (flutter/flutter#184994) 2026-05-19 chingjun@google.com Update Vulkan enum values (flutter/flutter#186694) 2026-05-19 1961493+harryterkelsen@users.noreply.github.com fix(web): Fixes CSS override detection when the browser has a default font size (flutter/flutter#186474) 2026-05-19 30870216+gaaclarke@users.noreply.github.com adds linux impeller hello world integration test (flutter/flutter#186715) 2026-05-19 jason-simmons@users.noreply.github.com Update the list of binaries in the code signing verification test to include new Dart snapshots (flutter/flutter#186754) 2026-05-19 brunocorona.alcantar@gmail.com Make EdgeDraggingAutoScroller respect ScrollPhysics (flutter/flutter#186541) 2026-05-19 bkonyi@google.com [ Widget Preview ] Fix inspector split resize focus loss over WebViews (flutter/flutter#186432) 2026-05-19 engine-flutter-autoroll@skia.org Roll Packages from b9bdd37 to ade10ca (1 revision) (flutter/flutter#186746) 2026-05-19 jason-simmons@users.noreply.github.com Manual Dart roll from 8e30b88e4d5a to 66873d2da857 (flutter/flutter#186690) 2026-05-19 bkonyi@google.com [ Widget Preview ] Improve zoom behavior and add zoom slider (flutter/flutter#186422) If this roll has caused a breakage, revert this CL and stop the roller using the controls here: https://autoroll.skia.org/r/flutter-packages Please CC stuartmorgan@google.com on the revert to ensure that a human is aware of the problem. To file a bug in Packages: https://github.com/flutter/flutter/issues/new/choose To report a problem with the AutoRoller itself, please file a bug: https://issues.skia.org/issues/new?component=1389291&template=1850622 Documentation for the AutoRoller is here: https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

flutter#186389) ## Summary `AnimatedList.separated.removeItem` used the sliver's total child count (`_itemsCount`) to decide whether the just-removed item was the last item in the list. That count still includes children animating out from earlier `removeItem` calls, so while a previous removal was in flight the new tail of the visible list was misclassified as a middle item. The separator removal then targeted the wrong index, and once the item being removed was added to `_outgoingItems` the next `_indexToItemIndex` lookup ran past `_itemsCount` and tripped the `itemIndex >= 0 && itemIndex < _itemsCount` assertion inside `_SliverAnimatedMultiBoxAdaptorState.removeItem`. The fix captures `_itemsCount - _outgoingItems.length` before mutating the sliver and uses that visible count for both the "are there enough items to have a separator?" and "is this the new last item?" checks. When no removal is in flight `_outgoingItems` is empty and behavior is unchanged, so existing `.separated` coverage and the `removeAllItems` regression test from flutter#176452 continue to pass. ## Related issues Fixes flutter#186361. Related prior fix for the non-separated `removeAllItems` variant: flutter#176452 (issue flutter#176362). ## Test plan - [x] Added a regression test in `packages/flutter/test/widgets/animated_list_test.dart` that reproduces the assertion on master and passes with this change: starts a 10s removal on a non-last item, pumps part of the animation, then removes the new last item, and asserts no exception is thrown. - [x] `flutter test packages/flutter/test/widgets/animated_list_test.dart packages/flutter/test/widgets/animated_grid_test.dart` — 34 tests pass, including the existing comprehensive `AnimatedList.separated` test and the `removeAllItems` regression test from flutter#176452. - [x] `dart analyze` on the touched files: no issues. - [x] Manually verified with the reproducer from the issue: before the fix the assertion fires in the debug console; after the fix the second removal animates out cleanly. ## AI disclosure Written with assistance from Claude Code (Anthropic). I reviewed every change, ran the tests, and verified the fix manually before submitting. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [ ] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [ ] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord].  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [AI contribution guidelines]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#ai-contribution-guidelines [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

