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

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
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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].

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…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].

<!-- Links -->
[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

…oval

_AnimatedScrollViewState._computeItemIndex derived the underlying sliver
index for insertItem/removeItem from the raw _itemsCount, which still
includes items whose removal animation is still in flight (_itemsCount
only decrements once that animation completes). insertItem's index
parameter is defined against the settled, post-removal item count (per
the class's own _indexToItemIndex contract), so computing it from the
raw count double-counted the still-animating-out items once
_indexToItemIndex shifted past them, overshooting _itemsCount and
tripping the "itemIndex >= 0 && itemIndex <= _itemsCount" assertion.

This is the same root cause as flutter#186361/flutter#186389, which fixed the
"are there enough items for a separator" check in removeItem's wrapper
but left _computeItemIndex itself using the raw count. Removing the
last item and immediately inserting a new one at the same index (the
documented, single-frame remove+insert pattern) hit the gap.

Exclude in-flight outgoing items from _computeItemIndex's count so all
of its callers (insertItem, insertAllItems, removeItem) agree with
_indexToItemIndex on what "settled item count" means.

Fixes flutter#179029

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>