…r#11747) flutter/flutter@259aeae...e03b91f 2026-05-20 engine-flutter-autoroll@skia.org Roll Packages from ade10ca to 1dfbada (6 revisions) (flutter/flutter#186811) 2026-05-20 brunocorona.alcantar@gmail.com Fix AnimatedList.separated assert when removing last item mid-removal… (flutter/flutter#186389) 2026-05-20 engine-flutter-autoroll@skia.org Roll Skia from d45969a5752e to 5f4f454b9662 (2 revisions) (flutter/flutter#186809) 2026-05-20 engine-flutter-autoroll@skia.org Roll Fuchsia Linux SDK from -F9Ci3Opxt06MixDl... to iKCvaD58jKStYGla0... (flutter/flutter#186796) 2026-05-20 engine-flutter-autoroll@skia.org Roll Skia from 19ad9707e5c6 to d45969a5752e (2 revisions) (flutter/flutter#186792) 2026-05-20 engine-flutter-autoroll@skia.org Roll Skia from 3471ebf5af0c to 19ad9707e5c6 (9 revisions) (flutter/flutter#186772) 2026-05-20 mdebbar@google.com [web] Refactor webparagraph painters to separate CK properly (flutter/flutter#186684) 2026-05-19 31859944+LongCatIsLooong@users.noreply.github.com Enable Swift testing in the iOS embedder (flutter/flutter#185712) 2026-05-19 mdebbar@google.com [web] Rename WebParagraph goldens (flutter/flutter#186680) 2026-05-19 engine-flutter-autoroll@skia.org Roll Skia from f1b406860c5e to 3471ebf5af0c (5 revisions) (flutter/flutter#186745) 2026-05-19 154381524+flutteractionsbot@users.noreply.github.com Revert "Ship gen_snapshot for linux-arm64 hosts targeting Android" (flutter/flutter#186693) 2026-05-19 bkonyi@google.com [ Tool ] Remove legacy analytics code (flutter/flutter#184994) 2026-05-19 chingjun@google.com Update Vulkan enum values (flutter/flutter#186694) 2026-05-19 1961493+harryterkelsen@users.noreply.github.com fix(web): Fixes CSS override detection when the browser has a default font size (flutter/flutter#186474) 2026-05-19 30870216+gaaclarke@users.noreply.github.com adds linux impeller hello world integration test (flutter/flutter#186715) 2026-05-19 jason-simmons@users.noreply.github.com Update the list of binaries in the code signing verification test to include new Dart snapshots (flutter/flutter#186754) 2026-05-19 brunocorona.alcantar@gmail.com Make EdgeDraggingAutoScroller respect ScrollPhysics (flutter/flutter#186541) 2026-05-19 bkonyi@google.com [ Widget Preview ] Fix inspector split resize focus loss over WebViews (flutter/flutter#186432) 2026-05-19 engine-flutter-autoroll@skia.org Roll Packages from b9bdd37 to ade10ca (1 revision) (flutter/flutter#186746) 2026-05-19 jason-simmons@users.noreply.github.com Manual Dart roll from 8e30b88e4d5a to 66873d2da857 (flutter/flutter#186690) 2026-05-19 bkonyi@google.com [ Widget Preview ] Improve zoom behavior and add zoom slider (flutter/flutter#186422) If this roll has caused a breakage, revert this CL and stop the roller using the controls here: https://autoroll.skia.org/r/flutter-packages Please CC stuartmorgan@google.com on the revert to ensure that a human is aware of the problem. To file a bug in Packages: https://github.com/flutter/flutter/issues/new/choose To report a problem with the AutoRoller itself, please file a bug: https://issues.skia.org/issues/new?component=1389291&template=1850622 Documentation for the AutoRoller is here: https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

flutter#186389) ## Summary `AnimatedList.separated.removeItem` used the sliver's total child count (`_itemsCount`) to decide whether the just-removed item was the last item in the list. That count still includes children animating out from earlier `removeItem` calls, so while a previous removal was in flight the new tail of the visible list was misclassified as a middle item. The separator removal then targeted the wrong index, and once the item being removed was added to `_outgoingItems` the next `_indexToItemIndex` lookup ran past `_itemsCount` and tripped the `itemIndex >= 0 && itemIndex < _itemsCount` assertion inside `_SliverAnimatedMultiBoxAdaptorState.removeItem`. The fix captures `_itemsCount - _outgoingItems.length` before mutating the sliver and uses that visible count for both the "are there enough items to have a separator?" and "is this the new last item?" checks. When no removal is in flight `_outgoingItems` is empty and behavior is unchanged, so existing `.separated` coverage and the `removeAllItems` regression test from flutter#176452 continue to pass. ## Related issues Fixes flutter#186361. Related prior fix for the non-separated `removeAllItems` variant: flutter#176452 (issue flutter#176362). ## Test plan - [x] Added a regression test in `packages/flutter/test/widgets/animated_list_test.dart` that reproduces the assertion on master and passes with this change: starts a 10s removal on a non-last item, pumps part of the animation, then removes the new last item, and asserts no exception is thrown. - [x] `flutter test packages/flutter/test/widgets/animated_list_test.dart packages/flutter/test/widgets/animated_grid_test.dart` — 34 tests pass, including the existing comprehensive `AnimatedList.separated` test and the `removeAllItems` regression test from flutter#176452. - [x] `dart analyze` on the touched files: no issues. - [x] Manually verified with the reproducer from the issue: before the fix the assertion fires in the debug console; after the fix the second removal animates out cleanly. ## AI disclosure Written with assistance from Claude Code (Anthropic). I reviewed every change, ran the tests, and verified the fix manually before submitting. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [ ] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [ ] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord].  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [AI contribution guidelines]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#ai-contribution-guidelines [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

flutter-dashboard Bot added the CICD Run CI/CD label May 12, 2026

github-actions Bot added framework flutter/packages/flutter repository. See also f: labels. f: scrolling Viewports, list views, slivers, etc. labels May 12, 2026

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Fix AnimatedList.separated assert when removing last item mid-removal (…

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…flutter#186361)

mbcorona dismissed navaronbracke’s stale review via 6e4a7c7 May 12, 2026 13:39

mbcorona force-pushed the fix-animated-list-separated-186361 branch from 8871713 to 6e4a7c7 Compare May 12, 2026 13:39

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Merged via the queue into flutter:master with commit 31f6edd May 20, 2026
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Roll Flutter from 259aeae0e0ba to 31f6edd8f6de (20 revisions) flutter/packages#11746

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Roll Flutter from 259aeae0e0ba to e03b91f1fe34 (21 revisions) flutter/packages#11747

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Fix AnimatedList.separated assert when removing last item mid-removal…#186389

Fix AnimatedList.separated assert when removing last item mid-removal…#186389
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mbcorona commented May 12, 2026

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